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Iron Catalysis in Organic Synthesis Ingmar Bauer and Hans-Joachim Kn€olker* Department Chemie, Technische Universit€at Dresden, Bergstraße 66, 01069 Dresden, Germany

CONTENTS 1. Introduction 2. Substitution Reactions 2.1. Nucleophilic Substitution at sp3-Carbon Centers 2.1.1. Nucleophilic Substitution of Benzylic CX Bonds 2.1.2. Nucleophilic Substitution of Allylic CX Bonds 2.1.3. Nucleophilic Substitution of Propargylic CX Bonds 2.1.4. Nucleophilic Substitution of Nonactivated C(sp3)X Bonds 2.1.5. Nucleophilic Substitution of Sugar Derivatives 2.2. Electrophilic Aromatic Substitution 2.2.1. CHeteroatom Bond Forming Reactions 2.2.2. CC Bond Forming Reactions 2.3. Radical Aromatic Substitution 2.4. CC Bond Formation by Cross Coupling Reactions 2.4.1. CX/CMetal Cross Coupling Reactions 2.4.2. CMetal/CMetal Homocoupling and Cross Coupling Reactions 2.4.3. CX/CX Homocoupling and Cross Coupling Reactions 2.4.4. CC Bond Formation by CH Bond Activation 2.4.5. Decarboxylative and Decarbonylative Coupling r 2015 American Chemical Society

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2.5. CHeteroatom Bond Forming Cross Coupling Reactions 2.5.1. CX/Heteroatom Cross Coupling Reactions 2.5.2. CMetal/Heteroatom Cross Coupling Reactions 2.5.3. CH/Heteroatom Cross Coupling Reactions 3. Addition Reactions 3.1. Carbometalation 3.1.1. Carbometalation of Olefins 3.1.2. Carbometalation of Alkynes 3.1.3. Carbometalation of Allenes 3.2. Hydroalkylation and Hydroalkenylation 3.3. Hydroalkynylation 3.4. Twofold CC Bond Forming Additions 3.4.1. Cyclizing Arylalkylation and Arylacylation 3.4.2. Annulations 3.5. CC and CHeteroatom Bond Forming Additions 3.5.1. Haloalkylation 3.5.2. Acylations 3.5.3. Other CC and CHeteroatom Bond Forming Addition Reactions 3.6. Carboxylation and Carbonylation 3.7. Ring Opening Reactions 3.7.1. SN-Type Reactions 3.7.2. SN0 -Type Reactions 3.8. Intermolecular Ring Expansions 3.9. Polymerization 3.9.1. Olefin Polymerization 3.9.2. ATRP 3.9.3. Ring-Opening Polymerization 3.9.4. Ring-Opening Copolymerization 3.9.5. Other Polymerizations 3.10. CHeteroatom Bond Forming Additions to CC Double and Triple Bonds 3.10.1. Hydration, Hydroalkoxylation, and Hydrothiolation 3.10.2. Hydroamination and Hydroamidation

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Received: August 5, 2014 Published: March 09, 2015 3170

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Chemical Reviews 3.10.3. 3.10.4. 3.10.5. 3.10.6.

4. 5.

6.

7.

Hydrophosphination Hydroboration Haloalkoxylation Halolactonization and Halolactamization 3.10.7. Haloamination and Halosulfonation 3.10.8. Other CHeteroatom Bond Forming Addition Reactions 3.11. Insertion of Carbene, Nitrene, and Related Species 3.11.1. Addition Reactions with Carbenes 3.11.2. Addition Reactions with Nitrenes Elimination Reactions at Carbonyl Groups and Analogues 5.1. Addition to Aldehydes, Ketones, and Heteroanalogues 5.1.1. Aldol-Type Reactions 5.1.2. Allylation 5.1.3. Alkynylation 5.1.4. Arylation 5.1.5. Reformatsky-Type Reaction 5.1.6. Wittig-Type Reactions 5.1.7. Other CC Bond Forming Reactions at Carbonyl Groups 5.1.8. Reactions with Heteroatom Nucleophiles 5.2. Reactions of Acetals and Analogues 5.2.1. Allylation 5.2.2. Hydrolysis of Acetals 5.2.3. Glycosylations 5.3. Reactions at Carboxylic Acid Derivatives and Analogues 5.4. Reactions of Carbonic Acid Derivatives 5.5. Conjugate Addition to Carbonyl Groups and Analogues 5.5.1. Carbon Nucleophiles 5.5.2. Heteroatom Nucleophiles 5.6. Synthesis of Heterocycles 5.6.1. Five-Membered Heterocycles 5.6.2. Six-Membered Heterocycles 5.6.3. Seven-Membered Heterocycles Cycloadditions and Alder-Ene-Type Reactions 6.1. Cycloadditions 6.1.1. [2 + 1] Cycloaddition 6.1.2. [2 + 2] Cycloaddition 6.1.3. [2 + 2 + 1] Cycloaddition 6.1.4. [3 + 2] Cycloaddition 6.1.5. [2 + 2 + 2] Cycloaddition 6.1.6. [4 + 2] Cycloaddition 6.1.7. [5 + 2] Cycloaddition 6.2. Intermolecular Alder-Ene-Type Reactions Isomerizations and Rearrangements 7.1. Isomerization 7.1.1. Allyl AlcoholCarbonyl Isomerization

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7.1.2. Olefin Isomerization 7.1.3. Other Isomerizations and Rearrangements 7.2. Cycloisomerization 7.2.1. Intramolecular Alder-Ene Reactions 7.2.2. Nazarov Cyclization 7.3. Intramolecular Ring Expansion 8. Metathesis Reactions 8.1. Olefin Metathesis 8.2. CarbonylAlkyne Metathesis 9. Reductions 9.1. Hydrogenation of Alkenes and Alkynes 9.2. Hydrosilylation and Hydromagnesiation of Alkenes and Alkynes 9.3. Reduction of Alkenes with Other Reductants 9.4. Hydrogenation of Carbonyl Groups and Derivatives 9.4.1. Hydrogenation and Transfer Hydrogenation of Ketones and Aldehydes 9.4.2. Hydrogenation of Imines 9.4.3. Hydrogenation of Other Carbonyl Compounds 9.5. Hydrosilylation of Carbonyl Compounds and Derivatives 9.5.1. Hydrosilylation of Aldehydes, Ketones, and Derivatives 9.5.2. Reductive Amination and Amidation by Hydrosilylation 9.5.3. Hydrosilylation of Carboxylic Acids, Esters, and Chlorides 9.5.4. Hydrosilylation of Amides 9.6. Reduction of CdO with Other Reductants 9.7. Reductive Amination 9.8. Reductive Etherification 9.9. Hydrogenation of Heteroarenes 9.10. Reduction of Alcohols 9.11. Reduction of Ethers 9.12. Hydrodehalogenation 9.13. Reductive CC Coupling 9.14. Other Reductions at Carbon 9.15. Reduction of Heteroatoms 9.15.1. Reduction of Nitro Groups 9.15.2. Reduction of Azides 9.15.3. Reduction of Sulfonamides 9.15.4. Reduction of Sulfoxides 9.15.5. Reduction of Other HeteroatomContaining Functional Groups 10. Oxidations 10.1. Oxidation of C(sp3)H Bonds 10.2. Oxidation of C(sp2)H Bonds 10.2.1. Oxidation of Alkenes 10.2.2. Oxidation of Arenes 10.3. BaeyerVilliger Oxidation 3171

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Chemical Reviews 10.4. Wacker Oxidation 10.5. CC Bond Forming Oxidation Reactions 10.6. Oxidative CC Bond Cleavage 10.7. Other Oxidations 11. Various Cyclization and Annulation Reactions 12. Domino Reactions 13. Miscellaneous 14. Conclusions and Outlook Author Information Biographies Acknowledgment References

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1. INTRODUCTION The present review “Iron Catalysis in Organic Synthesis” covers the literature until the end of June 2014 and represents a comprehensive update of a previous article published by Bolm et al. in this journal in 2004 entitled “Iron-Catalyzed Reactions in Organic Synthesis”.1 Since the appearance of this previous review, a tremendous development in the field of iron catalysis has taken place. This applies especially to the chemistry covered in sections 2.1, 2.4, 2.5, and 9. But also in other areas the number of reports applying iron compounds as catalysts has exploded. It appears now that iron-catalyzed reactions cover almost the full scope of transformations which are presented in organic textbooks. Since 2004 several general reviews and highlight articles on iron catalysis have been published,29 the most comprehensive one being Plietker’s book Iron Catalysis in Organic Chemistry published in 2008.9 In addition, a large number of reviews, accounts, highlight articles, and book chapters have appeared with the focus on certain aspects of iron catalysis. This may be, for example, a specific class of iron catalysts, a specific reaction, or a reaction principle. Thus, the application of iron(III) chloride, as the most abundant iron compound in organic synthesis, has been the subject of reviews by Martín10 and Yinjuan.11 More generally, Lewis acid catalysis by iron compounds has been reviewed by Padron and Martín.12 The field of iron-catalyzed asymmetric synthesis has been summarized by Gopalaiah13 and Wills.14 Iron complexes with N-heterocyclic carbene (NHC) ligands as catalysts have been the topic of a recent article by Bezier, Sortais, and Darcel.15 Most recently, Herrmann, K€uhn, and co-workers provided a review on ironNHC complexes including their catalytic activity.16 Catalysis with iron pincer complexes has been covered by Bhattacharya and Guan.17 A microreview by DesageEl Murr, Fensterbank, and Malacria presents the fascinating chemistry of iron catalysis with noninnocent (redox active) ligands.18 Peters et al. summarized reports on the application of ferrocenes and half-sandwich iron complexes with the iron core involved in the catalytic process.19 Chiral ferrocenes with appropriate donor groups have also been applied as ligands for asymmetric reactions catalyzed by other metal catalysts.1923 In addition, ferrocenes have been used for the construction of planar chiral Lewis and Brønsted base catalysts. In particular, planar chiral 4-(dimethylamino)pyridine (DMAP) derivatives with a ferrocene backbone have been extensively investigated.2436 It should be noted that catalytic processes without iron participation, exploiting only the planar chirality of ferrocenes, are not comprehensively covered in the present review.

Moreover, iron-catalyzed reactions are often included in more general reviews on (transition) metal-catalyzed reactions. Depending on their focus they are mentioned in the respective section of this article. Examples of iron cocatalysis with other metals are provided as well. For example, iron/copper cocatalyzed reactions have been outlined by Jiao.37 In consequence of the recent development, the section on cross coupling reactions has been extended significantly as compared to the previous review.1 It includes two new subsections on iron-catalyzed CH bond activation reactions, an intriguing area that has emerged only recently. The rapid development in this field has been documented by a review in this journal provided by Shi et al. in 2011.38 Unlike the review by Bolm,1 the present article also includes a section on oxidation reactions (section 10). However, due to the vast number of publications in this area which is also scattered over biochemistry and inorganic chemistry, the section on oxidations covers only selected reactions in combination with a compilation of review articles. The use of iron compounds as catalysts in organic synthesis is attractive for a number of reasons. It is the most abundant metal in the earth’s crust after aluminum and therefore is much cheaper than the precious metals that are often applied. The considerable increase of prices for many transition and rare earth metals over the past decade demands cheap alternatives. On the other hand, various iron compounds are incorporated in biological systems. During the evolutionary process this metal has become an integral part of many metabolic processes including safe disposal pathways. As a result, a relatively low toxicity of many iron species is observed, which is of importance for many applications especially in the pharmaceutical industry, the food industry, and cosmetics. Regarding the catalytic efficiency and broad applicability, at present iron is still behind palladium as the most versatile catalytic metal in organic synthesis. However, the tremendously increasing number of publications demonstrates that iron is catching up. Unlike palladium, iron can adopt oxidation states from 2 to +5 (rarely +6). Thus, in low oxidation states it may be operative as an iron-centered nucleophile and catalyze reactions such as nucleophilic substitutions, additions to carboxylic substrates, hydrogenation/hydrosilylation, cycloisomerization, and others (see for example refs 39 and 40). In contrast to this rather new area, iron Lewis acid catalyzed reactions12 have been known for a long time, for example electrophilic aromatic substitutions. This applies to iron catalysts in the common oxidation states +2 and +3. An extensively investigated research area deals with iron-catalyzed oxidation reactions preferably of nonactivated CH bonds. This rapidly developing field relies on iron in high oxidation states (+3, +4, or even +5). Iron features not only a broad spectrum of oxidation states but also the ability to transfer one or two electrons to a substrate. This opens up the possibility either for radical reactions and also for two-electron transfer processes which commonly proceed in metal-catalyzed coupling reactions, such as oxidative addition and reductive elimination. For cross coupling processes, the redox couple Fe(I)/Fe(III) has emerged as a potent system (see section 2.4). Moreover, the diverse possibilities provided by the large number of accessible oxidation and spin states can be even extended by applying designed ligands which interfere actively in the catalytic process (redox active ligands). Having this potential in mind, one can predict that in the coming years we will see a large number of fascinating new iron-catalyzed reactions and their application in organic synthesis. 3172

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2. SUBSTITUTION REACTIONS 2.1. Nucleophilic Substitution at sp3-Carbon Centers

Nucleophilic substitutions represent a major field in organic chemistry. The most abundant reaction is the nucleophilic substitution at sp3-carbon centers. Depending on the nucleophile, this transformation can be used for the formation of carbonheteroatom bonds and carboncarbon bonds. Functional group transformations and the construction of carbon skeletons are achieved by this type of reaction. While in classical organic chemistry a broad variety of reactions of this type is known, only a few have been reported to be catalyzed by iron species. The very important CC bond forming reactions of organometallic reagents with C(sp3) electrophiles are not covered in this section. These transformations are described in section 2.4.1.4. Likewise, reactions with aromatic nucleophiles are discussed in section 2.2, for example, the FeCl3-catalyzed arylation of benzyl alcohols and benzyl carboxylates.41 Ringopening nucleophilic substitutions are regarded as addition reactions and thus are described in section 3.7. The oldest known iron-catalyzed nucleophilic substitutions at sp3-carbon centers are iron-catalyzed FriedelCrafts alkylations.42 However, until 2004, and thus within the coverage of the review by Bolm, only a few examples of iron-catalyzed nucleophilic substitutions had been reported.1 After 2004, these transformations, and in particular allylation reactions, have experienced a rapid development and were summarized in a number of reviews, highlight articles, and accounts.3,4,39,4350 Iron-catalyzed substitution reactions are very much focused on electrophiles with π-activated leaving groups (benzyl, allyl, and propargyl). A large number of conditions have been described to achieve such transformations even with poor leaving groups such as hydroxy or alkoxy (cf. sections 2.1.12.1.3). In contrast, a much more limited number of reports deals with the substitution of nonactivated aliphatic electrophiles (cf. section 2.1.4). 2.1.1. Nucleophilic Substitution of Benzylic CX Bonds. Benzylic groups are especially activated for nucleophilic substitution due to the facile formation of benzylic cations in the case of SN1 processes or for SN2 processes, by the overlap of the transition state quasi p-orbital with the π-system of the arene. Even benzylic hydroxy groups have been reported to undergo nucleophilic substitution by alcohols in the presence of iron(III) nitrate nonahydrate (Scheme 1).51 Thus, the reaction of benzhydrol with various alcohols proceeds under neat conditions at moderate temperatures of 7080 °C to give diphenylmethyl (DPM) ethers, mostly in high yields offering a new and green variant for the introduction of the DPM protecting group. In a similar way, benzylic hydroxy groups could be substituted by activated methylene compounds using iron(III) chloride as catalyst. The reaction has been reported independently by Beller (Scheme 2, eq a)52 and shortly thereafter by Jana (Scheme 2, eq b).53 This CC bond forming reaction is of interest as usually alcohols are only poor electrophiles in SN reactions due to the

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poor leaving group character of the hydroxy group. It is noteworthy that the reaction proceeds under very mild conditions, either in nitromethane at usually 50 °C using iron(III) chloride hexahydrate as catalyst,52 or in dichloromethane at reflux with catalytic amounts of anhydrous iron(III) chloride,53 providing mostly high to excellent yields of the alkylated 1,3-dicarbonyl compounds in both cases. In addition, Jana et al. also introduced cinnamyl alcohol as a representative of an allylic alcohol to alkylate acetylacetone in 58% yield (Scheme 2, eq c). A brief summary of the iron-catalyzed substitution of alcohols by carboncentered nucleophiles has been provided more recently.49 García and Moriel described a method for the benzylation of 1,3-dicarbonyl compounds by benzylic alcohols (cf. Scheme 2) using iron-containing ionic liquids on mesoporous carbon supports as catalyst.54 For the ionic liquid, the bistriflimide anion was crucial to get the substitution products in high yields. Among different carbon supports, those with a low microporosity showed the best performance. The carbon-supported catalyst could be recycled four times with no loss of activity. Similarly, the iron(III) chloride catalyzed reaction of benzhydrols or other benzyl alcohols with β-keto carboxylic acids led to alkylation with concomitant decarboxylation providing the corresponding benzylated ketones in high to excellent yields.55 Jana and co-workers utilized benzylic alcohols as electrophiles for SN reactions with carboxamides and sulfonamides using iron(III) chloride as catalyst (Scheme 3).56 This transformation proceeded in nitromethane, either at room temperature or at reflux affording benzylated amides in good to excellent yields. Vinylogous benzylic alcohols could be introduced as electrophiles accordingly. Reaction of aryl alkynes with benzylic alcohols under iron(III) chloride catalysis affords aryl ketones (Scheme 4).57 The yields for this transformation were generally moderate to good. 1-Ethynyl-4-methoxybenzene gave the best results with yields ranging from 62 to 80%, whereas the electron-deficient halosubstituted derivatives and phenylacetylene were less reactive (3953% yield).

Scheme 2

Scheme 1

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Scheme 6

Scheme 4

Scheme 7

Scheme 5

Scheme 8

Scheme 9 It has been proposed that the reaction proceeds via nucleophilic substitution of the benzylic hydroxy group by a second molecule of benzylic alcohol to form an intermediate dibenzyl ether (Scheme 5).57 Iron(III) Lewis acid activation of the hydroxy group increases its nucleofugacity. Analogous iron(III) activation of the ether is followed by nucleophilic attack of the acetylene displacing an iron alkoxide. Addition of water to the resulting vinyl cation affords the aryl benzyl ketone. It has been shown that the proposed intermediate dibenzyl ethers are transformed to aryl ketones as well, when treated with aryl alkynes under these conditions. Such a formal hydroxybenzylation of phenylacetylene with benzylic alcohols has also been achieved by Jefferies and Cook in the presence of iron(III) chloride as catalyst and silver hexafluoroantimonate as additive (Scheme 6).58 This procedure led to alkyl phenyl ketones in slightly higher yields than those achieved using the conditions reported by Jana et al. However, Cook and Jefferies postulated an alternative mechanism for this transformation (Scheme 6).58 In contrast, direct coupling of benzhydrols with terminal alkynes was achieved using iron(III) triflate as catalyst and trifluoromethanesulfonic acid as cocatalyst (Scheme 7).59

The reaction proceeded in 1,2-dichloroethane (DCE) at reflux to give alkynylated diarylmethanes in good to high yields. Cossy and Reymond demonstrated that the Ritter reaction of benzylic alcohols with nitriles and water can be catalyzed by iron(III) chloride hexahydrate (Scheme 8).60 They used this method for the synthesis of various benzylamides mostly in high yields. In addition, tert-butyl acetate could be employed as electrophile to provide the corresponding tert-butylamides (cf. Scheme 56). In a subsequent paper the group presented new conditions, which also allowed the conversion of unactivated cyclic alcohols (cf. section 2.1.4). Using unmodified magnetite as catalyst, a selective N-monoalkylation of primary arylamines with benzylic alcohols has been achieved in moderate to excellent yields (Scheme 9).61 3174

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Chemical Reviews Scheme 10

Scheme 11

Mechanistically, the reaction proceeds via a hydrogen autotransfer process including dehydrogenation of the benzylic alcohol to a benzaldehyde followed by imine condensation and hydrogenation of the imine. Aliphatic alcohols do not react under these conditions. Aliphatic amines showed only a low reactivity in a competitive experiment and led preferentially to imines. More recently, the group of Shi presented an NiCuFeOx catalyst for the alkylation of ammonia or amines with benzylic alcohols using the same principle of hydrogen autotransfer (Scheme 10).62 The amine component was not restricted to arylamines (eq a). Primary and secondary aliphatic amines have been converted with similar success (eq b). Ammonia could be double benzylated under these conditions (eq c). Due to its magnetic properties, the catalyst can be easily recovered and reused. The electrophile is not limited to benzylic alcohols, as other primary and secondary alcohols have also been successfully employed (cf. section 2.1.4, Scheme 59). The allylation of benzylic alcohols and benzylic halides has been reported by Liu and co-workers (Scheme 11).63 Reaction of these substrates with allyltrimethylsilane in the presence of catalytic amounts of iron(III) chloride hexahydrate in dichloromethane at room temperature provided the allylated products often in high to excellent yields. The sulfonation of benzylic and vinylogous benzylic alcohols with sodium arenesulfinates was achieved in the presence of iron(III) chloride as catalyst and chlorotrimethylsilane in dichloromethane at reflux (Scheme 12).64 High yields have been reported for secondary benzylic alcohols, whereas primary

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Scheme 12

benzylic alcohols gave only moderate results (Scheme 12, eq a). The reaction of vinylogous benzylic alcohols occurred under ipsosubstitution (eq b). An unanticipated attack at the double bond with concomitant double bond isomerization was observed for homoallylic benzyl alcohols (eq c). The mechanistic explanation via formation of an intermediate π-allyliron complex from an unactivated olefin, however, appears to be equivocal. Carboxamides have been exploited as leaving groups in a combined benzylic/vinylogous benzylic position. Thus, the reaction of N-allyl amides with 1,3-dicarbonyl compounds using iron(III) chloride as catalyst led to CC bond formation in an SN2 and SN20 fashion (Scheme 13, eq a).65 Moreover, these authors succeeded in using cyclohexanone as a nonactivated nucleophile for the substitution of an N-allyl oxazolidinone (eq b). After addition of catalytic amounts of acetylacetone (CH2Ac2), allyltrimethylsilane could also be employed as nucleophile for this substitution reaction (eq c). The research group of Tian exploited N-benzyl sulfonamides as electrophiles in the decarboxylative benzylation of β-keto acids (Scheme 14).66 Iron(III) chloride was identified as a useful catalyst for this transformation which proceeds in 1,2-dichloroethane (DCE) at 60 °C. A series of homobenzyl ketones was obtained mostly in good to high yields. Organosilicon compounds, such as trimethylsilyl cyanide, allyltrimethylsilane, and 2-silyl-substituted benzofuran and 1-methylindole, have been employed as carbon nucleophiles for the iron-catalyzed SN reaction with benzylic acetates (Scheme 15).67 The transformation was carried out in the presence of iron(III) chloride as catalyst and silver triflate as additive in 1,2dichloroethane at temperatures varying from room temperature to reflux and afforded the benzylated products in good to very high yields. Under the same conditions, azidation of the benzylic substrates by reaction with trimethylsilyl azide was reported. Using iron(III) chloride as catalyst, a Ritter-type reaction of benzylic alcohols with cyanamides in 1,2-dichloroethane at reflux has been reported (Scheme 16).68 A rather high catalyst loading of 30 mol % was required in order to get the benzyl-substituted ureas in high yields. In addition, allyl and tert-butyl alcohols have been successfully applied as substrates. Benzyl trialkylsilyl ethers have been converted to the corresponding azides by reaction with trimethylsilyl azide (Scheme 17).69 3175

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Scheme 17

Scheme 18

Scheme 19

Scheme 14

Scheme 15

Scheme 16

The reaction is catalyzed by iron(III) chloride or iron(III) bromide and provides benzyl azides mostly in high yields under

mild conditions. Secondary and tertiary benzyl silyl ethers were transformed chemoselectively in the presence of primary benzyl silyl ethers. In addition, vinylogous benzylic ethers (allylic and propargylic) were shown to be useful substrates for this transformation. Moreover, the authors were able to extend the scope of the substrates to benzyl methyl ethers and unsymmetrical dibenzyl ethers (Scheme 18).70 Other nucleophiles, such as allyltrimethylsilane, 1-phenyl-2-trimethylsilylacetylene, and trimethylsilyl cyanide, could be employed as well. The reaction was performed in dichloromethane at room temperature using iron(III) chloride as catalyst. Sun and co-workers reported an unusual β-alkylation of benzylic alcohols with primary alcohols (mostly benzylic but also aliphatic) which is catalyzed by ferrocenecarboxaldehyde (Scheme 19).71 This formal substitution of primary alcohols by a carbon nucleophile may be rationalized by intermediate oxidation of both alcohols to the carbonyl compounds via hydride transfer to the ferrocene catalyst. Subsequent aldol condensation leads to α,βunsaturated ketones which are reduced by the hydride from the (hydrido)iron complex. The practicability of this protocol could be demonstrated by several β-alkylation reactions of secondary alcohols with primary alcohols proceeding in high yields. The scope of carbon nucleophiles successfully used for iron-catalyzed SN reactions with benzhydrol and combined 3176

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Scheme 23

Scheme 21

Scheme 22

benzylic/propargylic alcohols has been extended to a 1-silyl-3boryl-2-alkene (Scheme 20).72 This reaction provides 1,5-enynes which constitute valuable building blocks for the construction of highly functionalized carbocyclic structures. Ferrocenylmethanol could be substituted with S-, N-, P-, and C-nucleophiles under similar conditions (Scheme 21).73 The iron complex [Fp]+[OTf] [Fp = Cp(CO)2Fe] has been applied as catalyst for this transformation. The corresponding thioethers (eq a) and amines (eq b) were obtained in very high yields. Selectivity for sulfenylation over alkoxylation has been demonstrated using 2-mercaptoethanol as ambident nucleophile (eq a). In contrast, reaction of ferrocenylmethanol with diphenylphosphine provided the tertiary phosphine only in low yield along with a significant amount of bis(ferrocenylmethyl) ether (not shown). Electron-rich heteroarenes have been employed as carbon nucleophiles to give the alkylated heteroarenes in moderate to good yields (eq c).

para-Substituted arylamines and sulfonylamides have been alkylated with benzylated dibenzoylmethanes in the presence of catalytic amounts of iron(III) chloride hexahydrate in 1,2dichloroethane at elevated temperatures (Scheme 22).74 Even though this method provides the products mostly in moderate to good yields, it nicely complements existing procedures for amine alkylation. Electron-deficient anilines with an unsubstituted para-position afford preferentially the corresponding FriedelCrafts alkylation products. It is noteworthy that the 1,3-dicarbonyl moiety functions as a leaving group in this procedure. Such CC bond activating reactions have been summarized in a recent review by Plietker and Klein.75 Benzyl methyl ethers have been utilized for the alkylation of N-nucleophiles (Scheme 23).76 The reaction is catalyzed by iron(III) chloride at room temperature. With p-toluenesulfonamide and 4-nitroaniline as nucleophiles the alkylated products are obtained in high yields (eq a). In addition, azidation of a benzyl methyl ether was achieved using trimethylsilyl azide (eq b). It is noteworthy that vinylogous benzyl methyl ethers lead to ipsosubstitution (eq c), whereas (1-methoxyallyl)benzene reacts with p-toluenesulfonamide in an SN20 fashion under these conditions (eq d). In a comparative study, Samec and Biswas disclosed that iron(III), bismuth(III), and gold(III) catalysts exhibit higher activities for the nucleophilic substitution of benzylic, allylic, and propargylic alcohols with C-, N-, and S-nucleophiles, while the reaction with O-nucleophiles was more efficient under rhenium(I), rhenium(VII), palladium(II), or lanthanum(III) catalysis.77 Mn2O3-impregnated γ-Fe2O3 nanoparticles have been found to be active as catalysts for the benzylation of primary arylamines with benzyl alcohol.78 Gu and co-workers described the nucleophilic substitution of benzhydrols by acetophenones using iron(III) triflate as catalyst (Scheme 24).79 The reaction was carried out in chlorobenzene at 130 °C and afforded the alkylated benzhydrols in moderate to good yields. The allylation of benzyl methyl ethers with allyltrimethylsilane using iron(III) chloride as catalyst has been described by Fan and 3177

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Scheme 26

Scheme 25

Scheme 27

co-workers (Scheme 25, eq a).80 The reaction was performed in dichloromethane at room temperature and provided a large number of allylated products in high to excellent yields. It should be noted that this transformation is successful only when the leaving methoxy group is located at a secondary or tertiary benzylic carbon atom. In addition, some allyl methyl ethers gave analogous reactions in similar yields. Using the same set of reaction conditions with trimethylsilyl cyanide as nucleophile, the same substrates could be subjected to cyanation providing a range of benzyl cyanides in high yields (eq b).81 2.1.2. Nucleophilic Substitution of Allylic CX Bonds. Allylic and propargylic electrophiles can be attacked by nucleophiles at the allylic or at the conjugate position leading to regioisomeric products. In the case of concerted SN2 or SN20 processes a stereoselective formation of the products is anticipated. Alternatively, in the presence of very good leaving groups and/or strong Lewis acid catalysts, the intermediacy of allylic cations may lead to a mixture of regio- and stereoisomeric products. However, depending on the substrates, a stereoselective formation of single regioisomers can be achieved also under these conditions. Examples for all of these processes have been reported in the presence of iron catalysts. Depending on whether σor π-allyl intermediates are formed or the iron acts only as Lewis acid catalyst, a different regio- and stereochemical outcome is observed. 2.1.2.1. C-Nucleophiles. The conjugate addition of arenes to allylic alcohols bearing an electron withdrawing group was achieved in the presence of an FeCl3-doped K-10 montmorillonite clay (Scheme 26).82 A comparable transformation with chloride as nucleophile had been reported before using allylic acetates and stoichiometric amounts of iron(III) chloride.83 The substrates are readily available by the BaylisHillman reaction of α,β-unsaturated carbonyl compounds with the corresponding benzaldehyde. E- or Z-olefins were obtained, depending on the electron withdrawing group at the double bond. The 2-methoxycarbonyl-substituted derivatives provided

predominantly the E-olefins (eq a), whereas acrylonitriles led to the Z-isomers (eq b). Instead of Lewis acidic iron compounds, nucleophilic Fe(II) ferrates have also been employed as catalysts for SN reactions. The iron nucleophile displaces the leaving group and is subsequently substituted by the organic nucleophile leading to the stable product. Based on previous work by Roustan84,85 and Xu,86,87 Plietker88 developed an iron-catalyzed ipso-substitution of allyl isobutyl carbonates with activated methylene nucleophiles in good yields and high regioselectivity (Scheme 27). The product of conjugate addition was formed in only minor amounts. The active nucleophilic catalyst in this transformation is the Hieber-type complex [Bu4N][Fe(CO)3NO] (TBAFe). Addition of a triphenylphosphine ligand led to an increase in stability and reactivity of the iron complex. The leaving group plays the role of an in situ formed base for activation of the methylene pronucleophile (salt-free conditions). The reaction proceeds with retention of configuration as proven by using an enantiomerically enriched carbonate (94% enantiomeric excess (ee)). Only a slightly decreased enantiomeric excess of the product (79% ee) with retained configuration at the stereogenic center was observed.88 This stereochemical outcome was rationalized by a σ-allyl mechanism as shown in Scheme 28. The SN20 substitution of the allylic carbonate by the nucleophilic iron species (ionization) is followed by a second SN20 reaction with the organic nucleophile. An intermediate η3-allyliron complex (π-allyl mechanism) is excluded as this would result in a mixture of regioisomers. By quantum chemical calculations, Plietker and co-workers recently demonstrated that the iron center in TBAFe is better described as Fe(0) rather than Fe(II).89 In this complex the ligand may be regarded as NO with two π-bonds to the metal; no indication of an FeN σ-bond was found. 3178

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Chemical Reviews Scheme 28

REVIEW

Scheme 30

Scheme 31

Scheme 29

In recent years, N-heterocyclic carbene (NHC) ligands have been frequently applied in metalorganic synthesis. This holds true also for iron-catalyzed reactions.15 The allylic alkylation described above could be further improved by employing N-heterocyclic carbene (NHC) ligands and tert-butyl methyl ether (MTBA) as solvent, thus enabling the introduction of stoichiometric amounts of nonstabilized or reactive nucleophiles.90 Using a tert-butyl-substituted NHC ligand the ipsoproduct was obtained following the σ-allyl mechanism (double SN20 reaction). The geometry of the double bond was conserved under these conditions. In contrast, the corresponding mesityl-substituted NHC ligand afforded the product of an overall conjugate substitution via intermediate π-allyliron complexes. The TBAFe-catalyzed allylic alkylation in the presence of NHC ligands has also been applied and leads to an overall alkoxyallylation of activated olefins as outlined in Scheme 415 in section 3.5.3.91 Moreover, this reaction was exploited for the prenylation of a 1,3-dicarbonyl intermediate during the total synthesis of hyperibone I.92 In addition to TBAFe, structurally defined π-allyliron complexes have also been successfully employed as catalysts for the nucleophilic substitution of allyl carbonates with activated methylene carbon nucleophiles (Scheme 29).93 The transformation proceeds in the presence of an NHC ligand; otherwise the allyliron complex remains inactive. As compared to the TBAFe/NHC catalyst described above, this system shows a higher activity presumably due to a better solubility in organic solvents. The reaction probably follows a π-allyl mechanism as concluded from the regiochemical outcome. The nucleophile attacks preferentially the less hindered position of the allylic intermediate.

Another example of an iron(II)-catalyzed allylic alkylation of carbon-centered nucleophiles with allylic carbonates has been described more recently by He and co-workers.94 Catalytic amounts of TBAFe were employed in combination with N,N,N0 ,N0 -tetramethylethylenediamine (TMEDA) as ligand to form an iron(II) TMEDA complex. The reaction was carried out in toluene at 80 °C and afforded the allylation products mostly in high yields and with good regioselectivity (Scheme 30). Based on earlier work by Nicholas et al.,95,96 Åkermark and Sj€ogren investigated an amine-promoted, Fe2(CO)9-catalyzed allylic alkylation of sodium diethyl methylmalonate with various allylic acetates.97 They observed mainly ipso-substitution with preserved configuration of the double bond. A combination of nonacarbonyldiiron and triphenylphosphine has been reported to catalyze the nucleophilic substitution of allyl acetate by zinc enolates (Scheme 31).98 The latter have been prepared in situ by asymmetric copper-catalyzed conjugate addition of dimethylzinc to cycloalkenones. The allylation proceeds under mild conditions at room temperature with excellent diastereoselectivity and good enantioselectivity affording allylated cycloalkanones in moderate to good yields. A cationic cyclization of allylic alcohols with olefins in the presence of iron(III) chloride has been used for the construction of the indene ring system (Scheme 32).99 A variety of 1-vinyl-1Hindenes could be synthesized in very good yields. Moreover, this method has been successfully applied to the synthesis of (()-jungianol and epi-jungianol. The iron-catalyzed reaction of Grignard reagents with 1-methoxymethyl-substituted 3-(hydroxymethyl)cyclopropenes led to methylenecyclopropanes via an SN20 -type process (Scheme 33).100 Iron(III) acetylacetonate was identified as a useful catalyst leading to anti-addition with respect to the hydroxymethyl moiety. In contrast, a copper-catalyzed version led to the syn-addition products. 2.1.2.2. Heteroatom Nucleophiles. The iron-catalyzed allylation protocol developed by Plietker (cf. Scheme 27) could be 3179

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Chemical Reviews Scheme 32

REVIEW

Scheme 35

Scheme 36

Scheme 33

Scheme 37

Scheme 34

adapted to heteroatom nucleophiles.101 Thus, allylation of primary arylamines with various allylic carbonates was achieved in a regio- and stereoselective manner using the iron(II) catalyst [Bu4N][Fe(CO)3NO] (TBAFe) in the presence of triphenylphosphine and piperidine hydrochloride (Pip 3 HCl) (Scheme 34). Reaction of enantiopure allyl carbonate substrates occurred with retention of configuration, a fact that can be explained by a double SN20 mechanism (cf. Scheme 28) via a σ-allyl iron intermediate.102 The sulfonation of tertiary allylic carbonates has been accomplished with various aryl sulfinates in the presence of TBAFe (Scheme 35).103 This transformation leads to an ipsosubstitution in good to excellent regioselectivity. Reaction of an

(R)-linalool-derived carbonate with sodium phenyl sulfinate led to retention of configuration with almost no loss of enantiomeric purity. Cossy et al. utilized a 6-exo-trig ring closing nucleophilic SN20 substitution of ζ-amino- and ζ-hydroxyallyl alcohols and acetates catalyzed by iron(III) chloride hexahydrate for the construction of cis-2,6-disubstituted piperidines and tetrahydropyrans (Scheme 36).104,105 Thermodynamic equilibration of the cis/ trans-2-alkenyl 6-substituted piperidines and tetrahydropyrans led to the more stable cis-isomers in high yields and very good diastereoselectivity. The same transformation has been applied to the diastereoselective synthesis of cis-isoxazolidines starting from N-protected δ-aminooxy allylic acetates.106 A binuclear iron complex derived from TBAFe has been employed for a regioselective allylic sulfenylation (Scheme 37).107 Aromatic and aliphatic thiols were treated with an allylic carbonate in the presence of the iron(II) complex to give allyl aryl and allyl 3180

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Chemical Reviews Scheme 38

REVIEW

Scheme 40

Scheme 39 Scheme 41

alkyl thioethers in excellent yields. A high degree of α-substitution and full retention of the configuration was achieved under these conditions. α-Sulfonyl succinimides are versatile sulfinate donors to allow sulfonation of allylic carbonates catalyzed by the Hieber-type ferrate TBAFe in the presence of an NHC ligand under mild conditions (Scheme 38).108 Isobutyl 1,1-dimethylallyl carbonate afforded the ipso-substitution product in very good selectivity. Other allylic carbonates, however, showed a less pronounced regioselectivity. Performing this reaction with addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in dimethoxyethane (DME) at 60 °C led to a domino iron-catalyzed allylic sulfonation/aminecatalyzed isomerization. These conditions provided selectively E-vinyl sulfones in very good yields.109 TBAFe in combination with PPh3 was demonstrated to catalyze the decarboxylative allylation of phenols which generates allyl phenyl ethers (Scheme 39).110 The reaction was found to be regioselective but not regiospecific. Thus, cinnamyl and 1-phenylallyl carbonates gave the same linear allylic ethers, whereas both a crotyl carbonate and the corresponding 1-methylallyl carbonate provided the branched products. The result was rationalized by the presumed intermediacy of π-allyliron complexes which would be identical for both substrate couples. This explanation is in contrast to the σ-allyl mechanism assumed by Plietker for his allylic alkylation with isobutyl 1,1-dimethylallyl carbonate (cf. Scheme 28).88 An intramolecular iron(III)-catalyzed SN20 allylic amination has been exploited for the synthesis of substituted dihydroquinolines and quinolines (Scheme 40).111 The reaction of N-protected 1-(2-aminophenyl)-prop-2-en-1-ols in the presence of iron(III) chloride hexahydrate in dichloromethane at room temperature provided a large number of 2- and 4-substituted 1,2dihydroquinolines in moderate to excellent yields (eq a). Treatment of the crude product with sodium hydroxide in ethanol at reflux led to aromatization via elimination of p-toluenesulfinic acid and afforded the corresponding quinolines (eq b). Benzoylprotected dihydroquinolines were not aromatized under these conditions.

Scheme 42

Plietker and co-workers also reported the O-allylation of phenols with allylic carbonates. They employed TBAFe as catalyst in the presence of a triazolium-based redox-active abnormal N-heterocyclic carbene (aNHC) ligand and potassium tert-amylate as base (Scheme 41).112 The reaction proceeds with almost complete ipso-selectivity and thus complements the protocol of Tunge and co-workers (cf. Scheme 39).110 In most cases, high to excellent yields of allyl aryl ethers were obtained. The group of Najera presented a protocol for the substitution of free allyl alcohols by various N- and C-nucleophiles using iron(III) chloride hexahydrate as catalyst (Scheme 42).113 In general, this transformation proceeds in dioxane at 50 °C. The iron-catalyzed process was compared to a Brønsted acid catalyzed analogue using trifluoromethanesulfonic acid which gave similar results, in many cases even with slightly higher yields. The intermediacy of a carbocationic intermediate was assumed. 3181

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Chemical Reviews Scheme 43

REVIEW

Scheme 45

Scheme 44

The regioselectivity leading to the more stable allylic product was explained by an isomerization of the allylic alcohol to the more stable isomer prior to the nucleophilic substitution (cf. eqs a and b). In a subsequent work, the authors were able to perform this transformation in aqueous solution.114 The yields were slightly lower than in organic solution, and higher catalyst loadings and temperatures were required. An isomerization of allylic alcohols to the more stable congener prior to substitution was observed as well under these conditions (eqs a and b). 2.1.3. Nucleophilic Substitution of Propargylic CX Bonds. 2.1.3.1. Heteroatom Nucleophiles. A clean ipsosubstitution was observed for the iron(III) chloride catalyzed reaction of propargylic acetates with primary and secondary alcohols to give propargylic ethers (Scheme 43, eq a).115 The excellent regioselectivity, the short reaction times, the mild conditions, and the cheap and abundant catalyst are the advantages of this approach compared to reactions catalyzed by other transition metals, such as cobalt, rhenium, ruthenium, titanium, and gold. The reaction proceeds even with the corresponding propargylic alcohols (eq b).116 A large variety of heteroatom nucleophiles, including alcohols, thiols, and carboxamides, but also carbon nucleophiles (cf. Scheme 51), could be transformed into the corresponding propargylic products. The iron-catalyzed nucleophilic ipso-substitution of propargylic alcohols with N-protected hydroxylamines has been exploited for the synthesis of isoxazolines and isoxazoles (Scheme 44).117,118 In a first step, nucleophilic substitution of the hydroxy group occurs in the presence of iron(III) chloride as catalyst forming an intermediate propargylic hydroxylamine which could also be isolated. Subsequent reaction of the

sulfonyl-protected propargylic hydroxylamines with triethylamine in a one-pot procedure induces a ring closure and elimination of phenylsulfinic acid affording isoxazoles. It was assumed that elimination occurs first to give an oxime intermediate which finally undergoes cyclization. The propargylic hydroxylamine intermediate could also be subjected to a gold(III)-catalyzed cyclization in the presence of catalytic amounts of pyridine. In this case, elimination was suppressed and isoxazolines were formed mostly in high yields.118 Using Cbz-protected amines and DMAP as base for the gold-catalyzed cyclization, the isoxazolidines were obtained in slightly lower yields.117 In addition, the iron-catalyzed propargylic amination has been combined with an iodo cyclization in the presence of iodine monochloride to give 4-iodoisoxazolines and 4-iodoisoxazoles, respectively.118 By introduction of a palladium(0) catalyst, the iron-catalyzed propargylic substitution/cyclization protocol has been extended to an uninterrupted one-pot substitution/cyclization/cross coupling/hydrogenolysis/oxidation sequence to afford 3,4,5trisubstituted isoxazoles (Scheme 45).119 The overall yields for this four-step sequence are remarkably high, ranging from 40 to 84%. In analogy to the propargylation of N-protected hydroxylamines, triazoles can be employed as nucleophiles. Thus, reaction of triazoles with 1-monosubstituted propargylic alcohols in the presence of catalytic amounts of iron(III) chloride provided the corresponding ipso-substitution products (Scheme 46).120 Depending on the nature of the triazole nucleophile either N-1 or N-2 substitution was observed. Benzotriazoles gave exclusively N-1 substitution (eq a). 4-Phenyl-2H-1,2,3-triazole and 4-benzoyl-5-phenyl-2H-1,2,3-triazole predominantly led to the N-2 substituted products (eq b). In addition, three examples of 1,1-disubstituted propargylic alcohols as substrates have been reported to give the corresponding propargylated 1,2,3triazoles. The same authors reported that the iron(III) chloride catalyzed reaction of triazoles with sterically more encumbered 1,1-disubstituted propargylic alcohols proceeds in an SN20 fashion to afford allenyltriazoles (Scheme 47).121 Benzotriazole as nucleophile led to 1-(allenyl)benzotriazoles (eq a), whereas 4-substituted 2H-1,2,3-triazoles led to mixtures of N-2-allenyl and N-1-allenyl substituted triazoles (eq b). The former were favored with a minimum ratio of 4:1. 3182

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Chemical Reviews Scheme 46

REVIEW

Scheme 48

Scheme 49

Scheme 47

Scheme 50

The first attempts to achieve a dehydrative etherification of propargylic alcohols have been reported by Zanotti and co-workers (Scheme 48).122 They employed the iron complex [Fp][OTf] as catalyst for this transformation (Fp = CpFe[CO]2). The method was basically developed for the etherification of ferrocenylmethanol, which resembles to some extent a benzylic alcohol. With this substrate moderate to very good yields could be achieved with various propargylic alcohols (Scheme 48, eq a). Benzyl alcohol, p-nitrobenzyl alcohol, and phenol were also propargylated in good yields, whereas aliphatic alcohols provided only poor results (eq b). Under these conditions, even the etherification of ferrocenylmethanol with aliphatic alcohols, allyl alcohol, and benzyl alcohol was accomplished in good to very good yields (eq c).

Zhan and co-workers described an iron-catalyzed domino propargylic substitution/aza-MeyerSchuster rearrangement/ elimination reaction (Scheme 49).123 Thus, they treated propargylic trimethylsilylacetylenes with p-toluenesulfonyl hydrazide in the presence of catalytic amounts of iron(III) chloride and obtained acrylonitriles in high to excellent yields. A gram-scale experiment confirmed the yield of a small scale batch and demonstrated the synthetic utility of this procedure. Unsymmetrically substituted propargyl alcohols led to mixtures of E- and Z-olefins. Replacing the silyl substituent by an aryl or alkyl group changed the course of the reaction and led to 3,4,5-trisubstituted pyrazoles (Scheme 50).124 This outcome is explained by the fact that elimination of TsNHTMS leading to the cyanation cannot occur in this situation. Instead, p-toluenesulfinic acid is eliminated and the resulting diazo compound cyclizes to the pyrazole followed by a [1,5]-sigmatropic aryl shift. It is noteworthy that alkyl groups migrated to the nitrogen atom leading to 1,3,5-trisubstituted pyrazoles, albeit in lower yields. 2.1.3.2. C-Nucleophiles. In analogy to the reaction with heteroatom nucleophiles (cf. Scheme 43), Zhan and co-workers described the iron(III) chloride catalyzed ipso-substitution of propargylic alcohols with electron-rich arenes and heteroarenes 3183

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Chemical Reviews Scheme 51

REVIEW

Scheme 54

Scheme 55

Scheme 52

Scheme 56

Scheme 53

Scheme 57

as carbon nucleophiles (Scheme 51, eq a).116 Accordingly, allyltrimethylsilane could be employed, which led to 1,5-enynes as versatile building blocks for carbocyclic compounds (eq b). Jana and co-workers demonstrated the feasibility of an iron(III) chloride catalyzed SN2 reaction of propargylic alcohols with β-dicarbonyl compounds (Scheme 52).125 The reaction proceeded under mild conditions in dichloromethane at room temperature and afforded the ipso-substituted products in high yields. Zhan and co-workers treated propargylic alcohols with alkynylsilanes in nitromethane in the presence of catalytic amounts of iron(III) chloride (Scheme 53).126 This ipso-substitution protocol led to a series of 1,4-diynes in high yields. Tertiary propargylic alcohols were treated with α-oxo ketene dithioacetals under iron(III) bromide catalysis to give gembis(alkylthio)vinylallenes by a formal SN20 displacement of the hydroxy group (Scheme 54).127 A variety of vinyl allenes could be obtained by this procedure in high yields. 2.1.4. Nucleophilic Substitution of Nonactivated C(sp3)X Bonds. The archetypal and oldest example of an iron-catalyzed SN reaction other than an ArSE process is the Finkelstein reaction reported by Miller and Nunn in 1976 (Scheme 55).128 Tertiary alkyl chlorides could be transformed to the corresponding iodides in the presence of sodium iodide

and catalytic amounts of iron(III) chloride. The latter assists the removal of the leaving group by its Lewis acid activity. The reaction proceeds in carbon disulfide at room temperature. Suitable substrates include also benzyl and acyl chlorides. Catalyst loadings of 1.97.7 mol % were applied to give alkyl iodides in 95100% yield. Cossy and co-workers established a method for an ironcatalyzed Ritter reaction (Scheme 56, cf. also Scheme 8).60 They treated tert-butyl acetate with nitriles and water in the presence of catalytic amounts of iron(III) chloride hexahydrate to obtain tert-butyl amides in good yields. Various sulfonates could be transformed to the corresponding halides using trimethylsilyl halides as halide source and catalytic amounts of Fe(acac)3. A typical example is presented in Scheme 57.129 In general, 8-quinolinesulfonates and 2-pyridinesulfonates turned out to be more reactive than mesylates. The stereochemical outcome of the reaction varied depending on the substrate and the reaction conditions. In many cases, an overall retention of the configuration was observed. Aliphatic 8-quinolinesulfonates led to inversion under catalytic conditions. In addition to the iron-catalyzed transformation, the reaction was also investigated using stoichiometric amounts of iron(III) halides. Under these conditions, a chemoselective substitution of secondary sulfonates in the presence of primary sulfonates was achieved. 3184

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Chemical Reviews Scheme 58

REVIEW

Scheme 60

Scheme 59 Scheme 61

An iron-catalyzed three-component reaction involving an alkyne and an amine alkylation has been reported by the group of He (Scheme 58).130 They treated terminal arylalkynes, dichloromethane, and a secondary amine in the presence of 1,1,3,3-tetramethylguanidine (TMG) and catalytic amounts of iron(III) chloride in acetonitrile at 100 °C to prepare aromatic propargylamines in high yields. Shi and co-workers were able to perform the alkylation of various amines (primary arylamines, primary and secondary alkylamines, and ammonia) with primary and secondary alcohols (Scheme 59) (for benzyl alcohols see section 2.1.1, Scheme 10).62 They employed a self-prepared nickel-, copper-, and ironcontaining catalyst (NiCuFeOx) for this transformation. Primary and secondary amines were monoalkylated (eqs a and b), whereas ammonia was double-alkylated under these conditions (eq c). However, intramolecular double alkylation of primary amines with diols could be achieved providing N-heterocyclic compounds (eq d). In general, the alkylated amines were obtained in good to excellent yields. Enthaler and Weidauer developed a protocol for the benzoyl chloride mediated depolymerization of polyethers which is catalyzed by iron(II) chloride tetrahydrate (Scheme 60).131 This transformation was performed using neat conditions at 100 °C and provided the corresponding 2-chloroethyl benzoates in good

to high yields. The products can potentially be transformed into monomers for new polymerization reactions. In a subsequent work, the authors extended the scope of the reagents to fatty acid chlorides.132 The resulting 2-chloroethyl carboxylates could be obtained in good yields. In addition, a ring-closing depolymerization of poly(tetrahydrofuran) has been achieved by the same authors.133 Iron(III) chloride proved to be a useful catalyst for this transformation which was conducted at 100180 °C providing THF in high purity and good yields. An interesting iron-catalyzed version of the Mitsunobu reaction has been developed by Taniguchi and co-workers (Scheme 61). 134 Their method uses catalytic amounts of ethyl 2-(3,4-dichlorophenyl)hydrazinecarboxylate as precursor for a DEAD equivalent and iron(III)-phthalocyanine as catalyst. The hydrazinecarboxylate is oxidized in situ to an azocarboxylate by the iron complex and air as terminal oxidant. This protocol could be applied in good to high yields to the substitution of various primary and secondary alcohols by carboxylates and N-nucleophiles. In most cases, inversion of the configuration was achieved in high enantiomeric excess. The Ritter reaction of cyclic secondary alcohols with acetonitrile could be performed using iron(III) chloride in combination with silver hexafluoroantimonate as a catalytic system (Scheme 62).58 The corresponding carboxamides were obtained in moderate yields. As expected, benzylic substrates performed much better under these conditions (two examples, 8387%) (cf. also section 2.1.1). 3185

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Chemical Reviews Scheme 62

REVIEW

Scheme 65

Scheme 63

Scheme 64

2.1.5. Nucleophilic Substitution of Sugar Derivatives. An O-glycosylation of glycals can be achieved by an SN10 -type substitution, the so-called Ferrier rearrangement, to provide 2,3-unsaturated O-glycosides. An iron(III) chloride catalyzed version of this transformation has been reported by Bessodes135 and Krohn.136 Later, Tilve and co-workers developed a Ferrier rearrangement by reaction of 3,4,6-tri-O-acetylD-glucal with various alcohols in the presence of an iron(III) chloride based ionic liquid.137 More recently, Zhang et al. treated perbenzyl and peracetyl glycals with various alcohols in the presence of catalytic amounts of iron(III) sulfate hydrate with or without microwave irradiation and obtained 2,3-unsaturated D-O-glycosides with exclusive α-selectivity in high yields (Scheme 63).138 Chen and Wang described an iron(III) triflate catalyzed Ferrier type I rearrangement which proceeds by reaction of peracetylated glucal with a variety of alcohols (Scheme 64).139 They obtained a series of 2,3-unsaturated-O-glycosides in high yields and high α-selectivity. 2.2. Electrophilic Aromatic Substitution

Iron salts, in particular iron(III) chloride, have been frequently applied as Lewis acid catalysts for electrophilic aromatic substitutions.10,42,140143 For example, the formation of triphenylmethane from benzaldehyde and benzene in the presence of stoichiometric amounts of iron(III) chloride was already reported by Schaarschmidt in 1925.144 Catalytic applications have also been known for a long time. Recent reviews on this topic have been provided by Bolm1 and Beller.145 2.2.1. CHeteroatom Bond Forming Reactions. In 2004, Samant and co-workers reported their studies on the sulfonylation of arenes using both stoichiometric and catalytic amounts

of iron(III) chloride based ionic liquids (Scheme 65).146 The reaction of p-toluenesulfonyl chloride and benzenesulfonyl chloride with arenes in the presence of an iron-containing ionic liquid required rather high temperatures but afforded the corresponding sulfone products in high yields. In addition, the reaction could also be carried out under microwave irradiation with very short reaction times (typically 35 min). The sulfenylation of arenes can be achieved with disulfides.147 Thus, 1,3,5-trimethoxybenzene has been treated with various disulfides in the presence of 20 mol % iron(III) bromide in DMF at 145 °C. Modified reaction conditions (addition of catalytic amounts of iodine and acetonitrile as solvent) have been applied to the sulfenylation and selenylation of phenylpyrazoles.148 In this case, the reaction temperature could be decreased to 80 °C. The procedure opened up the way to the synthesis of a variety of derivatives of the insecticide fipronil. Iron(III) chloride has been employed as catalyst for the bromination of aryl azides using NBS. Starting from 1-azido-4chlorobenzene, this mild method affords regioselectively 1-azido2-bromo-4-chlorobenzene in 84% yield with no significant decomposition of the azide function.149 The research group of He developed an iron-catalyzed protocol for the oxidative iodination of electron-rich arenes.150 A series of methoxy-substituted arenes was treated with 0.55 equiv of iodine in the presence of iron(III) nitrate nonahydrate in ethylene glycol in a carbon dioxide/oxygen (1 MPa/0.2 MPa) atmosphere. The method afforded monoiodoarenes in high yields. The reaction of benzene with selenium oxychloride (SeOCl2) catalyzed by iron(III) chloride led to a mixture of diphenyl selenide (Ph2Se) and diphenyl diselenide (PhSeSePh) via electrophilic aromatic substitution and concomitant autoredox processes.151 Arylhydrazides have been synthesized by an iron-catalyzed reaction of electron-rich arenes with azodicarboxylates.152 The reaction was carried out in acetonitrile at room temperature using iron(III) chloride as catalyst and provided arylhydrazides in high yields within a reaction time of 2 min. Fu et al. described the reaction of phenols with N-(arylthio)succinimides catalyzed by iron(III) chloride or boron trifluoride etherate.153 This procedure led to diaryl sulfides in good to excellent yields with the regioselectivity usually observed for ArSE processes. Advantages of this protocol are the absence of any ligands and additives, broad functional group tolerance, and ambient reaction temperature. 3186

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Chemical Reviews Scheme 66

REVIEW

Scheme 67

Scheme 68

2.2.2. CC Bond Forming Reactions. 2.2.2.1. C(sp3) and CdO(NR) Electrophiles. In 2003, Sugunan and co-workers presented an iron-catalyzed benzylation of toluene with benzyl alcohol relying on an iron-incorporated sulfated zirconia system.154 This heterogeneous catalyst could be reused several times without loss of activity. However, in general conversion, chemoselectivity (formation of dibenzyl ether as major byproduct), and regioselectivity were only poor. The reaction of indole with aldehydes to bis(indolyl)methanes has been reported to proceed in an ionic liquid containing catalytic amounts of iron(III) chloride hexahydrate (Scheme 66).155 The yields for this transformation were excellent for many examples. Moreover, the ionic liquid together with the iron catalyst could be separated by extraction with diethyl ether. The recovered system could be reused directly for subsequent runs with yields dropping only slightly from 96 to 87%. In contrast, In(OTf)3 as a comparable catalyst showed only half of its efficiency during the third run and none in the fourth. For the same reaction, a polyindole iron salt has been successfully introduced as catalyst.156 A variety of bis(indolyl)methanes derived from aromatic, vinylic, and aliphatic aldehydes was obtained in excellent yields. As mentioned for the previous method, the catalyst could be recovered and reused up to five times without significant loss of activity. More recently, Seyedi demonstrated that an iron(III)salen complex can be employed for the same transformation.157 The reaction proceeded in glycerol under microwave irradiation and provided bis(indolyl)methanes in high yields. In situ generated iron(III) dodecyl sulfate has been successfully employed as a Lewis acidsurfactant catalyst for this reaction to afford the corresponding bis(indolyl)methanes in mostly excellent yields.158 Instead of aldehydes (cf. Scheme 66), primary alcohols can also be employed as electrophiles in the reaction with indoles.159 Sekar and co-workers used iron(II) chloride in combination with (()-1,10 -binaphthyl-2,20 -diamine (BINAM) as a catalytic system for this transformation. Dicumyl peroxide served as oxidant to generate the aldehyde from the alcohol. A series of bis(indolyl)methanes could be synthesized by this procedure. The FriedelCrafts acylation of benzene, toluene, and bromobenzene with acetic anhydride has been investigated using stoichiometric amounts of iron(III) chloride containing pyridinium-based ionic liquids.160 High conversions at relatively low temperatures were achieved. The ionic liquid together with the iron(III) chloride could be recovered and reused three times with only minor loss of activity. Benzyl esters and also benzyl alcohols have been reported to react smoothly in the presence of iron(III) chloride with a large variety of arenes to afford diarylmethanes (Scheme 67).41 In most

Scheme 69

cases the reaction proceeded in excellent yields with both electron-rich and electron-deficient arenes. The FriedelCrafts reaction was usually carried out in dichloromethane, in nitromethane, or in an excess of the arene with no need for strong acids or bases. In most cases a reaction temperature of 50 °C was sufficient to achieve excellent results. In addition, several functional groups such as aldehydes, esters, halides, hydroxyl groups, methoxy groups, thiophenes, and furans were tolerated. In many cases, an excellent regioselectivity was observed in agreement with the directing effects of the particular substituents. The iron(III) chloride catalyzed benzylation of phenols under microwave conditions has been described as part of a benzylation/cyclization cascade leading to xanthenes (cf. section 2.5.1, Scheme 332).161 Arenes and heteroarenes could be alkylated with propargylic acetates using iron(III) chloride as catalyst (Scheme 68).162 The reaction proceeds under mild conditions at room temperature in acetonitrile providing the propargyl arenes in high yields and with complete regioselectivity. A consecutive FriedelCrafts hydroxyalkylation/alkylation has been reported by Wu et al., who treated arenes with aromatic aldehydes in the presence of iron(III) chloride as catalyst (Scheme 69).163 This method led to symmetrical triarylmethanes in high yields. Consecutive addition of two different arenes to the aldehyde afforded triarylmethanes with three different aryl units in a one-pot procedure. Jana et al. developed an iron-catalyzed method for the allylation (Scheme 70, eq a) and benzylation (eq b) of indoles using allylic and benzylic alcohols as electrophiles.164 The reactions were performed in acetonitrile at room temperature in the 3187

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Scheme 70

Scheme 73

Scheme 71

Scheme 74

Scheme 72

presence of catalytic amounts of iron(III) chloride. The allylation proceeded in good yields and the benzylation provided the products in high to excellent yields. An intramolecular FriedelCrafts alkylation is part of an ironcatalyzed transformation of 2-alkylcinnamaldehydes to 2-alkyl-1alkoxyindenes (Scheme 71).165 The cinnamaldehyde was treated with a trialkyl orthoformate to form the corresponding dialkyl acetal intermediate which cyclizes to the 1-alkoxy-1H-indene and subsequently was isomerized and hydrolyzed to the 2-alkylindanone. Benzyl methyl ethers act as benzylating agents for electronrich arenes in the presence of 10 mol % iron(III) chloride as catalyst (Scheme 72).166 An excess of arenes was applied to serve as substrate and solvent. Several unsymmetrical diarylmethanes have been obtained in good to high yields. Toluene, o-xylene, and m-xylene led to mixtures of regioisomers. Aryl-substituted aziridines are useful as electrophiles in an iron-catalyzed FriedelCrafts reaction with electron-rich arenes (Scheme 73, eq a).167 The reaction was reported to proceed in

nitromethane at room temperature in the presence of catalytic amounts of iron(III) chloride. Several examples for this transformation have been reported to afford alkylated arenes in good yields. In the same paper, Wu et al. described the reaction of electron-rich arenes with arylimines using the same conditions. Depending on the substrate, monoarylated (eq b) or diarylated products were obtained. Aryl glycidyl ethers have been cyclized in the presence of iron(III) bromide either as sole catalyst or in combination with silver triflate to afford 3-chromanols in high to excellent yields (Scheme 74).168 Depending on the substrate, the reaction with benzyl glycidyl ethers provided either tetrahydrobenzo[c]oxepin-4-ols, by an analogous FriedelCrafts-type cyclization, or 4-diarylmethyl-1,3-dioxolanes by Lewis acid induced rearrangement (not shown). A diastereoselective iron-catalyzed FriedelCrafts alkylation has been described by Bach and Stader as part of their total synthesis of ()-podophyllotoxin (Scheme 75).169 They studied the reaction of a chiral benzylic alcohol with a 1,3-benzodioxole and found that simple iron(III) chloride was superior to Bi(OTf)3 and AuCl3 as catalyst for this SN1-type transformation. The product was obtained in an excellent yield of 99% and with a diastereoisomeric ratio of 94:6. Iron(III) chloride in toluene catalyzes the intramolecular FriedelCrafts reaction of aryl-substituted allyl alcohols to indenes (Scheme 76).170 This method has been established by a number of examples providing the products in high yields. In order to achieve exclusive γ-substitution, the allylic position should be sterically blocked by a second substituent. The group of Zhou described an intramolecular Friedel Crafts-type reaction of aryl-containing propargylic alcohols (Scheme 77).171,172 Iron(III) chloride was used as catalyst in nitromethane as solvent. It is noteworthy that at low temperatures (1015 °C) allenyltetrahydronaphthalenes are formed (eq a), whereas a reaction temperature of 60 °C promotes 3188

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Chemical Reviews Scheme 75

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Scheme 78

Scheme 79 Scheme 76

Scheme 80 Scheme 77

double bond isomerization leading to dihydronaphthalenes (eq b). For both variants several examples providing high yields of products have been presented. In addition, aryl-tethered propargylic alcohols containing a sulfonamide bridge afforded allenyltetrahydroisoquinolines (eq c). At the same time, an intramolecular allylation of arenes with pendent allylic alcohol moieties was developed by Bandini and

co-workers (Scheme 78).173 Iron(III) chloride in nitromethane proved to be superior over a variety of other Lewis acid catalysts. A number of tetrahydronaphthalenes could be generated by this procedure in good to high yields (eq a). In addition, amidetethered allylic (eq b), benzylic, and propargylic alcohols have been converted to tetrahydroisoquinolines. A homobenzylic amide afforded a tetrahydrobenzo[d]azepine (not shown). A slight decrease in efficiency was observed when highly pure iron(III) chloride (99.99% metal purity) was applied. The authors ruled out that this effect was caused by an impurity of copper. A domino FriedelCrafts hydroxymethylation/alkylation procedure has been exploited for the synthesis of substituted xanthenes (Scheme 79).174 For this transformation, 2-aryloxybenzaldehydes were treated with electron-rich arenes or heteroarenes in the presence of catalytic amounts of iron(III) chloride hexahydrate to give directly 9-substituted xanthenes. An iron-catalyzed intramolecular FriedelCrafts reaction of geminal diacetates derived from 2-alkylcinnamaldehydes has been reported by Womack and co-workers (Scheme 80).175 The cinnamaldehydes were treated with acetic anhydride in the presence of catalytic amounts of iron(III) chloride under neat conditions at room temperature to give acetylated indenols which can be hydrolyzed to indanones via ester cleavage and subsequent double bond isomerization. 3189

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Scheme 84

Scheme 82 Scheme 85

Scheme 83

Scheme 86

The reaction of 2-acetylfuran with carbon tetrachloride and aliphatic alcohols in the presence of 1 mol % iron(III) acetylacetonate provided 5-acetyl-2-furancarboxylic acid alkyl esters (Scheme 81).176 It can be assumed that an initial trichloromethylation of the furan occurs followed by alcoholysis to the corresponding esters. 3-Ketoarylamides have been subjected to an intramolecular hydroxyalkylation with concomitant dehydration (Scheme 82).177 This iron(III) chloride catalyzed cyclization provides access to antiviral 4-alkylidenequinoline-2-ones in good to high yields. Two successive iron-mediated FriedelCrafts reactions have been applied to an efficient four-step sequence to a prostaglandin EP4 receptor antagonist (Scheme 83).178 For the benzylation of 2,4-dimethylthiophene, substoichiometric amounts of iron(III) chloride in combination with methanesulfonic acid proved to be superior to H2[PtCl6], Bi(OTf)3, IrCl3, and ZnCl2 catalysts. The subsequent step was a FriedelCrafts amidation of the thiophene with a substituted isocyanate and was promoted by stoichiometric amounts of iron(III) chloride. Aldehydes can be introduced as electrophiles in a double FriedelCrafts-type reaction with 2-naphthol (Scheme 84).179 A solvent-free iron(III) chloride catalyzed procedure has been reported to provide 14-aryl(alkyl)-14H-dibenzo[a,j]xanthenes in high to excellent yields. Subsequently, the same transformation has been reported by Zhang and co-workers employing iron(III) triflate as catalyst also under neat conditions.180

Recently, the iron-catalyzed nucleophilic substitution of electrophiles bearing 1,3-dicarbonyl units as leaving groups has been described. This CC bond breaking process can be catalyzed by iron(III) chloride under very mild conditions (Scheme 85, Scheme 399).181 Electron-rich arenes and heteroarenes have been employed as nucleophiles for this transformation (Scheme 85). Reaction of indoles, furans, thiophenes, phenol, and 2-naphthol with 2-benzhydryl-1,3-diphenylpropane-1,3dione gave the diphenylmethyl-substituted arenes in high yields (eq a). The scope of electrophiles has been studied using 5-bromoindole as a model substrate (eq b). In all cases the electrophilic carbon was of benzylic, allylic, or propargylic nature. For benzylic electrophiles both electron-rich and electrondeficient arenes could be employed. The latter had to be treated with the bromoindole at 50 °C to achieve high conversions. A recent summary of iron-catalyzed CC bond activating reactions including electrophilic aromatic substitutions has been published by Klein and Plietker.75 A FriedelCrafts alkylation step is also involved in the following iron(III) triflate catalyzed three-component reaction for the synthesis of indenoquinolines (Scheme 86).182 Damavandi and Sandaroos treated aryl aldehydes with arylamines and 1,3-indanedione under solvent-free conditions at 3190

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Scheme 89

Scheme 88

Scheme 90

90 °C in the presence of 10 mol % iron(III) triflate. The corresponding dihydroindenoquinolines were obtained in high yields. An iron-catalyzed domino process of alkylamide oxidation by di-tert-butyl peroxide and subsequent FriedelCrafts alkylation of the tert-butyl hemiaminal intermediate provided amidomethylarenes (cf. Scheme 286).183 Alkylamines could be coupled with arenes in an analogous way (cf. Scheme 288).184 Overall, these transformations represent an oxidative CArCAlk coupling by double CH bond activation. Iron(III) chloride hexahydrate can be employed to catalyze a PictetSpengler cyclization of 2-(indol-1-yl)phenylamines with aromatic aldehydes (Scheme 87).185 5,6-Dihydro-indolo[1,2-a]quinoxalines with promising antifungal properties have been obtained in moderate to excellent yields by this procedure. Liang and co-workers described an iron-catalyzed reaction of N-methylanilines and aliphatic aldehydes to give 4-(4-aminophenyl)-substituted 1,2,3,4-tetrahydroquinolines in one pot (Scheme 88).186 This transformation includes a Mannich reaction followed by an intramolecular FriedelCrafts hydroxyalkylation and an intermolecular FriedelCrafts alkylation. The (2,3-trans-2,4-trans)-isomer was predominantly formed, however, in most cases with only moderate selectivity. The group of Enthaler synthesized an iron catalyst supported on a polyformamidine material and demonstrated its activity for the FriedelCrafts alkylation of anisole with adamantyl bromide.187 Benzyl thiocyanates have been employed as electrophiles for the iron(III) bromide catalyzed benzylation of arenes.188 The reaction was performed in 1,2-dichloroethane at 80 °C and provided unsymmetrical diarylmethanes in good yields. Gu and Li reported an iron-catalyzed three-component reaction of salicylaldehydes, 1,3-cyclohexanediones, and aryl nucleophiles leading to 9-arylated 2,3,4,9-tetrahydro-1Hxanthen-1-ones (Scheme 89).189 This domino process includes a FriedelCrafts-type benzylation step with iron(III) chloride as catalyst. In order to verify the activity of iron as catalyst,

Scheme 91

the outcome of the reaction has been confirmed by using ultrapure iron(III) chloride. Common metal impurities such as copper and palladium salts gave only poor yields when introduced in catalytic amounts. FriedelCrafts cyclization of biarylmethanol derivatives led to fluorenes (Scheme 90).190 The reaction proceeds in the presence of iron(III) chloride as catalyst in nitromethane at room temperature. Li et al. reported an intramolecular FriedelCrafts reaction of o-aryloxybenzaldehydes with subsequent oxidation to provide xanthones (Scheme 91).191 Iron(III) chloride hexahydrate was employed as catalyst and 2,3-dichloro-5,6-dicyano-pbenzoquinone (DDQ) served as oxidant. Several xanthones were obtained in high yields. 3-Cyano- or alkoxycarbonyl-substituted indolizines were treated with aldehydes in the presence of catalytic amounts of iron(III) chloride hexahydrate to afford bis(indolizinyl)methanes in high yields (Scheme 92).192 The transformation resembles to some extent the corresponding transformation of indoles with aldehydes (cf. Scheme 66). The reaction of 2-substituted indoles with phenylacetaldehyde to form (E)-3-styrylindoles is catalyzed by iron(III) chloride in the presence of ethanol (Scheme 93.).193 The reaction presumably proceeds via diethyl acetals and bis(indole) intermediates. A variety of 3-styrylindoles could be synthesized in good to high yields, thus demonstrating the utility of this method. 3191

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Scheme 95

Scheme 93

Scheme 96

Scheme 94

Ollevier et al. described an iron-catalyzed asymmetric ring opening of 1,2-diaryl-substituted meso-epoxides with indoles which provided the corresponding 2-(indol-3-yl)-ethanols in high yields and excellent enantioselectivities (Scheme 94).194 This transformation was achieved using iron(II) perchlorate hexahydrate as catalyst in combination with a chiral bipyridine ligand. FriedelCrafts alkylation of electron-rich arenes with benzylic, allylic, and propargylic alcohols was achieved with a cationic iron(III) porphyrin catalyst (Scheme 95).195 The alkylated arenes were obtained in good to excellent yields with ortho/para selectivities ranging from 1:1 to 11:1. A continuous flow alkylation of toluene with benzyl chloride was described using mechanochemically synthesized alumosilicate-supported iron oxide nanoparticles as catalyst.196 In contrast, the use of benzyl alcohol as alkylating agent led to the formation of dibenzyl ether. In the presence of iron(III) chloride hexahydrate, secondary propargylic alcohols react with phenols (Scheme 96).197 The resulting propargylated hydroxyarenes can readily cyclize in the presence of base leading to benzofurans in a one-pot process (eq a). Due to the different steric demand, tertiary propargylic alcohols formed 3H-benzo[f]chromenes on reaction with 2-naphthols (eq b). The latter domino process did not require the addition of an external base. Addition of iodine as an electrophile led to the formation of 2-iodo-3H-benzo[f]chromenes (eq c).

Scheme 97

Bioactive pyrazolo[3,4-b]pyridine-6(7H)-ones have been synthesized starting from 5-aminopyrazoles, aromatic aldehydes, and Meldrum’s acid employing an recyclable iron(III) montmorillonite (Fe3+/K-10) catalyst (Scheme 97).198 The reaction was performed under “on water” conditions and provided a large number of pyrazolo[3,4-b]pyridin-6(7H)-ones in excellent yields. Sajiki and Sawama et al. described a mild benzylation of electronrich arenes with benzyl trimethylsilyl ethers using iron(III) chloride as catalyst in 1,2-dichloroethane at room temperature 3192

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Chemical Reviews Scheme 98

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Scheme 101

Scheme 99 Scheme 102

Scheme 103 Scheme 100

(Scheme 98).199 A broad range of the corresponding diarylmethanes is available by this method. Complete double alkylation was achieved when the alkylation reagent was used in excess. Cook and Jefferies established a protocol for the inter- and intramolecular alkylation of arenes with secondary alcohols using catalytic amounts of iron(III) chloride in the presence of silver hexafluoroantimonate (Scheme 99).200 Alkylated arenes including six- to eight-membered benzannulated cycloalkanes are readily accessible by this procedure. An iron-catalyzed reductive FriedelCrafts benzylation of a large number of arenes with aromatic ketones and aldehydes has been reported by Leino et al. (Scheme 100).201 Iron(III) chloride proved to be an efficient catalyst for this transformation. The reaction is believed to proceed via an initial hydrosilylation of the carbonyl group and subsequent transformation of the intermediate silyl ether to the corresponding chloride with trimethylsilyl chloride as chlorinating agent. The resulting benzyl chloride serves as FriedelCrafts electrophile. A chelation-induced but remote CH allylation of 8-amidoquinolines with allyl alcohols has been reported by Zeng and Cong (Scheme 101).202 Using iron(III) chloride as a catalyst in 1,2-dichloroethane at 140 °C, a series of 5-allylated quinolines could be prepared. In contrast, a low-valent iron catalyst, generated from Fe(acac)3 and a Grignard reagent, led to the 4-allylated products. The configuration of the double bond was retained in the course of this transformation.

2.2.2.2. Hydro- and Carboarylation of Alkenes and Alkynes 2.2.2.2.1. Alkene Electrophiles. The group of Beller developed a protocol for the synthesis of 1,1-diarylalkanes by iron(III) chloride catalyzed hydroarylation of styrenes (Scheme 102).203 This FriedelCrafts-type reaction works with electron-rich arenes and thiophenes. However, in many cases significant amounts of regioisomeric products were formed. Among various metal salts and heteropoly acids, iron(III) chloride in water was found to be an efficient catalyst for the alkylation of indole with methyl vinyl ketone at the 3-position.204 Itoh and co-workers were the first to report efficient general conditions for the alkylation of indole with various vinyl ketones under iron catalysis (Scheme 103).205 They used iron(II) tetrafluoroborate hexahydrate as catalyst in acetonitrile or an ionic liquid as solvent. An excess of the alkylating agent led to double alkylation. Accordingly, the reaction of pyrroles with vinyl ketones under similar conditions but at 60 °C led to multiplealkylated pyrroles in good yields.206 2-Acetylpyrrole could be readily converted to the 4,5-dialkylated product. Alternatively, the authors also employed alumina-supported iron(III) perchlorate as catalyst. Lai, Xu, and co-workers reported the alkylation of indoles with chalcones using iron(III) chloride as catalyst in combination with acetylacetone as ligand precursor (Scheme 104).207 The reaction proceeded in methanol at room temperature and provided 3-alkylated indoles in high yields. A double alkylation of indoles with two different vinyl ketones has been described by Itoh et al. (Scheme 105).208 The reaction 3193

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Scheme 107

Scheme 105

Scheme 108

Scheme 106

was performed stepwise (not shown) or in a one-pot procedure. The first alkylation is very fast and occurs at the 3-position, whereas the second reaction is much slower and leads to alkylation at the 2-position. Iron(II) tetrafluoroborate hexahydrate and iron(III) perchlorate hydrate were the catalysts of choice. Moderate to good yields have been reported for this transformation. Radical scavengers did not inhibit the reaction and thus a radical mechanism was excluded in favor of a Lewis acid activation by the iron catalyst. An enantioselective alkylation of indoles with β-aryl α0 -hydroxyenones has been accomplished using a catalyst system consisting of iron(III) chloride, silver(I) triflate, and a chiral phosphoric acid (Scheme 106).209 The method provided

3-alkylated indoles in high yields and good enantioselectivities. The best results were obtained with arylenones bearing electronwithdrawing groups at the para-position of the aromatic substituent. It is noteworthy that a range of metal salts other than iron were less active and gave almost no enantioselectivity. The group of Zhang developed a protocol for an iron(III) chloride catalyzed alkylation of indoles with enamides (Scheme 107).210 The reaction proceeded regioselectively at the α-position of the enamides affording the 3-amidoalkylated indoles in high yields (eq a). In addition, 1,1-bis(indolyl)ethanes were obtained using N-methyl-N-vinylacetamide and an excess of indole (eq b). The reaction may be regarded as a domino ArSE/SN process. Nucleophilic displacement of the benzylic amide moiety from the alkylated intermediate by a second molecule of indole occurs subsequent to the hydroarylation. The method could be optimized to an efficient protocol for the synthesis of 1,1-bis(indolyl)ethanes in high yields. The Michael addition of indoles to nitroolefins has been achieved under solvent-free conditions using iron(III) chloride hexahydrate as catalyst (Scheme 108).211 The method gave access to a variety of 3-substituted indoles in excellent yields. It should be noted that catalyst loadings as low as 0.01 mol % were effective for this transformation. Indolylnitroalkenes have been applied as electrophiles for the FriedelCrafts alkylation of anilines in the presence of 20 mol % iron(III) chloride in 1,2-dichloroethane at 80 °C (Scheme 109).212 This procedure led to a variety of addition products in good to high yields. 2.2.2.2.2. Alkyne Electrophiles. An iron-catalyzed hydroarylation of aryl alkynes has been reported by Lu and co-workers (Scheme 110).213 This reaction was performed in nitromethane mostly at room temperature up to a maximum of 60 °C using iron(III) chloride as catalyst. Internal alkynes preferentially afforded Z-olefins. 3194

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Chemical Reviews Scheme 109

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Scheme 113

Scheme 110

Scheme 111 Scheme 114

Scheme 112

An intramolecular version of this reaction has been described by Campagne and co-workers (Scheme 111).214 In the presence of catalytic amounts of iron(III) chloride hexahydrate alkynetethered arenes were cyclized at ambient temperatures to dihydronaphthalenes in high yields. In analogy, tosylated N-propargylanilines could be converted to the corresponding 1,2-dihydroquinolines via a 6-endo-dig cyclization using iron(III) triflate as catalyst (Scheme 112, eq a).215 In contrast to the common ArSE reactivity, electron-rich anilines gave low yields in this intramolecular hydroarylation of alkynes, whereas electron-deficient arenes could be converted with high efficiency. Reversely, electron-rich aryl substituents bound directly to the alkynes led to high conversion. Alkynes with

electron-deficient aryl substituents showed no reaction. The protocol could also be applied to the cyclization of o-alkynyl biaryls to phenanthrenes (eq b). The reaction of electron-poor biaryls led to the corresponding products in high yields. A mechanism via an iron-substituted vinyl cation was proposed. An iron-mediated three-component reaction of terminal alkynes, benzylic alcohols, and arenes led to trisubstituted Z- or E-olefins (Scheme 113).216 The catalytic system for this transformation consisted of iron(III) chloride, silver nitrate, and trifluoromethanesulfonic anhydride or simply iron(III) chloride. The reaction proceeded with a pronounced diastereoselectivity. The three-component catalyst mixture and low temperatures (20 to 10 °C) led to the E-isomers (eq a), whereas higher temperatures (80 °C) and iron(III) chloride as promoter provided preferentially the Z-product (eq b). Propiolic acids could be hydroarylated with various arenes using iron(III) chloride as catalyst and silver(I) triflate as cocatalyst (Scheme 114).217,218 The resulting E- and Z-cinnamic 3195

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Scheme 117

Scheme 118

Scheme 116

acids were obtained in a wide range of yields with mostly low diastereoselectivity (eq a). In continuation of this work, the authors demonstrated the applicability of this protocol to the synthesis of coumarins employing ortho-unsubstituted phenols as nucleophiles (eq b).219 In this case, better yields could be obtained for the domino hydroarylation/lactonization process as compared to the simple hydroarylation of alkynes presented above (eq a). This iron-catalyzed process is superior to a related palladium-catalyzed protocol for the synthesis of coumarins.220 Under similar conditions, double hydroarylation was observed using indoles as nucleophilic partners.221 The corresponding 3,3-di(indolyl)propionic acids were obtained in high yields (eq c). Zhou and co-workers applied iron(III) chloride hexahydrate as catalyst for a domino benzylation/FriedelCrafts cyclization reaction (Scheme 115).222 Thus, benzylic alcohols were treated with arylalkynes in 1,2-dichloroethane at room temperature in the presence of 5 mol % of the catalyst to give indenes in high yields (eq a). Electron-deficient alkynes and electron-rich benzylic alcohols easily undergo a second benzylation. This effect could be exploited for a rational synthesis of benzylated indenes by using the benzylating agent and alkyne in a ratio of 2:1 (eq b). In the presence of an oxidant, preferably NBS, diarylmethanes could be employed as precursors of benzylic electrophiles (Scheme 116).223 Thus, Chen et al. established a protocol for the synthesis of indenes from alkynes and diarylmethanes using iron(II) chloride as catalyst and NBS as oxidant. The formation of intermediate diarylmethyl bromides was discussed which form benzylic cations in the presence of the iron catalyst. In analogy to

the example above (cf. Scheme 115), the cyclizing carboarylation of the alkyne was accompanied by benzylation of the arene with a second molecule of diarylmethyl bromide. Besides various heteroatom nucleophiles, Leyva-Perez and Corma also reported the addition of mesitylene and o-xylene to ethynylbenzene and p-chlorostyrene, respectively.224 They successfully applied iron(III) triflate or triflimide as catalysts for this transformation. The intramolecular hydroarylation of aryl alkynyl sulfides and sulfonamides has been reported to proceed efficiently under mild conditions (1,2-dichloroethane, room temperature) in the presence of catalytic amounts of iron(III) chloride and silver(I) triflate (Scheme 117).225 The resulting products, cyclic vinyl sulfides and vinylsulfonamides, respectively, have been obtained in moderate to high yields. Double hydroarylation of phenylene bis(propargyl) ethers could be realized to give dihydropyrano[2,3-g]chromenes. 2.3. Radical Aromatic Substitution

The iron-catalyzed reaction of aminoboranes with arenediazonium salts gave access to arylboronates (Scheme 118).226 This transformation was described by the group of Pucheault applying ferrocene as catalyst. In a first step, the diazonium salt was treated with an aminoborane to give the aryl(amino)borane intermediate. Subsequent treatment with methanol and then pinacol led to the pinacolyl benzeneboronates in a one-pot procedure. A radical process via initial formation of aryl radicals from the diazonium salt and the iron(II) catalyst was discussed. A formal intramolecular dehydrogenative cross coupling has been realized by Studer and co-workers via a base-promoted homolytic aromatic substitution process (Scheme 119).227 Thus, biphenyl-2-carbaldehydes were cyclized in the presence of catalytic amounts of ferrocene as a single-electron-transfer 3196

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Chemical Reviews Scheme 119

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Scheme 122

Scheme 123

Scheme 120

Scheme 121

(SET) reagent and tert-butyl hydroperoxide (TBHP) as oxidant providing fluorenones in high yields (eq a). Analogously, 2-aryloxybenzenecarbaldehydes afforded xanthones under the same conditions (eq b). A scavenging experiment with 2,2,6,6tetramethylpiperidin-1-oxyl (TEMPO) showed that radicals are involved in this process. This transformation did not require addition of an external base because hydroxide ions as base promotors were generated in situ by SET from the hydroperoxide. Base-promoted homolytic aromatic substitution was also exploited for the synthesis of phenanthridines from 2-isocyanobiphenyls and aryl aldehydes (Scheme 120).228 This reaction was performed using iron(III) chloride as catalyst and tert-butyl hydroperoxide as oxidant in acetonitrile at 90 °C and provided 6-aroylphenanthridines in good yields. An unusual substitution of aryl and heteroaryl bromides by chloride has been reported by Chen and co-workers (Scheme 121).229 They treated aryl bromides with 5 equiv of sodium chloride under photolytic conditions in the presence of 20 mol % iron(III) chloride in acetonitrile and achieved high conversions for many substrates. A radical additionelimination mechanism was discussed with the initial formation of chlorine radicals by action of the iron catalyst under UV irradiation. 2.4. CC Bond Formation by Cross Coupling Reactions

2.4.1. CX/CMetal Cross Coupling Reactions. Transition metal catalyzed cross coupling of carbon electrophiles and

carbon nucleophiles in various hybridization states has become a standard tool in synthetic organic chemistry.230236 The field is mainly dominated by palladium- and nickel-catalyzed reactions. Iron-catalyzed cross coupling stepped in when Kharasch and Fields reported their studies on the reaction of Grignard reagents with organic halides in the presence of metallic halides in 1941.237 They succeeded in the synthesis of biphenyl from phenylmagnesium bromide and bromobenzene in 47% yield using iron(III) chloride as catalyst. The group of Kochi developed the first iron-catalyzed cross coupling reactions of alkenyl halides with Grignard reagents.238241 It is noteworthy that this occurred prior to the emergence of palladium- and nickelcatalyzed cross coupling reactions. However, in the course of the following decades, the importance of the iron catalysis fell much behind those of the other metals. A renaissance began around the new millennium, and the new increase in publications on this subject until 2004 was summarized by Bolm and coworkers.1 Since then, this field has become very popular with the development of many efficient and reliable procedures which found application in the synthesis of complex natural products. A number of reviews, accounts, and highlight articles emphasize this tremendous development.2,3,8,44,242261 The following sections outline the recent development since 2004 including a retrospection on important earlier work. 2.4.1.1. Alkenyl and Alkynyl Electrophiles 2.4.1.1.1. Halide Electrophiles. The first examples for the alkenylation of alkyl Grignard reagents with vinyl bromides using catalytic amounts of iron(III) chloride were reported as early as 1971 by Kochi and Tamura (Scheme 122).238240 Starting from E-/Z-vinyl bromides, the reaction proceeded with retention of configuration and afforded the alkylated olefins in good to high yields.239 Subsequently, the conditions for cross coupling of alkenyl halides with alkylmagnesium reagents could be significantly improved by Molander262 and Cahiez.263 The scope of nucleophiles was extended by introducing various arylmagnesium compounds.262,264 Alami and Figadere reported the reaction of 1,1-dichloro-1-alkenes with Grignard reagents in the presence of Fe(acac)3 providing the disubstituted products.265 Conjugated enynes are prominent motifs in many bioactive molecules and important drug intermediates. Their accessibility by an iron-catalyzed cross coupling reaction of alkynyl Grignard reagents and alkenyl bromides or triflates has been demonstrated by Nakamura et al. (Scheme 123).266 A broad scope of substrates has been employed affording the corresponding products in high 3197

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Chemical Reviews Scheme 124

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Scheme 127

Scheme 125 Scheme 128

Scheme 126

to excellent yields. Lithium salts accelerate the reaction significantly. The authors proposed the redox couple Fe(0)/Fe(II) as driving force in this catalytic system (cf. Scheme 211). Using the protocol of Cahiez,263 the iron-catalyzed stereoselective KumadaCorriu cross coupling of a bromostyrene with an aryl Grignard reagent has been used as a key step in the synthesis of the anticancer agent combretastatin A-4 (Scheme 124).267 Alkynyl compounds can also function as electrophiles in ironcatalyzed cross coupling reactions. Thus, alkynyl bromides have been treated with Grignard-derived aryl cuprates in the presence of catalytic amounts of Fe(acac)3 to afford alkynylarenes in moderate to high yields (Scheme 125).268 The method has been applied to the synthesis of combretastatin analogues. More recently, iron(II) arylthiolates have been shown to be potent precatalysts for the coupling of alkenyl halides with alkyl or aryl Grignard reagents (Scheme 126).269 This modification avoids the use of NMP as cosolvent which has been classified as reprotoxic. Complete retention of the configuration at the double bond is observed for this transformation providing the substituted olefins in high yields. A domino process involving an iron-catalyzed preformation of an aryl Grignard reagent and subsequent iron-catalyzed coupling

with alkenyl bromides has been described by the group of von Wangelin (Scheme 127).270 Iron(III) chloride in combination with TMEDA as ligand served as a catalytic system for both the aryl Grignard formation and the cross coupling reaction (eq a). Electron-rich aryl bromides, heteroaryl bromides, and aryl bromides with polar substituents required an iron-free preformation of the aryl Grignard reagent (eq b). For both methods efficient couplings have been reported affording the corresponding styrenes in good to high yields. For specific substrates, e.g., activated aryl bromides and 1-substituted alkenyl bromides, the cross coupling can be carried out directly with no need for a preformation of the aryl Grignard species (cf. section 2.4.3, Scheme 228) The cross coupling of an alkenyl chloride with an arylmagnesium bromide in the presence of Fe(acac)3 has been exploited for a large-scale synthesis of the calcimimetic agent and calciumsensing receptor antagonist cinacalcet hydrochloride.271 1-Arylvinyl iodides but also chlorides, bromides, and triflates have been transformed to the corresponding 1,1-disubstituted olefins by iron-catalyzed cross coupling with alkyl and aryl Grignard reagents (Scheme 128).272 A catalyst system consisting of iron(III) chloride and copper(I) thiophene-2-carboxylate (CuTC) was used for this reaction. A cross coupling of alkenyl halides and Grignard reagents affording alkanes in moderate to excellent yields has been developed by Thomas and co-workers (Scheme 129).273 This result of a formal C(sp3)C(sp3) cross coupling was achieved using a dipyridyldiiminoiron(II) complex as catalyst and lithium N,N-dimethylaminoborohydride as reducing agent. Zorin et al. reported the coupling of ethyl (4E)-5-chloropent4-enoate with three different alkyl and aryl Grignard reagents in 3198

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Chemical Reviews Scheme 129

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Scheme 131

Scheme 130

Scheme 132 the presence of 2 mol % iron(III) acetylacetonate.274 The reaction was performed in a mixture of THF and N-methylpyrrolidone at room temperature to afford ethyl (4E)-alkenoates in 71, 73, and 81% yields, respectively. This procedure has been applied to the synthesis of (4E)-tridec-4-en-1-yl acetate which constitutes the sex pheromone of the tomato pinworm.275 In a subsequent work, the same authors described the cross coupling of a series of 3-chloroprop-2-en-1-amines with alkyl and aryl Grignard reagents in the presence of catalytic amounts of Fe(acac)3 and NMP as additive.276 The resulting allylamines were obtained in high yields and high diastereoselectivities with retention of configuration at the double bond. Farinola and co-workers treated bis(2-bromovinyl)benzenes with aryl Grignard reagents using Fe(acac)3 as catalyst.277 The reaction was conducted in THF at room temperature without addition of NMP providing the bisarylated products in moderate to high yields. 2.4.1.1.2. Sulfonate Electrophiles. An impressive variety of alkenyl triflates has been treated with aromatic and aliphatic Grignard reagents to afford the cross-coupled products in good to high yields (Scheme 130).278 The reaction was performed under very mild conditions in THF/NMP at 30 °C using Fe(acac)3 as catalyst. Functional groups such as esters, enones, ethers, carbamates, acetals, and lactones were tolerated in the electrophilic substrate. Thus, the catalyzed reaction suppresses the attack of the Grignard reagent at other electrophilic groups. F€urstner et al. propose that a highly reduced metal cluster of the formal composition [Fe(MgX)2]n is formed by reduction of the precatalyst with the Grignard reagent. This species would then induce a catalytic cycle involving an Fe(II)/Fe(0) redox couple as depicted in section 2.4.1.6 (cf. Scheme 209). The reduction of iron in the precatalyst to a formal oxidation state of II is believed to occur via β-hydride elimination, and thus, the reaction should not proceed with methylmagnesium halides. However, methyl magnesium bromide also gives a clean reaction. In this case ironate complexes may act as nucleophilic partners. Accordingly, dilithium tetramethylferrate could be employed to methylate alkenyl triflates in high yields.

Alternatively, arylcopper reagents which were obtained from the corresponding Grignard reagents by treatment with copper(I) cyanidedi(lithium chloride) complex can be employed as nucleophiles (Scheme 131).279 They react with alkenyl sulfonates in the presence of catalytic amounts of Fe(acac)3 to afford substituted styrenes in high yields (eq a). The scope of the electrophiles was extended to cyclic and acyclic dienyl sulfonates providing the corresponding arylated dienes (eqs b and c). The Fe(acac)3-catalyzed methylation of vinyl triflates with methylmagnesium bromide has been applied to the synthesis of a number of natural products. Building blocks for the synthesis of latrunculins A,280,281 B,281 and 16-epi-B281 and hence for the formal syntheses of latrunculins C, M, and S have been prepared by this procedure (Scheme 132). Once the synthetic pathway had been established, a large number of latrunculin analogues could be generated and tested for their actin-binding properties.282 An iron-catalyzed methylation of a vinyl triflate was also the final step of the total synthesis of the cadinane sesquiterpene ()-α-cubebene (Scheme 133).283 Similarly, a number of sesquisabinene and sesquithujene terpenes was synthesized employing a Fe(acac)3-catalyzed vinyl 3199

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Chemical Reviews Scheme 133

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Scheme 135

Scheme 134

Scheme 136

triflate/methylmagnesium bromide cross coupling.284 Another very impressive example for the cross coupling of vinyl triflates with Grignard reagents was the synthesis of the right-hand segment of ciguatoxin, a principal toxin of ciguatera poisoning (Scheme 134).285 This transformation demonstrates that the iron-catalyzed cross coupling of vinyl triflates has become an efficient technique which is applicable to the total synthesis of very complex natural products even at a late stage. More recently, the iron-catalyzed coupling of vinyl triflates and isobutylmagnesium bromide has been exploited for the synthesis of metabolites of the parasitic fungus Antrodia camphorata.286 Thus, the first total synthesis of antrocinnamomin D and new syntheses of antrodins A and B have been reported. Another application of the iron-catalyzed coupling of vinyl triflates has been reported for the construction of chiral C2-symmetric bicyclo[2.2.1]hepta-2,5-dienerhodium complexes as catalysts for the enantioselective 1,4- and 1,2-additions of phenylboronic acid to cyclic enones and N-sulfonylimines.287 F€urstner demonstrated that the homoleptic alkyl ferrate complex [(Me4Fe)(MeLi)][Li(OEt2)2] is a potent catalyst for the cross coupling of alkenyl triflates with methylmagnesium bromide.288 Various aliphatic and aromatic Grignard reagents have been coupled with bicyclic alkenyl triflates in the presence of catalytic amounts of Fe(acac)3 by Tam et al. (Scheme 135).289,290 High yields were achieved for this transformation which was carried out in DMPU or THF/NMP at 25 °C. A catalytic mechanism via FeMg clusters involving iron in the oxidation states 0 and II has been proposed by F€urstner et al.288,291,292 (see Scheme 209). This method has been applied to the introduction of the second substituent during the synthesis of enantiomerically pure C1-symmetrical 2,5-disubstituted bicyclo[2.2.2]octadiene (bod) ligands.293 E- and Z-vinyl tosylates with an additional methoxycarbonyl group could be efficiently coupled with Grignard reagents to

afford the corresponding E- and Z-trisubstituted α,β-unsaturated methyl esters with retention of configuration.294 Two examples out of 30 are presented in Scheme 136. Simple iron(III) chloride worked best as catalyst for the transformation of E-vinyl tosylates, whereas tris(dibenzoylmethane)iron(III) [Fe(dbm)3] was used for the Z-congeners. The reaction proceeded under mild conditions in THF at 05 °C affording the alkylated products in high to excellent yields. 2.4.1.1.3. Phosphate Electrophiles. Leaving groups other than sulfonates or halides have also been successfully applied to iron-catalyzed C(sp2)X/CMet cross coupling reactions. Cahiez et al. were the first to describe an example of a Fe(acac)3catalyzed reaction of a vinyl phosphate with butylmagnesium bromide.263 The use of vinyl phosphates is of special value as the halide congeners are often more difficult to prepare. Vinyl phosphates obtained from 4-piperidones have been treated with butylmagnesium bromide in the presence of Fe(acac)3 to afford 4-butyl-1,2,3,6-tetrahydropyridines in good yields.295 An application to natural product synthesis has been reported by Evans and co-workers for the synthesis of the k-opioid receptor antagonist salvinorin A (Scheme 137).296 A broader study on the reaction of vinyl phosphate electrophiles with alkyl and aryl Grignard reagents was subsequently carried out by Cahiez and co-workers (Scheme 138).297 A large number of substrates could be transformed in high yields using Fe(acac)3 as catalyst (eq a). E- and Z-olefins could be kinetically differentiated by this reaction. In mixtures of E- and Z-olefins the E-isomer reacts exclusively and stereoselectively (eq b). Alkyl chlorides, esters, ketones, and nitriles were tolerated. In addition, a one-pot procedure including the formation of vinyl phosphates from cyclic ketones and subsequent cross coupling with Grignard reagents was reported as well. 3200

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Chemical Reviews Scheme 137

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Scheme 139

Scheme 138 Scheme 140

This method could be applied successfully to conjugated dienyl- and trienylphosphates.298 Only 1 mol % iron(III) acetylacetonate was required to catalyze the stereoselective reaction which afforded the cross-coupled products in high yields. Terminal dienes are characteristic structural units in many insect pheromones. A very efficient access to the pheromone of Diparopsis castanea was accomplished in this work. 2.4.1.1.4. Carboxylate Electrophiles. Carboxylates represent readily available leaving groups. A broad study on iron-catalyzed cross coupling of several vinyl pivalates with a number of primary alkyl Grignard reagents has been reported by Shi et al. (Scheme 139).299,300 Surprisingly, the reaction was sensitive toward the counterion of the Grignard reagent. Primary alkylmagnesium chlorides reacted smoothly, whereas the corresponding bromides gave very poor results. In the presence of an excess of lithium chloride, an efficient coupling of the bromo Grignard reagent occurred with no need to add an additional ligand (eq a). Addition of NHC ligands proved to give better yields for the coupling of hexylmagnesium chloride (93% instead of 79% yield) (eq b). Cross coupling of a variety of cyclic and acyclic vinyl pivalates with hexylmagnesium chloride afforded the products in high yields. However, methyl Grignard reagents, as well as secondary and tertiary Grignard reagents, led to no significant conversion. An iron-catalyzed ring opening and alkenylX/methyl MgBr cross coupling has been presented by F€urstner and coworkers (Scheme 140).301 They treated 2-pyrones with methylmagnesium bromide in the presence of iron(III) acetylacetonate

Scheme 141

and obtained dienyl carboxylates with retention of configuration at both double bonds. Warming the reaction mixture to ambient temperature led to inversion of the configuration at C-2. The lactone moiety acts as a nontraditional leaving group. However, the mechanistic proposal envisages a conjugate addition rather than an oxidative addition/reductive elimination sequence. The method was applied to the total synthesis of the cytotoxic tryptamine derivative granulatamide B and the E/Z-diene carboxylate fragment of the marine macrolide pateamine A. 2.4.1.1.5. Chalcogen Electrophiles. Alkenyl aryl sulfides represent also useful electrophiles for iron-catalyzed cross coupling reactions with Grignard reagents (Scheme 141).302 The reaction occurs chemoselectively at the alkenyl site, whereas the arylS bond was almost not affected. However, in six examples 3201

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Chemical Reviews Scheme 142

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Scheme 144

Scheme 145 Scheme 143

tested the yields were mostly moderate and a broad scope of substrates has not been used yet. Alkenesulfonyl chlorides can be treated with alkyl or aryl magnesium halides in the presence of Fe(acac)3 as catalyst to provide the substituted olefins in moderate to good yields (cf. Scheme 181).303 It is noteworthy that the somewhat exotic vinyl selenides and tellurides are even better electrophiles for the iron-catalyzed cross coupling with alkyl- and arylmagnesium bromides (Scheme 142).304 This reaction was reported to proceed with retention of configuration affording the substituted alkenes in high yields. 2.4.1.2. Aryl Electrophiles 2.4.1.2.1. Alkyl Grignard Nucleophiles. Some examples of iron(III) acetylacetonate catalyzed cross coupling reactions of aryl halides with Grignard reagents have already been reported by Pridgen et al. in 1989.305 They required an ortho-imino group as activating function and obtained ortho-substituted benzaldehydes after acidic hydrolysis. The feasibility of iron-catalyzed cross coupling reactions of alkyl Grignard reagents with aryl and heteroaryl electrophiles has been demonstrated by F€urstner et al. (Scheme 143).291,292 The authors described a broad range of examples providing the coupling products in high to excellent yields (eqs a and b). Electron-deficient arenes were more reactive than electron-rich derivatives. Aryl chlorides, triflates, and even tosylates as electrophiles afforded higher yields than aryl bromides and iodides. Thus, a complementary approach to the corresponding palladium-catalyzed reactions was disclosed. The scope of heteroaryl electrophiles included pyridine, pyrimidine, triazine, quinoline, isoquinoline, β-carboline, purine (eq b), pyridazine, pyrazine (eq c), quinoxaline, quinazoline, uracil,

thiophene, and benzothiazole. A detailed procedure has been published in Organic Syntheses for the multigram scale synthesis of the liquid crystalline material 4-nonylbenzoic acid (see also ref 8).306 Similar reaction conditions have been applied to the cross coupling of aryl Grignard reagents with heteroaryl chlorides to afford the arylated products (eq c). Selective monoalkylation of dichloroarene and dichloroheteroarene derivatives has been achieved in good yields demonstrating the superiority of this method over the related nickel- and palladium-catalyzed procedures (Scheme 144). The former tend to the formation of dialkylated products, whereas palladium catalysts are mostly unreactive.278 Starting from an aryl triflate, this method has been applied to the synthesis of the immunosuppressive agent FTY720 (Scheme 145).307 More recently, the same target was synthesized by an iron-catalyzed cross coupling of an aryl chloride with the same Grignard reagent.308 An iron-catalyzed selective monoalkylation of 2,6dichloropyridine with an alkyl Grignard reagent has been applied to the synthesis of the macrocyclic spermidine alkaloid ()-isooncinotine.309 A consecutive ironsalen-catalyzed introduction of two different alkyl groups in the reaction with 2-chloro-6-triflyloxypyridine was the key step en route to the odoriferous alkaloid (R)-(+)-muscopyridine (Scheme 146).310 Similarly, but not in a one-pot procedure, a variety of 2,5dialkylated pyrazines have been synthesized as reference compounds for the identification of natural products from the myxobacterium Chondromyces crocatus and several marine bacteria. In this case, tris(acetylacetonato)iron was employed as catalyst for the cross coupling of chloropyrazines with alkyl Grignard reagents (Scheme 147).311 A dichotomy of regioselectivity was observed for the cross coupling of 6,8-dichloropurines with phenylboronic acid and methylmagnesium bromide catalyzed by palladium or iron, respectively (Scheme 148).312 The palladium-catalyzed Suzuki Miyaura coupling with phenylboronic acid gave 8-chloro-6phenylpurine. In contrast, Fe(acac)3-catalyzed methylation in THF/NMP at room temperature afforded the 6-chloro-8methylpurine as the major product in moderate yield. The same tendency could be observed with 2,6,8-trichloropurines. Monomethylation with methylmagnesium bromide in 3202

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Chemical Reviews Scheme 146

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Scheme 149

Scheme 150

Scheme 147

Scheme 148

the presence of Fe(acac)3 as catalyst occurs preferably at C-8. However, only a low selectivity and thus a low yield was achieved.313 Cross coupling of 2,6-dichloro-9-methylpurine afforded selectively 2-chloro-6,9-dimethylpurine in good yield.314 Treatment of a silylated 2,4-dichloropyrimidin-5-yl C-20 -deoxyribonucleoside with 1.5 equiv of methylmagnesium

chloride in the presence of Fe(acac)3 gave regioselectively the 2-chloro-4-methylpyrimidinyl nucleoside in 46% yield (Scheme 149). Substitution of both chlorine atoms was achieved when 4 equiv of the Grignard reagent were applied.315 4-Alkylated and 4-phenylated pyrrolo[3,2-c]quinolines have been obtained by iron-catalyzed cross coupling of the corresponding 4-chloro precursors with Grignard reagents.316 Heteroaryl tosylates and also the corresponding phosphates have been coupled successfully with alkyl Grignard reagents. The reactions were catalyzed by Fe(acac)3 at low temperature in a THF/NMP mixture.317 In addition, the coupling of a quinoxalin-2-yl dimethylsulfamate with phenylmagnesium bromide has been reported in the presence of Fe(acac)3 and TMEDA. In analogy to alkenyl pivalates, the group of Shi reported the coupling of 2-naphthyl pivalate and 2-naphthyl N,N-dimethyl carbamate with hexylmagnesium chloride in the presence of iron(II) chloride and an NHC ligand to afford 2-hexylnaphthalene in 40 and 80% yields, respectively (cf. Scheme 139).299 The coupling of aryl, alkenyl, and alkyl electrophiles with alkyl and aryl magnesium bromides has been investigated using lowvalent iron complexes. It was shown that the oxidation state of the iron precatalyst was not decisive for its performance but the presence of a labile coordination environment is important. In particular, good results could be achieved with an anionic bis(anthracene)iron(I) complex.318 2-Bromofuran was coupled with Grignard reagents in the presence of catalytic amounts of Fe(acac)3 and N,N0 -dimethylN,N0 -propylene urea (DMPU) as an additive.319 Moderate yields were achieved with primary and secondary alkylmagnesium halides as substrates. It is important to note that the corresponding palladium-catalyzed reaction did not afford 2-alkylfurans. The iron-catalyzed coupling of aryl Grignard reagents with 2-bromofuran provided 2-arylated furans in only low yields. 3203

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Chemical Reviews Scheme 151

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Scheme 154

Scheme 152

Scheme 153

Scheme 155

Nonactivated aryl chlorides, for example those without electron-withdrawing groups, could be coupled with primary and secondary alkyl Grignard reagents using iron(II) chloride tetrahydrate in the presence of N-heterocyclic carbene ligands (Scheme 150).320 Primary alkylmagnesium chlorides coupled in excellent yields, whereas cyclohexyl magnesium chloride provided the alkylated arenes in moderate yield. The iron-catalyzed cross coupling of aryl chlorides with alkyl Grignard reagents using the conditions described by F€urstner has been exploited for the construction of pentapodal ω-functionalized corannulene derivatives which are soluble in common organic solvents (Scheme 151).321 The yields for this 5-fold transformation were surprisingly high. In analogy to the coupling of alkenyl halides with Grignard reagents (cf. Scheme 126), Fe(II) arylthiolates can be used as precatalysts for the reaction of aryl chlorides with butylmagnesium chloride.269 Both electron-rich and electron-poor aryl chlorides, in particular o-chlorostyrenes, could be coupled efficiently with alkyl Grignard reagents using Fe(acac)3 as catalyst (Scheme 152).322 The reaction was performed in THF/NMP (10:1) at 30 °C with only 2 mol % of the catalyst. Various functional groups were tolerated, and for the coupling of Grignard reagents with o-chlorovinylarenes, even an aryl bromide without a vinyl group in close proximity stayed intact. This observation emphasizes the essential role of the olefinic moiety for the coupling. A regioselective Fe(acac)3-catalyzed functionalization of dihalo(hetero)arenes with alkyl Grignard reagents has been described by Malhotra and co-workers (Scheme 153).323 Due to the high degree of selectivity, two consecutive alkylations with

two different alkylmagnesium reagents could be performed in a one-pot procedure. The group of Fox introduced TMEDA as a cheap and readily removable ligand for the iron-catalyzed coupling of aryl chlorides with alkyl Grignard reagents.324 This led to very low catalyst loadings (0.11 mol %) and high conversions for a number of substrates containing ester, amide, and trifluoromethyl groups. Aryl sulfamates and carbamates were also used as electrophiles for the cross coupling with alkylmagnesium chlorides (Scheme 154).258,325 The reaction was catalyzed by iron(II) chloride in the presence of an N-heterocyclic carbene ligand. The scope of the electrophiles ranged from electron-deficient heterocycles and electron-poor arenes to arenes with electron-donating groups. The advantage of this method is the ready availability of the substrates from the corresponding phenolic compounds. Moreover, sulfamates and carbamates are inert to various classical cross coupling reactions and may function as ortho-directing moieties. This enables a chemoselective sequential functionalization of arenes as demonstrated in Scheme 155. In a first step, the 4-chloroaryl carbamate was subjected to an ortho-selective 3204

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Chemical Reviews Scheme 156

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Scheme 158

Scheme 159

Scheme 157

lithiation and methylation. Subsequently, the chloro substituent was exploited for a nickel-catalyzed SuzukiMiyaura coupling, and finally, the iron-catalyzed Kumada-type alkylation described above provided the substituted biphenyl. Similar conditions for the coupling of aryl sulfamates and tosylates with alkyl Grignard reagents have been described almost simultaneously by Cook and Agrawal (Scheme 156).326 They used iron(III) fluoride trihydrate as catalyst in the presence of an NHC ligand. In contrast to the previous procedures, dichloromethane as additive was not required. For the isopropyl nucleophile, the use of the fluoride anion was crucial to suppressing isomerization and obtaining a good branched-to-linear ratio. Wang and Guo demonstrated that aryltrimethylammonium triflates can be used as electrophiles for the iron-catalyzed cross coupling with alkyl Grignard reagents (Scheme 157).327 This reaction was performed under the standard conditions with iron(III) acetylacetonate as catalyst in a THF/NMP mixture at room temperature. A large number of alkylated arenes could be obtained in good to excellent yields. The iron(III) acetylacetonate-catalyzed coupling of a 2-chloropurine with n-butylmagnesium chloride has been applied as a key step in the synthesis of the selective adenosine A2A receptor antagonist ST1535.328 2.4.1.2.2. Aryl Metal Nucleophiles. In their initial papers on the iron-catalyzed cross coupling of aryl electrophiles with Grignard reagents F€urstner et al. could not achieve an efficient coupling with aryl-, allyl-, and alkenyl Grignard reagents. They attributed this negative result to the inability of these reagents to reduce the iron precatalyst due to a lack of β-hydrogen atoms.291

Employing phenylmagnesium bromide mainly led to the formation of biphenyl by homocoupling. This phenomenon had already been observed by Kharasch.237,329 Catalytic amounts of transition metal salts including iron(III) chloride induce the homocoupling of aryl Grignard reagents when aryl halides are present as oxidizing agents. However, efficient cross coupling with aryl Grignard reagents was achieved using electron-deficient heteroaryl chlorides as substrates.292 Figadere et al. reported the cross coupling of phenylmagnesium bromide with chloroquinolines, bromoquinolines, and bromopyridines in the presence of catalytic amounts of Fe(acac)3.330 This reaction was achieved in THF at 30 °C without addition of NMP to give the phenylated heterocycles in moderate to good yields. The same conditions have been used by Ple and co-workers for the coupling of chlorinated pyridines, pyrimidines, pyrazines, and pyridazines with phenylmagnesium bromide and pyridine-3-yl-magnesium chloride.331 The phenylated or pyridine-substituted heterocycles have been obtained in moderate yields. The formation of homodimers could be efficiently suppressed by employing aryl and heteroaryl copper reagents as nucleophiles. Thus, a broad range of organocopper compounds derived from Grignard reagents by reaction with CuCN 3 2LiCl was treated with various aryl iodides to afford the cross-coupled products (Scheme 158).332 In contrast to F€urstner’s alkylation conditions, the reactivity of the aryl electrophiles decreased from iodides via bromides to chlorides. Aryl triflates reacted even slower, whereas tosylates were inert under these conditions. In addition to the aryl halides also heteroaryl halides could be employed. Moreover, a pyridine copper reagent was also 3205

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Chemical Reviews Scheme 160

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Scheme 162

Scheme 163

Scheme 161

reported to undergo cross coupling reaction with an aryl iodide. Due to the use of the milder copper reagents as nucleophiles, the presence of various functional groups, such as ketones, esters, nitriles, amides, and acetals, was tolerated in the molecule. This reaction proceeded even more smoothly with aryl iodides bearing amide functions (Scheme 159). Functionalized biphenyls (eq a) and 8-phenylquinolin-2(1H)-ones (eq b) were obtained in high yields by this transformation.333 A highly efficient coupling of aryl chlorides with aryl Grignard reagents has been reported by Hatakeyama and Nakamura (Scheme 160).334,335 They used a catalytic system which was formed prior to the cross coupling reaction. Iron trifluoride trihydrate was reduced with ethylmagnesium bromide in the presence of an NHC ligand. The resulting catalytic system suppressed the homocoupling and provided excellent yields of the cross coupling products. As observed for the iron-catalyzed cross coupling of aryl halides with alkyl Grignard reagents under F€urstner’s conditions,291,292,306 chlorides were more reactive than bromides, iodides, and triflates. Complementary reactivity was observed with cobalt and nickel fluorides as catalysts. Aryl fluorides did not react at all. In addition to the usual aryl chlorides, the heteroaryl derivatives 2-chloroquinoline and 2-bromopyrimidine could be used successfully for cross coupling with mesitylmagnesium bromide, tolylmagnesium bromide, and thiophen-2-ylmagnesium bromide, respectively.335 Nonactivated o-chlorostyrenes have been successfully coupled with several arylmagnesium bromides (Scheme 161, eq a).336 Unsubstituted m- and p-chlorostyrenes gave only very poor

results in this transformation. However, introduction of methyl or phenyl substituents to the vinyl group, namely the use of 3-chloro- or 4-chloroisopropenylbenzene (eq b), or (E)-1chloro-4-styrylbenzene, led to a good conversion. Especially beneficial was the use of the less basic 4-fluorophenylmagnesium bromide as a nucleophilic partner (see example in Scheme 161). The activating effect of the vinyl group was attributed to an initial coordination of the olefin to the iron followed by haptotropic migration into proximity of the CCl bond. The arylation of heterocyclic chlorides or bromides has been reported by Knochel et al. using iron tribromide as catalyst (Scheme 162).337 The reaction proceeded mostly within minutes in a mixture of THF and MTBE (methyl tert-butyl ether) at room temperature. A large variety of substrates and aryl Grignard reagents could be coupled in high yields. Several electron-withdrawing and electron-donating functionalities were tolerated, such as dimethylamino, tert-butoxycarbonyloxy (OBoc), or methoxy groups. The method could be further improved by addition of isoquinoline as ligand (Scheme 163).338 As a consequence the reaction time could be decreased and the yields increased. A variety of complex halogenated pyridines, pyrimidines, and 1,3,5-triazines could be treated successfully with aryl Grignard reagents. The coupling of 2-bromo-3-(but-3-enyl)pyridine with phenylmagnesium chloride provided the phenylated product together with a cyclized product in a ratio of 4:1. This radical clock experiment indicated the occurrence of radical intermediates. Mixtures of cross-coupled p-terphenyl and homocoupled biphenyl were obtained by reaction of phenylmagnesium bromide with either 1,4-dibromobenzene or 1-bromo-4-chlorobenzene in THF at room temperature using iron(III) chloride as catalyst without an additional ligand.339 Chua and Duong described an iron(III) tert-butoxideNHC system as an active catalyst for the cross coupling of aryl chlorides with aryl Grignard reagents.340 A broad range of biaryls could be synthesized by this method in high to excellent yields. The SuzukiMiyaura coupling of aryl chlorides and bromides with phenylboronic acid could be catalyzed by iron(III) chloride when high pressure (15 000 bar) was applied.341 3206

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Chemical Reviews Scheme 164

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Scheme 166

Scheme 165 Scheme 167

2-Bromopyridines and 2-bromopyrimidine were subjected to a SuzukiMiyaura coupling with sodium tetraphenylborate in the presence of a mixed FeZn catalyst to afford the heterobiphenyls in good to high yields.342 Das and co-workers employed nanocrystalline nickel ferrite as catalyst for the coupling of arylboronic acids with iodobenzene to afford biphenyls in excellent yields.343 2.4.1.2.3. Alkynyl Nucleophiles. Protected terminal arylalkynes, such as 4-aryl-2-methylbut-3-yn-2-ols, have been coupled with aryl iodides (Scheme 164).344 The reaction was performed in water at 140 °C in a sealed tube in the presence of catalytic amounts of iron(III) chloride hexahydrate, a cationic bipyridyl ligand, potassium hydroxide as base, and zinc powder as reductant. Symmetrical and nonsymmetrical diarylalkynes were obtained in good to excellent yields. 2.4.1.3. Acyl Electrophiles. In an early work, Cook et al. could show that FeCl3 as catalyst for the acylation of Grignard reagents by carboxylic chlorides is superior to several alternative metal chlorides including CuCl, AlCl3, and CoCl2.345 Subsequently, this reaction became a reliable tool for the construction of ketones, especially by the contributions of Marchese346349 and F€urstner.278 In addition to the iron-catalyzed acylation reactions described in the previous review by Bolm and coworkers,1 a few new papers appeared on this topic. Knochel et al. reported the use of acyl cyanides as acylating agents for Grignard reagents (Scheme 165).350 The reaction was catalyzed by Fe(acac)3 and provided the corresponding ketones in high yields. The acyl cyanides were shown to be more efficient acylating agents as compared to the acyl chlorides and provided significantly better yields in this transformation. Kang et al. employed α,β-unsaturated acyl cyanides for this transformation and obtained the substrates from a triethylaminecatalyzed redox cyanation.351 Imidoyl chlorides could be transformed into the corresponding imines by an Fe(acac)3-catalyzed coupling with Grignard reagents (Scheme 166).352 The reaction worked also in the

presence of ester functionalities and Weinreb amides. Moreover, depending on the reaction conditions, either the Weinreb amide or the imidoyl chloride could be addressed by the Grignard reagent. The presence of the iron catalyst favors the formation of the imine, whereas the ketone is formed without catalyst. The reaction has been exploited for the synthesis of clozapine analogues. The reaction of an acyl chloride with an alkylmagnesium bromide has been successfully applied to the total synthesis of the macrolides latrunculin A,280,281 B,281 and 16-epi-B,281 and the musk odorant (R,Z)-5-muscenone (Scheme 167).353 2.4.1.4. Alkyl Electrophiles 2.4.1.4.1. Aryl Nucleophiles. Alkyl electrophiles are more challenging electrophiles for metal-catalyzed cross coupling reactions with metal organyls. They are less reactive than aryl halides and β-hydride elimination is a possible side reaction. In addition to the two methods described by Hayashi354 and Nakamura,355 several new papers have appeared on this topic since then. F€urstner and Martin introduced the ferrate complex [Li(tmeda)]2[Fe(C2H4)4] as catalyst for this transformation (Scheme 168).288,356 It was chosen as a well-defined mimic for low-valent iron species which might be formed from Fe(II) or Fe(III) precatalysts by reduction with Grignard reagents. A wide range of primary and secondary alkyl bromides and iodides could be successfully coupled with arylmagnesium bromides in high yields. Allylic and propargylic bromides and chlorides as substrates gave similar results. Ironsalen complexes were shown to catalyze the cross coupling of alkyl halides with arylmagnesium halides providing 3207

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Chemical Reviews Scheme 168

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Scheme 171

Scheme 169

Scheme 172

Scheme 170

the alkylated arenes (Scheme 169).357 β-Hydride elimination was efficiently suppressed under the mild reaction conditions reported. In addition, it has been shown that iron(III) chloride is a suitable precatalyst for the cross coupling of aryl Grignard reagents with alkyl halides bearing β-hydrogen atoms (Scheme 170). Addition of amine ligands was shown to lead to an efficient coupling (eq a).358 The authors favor a radical mechanism for this transformation. Support for this hypothesis comes from the fact that a bromomethylcyclopropane does not give the cyclopropyl product as would be the expectation for an oxidative addition (radical clock experiment, cf. Scheme 217). Instead the ring-opened terminal alkene was formed presumably via a methylcyclopropyl radical.

Moreover, 6-bromohexene gave predominantly the cyclized cyclopentane derivative upon cross coupling with phenylmagnesium bromide in the presence of FeCl3 and a phosphine or a phosphite ligand.358 The reaction also proceeded efficiently when monoand bidentate phosphine, phosphite (eq b), or arsane ligands were added.358 Alternatively, in the presence of carbene ligands the coupling was also highly efficient (eq c). Iron nanoparticles were also excellent catalysts for this transformation. They were prepared in a preliminary step and stabilized by polyethylene glycol (PEG) or 1,6-bis(diphenylphosphino)hexane or prepared in situ.359 The Kumada-type coupling of alkyl halides with aryl Grignard reagents was efficiently performed in the presence of Nakamura’s FeCl2(3,5-tBu2-SciOPP) catalyst360 (Scheme 171).361 Primary, secondary, and tertiary alkyl halides could be converted in high yields. The intermediacy of radical species was strongly implied by a radical clock experiment with (iodomethyl)cyclopropane. No (cyclopropylmethyl)-substituted arene was observed in this reaction but exclusively the ring-opened terminal olefin (cf. the related reaction with (bromomethyl)cyclopropane in Scheme 217). Yamaguchi and Asami synthesized an iron(III) complex with a tridentate β-aminoketonato ligand. This complex could be successfully applied as catalyst for the cross coupling of alkyl halides with aryl Grignard reagents.362 Simple Fe(acac)3 was shown to catalyze the arylation of α-bromocarboxylic acid derivatives with aryl Grignard reagents (Scheme 172).363 The reaction proceeded under very mild conditions at 78 °C and afforded the corresponding phenylacetic acid derivatives in moderate to high yields. Nakamura et al. established conditions for the cross coupling of alkyl halides with arylzinc reagents.364 Iron(III) chloride was used as precatalyst and TMEDA was required as ligand. The presence of a magnesium salt proved to be crucial for the success of this iron(III)-catalyzed coupling reaction. Nucleophiles could be symmetrical diarylzinc compounds (Scheme 173, eq a) or trimethylsilylmethyl(het)arylzinc compounds (eq b). This Negishi-type transformation provided alkylated (het)arenes under mild conditions in excellent yields. 3208

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Chemical Reviews Scheme 173

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Scheme 174

Scheme 175

Scheme 176

Replacing the TMEDA additive by catalytic amounts of 1,2bis(diphenylphosphino)benzene (DPPBz) enabled the selective coupling of polyfluorinated arylzinc reagents with primary and secondary alkyl halides (Scheme 173, eq c).365 The C(sp2)F bond cleavage was efficiently suppressed under these conditions. The obtained polyfluorinated arenes are of importance for liquid crystals, drugs, agrochemicals, and dyes. In addition, alkyl tosylates could be employed for the cross coupling reaction with arylzinc reagents (eq d).366 The latter were prepared in situ from the corresponding Grignard reagents or aryl lithium compounds and a zinc halideTMEDA complex. Functionalized arenes were introduced via the corresponding zinc organyl bearing a nontransferable trimethylsilylmethyl group as a second substituent (eq e). In situ generated arylzinc reagents could also be successfully employed for the cross coupling with α-halo-β,βdifluoroethyl substrates (Scheme 174).367 The catalyst system consisted of iron(II) chloride, N,N,N0 ,N0 -tetramethylethylenediamine (TMEDA), and 1,3-bis(diphenylphosphino)propane (DPPP). Dehalofluorination of the substrates could mostly be inhibited. In addition, arylaluminum reagents have been employed for cross coupling reactions with alkyl halides catalyzed by FeCl2(DPPBz)2.368 Among the various arylaluminum species which are in dynamic equilibrium, aluminum “ate” complexes are the active components. The equilibrium strongly depends on the coexisting in situ generated magnesium salt. In continuation of this work, the group of Nakamura was able to develop adequate conditions for an iron-catalyzed

SuzukiMiyaura coupling of alkyl halides with aryl borates (Scheme 175).360 Two novel iron(II) complexes with sterically hindered bidentate bisphosphine ligands have been applied to this transformation. Additionally, a magnesium cocatalyst was required. A mechanism involving the redox couple Fe(II)/ Fe(III) was proposed. Oxidation of iron(II) to iron(III) is accompanied by release of an alkyl radical which combines with an aryl radical from an intermediate aryliron complex, thus forming the cross coupling product (cf. the mechanism for alkylX/arylMgX coupling in Scheme 216). Another iron-catalyzed SuzukiMiyaura coupling of alkyl, benzyl, and allyl electrophiles with arylboronic esters has been described by Bedford and co-workers (Scheme 176).369 In distinction from Nakamura (cf. Scheme 175),360 they were able to demonstrate that iron(III) acetylacetonate or simple irondiphosphine complexes are potent catalysts for this transformation. The boronic esters were activated by alkyllithium reagents. Application of this method to a series of substrates provided the cross coupling products in good to high yields. Cross coupling of alkyl halides and aryl Grignard reagents has also been carried out in a biphasic system using the ionic liquid 3209

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Chemical Reviews Scheme 177

REVIEW

Scheme 179

Scheme 178 Scheme 180

butylmethylimidazolium tetrachloroferrate (bmim-FeCl4) as catalyst (Scheme 177).370 After completion of the reaction, the product mixture could be easily separated from the catalyst by simple decantation. The catalyst was air-stable and could be recycled. An example is presented in which the catalyst is recycled four times without losing much of its activity. Cahiez and co-workers developed an efficient method for the coupling of primary and secondary alkyl halides with aryl Grignard reagents.371 They introduced two new catalytic systems, one consisting of Fe(acac)3/hexamethylenetetraamine (HMTA)/TMEDA (Scheme 178, eq a) and the other one using the new complex [(FeCl3)2(tmeda)3] (eq b). The practicability of the methods was demonstrated with several examples affording the products in high yields. sec-Butylbenzene could be prepared by both methods on large scale. A single-electron transfer from an iron(0) intermediate to the alkyl halide was proposed (cf. section 2.4.1.6, Scheme 215). Basically the same conditions have been applied by Zhang et al. for the synthesis of a number of alkylated biphenyls by reaction of alkyl bromides with biphenyl-4-ylmagnesium bromide.372 A variety of nonfunctionalized primary and secondary alkyl halides bearing β-hydrogens could be coupled with aryl Grignard reagents using a newly synthesized Fe(III) amine bis(phenolate) complex as catalyst (Scheme 179).373 Yields varied from low to excellent. A similar but dimeric iron(III) catalyst was demonstrated to catalyze the double cross coupling of dichloromethane with aryl Grignard reagents.374 However, high yields of the diphenylmethane product were only achieved with o-tolylmagnesium bromide as nucleophile. 1,2-Diarylethanes frequently appeared as byproducts resulting from a radical coupling mechanism. Iron(III) chloride was also tested as catalyst for this reaction affording the double cross coupling product in moderate yields along with significant amounts of 1,2-diarylethane side products.

A prominent exception was again o-tolylmagnesium bromide which provided the di(o-tolyl)methane in 90% yield and less than 1% 1,2-di-o-tolylethane. Substoichiometric amounts of FeCl2 3 2LiCl have been applied to promote a highly stereoconvergent cross coupling reaction of cyclic iodohydrine derivatives and arylmagnesium bromides (Scheme 180, eq a).375 The thermodynamically more stable trans-isomers were formed in excellent selectivity. Nonprotected chlorocyclohexanols have been coupled with aryl aluminates as nucleophiles in the presence of FeCl2(TMS-SciOPP)360 as catalyst (eq b).376 High yields and mostly high diastereoselectivities were achieved under these conditions. Interestingly, alkanesulfonyl chlorides could be applied as electrophiles for the coupling with Grignard reagents (Scheme 181).303 This desulfinylative cross coupling proceeded under the standard conditions originally described by F€urstner,291,292 with Fe(acac)3 as catalyst in a mixture of THF and NMP, however, at elevated temperatures. The best results were obtained with aryl Grignard reagents affording the alkylated arenes in good yields. Alkenylmagnesium halides provided the products in only low to moderate yields. 3210

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Chemical Reviews Scheme 181

REVIEW

Scheme 184

Scheme 182

Scheme 183

Scheme 185

The anionic iron(II) complex [Fe(IPr)Br3](HIPr) 3 PhMe bearing an NHC ligand was reported to be an efficient catalyst for the cross coupling of cyclohexyl bromide with p-tolylmagnesium bromide. Catalyst loadings of 0.51 mol % were sufficient to achieve this transformation in excellent yields of 8996%.377 The same benchmark reaction was used to investigate the performance of isolated and fully characterized chelating diand tetra-NHC iron(II) complexes.378 Activities, however, were found to be lower than for the above-mentioned related complexes with two monodentate NHC ligands. A broad variety of primary, secondary, and tertiary alkyl chlorides were treated with various arylmagnesium bromides to provide the cross coupling products in high yields (Scheme 182).379 Even polychloroalkanes which often tend to give side reactions could be converted in high yields. FeCl3 was used as precatalyst in the presence of the NHC ligand IPr 3 HCl. Slow addition of the Grignard reagent turned out to be crucial for an efficient transformation. A dinuclear iron(II) complex bearing NHC ligands has been reported to induce the cross coupling of primary alkyl fluorides with aryl Grignard reagents (Scheme 183).380 In general, high to very high yields were achieved for this transformation. Most probably, radical intermediates are involved in this reaction as (fluoromethyl)cyclopropane as a substrate provides not only the (cyclopropylmethyl)-substituted arene but even higher amounts of the ring-opened terminal alkene and the aromatic homocoupling product. Hydrodefluorination, as developed by

Holland and co-workers as the main reaction,381 occurs only to a minor extent. In contrast to 1,2-bis(diphenylphosphino)benzene (DPPBz), bis(diphenylphosphino)ethane (DPPE) turned out to be less efficient as a ligand for iron-catalyzed cross coupling reactions. However, the group of Bedford could demonstrate that the isolated halo(dppe)2Fe(I) and (CH3CN)2(dppe)2Fe(II) complexes performed very well as catalysts for the coupling of benzyl and alkyl electrophiles with various organometallic reagents (Scheme 184).382 Thus, the electronic properties of the ligand may be less important for the reactivity of the corresponding iron complexes. The metalated aryl nucleophile can also be generated in situ. Thus, iron-catalyzed alkylations of heteroarenes and some electron-deficient arenes by primary and secondary alkyl bromides and iodides have been achieved by Daugulis and Tran (Scheme 185).383 The reaction proceeded under mild conditions in THF at room temperature using iron(III) chloride as catalyst, trans-N,N0 -dimethylcyclohexane-1,2-diamine as ligand, and TMPMgCl 3 LiCl (TMPMgCl = 2,2,6,6-tetramethylpiperidinylmagnesium chloride). The latter most likely functions as metalating agent as confirmed by comparative deuteration experiments. A pronounced regioselectivity was observed; in particular the directing effect of a 3-methoxy group has been exploited for pyridine derivatives to afford the 2-substituted products. 3211

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Chemical Reviews

REVIEW

Scheme 187

Figure 1. Iron(III)-containing imidazolium salts.

Scheme 186

Scheme 188

Six different iron(III)-containing imidazolium salts of the type shown in Figure 1 have been synthesized and studied for the model transformation: a reaction of cyclohexyl bromide and p-tolylmagnesium bromide.384 While all iron-containing imidazolium salts gave high to excellent conversions, it could be shown that the efficiency of the catalyst can be adjusted by modification of the substituents at the imidazole ring. Based on an extensive optimization work, Denmark and Cresswell established a cross coupling of secondary alkyl phenyl sulfones with aryl Grignard reagents using iron(III) acetylacetonate as catalyst. The reaction proceeds in the presence of 8 equiv of TMEDA in cyclopentyl methyl ether (CPME) at room temperature providing the coupling products mostly in good yields (Scheme 186, eq a).385 Moreover, unactivated alkyl aryl thioethers have also been applied as electrophiles for this transformation. The best results were obtained with a thioether bearing a 2-pyridyl directing group (eq b). Alkyl halides have been treated with arylmagnesium bromides using bis(phenol)-functionalized benzimidazolium-based iron(III) catalysts.386 For one of the catalysts, the recycling has been demonstrated. Only a moderate loss of activity was observed after nine runs. Even bulky aryl Grignard reagents were coupled with alkyl halides using a sterically less crowded 1,2-bis(diethylphosphino)ethane (DEPE) iron(II) complex as precatalyst (Scheme 187).387 The same applies to neopentylic electrophiles in reaction with mesitylmagnesium bromide. The authors showed that the procedure works for a variety of alkyl iodides. But also bromides and tosylates could be treated with ortho-substituted or ortho,orthodisubstituted aryl Grignard reagents affording the alkylated arenes in high yields. 2.4.1.4.2. Alkenyl Nucleophiles. The first iron-catalyzed cross coupling reactions of alkyl halides with alkenyl Grignard reagents have been published simultaneously by the groups of Cossy388 and Cahiez.389 The latter introduced a catalytic system consisting of Fe(acac)3, HMTA (hexamethylenetetramine), and TMEDA (N,N,N0 ,N0 -tetramethylethylenediamine) that they had reported

Scheme 189

earlier371 for the coupling of alkyl halides with aryl Grignard reagents (Scheme 188, cf. Scheme 178).389 In several examples it has been demonstrated that alkylated olefins are provided in high yields. An alkyl chloride gave only a poor result. The configuration of the olefin was retained with high selectivity in the course of this reaction. Cossy et al. independently reported the same transformation. They performed the reaction in the presence of catalytic amounts of FeCl3 and 1.9 equiv of TMEDA (Scheme 189).388 Several examples of alkyl bromides and iodides have been presented demonstrating the applicability of the method. Several functional groups such as acetals, ethers, esters, amides, and silyl ethers were tolerated. In some cases, elimination was found to be a side reaction. In agreement with Cahiez’s389 observation, an alkyl chloride did not give a satisfactory result. The utility of this method could be demonstrated by application to the synthesis of an E-trisubstituted double bond in the course of the synthesis of the spiroketal core of spirangien A.390 A formal total synthesis of this compound including the same iron-catalyzed cross coupling step has been reported by Rizzacasa et al.391 The group of Tomioka applied the FeCl3catalyzed coupling of 2-isopropenylmagnesium bromide with a brominated bicyclic carbamate to the formal synthesis of (+)allokainic acid.392 Recently, the formal total synthesis of the renin inhibitor aliskiren was reported using an FeCl3-catalyzed coupling of a primary alkyl bromide and vinylmagnesium bromide.393 Cossy and Reymond applied the iron-catalyzed cross coupling of C-bromo mannopyranosides with a vinyl Grignard reagent to 3212

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Chemical Reviews

REVIEW

Scheme 190

Scheme 192

Scheme 191

Scheme 193

their approach toward the synthesis of the C31C52 fragment of ent-amphidinol 3 (Scheme 190).394 Iron(III) chloride in combination with TMEDA was used as a catalytic system in THF at room temperature. Slow addition of the Grignard reagent and the ligand was essential to obtain the peracetylated transtetrahydropyran derivative in 72% yield and with full diastereoselectivity. In analogy to the coupling of alkyl halides with arylzinc reagents (cf. Scheme 173), Nakamura and co-workers could employ similar reaction conditions for the transformation of alkenylzinc nucleophiles (Scheme 191).395 The method works reliably with a number of substrates providing excellent yields. Moreover, the configuration of the initial alkenyl bromide was retained during formation of the organozinc reagent and the cross coupling reaction. Functional groups such as esters, nitriles, and carbamates were tolerated under these conditions. Alkyl bromides reacted chemoselectively in the presence of aryl bromides. Various nonactivated alkyl chlorides and bromides could be treated with alkenylboron reagents with complete stereoselectivity (Scheme 192).396 The two sterically congested (bisphosphine)iron complexes which had already been developed for the cross coupling of alkyl halides with arylboron reagents (cf. Scheme 175) proved to be effective catalysts for this reaction. The transformation proceeds in high yields and tolerates ester, carbamate, and nitrile groups. The trifluoromethylation of terminal olefins has been achieved using Togni’s reagent II and potassium vinyltrifluoroborates in the presence of catalytic amounts of iron(II) chloride (Scheme 193).397 Especially 2-arylvinyltrifluoroborates gave good to high yields. The E-configuration of these substrates was retained with high selectivity. (E)-2-Alkylvinyltrifluoroborates showed a slight drop in their stereointegrity during this transformation and afforded mixtures of E/Z-isomers with

Scheme 194

selectivities ranging from 67:33 to 83:17. The stereoconvergence of the reaction was shown by using a (Z)-2-arylvinyltrifluoroborate which provided exclusively the E-product. Thus, a mechanism involving a transmetalation/reductive elimination was ruled out in favor of a mechanism via radical or carbocationic intermediates. 2.4.1.4.3. Alkynyl Nucleophiles. A C(sp3)C(sp) bond forming Sonogashira-type coupling of alkyl halides with alkynyl Grignard reagents was reported by Nakamura and co-workers using their sterically highly congested (bisphosphine)iron(II) complex [FeCl2(3,5-tBu2-SciOPP)] (Scheme 194).398 Generally, this method provides the substituted alkynes in high yields. 3213

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Chemical Reviews Scheme 195

REVIEW

Scheme 197

Scheme 196

Scheme 198

Fine-tuning of the reaction conditions enabled the selective coupling of primary and secondary alkyl halides in the presence of alkenyl triflates which reverses the reactivity commonly observed. A radical mechanism involving an SET from the alkyl halide to the iron center was proposed. Hu and co-workers used simple iron(II) bromide as catalyst for the coupling of alkyl bromides and iodides with alkynyl Grignard reagents (Scheme 195).399 The reaction was conducted in THF/NMP at room temperature under ligand-free conditions. A large variety of cyclic and acyclic secondary alkyl halides but also some primary alkyl halides could be alkynylated by this method. 2.4.1.4.4. Alkyl Nucleophiles. Coupling of two sp3-carbon centers appears to be the most obvious reaction in organic chemistry. However, this transformation still remains a challenge, even though significant progress has been made using organometallic chemistry. The first useful iron-catalyzed C(sp3) C(sp3) Kumada-type cross coupling of primary and secondary alkyl halides and alkyl Grignard reagents has been reported by Chai and co-workers (Scheme 196).400 Iron(II) acetate in combination with Xantphos was used as catalyst. The yields were only moderate, but the feasibility of this highly important transformation could be pointed out. Alkylation of the polychlorinated solvents dichloromethane, chloroform, and carbon tetrachloride with Grignard reagents has been achieved using an iron(III) complex with a tridentate ligand.401 The aim of this work was the removal of toxic solvents. Thus, no effort was undertaken to increase the selectivity of this reaction which afforded complex mixtures of alkylated products. An iron-catalyzed C(sp3)C(sp3) SuzukiMiyaura coupling has been developed by the group of Nakamura (Scheme 197).402 Primary and secondary alkyl bromides were treated with tetraalkylborates using Fe(acac)3/Xantphos as the catalytic system. The latter were formed in situ by hydroboration of the corresponding olefins and subsequent activation with isopropylmagnesium bromide. This method provided long-chain alkanes in good yields. Cyanide, ester, and chloride groups were tolerated in this transformation. A radical clock experiment using (bromomethyl)cyclopropane as electrophile afforded the ring-opened terminal

Scheme 199

olefin revealing that radical species are involved in the mechanism of this process (cf. Scheme 219). Cardenas and co-workers were able to couple alkyl iodides with alkyl Grignard reagents in the presence of catalytic amounts of iron(II) acetate and an NHC ligand (Scheme 198).403 The resulting coupling products were obtained in good yields and the formation of elimination products was largely suppressed. Iron(I) species and radical intermediates have been proposed to be involved in the catalytic cycle. A protocol for the cross coupling of homobenzylic methyl ethers with alkyl Grignard reagents has been established by Shi and co-workers (Scheme 199).404 Iron(II) fluoride was introduced as catalyst and tricyclohexylphosphine as ligand. The reaction was performed in o-xylene at 120 °C and afforded the C(sp3)C(sp3) coupling products in good yields. Presumably, this transformation includes an initial elimination of methanol to give a styrene intermediate which further undergoes carbometalation by the Grignard species with subsequent protonation. The trapping with electrophiles different from H+ leads to functionalization at the benzylic position. 2.4.1.5. Benzyl, Allyl, and Propargyl Electrophiles. Early examples for the reaction of propargyl chlorides with Grignard reagents in the presence of iron(III) chloride providing allenes have been reported by the group of Pasto.405,406 It has been suggested that the mechanism involves a low-valent iron species that substitutes the halide in an SN20 fashion. Alternatively, the CX substrate can undergo an oxidative addition to the iron 3214

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Chemical Reviews complex followed by propargylallenyl rearrangement. Transmetalation and reductive elimination are the final steps for both alternative pathways leading to the substituted allenes. More than a decade later, the iron-catalyzed reaction of allylic phosphates with Grignard reagents has been reported by Yamamoto and co-workers.407,408 High preference for the SN2 product over the SN20 isomer was observed in most cases. At the same time, Fe(acac)3 was found to be the best catalyst for the coupling of allyl, propargyl, and allenyl halides with Grignard reagents to afford bicyclo[1.1.0]butanes in moderate yields.409 Excellent yields were obtained for the coupling of allyl, benzyl, and propargyl chlorides and bromides with phenylmagnesium bromide using the ferrate complex [Li(tmeda)]2[Fe(C2H4)4] as catalyst (cf. Scheme 168). Allene formation has been observed only to a minor extent when using propargylic electrophiles.288,356 Benzyl bromide could be coupled with p-tolyl and o-tolylmagnesium bromide in 50 and 68% yields, respectively, using Kozak’s Fe(III) aminebis(phenolate) halide complex (cf. Scheme 179).373 The Fe(acac)3-catalyzed desulfinylative coupling of phenylmethanesulfonyl chloride and (E)-2-phenylethenesulfonyl chloride with alkyl and aryl Grignard reagents affords the cross coupling products in good yields (cf. Scheme 181).303 In a follow-up paper the authors established a method for the desulfinylative allylation of Grignard reagents using Fe(acac)3 as precatalyst (Scheme 200).410 In general, high yields could be achieved. Employing but-2-ene-1sulfonyl and but-3-ene-2-sulfonyl chloride as electrophiles led to the same mixture of regioisomers (85:15 to 9:1) in favor of the linear product. This result is in agreement with a mechanism involving η3-allyliron complexes. A variety of benzyl halides has been subjected to a Suzuki Miyaura coupling with sodium or potassium tetraarylborates using a mixed ironzinc catalytic system (Scheme 201, eq a).342 Using the same iron(II) complex as catalyst, the coupling of benzyl halides and phosphates with aryl zinc reagents has been reported to give high yields for the examples investigated (Scheme 201, eq b).411 It is noteworthy that aryl halides are inert under these conditions. Thus, a procedure orthogonal to palladium-catalyzed coupling reactions has been developed. Recently, Bedford et al. demonstrated that isolated halo(dppe)2Fe(I) and (CH3CN)2(dppe)2Fe(II) complexes are efficient catalysts for the coupling of benzyl electrophiles with organozinc, organoboron, organoaluminum, and organoindium reagents (Scheme 184, eq b).382 Functional groups such as esters, nitriles, and aryl halides are tolerated. A practical procedure for the iron-catalyzed allylation of aryl bromides has been developed by the group of von Wangelin (Scheme 202).412 Most notably, the generation of the Grignard reagent and the coupling reaction was carried out in a one-pot procedure. Iron(III) acetylacetonate was employed as catalyst without any additive. Besides allyl acetate, several other allylic electrophiles (e.g., allyl chlorides, bromides, phosphates, thioethers, tosylates, and carbonates) have been found to be suitable substrates. For a broad spectrum of electron-rich and electrondeficient arenes, the allylarene coupling products have been obtained in good yields. Functional groups (e.g., methoxy, halide, pivalate, acetal, amine, and pyridine) were tolerated. A selectivity of at least 9:1 in favor of the α-product was observed using prenyl acetate as electrophile. However, only moderate yields were achieved for this transformation. The crotylation showed a lower selectivity but higher yields, whereas cinnamyl acetate provided

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Scheme 200

Scheme 201

Scheme 202

exclusively the linear (α) product in good yields. The method could be extended by an iron-catalyzed hydrogenation step directly after the cross coupling, thus providing alkylarenes in a one-pot procedure. Nakamura and co-workers studied the cross coupling of benzyl halides with aryl Grignard reagents depending on the electronic properties of the iron(II) precatalyst by introducing different bisphosphine ligands.413 It could be demonstrated that electronrich ligands lead to an efficient cross coupling reaction, whereas electron-deficient ligands led to reductive homocoupling of the benzyl halides. A variety of arylmagnesium bromides was treated with benzylic chlorides in the presence of a designed Fe(II) catalyst with two electron-rich bisphosphine ligands in its coordination sphere (Scheme 203). An iron(III) complex with a tridendate aminebis(phenolate) ligand was synthesized by Kozak et al. and used as catalyst for the cross coupling of benzyl bromides and chlorides with aryl Grignard reagents (Scheme 204).414 The yields varied from low to very high, and often homocoupling byproducts were obtained in significant amounts. Dibenzyl homocoupling products may be formed by a radical process. 3215

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Scheme 206

Scheme 207

Scheme 204

Scheme 208

Scheme 205

The coupling of 4-ethynyloxetan-2-ones with alkyl and aryl Grignard reagents resulted in ring opening to afford 4-alkynoic acids in high yields (Scheme 205).415 Iron(III) chloride hexahydrate proved to be an efficient catalyst for this transformation. Minor amounts of allenes were obtained as byproducts. The observed inversion of the configuration at the β-carbon was rationalized by an SN2-type process, in contrast to the oxidative addition/reductive elimination sequence of low-valent iron species which applies to many cross coupling reactions (cf. 2.4.1.6). The cross coupling of allyl ethers with ethylmagnesium chloride has been described by von Wangelin et al. (Scheme 206).416 Iron(II) chloride has been identified as a useful catalyst for this transformation which can be utilized as deallylation protocol. The liberated phenols or alcohols have often been obtained in high to excellent yields. Functional groups, such as halides, esters, amines, ethers, and olefins, are well-tolerated. An iron-catalyzed SuzukiMiyaura coupling of allylic and benzylic halides with arylboronic esters, which were activated by alkyllithium reagents, has been reported by Bedford et al. (cf. Scheme 176).369

2.4.1.6. Mechanistic Considerations 2.4.1.6.1. Mechanisms with Double Electron Transfer. Several proposals have been made for the mechanisms of ironcatalyzed CX/CMet cross coupling reactions. This section outlines the proposed mechanisms irrespective of the nature of electrophile and nucleophile. The first mechanistic proposal for the iron-catalyzed cross coupling of alkenyl halides with Grignard reagents was made by Kochi based on substantial experimental evidence (Scheme 207).241,417,418 In analogy to the palladiumand nickel-catalyzed cross coupling reactions,233 in particular the Kumada coupling,419 it includes an oxidative addition of the electrophile to the iron center, a transmetalation, and a reductive elimination as basic steps. Although the iron-catalyzed reaction was discovered one year before the palladium-catalyzed reaction, it has gained much less attention in the following years. Kochi proposed that iron(I) and iron(III) complexes are the on-cycle catalysts in this process. Iron(I) is initially formed by reduction of Fe(III) or Fe(II) precatalysts by the Grignard reagent. Oxidative addition of the vinyl bromide generates an alkenyliron(III) complex. The bromide ligand is exchanged against an alkyl ligand from the Grignard reagent (transmetalation step). Finally, a reductive elimination provides the coupling product and regenerates the Fe(I) species. F€urstner and co-workers favored a catalytic cycle based on the interconversion of Fe(II) and Fe(0) species for their coupling of aryl halides with Grignard reagents.288,291,292 Such a process would require an initial reduction of the iron(III) or iron(II) salt by the Grignard reagent to give a highly nucleophilic [FeII(MgX2)]n species which had been previously reported by Bogdanovic and coworkers (Scheme 208).420,421 3216

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Chemical Reviews According to this mechanism, Grignard reagents which are not prone to β-hydride elimination should not be able to generate the Fe(II) species required for the catalytic cycle. This is in agreement with the observation that methylmagnesium bromide did not react with methyl 4-chlorobenzoate in the presence of Fe(acac)3, FeCl3, or FeCl2 as precatalyst, whereas higher alkyl Grignard reagents gave a smooth reaction.291,292 The iron(II) “super-ate” complexes [(Me4Fe)(MeLi)][Li(OEt2)]2 and [Ph4Fe][Li(OEt2)][Li(1,4-dioxane)], which are formed by reaction of methyllithium and phenyllithium with FeCl3, may represent similar complexes formed from the corresponding Grignard reagents. It has been shown that these isolated iron(II) “super-ate” complexes transfer the organic group only to reactive acyl chlorides and alkenyl triflates but inefficiently to aryl chlorides.288,422 This result is in contrast to earlier claims that in situ formed “ate” complexes of the putative composition “Me4FeLi2” can methylate vinyl bromides.423 Ironmagnesium clusters of the composition [Fe(MgX2)]n with iron in the formal oxidation state II can effect a catalytic cycle similar to those established for most of the palladium- and nickel-catalyzed cross coupling reactions (Scheme 209).291,292 The aryl halide does not undergo an oxidative addition but a formal σ-bond metathesis with the Fe(II)MgX cluster with concomitant oxidation of the iron center. The magnesium halide is released in this process. The resulting Fe(0) complex can be alkylated by the Grignard reagent. Subsequent reductive elimination affords the cross coupling product and regenerates the Fe(II) species. Ironmagnesium clusters cannot be obtained in a welldefined and reproducible form. F€urstner and co-workers synthesized several lithium ferrate complexes with iron in the formal oxidation states 0 and II as model compounds.288 These compounds efficiently catalyzed the cross coupling of aryl chlorides with aromatic Grignard reagents in addition to the coupling of alkyl (cf. Scheme 168), benzyl, allyl, and propargyl

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halides (section 2.4.1.5). Stoichiometric oxidative addition of allyl and phenyl halides to these complexes afforded η3-allyl- and phenyliron complexes, respectively. As low-valent iron complexes in the oxidation states II and 0 were the most effective catalysts for the cross coupling of aryl chlorides with Grignard reagents, it was assumed that a catalytic cycle involving the interconversion of Fe(II) to Fe(0) species is dominant in this system (Scheme 209). This interconversion of Fe(II) to Fe(0) species was attributed to a very fast oxidative σ-bond metathesis of the organohalide and the highly nucleophilic Fe(II) species. However, F€urstner also emphasized that other catalytic pathways may be effective as well or may be interconnected with the one mentioned above (Scheme 210). This includes the redox couples Fe(0)/Fe(II) (Scheme 210, center cycle) and Fe(I)/Fe(III) (Scheme 210, left cycle). Fe(I) complexes might be formed by homolysis of Fealkyl bonds in Fe(II) compounds.288 For the iron-catalyzed enyne cross coupling reaction (cf. Scheme 123) Nakamura et al. proposed a mechanism involving Fe(0) and Fe(II) alkynyl complexes (Scheme 211).266 The alkynyl Grignard reagent reduces the Fe(III) precatalyst to form Fe(0) alkynyl complexes. In the absence of lithium salts, this reduction may be slow due to the high stability of Fe(II) alkynyl “ate” complexes. Oxidative addition of the alkenyl halide affords an alkenylalkynyl Fe(II) complex which on reductive elimination forms the enyne and an alkynyl Fe(0) species. The latter adds one more alkynyl ligand in a transmetalation process from the alkynyl Grignard reagent to regenerate the initial alkynyl Fe(0) complex. The group of Nakamura also investigated the reaction of aryl chlorides with aryl Grignard reagents in the presence of FeF3 or FeF2 and NHC ligands (cf. Scheme 160). They have done a Scheme 211

Scheme 209

Scheme 210

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Scheme 212

variety of control experiments and computational calculations to evaluate the mechanism of this transformation.335 The catalytic cycle involving the iron oxidation states 0 and +II (Scheme 212, right), as it is established for many palladium- and nickelcatalyzed cross coupling reactions, was compared with a ferrate mechanism involving the formal oxidation states Fe(II)/Fe(IV) (Scheme 212, left). Density functional theory (DFT) calculations revealed that in the presence of fluoride ligands the initial reduction of the precatalyst by the Grignard reagent following a transmetalation/reductive elimination sequence with formation of a homocoupling product is suppressed due to the strong coordination of the fluoride ligand to the metal center (Scheme 212, left top). Moreover, the reductive elimination step in the catalytic cycle is presumed to be disfavored in the case of the Fe(0)/Fe(II) system. A reaction pathway with reasonable activation barriers could be determined by calculations to involve an oxidative addition of the aryl halide to the highvalent heteroleptic ferrate complex A affording the Fe(IV) species B (Scheme 212, left). Subsequent fast reductive elimination provides the asymmetric biaryl compound and an Fe(II) complex C. Transmetalation of the Grignard reagent regenerates the ferrate complex A. The iron-catalyzed cross coupling of aryl electrophiles with alkyl Grignard reagents has been investigated by Norrby et al. using Hammett and computational studies.424,425 They concluded that the redox couple Fe(I)/Fe(III) is responsible for the catalysis. Lower valent iron species may be active precatalysts and are most active for oxidative addition, but they cannot be regenerated under the reaction conditions. Thermodynamics disfavors the reductive elimination to form Fe(II) or Fe(I) species with ΔG values of 195 and 94 kJ mol1, respectively. Formation of Fe(0) appears to be possible by thermodynamics, but the activation barrier of ΔG# = 191 kJ mol1 is rather high. Only the reductive process leading to Fe(I) gives reasonable thermodynamic (ΔG = 181 kJ mol1) and kinetic (ΔG# = 10 kJ mol1) values and thus is favored by the authors. For the preferred Fe(I)/Fe(III) system oxidative addition is the ratelimiting step. Transmetalation from the Grignard reagent is fast and may proceed before or after the oxidative addition with little difference in energy. The same conclusion was drawn from low temperature studies of the reaction of aryl electrophiles with alkyl Grignard reagents.426 Kinetic investigations were in agreement with an Fe(I)/Fe(III) catalytic cycle. Moreover, it could be

shown that an excess of Grignard reagent leads to reduced iron complexes which are less active in the cross coupling reaction. Computational studies revealed that in a hypothetical Fe(0)/ Fe(II) catalytic cycle all reaction steps are less efficient than in the Fe(I)/Fe(III) counterpart. This result was confirmed by a more recent study including the quantification of product formation. It has been disclosed that the Grignard reagent can reduce the iron precatalyst only to the Fe(I) state.427 Calculations revealed that the spin state of the active catalysts was S = 3/2, even though iron(III) precatalysts with spin states S = 5/2 were applied. The change of the spin state occurs after the first transmetalation. In contrast, Ren and co-workers calculated a reaction pathway for the cross coupling of an aryl chloride with hexylmagnesium bromide by the inorganic Grignard reagent [Fe(MgX)2] according to the mechanism proposed by F€urstner (cf. Scheme 209).428 They found that oxidative addition of the aryl halide to the FeMg cluster may proceed without concomitant elimination of MgX2. Moreover, the addition of the Grignard reagent (transmetalation) is facilitated when MgX2 is not removed from the Fe(0) intermediate. In agreement with Norrby and co-workers,424 the reductive elimination was identified as the rate-determining step in the Fe(II)/Fe(0) catalytic cycle. Even though the energies of the complexes before and after reductive elimination are almost equal in solution, the calculated activation energies of 32.40 kcal mol1 in the gas phase and 28.13 kcal mol1 in THF would not allow such reaction to proceed at room temperature. The importance of an Fe(I) complex could also be demonstrated by the group of Bedford.429 From the amount of diaryl homocoupling product which is formed during the reduction of the precatalyst, they deduced that Fe(I) is the lowest kinetically relevant oxidation state. Three (dppbz)2Fe(I) complexes were synthesized and employed for the cross coupling of benzyl, pyridyl, and cycloheptenyl electrophiles with arylzinc reagents (Scheme 213). The cross coupling products were obtained in excellent yields, which were comparable to those reported in the original literature employing the Fe(II) precatalysts.411 The halo(dppbz)2Fe(I) complexes (e.g., 2) are tentatively oncycle catalysts, whereas the corresponding aryl(dppbz)2Fe(I) complex (e.g., 1) may be an off-cycle catalyst. Bauer and Werner also confirmed the Fe(I)/Fe(III) catalytic cycle of iron-catalyzed cross coupling reactions based on X-ray absorption experiments.430 According to their investigations the 3218

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Chemical Reviews Scheme 213

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Scheme 214

Scheme 215 catalytically active species are iron clusters with three to four Fe(I) centers. The group of Bedford investigated the iron-catalyzed Kumada coupling of alkyl halides with bulky aryl Grignard reagents in the presence of TMEDA as ligand.431 They suggested that homoleptic “ate” complexes of the type [Fe(aryl)3] are the catalytically active species rather than neutral complexes bearing TMEDA ligands. In systems with bulky aryl Grignard reagents, an oxidation state of +II appears to be the lowest possible for iron due to resistance to reductive elimination. For the nonhindered tolyl complex, however, the spin state S = 1/2 has been observed by EPR measurements which is consistent with an Fe(I) complex. Based on cyclic voltammetry, but also on 1H NMR and EPR measurements, Lefevre and Jutand identified an iron(II) complex and an iron(I) complex as products of the reduction of Fe(acac)3 with excess phenylmagnesium bromide.432 While [PhFe(acac)(thf)n] proved to be inactive toward aryl and heteroaryl halides, the corresponding iron(I) complex [PhFe(acac)(thf)] reacted with the aryl and heteroaryl electrophiles according to two different mechanisms. Reaction with aryl halides followed an innersphere monoelectronic reduction pathway leading to the dehalogenated arenes via intermediate aryl radicals and Grignard homocoupling products. In the case of electron-deficient heteroaryl chlorides, the reaction follows the oxidative addition/reductive elimination pathway leading to the cross coupling products. In conclusion, several reasonable pathways have been postulated for iron-catalyzed cross coupling reactions. However, strong evidence for the Fe(I)/Fe(III) catalytic cycle has been accumulated in recent years by computational and spectroscopic methods as well as experimental observations. 2.4.1.6.2. Mechanisms with Single-Electron Transfer. Oxidative addition of alkyl halides to metal complexes, as outlined above, should proceed with retention of configuration. Thus, the stereoconvergence observed for the arylation of trans- and cis-1-bromo-4-tert-butylcyclohexane, with a very high selectivity in favor of the trans-product (96:4), implied the intermediacy of radical species.355 F€urstner et al. observed racemization during the reaction of (R)-2-bromooctane with phenylmagnesium bromide. In addition, the fact that a cyclic alkyl iodide bearing a terminal double bond undergoes ring closure prior to cross coupling can be attributed to the action of radical intermediates.356 A simplified mechanism involving radical intermediates

was presented by Bedford (Scheme 214).433 It starts with the reduction of the Fe(III) precatalyst by the Grignard reagent, for example via β-hydride elimination. Grignard reagents without a β-hydrogen atom, such as arylmagnesium compounds, can similarly reduce the iron center by reductive elimination under formation of biaryls. The reduced iron complex can undergo a oneelectron oxidation forming an alkyl radical. This radical may stay in the solvent cage during the following transmetalation step. Depending on the stability of the radical, it can undergo side reactions. Transmetalation of the aryl ligand from the Grignard compound to the iron complex is followed by elimination of the cross coupling product with concomitant reduction of the iron center. Cahiez and co-workers presented a mechanistic proposal for their alkylX/arylMgX cross coupling (cf. Scheme 178) that partly combines F€urstner’s FeMg clusters with radical intermediates (Scheme 215).371 According to their suggestion, transmetalation occurs prior to electron transfer. In fact, oxidative addition of the alkyl halide to the iron complex is split into two processes. The first is a one-electron transfer generating a radical anion which subsequently adds the alkyl moiety to the iron center with concomitant release of MgX2. Finally, reductive elimination affords the cross coupling product and regenerates the Fe(0) catalyst. In the course of the catalytic cycle, the iron center interchanges between oxidation states 0, +I, and +II. 3219

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Chemical Reviews Scheme 216

Nagashima and co-workers proposed a mechanism for the cross coupling of alkyl halides with aryl Grignard reagents in the presence of TMEDA.434 It involves the formation of a diaryliron(II) complex (1) with a chelating TMEDA ligand from iron(III) chloride and the Grignard reagent (Scheme 216). Such a complex could be isolated and the structure was confirmed by X-ray crystallography. The isolated diaryl(tmeda)iron(II) complex was treated with an alkyl halide to give the alkylated arene and an aryl(bromo)(tmeda)iron(II) complex of type 3. Such a formal σ-bond metathesis may proceed via a one-electron oxidation of the Fe(II) complex to give the corresponding diaryl(halogeno)iron(III) complex and an alkyl radical in the proximity of the iron complex. The catalytic cycle proceeds from formal abstraction of an aryl radical from the diaryl(halogeno)iron(III) complex 2 by the neighboring alkyl radical to give the alkylated arene and a reduced aryl(halogeno)iron(II) complex 3 that undergoes transmetalation to release the initial diaryliron(II) species. It is noteworthy that this mechanism involves only the common oxidation states +2 and +3 and no low-valent iron species. Radical clock experiments have been carried out to estimate the lifetime of the radical (Scheme 217).434 It turned out to be too short to allow radical addition to terminal olefins as a side reaction (eq a). No cyclopentyl product was observed when 1-bromo-5-hexene was used as electrophile. However, a faster opening of the cyclopropylmethyl radical was observed using cyclopropylmethyl bromide as electrophile leading to a butenylsubstituted arene (eq b). A similar mechanism has been proposed for the Suzuki Miyaura coupling of alkyl halides with aryl borates in the presence of a (diphosphine)iron(II) complex (cf. Scheme 175).360 Neidig and co-workers investigated this transformation and the related Kumada-type coupling with the same SciOPP ligand by means of M€ossbauer spectroscopy, magnetic circular dichroism (MCD), and DFT methods and identified an FeAr2(SciOPP) complex as a catalytically active species.435 In accordance with the iron(II)/iron(III) catalytic mechanism, this species was shown to react with primary alkyl halides forming the corresponding FeXAr(SciOPP) complex along with the cross-coupled alkylarene. The missing iron(III) intermediate with a proximal alkyl radical is probably a very short-living species and thus not detectable by M€ossbauer spectroscopy. The authors excluded an alternative iron(IV) intermediate due to the radical clock experiments and the unfavorable oxidation of bisphosphine-ligated iron(III) complexes. More recently, Neidig et al. described the isolation and characterization of a tetramethyliron(III) ferrate complex obtained by reduction of iron(III) chloride with methylmagnesium bromide.436 Upon warming, this square-planar S = 3/2 species

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Scheme 217

Scheme 218

was transformed into the S = 1/2 species originally proposed by Kochi241,417 as active intermediate in the cross-coupling. A slightly modified mechanistic variant was presented for the cross coupling of nonactivated alkyl chlorides with aryl Grignard reagents in the presence of an iron catalyst with NHC ligands (Scheme 218, cf. Scheme 182).379 It was assumed that an Fe(II)ate complex is the catalytically active species due to the increased reducing potential as compared to the neutral compound. A similar mechanism was proposed for the iron-catalyzed cross coupling of for example trans-4-chlorocyclohexanol with aryl aluminum reagents (cf. Scheme 180, eq b).376 Most of the radical mechanisms presented above are characterized by an interconversion of iron complexes which differ only in one oxidation state (cf. Schemes 214, 216, and 218), which means that no full oxidative addition of the electrophile to a low-valent iron complex occurs. In contrast, the mechanism presented by Cahiez (Scheme 215) includes a two-step oxidation of the iron complex by stepwise addition of the alkyl halide.371 In agreement with this proposal Nakamura and co-workers proposed a catalytic cycle for their C(sp3)C(sp3) Suzuki Miyaura coupling (Scheme 219, cf. Scheme 197).402 A low-valent 3220

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Chemical Reviews Scheme 219

iron complex transfers an electron to the alkyl halide. The halide thus formed will be ligated to the complex. Subsequently, another oxidation transforms the alkyl radical to a carbanion which also coordinates to the iron center. In total, an oxidative addition of the alkyl halide to the iron complex is achieved via intermediate radical species which may be prone to side reactions. Reductive elimination of the product regenerates the lowvalent iron complex. A radical intermediate has also been proposed as one possibility to describe the mechanism of the iron-catalyzed Negishi coupling of benzyl halides and phosphates with aryl zinc reagents (cf. Scheme 201, eq b, and Scheme 220).411 2.4.1.6.3. Mechanisms without Electron Transfer. Another possible way to couple alkyl halides with aryl metal reagents may involve a σ-bond metathesis of aryliron complexes with the organohalide. Such a mechanism has been presented as one option by Bedford and co-workers for their Negishi coupling of benzyl halides with aryl zinc reagents (cf. Scheme 201, eq b, and Scheme 220).411 Transmetalation of an aryl ligand from the arylzinc reagent to the dihaloiron(II) complex forms an aryliron(II) complex which subsequently loses an halide ligand. The vacant coordination site is occupied by σ-bonding of the benzyl halide. At this point two alternative pathways appear to be possible. A single-electron transfer (SET) from the iron to the benzyl halide would form an iron(III) complex with a benzyl radical in the solvent cage. Abstraction of an aryl radical leads to the cross coupling product with concomitant reduction of the iron. Alternatively, a concerted σ-bond metathesis would lead directly to the same products. The latter pathway proceeds without a change of the oxidation state at the iron center. 2.4.1.7. Recent Applications to the Synthesis of Biologically Active Compounds. The Fe(acac)3-catalyzed coupling of aryl and alkenyl electrophiles with alkyl Grignard reagents has been exploited for the synthesis of a number of biologically active molecules and natural products, such as pyridazine-containing C-aryl-glucoside SGLT2 inhibitors,437 cylindrocyclophanes A and F,438 (+)-asteriscanolide,439 and various HCV (hepatitis C virus) inhibitors derived from noricumazole A.440 Recently, the alkylation of a 2-chloropyridine has been applied to the construction of a new heterocyclic dual NK1/serotonin receptor antagonist (Scheme 221).441 The Fe(acac)3-catalyzed cyclopropylation step using F€urstner’s conditions proceeded in excellent yield. 2.4.2. CMetal/CMetal Homocoupling and Cross Coupling Reactions. Homocoupling of phenylmagnesium

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Scheme 220

Scheme 221

bromide in the presence of bromobenzene and iron(III) chloride affording biphenyl in 47% yield has already been observed in 1941 by Kharasch and Fields.237 This reaction was further developed by Cahiez et al.,442 who described the coupling of aryl Grignard reagents to symmetrical biaryls in the presence of iron(III) chloride as catalyst and 1,2-dihaloethanes as oxidants (Scheme 222, eq a). The arylmagnesium halides were prepared in a preliminary step by reaction of aryl iodides and isopropylmagnesium bromide. Methoxy, dimethylamino, ester, nitrile, and nitro groups were tolerated under these conditions. 3-Pyridylmagnesium bromide could be employed analogously giving 3,30 -bipyridyl in 82% yield. An intramolecular version of this reaction was described as well and applied to the construction of the natural product N-methylcrinasiadine (eq b). This transformation formally represents a cross coupling leading to an unsymmetrical biaryl derivative. The mechanism proposed for this transformation involves the reduction of Fe(III) by the Grignard reagent to a low-valent iron species (Scheme 223). Oxidation states of +1, 0, or 2 may be involved. Subsequent reaction with the dihaloethane leads to an oxidation of the iron in two steps. Transmetalation transfers two aryl ligands from the Grignard reagent to the iron and is followed by reductive elimination releasing the biaryl and a reduced iron complex. 3221

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Chemical Reviews Scheme 222

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Scheme 224

Scheme 225

Scheme 226

Scheme 223

Independently from the work of Cahiez on the iron-catalyzed homocoupling of Grignard reagents,442 Hayashi et al. reported almost the same conditions for this transformation.443 1,2-Dichloroethane was used as oxidant and FeCl3 as catalyst. The reaction was carried out in diethyl ether and led to the biaryl products in good to excellent yields. In continuation of their work, Cahiez et al. were able to show that dry air can be used as oxidant for this transformation.444 These conditions also allowed the stereoselective homocoupling of alkenyl Grignard reagents in good yields. Almost at the same time, Lei and Liu reported the iron-catalyzed homocoupling of Grignard reagents using oxygen as oxidant and in the presence of an FeCl3/bipyridine catalyst system.445 Phosphonium-based ionic liquids have been used as reaction media for the homocoupling of aryl and heteroaryl Grignard reagents.446 This transformation in the presence of catalytic amounts of iron(III) chloride and 1,2-dichloroethane as oxidant provided biaryls in high to excellent yields. The cross coupling of arylzinc and alkylzinc reagents could be successfully performed using Fe(acac)3 as catalyst and 1,2-dibromoethane as oxidant (Scheme 224).447 Primary and secondary dialkylzinc reagents have been applied to afford the corresponding alkylarene coupling products in moderate to high

yields. Ester and secondary amide functions were proven to be inert under these conditions. Ren et al. applied the Fe(acac)3-catalyzed homocoupling of different triazenyl-substituted arylmagnesium chlorides to the synthesis of precursors for 1,8-diarylcarbazoles in moderate yields.448 The homocoupling of p-tert-butylphenylboronic acid in the presence of a nanocrystalline nickel ferrite catalyst was recently described by Das and co-workers.343 2.4.3. CX/CX Homocoupling and Cross Coupling Reactions. The reductive homocoupling of benzyl, allyl, phenacyl, and phenyl bromides has been reported by Sen et al. using stoichiometric amounts of the iron(0) complex [CpFe(cod)][Li(tmeda)].449 An intramolecular Ullmann-type biaryl coupling could be achieved using an electron-transfer system consisting of catalytic amounts of Me3FeLi and magnesium as stoichiometric reducing agent.450 Thus, bis(2-bromophenyl)methylamine could be cyclized to afford the corresponding carbazole in 88% yield (Scheme 225). Carbazole alkaloids represent an important class of biologically active compounds which have been isolated from diverse natural sources.451453 The same transformation was reported by F€urstner et al. employing stoichiometric amounts of [(Me4Fe)(MeLi)][Li(OEt)2] in THF at 40 °C.422 It was suggested that complexes of this type may represent the catalytically active species for the iron-catalyzed coupling shown in Scheme 225. Instead of ironate complexes, Fe(acac)3 or Fe(dbm)3 can be employed as precatalysts for this type of reaction. Several aryl bromides, benzyl bromide, and two representatives of alkyl bromides have been applied successfully for a homocoupling using these Fe(III) species in the presence of magnesium turnings (Scheme 226).454 In contrast to the example above (cf. Scheme 225),450 initial formation of a Grignard reagent was postulated in the present case. The resulting arylmagnesium compound would then react with an aryl halide in terms of a CX/CMet coupling (cf. section 2.4.1). This hypothesis was 3222

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Scheme 227

Scheme 229

Scheme 228

Scheme 230

supported by coupling of phenylmagnesium bromide and bromobenzene under the standard conditions to give biphenyl in 73% yield. The separate preparation of Grignard reagents is not required, nor is the addition of oxidizing agents such as 1,2-dihaloalkanes. The latter would be essential if the reaction proceeded via a homocoupling of Grignard reagents according to section 2.4.2. Due to the ease of handling and the low costs of the required materials, this reaction has been elaborated even for undergraduate courses using Fe(acac)3 as catalyst.455 An impressive iron-catalyzed one-pot direct cross coupling of aryl halides with alkyl halides has been described by the group of von Wangelin using FeCl3 as precatalyst in the presence of magnesium metal and TMEDA as ligand (Scheme 227).456 This transformation affords the cross coupling products in good yields along with only minor amounts of biaryls (99.99%) iron(III) chloride was employed.586 However, addition of trace amounts of copper(I) oxide (5 ppm Cu2O) to the ultrapure iron catalyst restored the yields initially obtained by using >98% pure iron(III) chloride.586 This observation and a subsequent study indicated that in fact copper plays the crucial role in this catalysis.587 The coupling of carboxamides with aryl iodides was achieved by heating the components in toluene solution at 135 °C in a sealed tube in the presence of catalytic amounts of iron(III) chloride (>98%), DMEDA as ligand, and potassium tert-butoxide as base (Scheme 315, eq a).588 High yields have been reported for this BuchwaldHartwig analogous reaction. As described for the reaction above, the use of ultrapure iron(III) chloride (>99.99%) gave only traces of the coupling products. Addition of trace amounts of copper(I) oxide (5 ppm Cu2O) restored the yields obtained with the original iron catalyst, suggesting that the copper impurities are important for the catalytic activity.

In a subsequent publication, the authors applied this method to the synthesis of diarylamines (Scheme 315, eq b).589 They treated acetanilides with iodoarenes in the presence of catalytic amounts of iron(III) chloride, DMEDA, and 2 equiv of cesium carbonate in toluene at 135 °C in a sealed tube. The initially formed N,N-diarylacetanilides were treated with sodium methoxide in a one-pot procedure to give the diarylamines. It could be demonstrated that coupling of sulfoximines with aryl iodides can be achieved under similar conditions (Scheme 316).590 A number of N-arylsulfoximines was synthesized in high yields. Aryl bromides led only to low yields in this transformation. A recyclable heterogeneous graphite-supported iron catalyst has been introduced by Rao et al. for the arylation of aromatic and aliphatic amines, benzamide, thiobenzamide, pyrazole, imidazole, benzimidazole, and indole (Scheme 317).591 Aryl iodides and, with slightly lower efficiency, also aryl bromides have been applied as arylating agents. The catalyst could be reused at least five times without losing much of its efficiency. Teo and the group of Kwong presented methods for the N-arylation of pyrazole in aqueous medium (Scheme 318).592,593 Teo treated pyrazole, 3-methylpyrazole, indole, 7-azaindole, and benzamide with aryl iodides in aqueous solution at 125 °C using 3241

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Scheme 322

Scheme 320

Scheme 323

Scheme 321

iron(III) chloride as catalyst in the presence of DMEDA and potassium phosphate (eq a).592 The arylated N-nucleophiles were obtained in moderate to high yields. Kwong et al. treated the same substrates with aryl iodides in the presence of iron(III) chloride as catalyst in combination with N,N0 -dimethylcyclohexane1,2-diamine as ligand and potassium phosphate as base in water in a sealed tube at 135 °C.593 Compared to the former method, higher yields of N-arylated N-heteroarenes were obtained under these conditions (eq b). Aryl- and vinyl(trimethoxy)silanes have been treated with imidazoles and triazoles using catalytic amounts of copper and iron(III) chloride in the presence of TBAF under air to afford the arylated or vinylated N-heterocycles in moderate to high yields (Scheme 319).594 Using either only Cu as catalyst or only FeCl3, the yields were lower. A direct iron-catalyzed coupling of aryl- and alkylamines and N-heterocycles with aryl iodides has been developed by the group of Liu (Scheme 320).595 The catalytic system consisted of Fe2O3 as active species and L-proline as ligand. Under these conditions a variety of amines could be arylated. Aryl bromides were also reactive as demonstrated for three examples. Chlorobenzene gave only a moderate conversion in the reaction with morpholine. 1-Bromo-4-chlorobenzene reacted selectively at the bromine position. An alternative procedure using Cu(acac)2 as cocatalyst under microwave irradiation was subsequently reported by the same authors.596 As typical for microwavepromoted reactions, the reaction times were very short. Darcel et al. used a combined ironcopper catalysis in a solution of aqueous ammonia in ethanol in the presence of sodium hydroxide for the coupling of aryl iodides with anilines (Scheme 321).597 The catalyst system consisted of iron(III) oxide and copper(I) iodide.

It has been demonstrated that imidazoles can be coupled with (E)-vinyl bromides using catalytic amounts of iron(III) chloride and potassium phosphate as base (Scheme 322).598 Z-Olefins were predominantly formed under these conditions. In contrast, (E)-vinyl chlorides preferentially led to the E-products. The iron-catalyzed intramolecular arylation of amidines has been exploited for the synthesis of 1,2,4-benzothiadiazine 1,1dioxide and quinazolinone derivatives (Scheme 323).599 In the first example 2-bromobenzenesulfonamides were treated with amidines in the presence of catalytic amounts of iron(III) chloride and cesium carbonate as base (eq a). This domino process involves transamidation of the sulfonamide and iron-catalyzed arylX/ NH cross coupling. A variety of 1,2,4-benzothiadiazine-1,1dioxides has been obtained under these conditions in high yield. Analogously, 2-bromobenzenecarboxylic acids undergo an amidation/cross coupling cascade when treated with amidines in the presence of the iron catalyst and base (eq b). The corresponding quinazolin-4-ones were formed in high yields. Parallel to this work, the group of Liu published a very similar transformation.600 They treated 2-iodobenzenecarboxylic acid with amidines using four different additive combinations under microwave heating (Scheme 324). Two of the methods included water as solvent. Slightly better results were obtained in DMF solution. In general, high yields of the quinazolin-4-ones were obtained. Ynamides have been obtained by an iron-catalyzed cross coupling of alkynyl bromides with carboxyl or sulfonylamides (Scheme 325).601 3242

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Scheme 327

Scheme 325

Scheme 326

Iron(III) chloride hexahydrate in combination with DMEDA was used as catalytic system. The reaction proceeded in toluene in the presence of potassium carbonate as base and provided the ynamides in excellent yields. In addition to the amides, indoles could be N-alkynylated under the same conditions. In a subsequent work, the authors described the amination of aryl and heteroaryl bromides with magnesium amides using iron(II) chloride as catalyst (Scheme 326).602 The reaction proceeded best in the presence of lithium bromide in xylene at 140 °C. The amide component was usually a diarylamide leading to unsymmetrical triarylamines. Diarylamines were obtained from N-aryl-Nsilyl magnesium amides with concomitant loss of the silyl group. Investigations with stoichiometric amounts of a newly synthesized iron(II) diamide complex and DFT calculations revealed that an Fe(II)Fe(IV) catalysis may be operating in this system. In a mechanistic work, Jutand et al. could provide evidence that an [Fe(0)(phen)2] complex formed by cyclic voltammetry

undergoes oxidative addition to aryl halides generating [ArFe(II)X(phen)2].603 The same applies to an [Fe(0)(dmeda)2] complex. However, these complexes rather behave as nucleophiles (for example with H+ and CO2) than react with N-nucleophiles, such as imidazoles and pyrazoles. In addition, a reduction of {[Fe(acac)3] + 2phen} and {FeCl3 + 2dmeda} to an Fe(0) species by the N-nucleophiles alone is not possible, even in the presence of base. These obervations explain why the systems {[Fe(acac)3] + 2phen} and {FeCl3 + 2dmeda} are inefficient precatalysts for CN cross coupling reactions. The amination of iodoarenes with ammonia has been achieved using copper ferrite nanoparticles as catalyst.604 The transformation was carried out in poly(ethylene glycol) PEG-400 in the presence of potassium phosphate as base at 100 °C to afford a series of arylamines in good to high yields. The reaction of aryl and heteroaryl halides with aminocoumarins or vice versa of halocoumarins with arylamines has been achieved in the presence of nanocrystalline nickel ferrite as catalyst.343 In addition, the reaction of iodobenzene with o-aminophenol led to the resulting arylamines in very high yields. In 2009, Buchwald and Bolm demonstrated that many of the “iron-catalyzed” CX/NH coupling reactions are in fact triggered by copper impurities.586,605 Thus, they could show that the arylation of pyrazole, benzamide, phenol, and thiophenol proceeds only with drastically decreased efficiency when ultrapure FeCl3 (>99.99%) was employed. Addition of trace amounts of copper(I) oxide (510 ppm Cu2O) restored the yields originally achieved. One should keep this observation in mind for many of the reactions, discussed above or in other sections of this review, which have not been tested using copper-free iron catalysts. 2.5.1.2. CX/O, S, Se, Te Cross Coupling Reactions. Bolm et al. varied the conditions of the arylation of N-nucleophiles (cf. Scheme 314) to achieve an arylation of O-nucleophiles, and thus developed an efficient protocol for the coupling of phenols with aryl iodides to give diaryl ethers in high yields (Scheme 327, eq a).606 The reaction was carried out in DMF at 135 °C in the presence of catalytic amounts of iron(III) chloride and cesium carbonate as base. The best results have been obtained by addition of the chelating ligand 2,2,6,6-tetramethylheptane3,5-dione (TMHD). Replacing the iron catalyst of >98% purity by an ultrapure sample of FeCl3 (>99.99%) resulted in a pronounced decrease of the yield from 85 to 32%.586 Addition of trace amounts of copper(I) oxide (10 ppm Cu2O) led to 3243

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Scheme 329

Scheme 330

almost quantitative yields, suggesting that copper catalysis is important in this case. Taillefer and Xia reported almost identical conditions and results for this reaction in an independent work.607 Aromatic and heteroaromatic thiols gave di(hetero)aryl thioethers by reaction with aryl iodides in the presence of catalytic amounts of iron(III) chloride (Scheme 327, eq b).608 Instead of TMHD, cesium carbonate, and DMF, DMEDA was used as ligand, sodium tert-butoxide as base, and toluene as solvent. Unlike the conversion of phenols, thiophenols did not react with bromobenzene under these conditions. Chlorides were unreactive in both procedures. Ultrapure iron(III) chloride (>99.99%) was almost inactive as catalyst (2% yield).586,605 However, a very efficient catalysis could be achieved by addition of trace amounts of copper(I) oxide (100 ppm Cu2O) to the ultrapure iron(III) chloride. An intramolecular version of this method led to the synthesis of 2-arylbenzoxazoles (Scheme 328, eq a).609 For this purpose N-(2-bromophenyl)- or N-(2-iodophenyl)benzamides were subjected to the same reaction conditions but slightly lower temperature (120 °C) providing the cyclized products in high yields. In a subsequent work, the substrate scope was extended to aryl 2-bromobenzyl ketones affording 2-arylbenzo[b]furans (Scheme 328, eq b).610 With a slightly lower efficiency, this transformation could also be performed in the presence of very low amounts of copper(II) chloride, indicating the influence of copper impurities in the iron catalyst of 98% purity. Recently, this type of transformation has been reported by Wang and co-workers using recyclable copper ferrite nanoparticles as catalyst.611 In most cases, excellent yields could be achieved with this system. Tsai and co-workers also described a protocol for the ironcatalyzed coupling of aryl iodides with thiophenols providing diaryl thioethers in high yields (Scheme 329).612 The reaction was performed in water at reflux using a combination of iron(III) chloride hexahydrate and a cationic 2,20 -bipyridyl ligand as catalytic system. Coupling with benzyl mercaptan was considerably less efficient. An alternative procedure developed by Lee and co-workers is also useful for the conversion of alkylthiols (Scheme 330).613 They employed iron(III) chloride as catalyst but replaced the DMEDA ligand by Xantphos. Thus, a number of alkyl aryl thioethers could be obtained in high yields. Aryl bromides and iodides could be hydrolyzed to phenols using iron(III) chloride as catalyst and DMEDA as ligand at

Scheme 331

Scheme 332

180 °C in an autoclave (Scheme 331).614 This reaction in aqueous solution provided phenols in good to high yields. A one-pot benzylation/cyclization of phenols with 2-bromobenzyl acetates, bromides, or carbonates has been described to give xanthenes in high yields under microwave irradiation (Scheme 332).161 The process is catalyzed by iron(III) chloride. Benzylation of the phenols was achieved by stirring the two starting materials in the presence of the iron catalyst without solvent for 10 min. Subsequent addition of DMF and cesium carbonate and heating at 130 °C for a further 10 min under microwave irradiation induced the cyclization to xanthenes. The iron-catalyzed reaction of 2-iodoanilines with arylisothiocyanates led to 2-aminobenzothiazoles (Scheme 333).615 Iron(III) fluoride was employed as catalyst and 1,10-phenanthroline as ligand. In general, the benzoisothiazoles were obtained in high yields. 3244

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Scheme 336

Scheme 337

Scheme 334

Scheme 338

Scheme 335

Peng and co-workers achieved the same transformation (reaction of 2-iodoaniline with isothiocyanates) under phase transfer conditions using iron(III) chloride as catalyst and also 1,10-phenanthroline as ligand.616 DABCO (1,4-diazabicyclo[2.2.2]octane) was added as base, and the reaction was performed in water at 80 °C in the presence of octadecyltrimethylammonium chloride (PTC-4). A combined FeCu catalysis was found to be effective for the coupling of bromoarenes with phenols to give diaryl ethers (Scheme 334).617 The reaction was performed in DMF at 135 °C using a catalyst system consisting of copper(I) iodide and iron(III) acetylacetonate. Similar conditions have been described by Mao and co-workers for the reaction of diiodoarenes with phenols or thiophenols to provide dimeric aryl ethers or sulfides (Scheme 335).618 Their catalyst system also consisted of iron(III) acetylacetonate and copper(I) iodide. Alternatively, the same products could be obtained by reaction of dihydroxybenzenes with iodoarenes. Zhou, Chen, and Jiang used this method for the construction of aryl ether macrocycles.619 Several ortho- and meta-bridged aryl ether macrocycles of different ring sizes could be obtained in comparably good yields. Vinyl halides were treated with aromatic and aliphatic thiols using iron(III) chloride in combination with Xantphos as catalytic system (Scheme 336).620 The reaction was performed

in dioxane or DMF at 135 °C in the presence of KOtBu as base, providing vinyl sulfides in moderate to good yields from both vinyl iodides and bromides. A vinyl chloride proved to be less reactive, affording vinyl sulfides only in low to moderate yields. Diselenides and ditellurides could be synthesized by an ironcatalyzed cross coupling of aryl iodides with elementary selenium or tellurium, respectively (Scheme 337).621 Graphene oxide based nano-Fe3O4 ([email protected]) was used as recyclable catalyst. This catalyst can be easily removed due to its magnetic properties and reused several times without losing much of its activity. The yields for this transformation were mostly very high. Copper ferrite nanoparticles could be applied as catalysts for Ullmann-type CO coupling reactions (Scheme 338).622 Thus, aryl iodides and bromides were treated with phenols to give diaryl ethers in high yields. Even though the reactivity of aryl bromides has been proven to be sufficient for this coupling, 1-bromo-4-iodobenzene could be coupled selectively at the iodo position. The method tolerated a range of functional groups, such as free amino groups, keto groups, cyano groups, and the corresponding less reactive halides. Moreover, the catalyst could be easily removed by an external magnet and reused. A significant loss of activity over six runs was observed when 5 mol % of the catalyst was employed. In contrast, almost no decrease in efficiency was detected during the first five runs starting with 10 mol % of the catalyst. The same catalyst species was employed by Sreedhar and coworkers for the reaction of aryl and alkyl iodides or bromides with arylsulfinic salts (Scheme 339).623 Using 1,10-phenanthroline as ligand and DMF as solvent at 110 °C afforded the corresponding sulfones in high yields. Analogously, arylsulfinic acids could also be coupled with aryl boronic acids under the same conditions (see section 2.5.2). 3245

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Chemical Reviews Scheme 339

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Scheme 342

Scheme 340 Scheme 343

Scheme 341

In analogy to the reaction with aminocoumarins (cf. section 2.5.1.1), iodobenzenes could be coupled with hydroxycoumarins or phenols using nanocrystalline nickel ferrite as catalyst providing the corresponding ethers in excellent yields.343 2.5.1.3. CX/B Cross Coupling Reactions. Chavant and coworkers published an ironcopper-cocatalyzed procedure for the borylation of aryl bromides with pinacolborane (Scheme 340).624 Stoichiometric amounts of sodium hydride as base were essential as well as TMEDA as ligand to give the borylated arenes in good yields. The borylation of alkyl halides with bis(pinacolato)diboron was reported by the group of Cook (Scheme 341).625 Iron(III) acetylacetonate in combination with TMEDA was applied as a catalytic system. In addition, an excess of Grignard reagent was crucial for this reaction. Using this protocol, a large number of alkylboronates could be synthesized in a wide range of yields. The utility of this method was demonstrated by a gram-scale synthesis of a secondary alkylboronate in 87% yield. 2.5.2. CMetal/Heteroatom Cross Coupling Reactions. Iron powder has been reported to catalyze the cross coupling of arylboronic acids with diselenides and ditellurides (Scheme 342).626 The reaction proceeded without ligand in DMSO at 130 °C. Unsymmetrical arylselenides and aryltellurides were obtained in high yields. An iron-catalyzed coupling of zinc amides with Grignard reagents has been described by Nakamura et al. (Scheme 343).627 The zinc amides were prepared from primary amines, preferably anilines, by sequential treatment with butyllithium and ZnCl2 3 TMEDA. This intermediate was then treated with arylzinc reagents in the presence of 1,2-dichloroisobutane (DCIB) and catalytic amounts of Fe(acac)3. Under these conditions secondary amines could be obtained in high yields.

The addition of in situ generated aryl Grignard reagents to trimethylsilyl chloride has been achieved in the presence of catalytic amounts of iron(III) chloride and stoichiometric amounts of TMEDA.628 Thus, 1,2-bis(trimethylsilyl)benzenes have been obtained by reaction of o-dibromoarenes with magnesium and trimethylsilyl chloride. Sreedhar and co-workers described a copper ferrite nanoparticlecatalyzed coupling of aryl- and alkenylboronic acids with arylsulfinic acid salts.623 The reaction was performed in DMF at 110 °C in the presence of 1,10-phenanthroline as ligand. Diaryl and alkenyl aryl sulfones could be obtained in good yields. Under the same conditions arylsulfinic acids could be coupled with aryl and alkyl halides (cf. Scheme 339). In an independent study, Das et al. employed nanocrystalline nickel ferrite as catalyst for the coupling of arylboronic acids with amines and phenols to afford arylamines and diaryl ethers, respectively, in high yields.343 2.5.3. CH/Heteroatom Cross Coupling Reactions. CH/Het(H) cross coupling reactions can also be classified as oxidation of CH bonds. This appears plausible in particular for the hydroxylation and amination (with ammonia) of CH bonds. Due to the large number of studies in this area only a list of reviews is provided in section 10. However, CH/CO and CH/CN coupling reactions with substituted O- and N-nucleophiles are covered in the following section. 2.5.3.1. CH/N,P Cross Coupling Reactions. An iron(II) phthalocyanine-catalyzed allylic amination with phenylhydroxylamine had already been reported by Jørgensen and Johannsen in 1994.629 Later in the same year, Nicholas and Srivastava described the same transformation using various iron(II) and iron(III) salts as catalysts.630 Benzylic CH bonds have been used for oxidative coupling reactions with benzamides and benzenesulfonamides in the presence of iron(II) chloride as catalyst and N-bromosuccinimide (NBS) as oxidant (Scheme 344).631 A variety of benzylated benzamides and sulfonamides could be synthesized by this protocol. N,N-Dimethylanilines can undergo a dehydrogenative cross coupling with dialkyl H-phosphonates providing α-aminophosphonates (Scheme 345).632,633 The reaction is catalyzed by iron(II) 3246

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Scheme 348

Scheme 345

Scheme 346 Scheme 349

Scheme 347

chloride in the presence of tert-butyl hydroperoxide and proceeds under mild conditions in methanol at room temperature. Various substrates with electron-withdrawing and electrondonating substituents at the arylamine could be transformed in high yields. The α-CH bond of ethers could be activated for a cross coupling reaction with azoles in the presence of iron(III) chloride and tert-butyl hydroperoxide (Scheme 346).634 A variety of imidazoles but also a pyrazole and a triazole could be treated with cyclic and acyclic ethers to afford the 1-substituted azole derivatives in high yields. An iron(III)/copper(II) cocatalyzed intramolecular aromatic amination has been described by Wang et al. (Scheme 347).635 They converted N-aryl-2-aminopyridines into pyrido[1,2-a]benzimidazoles using iron(III) nitrate nonahydrate and copper(II) acetate as catalysts and oxygen as terminal oxidant. The reaction proceeded in DMF in the presence of pivalic acid. The mechanistic proposal for this reaction is based on the oxidation of a pyridylcopper(II) intermediate into the corresponding copper(III) complex by the iron(III) catalyst (Scheme 348).635 The increased electrophilicity of Cu(III) facilitates an ArSE reaction with the phenyl substituent. Reductive elimination of the aryl(pyridyl)copper(III) complex generates the CN bond of the

Scheme 350

benzimidazole and releases Cu(I) which is reoxidized to Cu(II) by oxygen. Similarly, 3-aryl-4-hydroxycoumarins can be cyclized in the presence of stoichiometric amounts of iron(III) chloride to give coumestan derivatives in high yields.636 The decarbonylative coupling of various benzoxazoles and 1,3,4-oxadiazoles with formamides was achieved using iron(III) chloride as catalyst in the presence of imidazole as base and air as oxidant (Scheme 349).637 In addition, the amination of azoles could also be carried out with amines and stoichiometric amounts of iron(III) chloride. Using different reaction conditions (20 mol % TBAI, aqueous TBHP, 1,2-dichloroethane, AcOH, 90 °C), it has been shown recently that the decarbonylative coupling of benzoxazoles with formamides can be achieved transition metal free.638 Qiu and co-workers developed a method for coupling imidazoles with benzylic compounds (Scheme 350).639 This reaction was accomplished using iron(II) chloride as catalyst and di-tert-butyl peroxide as oxidant. A large variety of benzylated imidazoles could be obtained in moderate to high yields. N-Substituted pyrrolidin-2-ones have been coupled at the 5-position with (hetero)aryl amides or sulfonyl amides to give 3247

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Scheme 354

Scheme 355

Scheme 352

Scheme 353

4-amidopyrrolidin-2-ones (Scheme 351).640 Optimized conditions for this transformation include iron(II) chloride tetrahydrate as catalyst, 6,60 -dimethyl-2,20 -bipyridine as ligand, and tert-butyl hydroperoxide as oxidant. An iron(II) chloride catalyzed N-aminomethylation of azoles has been described by Chen et al. using N-alkylamides, N,Ndialkylamides, or N,N-dialkylalkyl sulfonamides as aminomethylating agents (Scheme 352).641 This dehydrogenative coupling required di-tert-butyl peroxide as oxidant. A series of mostly imidazoles and benzimidazoles was treated with a variety of amides and sulfonamides to afford the substituted azoles in high yields. Yoshikai and Deb developed a cyclization of 20 -arylacetophenone O-acetyloximes in the presence of catalytic amounts of Fe(acac)3 in glacial acetic acid at 80 °C (Scheme 353).642 This method afforded a series of substituted phenanthridines in high yields. The authors discussed a FriedelCrafts-type reaction and a 6π-electrocyclization as potential mechanistic key steps. Maes and co-workers described an iron-catalyzed cyclization of 5-(pyridin-2-ylamino)pyrimidine-2,4(1H,3H)-diones to pyrido[1,2-e]purines (Scheme 354).643 The reaction requires iron(II) chloride tetrahydrate as catalyst in rather high loadings (1540 mol %) and oxygen as oxidant. The iron catalyst proved to be more efficient than copper(II) acetate. Gade and co-workers developed an enantioselective azidation of β-ketoesters and 3-aryl-2-oxindoles using a T-shaped azidoiodinane as azide source (Scheme 355).644 The reaction with β-ketoesters proceeded in the presence of catalytic amounts of

Scheme 356

a chiral pincer (boxmi)iron(II) complex and a silver carboxylate (eq a). Oxindoles were treated with the azidoiodinane using a catalyst formed in situ from iron(II) propionate and the chiral boxmi ligand (eq b). The azides were obtained in high yields and enantiomeric excesses of about 90% for a variety of substrates. Diarylhydrazones and hydrazones of α,β-unsaturated ketones can be cyclized by an iron-catalyzed CH/NH coupling to afford 1,3-diaryl-substituted 1H-indazoles (Scheme 356, eq a) and trisubstituted 1H-pyrazoles (eq b), respectively. 645 3248

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Chemical Reviews Scheme 357

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Scheme 360

Scheme 361

Scheme 358

Scheme 362

Scheme 359

For this transformation, Bao et al. developed a protocol using iron(III) bromide as catalyst and oxygen as terminal oxidant. Nakamura and Ilies developed a method for the coupling of N-chloroamines with arenes bearing an 8-quinolinylamide group (Scheme 357).646 Iron(III) acetylacetonate in combination with a bidentate phosphine ligand proved to be a suitable catalyst system. Addition of a slight excess of Grignard reagent was required for amide deprotonation/ortho-metalation of the substrate to form an active tridentate aryliron(III) complex which was quenched by the nitrogen electrophile. Slow addition of the aminating reagent was crucial to avoid direct reaction with the Grignard reagent. The coupling occurred ortho to the directing group and provided anthranilic acid derivatives in mostly excellent yields. The C-1 position of isochromane derivatives has been successfully coupled with anilines employing iron(II) chloride tetrahydrate as catalyst and tert-butyl hydroperoxide as oxidant (Scheme 358).647 The corresponding cyclic hemiaminals have been obtained under relatively mild conditions in toluene at 75 °C in mostly moderate to good yields. It was assumed that the C-1 position is oxidized to the corresponding carbenium oxonium ion which is subsequently trapped by the N-nucleophile. Li and Gu reported the oxidative cyclization of O-benzylated 2-aminophenols to provide 2-arylbenzoxazoles (Scheme 359).648 The reaction was conducted in the presence of iron(II) bromide as catalyst and di-tert-butyl peroxide as oxidant. A series of 2-arylbenzoxazoles with various substituents, such as halide, cyanide, and methoxy, could be obtained in high yields.

An iron-catalyzed amination of C(sp3)H bonds adjacent to the nitrogen of amide groups, preferably N-methyl-2-pyrrolidone, with aniline derivatives has been reported by Bao and co-workers (Scheme 360).649 They employed iron(III) chloride as a catalyst and tert-butyl hydroperoxide as terminal oxidant. The corresponding aminals were obtained in mostly moderate to good yields. Bolm et al. described a direct CH/NH coupling of sulfoximines with diarylmethanes using iron(III) bromide as catalyst and di-tert-butyl peroxide as oxidant under neat conditions (Scheme 361).650 Several N-diarylmethylated sulfoximines were obtained in high yields. Cross coupling of arenes and heteroarenes with an succimidylN-oxy perester led to a range of N-aryl succinimides which can be easily deprotected to the corresponding amino(hetero)arenes (Scheme 362).651 The transformation was performed under mild conditions in dichloromethane at 50 °C in the presence of ferrocene as catalyst. The process may follow a radical pathway with ferrocene as a single-electron shuttle. 2.5.3.2. CH/O, S, Se, Te Cross Coupling Reactions. Pearson and Kwak developed a method for the oxidative cyclization of chiral N-(hydroxyethyl)pyrrolidines to hexahydropyrrolo[2,1-b]oxazoles (Scheme 363).652 A tricarbonyl(cyclohexadiene)iron complex was employed as catalyst and trimethylamine N-oxide as oxidant. The products were obtained in mostly high yields and high to complete diastereoselectivity. The group of Beller was able to perform an iron-catalyzed hydroxylation of β-ketoesters with hydrogen peroxide as oxidant (Scheme 364).653 Iron(III) chloride hexahydrate was found to be the most active iron-containing catalyst for this transformation. Seven examples with excellent yields for the α-hydroxy β-ketoesters have been presented. Iron(III) fluoride has been shown to efficiently catalyze the sulfenylation of indoles in the presence of iodine (Scheme 365).654 3249

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Chemical Reviews Scheme 363

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Scheme 367

Scheme 364

Scheme 365 Scheme 368

Scheme 366

Various aromatic and aliphatic disulfides have been employed in the reaction with substituted indoles to afford 3-sulfenylindoles in mostly high to excellent yields. Accordingly, selenylation of indoles was achieved under the same conditions using 1,2-diphenyldiselane as reagent. A Lewis acid catalyzed and copper(II)-mediated reaction of heteroarenes, such as thiazoles and oxazolines, with thiols has been described by Gao et al. (Scheme 366, eq a).655 Ag(I), Ni(II), and Fe(II) have been investigated as catalysts wherein Ag(I) proved to be most efficient for the majority of substrates. For selected examples, however, iron(II) fluoride was superior. It is noteworthy that DMSO could be employed as thiomethylation reagent (eq b). The best catalyst for this transformation was silver fluoride, but iron(II) fluoride gave reasonable results for some examples. Urabe and co-workers developed an iron-catalyzed protocol for the peroxylation of benzyl, allyl, and propargyl ethers to give

tert-butyl peroxyacetals (Scheme 367, eqs ac).656 The ethers were treated with tert-butyl hydroperoxide in the presence of Fe(acac)3 and molecular sieves in acetonitrile at 80 °C. The peroxyacetals were obtained in good to very high yields. In addition, ethylene acetals could be transformed to the corresponding peroxyorthoesters (eq d). Iron(III) chloride in combination with sodium peroxodisulfate as oxidant and pyridine as base could be employed for the oxidative cyclization of N-arylbenzothioamides to give 2-arylbenzothiazoles (Scheme 368).657 In addition, N-aryl alkylthioamides and thioureas could also be cyclized in moderate to excellent yields. The group of Jiao described the dehydrogenative coupling of aryl propargyl azides with carboxylic acids (Scheme 369).658 Iron(II) chloride functioned as catalyst and DDQ was employed as oxidant. The azides were prepared in a one-pot procedure from the corresponding chlorides which improved the yields in comparison to the two-step synthesis. On the other hand, a CO coupling could not be achieved directly from the propargyl chlorides, demonstrating the assisting role of the azide for the CH bond activation. The acyloxylated propargyl azides were obtained in good yields. They could be further transformed to 1,2,3-triazoles by heating in methanol or to 3-alkoxy-3-arylacroleins when treated with sodium hydroxide in methanol. An oxidative C(sp3)H/OH coupling of 2-aminophenols with toluenes followed by an oxidative C(sp3)H/NH cyclization and aromatization has been achieved with iron(II) bromide as catalyst and di-tert-butyl peroxide as oxidant (Scheme 370).659 This domino reaction led to the synthesis of 2-arylbenzoxazoles in moderate to good yields. 3250

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Chemical Reviews Scheme 369

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Scheme 373

Scheme 374

Scheme 370

Scheme 371

Scheme 372

Sun et al. established a protocol for the formal amination of benzoxazoles in the 2-position (Scheme 371).660 The reaction consists of two steps which could be conducted in a one-pot procedure: ring opening to form an amidine intermediate and iron-catalyzed oxidative cyclization. Iron(II) chloride was identified as an efficient catalyst, and cheap hydrogen peroxide could be employed as oxidant. Excellent yields were reported for a number of examples. This transformation is reminiscent of the decarbonylative amidation of benzoxazoles reported by Wang et al. (cf. Scheme 349). 1,4-Dioxane was coupled with various salicylaldehydes in the presence of catalytic amounts of nonacarbonyldiiron and tertbutyl hydroperoxide as oxidant (Scheme 372).661 The resulting cyclic acetals were obtained in good to high yields. The method may be of importance for the selective protection of phenolic hydroxy groups adjacent to carbonyl functions. At the same time Han and co-workers presented a similar transformation using carboxylic acids as O-nucleophiles for the coupling with 1,4-dioxane or 1,3-dioxolane.662 They introduced iron(III) acetylacetonate as catalyst and di-tert-butyl peroxide as

oxidant. This method led to 1,4-dioxan-2-yl and 1,3-dioxolan-4-yl carboxylates in high to excellent yields. Magnetite nanoparticles have been demonstrated to catalyze the reaction of terminal alkynes with diselenides or ditellurides.663 This method provided alkynyl selenides or tellurides in good yields. The magnetic properties of the catalyst enabled an easy recovery and reuse. 2.5.3.3. Other CH/Het Cross Coupling Reactions. An ironcatalyzed fluorination of benzylic CH bonds has been elaborated by Lectka and co-workers using Selectfluor as electrophilic fluorinating reagent (Scheme 373).664,665 In the presence of catalytic amounts of iron(II) acetylacetonate the transformation could be conducted in acetonitrile at room temperature to afford the fluoroalkylbenzenes in good yields. No α-fluorination of carbonyl functions was observed in the presence of the iron catalyst. The more common α-fluorination of carbonyl compounds could also be achieved in the presence of an iron catalyst in combination with silver perchlorate (Scheme 374, eq a).666 Che and co-workers developed an enantioselective variant using N-fluorobenzenesulfonimide (NFSI) as electrophilic fluorine source and a chiral bipyrrolidine-based salaniron complex as catalyst. A large number of preferably bulky α-fluorinated β-oxo esters were obtained in excellent yields and mostly high enantioselectivities. In addition, the same catalyst system could be employed for the α-hydroxylation of β-oxo esters with an oxaziridine reagent (eq b). This procedure also afforded α-hydroxy-β-ketoesters in high yields but slightly lower enantiomeric excess. A dehydrogenative borylation of furans and thiophenes with pinacolborane has been reported by Tatsumi and Ohki using a coordinatively unsaturated NHC iron complex as catalyst 3251

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Chemical Reviews Scheme 375

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Scheme 377

Scheme 378 Scheme 376

Scheme 379

(Scheme 375).667 The hydrogen equivalent evolved was trapped by tert-butylethylene. The reaction proceeded regioselectively in the 2(5)-position providing borylated heterocycles in mostly high yields. Mankad and co-workers demonstrated that heterobimetallic CuFe and ZnFe complexes can be utilized as catalysts for the borylation of arenes (Scheme 376).668,669 Thus, they treated the arenes, which also served as solvents, with pinacolborane (HBpin) under photolytical conditions in the presence of the heterobimetallic catalyst. The most efficient catalyst proved to be (IPr)CuFeCp(CO)2, which provided the arylboranes in moderate to high yields.

3. ADDITION REACTIONS 3.1. Carbometalation

While the previous comprehensive review on iron-catalyzed reactions by Bolm et al.1 mentions only two noteworthy examples for the addition of organometallic reagents to olefins or acetylenes, namely the seminal works of Nakamura670 and Hosomi,671 ever since the field has been developed considerably. Some of the recent publications discussed herein have also been introduced in other reviews and highlight articles which have appeared since 2004, however, often summarized under a different aspect.8,243,255,672 Thus, Darcel et al. compiled a chapter on ironcatalyzed carbometalations in an article focusing on iron catalysis with N-heterocyclic carbene ligands.15 The iron-catalyzed hydromagnesiation of olefins has been subject of a recent minireview by Thomas.673

3.1.1. Carbometalation of Olefins. Kotora and co-workers described an alkylative cyclization of 2-chloro-α,ω-dienes in the presence of iron(III) chloride, a phosphine ligand, and a trialkylaluminum reagent as nucleophile (Scheme 377).674 The proposed mechanism includes intermolecular and intramolecular carbometalation steps followed by β-chloride elimination of an intermediate β-chloroalkyliron complex. The same transformation was subsequently shown to proceed also in the presence of catalytic amounts of a (trichloro)[bis(imidazolonyl)pyridine]iron complex as catalyst.675 The corresponding spiroannulated fluorenes were obtained mostly in low yields. In the presence of Fe(acac)3 as catalyst, cyclopropenes can undergo a carbometalation reaction with trialkylaluminum reagents followed by ring opening (Scheme 378.).676 The feasibility of this domino process could be demonstrated by the synthesis of a variety of tri- and tetrasubstituted alkenes, for example, α,β,β0 -trisubstituted vinylsilanes and -stannanes. A high degree of regio- and stereoselectivity was achieved providing the functionalized alkenes in high yields. Carbozincation of oxa- and azabicyclic alkenes has been accomplished using iron(III) chloride as catalyst in the presence of a diphosphine ligand (Scheme 379).677 The carbometalated intermediates have been trapped by protons but also other electrophiles, such as iodine, allyl bromide, and acetyl chloride. 3252

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Chemical Reviews Scheme 380

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Scheme 382

Scheme 381

Scheme 383

The reaction proceeded with high diastereoselectivity and provided arylated bicycles in high yields. 3.1.2. Carbometalation of Alkynes. An ironcoppercatalyzed hydroarylation of alkynes with aryl Grignard reagents has been reported by the group of Hayashi (Scheme 380).678 They employed Fe(acac)3 as catalyst and copper(I) bromide as cocatalyst in the presence of tributylphosphine. After quenching of the carbometalated intermediates with water, E-olefins were obtained in good yields and with high diastereoselectivities. In a subsequent work the authors demonstrated that this transformation can be efficiently conducted using Fe(acac)3 as catalyst in the presence of an NHC ligand (Scheme 381).679 In general, the yields were superior as compared to the FeCu cocatalyzed variant; however, the diastereoselectivity was less pronounced. Tetrasubstituted olefins could be constructed by quenching the intermediate carbometalated products with electrophiles such as allyl bromide and iodine. An interesting one-pot carbomagnesiation/cross coupling sequence was achieved by treating the alkenyl Grignard intermediate with phenyl iodide in the presence of a nickel catalyst. Ready et al. have treated propargylic and homopropargylic alcohols with an excess of alkyl- and arylmagnesium reagents in the presence of an iron(III) catalyst and obtained after hydrolysis of the carbometalated products allylic and homoallylic alcohols in good yields (Scheme 382).680 In addition, alternative electrophiles have been introduced. For example, trapping with benzaldehyde provided the corresponding hydroxybenzylated olefins (eq b). Hayashi and Shirakawa elaborated two procedures for the iron-catalyzed carbolithiation of arylalkynes (Scheme 383).681 The first procedure, with iron(III) chloride as catalyst and two ligands, TMEDA and triphenylphosphine, is used for alkyllithium reagents (eq a). The second procedure, with Fe(acac)3 as catalyst, copper(I) bromide as cocatalyst, and tributylphosphine as ligand, is applied to aryllithium reagents (eq b). In a subsequent work, they were able to conduct aryl- and alkenyllithiation of internal alkynes in a Fe(acac)3-catalyzed process.682

A limitation was the necessity of alkyl substituents in ortho- or cis-position of the aryl- or alkenyllithium reagents, respectively. Shirakawa and Hayashi also established an iron/copper cocatalyzed exchange reaction between terminal alkenes and cyclopentylmagnesium bromide which formally constitutes a hydromagnesiation of olefins (Scheme 384, eq a).683 Additionally, tributylphosphine was required as ligand. Even a chloroaryl function was tolerated under these conditions. The resulting 1-alkyl Grignard reagents were trapped with a series of electrophiles to afford isolable products. In another iron/coppercatalyzed procedure, the Grignard reagent was treated with arylalkynes to provide the alkylmagnesiation products which after methanolysis led to substituted alkenes (eq b). Even though the yields were generally high, it should be noted that in many cases the stereoselectivity was only low. After optimizing the two reactions separately, the authors eventually established conditions which enabled performing them in a one-pot procedure (eq c). An intramolecular carbometalation has been described by Urabe and co-workers.684 They reductively cyclized enynes in the presence of catalytic amounts of iron(II) chloride and an excess of tert-butylmagnesium chloride (Scheme 385). This procedure led to a number of alkylidenecyclopentanes and alkylidenecyclohexanes. It was proposed that an iron(II)magnesium cluster is formed which undergoes oxidative cyclization with the enyne to generate a ferracyclopentene (Scheme 386). Transmetalation with additional Grignard reagent would lead to a double magnesiated alkylidenecyclopentane which is quenched by electrophiles. 3253

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Chemical Reviews Scheme 384

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Scheme 387

Scheme 388

Scheme 385

Scheme 386

This structure was supported by the feasibility of a double allylation with allyl bromide. The same type of transformation was reported by Yang and co-workers.685 They treated N-protected N-allylpropargylamines or allyl propargyl ethers with a combination of diethylzinc and magnesium bromide etherate in the presence of catalytic amounts of iron(II) chloride and an iminopyridine ligand. This method led to 3-benzylidene-substituted pyrrolidines and tetrahydrofurans in moderate to good yields. Nakamura and co-workers described an iron-catalyzed carbomagnesiation of conjugated diynes with Grignard reagents affording substituted conjugated enynes (Scheme 387).686 The reaction proceeded in the presence of catalytic amounts of iron(II) chloride with no need for additional ligands. The vinylmagnesium intermediate could be trapped by protons or subjected to other electrophiles. Thus, subsequent palladium- or nickel-catalyzed cross coupling reactions, or zirconium-catalyzed iodination, could be carried out directly with these intermediates to obtain polysubstituted enynes. 3.1.3. Carbometalation of Allenes. Ma and co-workers described the Fe(acac)3-catalyzed carbometalation of 2,3allenoates which after hydrolysis afforded β,γ-unsaturated esters under very mild conditions (Scheme 388).687 The reaction proceeded with high regio- and stereoselectivity and provided the trisubstituted olefins in high yields. The feasibility of trapping the intermediate magnesium dienolate with other electrophiles was proven by reaction with acrolein, acetyl chloride, and methyl chloroformate. In a subsequent paper the authors could demonstrate that the reaction of the magnesium dienolate with electrophiles may lead to E- or Z-olefins depending on the nature of the electrophile and the reaction conditions.688 For example, trapping with allyl acetates in the presence of a palladium(0) catalyst or with acid chlorides led to the Z-olefins. In contrast, E-olefins were obtained using allyl carboxylates without additional metal catalyst or with allylic bromides in the presence of copper(I) bromide. This selectivity was rationalized by an isomerization of π-allylmetal intermediates. Starting from 1-(trimethylsilyl)allene-1-carboxylates, the ironcatalyzed reaction with Grignard reagents provided a stereoselective access to polysubstituted α-methoxycarbonyl allyl silanes which have been further treated with electrophiles to give α,β-unsaturated esters.689 3.2. Hydroalkylation and Hydroalkenylation

Beller and co-workers described an iron-catalyzed method for the direct addition of 1,3-dicarbonyl compounds to styrenes 3254

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Chemical Reviews Scheme 389

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Scheme 392

Scheme 390

Scheme 393

Scheme 391 Scheme 394

in Markovnikov orientation (Scheme 389).690 Iron(III) chloride hexahydrate was identified as an efficient catalyst for this transformation which provided a variety of alkylated 1,3-dicarbonyl compounds in good yields. An intermolecular 1,4-hydrovinylation of 1,3-dienes was achieved by reaction with terminal alkenes in the presence of iron(II) chloride, an iminopyridine ligand, and magnesium metal (Scheme 390).691 It should be noted that often 1 or 2 mol % of the iron catalyst was sufficient to promote the transformation. 1,4-Dienes were obtained in high yields with complete stereoselectivity for both double bonds. Some examples of unsymmetrical 1,3-dienes could be converted with high regioselectivity. A reductive cyclization of 1,6-enynes and diynes to afford substituted cyclopentanes has been established by Chirik et al. (Scheme 391).692 The transformation was achieved under hydrogen pressure using the [bis(imino)pyridine][bis(dinitrogen)]iron complex [(iPrPDI)Fe(N2)2] as catalyst. In a subsequent work, the authors synthesized and characterized a number of bis(imino)pyridine iron metallacycles (Scheme 392).693 This, in combination with computational results, led to conclusions on the electronic structure of

important intermediates of the iron-catalyzed hydrogenative cyclization. The proposed mechanism involves the ligand as a redox active moiety. Thus, oxidative cyclization leads to a oneelectron oxidation of both the iron center (II f III) and the ligand (triplet diradical bis(imino)pyridine dianion f doublet monoradical bis(imino)pyridine monoanion; [PDI]2 f [PDI]). Hydrogenation forms an alkyl(hydrido)iron intermediate with iron probably remaining in oxidation state +III. Subsequent reductive elimination releases the product and reduces both the iron center and the ligand. Notably, using the same ligand, a catalytically active iron(I)/iron(III) redox couple was proposed for the [2 + 2] cycloaddition of α,ω-dienes (cf. Scheme 673). Tu and co-workers succeeded in the development of an iron(III) chloride catalyzed hydrohydroxyalkylation of styrene or 1,1-diphenylethene by reaction with primary alcohols (Scheme 393).694 This transformation includes a C(sp3)H bond activation α to the hydroxy function. A variety of secondary alcohols could be synthesized in high yields by this procedure. An iron(IV) hydrido complex and alkyl radicals were proposed as intermediates. The dimerization of styrenes has been achieved by the group of Corma employing a catalytic system consisting of iron(III) chloride in combination with silver(I) triflate or bis(triflyl)imide (Scheme 394).695 The head-to-tail E-olefins were formed with full regio- and diastereoselectivity in good to excellent yields. Moreover, catalyst recovery and reuse was possible using a (phosphine)iron(III) complex. 3255

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Chemical Reviews Scheme 395

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Scheme 398

Scheme 399

Scheme 396

Scheme 397

An iron-catalyzed addition of 1,3-dicarbonyl compounds to styrenes, cyclohexa-1,3-dienes, dihydropyrans, and dihydrofurans has been developed by Campagne and Prim.696 The reaction proceeds in 1,2-dichloroethane at 80 °C using iron(III) chloride hexahydrate as catalyst. Styrenes could be converted in good to high yields. The other substrates gave mostly moderate yields. The main focus of this work, however, was the addition of N-nucleophiles to olefins (see section 3.10.2, Scheme 471). An intramolecular radical hydroalkylation of unsaturated organic halides has been achieved using iron(II) chloride as catalyst in the presence of sodium borohydride as stoichiometric reductant (Scheme 395).697 The initially formed radical undergoes 5-exo-trig or 6-exo-dig cyclization to give tetrahydrofurans, cyclopentanes, or tetrahydropyrans, respectively. An anionic (hydrido)iron(I) complex generated from the iron(II) precatalyst by reduction with sodium borohydride acts as single electron donor for the halide. Aryl iodides and tertiary alkyl iodides, as well as primary and secondary alkyl bromides without an adjacent double bond, were readily hydrodehalogenated under these conditions. Yu and co-workers described the alkylation of α-oxo ketene dithioacetals by styrene derivatives (Scheme 396).698 The reaction was catalyzed by iron(III) triflate and led to the benzylated ketene dithioacetals in good yields. 3.3. Hydroalkynylation

Iron(III) triflate has been proven to be an efficient catalyst for the hydroalkynylation of olefins (Scheme 397).699

Thus, terminal arylalkynes were treated with norbornenes and styrenes to afford the addition products in good yields. Regarding the substrate scope and the robustness of the reaction, this method may complement a related nickel-catalyzed process.700 The same transformation has also been described by LeyvaPerez and Corma working with slightly lower amounts of iron(III) triflate (2 mol %) in 1,2-dichloroethane at 80 °C (see also Scheme 468).224 Dash and co-workers developed an iron-catalyzed method for the head-to-head coupling of terminal aryl alkynes (Scheme 398).701,702 The best conditions included 30 mol % iron(III) chloride (99.9%) in the presence of DMEDA or DPPE as ligands and potassium tert-butoxide as base. The reaction proceeded smoothly with anti-Markovnikov selectivity. A moderate E-selectivity was observed. Quenching of the reaction by a radical scavenger suggested a radical mechanism. 3.4. Twofold CC Bond Forming Additions

3.4.1. Cyclizing Arylalkylation and Arylacylation. The group of Li described the reaction of alkenes and alkynes with 2-benzhydryl-1,3-diphenylpropane-1,3-dione to give dihydroindenes and indenes via nucleophilic displacement of the 1,3-dicarbonyl moiety and subsequent FriedelCrafts-type ring closure (Scheme 399, cf. Scheme 85).181 Alkenes were treated at 50 °C, whereas alkynes required 80 °C to achieve good yields. The pronounced diastereoselectivity of this transformation was explained by formation of the thermodynamically most stable product. Thus, both trans- and cis-β-methylstyrenes afforded exclusively the same dihydroindene isomer. Similarly, Chen et al. described an indene synthesis from alkynes and diarylmethanes using iron(II) chloride as catalyst in the presence of overstoichiometric amounts of NBS (Scheme 116).223 This transformation involves an addition of an in situ generated diarylmethyl electrophile to the alkyne and subsequent Friedel Crafts-type cyclization. The iron-catalyzed intramolecular carboarylation of N-methyl-Narylacrylamides led to the synthesis of oxindoles (Scheme 400).703 Thus, Li and co-workers could demonstrate that enamides readily react with ethers, thioethers, and N-methylpiperidine in a cyclization 3256

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Chemical Reviews

REVIEW

Scheme 400

Scheme 402

Scheme 401

Scheme 403

Scheme 404

reaction leading to 2-oxindoles. This transformation includes activation of a C(sp3)H bond adjacent to a heteroatom and a Friedel Crafts-type C(sp2)H activation. The procedure requires catalytic amounts of iron(III) chloride, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and tert-butyl hydroperoxide as oxidant. Investigation of the kinetic isotopic effect (kH/kD = 1.0) and inhibition of the reaction upon addition of radical scavengers implied a radical mechanism for this transformation. The same kind of substrates underwent a methylarylation when the reaction was performed in DMSO in the presence of dicumyl peroxide, which delivered the methyl group, iron(II) acetate as catalyst, and DABCO as ligand (Scheme 401, eq a).704 This method provided 3-substituted 3-ethylindolin-2-ones in mostly good yields. In a parallel endeavor, the group of Cheng described the same transformation with iron(II) chloride as catalyst and di-tert-butyl peroxide as methyl transferring agent and oxidant (eq b).705 An additional ligand was not required. A variety of functional groups, such as cyano, nitro, ester, bromo, chloro, and trifluoromethyl, were tolerated. Yields were slightly higher than in the previous case but higher catalyst loadings were reported. Li and Song described the addition of benzyl alcohols to N-aryl acrylamides (Scheme 402).706 This cyclizing arylacylation of the olefinic unit proceeded in the presence of iron(II) acetate as catalyst. tert-Butyl hydroperoxide was employed as oxidant. A series of 3-(2-oxoethyl)oxindoles was obtained in good to high yields. As in the related reaction of N-aryl acrylamides with carbazates (cf. Scheme 426), a radical mechanism was proposed. Initially, an α-hydroxyalkyl radical is formed from the alcohol which subsequently adds to the olefin followed by intramolecular radical aromatic substitution and oxidation of the alcohol to the ketone. More recently, an aryldifluoromethylation of the same substrates was achieved by treatment with PhSO2CF2I in the presence

Scheme 405

of hydrogen peroxide and catalytic amounts of ferrocene.707 This transformation provides difluoromethylated oxindoles in high yields and under mild reaction conditions (DMF/THF, 60 °C). 3.4.2. Annulations. An intramolecular addition of 2-biaryl, 2-heteroarylphenyl, or 2-alkenylphenyl Grignard reagents to alkynes has been achieved by Nakamura et al. (Scheme 403).708 Tris(acetylacetonato)iron in combination with 4,40 -di-tert-butyl2,20 -bipyridine (DTBPY) was used as catalyst system for this [4 + 2] benzannulation. In addition, the presence of the oxidant 1,2-dichloroisobutane (DCIB) was required. This procedure led to a variety of phenanthrenes, naphtho[b]heteroarenes, and one naphthalene in good to excellent yields. This method could be extended to a [2 + 2 + 2] annulation of Grignard reagents with internal alkynes (Scheme 404).709 Fe(acac)3 in combination with 1,10-phenanthroline (phen) functioned as catalytic system and 1,2-dichloroisobutane (DCIB) as oxidant. The transformation afforded polysubstituted naphthalenes in moderate to good yields. Closely related, the annulation of arylindium reagents with two molecules of alkynes has been achieved using iron(III) chloride as catalyst (Scheme 405).710 In addition, 3257

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Chemical Reviews Scheme 406

REVIEW

Scheme 409

Scheme 410 Scheme 407

Scheme 408

1,2-bis(diphenylphosphino)benzene (DPPBz) was added as ligand and (trimethylsilyl)methylmagnesium chloride as activating reagent. Interestingly, no external oxidant was required for this overall oxidative coupling of arylindium compounds and alkynes. The oxidative carboarylation of terminal alkynes (Scheme 406, eq a) and alkenes (eq b) with N-alkylanilines in the presence of di-tert-butyl peroxide afforded quinolines in good to high yields.711 The transformation was catalyzed by iron(III) chloride. It was proposed that in a first step the amine is oxidized to an iminium function which then undergoes addition to the alkene or alkyne with subsequent FriedelCrafts-type ring closure and aromatization. 3.5. CC and CHeteroatom Bond Forming Additions

3.5.1. Haloalkylation. The group of Liu developed an iron(III) chloride catalyzed method for the haloalkylation of arylalkynes (Scheme 407).712 The reaction proceeded in a regioselective manner with the halogen atom occupying the position α to the phenyl ring of the original alkyne. Regarding the diastereoselectivity, a pronounced preference for the E-isomer was observed. The addition of 1,3-dithiane and chlorine to styrenes has been achieved in the presence of iron(III) chloride as catalyst (Scheme 408).713 The reaction was performed using

N-chlorosuccinimide as chlorine source in dichloroethane under mild conditions and provided the chloro-dithianylated styrenes in high yields. The formation of 2-chloro-1,3-dithiane was proposed which adds to the styrene following a radical pathway. An iron-catalyzed 1,2-addition of perfluoroalkyl iodides to terminal alkynes and alkenes has been achieved by Hu and coworkers (Scheme 409).714 The reaction proceeded in dioxane at 60 °C in the presence of cesium carbonate as base and iron(II) bromide as catalyst. The products of the alkyne addition were obtained in high yields and with high E-selectivity. In some examples, the reaction could be performed solely by induction with cesium carbonate with no need for the iron salt. 3.5.2. Acylations. Acyl chlorides can be added to terminal alkynes forming (Z)-β-chlorovinyl ketones. Wang, Li, and coworkers described this reaction using iron(II) bromide as catalyst (Scheme 410).715 The regio- and stereochemical outcome of this transformation is completely controlled. Thus, the acyl group adds in a syn fashion to the H terminus of the alkyne leading to the corresponding (Z)-olefins. A large set of substrates has been converted in mostly high to excellent yields. Milder reaction conditions have been reported by Cheng and co-workers.716 In the presence of catalytic amounts of iron(III) chloride in chloroform at 0 °C, terminal alkynes reacted with acyl chlorides with concomitant transfer of the chlorine to the alkynyl group affording β-chloroalkenyl ketones (Scheme 411, eq a). The reaction proceeded also with very high selectivity for the Z-isomers. Analogously, internal alkynes could be employed for this transformation (Scheme 411, eq b). However, in this case the E/Z-selectivity was rather poor with the E-product dominating. In order to avoid the chlorine transfer in case of terminal alkynes, the authors introduced silylated alkynes as nucleophiles. With this method in hand they were able to obtain alkynyl ketones in mostly high yields (Scheme 411, eq c). Li and co-workers presented an iron(II) chloride catalyzed oxidative acylation of alkenes using hydroperoxides as oxidants (Scheme 412).717 The reaction afforded β-peroxy ketones 3258

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Chemical Reviews Scheme 411

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Scheme 414

Scheme 415

Scheme 412

Scheme 413

which were further converted to α-epoxy ketones in a DBU-catalyzed process. Both transformations could be combined to a one-pot procedure. An N-bridged diironphthalocyanine, which Sorokin and coworkers frequently employed for the oxidation of CH bonds (cf. sections 10.1 and 10.2), catalyzes the hydroacylation of olefins with acetaldehyde (Scheme 413).718 This transformation was performed with an excess of acetaldehyde in a high pressure tube at 60 °C in the presence of only 0.01 mol % of the catalyst. A pronounced regioselectivity was observed providing methyl ketones in good yields. The regioselective outcome is in agreement with a radical mechanism. The mechanistic proposal

includes the formation of acyl radicals from the aldehyde via hydrogen abstraction by an iron(IV)oxo species. An iron-catalyzed cyclizing acylation/arylation reaction of N-aryl acrylamides with aldehydes has been reported by the group of Li (Scheme 414).719 The reaction proceeded with iron(III) chloride as catalyst under oxidative conditions using tert-butyl hydroperoxide as oxidant and afforded 3-substituted oxindoles in high yields (cf. Schemes 400 and Scheme 401). A radical mechanism via initial formation of acyl radicals was discussed for this transformation. 3.5.3. Other CC and CHeteroatom Bond Forming Addition Reactions. The group of Plietker developed a protocol for a decarboxylative alkoxyallylation of activated double bonds by allylic carbonates with high regioselectivity (Scheme 415).91 The ferrate complex Bu4N[Fe(CO)3(NO)] (TBAFe) functioned as catalyst in combination with an NHC ligand. The transformation was performed under mild conditions and tolerated several functional groups such as ester, halide, nitro, and ether groups. Moreover, the method could be extended to a three-component reaction using tert-butyl 1,1-dimethylallyl carbonate as allylating reagent, which does not transfer the tert-butyl group, and an external alcohol (eq b). The same catalyst could be applied to a phosphono-allylation of activated olefins (Scheme 416).720 The latter were treated with allylic carbonates and dimethylphosphonate in the presence of catalytic amounts of TBAFe and an NHC ligand in a one-pot procedure. A variety of highly functionalized phosphonates could be generated in moderate to high yields. In most cases a mixture of regioisomers was obtained with the ipso-isomer dominating. An iron-catalyzed oxidative cycloaddition of phenols and styrenes has been reported by Lei and co-workers (Scheme 417).721 Thus, electron-rich phenols reacted with styrenes in the presence of iron(III) chloride as catalyst and DDQ as oxidant to afford dihydrobenzofurans in high yields. The reaction probably follows a 3259

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Scheme 419

Scheme 420

Scheme 417

Scheme 421

Scheme 418

radical pathway. According to the proposed mechanism the iron catalyst acts as Lewis acid activator to stabilize the mesomeric cyclohexadienonyl form of the phenoxyl radical intermediate. In an independent work, the same transformation was reported by Pappo and co-workers.722 They applied iron(III) chloride hexahydrate as catalyst and di-tert-butyl peroxide as stoichiometric oxidant. In contrast to the report above, they postulated a mechanism that involves the iron as the redox active center switching between oxidation states +II, +III, and +IV. It is noteworthy that, under similar conditions at higher concentrations and with iron(II) chloride as catalyst, 2-hydroxynaphthalenes underwent an oxidative coupling to 1,10 -bi-2,20 -naphthols (BINOLs). They further reacted with styrenes in an oxidative cyclization with concomitant dearomatization of one naphthyl unit. Based on earlier work on the iron-mediated cyclization of N-aryl-N-(2-alkynyl)toluenesulfonamides,723 Majumdar and co-workers developed an iron-catalyzed formal cyclizing hydroxyalkenylation of alkynyl-tethered pyrimidinediones leading to spiropyrimidinediones (Scheme 418).724 Iron(III) chloride has been identified as the best catalyst for this transformation. The utility of the protocol could be demonstrated by several highly efficient syntheses of spiropyrimidinediones. The group of Jiao described a copper/iron cocatalyzed oxidative coupling of styrenes with arylhydrazines under oxygen atmosphere (Scheme 419).725 A radical addition to the styrene is probably a key mechanistic step of this transformation. Oxygen plays multiple roles, as it is involved in the radical formation,

serves as an oxidant for reduced catalyst species, and is the reagent that provides the oxygen atoms for the product. The method delivered symmetrical and unsymmetrical benzils in good yields. Jiao and co-workers also presented an intramolecular arylsulfonation of N-methyl N-aryl acrylamides with benzenesulfinic acid in the presence of catalytic amounts of iron(III) nitrate nonahydrate (Scheme 420).726 This method provided good to high yields of 3-substituted 3-(sulfonylmethyl)oxindoles. Zhang and co-workers described an iron-catalyzed procedure for the cyclization of aryl enynes with disulfides or diselenides (Scheme 421).727 Iron(III) chloride was identified as a suitable catalyst for this reaction. The reaction was performed in nitromethane at 120 °C in the presence of catalytic amounts of dibenzoyl peroxide (BPO) and 2 equiv of iodine. Polysubstituted naphthalenes were obtained in moderate to good yields. An iron-catalyzed reaction of 4 equiv of aromatic sodium sulfinates with 1 equiv of diarylalkynes provided triarylvinyl sulfides in good yields, albeit with almost no diastereoselectivity.728 The reaction was performed in dioxane/water (3:1) at 120 °C using iron(II) sulfate heptahydrate in combination with 1,8-naphthalenediamine as catalytic system. In addition, equimolar amounts of trifluoroacetic acid and methanesulfonic acid were required. The sodium sulfinates acted as sulfenylation reagents and as the source of the aryl group to be transferred. 3.6. Carboxylation and Carbonylation

Dodecacarbonyltriiron has been successfully introduced as catalyst for the carbonylation of alkynes in the presence of 3260

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Chemical Reviews Scheme 422

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Scheme 424

Scheme 425

Scheme 423

ammonia or primary amines.729 The transformation provided succinimides in moderate to high yields. In a subsequent paper the authors extended the substrate scope to a wide variety of 1,2di(hetero)arylalkynes which have been generated by a palladiumcatalyzed Sonogashira coupling of heteroaryl bromides with arylalkynes (Scheme 422).730 The iron-catalyzed aminocarbonylation protocol provided trans-3,4-substituted succinimides in mostly good to high yields. Oxidation with 2,3-dichloro-5,6dicyano-p-benzoquinone (DDQ) led to the corresponding maleimides. This method may provide a short access to biological active bis(indolyl)maleimides and indolocarbazole alkaloids, which represent interesting lead structures for the pharmaceutical industry.451453 A first application was provided by the synthesis of an arcyriarubin analogue.730 Subsequently, the protocol was applied as a key step in the synthesis of himanimides A and B.731 In a subsequent work the authors could verify this reaction to allow a selective monoaminocarbonylation of terminal alkynes (Scheme 423).732 Using dodecacarbonyltriiron as catalyst in combination with a bis(imine) ligand and triethylamine as base, they were able to synthesize several (E)-acrylamides and (E)-cinnamides from terminal alkynes and amines in the presence of carbon monoxide. The reaction was highly chemo- and regioselective giving the formal anti-Markovnikov product in good to high yields. This constitutes an orthogonal method to most known palladium-catalyzed carbonylations of alkynes.

Internal alkynes also reacted but afforded mixtures of E- and Z-products. In contrast to the double carbonylation presented above, the formation of (hydrido)iron intermediates was presumed in the present case. It is known that such species are formed from carbonyliron complexes in the presence of triethylamine.733 The group of Taniguchi described the hydroxy methoxycarbonylation of alkenes with methyl carbazate in the presence of catalytic amounts of ironphthalocyanine [Fe(Pc)] under air (Scheme 424).734 Methoxycarbonyl radicals are formed under these conditions which add to the alkenes. The resulting alkyl radicals are trapped by oxygen which ends up as a hydroxy group in β-position to the alkoxycarbonyl moiety. Applying this procedure, a variety of terminal alkenes could be converted to β-hydroxy esters in mostly good to high yields. Hydrocarboxylation of styrenes has been achieved by reaction with ethylmagnesium bromide and carbon dioxide in the presence of iron(II) chloride and a bis(imino)pyridine ligand (Scheme 425).735 A low catalyst loading of 1 mol % was sufficient to promote this reaction affording α-aryl carboxylic acids in almost perfect regioselectivity and good to excellent yields. A possible mechanism was proposed involving formation of an alkyliron complex by transmetalation of the Grignard reagent: β-hydride elimination to provide a (hydrido)iron species which subsequently adds to the styrene in terms of a hydrometalation. Another transmetalation leads to a benzyl Grignard compound which is trapped by carbon dioxide. A cyclizing arylalkoxycarbonylation of N-aryl acrylamides catalyzed by iron(II) chloride tetrahydrate has been presented by Li, Du, and co-workers (Scheme 426).736 Alkoxycarbonyl radicals were generated from the corresponding carbazates. This protocol led to the synthesis of a series of 3-alkoxycarbonylmethyl3-methyloxindoles in good yields. The same transformation was independently reported by Yu and co-workers.737 Their method utilizes ethyl acetate as solvent and requires addition of 4-cyanopyridine as ligand. The research group of Zhu developed a procedure for a cyclizing 3261

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Scheme 429

Scheme 427

Scheme 430

Scheme 428

arylcarboxylation of 2-isocyanobiphenyls (Scheme 427).738 The substrates were reacted with carbazates as alkoxycarbonyl radical precursors in the presence of catalytic amounts of Fe(acac)2 and tert-butyl hydroperoxide as stoichiometric oxidant. Using this method, a series of phenanthridine-6-carboxylates was synthesized in high yields. The reaction probably proceeds via addition of the alkoxycarbonyl radical to the isocyanide and subsequent radical aromatic substitution. 3.7. Ring Opening Reactions

3.7.1. SN-Type Reactions. Nucleophilic ring opening of oxiranes is a widely applied method in synthetic chemistry to set up CC and CHet bonds. The reaction can be catalyzed by Lewis acids. A first attempt to introduce iron(II) salts as Lewis acidic catalysts was reported by Yamashita.739 In this first study, even the enantioselective ring opening of meso-oxiranes was studied with various metal(II) D-tartrates. However, iron(II) D-tartrate gave only a moderate yield and almost no chiral induction. Very good yields could be achieved for the alcoholysis of oxiranes catalyzed by iron(III) chloride740 and for the ring opening of oxiranes with different nucleophiles in the presence of FeCl3 3 6H2O absorbed on silica gel.741 The enantioselective ring opening of meso-oxiranes could be significantly improved by Ollevier et al.742,743 They treated meso-styrene oxides with various anilines in the presence of catalytic amounts of Fe(ClO4)2 3 6H2O and Bolm’s chiral bipyridine ligand744 to obtain chiral β-amino alcohols in very good yields with excellent enantioselectivities (Scheme 428).

Similarly, meso-N-aryl aziridines could be ring opened by a variety of amines to give trans-1,2-diamines in excellent yields (Scheme 429). The reaction was catalyzed by a dicationic iron complex generated in situ from FeCl2(mep) and AgSbF6.745 A ring opening of styrene oxides by anilines was part of a domino process leading to 3-arylquinolines in fairly good yields (Scheme 430).746 Two equivalents of styrene oxides were required to react with anilines in the presence of substoichiometric amounts of iron(III) chloride. One equivalent of styrene oxide undergoes a CC cleavage reaction contributing only one C atom to the quinoline product. The rest of the molecule is transformed to benzaldehyde. A mechanism for this unusual transformation was proposed by the authors as shown in Scheme 431. It involves an Fe(I) Fe(III) redox couple as catalytically active species. Nucleophilic ring opening of the iron(III)-activated oxirane by the aniline nitrogen atom provides a β-hydroxyethylaniline which is dehydrated to the corresponding enamine. Nucleophilic attack of the latter at a second molecule of iron(III)-activated styrene oxide leads to a first CC bond formation under regeneration of an enamine moiety. β-Hydride elimination oxidizes the alkoxide to the corresponding ketone, releasing a dichloro(hydrido)iron(III) species which on reductive elimination forms an iron(I) complex. Subsequent activation of an aryl CH bond ortho to the aniline nitrogen atom occurs by oxidative addition to the low-valent iron(I) species generating an iron(III) complex. The iron(III) complex is attacked by the carbonyl group forming a Lewis acidLewis base complex which undergoes a hydride shift. The nine-membered ferracycle can extrude one molecule of benzaldehyde in an elimination process. Examples for aldehyde insertions into carbonmetal bonds have been reported in the literature for a series of metals.747751 Thus, the present reverse process for an iron(III) alkoxide appears to be reasonable. Reductive elimination of FeCl from the seven-membered ferracycle leads to CC bond formation and provides the dihydroquinoline which is finally aromatized in the presence of air. γ,δ-Epoxy-α,β-unsaturated esters and amides were treated with Grignard reagents to give homoallyl alcohols in good yields 3262

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Chemical Reviews Scheme 431

REVIEW

Scheme 433

Scheme 434

Scheme 432 Scheme 435

as single diastereoisomers (Scheme 432).752 The nucleophile attacks regioselectively at the allylic epoxide carbon with inversion of the configuration at this center. This observation was explained by the formation of an intermediate η3-allyliron complex which stereoselectively transfers an alkyl or aryl ligand to the terminus of the allyl ligand by reductive elimination. 3.7.2. SN0 -Type Reactions. A review on SN20 reactions with Grignard reagents including iron-catalyzed ring openings of propargyl epoxides was provided by Kar and Argade.753 In this field, F€urstner et al. developed an efficient iron-catalyzed ring opening addition of alkynyl oxiranes with Grignard reagents providing 2,3-allenols in high yields (Scheme 433).754 The synallenols were formed with moderate to good diastereoselectivity. The approach is complementary to the related transformation of organocopper reagents with alkynyl oxiranes leading to antiallenols as major products. This method was successfully applied to the first total synthesis of the marine natural product amphidinolide X (Scheme 434) and its biogenetic precursor amphidinolide Y. Moreover, the

non-natural 19-epi-amphidinolide X was synthesized demonstrating the flexibility of this synthetic approach.755,756 During the course of the synthetic sequence, the central chirality of an oxirane, obtained by either Sharpless or Shi epoxidation, was transferred to the axially chiral allene and finally to another centrally chiral C(sp3) center which ends up as the tetrasubstituted C-19 carbon atom of the product. Grignard reagents could also be added to activated vinylcyclopropanes (Scheme 435).757 A low-valent iron catalyst generated from Fe(acac)3 and the alkyl Grignard species enables the regioselective attack of the nucleophile at the double bond in an SN20 fashion. In the absence of the catalyst, a rapid 1,2-addition of the Grignard reagent was observed instead. Good diastereoselectivity in favor of the E-olefin was achieved. The authors provided hints that a direct nucleophilic addition mechanism is preferred over radical or η3-allyliron based pathways. The intermediate magnesium enolate was treated with propargyl or allyl bromide to give dialkylated malonates. 3263

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Chemical Reviews Scheme 436

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Scheme 438

Scheme 439 Scheme 437

Irie and co-workers demonstrated that iron oxide hydroxide is an efficient catalyst for the ring opening of 5,6-disubstituted 7-oxabicyclo[2.2.1]hept-2-enes with acetyl bromide (Scheme 436).758 This transformation initially forms cyclohexenes with two leaving groups, which could be further transformed to 5,6-meso-cyclohexa1,3-dienes via a zinc mediated reductive 1,4-elimination. Plietker and co-workers applied the nucleophilic ferrate Bu4N[Fe(CO)3(NO)] (TBAFe) as catalyst for the ring opening of vinylcyclopropanes.759 As nucleophiles they introduced methylene compounds such as malononitrile, ethyl 2-cyano-2phenylacetate, 2-cyanocyclopentanone, and an oxazol-5(4H)one (Scheme 437). The addition products were obtained in good to excellent yields with high preference for the SN20 -derived linear isomer over the branched isomer obtained by 1,2-addition. Moreover, a pronounced diastereoselectivity in favor of the E-configured olefin was obtained in all cases. In contrast to the addition of Grignard reagents to vinylcyclopropanes reported by F€urstner (cf. Scheme 435),757 this reaction probably proceeds via η3-allyliron intermediates. These intermediates can also undergo a formal [3 + 2] cycloaddition with Michael acceptors or tosylimines providing vinylcyclopentanes or vinylpyrrolidines, respectively. The practicability of the traceless allylation of activated methylene compounds in the presence of TBAFe was demonstrated by introduction into two different reaction sequences. First it was combined with Plietker’s standard allylic substitution to extend the carbon chain. The second sequence was completed by a nucleophilic ring opening of an azlactone that played the role of the nucleophile in the traceless allylation. Treatment of 2-phenyl- or 2,2-diphenylcyclopropyl acetylene with water in the presence of catalytic amounts of iron(III) triflimide in dioxane at 80 °C led to homoallenyl alcohols and vinyl allenes (Scheme 438).760 In contrast to the corresponding

gold(I)-catalyzed process forming cyclopropyl methyl ketones, the intermediate iron alkyne complexes display a profound vinyl carbocationic character which enables the observed ring opening of the cyclopropyl moiety. Sawama, Sajiki, and co-workers established an iron-catalyzed method for the ring opening of tetrahydrofuran, 1,3-dihydro-2benzofuran (phthalane), and lactone derivatives with trimethylsilyl azide (Scheme 439, eq a).761 These reactions were catalyzed by iron(III) chloride and proceeded under mild conditions in dichloromethane at room temperature giving azidated linear alcohols, benzyl alcohols, or carboxylic acids in high yields. Moreover, allyltrimethylsilanes could be employed as nucleophiles as well providing ring-opened CC coupling products (eq b). The alcoholysis of styrene oxide has been efficiently performed on sulfur- and iron-co-doped [TiNbO5] nanosheets at room temperature.762 Excellent yields of β-alkoxyalcohols were achieved for this transformation. Moreover, the catalyst could be easily recycled. In the fourth run it still afforded yields up to 98%. Magnetic Fe3O4 nanoparticles have been applied as recyclable catalysts for the ring opening of epoxides with amines.763 The corresponding β-amino alcohols have been obtained under neat conditions with high stereo- and regioselectivity and in high yields. 3.8. Intermolecular Ring Expansions

The achievements in the field of either intermolecular or intramolecular iron-catalyzed ring expansion reactions have been summarized by Hilt and Janikowski in 2008.764 In 2005, this group described a ring expansion of epoxides with olefins to form tetrahydrofurans (2458% yield) in a chemo- and regioselective fashion.765 The transformation was achieved in the presence of 20 mol % FeCl2(dppe), zinc, and triethylamine. A second method was described replacing the FeCl2(dppe) by a mixture of 20 mol % each of iron(II) chloride, triphenylphosphine, and an NHC ligand. The corresponding tetrahydrofurans were obtained

3264

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Scheme 442

Scheme 441

Scheme 443

in moderate yields (3750%) as mixtures of cis and trans isomers. In subsequent publications, the efficiency and scope of this transformation could be considerably improved.766,767 An in situ generated ironsalen complex was the key to this improvement. A wide variety of acceptor-substituted acyclic alkenes and cyclic dienes reacted with styrene oxides (Scheme 440). The reaction was chemoselective for acceptor- and aryl-substituted double bonds, while other double bonds and triple bonds stayed intact. Employing unsymmetrical alkenes, a pronounced regioselectivity was observed. However, the diastereoselectivity was only low to moderate in most cases. An iron(III) chloride catalyzed reaction of aziridines with terminal alkynes afforded 2-pyrrolines in mostly good yields (Scheme 441).768 The reaction proceeded under mild conditions in nitromethane at 20 °C. In addition, it could be combined with a hydrolytic ring opening to furnish γ-amino ketones in a one-pot protocol. The ring expansion of vinylcyclopropanes was achieved by treatment with electron-deficient olefins in the presence of the low-valent iron complex TBAFe and an NHC ligand (Scheme 442).759 Highly substituted cyclopentanes were obtained by this method (eq a). In addition, imines could be used instead of olefins leading to pyrrolidines (eq b). A short time later, Li, Wang, and co-workers developed a ring expansion of vinyl or aryl-substituted dimethyl cyclopropane-1,1dicarboxylates with aryl isothiocyanates (Scheme 443).769 The reaction proceeded in dichloromethane at room temperature and was promoted by iron(III) chloride as Lewis acid. The corresponding pyrrolidine-2-thiones were obtained in good yields. Based on their results for the iron-catalyzed ring expanding aminohydroxylation of olefins,770 Yoon and co-workers were able to extent this protocol to an asymmetric reaction (Scheme 444).771 Thus, they treated terminal olefins with oxaziridines in the presence of catalytic amounts of Fe(NTf2)2 in combination with a chiral bis(oxazoline) ligand and magnesium oxide as additive in benzene

Scheme 444

Scheme 445

at 0 °C to room temperature. This protocol provided access to the corresponding 1,3-oxazolidines in a highly regioselective, diastereoselective (cis product), and enantioselective (8595% ee) fashion. The reaction of various heterocumulenes with aziridines has been described by Punniyamurthy et al. using iron(III) nitrate nonahydrate as catalyst (Scheme 445).772 It should be noted that this transformation was performed “on water” as the organic reaction partners are not soluble in water. This method provided 3265

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Scheme 447

REVIEW

CO2 pressure of 2 bar to afford cyclic carbonates in high yields (eq a).774 For internal epoxides it was disclosed that the catalyst/ cocatalyst ratio influences the diastereoselectivity of the reaction. Thus, it was possible to develop conditions which enabled the selective formation of both cis- and trans-configured cyclic carbonates starting either from cis-2,3-epoxybutane (eqs b and c) or trans-2,3-epoxybutane (not shown). Other trans-epoxides could be converted selectively to trans-configured cyclic carbonates in excellent yields (eq d). Similar conditions were also employed for the synthesis of cyclic and polymeric cyclohexene carbonates.776 Depending on the catalyst and cocatalyst and their ratio, selective formation of the cyclic or the polymeric product could be achieved. The reaction of styrene oxide with carbon dioxide to the cyclic carbonate has also been performed using various iron(II) and iron(III) salicylaldimine, thiophenaldimine, and quinolinaldimine complexes as catalysts.777 The procedure features solvent-free conditions, tetrabutylammonium bromide as cocatalyst, and a relatively low CO2 pressure of 5 bar. Turnover numbers up to 2071 and turnover frequencies up to 209 h1 have been achieved with these catalyst systems. D€oring and co-workers introduced cationic phenylenediaminebased tetradentate iron(II) and iron(III) complexes for the synthesis of cyclic carbonates from epoxides and carbon dioxide.778 In the presence of neutral iron complexes as catalysts, TBAB had to be added (cf. Scheme 447) as the bromide plays the role of the nucleophile. However, using the cationic phenylenediamine-based iron(III) complex, no additive was required as the counteranion acted as nucleophile. For several epoxides an almost complete conversion and turnover numbers of about 500 were achieved. 3.9. Polymerization

access to 2-imino-substituted selenazolidines, imidazolidines, oxazolidines, and thiazolidines in good to high yields. Iron(III) chloride on alumina has proven to be an efficient catalyst for the formal [3 + 2] ring opening annulation of donoracceptor-substituted aminocyclopropanes with aldehydes (Scheme 446).773 This protocol provided efficient access to 2-aminotetrahydrofurans. Kleij and co-workers investigated the formation of cyclic carbonates by formal cycloaddition of carbon dioxide to terminal and internal epoxides (Scheme 447).774,775 Iron amine triphenolate complexes functioned as catalysts in combination with tetrabutylammonium bromide (TBAB) or iodide (TBAI) as additives. Terminal epoxides were converted under mild conditions in methyl ethyl ketone (MEK) at room temperature with a

Polymerization and polycondensation reactions play an important role in the chemical industry and are the basis for billion dollar sales figures. Thus, the search for new efficient, selective, environmentally benign, and low cost catalysts has been permanently on the agenda of industrial and academic research. Seminal contributions have been reported at the end of the 1990s with work on iron-catalyzed conventional alkene polymerizations by Brookhart779,780 and Gibson,781 and atom transfer radical polymerization (ATRP) processes by Sawamoto782 and Matyjaszewski.783 Further achievements in this field until 2004 have been documented by Bolm et al.1 Bianchini and Giambastiani summarized the developments in olefin polymerization catalyzed by iron and other late transition metals in 2006 and 2010, respectively.784,785 Bis(imino)pyridine complexes of iron and other metals and their performance in olefin polymerization have been the subject of reviews and book chapters by Gibson, Redshaw, and Solan in 2007 and 2010786,787 and Li and Gomes in 2011.788 A recent review on ironNHC complex catalysis also includes a chapter on polymerization reactions.15 Redshaw and Sun summarized recent developments in the field of iron complexes as catalysts for the oligomerization and polymerization of ethylene in a perspective article.789 The development of nonbis(imino)pyridine ligands for the iron-catalyzed oligomerization of ethylene has been summarized recently by Olivier-Bourbigou and coworkers.790 3.9.1. Olefin Polymerization. Due to the large number of review articles available on this subject (see above), the reader is referred to those for a comprehensive coverage. In the following section selected examples of the past decade are presented. 3266

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REVIEW

Scheme 448

Figure 2. NNP tridentate α-diimino(phosphine)iron complex.

Figure 3. Iron bis(iminoethyl)pyridine complexes with pendent alkenyl groups.

Scheme 449

An efficient polyethylene chain growth on zinc catalyzed by a bis(imino)pyridine iron/methylalumoxane (MAO) catalyst system has been investigated by Gibson et al. (Scheme 448).791,792 A fast and reversible exchange of the growing polymer chains between the zinc and the iron centers has been observed. After hydrolysis a Poisson distribution of linear alkanes was obtained. Alternatively, displacement of the polymer chains from Zn(polymer)2 via a nickel catalyzed displacement reaction led to linear α-olefins. Chirik and co-workers described cationic bis(imino)pyridine iron(II) alkyl complexes to catalyze olefin polymerization efficiently even without addition of methylalumoxane (MAO) (Scheme 449).793 This procedure provided polyethylene with a higher molecular weight and lower polydispersity than conventional methylalumoxane-activated catalysts. Iron(II) alkyl cations were discussed as propagating species. Herrmann and co-workers synthesized and tested a series of bis(imino)pyridine and monoiminoacetyl iron(II) complexes as catalysts for the oligomerization and polymerization of ethene and propene in the presence of modified methylaluminoxane (MMAO).794 With increasing bulk of the ortho-position of the N-aryl ring, an increase in molecular weight of the polymers was observed. A theoretical method for the prediction of the degree of ethylene oligomerization by bis(imino)pyridine iron catalysts has been developed by the group of de Bruin.795 Very active nonbis(imino)pyridine iron complexes for the oligomerization of ethylene have been reported by Small et al.796,797 High product purities of 1-hexene and 1-octene have been

Scheme 450

achieved. The highest turnover number of about 2.5  106 mol of C2/mol of Fe was reported for a NNP tridentate α-diimino(phosphine)iron complex (Figure 2). Activities in the range 104106 g (mol h atm)1 for nonbis(imino)pyridine iron complexes are still below the values originally obtained for bis(imino) pyridine systems [108 g (mol h atm)1] by Brookhart.779,780,790 Erker and co-workers prepared cobalt and iron bis(iminoethyl)pyridine systems with pendent alkenyl groups which functioned in combination with MAO as ZieglerNatta catalysts for ethene polymerization (Figure 3).798 The iron complexes were more active than the cobalt catalysts and gave mixtures of linear polyethylene and low molecular weight oligoethylenes. Polymerization of 1,3-dienes in the presence of trialkylaluminum and triphenylcarbenium tetrakis(pentafluorophenyl)borate as alkylating and dealkylating reagents, respectively, and iminopyridineiron complexes as catalysts has been reported by Ritter et al. (Scheme 450).799 According to the authors, the configuration of the polymers (E-1,4- or Z-1,4-polymers) could be controlled by the ligand (cf. eqs a and b). Polyisoprene of high 3267

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REVIEW

Scheme 452

Figure 4. 2,8-Bis(arylimino)-5,6,7,8-tetrahydroquinoline iron complex.

Scheme 451

molecular mass was synthesized from isoprene in excellent yield and good polydispersity. Analogous reactions with myrcene and farnecene demonstrated that 1,3-dienes can be polymerized chemoselectively in the presence of other isolated double bonds under these conditions. Several 2,8-bis(arylimino)-5,6,7,8-tetrahydroquinoline iron complexes have been synthesized by Redshaw, Hu, Sun, and co-workers (see example in Figure 4).800 After treatment with MAO or MMAO they showed high activity as catalysts for the polymerization of ethylene at 50 °C. Polypropylene with molecular weights up to 203 000 g/mol could be obtained with a catalyst activity of up to 2.4  107 g of (PE)/(mol of [Fe] h atm). 3.9.2. ATRP. The atom transfer radical polymerization (ATRP) has become a powerful tool for the construction of well-defined polymeric and oligomeric material.801805 An ATRP relies on the equilibrium between active radical intermediates and their dormant halide form mediated by a metal redox couple. Cu(I)/Cu(II) has been employed most frequently for this purpose. The first iron catalyzed ATRPs have been reported independently by Sawamoto782 and Matyjaszewski783 in 1997. The general mechanism of the ATRP, activator generated by electron transfer (AGET) ATRP, and reverse ATRP with an Fe(II)/Fe(III) couple, is outlined in Scheme 451. The three

methods differ only in the initiation step. In conventional ATRP the metal catalyst is introduced in its low oxidation state, in the present case an iron(II) complex. For AGET ATRP, the iron(II) species is generated by one-electron reduction of an iron(III) complex. Subsequently, the iron(II) complex abstracts a halogen atom from the organohalide with concomitant oxidation to an iron(III) species. An initiating radical is formed which adds to the monomer affording the corresponding alkyl radical bearing the initiator end group. For reverse ATRP the propagation radical is formed as in a conventional radical polymerization using standard radical initiators like AIBN. The propagating step, responsible for the controlled character of the ATRP, is identical for all three methods (Scheme 451). During this step the initially formed radical will add to a monomer molecule to form the extended polymer radical, which is identical to the propagation step of common radical polymerization. However, in ATRP this chain extending reaction competes with the abstraction of a halogen atom from the iron(III) halo complex leading to a polymeric halide as a dormant species. The dormant species can be reactivated by the reverse process. A fast equilibrium between active radical and dormant polymeric halide lies preferably on the side of the dormant species and keeps the concentration of the active radical low. This prevents chain termination and accounts for a narrow polydispersity. Besides the three basic ATRP methods outlined above, several variants, such as simultaneous reverse and normal ATRP (SR&NI), initiators for continuous activator regeneration (ICAR) ATRP, activator regenerated by electron transfer (ARGET) ATRP, and others, have been developed but will not be discussed in detail herein.803 In continuation of ironcatalyzed ATRP summarized in the previous review by Bolm,1 recent examples for this transformation are presented below. Ibrahim et al. investigated the ATRP of methyl methacrylate (MMA) with ethyl 2-bromoisobutyrate as initiator and iron(II) catalysts with tetradentate nitrogen ligands based on quinolinesubstituted 1,2-ethylenediamines.806 Increasing sterical demand of the ligand reduced the rate of the polymerization and increased the polydispersity. Gibson and co-workers employed several pentacoordinated iron(II) complexes with tridentate nitrogen ligands for the ATRP of styrene.807 They found an order of reactivity for the donor groups in the sequence alkylamine ∼ pyridine > alkylimine . arylimine > arylamine. The latter proved to be almost inactive for ATRP. The polymerization activity could be correlated with the redox potential of the corresponding iron complexes. Due to steric hindrance, the pentacoordinated iron(II) complexes which are in equilibrium with the hexacoordinated oxidized iron(III) counterparts were found to be less efficient than tetra-/ pentacoordinated species. Reverse ATRP of n-docosyl acrylate in DMF at 80 °C has been achieved using a bipyridine (BPY)/iron(III) chloride catalyst system in the presence of carbon tetrabromide and 3268

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Chemical Reviews 2,20 -azobis(isobutyronitrile) (AIBN) or benzoyl peroxide (Scheme 452).808,809 The living character of this process was indicated by the narrow polydispersity and by the linear relationship between molecular weight of the polymer and the conversion. A reverse ATRP of n-hexadecyl methacrylate has been described using the same initiating system CBr4/FeCl3/BPY in the presence of AIBN in DMF at 80 °C.810 The FeCl3/BPY catalytic system was also efficient for the conventional ATRP and reverse ATRP of n-hexadecyl methacrylate. The latter was performed with AIBN as radical initiator to provide a high molecular weight (77 500 g mol1) and relatively low Mw/Mn (down to 1.18) poly(n-hexadecyl methacrylate).810 Molecular weight data and kinetic studies revealed the living/controlled character of both processes. Analogously, a reverse ATRP of n-docosyl acrylate could also be performed with the FeCl3/BPY catalyst system in the presence of either AIBN or benzoyl peroxide.809 With AIBN, much higher Mn values could be achieved compared to the conventional ATRP, whereas benzoyl peroxide led to an uncontrolled polymerization. Tris(1,6-dioxaheptyl)amine was proven to be an active ligand for the iron-catalyzed ATRP of styrene using (1-chloroethyl)benzene, (1-bromoethyl)benzene, or tosyl chloride as initiators.811 Better control was achieved with iron(II) bromide as catalyst compared to iron(II) chloride. A higher polymerization rate but less control over polydispersity was observed for the tosyl chloride initiator in comparison to (1-chloroethyl)benzene and (1-bromoethyl)benzene. The catalytic system iron(II) chloride/hexamethylphosphoric triamide proved to be efficient for the ATRP of MMA using ethyl 2-bromoisobutyrate as initiator.812 Molecular weight distributions (Mw/Mn) lower than 1.20 were reported. Grassi and co-workers described an ATRP of styrene using bis(oxazoline) iron complexes Fe(box)Cl2 as catalysts and (1-bromoethyl)benzene as initiator.813 The reaction showed the characteristics of an ATRP but the polydispersity (1.401.58) revealed only modest control. More interestingly, Fe(box)Cl3 was employed as the first iron(III) catalyst for a reverse ATRP and gave effective control in styrene polymerization in the presence of 1,1,2,2-tetraphenyl-1,2-ethanediol as initiator. 2-(Diphenylphosphino)pyridine (DPPP) and 2-[(diphenylphosphino)methyl]pyridine (DPPMP) have been successfully employed as ligands for the iron-catalyzed ATRP of MMA and styrene with various initiators and solvents.814,815 Using DPPP as ligand, optimum conditions were found for the following ratio of [monomer]/[initiator]/[catalst]/[ligand] = [MMA]0/ [ethyl 2-bromoisobutyrate]0/[FeBr2]0/[DPPP]0 = 200:1:1:2 in p-xylene at 80 °C. Chain extension experiments proved the controlled nature of the polymerization. Good control on the polymerization of styrene was achieved by using the (1-bromoethyl)benzene/FeBr2/DPPMP system in DMF at 110 °C. A reverse ATRP of MMA using iron(III) bromide in combination with DPPP or DPPMP and AIBN as initiator was successfully carried out in toluene at 80 °C. An ionic iron complex as catalyst for the ATRP of styrene and MMA has been presented by Nagashima and co-workers.816 Most notably, the catalyst could be recovered and reused repeatedly without loss of activity. Various bisimine, diamine, and diphosphine ligated iron complexes have been studied as catalysts for the ATRP of styrene, methyl acrylate, and MMA.817,818 Due to suitable reversible oneelectron redox couples, alkylimine-based iron catalysts showed

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a good control in ATRP. In contrast, complexes with arylimine ligands are less reducing and showed redox couples with larger peak-to-peak separations in cyclic voltammetry. Gibson et al. have investigated a series of new α-diimine iron(II) complexes as catalysts for the polymerization of styrene.817 The α-diimine iron complexes with alkylimine substituents lead to in situ generated Fe(III) high-spin complexes (S = 5/2) and promote a controlled polymerization of styrene following an ATRP mechanism. In contrast, α-diimine iron complexes with arylimine substituents generate an iron(III) center with an intermediate-spin state (S = 3/2) and induce a polymerization of styrene and MMA via catalytic chain transfer (CCT) mechanism. Phosphine complexes were the strongest reductants in the series followed by the amines and then the imines. However, due to poor reversibility, they behaved as poor catalysts for ATRP. Sawamoto optimized the iron catalyst and the conditions for the ATRP of MMA and found that within a series of phosphine ligands the more basic compounds gave higher reaction rates and conversions.819 A low concentration of the catalyst also enabled a narrow polydispersity. Thus, tri-n-butylphosphine was identified to give the most active complex. In addition, the effect of the halide in the initiator and the iron complex were investigated. Combinations with the same halide in initiator and complex were superior over mismatched combinations. Bromide was found to be much better compared to the chloride initiated system. Thus, the complex FeBr2(PBu3)2 in combination with the bromide initiator [(MMA)2Br] exhibited the best performance in terms of activity (>90% conversion) and control (Mw/Mn = 1.21.3). The living character could also be demonstrated by block copolymerization with n-butyl methacrylate. The catalytic system consisting of iron(II) bromide and diphenyl-2-pyridylphosphine was employed for the ATRP of MMA.820 Several parameters of the reaction have been varied, such as initiator, solvent, and temperature. For most processes a linear increase of the number-average molecular weight and a low molecular weight distribution (Mw/Mn = 1.21.4) revealed the controlled character of the polymerization. A reverse ATRP of acrylonitrile could be achieved using iron(III) chloride coordinated by triphenylphosphine.821 The best control of molecular weight and distribution was achieved using a molar ratio of 1:2 of metal to ligand. The living character could be proven by using the obtained polyacrylonitrile as macroinitiator for a conventional CuCl2/2,20 -bipyridine-catalyzed ATRP of the same monomer. Noh and co-workers investigated the ATRP of MMA, methyl acrylate, n-butyl methacrylate, and styrene using iron(III) salts in combination with phosphine ligands as catalysts.822825 It is noteworthy that the reaction proceeded in the absence of any additional reducing agents and radical initiator but the phosphine ligands. Chain extension experiments and narrow polydispersity confirmed the living character of this polymerization. An activator generated by electron transfer (AGET) ATRP of styrene and MMA using iron(III) chloride, an alkyl chloride, and tri-n-butylphosphine was described by Kamigaito and coworkers.826 The phosphine ligand functioned as reducing agent in this system. Phosphazenium halides have been explored as cocatalysts for the iron(II)-catalyzed ATRP of MMA giving polymers with controlled molecular weights and narrow molecular weight distribution (Mw/Mn < 1.2) (Figure 5).827 It was assumed that an anionic complex is formed by mixing equimolar amounts of phosphazenium halides and iron(II) salts. 3269

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Scheme 453

Figure 5. Phosphazenium halides.

The catalyst could be removed almost quantitatively simply by washing with water (metal residue < 5 ppm). Matyjaszewski and Wang presented a method for the ligandfree ATRP of MMA.828 Well-controlled polymerization was achieved using iron(II) bromide as catalyst in polar solvents such as NMP, DMF, and acetonitrile which can dissolve the iron salt. Further improvement of the molecular weight distribution was accomplished by addition of small amounts of iron(III) bromide. Spectroscopic measurements during the iron-catalyzed ATRP of MMA in NMP revealed that KATRP decreases by about 3 orders of magnitude from MMA bulk polymerization to MMA polymerization in the presence of 92 mol % NMP.829 This observation may be attributed to the formation of catalytically less active Fe(II)/NMP species. Nonpolar solvents are suitable media for the iron(II) bromide catalyzed ATRP of MMA when tetrabutylammonium triflate is added.830 The weakly coordinating triflate anion helps to dissolve the iron salt and has no influence on its catalytic effect. The presence of iron(0) wire as a supplemental activator and reducing reagent (SARA) allowed introduction of only parts per million amounts of iron(II) bromide for the ATRP of MMA.831 The same beneficial effect of iron(0) on the catalyst concentration was observed for the ATRP of MMA with AIBN as initiator for continuous activator regeneration (ICAR). In an independent work, Cheng and Zhu also described a catalyst concentration of 34 ppm for the ICAR ATRP of MMA using iron(III) chloride hexahydrate in combination with triphenylphosphine as ligand, 1,4-bis(2-bromo-2-methylpropanoyloxy)benzene [(BMP)2B] as initiator, and AIBN as thermal radical initiator.832 The group of Matyjaszewski could reduce the catalyst loading to 5 ppm iron(III) bromide for the ICAR ATRP of styrene.833 1,10 -Azobis(cyclohexanecarbonitrile) (ACHN) was employed as thermal radical initiator for this polymerization conducted in anisole at 90 °C. An efficient iron-catalyzed ATRP of styrene with various triarylphosphine ligands has been reported by Matyjaszewski et al.834 The reducing ability of the phosphines allowed introduction of the iron catalyst in its high oxidation state, namely iron(III) bromide. The activity of the catalyst was enhanced with increasing electron-donating properties of the phosphine ligands. Similarly, iron(III) chloride or bromide in combination with phosphine or phosphite ligands proved to be effective for the ATRP of MMA without external initiator and reducing agent.835 A relatively narrow molecular weight distribution of benzylic > etheral > 3° C > 2° C . 1° C). Moreover, competing aziridination was much more suppressed in the iron system (insertion/aziridination > 20:1) as compared to a rhodium catalyst. In substrates with multiple allylic positions the reaction proceeded preferentially at the electron-rich, nonhindered olefinic sites. In general, the cyclic sulfamates could be obtained in good yields. Che and co-workers employed a heptacoordinated iron quinquepyridine complex for the intramolecular amination of preferentially benzylic C(sp3)H bonds by sulfamic acid esters (Scheme 517, eq a).930 In addition, examples for the insertion into tertiary and secondary aliphatic CH bonds have been presented as well. The aminating species was generated by reaction of the sulfamic acid esters with diacetoxyiodobenzene. This protocol allowed the synthesis of a variety of cyclic sulfamidates in high yields. The amination of benzylic, allylic, and tertiary and secondary aliphatic CH bonds was achieved with the same catalyst and (tosyl- or nosyliminoiodo)benzene as nitrogen source (eq b). Iron imide/nitrene complexes were proposed as catalytic intermediates and identified by high resolution mass spectrometry. A (dipyrromethene)iron complex has been demonstrated to catalyze the amination of toluene with adamantyl azide to give adamantylbenzylamine in 95% yield.931 A turnover number (TON) of about 10 was achieved with this catalyst at 60 °C. An isolated high-spin (S = 2) iron(III) complex with an imido radical ligand was discussed as the nitrene delivering species. It could be demonstrated that this complex is able to aminate toluene by CH insertion. Aliphatic azides having benzylic, allylic, or tertiary CH bonds in the 4-position could be readily cyclized under the influence of a

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Scheme 517

Scheme 518

sterically congested iron(II) dipyrrinato complex (Scheme 518).932,933 Thus, substituted pyrrolidines were obtained in good to excellent yields. Secondary and primary CH bonds reacted less efficiently, leading only to low yields of the cyclized products. A mechanism was proposed that includes the formation of a high-spin iron(III) imido species with significant radical character (Scheme 519). This radical may follow a stepwise pathway by hydrogen abstraction and radical recombination or alternatively a direct CH insertion of the nitrogen to provide the pyrrolidine iron(II) complex. Subsequent reaction with di-tertbutyldicarbonate releases the protected pyrrolidine and regenerates the catalytically active iron species. The intramolecular CH amination of benzenesulfonyl azides to sultams has been achieved using engineered natural P450 enzymes (Scheme 520).934 Arnold and co-workers demonstrated that the mutant P411BM3CIS-T438S effected this reaction in vitro with a total turnover number (TTN) of 383 and ee values up to 73% (eq a).935 The group of Fasan relied on the mutant P450BM3FL#62 as catalyst for this transformation.936 In contrast to a suggestion by Arnold, they could prove that cysteineheme ligated P450 enzymes can also be effective catalysts in this system. 3283

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Scheme 522

Scheme 523

Scheme 524

Scheme 520

Scheme 521

Liu and co-workers employed an iron(II) complex with the pentadentate N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine (N4Py) ligand for the intermolecular amination of benzylic and α-OR-substituted C(sp3)H groups using bromamine-T as nitrene source.937 The resulting alkylated tosylamides were formed in high yields under mild conditions in acetonitrile at room temperature.

4. ELIMINATION Beller et al. described an iron-catalyzed dehydration of amides to afford nitriles (Scheme 521).938 They employed the iron cluster [Et3NH][HFe3(CO)11] as catalyst for this transformation and diethoxy(methyl)silane as dehydrating agent. The utility of this method was demonstrated by the synthesis of various nitriles in high to excellent yields.

The same transformation could be realized using N-methylN-(trimethylsilyl)trifluoroacetamide as dehydrating agent and iron(II) chloride tetrahydrate as catalyst.939 Under mild conditions (THF, reflux), reaction of a broad range of substrates provided nitriles in mostly excellent yields. Li and co-workers demonstrated that fructose and sucrose can be dehydrated to provide 5-(hydroxymethyl)furfural in the presence of iron(III) choride in combination with tetraethylammonium bromide as catalytic system (Scheme 522).940 Thus, 5-(hydroxymethyl)furfural could be obtained in 86% yield by treating fructose in NMP at 90 °C in the presence of the catalyst. Subsequently, the dehydration of xylose was described in the biphasic system H2O/2-methyltetrahydrofuran (2-MTHF) using iron(III) chloride hexahydrate as catalyst and sodium chloride as additive.941 The resulting furfural was directly extracted into the 2-MTHF layer. The process could be combined with the fractionation of lignocellulose, demonstrating its potential for biorefinery systems. More recently, polymer-supported NHC iron(III) complexes have been reported to catalyze the dehydration of fructose.942 5-(Hydroxymethyl)furfural was obtained in 73% yield at 97% conversion. Iron(II) chloride in combination with 1,5-bis(diphenylphosphino)pentane (DPPPent) has been identified as a useful catalyst for the formal decarbonylation/dehydration reaction of carboxylic acids (Scheme 523).943 Potassium iodide and acetic anhydride were required in stoichiometric amounts. The reaction proceeded under rather drastic conditions at 250 °C in carbon monoxide atmosphere to provide terminal olefins in high yields. Small amounts of internal olefins were identified as byproducts. Propargylic alcohols could be dehydrated using iron(III) chloride in combination with 1,2,3-triazoles as catalytic system (Scheme 524).944 A variety of conjugated enynes could be synthesized by this method in high yields. 3284

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Chemical Reviews Scheme 525

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Scheme 527

Scheme 528 Scheme 526

5. REACTIONS AT CARBONYL GROUPS AND ANALOGUES For a long time, the Lewis acidity and also the Brønsted acidity of many iron salts and complexes have been exploited for the activation of carbonyl groups. A review on iron-catalyzed addition reactions to carbonyl groups has been compiled recently by Christoffers et al.945 5.1. Addition to Aldehydes, Ketones, and Heteroanalogues

5.1.1. Aldol-Type Reactions. An iron-catalyzed asymmetric Mannich-type reaction has been realized by Kobayashi and coworkers (Scheme 525).946 A ketene silyl acetal was treated with an imine in the presence of iron(II) chloride, (R)-3,30 -I2-BINOL as ligand, and ethyldiisopropylamine as base to afford β-aminoesters in high yields and with moderate enantiomeric excess. The same group also developed a protocol for an ironcatalyzed diastereoselective Mukaiyama-type aldol reaction in water (Scheme 526).947 Iron(III) chloride was found to be a useful Lewis acid catalyst for this transformation in the presence of a surfactant. An asymmetric variant was reported first by Mlynarski et al. in 2006.948 Reaction of aldehydes with silyl enol ethers in the presence of iron(II) chloride and a chiral bis(oxazoline) ligand in a mixture of ethanol and water at 0 °C provided the aldol products in high syn-selectivity and moderate enantioselectivity. In a subsequent work, the authors improved the efficiency of this transformation by employing a sterically hindered PyBox ligand (Scheme 527).949 This procedure led to excellent yields and also high enantioselectivities. A related zinc-based system was even more active and represented the most efficient catalyst for the aqueous asymmetric Mukaijama aldol reaction at that time. Ollevier et al. have used a chiral iron(II) bipyridine catalyst for the asymmetric Mukaiyama aldol reaction (Scheme 528).743,950 The reaction between various silyl enolates and aldehydes was

performed in a mixture of 1,2-dimethoxyethane (DME) and water (7:3) at 0 °C in the presence of catalytic amounts of iron(II) perchlorate hexahydrate, a chiral bipyridine ligand, and benzoic acid. The resulting aldol products were obtained in very high yields, syn-selectivity, and enantioselectivity. In 2013, Kobayashi et al. published an additional report on the iron-catalyzed asymmetric Mukaiyama aldol reaction of aromatic aldehydes, thiophenecarboxaldehyde, aliphatic aldehydes, and α,β-unsaturated aldehydes (Scheme 529).951 Depending on the substrates, two protocols have been established both using a chiral bipyridine ligand as source of chirality. The standard conditions employed pyridine as additive and iron(II) triflate as catalyst (eq a). In an alternative protocol, the reaction was performed in the presence of iron(II) perchlorate as catalyst and benzoic acid as additive (eq b). In general, high yields, excellent diastereoselectivities, and high enantioselectivities for the major diastereoisomers were achieved. The group of Kobayashi developed an efficient hydroxymethylation of 1,3-dicarbonyl compounds which is carried out in aqueous solution at room temperature in the presence of iron(III) nitrate nonahydrate as catalyst and using sodium dodecylbenzenesulfate (SDBS) as surfactant (Scheme 530).952 Following this protocol, several 1,3-dicarbonyl componds have been transformed to the corresponding hydroxymethylated products in high yields. A Biginelli-like Mannich reaction has been reported by Xia and co-workers (Scheme 531).953 They conducted a threecomponent reaction of aldehydes, ketones, and carbamates in 3285

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Scheme 532

Scheme 533

Scheme 530

Scheme 531

the presence of catalytic amounts of iron(III) chloride hexahydrate and substoichiometric amounts of trimethylchlorosilane. Under very mild conditions, in a solvent mixture of dichloromethane and diethyl ether at room temperature, they obtained β-(benzyloxycarbonylamino)aldehydes or ketones in moderate to good yields. The reaction of aromatic aldehydes with ethyl diazoacetate catalyzed by the Lewis acidic iron complex [(η5-C5H5)Fe(CO)2(THF)]+BF4 resulted in mixtures of 3-hydroxy-2-aryl acrylates and β-keto esters due to phenyl or hydride migration, respectively.954 However, compared to other Lewis and Brønsted acids, the iron complex provided better yields and the highest ratio in favor of the 3-hydroxyacrylates.

Kirchner et al. conducted this reaction with an air stable tridentate iron pincer complex and obtained in most cases selectively 3-hydroxyacrylates rather than β-oxoesters (Scheme 532).955 The reaction depends very much on the counterion. The best results were achieved with the tetrafluoroborate, whereas in the presence of most other counterions no reaction took place. The group of Khan elaborated a one-pot multicomponent iron(III) chloride catalyzed reaction of aromatic aldehydes, enolizable ketones, nitriles, and acetyl chloride leading to β-acetamido carbonyl compounds (Scheme 533).956 In a first step, aldol addition may take place with concomitant acetylation of the hydroxy group. Nucleophilic displacement of the acetate by the nitrile and addition of water to the intermediate nitrilium ion (Ritter-type reaction) leads to the amide function. Reactions with acetophenone gave very high to excellent yields. Mixtures of syn and anti diastereoisomers in high overall yields were obtained when β-dicarbonyl compounds were introduced as nucleophiles. Under these conditions, the reaction of nitrobenzaldehydes with methyl acetoacetate led to the simple Knoevenagel products. The same transformation has been reported subsequently by Behbahani and co-workers using iron(III) phosphate as catalyst.957 3,4,5,6,-Tetrachloro-o-benzoquinone was demonstrated to undergo aldol addition when treated with methyl ketones in the presence of 5 mol % iron(III) chloride.958 Heating of the aldol adducts to 130 °C led to ring expansion under formation of tropone derivates. Retro-Claisen condensation can be exploited to synthesize carboxylic esters or ketones by treatment of 1,3-diketones with alcohols. Jana et al. developed an efficient protocol for this transformation working under neat conditions with iron(III) triflate as catalyst (Scheme 534).959 They demonstrated the applicability of their procedure by the synthesis of a variety of carboxylic esters (eq a) and methyl ketones (eq b) in high yields. Moreover, the method could be integrated in a one-pot protocol including a nucleophilic substitution of 1,3-diketones with benzhydrols and subsequent retro-Claisen condensation to give methyl ketones (eq c). Under these conditions the Michael addition products of 1,3-diketones with chalcones underwent a 3286

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Chemical Reviews Scheme 534

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Scheme 536

Scheme 537

Scheme 538

Scheme 535

retro-Claisen reaction with subsequent intramolecular aldol condensation affording cyclohex-2-en-1-ones (eq d). In a parallel work, Venkateswarlu and co-workers reported the same reaction under very similar conditions using iron(III) chloride as catalyst (Scheme 535).960 They obtained a large number of carboxylic esters from acyclic and cyclic 1,3-diketones in excellent yields. Regioselective transformation of primary alcohols in the presence of secondary alcohols was achieved. In addition, silyl ethers could be directly employed as nucleophiles. An iron-catalyzed condensation of 2-methylquinolines, 2-methylpyridines, and 2-methylquinoxalines with N-sulfonyl aldimines has been reported by the group of Huang (Scheme 536).961 Using catalytic amounts of iron(II) acetate, they synthesized 2-alkenylated azaarenes with high E-stereoselectivity and in moderate to high yields. An aldol-type nucleophilic attack of dimedone at aldehydes is the first step in an iron-catalyzed three-component domino reaction of salicylaldehydes, dimedone, and aryl nucleophiles leading to 9-arylated 2,3,4,9-tetrahydro-1H-xanthen-1-ones

(cf. Scheme 89).189 N-Protected and free arylglycines were accessible by amino- and amidoalkylation of arenes (Scheme 537).962 Iron(III) chloride hexahydrate or iron(III) perchlorate are efficient catalysts for this Mannich-type reaction which was performed in nitromethane at 80100 °C. The versatility of this simple procedure could be demonstrated by the synthesis of a variety of N-protected arylglycines which are synthetically valuable building blocks. An efficient Mannich protocol converting aromatic aldehydes, arylamines, and ketones to β-aminocarbonyl compounds has been developed by Behbahani and Ziarani.963 They employed iron(III) phosphate as catalyst under neat conditions at room temperature. The same catalyst has been used for the condensation of aldehydes with enolizable ketones and nitriles in the presence of acetyl chloride to give β-amido carbonyl compounds (cf. Scheme 533).957 Jana et al. described an efficient iron-catalyzed procedure for a Henry reaction (Scheme 538).964 Aldehydes were treated with nitromethane or nitroethane in the presence of catalytic amounts of iron(III) chloride and piperidine to afford nitroolefins in good to excellent yields. This simple catalyst system enabled a concomitant activation of the aldehyde and the nucleophile. Many functional groups were tolerated. In addition, the Michael activity of the products could be exploited for one-pot syntheses of 3-alkylindoles, 3-nitrochromenes, and N-arylpyrroles (cf. section 5.6.1). The iron(III) complex obtained by reaction of iron(III) chloride with 5-chloro-3-[2-(4,4-dimethyl-2,6-dioxocyclohexylidene)hydrazinyl]-2-hydroxybenzenesulfonic acid was shown to be an efficient catalyst for the Henry reaction of aliphatic and 3287

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Chemical Reviews Scheme 539

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Scheme 542

Scheme 543

Scheme 540

Scheme 541

aromatic aldehydes with nitroethane in water (Scheme 539).965 This transformation led to the nitroaldol products in high yields but with moderate diastereoselectivities. Self-condensation of various enamides has been achieved in the presence of catalytic amounts of iron(III) chloride (Scheme 540).966 This aldol-type transformation led to highly substituted 1,3-diacetamides in good yields. Bauer and co-workers have synthesized two bis(imino)pyridine iron(II) and iron(III) complexes and used them as catalysts for the Mukaiyama aldol reaction (Scheme 541).967 The reaction was performed in dichloromethane at room temperature to give the silylated aldol products in mostly high to quantitative yields. It is noteworthy that the oxidation state of the iron complex had an impact on the diastereomeric ratio. Low or even no diastereoselectivity was obtained with the iron(II) catalyst, whereas the iron(III) congener gave a moderate excess of the syn-isomer in some cases.

Iron(III) chloride hexahydrate has been demonstrated to catalyze the vinylogous aldol condensation of aryl aldehydes with 6-methyl-2-oxo-4-aryl-1,2,3,4-tetrahydropyrimidine-5-carboxamides which are readily available Biginelli products (Scheme 542).968 This protocol provided the condensation products in high yields. Such products are attractive as they can be converted to pyrido[4,3-d]pyrimidines which display interesting biological activities.969972 2-Methylquinolines, 2-methylpyridines, and 1-methylisoquinoline have been treated with α-oxoesters using iron(II) acetate as catalyst (Scheme 543).973 This protocol afforded lactic acid derivatives in high yields. It was assumed that the iron salt forms an enamide complex which undergoes aldol-type addition to the carbonyl compound. 5.1.2. Allylation. The group of Chan elaborated an iron mediated procedure for the allylation of aryl aldehydes with allyl bromide in water.974 However, it required stoichiometric amounts of iron powder and sodium fluoride for its activation. Kishi et al. reported a catalytic asymmetric 2-haloallylation reaction of aldehydes by a catalyst system formed from iron tris(2,2,6,6-tetramethyl-3,5-heptanedionate) [Fe(TMHD)3], chromium tribromide, a chiral oxazoline ligand, an excess of manganese, and substoichiometric amounts of triethylamine (Scheme 544).975 The reaction was performed in the presence of 2,6-lutidine and an excess of trimethylsilyl chloride. The halogenated homoallylalcohols were obtained in good yields and with high enantiomeric excess. Allylic acetates could be added to ketones in the presence of manganese as reductant and FeBr2bipy as catalyst (Scheme 545).976 In the case of substituted allylic reagents, the branched regioisomers were formed preferentially; however, after longer reaction times equilibration occurred leading to higher portions of the linear isomers. Mohan and co-workers described an allylation of aldehydes with allyltrimethylsilane using iron(III) triflate hexahydrate as catalyst and additional alkoxysilane (cf. Scheme 571, eq b).977 Allylation of aromatic aldehydes with allyltriethoxysilanes was also achieved using iron(III) chloride as catalyst (Scheme 546).978 This HosomiSakurai-type transformation did not require 3288

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Chemical Reviews Scheme 544

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Scheme 548

Scheme 549 Scheme 545

Scheme 546

Scheme 547

stoichiometric amounts of a Lewis acid and led directly to homoallyl ethers under very mild conditions. The reaction of aromatic aldehydes with aniline and allyltributylstannane provided homoallylic phenylamines. Zhao et al. were able to perform this transformation in aqueous medium using iron(III) chloride as Lewis acid catalyst and sodium dodecyl sulfate as surfactant.979 The homoallylamines could be isolated in good to high yields. 5.1.3. Alkynylation. Propargylic amines have been synthesized in high yields by Wang et al. utilizing an iron-catalyzed three-component coupling of aldehydes, secondary amines, and terminal alkynes (Scheme 547).980 The reaction was conducted in a sealed tube in toluene at 120 °C using iron(III) chloride as catalyst. In an independent work, the group of Li described a similar procedure.981 They treated aldehydes, secondary cyclic amines,

and terminal alkynes under neat conditions in air at 70 °C also using iron(III) chloride as catalyst. Yields for this approach, however, were considerably lower. 5.1.4. Arylation. Iron(III) chloride in combination with 2-(di-tert-butylphosphino)biphenyl as ligand was identified as an efficient catalyst for the arylation of various electron-deficient aromatic aldehydes with arylboronic acids (Scheme 548).982 The resulting diarylmethanols could be isolated in excellent yields. 5.1.5. Reformatsky-Type Reaction. Durandetti showed that α-chloro- or α-bromoesters reductively add to ketones or aldehydes in the presence of metallic manganese as reductant and iron(II) bromide as catalyst (Scheme 549).983 The reaction was conducted under mild conditions in acetonitrile at 50 °C to afford 3-hydroxyesters in moderate to high yields. A minor preference for the formation of the anti diastereoisomer was observed. Similarly, α-chloropropionitrile could be coupled with ketones. 5.1.6. Wittig-Type Reactions. Tang and Zhou developed a protocol for the enantioselective synthesis of allenes by olefination of ketenes with ethyl diazoacetate (EDA) in the presence of catalytic amounts of tetra(p-chlorophenyl)porphyrin iron chloride [Fe(tcpp)Cl] (cf. Scheme 620) and a chiral phosphine (Scheme 550).984 The authors proposed the formation of an intermediate phosphorus ylide, which undergoes a Wittig reaction with the ketene. This method provided optically active allenic esters in high yields and excellent enantioselectivities. In a more recent application using the Fe(tcpp)Cl complex as catalyst, Tang et al. reported the synthesis of tetrasubstituted dienes and cyclopentadienes.985 An iron-catalyzed Wittig-type reaction of a phosphonium ylide, generated from a diazoketone and triphenylphosphine, with a ketene has been combined in a one-pot procedure with a Nazarov cyclization to provide β-methylenecyclopentenones (cf. Scheme 719).986 Reiser et al. presented a polyethylene glycol (PEG)-immobilized iron porphyrin complex as catalyst for the olefination of aldehydes with ethyl diazoacetate in the presence of triphenylphosphine 3289

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Chemical Reviews Scheme 550

REVIEW

Scheme 552

Scheme 553

Scheme 551

Scheme 554

Scheme 555

(Scheme 551).987 Using this catalyst, the products were obtained in high yields and with high E-selectivity. Moreover, it could be recycled almost quantitatively and reused for additional nine runs with only a slight drop in activity. 5.1.7. Other CC Bond Forming Reactions at Carbonyl Groups. Cyanohydrin alkyl ethers have been obtained by an iron(III) chloride catalyzed consecutive reaction of aromatic or aliphatic aldehydes with alkoxy(trimethyl)silanes and trimethylsilyl cyanide (Scheme 552).988 The transformation was performed as a one-pot procedure either under neat conditions or in dichloromethane and provided the alkylated cyanohydrins in high yields. Nakazawa and Itazaki presented a silylcyanation of aldehydes and ketones using Cp(CO)2FeMe as catalyst (Scheme 553).989 Triethylsilyl cyanide was generated from triethylsilane and acetonitrile under photolytic conditions and reacted directly with the carbonyl compound. The iron complex served a dual function as catalyst for the formation of trimethylsilyl cyanide and for the transfer of the cyano group to the carbonyl function. Ferrocenium hexafluorophosphate catalyzes the Strecker reaction of aldehydes with primary amines and trimethylsilyl

cyanide.990 The reaction was performed under solvent-free conditions to afford α-aminonitriles in good to excellent yields. The reaction of aryl aldehydes with styrenes has been demonstrated to produce 1,4-skipped dienes in high yields using a catalyst system consisting of iron(III) tosylate, p-toluenesulfonic acid, and methanol (Scheme 554).991 The process has been described as a deoxygenative and dehydrogenative cross coupling of aldehydes and olefins. It may involve an initial dimethylacetal formation followed by a Prins-type reaction and a Friedel Crafts-type alkylation of a second molecule of alkene. Unsymmetrical 1,2-disubstituted alkenes led to mixtures of E/E-, E/Z-, and Z/Z-isomers. Plietker et al. described a trifluoromethylation of aldehydes and ketones with trifluoromethyltrimethylsilane catalyzed by TBAFe in combination with DMAP (Scheme 555).992 The resulting trimethylsilyl-protected 2,2,2-trifluoroethan-1-ols were obtained in high yields. This protocol could be used for a domino trifluoromethylation/allylation of electron-deficient olefins (cf. Scheme 606). 5.1.8. Reactions with Heteroatom Nucleophiles. A three-component KabachnikFields reaction of aromatic aldehydes with primary amines and diethyl phosphite has been 3290

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Chemical Reviews Scheme 556

Scheme 557

reported to be catalyzed by iron(III) chloride (Scheme 556).993 The reaction was conducted in ethanol or under solvent free conditions and provided α-aminophosphonates in excellent yields. Nanoparticles of Fe3O4 are also potent catalysts for this transformation.994 The reaction proceeded under neat conditions at 50 °C to afford α-aminophosphonates in high yields. The catalyst could be easily removed and reused several times. An initial yield of 94% dropped only to 86% after nine cycles. Pramanik and Ghorai described the iron(III)-catalyzed synthesis of α-azido peroxides.995 By reaction of aldehydes or ketones with TBHP and trimethsilyl azide in the presence of 10 mol % iron(III) chloride the α-azido peroxides were obtained in yields ranging from 50 to 93% (31 examples). Iron(III) tosylate has been employed as catalyst for the synthesis of dimethyl and diethyl acetals from ketones and for the synthesis of vinyl ethers from cyclic β-diketones (Scheme 557).996 The acetalization reaction was performed with the corresponding alcohol in the presence of 3 equiv of trialkyl orthoformate and led to the dialkyl acetals in high yields (eq a). Analogously, cyclic diketones were treated with alcohols at reflux and provided β-keto vinyl ethers (eq b). Alternatively, β-keto vinyl ethers were synthesized from 1,3-dicarbonyl compounds and alcohols using an ionic liquid supported on γ-Fe2O3 nanoparticles as catalyst.997 The magnetically recoverable catalyst could be reused six times without significant loss of activity. An oxidative esterification of aldehydes has been achieved by Darcel et al. employing iron(III) perchlorate hydrate as catalyst and hydrogen peroxide as oxidant (Scheme 558).998 The reaction was conducted at room temperature using the corresponding alcohol as solvent. A variety of mostly aromatic aldehydes was treated with various alcohols to afford the carboxylic esters in high yields. The same group established an iron-catalyzed protocol for the condensation of N-sulfonylamides with aldehydes under neutral conditions in ethanolic solution at room temperature to provide N-sulfonylimines.999

REVIEW

Scheme 558

Scheme 559

The reaction of aldehydes with arylboronic acids under aerobic conditions, using iron(II) triflate as catalyst in the presence of an NHC ligand and potassium tert-butoxide as base, led to carboxylic esters (Scheme 559).1000 On the basis of mechanistic experiments it was assumed that phenolic species are formed from the arylboronic acids which undergo oxidative esterification with the aldehyde. Initial formation of carboxylic acids could be excluded as benzoic acid did not react under the present conditions. Transacetalization of glycosides and thioglycosides of monosaccharides with benzaldehyde dimethylacetal or p-methoxybenzaldehyde dimethylacetal was achieved in the presence of catalytic amounts of iron(III) chloride.1001 The reaction was regioselective giving the 4,6-O-arylidenation products in very high yields. Disaccharides required stoichiometric amounts of the iron salt. Iron(III) chloride could be applied as catalyst for an efficient one-pot orthogonal protection of persilylated D-glycopyranosides (Scheme 560).1002 Formation of 4,6-O-benzylidene acetal, reductive benzylation in the 3-position using triethylsilane, and subsequent addition of an acyl electrophile provided the 2-acylated derivative (eq a). Alternatively, the initially formed cyclic acetal could be reductively opened by addition of additional triethylsilane and another portion of iron(III) chloride, thus providing 3,6-dibenzylated glucopyranosides (eq b). These procedures have been applied to the regioselective protection of α,α-D-trehalose and α-methyl maltoside. Regioselective orthogonal protection of α,α-D-trehalose was further advanced to a synthesis of a tetra-O-acylated sulfolipid as a model of sulfolipids I and IV (SL-I, SL-IV) of Mycobacterium tuberculosis (Scheme 561).1003 This procedure may be useful for the synthesis of derivatives of the maradolipids, novel 6,60 -di-Oacyl-α,α-D-trehalose glycolipids isolated from the dauer larvae of Caenorhabditis elegans.1004,1005 Primary amides can be readily obtained by reaction of aldehydes with hydroxylamine in the presence of catalytic 3291

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Chemical Reviews Scheme 560

REVIEW

Scheme 563

Scheme 564

Scheme 561

Scheme 562

amounts of iron(III) chloride (Scheme 562).1006 The utility of the method has been demonstrated by a number of applications including α-amino acid amides and sugar derivatives. It is noteworthy that the configuration of α-chiral aldehydes was not lost during this procedure. Oxidative amidation of aldehydes was also achieved by treatment with primary or secondary amines in the presence of tertbutyl hydroperoxide and iron(II) sulfate heptahydrate as catalyst

(Scheme 563).1007 The transformation was performed in acetonitrile at 60 °C. The utility of this method was demonstrated by a large number of examples. Primary and secondary amides were obtained in moderate to high yields. Very recently, Sreedhar and co-workers described the oxidative amidation of aldehydes using amine hydrochlorides in the presence of catalytic amounts of CuFe2O4 nanoparticles with catalytically active copper centers.1008 A simple magnetic separation of the catalyst and reuse up to four times did not result in a significant decrease of activity. A large number of carboxamides was obtained in 7080% yield. De Luca and Porcheddu established an iron-catalyzed method for the amidation of aldehydes with N-chloroamines (Scheme 564, eq a).1009 The reaction was conducted in acetonitrile at reflux in the presence of tert-butyl hydroperoxide as stoichiometric oxidant and iron(III) chloride as catalyst. A radical mechanism was proposed which was supported by the isolation of a TEMPO adduct when adding TEMPO as radical scavenger. For a number of examples, the products were obtained in high yields. In a subsequent work, the authors demonstrated that benzyl alcohols can be introduced instead of aldehydes (Scheme 564, eq b).1010 The reaction proceeds via a preliminary oxidation of the benzyl alcohol to the benzaldehyde. The N-chloroamines were generated in situ from the corresponding amines and N-chlorosuccinimide (NCS). In a related work, Bantreil and co-workers also reported an oxidative amidation of benzyl alcohol (Scheme 565).1011 They treated benzyl alcohol with primary or secondary amine hydrochlorides in the presence of catalytic amounts of iron(II) chloride tetrahydrate and tert-butyl hydroperoxide as oxidant. The resulting benzamides were obtained in good yields. Tertiary amines have been employed for the oxidative amidation of aldehydes (Scheme 566).1012 The iron(II) chloride 3292

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Chemical Reviews Scheme 565

REVIEW

Scheme 568

Scheme 566

Scheme 569

Scheme 567

Scheme 570

catalyzed reaction proceeds with loss of one alkyl group from the tertiary amine probably via oxidation to the iminium species and formation of a hemiaminal. tert-Butyl hydroperoxide serves as terminal oxidant and the products were obtained in moderate to high yields. Consequently, the authors could also apply these reaction conditions to the amidation of anhydrides with tertiary amines (cf. Scheme 586).1013 Chen et al. established a catalytic enantioselective hydrophosphonylation of aldehydes (Pudovik reaction) using a newly synthesized camphor-based dinuclear iron(III) complex (Scheme 567).1014 They were able to obtain α-hydroxy phosphonates in high yields and mostly very high enantioselectivities from aromatic, heteroaromatic, α,β-unsaturated, and aliphatic aldehydes. The group of Xu described an iron-catalyzed oxidative condensation of amines and the condensation of alcohols with amines to provide imines (Scheme 568).1015 The reaction was performed using iron(III) nitrate nonahydrate as catalyst in combination with TEMPO, and air was the terminal oxidant. Several variants of this transformation have been reported. The simplest one was a direct oxidation of secondary amines under these conditions without formation of new CN bonds. Homocondensation of primary amines led to imines with CN bond

formation (eq a). Anilines and benzylamines would undergo a cross condensation to form arylimines. The reaction of benzyl alcohols with amines afforded imines in high yields (eq b). The condensation of 1,3-dicarbonyl compounds with thiols leads to β-sulfenylated α,β-unsaturated carbonyl compounds. Singh and Kumari employed iron(III) chloride as an efficient catalyst for this reaction.1016 A variety of different thiols and 1,3-diones could be converted in high yields. An oxidative imide formation has been achieved by reaction of carboxamides with aldehydes in the presence of catalytic amounts of iron(II) bromide and tert-butyl hydroperoxide as oxidant (Scheme 569).1017 This method opens up a simple route for the synthesis of unsymmetrical imides. Lee and co-workers treated aldehydes with thiols in the presence of iron(II) bromide and tert-butyl hydroperoxide as terminal oxidant (Scheme 570).1018 The transformation was conducted in aqueous solution at 110 °C or in acetonitrile at 90 °C in a closed vessel to afford thioesters in good yields. Renaud and co-workers described the condensation of amines with β-ketoesters in the presence of catalytic amounts of FeCl3 3 6H2O to afford a broad range of β-aminoacrylates.1019 The group of Ji achieved the same transformation under neat conditions using iron(III) triflate as catalyst.1020 5.2. Reactions of Acetals and Analogues

5.2.1. Allylation. Mohan and co-workers described the allylation of acetals with allyltrimethylsilane using catalytic 3293

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Chemical Reviews Scheme 571

REVIEW

Scheme 574

Scheme 575

Scheme 572

Scheme 573 Scheme 576

amounts of iron(III) tosylate hexahydrate (Scheme 571, eq a).977 This reaction proceeded under mild conditions in acetonitrile at room temperature affording the corresponding homoallyl ethers in high yields. Aldehydes could be converted accordingly when supplemental alkoxysilane was added (eq b). 5.2.2. Hydrolysis of Acetals. Iron(III) tosylate hexahydrate has also been employed as catalyst for the deprotection of aromatic and conjugated acetals in water (Scheme 572).1021 The simple procedure was performed at room temperature or at reflux and provided the free aldehydes or ketones in high to excellent yields. Analogously, tetrahydropyranyl (THP) ethers could be efficiently cleaved in the presence of catalytic amounts of iron(III) tosylate in methanol at room temperature.1022 An iron(III) chloride catalyzed aminolysis of thioacetals has been described by Bi and co-workers (Scheme 573).1023 They treated β-carbonyl 1,3-dithianes with ammonia or primary or secondary amines in the presence of 10 mol % iron(III) chloride to afford β-enaminones in a stereoselective manner. Accordingly, treatment with hydrazine hydrate led to 3,4-disubstituted pyrazoles (cf. Scheme 626). An elegant oxidative iron-catalyzed deprotection of dithioacetals has been elaborated by the group of Kirihara (Scheme 574).1024 Thus, in the presence of iron(III) acetylacetonate as catalyst, stoichiometric amounts of sodium iodide, and 30% hydrogen peroxide, dithioacetals were readily hydrolyzed to the parent ketones or aldehydes. The reaction proceeded in a mixture of ethyl acetate and water at room temperature.

According to the proposed mechanism, the reaction is initiated by the oxidation of iodide to an iodonium ion which attacks the sulfur atom (Scheme 575). Subsequent ring opening and attack of water at the activated thiocarbonyl group generates the carbonyl compound with release of 1,2-dithiolane and iodide. 5.2.3. Glycosylations. An early stereoselective α-glycosylation of alcohols and protected sugars using stoichiometric amounts of iron(III) chloride has been reported by Nuhn et al.1025 Stoichiometric amounts of iron(III) chloride have also been shown to mediate the formation of 1,6-anhydro-Dglucopyranosides from per-protected 6-O-benzyl α/β-D-glucopyranosides by debenzylation and intramolecular glycosidation.1026 1,2-Trans-glycosyl azides are available by nucleophilic substitution of glycosyl β-peracetates with trimethylsilyl azide employing iron(III) chloride as catalyst (Scheme 576).1027 A number of mono-, di-, and trisaccharides were obtained in high yields. Subsequently, the glycosyl azides have been subjected to [3 + 2] cycloaddition with acetylenes forming the corresponding triazoles. Interestingly, this reaction was also shown to be catalyzed by iron(III) chloride in combination with copper powder (cf. section 6.1.4). Boyer and Beau reported an efficient glycosylation of peracetylated β-D-N-acetylglucosamine under microwave conditions using Fe(OTf)3 3 6.2DMSO as a catalyst and 2,4,6-tri-tertbutylpyrimidine (TTBP) as base.1028,1029 Various β-(1f6) and β-(1f3) GlcNAc glycosides of menthol and various 3294

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Chemical Reviews Scheme 577

REVIEW

Scheme 580

Scheme 578

Scheme 581

Scheme 579

protected glucose derivatives were obtained in mostly high yields. An example for the iron-catalyzed formation of disaccharides is given in Scheme 577. Glycosidic allylation and propargylation of various peracetylated mono- and disugars has been achieved by Paix~ao et al. using iron(III) chloride as catalyst (Scheme 578).1030 A pronounced selectivity for the anomeric position could be observed providing the glycosidic products in moderate to good yields under very mild conditions. 5.3. Reactions at Carboxylic Acid Derivatives and Analogues

This section may intersect with section 2.4.1.3. Cross coupling of acyl electrophiles with organometallic reagents are referred to the former section, while reactions of carboxylic acid derivatives with other carbon and heteroatom nucleophiles are discussed below. Williams and co-workers reported the reaction of primary amines with nitriles using iron(III) nitrate nonahydrate as catalyst (Scheme 579).1031 The reaction was performed under solvent-free conditions at 125 °C to afford amides with a very high degree of conversion. Plietker and co-workers studied transesterification reactions using the nucleophilic iron complex [Bu4N][Fe(CO)3(NO)] (TBAFe) as catalyst.46,47 In a first paper, they focused on vinyl acetates as acylation agents and achieved good to high yields for this transformation with a variety of alcohols.1032 A drawback of this method was a sometimes difficult accessibility of vinyl carboxylates for a variation of the acyl component. Thus, in a subsequent work, they introduced p-chlorophenyl esters as readily available acyl donors (Scheme 580, eq a).1033

Transesterification of these substrates was also efficiently catalyzed by TBAFe. A variety of different primary and secondary alcohols and different p-chlorophenyl esters could be converted using this protocol to afford the corresponding transesterification products in high yields. In addition, simple methyl esters bearing electronegative substituents in α-positions could also be employed (eq b). No racemization of α-chiral acyl donors has been observed, which renders this method useful for an application to the construction of complex natural products. Consequently, an efficient iron-catalyzed thioesterification of p-chloroaryl esters has been elaborated by the same authors (Scheme 581).1034 Using TBAFe in catalytic amounts, the reaction between p-chloroaryl esters and aliphatic and aromatic thiols was performed in dioxane at 80 °C to afford thioesters in mostly high to excellent yields (eq a). No racemization of labile stereogenic centers was observed during this process. In addition, this method could be combined with an intramolecular amidation of the formed thioesters (eq b). This domino process can be 3295

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Chemical Reviews Scheme 582

REVIEW

Scheme 584

Scheme 585 Scheme 583

regarded as a native chemical-ligation-type peptide coupling. The primary acyl donor is activated by thioester formation followed by intramolecular amidation. Selenoesters could be synthesized in high yields by an iron(II) chloride catalyzed reaction of acyl chlorides with diselenides.1035 The reaction was performed in dioxane at 100 °C in the presence of stoichiometric amounts of magnesium dust. Taniguchi and co-workers established an esterification methodology based on an iron-catalyzed oxidative triphenylphosphine activation (Scheme 582).1036 Thus, carboxylic esters were readily generated by treating carboxylic acids and alcohols in the presence of triphenylphosphine and catalytic amounts of iron(II)phthalocyanine [Fe(Pc), for structure see Scheme 61] and 4-methoxypyridine N-oxide (MPO) in acetonitrile at reflux in air. The utility of this method could be demonstrated by several examples providing carboxylic esters in moderate to high yields. It was assumed that triphenylphosphine is oxidized to a radical cation by a hydroxoiron(III) or an oxoiron(IV) complex. The phosphoryl radical cation reacts with the carboxylate anion in the presence of air to an acyloxyphosphonium intermediate. Displacement of phosphine oxide by the pyridine N-oxide forms an activated ester which is finally attacked by the alcohol to give the carboxylic ester. The transesterification of methyl and ethyl esters was achieved using Fe(acac)3 as catalyst in the presence of catalytic amounts of sodium carbonate (Scheme 583).1037 The reaction was conducted in heptane, and the released alcohol was removed by azeotropic reflux using a DeanStark apparatus. A wide range of carboxylic esters and alcohols was subjected to these conditions affording the transesterification products in mostly very high to excellent yields. A dimeric μ-alkoxy-bridged iron(III) complex was discussed as catalytically active species. A large variety of alcohols, phenols, and diols could be acylated with anhydrides using only 2 mol % iron(III) tosylate hexahydrate as catalyst (Scheme 584).1038 The reaction was performed at room temperature to provide the acetates, benzoates, and propionates in high yields. The esterification of carboxylic acids with a series of alcohols under solvent-free conditions at room temperature has also been achieved with a heterogeneous 1-glycyl-3-methyl imidazolium chloride iron(III) catalyst (Scheme 585).1039 The utility of this protocol has been demonstrated by a number of examples.

Scheme 586

It is noteworthy that the catalyst could be recycled and reused without loss of activity. Li and co-workers presented a method for the amidation of anhydrides with tertiary amines using iron(II) chloride as catalyst and aqueous tert-butyl hydroperoxide as oxidant (Scheme 586; see also amidation of aldehydes, Scheme 566).1013 A large variety of tertiary amines and various anhydrides could be converted in mostly high yields. Mechanistic studies suggested the formation of an α-amino peroxide which eliminates to an iminium ion. Addition of water provides the aldehyde (in most cases formaldehyde) and releases the secondary amine which undergoes the amidation reaction. Direct esterification of carboxylic acids with primary and secondary alcohols could be performed with iron(III) acetylacetonate as catalyst (Scheme 587).1040 The reaction was carried out in xylene at reflux without any dehydrating agent. Excellent yields were obtained for a variety of examples. Based on earlier work on the kinetic resolution of secondary alcohols,10411043 Fu and co-workers presented an efficient system which effects a dynamic kinetic resolution of such substrates by an acylation reaction (Scheme 588).1044 They applied a planar-chiral DMAP-derived ferrocene in combination 3296

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Chemical Reviews Scheme 587

REVIEW

Scheme 590

Scheme 588

Scheme 591

Scheme 589

with a ruthenium complex as catalytic system. The iron complex catalyzes the acylation of the alcohol with acetyl isopropyl carbonate as acylating reagent. It has been reported that the iron center can influence the catalytic activity of DMAP when the pyridine is directly connected to the ferrocene.19 The ruthenium complex effects the racemization of the unreacted enantiomer of the alcohol. The importance of this first nonenzymatic dynamic kinetic resolution by enantioselective acylation has been highlighted recently.1045 The group of Shimizu demonstrated that Fe3+-exchanged montmorillonite (Fe3+/K-10) is an effective catalyst for the transamidation of various primary carboxamides (Scheme 589).1046 The reaction was performed under neat conditions at 140 °C with 1 mol % of the catalyst. A broad scope of aliphatic, aromatic, and heteroaromatic amides was treated preferably with primary aliphatic and aromatic amines to provide the new carboxamides in high yields. Subsequently, Gamba-Sanchez and co-workers described the same type of transformation catalyzed by iron(III) nitrate nonahydrate.1047 The reaction was performed in toluene at reflux. Unlike the procedure of Shimizu, they were also able to introduce secondary and tertiary amides as substrates. A large variety of different amides, substituted ureas, and substituted phthalimides could be synthesized in high yields. 5.4. Reactions of Carbonic Acid Derivatives

Two efficient methods for the deprotection of allyloxycarbonyl-protected alcohols under neutral conditions have been

established by Plietker et al. (Scheme 590).1048 In a first procedure, TBAFe was used as catalyst in the presence of a phosphine ligand (eq a). The second method applied a binuclear TBAFe-derived iron complex (eq b). In both cases propane-2thiol was used as allyl scavenger. The two methods proved to be similarly effective, affording the free alcohols in high to excellent yields. A variety of functional groups was tolerated and stereogenic centers were not affected under these conditions. The guanylation of amines in the presence of iron(II) acetate as catalyst has been described by Royo and Pottabathula (Scheme 591).1049 Both electron-deficient and electron-rich anilines were converted in mostly excellent yields. Moreover, heteroaryl amines and cyclic aliphatic amines could be converted in similar yields. Borisov and Osmanov described the iron powder catalyzed chlorination of perfluoroalkenyl isothiocyanates to provide the so far unknown N-perfluoroalkenyl(chlorosulfanyl)formimidoyl chlorides in about 70% yield.1050 Iron(III) triflate proved to be an efficient catalyst for the Boc protection of primary and secondary aliphatic and primary aromatic amines.1051 Thus, amines were treated with di-tert-butyl dicarbonate under solvent-free conditions at room temperature to afford the monocarbamates in excellent yields within a few minutes. 5.5. Conjugate Addition to Carbonyl Groups and Analogues

5.5.1. Carbon Nucleophiles. The chemistry of ironcatalyzed Michael additions has already been intensively investigated during the late 1990s, in particular by the group of Christoffers.945,10521056 Iron(III) chloride hexahydrate was shown to be an excellent catalyst for the addition of β-dicarbonyl compounds to various Michael acceptors leading to products in high yields with low catalyst loadings and under mild reaction conditions (Scheme 592). Based on theoretical calculations a mechanistic proposal was presented that included the formation of a dionatoiron(III) 3297

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Chemical Reviews Scheme 592

REVIEW

Scheme 595

Scheme 593

Scheme 594

complex from the iron(III) salt and the β-dicarbonyl compound (Scheme 593).1057 Subsequent loss of a ligand and coordination of the Michael acceptor brings the reaction partners in close proximity and concomitantly activates the electrophile. Alkylation leads to a bicyclic iron complex which releases the product in favor of a new β-ketoester ligand. Electrospray mass spectrometric and other spectrometric (EXAFS, XANES, Raman, UVvis) investigations showed that the catalytically active species is rather a cationic bis(dionato)iron(III) complex which is formed from four molecules of iron(III) chloride and the dicarbonyl compound (Scheme 594).1058,1059 Concomitantly, three molecules of tetrachloroferrate(III) are formed which are inactive for this process. Thus, the higher efficiency of Fe(ClO4)3 3 9H2O compared to FeCl3 3 6H2O could be explained as no such inactive ferrates are formed. The method of Christoffers has been applied by Jung et al. for the construction of 3-acetyl-2,6-heptanedione, which they treated further with methanesulfonyl azide to obtain the corresponding α-diazo ketone along with a dihydropyrazole and a pyrazole.1060 Ferrocenium triflate, poly(vinylferrocenium triflates), and a block copolymer of the latter with isoprene have been demonstrated to catalyze the reaction of methyl vinyl ketone with ethyl

2-oxycyclopentane-1-carboxylate (cf. Scheme 592).1061 The rate constants for this process were of the same order of magnitude as for the iron(III) chloride catalyzed reaction. Another heterogeneous catalyst for the Michael reaction of β-ketoesters with vinyl ketones has been reported to be iron(III)-exchanged fluorotetrasilicic mica.1062 The reaction of methyl vinyl ketone with ethyl 2-oxycyclopentane-1-carboxylate (cf. Scheme 592) provided the Michael adduct in 99% yield even after the fourth run. A dual addition of methyl vinyl ketone to cyclohexane-2,5dione-1,4-dicarboxylates could be achieved using 10 mol % iron(III) chloride hexahydrate as catalyst (Scheme 595).1063 Subsequent annulation of the Michael adducts in an overall Robinson-type procedure provided s-indacenes in moderate to high yields and excellent diastereoselectivity. A chloride-free Michael addition of β-oxo esters to methyl vinyl ketone was accomplished using iron(II) tetrafluoroborate hexahydrate as catalyst in an ionic liquid.1064 The reaction of methyl vinyl ketone with ethyl 2-oxycyclopentane-1-carboxylate (cf. Scheme 592) provided the addition product in 95% yield. The product was extracted with ether. The remaining ironcontaining ionic liquid could be used for at least another 13 runs. Still a yield of 81% was achieved after the 14th run. Christoffers et al. applied their conditions for the ironcatalyzed Michael addition to the synthesis of lactam precursors which could be converted into spirocyclic tetrahydrocarbazoles and tetrahydroacridines by Fischer indole or Friedl€ander quinoline synthesis, respectively (Scheme 596).1065 Lai, Xu, and co-workers treated several chalcones with activated methylene compounds in the presence of iron(III) chloride as catalyst (Scheme 597, eq a).1066 In addition, a combination of iron(III) salts with chiral amines proved to be effective for the asymmetric Michael addition of 4-hydroxycoumarin to (E)-4-phenylbut-3-en-2-one. Employing iron(II) chloride tetrahydrate as catalyst and (1R,2R)-1,2-diphenylethane1,2-diamine [(R,R)-DPEN] as chiral ligand, the anticoagulant drug warfarin was obtained in 90% yield and an enantiomeric excess of 90% (eq b). The HosomiSakurai-type addition of allyltrimethylsilane to chalcones has been described to proceed at room temperature in the presence of iron(III) chloride as catalyst and trimethylsilyl chloride as additive (Scheme 598).1067 Grignard reagents can undergo an iron(II) chloride catalyzed 1,6-addition to 2,4-dienoates or 2,6-dienamides (Scheme 599).1068 The resulting magnesium dienolates have been quenched with ammonium chloride or methyl iodide to give (Z)-β,γ-unsaturated esters or amides in a regio- and stereoselective manner. 3298

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Chemical Reviews Scheme 596

REVIEW

Scheme 599

Scheme 600

Scheme 597

Scheme 598

Ma and co-workers established an Fe(acac)3-catalyzed procedure for the addition of Grignard reagents to 2,3-allenoates (Scheme 600).688 This reaction led to magnesium dienolates (eq a) which could be trapped with various electrophiles, such as acyl chlorides, allyl esters, bromides, or acetates. Depending on the reaction conditions, 3(Z)- or 3(E)-alkenoates were obtained with high diastereoselectivity. Acylation with carboxylic chlorides led exclusively to the α-acylated 3(Z)-alkenoates (eq b), while reaction with allyl carboxylates provided the corresponding

3(E)-isomers (eq c). In addition, stereospecific methods for the allylation of magnesium dienolates have been developed as well. Reaction of the dienolate nucleophiles with allylic acetates in the presence of Pd(PPh3)4 afforded the α-allylated 3(Z)-alkenoates in high diastereoselectivity. Treatment with allylic bromides under copper(I) catalysis led almost exclusively to the (E)isomers (not shown). The same group described the reaction of 2-trimethylsilyl-2,3allenoates with Grignard reagents in the presence of Fe(acac)3 as catalyst to afford trimethylsilyl-substituted (Z)-vinyl ketenes in excellent stereoselectivity (Scheme 601).1069 The presence of the silyl group is a prerequisite for the course of this addition/ elimination sequence compared to the simple Michael addition as presented in Scheme 600. Vinyl ketenes are versatile synthetic building blocks. Subsequent reaction with metal organyls provided allyl ketones. Stereodefined trienes and phenols have also been constructed from these precursors in the course of this work. Urabe and co-workers claimed that α,β,γ,δ-unsaturated amides undergo 1,4- or 1,6-addition with Grignard reagents depending on the presence of an iron catalyst (Scheme 602).1070 Reaction without iron catalyst afforded the 1,4-addition product (eq a). The intermediate enolate could be treated with alkyl halides to afford α,β-disubstituted γ,δ-unsaturated amides. In contrast, the presence of catalytic amounts of iron(II) chloride directed the Grignard nucleophile into the 5-position (eq b). Subsequent treatment with an alkyl electrophile provided α,δ-disubstituted β,γ-unsaturated amides in a one-pot procedure. The presence of 3299

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Chemical Reviews Scheme 601

Scheme 602

REVIEW

Scheme 603

Scheme 604

Scheme 605

chiral N-substituents leads to very high diastereoselectivities in both cases. Urabe and co-workers established an iron(II) chloride catalyzed addition of Grignard reagents to 2-alken-4-ynoates (Scheme 603).1071 After aqueous workup this procedure led to allenes in good to high yields. The same authors developed an iron-catalyzed method for the 1,6-addition of aryl Grignard reagents to α,β,γ,δ-unsaturated sulfones to give (Z)-4-aryl-alkenylsulfones (Scheme 604).1072 Iron(II) chloride was employed as catalyst. Generally, the products were obtained in good yields under the reported reaction conditions. The obtained (Z)-4-aryl-alkenylsulfones could be readily cyclized in a FriedelCrafts reaction to afford 1,4-dihydronaphthalenes. An interesting iron-catalyzed β-functionalization of tertiary amines via in situ enamine formation and addition to nitroalkenes has been reported by Oisaki and Kanai (Scheme 605).1073 The reaction proceeded in the presence of catalytic amounts of iron(III) chloride and DMAP using di-tert-butyl peroxide as oxidant. A variety of substituted cyclic enamines have been obtained by this procedure (eq a). Acyclic enamine products turned out to be unstable and have been immediately transformed to the corresponding amines by treatment with sodium cyanoborohydride (eq b). It should be noted that this method constitutes a valuable extension to the more common ironcatalyzed oxidative α-functionalization of alkylamines and amides via oxidation to the iminium salts and reaction with nucleophiles (see for example Schemes 275 and 360). The addition of indole to Michael acceptors has been described by Maleki et al.158 They used in situ generated iron(III) dodecyl sulfate as Lewis acidsurfactant catalyst for this transformation to generate a series of Michael adducts in excellent yields.

A domino trifluoromethylation/allylation of electron-deficient olefins has been achieved using the low-valent iron complex TBAFe as catalyst (Scheme 606).992 Thus, arylidenemalononitriles were treated with allyl acetates and trifluoromethyltrimethylsilane in the presence of the nucleophilic iron catalyst in 1,2-dimethoxyethane at 80 °C to provide the trifluoromethylated and allylated products. It was presumed that the reaction starts with an ionization of the allylic acetate by the iron species to liberate the acetate which in turn attacks the trifluoromethylsilane to generate a trifluoromethyl nucleophile. Addition of the latter in a Michael-type fashion to the olefin affords an enolate which is finally trapped by the allyliron intermediate. A hybrid magnetic material derived from k-carrageenan (a natural sulfated polysaccharide extracted from red seaweed) and Fe3O4 nanoparticles was shown to be an efficient catalyst for the Michael addition of aldehydes to nitroalkenes.1074 It is noteworthy that the individual components did not show catalytic activity. A significant preference for the formation of the synproduct was observed. Even though the polysaccharide support is chiral, no asymmetric induction of this reaction was achieved. However, a k-carrageenan system equipped with a chiral prolinol derivative led to high enantioselectivities (up to 93% ee). 3300

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Chemical Reviews Scheme 606

REVIEW

Scheme 609

Scheme 607

Scheme 610 Scheme 608

5.5.2. Heteroatom Nucleophiles. An aza-Michael addition was performed in aqueous solution using several transitionmetal-based Lewis acid catalysts. In a screening experiment iron(III) chloride heptahydrate was identified as the most efficient for the addition of aniline to ethyl acrylate along a long line of other metal salts. Some primary and secondary aliphatic amines could be treated with ethyl acrylate, acrylonitrile, and cyclohex-2-enone to afford the addition products in mostly excellent yields (Scheme 607).1075 Iron(III) chloride hexahydrate is a promising catalyst for the aza-Michael reaction of chalcones with weakly nucleophilic carbamates in the presence of trimethylsilyl chloride as activating additive.1076,1077 The reaction was performed utilizing a dual activation of the hard nucleophile (carbamates) and the soft electrophile (enone). However, indium(III) chloride but also palladium(III) chloride and rhodium(III) chloride outperformed iron(III) chloride as Lewis acidic catalysts. Yao et al. described a procedure for the iron-catalyzed sulfenylation of α,β-unsaturated ketones and esters (Scheme 608).1078 They worked under neat conditions in air at room temperature using anhydrous iron(III) chloride as catalyst. Several examples have been presented which gave very high to excellent yields. An enantioselective addition of thiols to Michael acceptors has been accomplished using iron(II) tetrafluoroborate and a chiral PyBox ligand (Scheme 609).1079,1080 Several thiols have been treated with (E)-3-crotonoyloxazolidin-2-one under very mild conditions in THF at 20 °C providing the thioethers in good to excellent yields and mostly high enantioselectivities. The group of Feng employed iron(II) complexes with chiral N,N0 -dioxide ligands as catalysts for the asymmetric Michael addition of 1-(4-methoxyphenyl) ethanone oxime to various α,β-unsaturated aldehydes (Scheme 610).1081 The reaction was

Scheme 611

combined with the reduction of the aldehydes to give hydroxy-substituted oxime ethers in moderate yields and good enantioselectivities. An asymmetric addition of thiols to α,β-unsaturated carbonyl compounds using a chiral saleniron(III) complex has been reported by White and Shaw (Scheme 611).1082 A broad scale of substrates could be converted in excellent yields and excellent enantioselectivities. The method could be applied to a short synthesis of the antiasthma agent (R)-montelukast. 3301

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Chemical Reviews Scheme 612

REVIEW

Scheme 614

Scheme 613 Scheme 615

5.6. Synthesis of Heterocycles

Scheme 616

A comprehensive summary on the iron-catalyzed synthesis of heterocycles has been provided very recently by Majumdar and co-workers.1083 Some examples have also been included in an account article by Wang.1084 Among other synthetic principles this involves reactions at carbonyl and carbonyl analogue groups as described in the following section. 5.6.1. Five-Membered Heterocycles. 5.6.1.1. Pyrroles and Furans. A PaalKnorr-type synthesis of pyrroles starting from 2,5-dimethoxytetrahydrofuran and primary amines, carboxamides, or sulfonamides was catalyzed efficiently by iron(III) chloride heptahydrate (Scheme 612).1085 Only 2 mol % of the catalyst was used to promote the reaction in aqueous solution at 60 °C. In general, the 1-substituted pyrroles were isolated in high yields. Organic solvents led to a much lower conversion. Brønsted acid catalysis using hydrogen chloride or acetic acid provided almost identical results. An iron-catalyzed Henry reaction has been reported by the group of Jana (cf. Scheme 538).964 Using these nitroalkenes as starting material, they developed a one-pot protocol for the threecomponent synthesis of substituted N-arylpyrroles by reaction with arylamines and 1,3-dicarbonyl compounds in nitromethane at reflux in the presence of iron(III) chloride as catalyst.1086 The synthesis of substituted N-(2-alkynylaryl)pyrroles by a slightly modified protocol has been described in a consecutive paper (Scheme 613).1087 These products have been transformed to pyrrolo[1,2-a]quinolines by a gold-catalyzed hydroarylation. Jana and co-workers have also developed an iron-catalyzed four-component coupling for the construction of pyrroles (Scheme 614).1088 Following their procedure, 1,3-dicarbonyl compounds, primary amines, aromatic aldehydes, and nitroalkanes were readily condensed to substituted pyrroles. Iron(III) chloride has been identified as the most potent catalyst for this transformation which proceeds in nitromethane at reflux. The multicomponent coupling involves the initial formation of two intermediates: a β-enamino ketone by condensation of the amine with the 1,3-dicarbonyl compound and a nitroalkene by Henry reaction of the aldehyde with the nitroalkane. Subsequent Michael addition of the two intermediates and ring closure lead to the pyrrole ring.

An intramolecular addition of tertiary enamides to ketones has been reported by Wang et al. (Scheme 615).1089 This Knorr-type process was catalyzed by iron(III) chloride and proceeded in dichloromethane at room temperature. After an 1,3-hydroxy rearrangement, probably via an elimination/addition mechanism, 5-hydroxy-1H-pyrrol-2(5H)-ones were obtained in excellent yields. Prins cyclizations of alkynyl acetals with stoichiometric amounts of iron(III) chloride have been reported by Li, Xie and co-workers.1090,1091 An iron-catalyzed variant has been presented by the groups of Li and Yu.1092 They cyclized alkynyl acetals in the presence of catalytic amounts of iron(III) chloride hexahydrate or iron(III) bromide with concomitant halogenation to afford halobenzylidene-substituted five-membered carboand heterocycles (Scheme 616, eqs a and b). Acetyl chloride or bromide were employed as halogen source. In the case of iron(III) bromide and acetyl bromide a double bromination was achieved (eq b). Conducting this reaction in acetone without acetyl halide led to the formation of cyclic enones (eq c). 3302

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Chemical Reviews Scheme 617

REVIEW

Scheme 620

Scheme 618

Scheme 621 Scheme 619

This method could be applied to the synthesis of a variety of six- to eight-membered carbo- and heterocycles. Calculations revealed that the latter process may proceed via an oxocarbenium species which cyclizes to an oxete intermediate. Subsequent electrocyclic ring opening leads to the cyclic enone. β-Lactam-substituted propargylic alcohols have been transformed to vinylpyrroles by heating at 85 °C in dichloroethane in a sealed tube (Scheme 617, eq a).1093 Iron(III) chloride was employed as catalyst for this transformation which probably includes a MeyerSchuster rearrangement step. Pyrroles were obtained in moderate to good yields. In contrast, heating of 2-azetidine-tethered allenic alcohols in dichloroethane at 80 °C in the presence of catalytic amounts of iron(III) chloride provided 4-allenyl γ-lactones in moderate yields (eq b). Balalaie and co-workers described a three-component reaction of primary amines, dialkyl acetylenedicarboxylates, and β-nitrostyrenes in the presence of 40 mol % iron(III) chloride for the synthesis of tetrasubstituted pyrroles (Scheme 618).1094 The course of this reaction involves a sequence of two Michael additions, cyclization, and oxidation. Bobade et al. treated pyridine-2-carbaldehyde with secondary cyclic amines and terminal acetylenes in the presence of catalytic amounts of Fe(acac)3 and tetrabutylammonium hydroxide (TBAOH) (Scheme 619).1095 A domino reaction consisting of imine formation, alkyne addition, and cycloisomerization led to 1-aminoindolizines in high yields.

A formal [4 + 1] cycloaddition of α,β-unsaturated ketones with an iron carbenoid generated from ethyl diazoacetate provided dihydrofurans in excellent yields (Scheme 620, eq a).1096 Tetra(p-chlorophenyl)porphyrin iron chloride [Fe(tcpp)Cl] and pyridine are employed as catalysts for this transformation. It should be noted that the reaction proceeded with complete diastereoselectivity in favor of the trans-dihydrofurans. Employing α,β-unsaturated imines as substrates, the corresponding dihydropyrroles could be obtained with equal efficiency and complete diastereoselectivity (eq b). An iron(III) chloride catalyzed one-pot reaction of α-hydroxy ketones and activated alkynes afforded substituted furans in high yields (Scheme 621).1097 In a first step, a DABCO-catalyzed oxa-Michael addition of the hydroxy ketone onto the acetylene occurred. After evaporation of the solvent, an intramolecular iron-catalyzed condensation of the intermediate Michael adduct led to the furan. Reaction of nitriles with Reformatsky reagents, the so-called Blaise reaction, provided zinc complexes of β-enamino esters as intermediates (Scheme 622).1098 Guan et al. were able to react these intermediates with nitroolefins in an iron(III) chloride catalyzed process. Thus, a number of substituted pyrroles could be synthesized in good yields. A tentative mechanism was proposed which included a Michael addition of the Blaise intermediate to the olefin, tautomerization, cyclization, and elimination. 3303

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Chemical Reviews Scheme 622

REVIEW

Scheme 625

Scheme 626

Scheme 623

Scheme 627

Scheme 624

Isotetronic acids have been constructed by an iron-catalyzed reaction of pyruvic acid derivatives with cyclic hemiacetals (Scheme 623).1099 The best catalyst system was an iron trifluoroacetate complex in combination with 2-mercaptopyridine. The transformation was conducted in 1,2-dichloroethane at 60 °C and provided substituted isotetronic acids in mostly moderate to good yields. An asymmetric version has been elaborated as well using a chiral 6,60 -dimethoxybinaphthyl phosphate as additive. An iron-catalyzed aza-Michael addition was involved in a three-component reaction of imines with two molecules of alkynoates (Scheme 624).1100 The process was initiated by a copper-catalyzed addition of a terminal alkynoate to generate a propargylamine. This species underwent an iron(III) chloride catalyzed aza-Michael addition to the same (see example) or a different (not shown) alkynoate. Cyclization of the resulting allyl propargyl amines and subsequent tautomerization led to substituted pyrroles. 5.6.1.2. Imidazoles, Pyrazoles, and Thiazoles. The reaction of 2-aminobenzenethiols with isothiocyanates could be carried out by using iron(III) nitrate nonahydrate as catalyst in water in the presence of sodium dodecylbenzenesulfonate (SDBS) as surfactant (Scheme 625).1101 This procedure provided a variety of 2-aminobenzothiazoles in good to high yields. In a parallel work, Peng and Ding described a similar reaction of 2-aminobenzenethiols with aryl isothiocyanates.1102

They introduced iron(III) sulfate monohydrate as catalyst, DABCO as base, and silica gel as additive. Their protocol also provided the corresponding 2-aminobenzothiazoles in high yields. 3,4-Disubstituted pyrazoles were obtained by reaction of β-carbonyl 1,3-dithianes with hydrazine hydrate using iron(III) chloride as catalyst (Scheme 626).1023 These conditions have also been applied to the stereoselective aminolysis of β-carbonyl 1,3-dithianes to give β-enaminones (cf. Scheme 573). Arylhydrazones have been treated with vicinal diols, which were used as solvent, under oxidative conditions to afford 1,3- and 1,3,5-substituted pyrazoles (Scheme 627).1103 Iron(III) chloride was used as catalyst in combination with acetylacetone as ligand. Oxidative conditions were generated by addition of tert-butyl hydroperoxide and an oxygen atmosphere. The authors demonstrated the utility of their method by employing a large number of diaryl hydrazones for the reaction with ethylene glycol to give 1,3-diarylpyrazoles in good yields (eq a). The reaction with 1,2-propanediol provided 1,3,5-trisubstituted pyrazoles in moderate yields (eq b). A copper/iron cocatalyzed intramolecular annulation of 2-aminopyridines and 1-aminoisoquinolines with alkynes has been described by Liu et al. (Scheme 628).1104 The reaction proceeded with high chemo- and regioselectivity affording imidazo[1,2-a]pyridines and imidazo[2,1-a]isoquinolines in good to high yields. The iron(III) salt participates as a redox 3304

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Chemical Reviews

REVIEW

Scheme 628

Scheme 631

Scheme 629

Scheme 632

Scheme 630

catalyst which reoxidizes copper(I) formed during a reductive elimination step. The resulting iron(II) species is reoxidized by atmospheric oxygen as terminal oxidant. A similar domino reaction of 2-aminopyridines and nitroalkenes affording imidazo[1,2-a]pyridines has been developed by Huang and Yan (Scheme 629).1105 Iron(II) chloride has been demonstrated to be an efficient catalyst for this transformation. 3-Methyl-2-arylimidazo[1,2-a]pyridines were formed probably via a reaction sequence involving Michael addition, 5-exo-trig cyclization, and elimination. Subsequently, Hajra and co-workers described the same transformation using iron(III) chloride as catalyst at a lower reaction temperature (80 °C).1106 The method could be applied to the synthesis of a variety of 2-arylated imidazo[1,2-a]pyridines including a precursor for the peptic ulcer drug zolimidine. Iron(III) chloride proved to be also a suitable catalyst for the three-component reaction of 2-aminopyridines, aldehydes, and nitroalkanes to afford imidazo[1,2-a]pyridines (Scheme 630).1107 A large variety of different substrates were treated to provide the heterocycles in good yields. This transformation includes a reaction sequence of imine formation, nitroalkane addition, 5-exotrig cyclization, and elimination of a nitrous acid equivalent. 1,10 -Bis(diphenylphosphino)ferrocene (dppf) was the most efficient iron catalyst for the reaction of benzylic alcohols with 2-nitroanilines to give 2-arylbenzimidazoles (Scheme 631).1108 This unusual transformation included a hydrogen transfer reaction from the benzylic alcohol to the nitro group forming benzaldehydes and o-phenylenediamines as intermediates. Subsequent condensation, ring closure, and dehydrogenation provided the benzimidazoles in mostly good yields. A similar transformation was discovered in a parallel work by Nguyen and co-workers (Scheme 632).1109 They treated

Scheme 633

2-nitroanilines with benzylamines in the presence of CoBr2 3 xH2O or FeCl3 3 6H2O as catalyst to afford 2-arylbenzimidazoles in high yields (eq a). Using FeCl3 3 6H2O in combination with sulfur as catalyst enabled the application of other alkylamines as reducing agents (eq b) or 2-nitrobenzamides as oxidants (eq c). Reaction of the latter with benzylamines led to quinazolin4(3H)-ones. The iron(III) chloride catalyzed reaction of N-aryl benzamidines with aldehydes and nitromethane provided 1,2,4-trisubstituted imidazoles in moderate to good yields (Scheme 633).1110 No additional base or ligand was required. The requisite oxidative conditions for this transformation were realized by working in air. Majumdar and Ghosh established a method for the preparation of coumarin- and quinolone-annulated thiazoles (Scheme 634).1111 Thus, they treated 5-bromo-6-carbonylaminosubstituted coumarins or quinolones with sodium sulfide in the presence of iron(III) chloride as catalyst in dimethylformamide at 120 °C. After HCl-promoted intramolecular condensation the substituted thiazoles were obtained in high yields. 3305

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Chemical Reviews Scheme 634

REVIEW

Scheme 636

Scheme 635

Behbahani and co-workers applied iron(III) phosphate as catalyst for the synthesis of 2-substituted 2-imidazolines from aldehydes and 1,2-ethylenediamine.1112 The reaction could be conducted in water at reflux affording the imidazolines in very high yields. Iron phosphate acts not only as a Lewis acid catalyst but also as a redox catalyst to oxidize the intermediate imidazolidine to the imidazoline. Atmospheric oxygen serves as terminal oxidant. Alternatively, nitriles could be transformed into the corresponding 2-substituted 2-imidazolines under solvent-free conditions at 100 °C using the same catalyst. Multisubstituted imidazoles were obtained by a regioselective formal [3 + 2] cycloaddition of N-aryl benzamidines with nitroolefins (Scheme 635).1113 The iron(III) chloride catalyzed process probably proceeds in a stepwise manner by initial Michael addition followed by nucleophilic ring closure and elimination. A variety of imidazoles could be obtained in moderate to high yields. 5.6.2. Six-Membered Heterocycles. 5.6.2.1. Piperidines, Pyridines, and Quinolines. An iron-catalyzed Friedl€ander reaction for the synthesis of quinolines has been reported by Wang and co-workers.1114 Thus, 2-aminobenzophenones were treated with aceto- and propiophenones in the presence of iron(III) chloride hexahydrate in an ionic liquid at 100 °C. A series of quinolines was obtained in good to high yields under these conditions. An aza-Prins cyclization using stoichiometric amounts of iron(III) chloride or bromides has been established by Padron and co-workers.1115 They treated homoallylic and homopropargylic tosylamines with aldehydes in the presence of the iron salt to afford 4-halogenated piperidines and Δ3-piperideines, respectively. In a subsequent paper, the authors were able to establish conditions which allowed the application of catalytic amounts of iron(III) chloride or acetylacetonate to promote this transformation (Scheme 636).1116 Instead of the iron salt, trimethylsilyl chloride or bromide was employed as halide source. In addition, homopropargylic alcohols have been converted as well. Thus, a series of 4-halogenated Δ3-piperideines (eq a) and 3,6-dihydro-2H-pyrans (eq b) have been obtained in mostly high yields. The reaction of homoallylamines with aldehydes provided 4-halogenated trans-piperidines with a pronounced diastereoselectivity (eq c).

Scheme 637

Scheme 638

Using iron(III) triflate as catalyst, alkenyl-tethered 3-hydroxyisoindolin-1-ones could be cyclized to the corresponding azacycloalkenes which were obtained in high yields as mixtures of double bond isomers (Scheme 637).1117 A domino reaction of imine formation, alkyne addition, intramolecular hydroarylation, and oxidation has been presented by the research group of Tu (Scheme 638).1118 This threecomponent reaction of aldehydes, anilines, and terminal alkynes was catalyzed by iron(III) chloride and proceeded in toluene at 110 °C affording 2,4-disubstituted quinolines in high yields. Wang and co-workers reported the same conversion also using iron(III) chloride as catalyst but with 1,2-dichloroethane as solvent at 120 °C in a sealed tube.1119 Bobade et al. applied for this transformation iron(III) acetylacetonate and tetrabutylammonium hydroxide as catalytic system and obtained a number of 2,4-disubstituted quinolines in high yields.1095 Yao and co-workers introduced iron(III) triflate as catalyst for this three-component quinoline synthesis under solvent-free conditions, which led to shorter reaction times and enabled a 3306

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Chemical Reviews

REVIEW

Scheme 639

Scheme 641

Scheme 640

Scheme 642

Scheme 643

recycling of the catalyst.1120 Using this procedure, they prepared a large number of quinolines in high yields. Li and co-workers established an iron-catalyzed synthesis of 3-acylquinolines.1121 They treated 2-aminoaryl aldehydes with alkynyl ketones in 1,4-dioxane at 80 °C in air in the presence of catalytic amounts of iron(III) chloride hexahydrate (Scheme 639). A variety of 3-acylquinolines was obtained in good to high yields. In addition, 2-aminoaryl ketones and α-substituted 2-aminobenzyl alcohols have been introduced as substrates to give the corresponding 4-substituted 3-acylquinolines in moderate yields. α-Sulfonamidoallenes have been shown to react with aldehydes forming Δ3-piperideines (Scheme 640, eq a) or pyrrolines (eq b), respectively.1122 The reaction was carried out in dichloromethane at 30 °C using substoichiometric amounts of iron(III) chloride and trimethylsilyl chloride as chloride source. After formation of a tosyliminium intermediate, an aza-Prins cyclization occurred. The differing regioselectivity of the cyclization was rationalized by the stabilizing effect of a phenyl substituent on the allyl cation leading to the pyrroline. In the absence of the phenyl substituent, cyclization to the six-membered ring via a vinyl cation intermediate is favored. N-Phenyl glycinates react with olefins in an iron-catalyzed process under oxidative conditions to give quinolines (see Scheme 847, eq b).1123,1124 Alternatively, this reaction can be split into a three-component transformation employing anilines, ethyl glyoxylate, and olefins (see Scheme 847, eq c). An iron(III) chloride catalyzed three-component reaction of arylamines, glyoxylic esters, and α-ketoesters has been developed by Wang, You, and co-workers (Scheme 641).1125 The reaction proceeded under very mild conditions in acetonitrile at room temperature. Several examples of quinoline-2,4-dicarboxylates could be accessed in moderate to high yields. A Hantzsch dihydropyridine synthesis by reaction of aryl aldehydes with β-keto esters and ammonia has been performed in glycerol solution under microwave irradiation using an

iron(III)salen complex as catalyst.157 For a number of different arylaldehyde substrates, this protocol led to the corresponding dihydropyridines in high yields. 5.6.2.2. Pyrans, Chromanes, and Xanthenes. The reaction of various 2-hydroxybenzaldehydes with 2,2-dimethoxypropane catalyzed by iron(III) chloride hexahydrate provided 2,4dimethoxy-2-methylchromanes in high yields (Scheme 642).1126 The transformation proceeds with complete diastereoselectivity in favor of the trans isomers. Maiti and co-workers described an iron-catalyzed annulation of salicylaldehydes with terminal alkynes to afford flavones (Scheme 643).1127 This approach exploited a dual catalysis by piperidine as organocatalyst and iron(III) chloride as Lewis acid activator. The utility of the method was demonstrated by the synthesis of various flavones in high yields. In a subsequent endeavor, the authors applied dialkyl acetylenedicarboxylates as substrates and obtained 2-oxo-2H-chromene-3-carboxylates under the same conditions in high yields.1128 The iron(III) chloride catalyzed reaction of salicylaldehydes with activated methylene compounds led to coumarin derivatives.1129 The reaction proceeded in ethanol at 80 °C and afforded various functionalized coumarins in high yields. 4-Hydroxytetrahydropyrans have been obtained by a Prins cyclization of homoallyl alcohols with aldehydes (Scheme 644).1130 The reaction was performed in the presence of catalytic amounts of iron(III) chloride and tert-butyldimethylsilyl chloride (TBSCl) in dichloromethane at 10 °C. It is noteworthy that exclusively the hydroxy group, which originates from the aldehyde, is introduced as nucleophile and not the chloride. This may be explained by an initial [2 + 2] cycloaddition step between the olefin and the carbonyl group leading to an oxetan intermediate which is intramolecularly opened by the hydroxy group. Good to excellent diastereoselectivities have 3307

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Chemical Reviews Scheme 644

REVIEW

Scheme 646

Scheme 645 Scheme 647

Scheme 648 been accomplished for this process. The carbon substituents adopt a cis-configuration in the product. Consequently, the hydroxy substituent is in a trans-position. Enantiopure homoallylic alcohols as substrates were obtained by an asymmetric nickel-catalyzed ene reaction. Their configuration was retained in the course of the reaction leading to enantiomeric ratios of the products greater than 99:1. A large variety of alkene and aldehyde substrates could be converted to give 2,4,6-trisubstituted tetrahydropyrans in mostly high to excellent yields. 4H-Pyrano[3,2-b]pyrroles have been obtained by a threecomponent condensation of 3-hydroxypyrroles, aryl aldehydes, and malononitrile catalyzed by iron(III) hydrogen sulfate.1131 The same authors described the utility of a bis(imino)pyridine iron(II) catalyst for a one-pot three-component reaction of naphthols, aromatic aldehydes, and acetophenone or dibenzoylmethane to construct 1H-benzo[f]chromenes (Scheme 645).1132 The p-nitro group of the bis(imino)pyridine iron(II) catalyst was reported to lead to a reduced electron density at the metal center and thus to an increased catalytic activity.1133 The reaction was performed under solvent-free conditions with ultrasonic irradiation and provided a series of benzochromenes in 8090% yield. The three-component reaction of 4-hydroxycoumarines with aldehydes and 1,3-dicarbonyl compounds has been efficiently catalyzed by the Lewis acidsurfactant combined catalyst (LASC) iron(III) dodecyl sulfate [Fe(DS)3] (Scheme 646).1134 Chromeno[4,3-b]chromenes were obtained in high yields. The catalyst was easily recovered by centrifugation and reused four times with almost equal efficiency. A similar transformation was reported by Gu and co-workers (Scheme 647).79 Reaction of cyclohexane-1,3-diones, salicylaldehydes, and acetophenones in the presence of iron(III) triflate in chlorobenzene at 130 °C afforded 4H-chromene derivatives in moderate to good yields. This domino process includes the

formation of a tetrahydroxanthen-1-on-9-yl cation intermediate and its alkylation by the acetophenone nucleophile. The three-component reaction of salicylaldehydes, malononitrile, and triethyl phosphite in water at room temperature to 2-amino-4-chromen-4-yl phosphonates has been catalyzed by highly dispersed chitosan-coated and sulfonated nano Fe3O4 coreshell structures (Fe3O4/CS-SO3H NPs).1135 The products were obtained in excellent yields. Due to its magnetic properties, the catalyst could be easily recovered and reused at least six times with only slight loss of activity. 5.6.2.3. Pyrimidines and Quinazolines 5.6.2.3.1. Biginelli Reaction. Based on their first report on an iron-mediated Biginelli reaction,1136 Lu and Bai described an iron-catalyzed condensation of β-ketoesters, aldehydes, and urea (Biginelli reaction) to construct 3,4-dihydropyrimidin-2(1H)ones (Scheme 648).1137 The iron(III) chloride hexahydrate catalyzed process provided the 3,4-dihydropyrimidin-2(1H)ones within a reaction time of 4 h in 5396% yield and thus proved to be superior to the classical Brønsted acid catalyzed procedure which afforded the products in 2050% yield after 18 h. The same reaction could be catalyzed by a ferrihydrite silica aerogel containing 1113% iron.1138 Three different 3,4dihydropyrimidin-2(1H)-ones were obtained in 3465% yield. Milder reaction conditions could be applied using acetonitrile as solvent and trimethylchlorosilane as additive.953 In addition, under these conditions the authors could extend the substrate 3308

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Chemical Reviews Scheme 649

REVIEW

Scheme 651

Scheme 650

Scheme 652

scope to ketones (Scheme 649).1139 A variety of 5-unsubstituted 3,4-dihydropyrimidin-2(1H)-ones could be synthesized in high yields. The Biginelli reaction of aldehydes, urea, and 3-oxobutanoates in the presence of tetraethyl orthosilicate has been shown to be effectively catalyzed by iron(III) chloride (Scheme 650).1140 A series of dihydropyrimidinones was obtained in high yields by this procedure. The group of Shah reported a Biginelli reaction of 4-methylN-(3-nitrophenyl)-3-oxopentanamide with aryl aldehydes and urea to produce the corresponding 3,4-dihydropyrimidin-2(1H)ones.1141 The reaction was catalyzed by iron(III) chloride hexahydrate. Trimethylsilyl iodide as additive was crucial in order to get the products in high yields. Shirini et al. described the iron(III) hydrogen sulfate catalyzed Biginelli reaction of benzaldehydes, urea, or thioureas, and methyl or ethyl acetoacetate.1142 The transformation was performed either in acetonitrile or under solvent-free conditions. In both cases the resulting 3,4dihydropyrimidin-2(1H)-ones and 3,4-dihydropyrimidin-2(1H)thiones were obtained in high yields. Fe3+montmorillonite K-10 has been introduced as reusable catalyst for a Biginelli reaction to produce dihydropyrimidinones with azo linkers.1143 Neto and co-workers developed two ion-tagged iron catalysts for the Biginelli reaction. The yields for this transformation were significantly improved working in an ionic liquid solution (Scheme 651).1144 A plethora of dihydropyrimidin-2-ones and dihydropyrimidine-2-thiones were synthesized by this method in high yields. Moreover, the catalyst could be recovered and reused at least seven times without loss of activity. Kinetic studies and HR-ESI-MS results indicated that, in this system, the iminium mechanism is favored over the enamine and the Knoevenagel mechanism. Very recently, Mohan et al. have shown that iron(III) tosylate hexahydrate is an efficient catalyst for the Biginelli reaction.1145 Aldehydes and also acetals could be converted to 3,4-dihydropyrimidin-2(1H)-ones and 3,4-dihydropyrimidin2(1H)-thiones in high yields. An iron(III)salen complex was also reported to catalyze this transformation in glycerol solution under microwave irradiation.157 5.6.2.3.2. Other Methods. The condensation of o-phenylenediamines with α-hydroxy ketones has been catalyzed by iron

exchanged molybdophosphoric acid to afford quinoxalines in high to excellent yields.1146 Similar to their synthesis of benzimidazoles (cf. Scheme 631), Deng and co-workers treated 2-nitro-N-arylbenzamides with benzylic alcohols in the presence of 1,10 -bis(diphenylphosphino)ferrocene (dppf) as catalyst to afford 2,3-diarylquinazolinones in good yields.1147 This unusual transformation includes reduction of the nitro group and concomitant oxidation of the benzylic alcohol. Quinazolines were also obtained by an oxidation/cyclization protocol starting from primary alcohols and o-aminobenzamide (eq a), o-aminobenzyl amine (eq b) or o-aminobenzenesulfonamide (eq c) (Scheme 652).1148 This transformation was achieved using iron(III) chloride as catalyst and tert-butyl hydroperoxide as oxidant affording quinazolinones, quinazolines, and benzothiadiazine derivatives in moderate to high yields. A three-component reaction of 1,3-dicarbonyl compounds, arylamines, and formaldehyde in the presence of iron(III) chloride as catalyst led to 1,3-diaryl-substituted hexahydropyrimidines as reported by Mukhopadhyay et al. (Scheme 653).1149 Using the same conditions, an analogous reaction of 2 equiv of dimedone with arylamines and formaldehyde afforded bis-spiro piperidines in high yields (not shown).1149 The synthesis of 4-imino-3,4-dihydroquinazolin-2-yl phosphonates via palladium-catalyzed reaction of carbodiimide, 3309

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Chemical Reviews Scheme 653

REVIEW

Scheme 656

Scheme 657 Scheme 654

Scheme 655

isocyanide, and phosphite has been reported by Wu and coworkers.1150 The yield could be significantly improved when catalytic amounts of iron(III) chloride were introduced. An iron(II) triflate catalyzed oxidative self-condensation of N-phenyl glycinates provided dihydroquinazolines. The reaction includes a complex sequence of amine oxidation, aminal formation, oxidation, FriedelCrafts reaction, and another oxidation and is therefore presented in section 12 (cf. Scheme 847, eq a).1124 Sarva and co-workers treated isatoic anhydride with various amidoximes in the presence of catalytic amounts of iron(III) chloride (Scheme 654).1151 This procedure led to quinazolin4(3H)-ones in high yields. 4H-Pyrimido[2,1-b]benzothiazoles have been obtained by an iron-catalyzed three-component reaction of 2-aminobenzothiazole with aldehydes and 1,3-diketones (Scheme 655).1152 Iron(III) fluoride has been identified as the most efficient catalyst for this transformation, which was conducted under neat conditions at 100 °C. Many annulated pyrimidines were obtained in excellent yields. 5.6.2.4. Pyrazines. Various pyrazine derivatives have been obtained by reaction of 1,2-diamines with 1,2-diketones in the presence of iron(III) chloride as catalyst.1153 Mostly very high to excellent yields were reported for this transformation, which was conducted in water at reflux. 5.6.3. Seven-Membered Heterocycles. An iron-mediated stereoselective cyclization of N-acyliminium ions with pendent

alkenyl groups has been described by Hong et al. (Scheme 656).1154 This aza-Prins-type haloalkylation of the olefin led to 1-benzazepines in high yields and high diastereoselectivity. In principle the reaction could be performed with catalytic amounts of iron(III) chloride, but with lower rates. Thus, 50 mol % iron(III) chloride was chosen for optimal conditions. Trimethylsilyl chloride was introduced as activating agent and chloride source. Bhattacharya and co-workers were able to set up an ironcatalyzed four-component reaction for the construction of benzodiazepinylphosphonates (Scheme 657, eq a).1155 These compounds show clostripain inhibitor activity and thus represent potential gas gangrene drugs. The synthesis starts from o-phenylenediamines which are condensed with two molecules of acetone and a dialkyl phosphite using iron(III) chloride as catalyst. A variety of benzodiazepinylphosphonates could be obtained by this procedure. Unsymmetrical o-phenylenediamines led to mixtures of regioisomers in varying ratios. Maleki employed Fe3O4/SiO2 nanoparticles as catalyst for the diazepine formation from 1,2-diamines, ketones, and isocyanides (Scheme 657, eq b).1156 A series of examples was described affording 1,4-diazepines in high to excellent yields. The catalyst could be recovered and reused up to five times with almost no loss in reactivity. Padron and co-workers developed an efficient protocol for a Prins-type cyclization of 5-hydroxypent-1-enes with aldehydes using iron(III) chloride or iron(III) acetylacetonate as catalyst and trimethylsilyl chloride as additive (Scheme 658).1157 They obtained 2,7-disubstituted 5-chlorooxepanes in high yields and excellent cis-selectivity with respect to the carbon substituents. The chlorine atom was not introduced stereoselectively but was removed during further elaboration of the oxepane skeleton. This method could be successfully applied to a three-step synthesis of (+)-isolaurepan. 3310

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Chemical Reviews Scheme 658

REVIEW

Scheme 660

Scheme 659

Scheme 661

6. CYCLOADDITIONS AND ALDER-ENE-TYPE REACTIONS 6.1. Cycloadditions

Iron-catalyzed cycloaddition reactions have been the subject of continuous scientific research over the past two decades. The development until 2004 has been summarized by Bolm.1 Since then, the topic of iron-catalyzed cycloadditions has been reviewed by Hilt and Janikowski.764 Cyclopropanation and aziridination of olefins with catalytically active iron carbene or nitrene complexes have been summarized by Zhou and co-workers.909 A recent review by Wan is exclusively devoted to the area of iron-catalyzed cycloaddition reactions.1158 Prominent examples including practical procedures are included in the section “Organoiron Chemistry” of the third manual Organometallics in Synthesis.8 6.1.1. [2 + 1] Cycloaddition. The present section describes iron-catalyzed [2 + 1] cycloaddition reactions, except the epoxidation of olefins. 6.1.1.1. Cyclopropanation. An extensive early study of the iron-catalyzed cyclopropanation of olefins with ethyl diazoacetate using various iron porphyrin complexes as catalysts has already been presented by the groups of Woo and Kodadek in 1995 (Scheme 659).1159 With rather high turnover numbers for some iron(II) and iron(III) porphyrin complexes, they obtained trans-cyclopropyl esters in good selectivities. The iron(III) porphyrin complexes have been reduced in situ to their iron(II) congeners by additional reducing agents or by ethyl diazoacetate itself. The cyclopropanation was selective for 1-alkenes. Under some conditions, a considerable amount of diethyl maleate as dimerization product of ethyl diazoacetate has been observed. Also before 2004, the main contributions to the area of ironcatalyzed [2 + 1] cycloadditions, in particular cyclopropanation,

aziridination, and epoxidation of aromatic aldehydes, have been made by the group of Hossain using the iron Lewis acid catalyst [(Cp)Fe(CO)2(THF)]+(BF4).1160 In 2003 Tagliatesta and Pastorini reported a cyclopropanation of styrenes by ethyl diazoacetate using iron(II) meso-tetra-(20 ,60 dichlorophenyl)porphyrin [Fe(TDCPP)] as catalytically active species which was formed in situ form the corresponding iron(III) complex (Scheme 660).1161 In contrast to the rhodium-catalyzed processes, a pronounced anti selectivity was observed which was attributed to a nonconcerted mechanism via radical intermediates. The role of DMSO as a stabilizer of intermediate radicals was discussed. Olefin cyclopropanation with diazoacetates in the presence of μ-oxo-bis[(salen)iron(III)] complexes has been described by Nguyen et al. (Scheme 661).1162,1163 The robust complexes allowed to perform the reaction without exclusion of air. It is noteworthy that also less reactive substrates such as α-methylstyrene, α-(trifluoromethyl)styrene, 1,1-diphenylethylene, methylenecyclohexane, n-butyl vinyl ether, and internal olefins, such as trans- and cis-β-methylstyrenes and ethylidenecyclohexane, could be converted successfully. Hossain and co-workers adsorbed their iron Lewis acidTHF adduct [CpFe(CO)2 3 THF]+BF4 onto silica gel and employed it for the heterogeneous catalysis of the cyclopropanation of 3311

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Chemical Reviews

REVIEW

Scheme 663

Scheme 664 Figure 6. Halterman iron porphyrin [FeHalt(Cl)].

Scheme 662

styrene with ethyl diazoacetate.1164 Compared to the homogeneous catalysis with the free complex (trans/cis = 15:85), the trans/cis ratio was dramatically altered in favor of the transcyclopropane (trans/cis = 67:33 [third to fifth runs]). Good diastereoselectivity in favor of the trans-isomer (>12:1) and also noticeable enantioselectivity (7486% ee) have been achieved employing the chiral Halterman iron porphyrin FeHalt(Cl) as catalyst (Figure 6).1165 The trans/cis ratio could even be improved by addition of organic bases such as pyridine (up to 33:1) and 1-methylimidazole to the reaction. Simonneaux and co-workers described an asymmetric cyclopropanation of styrenes with 2,2,2-trifluorodiazoethane or ethyl diazoacetate catalyzed by the D4-symmetric Halterman iron porphyrin [FeHalt(Cl), see Figure 6] which afforded the corresponding 1,2-disubstituted cyclopropanes in moderate yields (Scheme 662).1166 The diastereoselectivity in favor of the trans-isomer was in most cases excellent, whereas the enantioselectivity was only moderate. In addition, the catalyst has also been grafted on a solid support for heterogeneous catalysis. A water-soluble para-tetrasulfonated Halterman porphyrin iron complex has been introduced by Simonneaux for an asymmetric carbene transfer in water.1167 The cyclopropanation of styrene could be achieved in high yields, high trans selectivity, and also high enantioselectivity. Catalyst reisolation and reuse led to a slight decrease in efficiency, but still good results are obtained even after a fourth cycle. Kwong and co-workers described the enantioselective cyclopropanation of styrene with ethyl diazoacetate.1168 They employed catalytic amounts of iron(II) and iron(III) complexes with chiral C1- and C2-symmetric terpyridine ligands for this transformation and obtained the chiral phenylcyclopropanes in good yields, but in rather low trans selectivities and low to moderate enantioselectivities.

The cyclopropanation of ethene catalyzed by iron carbene complexes has been investigated by means of density functional calculation.1169 Two possible pathways have been identified. One is characterized by a direct attack of ethene at the carbene carbon of the complex which leads directly to the cyclopropane product. An alternative way starts with a ligand exchange to form an η2-ethene iron complex. This intermediate undergoes an oxidative cyclization to afford a ferracyclobutane which reductively eliminates a reduced iron species to release the product. Both pathways can be relevant depending on the ligand and the carbene substituent. Carreira and Morandi described a method for the ironcatalyzed cyclopropanation of olefins with a trifluoroethylcarbene equivalent generated via in situ diazotation of trifluoroethylamine hydrochloride, thus avoiding the preparation of (trifluoromethyl)diazomethane (Scheme 663).1170 The tetraphenylporphyrin iron(III) complex Fe(TPP)Cl (for structure, see Scheme 665) was successfully employed as catalyst. Moreover, it should be noted that the reaction could be performed in aqueous solution. The corresponding trifluoromethylcyclopropanes have been obtained in high yields and with complete trans selectivity. In a subsequent paper, the authors demonstrated the utility of this method for the synthesis of trifluoromethyl-substituted vinyl- and alkynylcyclopropanes.1171 Thus, 1-arylbuta-1,3-dienes or 1-aryl-but-3-ene-1-ynes were treated with trifluoroethylamine hydrochloride in the presence of sodium nitrite in water using Fe(TPP)Cl as catalyst. The reaction afforded chemoand stereoselectively trans-cyclopropanes in high yields. In a similar way, glycine ethyl ester hydrochloride has been introduced as carbene precursor when treated with sodium nitrite in the presence of Fe(TPP)Cl as catalyst.1172 This method led to trans-arylcyclopropanecarboxylic acid esters in good diastereoselectivity and good yields. Cyclopropanation of styrenes, dienes, and enynes by in situ formed diazomethane was achieved in concentrated aqueous potassium hydroxide in the presence of the iron(III) porphyrin complex Fe(TPP)Cl (Scheme 664).1173 Most astonishingly, the alkaline medium which is required for diazomethane formation from the water-soluble N-methyl-N-nitrososulfonamide did not interfere with the action of the iron carbene complex. This was explained by the immiscibility of the olefinic substrate with the aqueous layer. Thus, diazomethane is generated in the aqueous 3312

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Chemical Reviews

REVIEW

Figure 7. Boitrel’s iron porphyrin complex.1175

Scheme 665

Figure 8. Ironphthalocyanine aziridination catalyst.1184

Scheme 666

layer and subsequently transfers to the organic layer where it forms a carbenoid in the presence of the iron catalyst and effects the cyclopropanation reaction. The importance of the safe diazomethane generation and iron-catalyzed carbene transfer to olefins by the Carreira group has been emphasized in a recent highlight article.1174 A very active iron porphyrin complex (Figure 7) for the cyclopropanation of α-methylstyrene with ethyl diazoacetate has been recently introduced by Boitrel et al.1175 Under optimized conditions complete conversion of ethyl diazoacetate was achieved within 5 min at 0 °C in the presence of only 0.01% catalyst. The trans-cyclopropane was obtained in excellent yield (>99%) and diastereoselectivity (trans/cis = 98:2) and significant enantioselectivity (78% ee). The outstanding turnover frequency (TOF) of 120 000 h1 is the highest for a metalloporphyrinmediated cyclopropanation reported so far. Li and Moores et al. employed copper-plated iron nanoparticles for the cyclopropanation of styrenes with diazoesters.1176 The reaction was performed under neat conditions at 40 °C to afford cyclopropanes in good yields with some selectivity for the trans-product. 6.1.1.2. Aziridination. Iron porphyrin catalyzed nitrene CH insertion and aziridination of olefins with iminoiodobenzene derivatives as an analogue to the epoxidation with iodosylbenzene dates back to the beginning of the 1980s.11771179 The iron(III) porphyrin complex Fe(TPP)Cl was successfully employed as catalyst for the aziridination of olefins with bromamineT as nitrene source (Scheme 665).1180 A variety of styrenes but also cyclic alkenes and linear alkenes could be converted to the corresponding tosylaziridines in moderate to good yields. It should be noted that the alkene could be employed as the limiting substrate. The stereoselectivity for the formation of 1,2-disubstituted alkenes was only low.

Hossain and Redlich applied chiral ironPyBox complexes as catalysts for the reaction of ethyl diazoacetate with two different imines providing cis-aziridines in moderate yields and enantioselectivities.1181 The aziridination of olefins with N-tosyliminophenyl-λ3iodinane was achieved using a mixed-valent, dinuclear iron complex with an unsymmetrical hexavalent phenolic ligand.1182 The corresponding aziridines were obtained in moderate yields. In addition, thioanisole could be amidated to the corresponding N-tosyl sulfimine [Ph(Me)SNTs] in excellent yield under the same conditions. A p-phenylenediamine-based iron catalyst on polymer support has been applied to the aziridination of olefins with bromamine-T as a nitrene source to give aziridines in moderate yields.1183 Zhou and co-workers investigated ironphthalocyanines as catalysts for the aziridination of olefins with N-tosyliminophenyl-λ3-iodinane.1184 The 3-trifluoromethylphenoxy-substituted ironphthalocyanine complex depicted in Figure 8 was identified as the most efficient catalyst providing aziridines in good yields. Halfen et al. described the application of two iron(II) complexes with tridendate amine ligands, [(Me5dien)Fe(OTf)2] and [(iPr3TACN)Fe(OTf)2], respectively (Scheme 666), for the aziridination of styrene with N-tosyliminophenyl-λ3iodinane.1185 Both complexes, either with the linear triamine ligand or with the macrocyclic ligand, proved to be suitable catalysts. The reaction was carried out at room temperature with an excess 3313

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Chemical Reviews Scheme 667

of styrene. In the case of a 25-fold excess of styrene, the phenylaziridine was obtained in yields higher than 95%. Computational studies on the chemoselectivity of iminoiron porphyrin complexes suggest that (in contrast to the most often applied PhINTs reagent) electron-donating substituents at the nitrogen with small size will be powerful aziridination reagents, whereas electron-withdrawing substituents will preferably undergo CH bond activation (cf. section 3.11.2).1186 The computational and experimental investigations of this reaction indicated that two mechanistic pathways may be operative.1187 Both of them have an imidoiron(IV) precursor in common. An azaferracyclobutane intermediate was discussed to account for the formation of the cis-aziridine from a Z-olefin substrate. The trans-congener may be formed in a stepwise fashion via an open amidoiron complex. Aziridination of olefins with N-tosyliminophenyl-λ3-iodinane has also been successfully conducted using iron(II) triflate as catalyst (Scheme 667).1188 The corresponding aziridines have been obtained in moderate to high yields (eq a). Addition of chiral bis(imino)pyridine ligands led to an asymmetric induction; however, only low ee values were observed. Under the same conditions, silyl vinyl ethers were transformed to α-N-tosylamido ketones via ring opening of intermediate aziridines (eq b). Che and co-workers described the intermolecular aziridination of alkenes with N-tosyliminophenyl-λ3-iodinane (Scheme 668, eq a) and the intramolecular aziridination of sulfonamides (eq b) in the presence of recyclable terpyridineiron complexes.917,918 The robust protocol provided aziridines in generally high yields. Bolm and co-workers employed iron(II) triflate as catalyst for the aziridination of styrenes with iminoiodinanes in combination with quinaldic acid and an ionic liquid.1189 The reaction was conducted in acetonitrile at 85 °C with equimolar amounts of the iminoiodinanes to give a series of N-sulfonylaziridines in good to excellent yields. The Halterman iron porphyrin catalyst shown in Figure 6 has been employed for the asymmetric cyclopropanation of styrenes with diazoacetophenone.1190 This transformation afforded the corresponding products in good yields, high diastereoselectivities, and good enantioselectivities. A meso-tetrakis(pentafluorophenyl)porphyrinato iron(III) complex has been introduced as catalyst for the aziridination of olefins with various sulfonyl, aryl, and phosphoryl azides (Scheme 669).922 Conducting the transformation in 1,2dichloroethane at reflux provided the aziridines in high yields. By application of microwave conditions, the reaction rate could

REVIEW

Scheme 668

Scheme 669

be drastically increased and the conversion was completed within maximal 3 h. The same catalyst was also used for the sulfimidation of sulfides, allylic amidation, and amination of C(sp3)H bonds (cf. Scheme 512). A macrocyclic tetracarbene iron(II) complex has been synthesized, fully characterized, and introduced as catalyst for the aziridination of olefins with aryl azides (Scheme 670).1191 In general, the aziridines were obtained in moderate to high yields. Even a tetrasubstituted alkene could be transformed to the corresponding aziridine, however, only in low yield, whereas cis-cyclooctene was converted nearly quantitatively. Aziridination of styrene by reaction with organic azides has also been achieved with an iron(II) complex bearing a dipyrromethene ligand.931 For example, the reaction of styrene with adamantyl azide in the presence of 5 mol % dipyrromethene iron complex provided the corresponding aziridine in 85% yield, which corresponds to a TON of 17. 3314

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Chemical Reviews Scheme 670

REVIEW

Scheme 672

Scheme 673 Scheme 671

6.1.2. [2 + 2] Cycloaddition. Fu and co-workers developed an iron-catalyzed asymmetric formal [2 + 2] cycloaddition of ketenes with sulfonylimines (Staudinger β-lactam synthesis) to afford β-lactams in a stepwise process proceeding via ionic intermediates (Scheme 671).1192 They introduced a planar-chiral pyrrolidin-4-ylpyridine iron(II) complex as catalyst with a [Cp*Fe] group as chiral auxiliary and the ferrocene unit as strong electron donor which increases the nucleophilicity of the pyridine nitrogen atom.19 It is noteworthy that tosylimines gave predominantly cis-β-lactams, whereas triflylimines afforded selectively trans-β-lactams in high enantiomeric excess. Bis(imino)pyridine iron complexes have been demonstrated to be efficient catalysts for the intramolecular [2 + 2] cycloaddition of α,ω-dienes (Scheme 672).1193 The method takes advantage of the redox active character of the ligand. An initially proposed mechanism with iron remaining in oxidation state +II throughout the catalytic cycle was later revised returning the redox steps to the iron center by switching between iron(I) and iron(III) (vide infra). Recently reported experimental and computational data indicate an iron(I)/iron(III) couple to be operative in the catalytic cycle (Scheme 673).693 Thus, several potential intermediate bis(imino)pyridinebis(alkene)iron(I) and bis(imino)pyridine bis(alkyl)iron(III) complexes could be isolated and characterized. This also allowed the elucidation of their electronic characteristics. All these complexes include the bis(imino)pyridine ligand in a monoreduced (radical anion) state which enables the generation of the oxidation states of iron(I) and iron(III), respectively. The formal [2 + 2] cyclization proceeds via oxidative cyclization of the

Scheme 674

α,ω-diene ligand with the iron(I) center to form an iron(III) metallacycle which releases the cyclobutane by reductive elimination. During the related hydrogenative cyclization of enynes and diynes, the ligand may pass through a double reduced form which results in a catalytically operative iron(II)/iron(III) redox couple (cf. Scheme 392). Analogously, these complexes could also be employed for the [2 + 2] cycloaddition of ethylene and butadiene to form vinylcyclobutane.1194 The reaction proceeded via an oxidative cyclization to form a cyclic allyliron intermediate which could be isolated during a stoichiometric experiment. Reductive elimination led to the cyclobutane product. Waser and de Nanteuil could demonstrate that aluminasupported iron(III) chloride is an efficient catalyst for the [2 + 2] cycloaddition of enimides with alkylidene malonates (Scheme 674).1195 This procedure provided a range of aminocyclobutanes in high to excellent yields and mostly excellent diastereoselectivities. Aminocyclobutanes can be further elaborated to β-peptides as was exemplified with one of the products. 3315

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Chemical Reviews Scheme 675

REVIEW

Scheme 677

Scheme 676

Scheme 678

6.1.3. [2 + 2 + 1] Cycloaddition. Among the variants to construct five-membered rings by cycloaddition, the least frequently applied method relies on [2 + 2 + 1] cycloaddition reactions. This holds true even though a prominent example is given with the originally cobalt-mediated PausonKhand reaction.11961198 A closely related iron analogue, namely the cycloaddition of acetylene and carbon monoxide using stoichiometric amounts of pentacarbonyliron, even dates back to work by Reppe and Vetter in 1953.1199 The structures of the obtained (cyclopentadienone)iron complexes were later identified by Weiss/H€ubel and Wilkinson.1200,1201 The [2 + 2 + 1] cycloaddition of diynes with stoichiometric amounts of pentacarbonyliron has been largely established by Kn€olker et al. and further elaborated for the construction of arenes which are part of alkaloids or polycyclic aromatic compounds.8,12021213 A catalytic version has only been reported by Imhof et al. for the hetero-PausonKhand cycloaddition of cyclic imidates, carbon monoxide, and ethylene catalyzed by nonacarbonyldiiron (Scheme 675).12141216 A second imine moiety was required in proximity to the reacting imidate, suggesting that the substrate acts as a bidentate ligand for the iron. Computational results revealed that the ethylene will always attack the less stable imine group with less efficient delocalization of π-electron density. 6.1.4. [3 + 2] Cycloaddition. [3 + 2] Cycloaddition reactions with iron-functionalized 1,3-dipolaric compounds were reported early on. In 1982, Rosenblum et al. already described the reaction of (η1-allyl)Cp(CO)2Fe complexes with cycloalkenones to afford cyclopentane-annulated products.1217 In 2000, Nakamura and co-workers applied iron(III) perchlorate as catalyst for the cyclization of cyclopropanone thioacetals with pendent alkene moieties.1218 An iron-catalyzed formal [3 + 2] cycloaddition of styrenes with benzoquinone affording 2,3-dihydrobenzofuran derivatives was presented by Itoh and co-workers (Scheme 676).1219

The transformation was conducted in an ionic liquid (1-butyl3-methylimidazolium hexafluorophosphate = [bmim]PF6) and provided the products after a few minutes in high yields and high diastereoselectivities (eq a). Alternatively, the reaction also proceeded in acetonitrile at room temperature using an aluminasupported iron(III) perchlorate catalyst (eq b). In a subsequent work, the authors applied their method to the synthesis of optically active 2,3-dihydrobenzofurans.1220 Thus, they synthesized 3-hydroxymethyl-substituted 2,3dihydrobenzofurans using the method mentioned above and acylated the racemic mixture with vinyl acetate in the presence of a lipase. This led to a separable mixture of acylated and nonacylated products each in enantiopure form. The dimerization of 2,5-norbornadiene was effected by a catalytic system consisting of iron(III) acetylacetonate, triphenylphosphine, and diethylaluminum chloride to give the hexacyclic endo-endo dimer by a formal [3 + 2] cycloaddition.1221 Aluminum chloride as additive promoted the reduction of the iron to form iron(II) and iron(0) species. An asymmetric [3 + 2] cycloaddition of α,β-unsaturated carbonyl compounds with diaryl nitrones has been achieved in the presence of a chiral cyclopentadienyliron complex bearing bidentate pentafluoroarylphosphinite ligands (Scheme 677).1222 The isoxazolidine products were obtained in moderate yields as mixtures of regioisomers with complete endo selectivity and high enantioselectivity. Bolm and Bonnamour described an iron(II) acetate catalyzed [3 + 2] cycloaddition of nitriles with trimethylsilyl azide (Scheme 678).1223 They obtained 5-substituted tetrazoles in good yields. Using ultrapure iron(II) acetate as catalyst, the products were obtained in slightly lower yields. However, the effect was not as pronounced as to say that other metal impurities in the iron salt of standard quality are the actual catalysts for this reaction. Iron(III) chloride in combination with copper powder was shown to catalyze the [3 + 2] cycloaddition reaction of glycosyl 3316

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Chemical Reviews Scheme 679

REVIEW

Scheme 682

Scheme 680 Scheme 683

Scheme 681

azides with terminal alkynes forming triazoles (Scheme 679).1027 The glycosyl azides were obtained by an iron-catalyzed nucleophilic substitution of peracetylated saccharides with trimethylsilyl azide (cf. Scheme 576). Wang and co-workers developed an enantioselective cycloaddition of azomethine ylides with electron-deficient olefins (Scheme 680).1224 They introduced iron(II) chloride in combination with a prolinol as catalytic system. The reaction was carried out in acetonitrile at room temperature and delivered tetrasubstituted pyrrolidines in good yields, complete endoselectivity, and pronounced enantioselectivities. Feng and Liu treated α-isocyano esters with azodicarboxylates in the presence of iron(II) acetylacetonate and chiral bis(N-oxide) ligands (Scheme 681).1225 In a formal [3 + 2] cycloaddition 1,2,4-triazolines were formed in high yields and with high enantioselectivities. 6.1.5. [2 + 2 + 2] Cycloaddition. The applicability of carbonyliron complexes as catalysts for the trimerization of alkynes to form aromatic compounds was reported as early as

1960 by H€ubel.1226 Under these conditions, unsymmetrical acetylenes gave exclusively the arenes with an unsymmetric substitution pattern. Okamoto demonstrated the utility of an intramolecular cyclotrimerization of triynes using iron(III) chloride, iron(II) chloride, or their hydrates in combination with zinc powder and an NHC ligand as catalytic system (Scheme 682).1227,1228 Annulated benzenes could be obtained in mostly very high yields. The achievements of the group of Okamoto and others in iron-catalyzed [2 + 2 + 2] cycloaddition reactions has been summarized in a recent account article.1229 F€urstner and co-workers employed a low-valent ferrate or a diethylene Cp*iron(I) complex for the same type of reaction (Scheme 683, eq a).1230 In addition, they were able to develop an intermolecular variant with two diynes leading to an alkynetethered cyclopentane-annulated benzene (eq b). Terminal alkynes have been demonstrated to undergo intermolecular cyclotrimerization using iron(II) bis(imino)pyridine complexes as catalysts (Scheme 684).1231 In addition, zinc powder was required as reductant and zinc iodide, which probably activates the precatalyst by ligand exchange, was used. The reaction proceeded under mild conditions in acetonitrile at 50 °C and provided 1,2,4-trisubstituted benzenes with high regioselectivity and in high yields. Wan et al. established an iron-catalyzed method for the [2 + 2 + 2] cycloaddition of diynes and nitriles leading to pyridines (Scheme 685).1232 The transformation was achieved using a 3317

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Chemical Reviews Scheme 684

REVIEW

Scheme 687

Scheme 685 Scheme 688

Scheme 686 Scheme 689

catalyst system consisting of iron(II) iodide and 1,3-bis(diphenylphosphino)propane (DPPP). Zinc powder was required to activate the iron species by reduction. Under the mild reaction conditions, a series of substituted pyridines was obtained in good to excellent yields. The [2 + 2 + 2] cycloaddition of alkynenitriles with alkynes in the presence of catalytic amounts of iron(II) acetate, a bis(imino)pyridine ligand, and zinc as reductant has been reported by Louie et al. (Scheme 686).1233 This method enabled the synthesis of multisubstituted pyridines in generally good yields. Unsymmetrical alkynes led to mixtures of regioisomers. In addition, the same authors were able to demonstrate that under similar conditions diynes can be treated with cyanamides furnishing 2-aminopyridines in high yields (Scheme 687).1234 Unsymmetrical diynes show a good selectivity in favor of the pyridine with the amino group next to the smaller alkyne substituent. A short time later, Wan and co-workers described the same transformation using their catalytic system consisting of iron(II) iodide, 1,3-bis(diphenylphosphino)propane (DPPP), and zinc powder which they had successfully employed for the cyclotrimerization of diynes with nitriles (Scheme 688,

cf. Scheme 685).1235 This system allowed performance of the reaction under even milder conditions in THF at room temperature and provided pyridines in high yields. In analogy to the reaction of alkynenitriles with alkynes (cf. Scheme 686), the former can be converted to 2-aminopyrimidines in a regioselective [2 + 2 + 2] cycloaddition reaction with cyanamides according to Louie et al. (Scheme 689).1236 They used a similar catalytic system consisting of iron(II) iodide in combination with an bis(imino)pyridine ligand and zinc as reductant. In most cases, 2-aminopyrimidines were obtained in only moderate yields. The formation of benzene by cyclotrimerization of acetylene has been observed in the presence of a bis(imino)pyridineiron complex and methylaluminoxane.1237 A sterically more congested bis(imino)pyridineiron complex produced polyacetylene. In analogy to the reports by Wan and co-workers (cf. Scheme 685)1232 the [2 + 2 + 2] cycloaddition of diynes with nitriles has been achieved in toluene under microwave irradiation using a cationic naphthyl(Cp)iron(II) precatalyst in combination with a hemilabile bidentate phosphine ligand.1238 The method 3318

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Chemical Reviews Scheme 690

REVIEW

Scheme 692

Scheme 693

Scheme 691

led to a series of cyclopentane-annulated pyridines in good to high yields. 6.1.6. [4 + 2] Cycloaddition. Iron-catalyzed DielsAlder reactions, including asymmetric Lewis acid catalyzed DielsAlder reactions with various chiral ligands, have already been covered by Bolm in 2004.1 The review by Wan on iron-catalyzed cycloaddition reactions provides a nice overview on this topic up to 2012.1158 In 2004 Shibasaki and co-workers presented a new method for an iron-catalyzed asymmetric DielsAlder reaction between polysubstituted silyl vinyl ethers as dienes and N-acryloyl or N-crotyl oxazolidin-2-ones (Scheme 690).1239 As catalytic system they introduced iron(III) bromide in combination with a chiral aryl-PyBox ligand and silver hexafluoroantimonate as additive. This method enabled the construction of chiral polysubstituted cyclohexanones in high to excellent yields and good enantioselectivities. It has been applied as a key step to the total synthesis of ent-hyperforin.1240,1241 Sibi and co-workers introduced the concept of fluxional additives to iron-catalyzed [4 + 2] cycloaddition reactions (Scheme 691).1242 It is presumed that the basic additive will be forced into a chiral conformation by a chiral ligand and transfer the chiral information more efficiently to the substrate. Moreover, the fluxional additive can be often more easily modified than the chiral ligand. The authors investigated the DielsAlder

reaction of cyclopentadiene with N-acryloyl oxazolidin-2-one in the presence of iron(II) perchlorate, an (S)-phenyl bisoxazoline ligand, and various pyrazolidinones or N-benzoyl oxazolidin-2one as fluxional additives. They found that the additives led to an enhancement of the enantioselectivity. Intramolecular [4 + 2] cycloaddition of unactivated dienynes using a Cp(cod) ferrate as catalyst has been reported by F€urstner and co-workers (Scheme 692).1230 The corresponding 1,4cyclohexadienes could be obtained in moderate to good yields. Schaus and co-workers described an iron-catalyzed domino isomerization/hetero-DielsAlder reaction of 2H-chromenones leading to homodimers (Scheme 693, eq a) or cycloadducts with electron-rich dienophiles (eq b).1243 Iron(III) chloride hexahydrate was employed as a catalyst. For the heterodimerization substoichiometric amounts of the iron salt were required. The reaction afforded tetrahydrochromeno heterocycles in high yields and mostly with complete regioselectivity and moderate diastereoselectivity. A probable mechanism involves oxa-6π electrocyclic ring opening to a (Z)-o-quinone methide, equilibration to the E-isomer, and subsequent DielsAlder reaction with a 4H-chromenone (eq b) or a different dienophile (not shown). 3319

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Chemical Reviews Scheme 694

REVIEW

Scheme 696

Scheme 697

Scheme 695

Cationic porphyrin iron(III) complexes have been found to be active catalysts for the hetero-DielsAlder reaction of aldehydes with 1,3-dienes (Scheme 694).1244 The reaction was carried out in benzene at reflux to afford 3,6-dihydro-2H-pyrans. The utility of this method could be demonstrated by a broad range of examples affording the products in high yields. Substituents such as fluoride, chloride, bromide, nitro, nitrile, alkene, alkyne, and cyclopropyl were tolerated. Shortly thereafter, Matsubara and Kurahashi presented a cationic iron(IV) corrole complex as catalyst for the same transformation.1245 Dihydropyrans were obtained in many cases in excellent yields. In addition, an α,β-unsaturated aldehyde was also selectively converted to the dihydropyran derivative. Solventfree DielsAlder reactions of dienes with p-benzoquinones have been performed using iron(III) chloride on AerosilÒ silica as catalyst.1246 The reaction proceeded at room temperature within a short time and provided the cycloadducts in high yields. 6.1.7. [5 + 2] Cycloaddition. The only examples of ironcatalyzed [5 + 2] cycloaddition reactions have been reported by F€urstner et al. (Scheme 695).1230 They heated alkyne-tethered vinylcyclopropanes in toluene at 90 °C in the presence of catalytic amounts of their (diethene)(Cp)ferrate(0) and obtained the corresponding annulated 1,4-cycloheptadienes in good to excellent yields. Terminal and differently end-capped alkynes could be transformed likewise. 6.2. Intermolecular Alder-Ene-Type Reactions

An early example for this type of transformation has been provided by Takacs and co-workers with the coupling of allylic

Scheme 698

ethers and 2,3-disubstituted 1,3-butadienes resulting in the formation of 1,5-heptadienes.1247 Iron(III) chloride was found to be an effective catalyst for an acetal-ene reaction (Scheme 696).1248 Using this method, 1,1disubstituted olefins were treated with aliphatic or aromatic acetals to provide homoallyl ethers in high yields. Linear olefins give a moderate diastereoselectivity in favor of the trans product. Srivastava and co-workers described an iron-catalyzed nitroso-ene reaction (Scheme 697).1249 The nitroso compound was generated in situ from arylhydroxylamines and treated with α,β-unsaturated carbonyl compounds as ene component in the presence of catalytic amounts of iron(II) chloride tetrahydrate. The transformation provided aza BaylisHillman adducts in moderate to high yields. Mechanistically it might consist of three steps: oxidation of the hydroxylamine to the nitroso compound, nitroso-ene reaction, and reduction of the resulting allylhydroxylamine.

7. ISOMERIZATIONS AND REARRANGEMENTS 7.1. Isomerization

7.1.1. Allyl AlcoholCarbonyl Isomerization. The isomerization of unsaturated alcohols to carbonyl compounds mediated by carbonyliron complexes has already been observed by Emerson and Pettit as early as 1962. They treated butadiene(tricarbonyl)iron with strong acids (e.g., HBF4 or HClO4) and obtained η3-allyliron salts. Subsequent reaction with water led to the η2-(allyl alcohol)iron complexes which isomerized to the enol congener. Eventually, this unstable complex demetalated to release the ketone.1250 Logan and Damico were able to achieve this isomerization with catalytic amounts of pentacarbonyliron (Scheme 698).1251 3320

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Chemical Reviews Scheme 699

REVIEW

Scheme 701

Scheme 700

Scheme 702

Among others, Gree and co-workers contributed substantially to this field especially by combining the photoinitiated allyl alcoholcarbonyl isomerization with an aldol addition.1252,1253 They summarized the achievements in transition-metal-mediated allyl alcoholcarbonyl isomerization in 2003.1254 Another review article on this subject appeared in 2002 by Bouwman and coworkers.1255 The scope of the photoinitiated domino isomerization/aldol addition could be substantially increased by using (bda)Fe(CO)31256 and (cot)Fe(CO)3 as catalysts (Scheme 699).1257 The latter is advantageous, especially for more bulky aldehydes. The products were obtained in high yields and usually with high regioselectivities. In most cases, the diastereoselectivity with respect to syn and anti products was less pronounced. Computational studies revealed that the reaction preferably proceeds via formation of an (allyl alcohol)iron complex followed by a 1,3-hydrogen shift forming an η3-allyl(hydrido)iron intermediate which undergoes a second hydrogen shift to an enoliron complex (Scheme 700).1258 Demetalation releases the enol which isomerizes to the ketone without assistance of the metal. In the domino isomerization/aldol addition the aldehyde reacts with the liberated enol rather than with the coordinated enol.1259 Interestingly, this process could be best described as a carbonyl-ene reaction with simultaneous CC bond formation and proton transfer. A third pathway via coordination of the aldehyde was less favorable. The domino allyl alcoholcarbonyl isomerization/aldol reaction catalyzed by pentacarbonyliron could be applied to the transformation of vinylic furanoses into cyclopentenones, which constitutes a promising approach for the asymmetric synthesis of various types of natural products (Scheme 701, eq a).1260

Similarly, 1-hydroxyisobenzofurans or the corresponding silyl ethers could be converted into indanones (eq b).1261 Via a domino isomerization/aldol reaction the 1-silyloxyisobenzofurans afforded the products in high yields as mixtures of trans and cis isomers in ratios of 1.6:1 to 4.6:1. Gree, Yadav, and co-workers also used vinyl pyranoses for the iron-catalyzed domino allyl alcoholcarbonyl isomerization/ aldol reaction (Scheme 702).1262 This procedure provided short synthetic routes to cyclohexanones and cyclohexenones (eq a). It should be noted that similarly substituted cyclohexanones are not readily available by the classical Ferrier carbocyclization. The utility of this method was demonstrated by short syntheses of six different gabosine natural products as exemplified with gabosine A in Scheme 702 (eq b).1263 3321

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Chemical Reviews Scheme 703

REVIEW

Scheme 706

Scheme 704

Scheme 705

The allyl alcoholcarbonyl isomerization could also be combined with a Mannich reaction. Lactols were functionalized with a sulfinamide moiety which also served as chiral auxiliary. Pentacarbonyliron-catalyzed isomerization/Mannich reaction of the sulfinamides provided sulfinamidocyclopentanones (Scheme 703).1264 The sulfinamidocyclopentanones could be further elaborated to aminocyclopentitols. The method proved to be useful for the synthesis of a mannostatin A analogue as well as for isomers of fucosidase and glycosidase inhibitors. Gree and co-workers demonstrated that the iron-catalyzed domino isomerization/aldol reaction is compatible with protected amino functions as exemplified in Scheme 704.1265 Starting from amino acids, they applied this method to the synthesis of 3-piperidones and 3-piperidols, which represent promising building blocks for the synthesis of bioactive compounds. Renaud et al. have achieved an isomerization of γ-trifluoromethylated allylic alcohols to the corresponding ketones in the presence of 1 mol % tetrakis(isonitrile)iron(II) complex as catalyst. Using a (diacetonitrile)iron(II) catalyst with a tetradentate diiminodiphosphine ligand as catalyst, an enantioenriched allylic alcohol was converted to the ketone with transfer of chirality (Scheme 705).1266

7.1.2. Olefin Isomerization. The isomerization of olefins with stoichiometric12671270 or catalytic amounts12711273 of carbonyliron species has already been studied for several decades without becoming a standard tool in organic synthesis. More recently, the isomerization of olefins has also been observed by Chirik and co-workers during their studies on the hydrogenation and hydrosilylation of olefins.1274 In the absence of hydrogen and silanes, their bis(imino)pyridineiron complex induced an isomerization of alkenes. Thus, 1-hexene was converted to E/Z mixtures of 2- and 3-hexenes. The mechanism of the pentacarbonyliron-catalyzed photochemical alkene isomerization has been investigated by Harris and co-workers using DFT calculations and nano- and microsecond FTIR spectroscopy of Fe(CO)4(η2-1-hexene) in neat 1-hexene (Scheme 706).1275 Upon UV irradiation, one molecule of carbon monoxide is dissociated and the complex Fe(CO)3(η21-hexene) in a singlet spin state is formed. Migration of the allylic hydrogen to the iron center leads to the η3-allyl(hydrido)iron complex HFe(CO)3(η3-C6H11). Coordination of one molecule of 1-hexene and concomitant transposition of the hydride to the terminal position of the allylic ligand leads to the bis-alkene iron complex Fe(CO)3(η2-1-hexene)(η2-2-hexene). This species is thermodynamically favored over the bis-1-hexene complex, which is in equilibrium with the η3-allyl(hydrido)iron intermediate. Release of the 2-alkene product regenerates the initial Fe(CO)3(η2-1-hexene) species which will continue the catalytic cycle. A clean isomerization of 1-alkenes to 2-alkenes has been achieved under mild conditions in THF at room temperature using Fe(acac)3 in combination with phenylmagnesium bromide as catalytic system (Scheme 707).1276 The method was applied to various allylbenzenes, but also linear 1-alkenes could be isomerized to the 2-congeners without formation of mixtures of internal alkenes. 1,5-Cyclooctadiene was isomerized to the conjugated 1,3-isomer, and (Z)-stilbene was transformed to (E)-stilbene. The basic conditions tolerate functional groups such as halogens, ethers, amines, esters, and acetals. In addition, a one-pot procedure for the iron-catalyzed allylation of arylmagnesium bromide and subsequent isomerization to propenylbenzenes could be established. 3322

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Chemical Reviews Scheme 707

REVIEW

Scheme 710

Scheme 708

Scheme 711

Scheme 709

Monoisomerization of terminal double bonds to the neighboring position has also been achieved with dodecacarbonyltriiron as catalyst in the presence of aqueous potassium hydroxide.1277 A variety of terminal olefins could be converted to 2-alkenes in mostly excellent GC yields and pronounced E-selectivity. Potassium hydroxide could be substituted by potassium iodide, which resulted in a slightly lower catalyst activity. However, this may allow the conversion of base sensitive substrates. 7.1.3. Other Isomerizations and Rearrangements. Isomerization of 2-alkyl Grignard reagents to the 1-alkyl counterparts was achieved employing a cooperative iron/copper catalyst (Scheme 708).1278 The isomerized products were quenched with carbon dioxide to afford linear carboxylic acids or, alternatively, with a chlorosilane or benzaldehyde to give the linear alkyl silanes or benzyl alcohols, respectively. An iron(III) chloride catalyzed synthesis of 3-hydroxyphthalates by ring-opening aromatization of 7-oxabicyclo[2.2.1]hepta2,5-dienes has been reported by Sonoda and co-workers (Scheme 709).1279 The bicyclic substrates were readily available by DielsAlder reaction of furans with dimethyl acetylenedicarboxylate. A Schmidt-type rearrangement is the key step in an ironcatalyzed reaction of diarylmethanes or alkylarenes with organic

azides leading to N-alkylanilines (Scheme 710).1280 This transformation includes a CC bond cleavage which occurs during the rearrangement process in combination with hydrolysis of the resulting iminium species. The authors emphasized that this method may well be attractive for polystyrene degradation leading to the formation of synthetically valuable chemicals. The proposed mechanism starts with an oxidation of the benzylic carbon atom by DDQ to give the bisbenzylic cation. Nucleophilic attack by the azide followed by a 1,2-aryl shift from the benzylic carbon to the adjacent nitrogen atom (Schmidt rearrangement) provides an iminium ion, which on hydrolysis leads to the aldehyde and the secondary amine. α-Arylaldehydes have been reported to undergo [1,2]-aryl (Scheme 711, eq a) or [1,2]-alkyl (eq b) shifts catalyzed by iron(III) bromide.1281 The selectivity of the migration is tuned by electronic effects at the aromatic ring. Electron-rich aromatic or heteroaromatic substituents can stabilize an adjacent carbocation and thus lead to a selective migration of the geminal alkyl substituent (eq b). For both pathways, many examples could be presented leading to aryl ketones and alkyl ketones in high yields. Yoon and co-workers presented a method for the kinetic resolution of N-sulfonyl oxaziridines by using iron(II) chloride in combination with a chiral bis(oxazolidine) ligand as catalyst system (Scheme 712).1282 While one enantiomer was ring opened to the corresponding N-sulfonyl imide, the other could be reisolated in high enantiopurity. A special feature of this method is that no additional reagents are required. 3323

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Chemical Reviews Scheme 712

REVIEW

Scheme 714

Scheme 715

Scheme 713

7.2. Cycloisomerization

7.2.1. Intramolecular Alder-Ene Reactions. The chemistry of iron-catalyzed Alder-ene-type carbocyclizations has been mainly developed by Takacs et al. in the late 1980s and 1990s. They employed an iron(0)bipyridine complex as catalyst preferably for the [4 + 4] ene-type carbocyclization which could also be applied to a number of total syntheses of natural products.12831292 Tietze reported a [4 + 2] ene-type cyclization of a 1,7-diene catalyzed by iron(III) chloride.1293 Following the review by Bolm,1 iron-catalyzed intramolecular Alder-ene reactions have been treated in a concise review by Kotora,2 a book chapter by Hilt on iron-catalyzed cycloadditions and ring expansions,764 and in a review article by Yamamoto.1294 The cycloisomerization of 1,6-enynes with cyclic or acyclic alkene moieties has been reported to proceed in the presence of the iron(0)ate complex [CpFe(C2H4)2][Li(tmeda)] (Scheme 713).1230,1295 A variety of bicyclic annulated cyclopentanes (eq a) and monocyclic methylenevinylcyclopentanes (eq b) could be obtained by this method. The bicyclic ring system was constructed stereoselectively to furnish in most cases the trans product with the exocyclic double bond in E configuration (eq a). It is noteworthy that, in the 10- and 12-membered ring series,

the double bond in the ring appeared to be E-configured even though the starting material was introduced as a mixture of Z and E isomers. The acyclic enynes afforded trans-configured cyclopentanes in high diastereomeric excess (eq b). Enynes without substituents between the alkene and the geminal diethoxycarbonyl moieties could not be converted with the depicted (diethylene)iron(0) complex but with a similar (cod)iron(0)ate complex (cf. Scheme 692). Related cycloisomerizations of 1,6enynes to 1-vinylcyclopentenes (7988% yield) which are catalyzed by iron(III) chloride have been described by Echavarren and co-workers.1296 Iron(III) chloride hexahydrate was found to be a potent catalyst for an intramolecular carbonyl nitroso ene reaction (Scheme 714).1297 In the presence of hydrogen peroxide as oxidant, hydroxamic acids could be converted to the corresponding 3-hydroxyoxazolidin-2-ones in good yields. Substrates with an additional substituent at the allylic position led to a mixture of diastereoisomers. The Conia-ene cyclization of 1,3-dicarbonyl compounds with an alkynyl side chain has been described by Lee and Kim (Scheme 715).1298 Iron(III) chloride was a useful catalyst for this transformation. Depending on the substrate either a 5-exodig, 5-endo-dig, or 6-exo-dig cyclization was observed. Accordingly, alkylidenecyclopentanes but also cyclopentenes and an alkylidenecyclohexane, which isomerized to the corresponding cyclohexene, could be obtained by this method (eq a). In addition, the synthesis of vinylstannanes was achieved in high yields starting from tin enolates or alkynylstannanes, which could be generated from vinyl acetates (eq b). Recently, Shaw and White described an asymmetric Conia-ene cyclization in the presence of 7.5 mol % chiral iron(III)salen complex which affords alkylidenecycloalkanes in high enantioselectivities.1299 Similarly, Ratovelomanana-Vidal, Michelet, and co-workers developed an iron(III) acetylacetonate catalyzed Conia-enetype cyclization of aldehydes with a terminal alkyne group (Scheme 716).1300 The reaction was performed in 1,2dichloroethane at 100 °C in a sealed tube. Catalytic amounts 3324

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Chemical Reviews Scheme 716

REVIEW

Scheme 719

Scheme 717

Scheme 718

of cyclohexylamine were added which may account for enamine formation or alternatively as base to form the enolate in the course of the reaction. 7.2.2. Nazarov Cyclization. The Nazarov cyclization of 3-enoyl-substituted thiophenes has been accomplished in the presence of catalytic amounts of iron(III) chloride (Scheme 717).1301 The reaction proceeded diastereoselectively providing trans-5,6-dihydrocyclopenta[b]thiophen-4-ones in good to high yields. In contrast to earlier reported iron-mediated Nazarov cyclizations,1302,1303 this method does not require a directing silicon group. This strategy could be extended to pyrrole derivatives (Scheme 718).1304 Alumina-supported iron(III) perchlorate was found to be an efficient catalyst for this transformation. A variety of 2- or 3-enoyl-substituted pyrroles could be converted to the trans-4,5-dihydrocyclopenta[b]pyrrol-6-ones in high yields and excellent diastereoselectivity (eq a). The products could be further treated with vinyl ketones under the same

conditions, thus leading to a protocol for an efficient sequential iron-catalyzed Nazarov/Michael reaction (eq b). In a subsequent work, this one-pot reaction was performed in ionic liquids providing 5-disubstituted 4,5-dihydrocyclopenta[b]pyrrol-6ones in high yields even after repeated use of the catalyst.1305 An asymmetric variant has been developed by the same authors by using iron(III) perchlorate hexahydrate or triflate as catalyst in combination with the chiral bis(oxazoline) ligands PyBox-ip and PyBox-tb (cf. similar structures in Figure 18, section 9.5).1306 Thus, the cyclization of some divinyl ketones to the corresponding cyclopentenones was achieved in moderate to good yields and mostly moderate enantioselectivity. In addition, a domino Nazarov cyclization/fluorination protocol was established in the course of this work using N-fluorobenzenesulfonimide as electrophilic fluorinating agent. Recently, Itoh and co-workers applied their Nazarov protocol to the cyclization of indole, benzofuran, and benzo[b]thiophene derivatives.1307 Indolyl vinyl ketones were most efficiently cyclized in the presence of 5 mol % of the above-mentioned Fe(ClO4)3/Al2O3 catalyst in dichloromethane at reflux, while iron(III) chloride was employed for benzofurans and benzo[b]thiophenes. Generally good yields have been accomplished for the cyclopentanone-annulated indoles. Some benzofuran and benzo[b]thiophene derivatives were also obtained in good yields when working in 1,2-dichloroethane solution at 60 °C. Tang et al. developed a one-pot iron-catalyzed domino Wittig/ Nazarov reaction for the formation of β-methylenecyclopentenones (Scheme 719).986 Thus, they generated phosphonium ylides from diazoketones and triphenylphosphine in the presence of tetra(p-chlorophenyl)porphyrin iron(III) chloride (Fe(tcpp)Cl). Subsequently, an acyl chloride and triethylamine were added to form in situ the corresponding ketene, which on reaction with the ylide led to an allenyl ketone. After addition of trifluoroacetic acid, the Nazarov cyclization occurred affording the β-methylenecyclopentenones in high yields. An iron-mediated Nazarov cyclization of β-alkoxy divinyl ketones has been described by Shindo and Yaji.1308 The method provided α-exo-methylene cyclopentenones and was applied to the construction of the core skeleton of stemonamide and for the total synthesis of (()-xanthocidin.1308,1309 7.3. Intramolecular Ring Expansion

The topic of this section is also part of a book chapter on ironcatalyzed cycloadditions and ring expansion reactions by Hilt and Janikowski.764 These authors provided a first example for the ring expansion of epoxides upon reaction with pendent alkene 3325

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Chemical Reviews Scheme 720

REVIEW

Scheme 723

Scheme 724

Scheme 721

Scheme 722

functions.765 A hexahydrocyclopenta[c]furan was obtained in high yield using an iron(II)dppe catalyst in the presence of zinc and triethylamine. In a subsequent paper, a series of different ligands was screened.766 Finally, the scope of the method was explored using a variety of substrates with different double bond substituents (Scheme 720).1310 In addition, the linker between the epoxide and the alkene function was modified by introduction of ether and ester groups. In all cases, exclusively the cis-fused cyclopenta[c]furan derivatives were formed. Jiao and co-workers established an iron-catalyzed method for the ring expansion of alkynylcyclopropylalkanols to cyclobutanols (Scheme 721).1311 The reaction was conducted in a mixture of acetone and nitromethane at room temperature under an oxygen atmosphere. Iron(II) chloride was found to be the most active catalyst. Many other transition metal salts led to no conversion. A variety of substrates could be transformed with mostly good yields and good diastereoselectivity in favor of the trans-cyclobutanols. Nitrene iron complexes are likely to be intermediates in the ring expansion of 2-aryl-2H-azirines to indoles (Scheme 722).1312 This consecutive azirine ring opening/nitrene NH insertion was performed in THF at 70 °C under iron(II) chloride catalysis. A variety of 2,3-disubstituted indoles could be obtained in mostly moderate to high yields. Substituents such as bromo, fluoro, nitro, methoxy, trifluoromethyl, OTBS, alkenyls, and pivalates were tolerated.

2-Acyl-2H-azirines underwent a ring expansion to isoxazoles in the presence of substoichiometric amounts of iron(II) chloride upon heating in dioxane at reflux (Scheme 723).1313 This transformation was combined with the preceding azirine formation by treatment of enaminones with phenyliodine diacetate (PIDA) to afford a series of isoxazoles. Plietker and co-workers observed that acyl vinyl cyclopropanes undergo a ClokeWilson rearrangement in the presence of catalytic amounts of TBAFe to form dihydrofurans.1314 Quantum chemical investigations revealed that the NO ligand participates in this catalysis. Thus, electrons are transferred from a covalent FeN π-bond to the substrate resulting in a formal oxidation of the NO ligand and vice versa. A clear preference for an SN2 or SN20 pathway could not be established due to similar activation barriers.

8. METATHESIS REACTIONS 8.1. Olefin Metathesis

An iron-catalyzed olefin metathesis has not been practically realized so far. Cavallo and Poater, however, outlined the feasibility of this transformation by density functional theory calculations and compared it with the common rutheniumcatalyzed process.1315 The theoretical iron-catalyzed process shows a less endothermic reaction profile and a small reduction of the upper energy barriers. In contrast, Dixon and co-workers reasoned upon theoretical calculations that Grubbs-type iron carbene complexes might be rather poor catalysts for olefin metathesis due to a low iron carbene bond dissociation energy and thus a low probability to form the corresponding hypothetic intermediates.1316 8.2. CarbonylAlkyne Metathesis

An iron-catalyzed intramolecular formal alkynecarbonyl metathesis has been established by Jana and co-workers (Scheme 724).1317 This method has been utilized for the conversion of O-propargylsalicylaldehydes into 2H-chromenes 3326

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Chemical Reviews Scheme 725

in the presence of catalytic amounts of iron(III) chloride. A series of such products could be obtained in good to high yields. It was proposed that an oxacyclobutene intermediate is formed via a stepwise ionic mechanism which then undergoes an electrocyclic ring opening to provide the α,β-unsaturated carbonyl moiety. In a subsequent paper the authors applied this method to the construction of substituted phenanthrenes starting from 20 -alkynylbiphenyl-2-carbaldehydes (Scheme 725, eq a).1318 Substituents such as methoxy, fluoro, chloro, and ester groups were tolerated under these conditions. In addition, a ketone could also be employed as carbonyl component. Starting from an appropriately functionalized naphthalene precursor, a benzo[c]phenanthrene could be synthesized. Recently, the authors extended their methodology to the synthesis of benzo[b]oxepines and dibenzo[b,f]oxepines (eq b).1319 It was found that among the various Lewis and Brønsted acids which were tested only iron(III) chloride exhibited a catalytic effect. This method was further extended to the formation of 1,2dihydroquinolines and dihydrobenzo[b]azepines starting from N-propargyl and N-homopropargyl-substituted o-aminobenzaldehydes, respectively.1320 A series of N-tosyl-1,2-dihydroquinolines was synthesized in high yields. In addition, a one-pot protocol including the detosylation and aromatization of these products was developed.

9. REDUCTIONS Iron-catalyzed reductions have been extensively investigated during the past two decades. This is reflected by a number of reviews on this topic.3,4,8,15,13211329 A special focus has always been on reduction of olefins and carbonyl compounds by hydrogenation or hydrosilylation. But reductions with other substrates and/or reagents are also of importance in synthetic organic chemistry as will be illustrated in this section. 9.1. Hydrogenation of Alkenes and Alkynes

Catalytic hydrogenation of olefins in the presence of pentacarbonyliron dates back to the 1960s.1330,1331 Frankel and coworkers investigated the hydrogenation of methyl linoleate and methyl linolenate catalyzed by pentacarbonyliron. They found mainly monoenes as reaction products along with some minor amounts of the fully saturated methyl stearate. Recent advances in this area have been summarized by Chirik1324 and Thomas.1326 The hydrogenation of olefins under mild conditions has been reported by Chirik and co-workers using their bis(imino)pyridine bis(dinitrogen) iron complex (iPrPDI)Fe(N2)2 in toluene at room temperature (Scheme 726).1274 The reaction

REVIEW

Scheme 726

Figure 9. α-Diimine iron alkene complexes.

could be efficiently performed at a hydrogen pressure of 1 atm; however, routinely it was operated at a hydrogen pressure of 4 atm. Turnover frequencies up to 1814 h1 have been reported for the hydrogenation of 1-hexene. This exceeded the efficiency of palladium on carbon, Wilkinson’s catalyst (PPh3)3RhCl,1332 and Crabtree’s catalyst [(cod)Ir(PCy3)py]PF61333 under these conditions by far. However, it should be noted that the precious metal catalysts are usually applied in polar reaction media. The internal alkynes diphenylacetylene and 2-butyne have been hydrogenated stepwise to cis-alkenes and finally to alkanes using (iPrPDI)Fe(N2)2 as catalyst. Terminal alkynes did not form welldefined products under these conditions. The same authors found a low activity of α-diimine iron alkene complexes (Figure 9) for the hydrogenation of 1-hexene, whereas the corresponding arene and bis(α-diimine) complexes were inactive. A related alkyne complex showed only very low activity.1334 Replacing the methyl group in (iPrPDI)Fe(N2)2 by a phenyl moiety provided an even more active catalyst for the hydrogenation of 1-hexene.1335 However, for more hindered substrates such as cyclohexene and (+)-(R)-limonene the (iPrPhPDI)Fe(N2)2 catalyst (cf. Figure 18, section 9.5) was less active due to a deactivation pathway under formation of η6-aryliron complexes. In analogy to their bis(imino)pyridine iron complexes, the authors constructed bis(diisopropylphosphino)pyridine iron complexes as catalysts for the hydrogenation of 1-hexene and cyclohexene as model compounds.1336 Based on this pincer structure they isolated a dihydrido dinitrogen derivative that exhibited some activity for 1-hexene hydrogenation, but performed very poorly for the reduction of cyclohexene. Turning their attention to the original bis(imino)pyridine iron complexes, Chirik et al. investigated the substrate scope of the alkene hydrogenation at 4 atm hydrogen pressure.1337 The hydrogenation of allyl amines was efficient. Allylic and vinylic 3327

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Chemical Reviews Scheme 727

REVIEW

Scheme 729

Scheme 728

ethers were converted with the same TOF as the corresponding α-olefins. Carbonyl compounds showed diverse results. High TOF could be achieved for crotyl and cinnamyl esters. 5-Hexen2-one required higher temperatures (65 °C) to achieve a reasonable turnover, whereas (+)-dihydrocarvone, vinyl acetate, and allyl acetate gave no reaction. Oxidative addition of the CO bonds of ethers and esters has been identified as a principal deactivation pathway.1338 In continuation of their investigations, the group of Chirik synthesized a variety of dinuclear bis(imino)pyridine iron dinitrogen complexes such as [(MePDI)Fe(N2)]2(μ2-N2) (Scheme 727).1339 They were found to be efficient catalysts for the hydrogenation of α,β-unsaturated esters. The efficiency of the catalyst could be adjusted by the substituents at the aryl moiety. The compound with dimethylaryl substituents at the imine nitrogen was superior to the corresponding ethyl or isopropyl derivatives and proved to be one of the most active catalysts for olefin hydrogenation at that time. Compared to the bis(ketimino)pyridine iron complex (iPrPDI)Fe(N2)2 (cf. Scheme 726), a related bis(aldimino)pyridine iron butadiene complex showed a relatively poor catalytic activity for the hydrogenation of ethyl 3-methylbut-2enoate.1340 This was attributed to the more open coordination environment resulting in slower precatalyst activation and occurrence of deactivation processes. Peters and Daida described iron precatalysts of the type [PhBPiPr3]Fe-R and [PhBPiPr3]Fe(H)3(PR3) which have been employed for the hydrogenation of olefins and certain alkynes (Scheme 728).1341 The alkyliron(II) complexes of type A were

more active than the trihydrides of type B for the hydrogenation of styrene at room temperature under atmospheric hydrogen pressure. The redox couple Fe(II)/(IV) is probably responsible for the catalytic function. De Vries and co-workers demonstrated that iron nanoparticles are efficient catalysts for the hydrogenation of alkenes and alkynes.1342,1343 The reaction was carried out in THF at room temperature with a typical hydrogen pressure of 20 bar. The hydrogenation of norbornene, some terminal olefins, and cis-2hexene could be driven to full conversion under these conditions, whereas trans-2-hexene was only partly converted. Some terminal and internal alkynes could be transformed to the corresponding alkanes. Using a lower hydrogen pressure and shorter reaction times, the Z-alkenes could be obtained as main products along with the fully reduced alkanes. Iron nanoparticles supported on graphene have been employed as catalysts for the hydrogenation of various terminal and cyclic olefins and 1-phenyl-1-propyne.1344 The catalyst could be removed by magnetic decantation and reused without significant loss of activity. Small size iron(0) particles have been identified as products of the reaction of iron(III) chloride with ethylmagnesium chloride.1345 According to X-ray absorption spectroscopy these clusters contain about eight iron atoms and are probably stabilized by THF molecules. Unlike earlier assumptions and experimental evidence (cf. section 2.4.1.6), no magnesiumcontaining ligands have been observed.1345 These particles were shown to be suitable catalysts for the hydrogenation of allylbenzenes, styrenes, and aryl acetylenes (Scheme 729). Each of these substance classes required fine-tuned reaction conditions in order to achieve high to excellent yields. In addition, the ironcatalyzed hydrogenation could be combined with a preceding iron-catalyzed cross coupling of aryl Grignard reagents with allyl acetates (cf. Scheme 202) as reported by the same group.412 The hydrogenation of a large number of monosubstituted and 1,1- and 1,2-disubstituted aryl and alkyl alkenes has been achieved using iron(II) chloride in combination with a tetradentate iminopyridine ligand and isopropylmagnesium chloride as catalytic system (Scheme 730).1346 3328

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Chemical Reviews Scheme 730

REVIEW

Scheme 732

Scheme 731

Scheme 733

Figure 10. Ferraboratrane hydrogenation catalyst.

A selective transfer hydrogenation of terminal alkynes to alkenes has been reported by Beller and co-workers (Scheme 731).1347 Iron(II) tetrafluoroborate hexahydrate in combination with tetraphos as ligand has been employed as catalytic system and formic acid as hydrogen source. For a large variety of aromatic alkynes quantitative yields have been reported. Aliphatic and heteroaromatic alkynes could also be converted with excellent results. Peters and co-workers employed ferraboratranes (Figure 10) as catalysts for the hydrogenation of ethene, styrene, norbornene, and phenylacetylene with a rather low rate which induced detailed mechanistic studies.1348 It is noteworthy that the boron may act as a hydride shuttle accompanied by reversible FeB bond cleavage. The group of Beller used ultrasmall iron(0) nanoparticles as catalysts for the hydrogenation of alkenes and alkynes which they obtained from decomposition of {Fe[N(SiMe3)2]2}2 under dihydrogen.1349 A variety of terminal and cyclic alkenes and alkynes could be hydrogenated in often quantitative yields at room temperature with a hydrogen pressure of 10 bar. Noncyclic internal alkenes except trans-stilbene and internal triple bonds appeared to be nonreactive substrates under these conditions. A polymer-supported iron catecholate was found to be an effective catalyst for olefin hydrogenation either as iron(II) or in the oxidized iron(III) form.1350 The oxidation state did not

change under the hydrogenation conditions. Thus, dihydrogen activation was proposed to involve the ligand as well, possibly by a dearomatization reaction. Selective partial hydrogenation of alkynes providing (E)alkenes has been reported by Milstein et al. using an acridinebased P,N,P iron pincer complex as catalyst (Scheme 732).1351 The transformation was performed at 90 °C under a hydrogen pressure of 410 bar. High yields and mostly excellent E-selectivities were reported. A short time later, von Wangelin and co-workers described the same transformation but with Z-selectivity using iron(0) nanoparticles (generated by reduction of iron(III) chloride with ethylmagnesium chloride) in a biphasic solvent system consisting of heptane and an ionic liquid (Scheme 733).1352 The presence of a nitrile function, either as substituent in the ionic liquid or as acetonitrile additive, was essential for a high stereoselectivity. Starting from various aryl and alkyl alkynes, the products were obtained in high yields and in most cases with almost complete Z-selectivity. The ionic liquid prevents particle aggregation and allows for the separation of the catalyst layer by decantation. Reuse of this layer up to six times led to no decrease in catalytic activity. Transfer hydrogenation of alkynes with isopropanol has been reported using a (hydrido)iron complex bearing an SiH-functionalized cyclopentadienyl ligand.1353 Even though the corresponding alkenes were obtained in only moderate yields along with low amounts of the alkanes, an interesting mechanistic pathway may be realized by this complex. According to the 3329

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Chemical Reviews Scheme 734

REVIEW

Scheme 736

Scheme 735

Scheme 737

authors’ proposal, the hydrogen atoms were transferred consecutively via hydrometalation and reductive elimination steps. 9.2. Hydrosilylation and Hydromagnesiation of Alkenes and Alkynes

Iron-catalyzed hydrosilylations of olefins and carbonyl compounds have been covered already by Bolm in 2004 and in a more recent review by Zhang.1325 Chirik’s bis(imino)pyridineiron complex (iPrPDI)Fe(N2)2 (cf. Scheme 726) proved to be a suitable catalyst for the hydrosilylation of alkenes and alkynes (Scheme 734).1274 The reaction of olefins with phenylsilane or diphenylsilane was performed at room temperature in the presence of only 0.3 mol % (iPrPDI)Fe(N2)2. Terminal olefins were converted to the primary alkylsilanes. In terms of reactivity they are followed by 1,1-disubstituted olefins. The reaction of internal alkenes was much slower. Reaction of diphenylacetylene afforded the silylated cis-stilbene which was inert under the reaction conditions, probably due to steric encumbrance. In a subsequent study it could be demonstrated that the complex (iPrPhPDI)Fe(N2)2 (cf. Figure 18, section 9.5), wherein the methyl groups of the original complex (iPrPDI)Fe(N2)2 were replaced by phenyl groups, was more active for the hydrosilylation of 1-hexene (TOF = 930 h1) but less efficient for cyclohexene and (+)-(R)-limonene.1335 Moreover, the research group of Chirik reported a highly efficient hydrosilylation of terminal alkenes with tertiary silanes to primary alkylsilanes by using well-characterized bis(imino)pyridine(dinitrogen) iron complexes (Scheme 735).1354 In model reactions, the corresponding products were obtained in excellent yields using mild conditions and extremely low catalyst loadings (∼0.0040.05 mol %). Due to the perfect regioselectivity, functional group tolerance, and high activity, this method is attractive even for industrial applications in competition with established platinum- and rhodium-catalyzed processes. The group of Ritter described a 1,4-hydrosilylation of 1,3dienes with triethoxysilane using a bis(aryl)iron(II) precatalyst

Scheme 738

(Scheme 736).1355 Addition of an iminopyridine ligand formed in situ a bis(iminopyridine)iron(0) complex via reductive elimination. The hydrosilylation proceeded in high yields and excellent regioselectivity. In addition, a nearly complete selectivity in favor of the E-isomers was observed. Starting from the bis(iminopyridine)iron(0) complex, the authors proposed a mechanism that involves the dissociation of a pyridine and of an iminopyridine ligand (Scheme 737). Complexation of the diene in an η4-mode followed by oxidative addition of the silane leads to a hexacoordinated (hydrido)iron complex. The diene inserts into the FeSi bond forming an η3-allyl(hydrido)iron complex which undergoes haptotropic migration to an η1-alkyl(hydrido)iron species. Finally, the silylated alkene is released by reductive elimination. Nakazawa and co-workers treated divinyldisiloxanes with various hydrosilanes in the presence of catalytic amounts of CpFe(CO)2Me (Scheme 738).1356 An overall hydrosilylation was achieved, however, in an asymmetric manner. One of the 3330

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Chemical Reviews Scheme 739

REVIEW

Scheme 741

Scheme 740

Figure 11. Terpyridine and bis(imino)pyridine iron complexes.

double bonds was dehydrogenatively silylated while the other was hydrogenated. This unusual transformation provided the products in high yields. It can be explained by a stepwise silametalation of one double bond by an initially formed silyliron complex and subsequent β-hydride elimination. The resulting hydridoiron moiety transfers the hydride to the other double bond via hydrometalation, oxidative addition of hydrosilane, and reductive elimination under CH bond formation. The reduction of alkynes with triethoxysilane in the presence of catalytic amounts of nonacarbonyldiiron and tributylphosphine provided alkenes as mixtures of E and Z isomers (Scheme 739).1357 Usually, good conversions have been achieved in THF solution at 60 °C, conditions that afforded chemoselectively the alkene products within 48 h. For many examples the diastereoselectivity, mostly in favor of the Z product, was only low. Slightly higher Z/E ratios could be achieved by replacing tributylphosphine with the newly synthesized N,N-diphenyl-1H-pyrrol-1-amine-based monodentate phosphine ligands.1358 Plietker and co-workers introduced a (hydrido)iron(0) catalyst for the hydrosilylation of internal alkynes (Scheme 740).1359 Vinylsilanes were obtained in mostly very high to excellent yields and good to excellent stereoselectivities. The stereoselective course of the transformation could be controlled by the bulkiness

of the silane. In the reaction with diarylalkynes, sterically congested silanes led to (E)-vinylsilanes which after desilylation provided (Z)-alkenes (eq b). Phenylsilane, as a less bulky silane, predominantly formed (Z)-vinylsilanes and thus led to (E)-alkenes after desilylation (eq a). Nakazawa and co-workers synthesized several iron(II) complexes bearing unsymmetrical terpyridine ligands and employed them as catalysts for the hydrosilylation of olefins (Scheme 741).1360 In addition, catalytic amounts of sodium triethylborohydride were required to activate the catalyst. High turnover numbers of up to 1533 were reported for the reaction of 1-hexene and 1-octene with phenylsilane, diphenylsilane, and methylphenylsilane to form the corresponding anti-Markovnikov products. A simple increase of the amount of catalyst led to a selective double hydrosilylation at 100 °C. Parallel to this work, Chirik and co-workers also employed a terpyridine iron complex (Figure 11) for the anti-Markovnikov hydrosilylation of 1-octene. This terpyridine iron complex was found to be active when triethylsilane or (Me3SiO)2MeSiH was used as hydrosilylating agent.1361 No reaction was observed with triethoxysilane. In addition, the activity of several bis(imino)pyridine iron complexes was studied as well. Less hindered derivatives, as for example (EtPDI)Fe(CH2SiMe3)2 (Figure 11), showed a good activity in the hydrosilylation with 3331

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Chemical Reviews Scheme 742

REVIEW

Scheme 744

Scheme 743 Scheme 745

triethoxysilane or (Me3SiO)2MeSiH, whereas no reaction was observed with triethylsilane. The more hindered complexes, (iPrPDI)Fe(CH2SiMe3)2 and the bis(oxazoline) compound (R, R)-(iPrPyBox)Fe(CH2SiMe3)2 (Figure 11), showed no reactivity with all three silyl reagents. Ethylmagnesium bromide in the presence of catalytic amounts of iron(II) chloride was employed for the hydromagnesiation of diaryl alkynes (Scheme 742).1362 Trapping with protons provided the Z-olefins in high yields and high diastereoselectivity (eq a). In addition, other electrophiles could be added to the intermediate vinylmagnesium bromides to give the formal syn addition products with high selectivity and moderate to high yields (eq b). Moreover, conjugated diynes could be transformed to enynes in good yields, very high regioselectivity, and moderate stereoselectivity. Huang and Walter et al. employed phosphinite iminopyridine iron complexes for the hydrosilylation of a variety of terminal olefins with primary, secondary, and tertiary silanes (Scheme 743).1363 Good to excellent yields have been reported for this transformation which could also be performed in the presence of various carbonyl functional groups such as ketones (eq b), esters, and amides. Internal alkenes were not reactive

under these conditions which enabled selective transformations as demonstrated with 4-vinylcyclohexene (eq a). Iron(II) chloride in combination with bis(imino)pyridine ligands and Grignard reagents has been used as a catalytic system for the hydrosilylation of alkenes (eq a) and alkynes (eq b) with phenylsilane (Scheme 744).1364 Functional groups such as halide, amino, ester, ketone, imine, nitrile, and imidate were tolerated. A large variety of substrates could be converted in THF at room temperature in mostly high yields. Alkynes afforded selectively vinylsilanes under these conditions (eq b). Internal double bonds did not react in most cases. Thomas and co-workers developed a procedure for an ironcatalyzed hydromagnesiation of styrenes (Scheme 745).1365 This transformation was achieved by using iron(III) acetylacetonate as catalyst in combination with TMEDA. The resulting benzyl Grignard reagent was trapped by carbon dioxide to get the corresponding carboxylic acid. This protocol constituted the final step of an elegant synthesis of ibuprofen using exclusively iron-catalyzed reactions. 9.3. Reduction of Alkenes with Other Reductants

Prabhu and co-workers used aqueous hydrazine in the presence of iron(III) chloride hexahydrate as catalyst for the reduction of olefins and alkynes to the corresponding alkanes (Scheme 746).1366 An advantage of this method is the low excess of hydrazine required for a full conversion. Several substrates also bearing other reducible functional groups (nitro, azide, benzyl, ester, carboxylic acid, amide, and carbamate) could be converted in excellent yields. Terminal double bonds are more reactive and can be selectively reduced in the presence of internal double bonds. 3332

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Chemical Reviews Scheme 746

REVIEW

Scheme 748

Scheme 747

Scheme 749

An efficient method for the reduction of alkenes with sodium triethylborohydride in the presence of substoichiometric amounts of iron(II) triflate has been developed by Thomas et al. (Scheme 747).1367 Many substrates could be transformed to the alkanes in quantitative yield. The method worked best for monosubstituted and trans-1,2-disubstituted alkenes. Trisubstituted alkenes were converted in slightly lower yields. Moreover, terminal and internal alkynes could be reduced to alkanes in up to 84% yield with some alkene byproduct. In analogy, the same authors developed a protocol for the reduction of alkenes with sodium borohydride.1368 This transformation was achieved in ethanol solution at room temperature in the presence of catalytic amounts of iron(III) triflate. This simple protocol could be applied to a large variety of alkenes. Many unpolar alkenes could be converted in high yields. Some substrates, with functional groups such as hydroxy and keto, and also various styrenes led to the corresponding products in only moderate yields. 9.4. Hydrogenation of Carbonyl Groups and Derivatives

The hydrogenation of carbonyl compounds has long been dominated by rhodium, ruthenium, and iridium catalysts. Iron catalysts have also been studied for this purpose for about three decades. However, it is only recently that their efficiency can compete with noble metal catalysts for specific applications. The first examples of a reduction of carbonyl compounds catalyzed by iron complexes were reported by Marko and co-workers using carbonyl(hydrido)iron complexes as catalysts and carbon monoxide as reductant.1369 In a subsequent work, they applied hydrogen as reductant in the presence of catalytic amounts of pentacarbonyliron and reported high conversions for a number of aldehydes and ketones.1370 In the following years a number of different conditions and iron catalysts have been described for this transformation. Even though this subject has not been extensively covered in the parent review by Bolm,1 several other review and highlight articles are devoted to this topic.3,4,8,13211324,13271329

9.4.1. Hydrogenation and Transfer Hydrogenation of Ketones and Aldehydes. Gao and co-workers employed carbonyl iron complexes with chiral diaminodiphosphine ligands for the asymmetric transfer hydrogenation of aromatic ketones with isopropanol as hydrogen source and solvent (Scheme 748).1371,1372 The iron catalysts were formed in situ from the carbonyl(hydrido)iron cluster [Et3NH][HFe3(CO)11] and the corresponding ligands. The corresponding products were obtained in moderate to high yields for a number of substrates. Enantioselectivities were generally moderate to good, except for two sterically crowded ketones which were reduced with excellent enantioselectivity. The group of Beller introduced a three-component catalytic system consisting of dodecacarbonyltriiron or iron(II) chloride, 2,20 :60 200 -terpyridine (terpy), and triphenylphosphine for the transfer hydrogenation of aliphatic and aromatic ketones utilizing 2-propanol as hydrogen donor in the presence of a base (Scheme 749).1373 The corresponding alcohols were obtained in good to excellent yields without significant difference between the two iron catalysts. Deuteration experiments suggested a monohydride mechanism to be operating in this system. In a subsequent paper the authors introduced in situ generated iron porphyrin complexes as catalysts for the transfer hydrogenation of ketones with isopropanol.1374 Conversions of up to 99% and turnover frequencies of up to 642 h1 were reported for this biomimetic transformation. The method could be applied to 2-aryloxy- or 2-alkyloxy-substituted ketones with turnover frequencies of up to 2500 h1.1375 Nine different substrates were converted in good to excellent yields demonstrating the utility of this method. 3333

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Scheme 750

Figure 12. Diiminodiphosphine iron(II) complex (L = NCMe).

Scheme 751

Kn€olker’s iron complex1212,1376 has been demonstrated to be an efficient catalyst for the chemoselective hydrogenation of aldehydes, ketones, and imines in toluene at room temperature under moderate hydrogen pressure (3 atm) (Scheme 750).1377 Isolated CdC double bonds, CtC triple bonds, halides, nitro groups, epoxides, and ester functions were tolerated under these conditions. The transfer hydrogenation of acetophenone with isopropanol has also been successfully performed with this catalyst. The importance of this procedure has been emphasized in several highlight articles.4,1322,1378,1379 In order to get some insight into the mechanism of this reaction, Casey et al. synthesized alcohol iron complexes as potential intermediates of this reaction (Scheme 751).1380 These complexes easily lose the alcohol ligand. Under a hydrogen atmosphere, Kn€olker’s (hydrido)iron complex was regenerated. A concerted transfer of a proton from the hydroxy group of the ligand and hydride from the iron to the aldehyde was proposed based on intramolecular trapping experiments. In addition, theoretical investigations have provided evidence for the concerted hydrogen transfer.13811384 The following mechanism has

been proposed. In a first event, the ketone is attached to the hydroxy group of the complex via hydrogen bonding. Subsequently, hydride and proton are transferred to the ketone via a pericyclic transition state. The resulting alcohol stays loosely connected to the iron complex by a hydrogen bond to the carbonyl group and an agostic interaction between the CH bond and the iron. This intermediate may lose the alcohol to form a free coordination site at the iron center (middle pathway) which is eventually occupied by dihydrogen. Alternatively, dihydrogen will coordinate directly to this intermediate while keeping the product attached via a CdO 3 3 3 H—O hydrogen bond. In the latter case, dihydrogen activation occurs by assistance of the alcohol to form a (hydrido)iron complex with a hydrogen-bound alcohol. Release of the alcohol regenerates the initial catalyst. In the central pathway, the dihydrogen splitting into hydride and proton occurs without assistance of the alcohol and leads directly to the starting (hydrido)iron complex. Computational calculations on the effect of substituents revealed that electron-withdrawing substituents are kinetically and thermodynamically favorable for the hydrogenation of aldehydes with Kn€olker’s iron complex. In addition, replacing carbonyl ligands by phosphines with electron-donating groups should improve the catalytic activity.1383,1385 The properties of cyclopentadienone complexes and their application as catalysts for the reduction of carbonyl compounds and the oxidation of alcohols, respectively, have been summarized recently by Rodriguez and Quintard.1386 The group of Morris investigated the asymmetric hydrogenation and transfer hydrogenation of ketones using differently substituted diiminodiphosphine iron(II) complexes which are derived from the ligand used by Gao (cf. Scheme 748). In their initial work, they employed a well-defined diiminodiphosphine iron(II) complex (Figure 12) for the asymmetric hydrogenation of acetophenone and for the asymmetric transfer hydrogenation (ATH) of a variety of ketones and imines with isopropanol and achieved excellent conversions and moderate enantioselectivities.1387 A maximum turnover frequency of about 1000 h1 was reported, which was competitive with the best ruthenium catalysts. Spectroscopic and DFT studies suggested that unusual ferraaziridine intermediates are formed in the course of this reaction.1388 In a following work, Morris et al. introduced a new iron catalyst based on a more flexible diiminodiphosphine ligand which displayed an even higher activity (Scheme 752).1389,1390 Thus, turnover frequencies of up to 4900 h1 and enantiomeric excess values of up to 94% became feasible. Furthermore, several modifications of the ligands have been studied in terms of their influence on the catalytic activity of the iron complex. Morris et al. synthesized chiral and achiral iron(III) complexes with tetradentate diiminodiphosphine or diaminodiphosphine ligands and investigated their efficiency as catalysts for the hydrogenation of acetophenone.1391 Complexes without 3334

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Scheme 753

Figure 13. Diiminodiphosphine iron(II) complex (R = p-tolyl).

axial substituents on the diamine bridge showed a moderate activity for the hydrogenation, whereas the complexes with an axial phenyl substituent exhibited a low activity but led to a respectable enantioselectivity (61% ee). According to DFT calculations, a mechanistic action similar to the ruthenium catalysts was presumed. In the course of their search for active catalysts for the asymmetric transfer hydrogenation (ATH), the group of Morris further varied the substitution pattern of their diiminodiphosphine iron(II) complexes. Thus, various alkyl substituents have been introduced at the phosphorus, and the diamine backbone of the ligand has been modified.1392,1393 In addition, the stereoelectronic influence of various aryl groups at the phosphorus has been studied as well.1394 In comparison with earlier synthesized ligands, a narrow window of electronic and steric parameters for the phosphorus donors, as defined by CO stretches and Tolman cone angles, could be identified for a high catalytic activity. This research culminated in the development of a catalyst with a turnover frequency of 30 000 h1 for the ATH of acetophenone which constituted the highest activity of a catalyst for this transformation at that time (Figure 13). The role of the base in the ATH with the diiminodiphosphine iron(II) complexes (cf. Scheme 752) is the deprotonation of the ligand in α-position to the imine.1395 Such double deprotonated complexes could be synthesized and employed for the ATH of acetophenone without addition of an external base and showed a similar activity compared to the parent complexes in combination with an excess of base. Extensive kinetic and other investigations including isolation of intermediates and DFT calculations led to a mechanistic concept for the ATH of ketones with diiminediphosphine iron complexes of the type displayed in Figure 13 (Scheme 753).1396,1397 During an induction period, the precatalyst is activated by monodeprotonation in α-position to the imine

Figure 14. Amine(imine)(diphosphine)iron(II) complex.

affording an enamido complex. The subsequent hydride transfer from the isopropoxide to the imine moiety of the complex, probably via a stepwise inner-sphere mechanism, is slow and leads to an amido(enamido)iron(II) species. This complex constitutes an active catalyst for the ATH. It receives a proton and a hydride from isopropanol probably via a stepwise outersphere mechanism. The resulting amine(enamido)hydrido iron intermediate in turn transfers the hydrogen to the ketone. Recently, Morris et al. have developed highly active iron(II) catalysts for the asymmetric transfer hydrogenation of ketones and imines.1398 Application of amine(imine)(diphosphine)iron complexes (see example in Figure 14) for the ATH of ketones at 28 °C led to turnover frequencies of up to 200 s1 at 50% conversion and turnover numbers of up to 6100 at equilibrium. Moderate to excellent ee values have been reported for this transformation. The active amido(enamido)iron(II) complex (cf. Scheme 753) is formed directly by double deprotonation from the introduced amine(imine) precursor. This avoids the slow hydride transfer from isopropoxide to the amido(imine)iron species outlined above, and consequently, no induction period has been observed in this catalytic system. Furthermore, Morris and co-workers found that in fact iron(0) nanoparticles formed from the precatalysts are the catalytically active species in this ATH of ketones.1399 Support for this proposition derived from scanning transmission electron microscopy (STEM), superconducting quantum interference device (SQUID), and X-ray photoelectron spectroscopy (XPS) analyses. However, a parallel operation of homogeneous catalysis could not be ruled out completely. Royo and co-workers synthesized iron complexes with cyclopentadienyl-NHC chelating ligands [see for example 3335

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Scheme 754

Figure 16. Chiral (cyclopentadienone)iron complex.1407

Figure 17. Acetonitrile(cyclopentadienone)iron complex.1408

Figure 15. Iron borohydride pincer complex.

(Cp*-NHC)Fe(CO)I, Figure 18, section 9.5] and introduced them as efficient catalysts for the transfer hydrogenation of some ketones using isopropanol as hydrogen source.1400 The asymmetric transfer hydrogenation of aromatic and heteroaromatic ketones has been described by the group of Reiser employing chiral bis(isonitrile)iron(II) complexes as catalysts.1401 The reaction proceeded at room temperature in isopropanol as solvent and hydrogen source in the presence of potassium tertbutoxide as base. For both classes of substrates good to excellent conversions (GC) and mostly moderate enantioselectivities were achieved. Catlow and co-workers studied the mechanism of the trans-(diamine)(dihydrido)(diphosphine)iron(II)-catalyzed asymmetric hydrogenation of ketones by DFT analysis and compared it with that of the highly selective ruthenium catalysts introduced by Noyori.1402 They found the same intermediates for the iron system and similar activation energies and concluded that it should be possible to achieve high asymmetric inductions with the iron complexes as well. Chirally modified derivatives of Kn€olker’s iron complex1376 have been synthesized and employed for the light-induced enantioselective hydrogenation of acetophenone.1403 Yields of up to 90% (GC) and ee values of up to 31% were reported. It is noteworthy that the hydride transferring (hydrido)iron complex could be generated in situ from its carbonyl congener under hydrogen atmosphere, thus avoiding the handling of the rather labile species. A (hydrido)iron(II) pincer complex has been demonstrated to be a useful catalyst for the hydrogenation of ketones (Scheme 754).1404 The reaction proceeded under mild conditions in ethanol at room temperature. For a number of examples, a very low catalyst loading (0.05 mol %) was sufficient to provide the products in high yields. This corresponds to turnover numbers of up to 1880. The same authors introduced an iron borohydride pincer complex for the hydrogenation of ketones at low hydrogen pressure (4.1 atm) (Figure 15). Turnover numbers of up to 1980 were reported for this iron-catalyzed hydrogenation.1405 The reaction was performed under similar conditions as described above, and no additional base was required. A variety of

substrates could be converted to the corresponding alcohols in good to high yields. Benzaldehyde gave only low yields of benzylic alcohol, and α,β-unsaturated ketones led to mixtures of hydrogenated products in moderate yields. NHC iron complexes have been identified as catalysts for the transfer hydrogenation of 20 -acetonaphthone. Catalyst loadings as low as 0.1 mol % still provided the hydrogenated products in high yields.1406 A series of chiral (cyclopentadienone)iron complexes (see example in Figure 16) in combination with trimethylamine N-oxide has been investigated by Wills and co-workers for the ATH of acetophenone with formic acid.1407 Some catalysts provided good yields, but enantiomeric excess was only low. Funk and co-workers employed air-stable nitrile-ligated (cyclopentadienone)iron complexes originally synthesized by Kn€olker et al.1408 as catalysts for the transfer hydrogenation of aldehydes and ketones using isopropanol as hydrogen source (Figure 17).1409 The reaction afforded a large number of primary and secondary alcohols in mostly excellent yields. Gao and co-workers included the motif of diamino- and diiminodiphosphine ligands into a macrocyclic framework (Scheme 755).1410 These chiral ligands have been tested in combination with dodecacarbonyltriiron as catalytic system for the ATH of ketones with isopropanol (eq a). Excellent conversions as well as excellent enantioselectivities have been achieved for a large number of substrates. With a catalyst loading as low as 0.1 mol %, turnover frequencies of about 1000 h1 could be obtained. The same catalyst system has also been applied recently to the asymmetric hydrogenation of aromatic and heteroaromatic ketones (Scheme 755, eq b).1411 A large variety of substrates could be converted into the chiral alcohols in high yields and excellent ee values. Air- and water-stable tricarbonyl(cyclopentadienone)iron complexes have also been successfully employed for the transfer hydrogenation of a variety of aromatic aldehydes and ketones providing the corresponding primary and secondary alcohols in excellent yields (Scheme 756, eq a).1412 The active (hydrido)iron species were formed in situ from water under basic conditions (Hieber base reaction). Functional groups such as halides, methoxy, nitro, trifluoromethyl, trifluoromethoxy, amines, esters, amides, and N-, O-, and S-heterocycles are well tolerated. In addition, α,β-unsaturated aldehydes could be readily converted into allylic alcohols under these conditions (eq b). The chemoselective hydrogenation of the carbonyl group in β,γ-unsaturated 3336

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Scheme 757

Scheme 758

Scheme 756

aldehydes using Kn€olker’s catalyst has been rationalized in a computational study by Zhang, Sun, and co-workers.1384 Renaud and co-workers synthesized ionic derivatives of Kn€olker’s complex and applied them to the catalytic hydrogenation of aldehydes, ketones, and imines in pure water under mild conditions.1413 Moreover, it was shown that using water as solvent led to improved reaction rates. Selective hydrogenation of α,β-unsaturated aldehydes has also been achieved using low catalytic amounts of an iron(II) complex bearing a tetradentate phosphine ligand (Scheme 757).1414 Allylic alcohols were obtained in excellent yields. Analogously, aromatic, heteroaromatic, and alkyl aldehydes were quantitatively

converted to the corresponding alcohols. In the presence of other reducible functional groups, like olefins, ketones, and esters, a high chemoselectivity for the reduction of aldehydes was observed. The same transformation was achieved using the catalyst system and the conditions applied by Beller et al. to the transfer hydrogenation of terminal alkynes (cf. Scheme 731).1415 Iron(II) tetrafluoroborate hexahydrate in combination with tetraphos as ligand was employed as catalyst in only 0.4 mol %. Formic acid served as hydrogen source. The reaction proceeded in THF at 60 °C. Several allylic alcohols have been obtained in quantitative yield from the corresponding α,β-unsaturated aldehydes. An even larger number of aromatic, heteroaromatic, and aliphatic aldehydes could be treated to give quantitatively the primary alcohols. A dual iron-catalyzed process has been developed by Beller and co-workers for the hydrogenation of α-keto and α-imino esters (Scheme 758).1416 This method utilized the NAD(P)H model dihydrophenanthridine (DHPD) as hydrogen transferring agent. A combination of dodecacarbonyltriiron and iron(II) triflate was employed as catalyst. According to the proposed mechanism, the former catalyzes the hydrogenation of DHPD. The transfer hydrogenation is catalyzed by Lewis acids which can be the iron carbonyl or derived species or the iron(II) salt. 9.4.2. Hydrogenation of Imines. N-Phosphinylimines have been successfully subjected to an iron-promoted asymmetric 3337

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Chemical Reviews Scheme 759

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Scheme 761

Scheme 760

Scheme 762

transfer hydrogenation.1417 The carbonyl(hydrido)iron cluster [Et3NH][HFe3(CO)11] was employed as hydride transfer reagent in catalytic or stoichiometric amounts in combination with a diiminodiphosphine ligand. Using this method, a large number of phosphinic amides could be synthesized in high yields and excellent enantioselectivities. The same transformation was efficiently performed using Morris’ diiminodiphosphine iron(II) complexes (Scheme 759).1418 High conversions and excellent enantioselectivities have been accomplished with only low catalyst loadings. In a subsequent work, Beller et al. developed a catalytic system for the asymmetric hydrogenation of N-aryl ketimines, which is reminiscent of biocatalytic systems found in iron-based hydrogenases (Scheme 760).1419 It consisted of Kn€olker’s iron complex1376 as hydride donor working in cooperation with a binaphthyl-based phosphoric acid as chiral Brønsted acid. A large variety of N-aryl ketimines could be hydrogenated to amines in high yields and excellent enantioselectivities. Beller et al. combined the hydroamination of alkynes with an enantioselective hydrogenation to give chiral amines in high yields and high enantiomeric excess in a one-pot procedure (Scheme 761).1420 They employed a triple-catalyst system to achieve this transformation. The hydroamination was catalyzed by a gold(I) complex. The consecutive hydrogenation was catalyzed by combination of an axially chiral Brønsted acid for imine activation and Kn€olker’s catalyst1376 to mediate the addition of hydrogen.

9.4.3. Hydrogenation of Other Carbonyl Compounds. The reduction of carbon dioxide and bicarbonates has been reported using hydrido(phosphine)iron(II) complexes generated from iron(II) tetrafluoroborate and tetradentate phosphine ligands as catalysts (Scheme 762).1421,1422 Bicarbonates could be reduced to formates reaching turnover numbers of up to 7500 (eq a, ref 1422). Carbon dioxide was converted to alkyl formates in the presence of the corresponding alcohol and triethylamine as base (eq b, ref 1421). Formamides were obtained when the reaction was conducted in methanol or THF in the presence of a secondary amine (eq c, ref 1422). Turnover numbers of up to 5104 were achieved in these transformations. Milstein and co-workers introduced their iron pincer complex for the hydrogenation of bicarbonate and carbon dioxide 3338

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Scheme 765

Scheme 766

Scheme 764

(Scheme 763).1423 It should be noted that this transformation was carried out at relatively low hydrogen pressure and moderate temperatures. High turnover numbers of up to 788 have been achieved for the reduction of carbon dioxide, but the yields of sodium formate were only moderate. The key step of the mechanism is a direct hydride transfer from a trans-dihydridoiron complex to carbon dioxide (CO2 insertion). The catalyst is regenerated by complexation of dihydrogen and subsequent heterolytic cleavage upon attack of hydroxide or by dearomatization of the pyridine followed by proton migration. In a subsequent work, the authors could demonstrate that the same iron pincer complex can be applied as catalyst in combination with a base for the hydrogenation of trifluoroacetic acid esters under mild conditions providing the corresponding alcohols in often excellent yields (NMR yields of trifluoroethanol) (Scheme 764).1424 Beller and co-workers were able to hydrogenate unactivated esters in the presence of a hydridoborohydride P,N,P-pincer iron complex1425,1426 as homogeneous catalyst under base-free conditions (Scheme 765).1427 The reaction was conducted in THF at 100 or 120 °C under a hydrogen pressure of 30 bar to afford the corresponding alcohols in high yield. Moreover, a methyl ester group in a complex dodecapeptide as synthetic

intermediate to Alisporivir could by readily hydrogenated in the presence of an acetate function. Two months later, Fairweather and Guan submitted a paper on the same transformation using the same hydridoborohydride iron complex.1428 They conducted the reaction in toluene or THF at 115 °C under a hydrogen pressure of 10 or 16 bar to afford the corresponding alcohols also in high yields. Using their hydridoborohydride P,N,P-pincer iron complex as catalyst, Beller and co-workers described the selective hydrogenation of aliphatic and aromatic nitriles under mild conditions.1429 Noteworthy is that this complex represents the first example of a homogeneous catalyst for the selective hydrogenation of adiponitrile to 1,6-hexamethylenediamine, an important building block for the polymer industry. Very recently, Schneider and co-workers reported diverse applications of the hydrido borohydride P,N,P-pincer iron complex as catalyst for the hydrogenation of ketones.1430 The combination of a chiral Brønsted acid and Kn€olker’s catalyst1376 was employed for the enantioselective hydrogenation of 2H-1,4-benzoxazines to dihydro-2H-benzoxazines (Scheme 766).1431 9.5. Hydrosilylation of Carbonyl Compounds and Derivatives

Alternatively to the hydrogenation of carbonyl compounds using dihydrogen or hydrogen transfer reagents such as formic acid or isopropanol, carbonyl compounds can be reduced with silanes as hydride source. The resulting silyl ethers are often cleaved under the reaction conditions. If this is avoided, the hydrosilylation represents a convenient method to generate silylprotected alcohols directly from carbonyl compounds. Recently, iron-catalyzed hydrosilylation reactions have been reviewed by 3339

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Chemical Reviews Zhang1325 and Thomas.1326 For many examples, the hydrosilylation of carbonyl groups and derivatives can be achieved using simple iron salts (e.g., FeCl3) as catalysts,201,1432 or in some cases even without metal catalysts.1433 Thus, the justification for applying structurally sophisticated complexes as catalysts for nonasymmetric reductions appears questionable. 9.5.1. Hydrosilylation of Aldehydes, Ketones, and Derivatives. An early example of this kind of transformation was reported by Brunner and Fisch, who treated acetophenone with diphenylsilane in the presence of Cp(CO)Fe(II) complexes under thermal and photolytic conditions.1434,1435 They observed an exclusive formation of the silylated 1-phenylethanol rather than a silyl vinyl ether as originally expected in analogy to ruthenium-catalyzed reactions. Nishiyama and Furuta were able to establish appropriate conditions for the hydrosilylation of various ketones in the presence of catalytic amounts of iron(II) acetate and nitrogen-based ligands such as TMEDA (Scheme 767).1436 The corresponding alcohols were obtained in high yields upon hydrolytic workup. Initial experiments for the asymmetric hydrosilylation have been carried out using the chiral ligands PyBox-bn, Bopa-ip, and Bopa-tb (Figure 18, section 9.5). In most cases, the enantiomeric excess of the products was good (3779% ee). In a subsequent work, it was disclosed that a combination of iron(II) acetate and sodium thiophene-2-carboxylate promotes the hydrosilylation of aromatic and aliphatic ketones even more efficiently and leads to excellent yields for a variety of substrates.1437 Shortly after the first report of Nishiyama on the iron-catalyzed hydrosilylation of ketones in 2007,1436 the Beller group established conditions for an iron-catalyzed hydrosilylation of aldehydes.1438,1439 They applied iron(II) acetate as catalyst, tricyclohexylphosphine as ligand, and polymethylhydrosiloxane (PMHS) as stoichiometric reductant. A large number of aromatic, heteroaromatic, and aliphatic aldehydes could be converted to the primary alcohols in high to excellent yields. Consequently, Beller et al. employed chiral phosphine ligands for an iron-catalyzed asymmetric hydrosilylation of ketones.1439,1440 In this early report on the enantioselective iron-catalyzed hydrosilylation, they achieved already high enantioselectivities (up to 99% ee) for electron-rich aryl ketones using iron(II) acetate as catalyst and (S,S)-Me-DuPhos (Figure 18, section 9.5) as chiral ligand. Notably no other activators or additives were required. In a parallel endeavor, Gade et al. relied on bis(pyridylimino)isoindoles as framework for chiral ligands.1441 Using Fe(tetraphenyl-carbpi)(OAc) (Figure 18, section 9.5) as catalyst for the hydrosilylation of ketones with diethoxy(methyl)silane, they obtained the corresponding products in high yields and with high enantioselectivities. At the same time, the group of Chirik introduced their bis(imino)pyridine iron precatalysts (iPrPDI)Fe(CH2SiMe3)2 and (cyAPDI)Fe(CH2SiMe3)2 (Figure 18, section 9.5) for the racemic hydrosilylation of aldehydes and ketones with diphenylsilane. The transformation of some aryl methyl ketones and p-methylbenzaldehyde following this procedure afforded the corresponding alcohols in excellent yields, whereas aliphatic ketones gave only moderate results.1442 The low catalyst loading of 0.1 mol % for one of the precatalysts is a significant feature of this method. In extension of their catalyst screening for the hydrosilylation of ketones, Chirik et al. introduced various (RPyBox)Fe(CH2SiMe3)2, (RBox)Fe(CH2tBu)2, and (RBox)Fe(CH2SiMe3)2 complexes (Figure 18, section 9.5).1443 Employing 0.3 mol % of

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some of these catalysts at room temperature resulted in excellent yields for a variety of substrates. Various functional groups were tolerated but steric hindrance (o,o0 -dimethylaryl ketones) led to a complete loss in activity. The asymmetric induction was only low under these conditions. Activation of the catalyst with a neutral borane led to a significant improvement of enantioselectivity. Nikonov and Vyboishchikov synthesized nonclassical iron silyl dihydride complexes and showed the activity of one of them for the hydrosilylation of benzaldehyde (Figure 18, section 9.5).1444 The structure of these complexes and computational results indicated that the iron-catalyzed hydrosilylation proceeds via unprecedented silyl iron complexes bearing two μ2-H(FeSi) ligands. Complete reduction of aromatic and aliphatic ketones and aldehydes with PMHS has been reported by Campagne et al. applying iron(III) chloride hexahydrate as catalyst under microwave irradiation at 120 °C (Scheme 768).1445 The corresponding methylene compounds could be isolated in high yields. Using iron(II) acetate in combination with the Bopa-dpm ligand (Figure 18, section 9.5), excellent yields and mostly high enantioselectivities have been achieved for the asymmetric hydrosilylation of ketones and enones. A similar catalyst system derived from cobalt(II) acetate provided even better results with respect to the enantioselectivity.1446 Nishiyama and co-workers demonstrated that both enantiomers of the hydrosilylation product are accessible from the same enantiomer of a chiral ligand by a slight variation of the reaction conditions (Scheme 769).1447 Iron(II) acetate in combination with (S,S)-Bopa-dpm provided the R-enantiomer of the alcohol (eq a), whereas the S-enantiomer was obtained when the corresponding iron(III) precatalyst (FeCl2[(S,S)-Bopa-dpm]) was activated by zinc (eq b). In both cases, diethoxy(methyl)silane was used as stoichiometric reducing agent. The group of Nishiyama also applied the bis(oxazolinyl)phenyl-containing complex (S,S)-(PheBox-ip)FeBr(CO)2 (Figure 18, section 9.5) to the asymmetric hydrosilylation of 4-phenylacetophenone with diethoxy(methyl)silane.1448 Under optimized conditions, in hexane and using sodium acetate as additive, the corresponding alcohol was obtained in an excellent yield (99%) and an enantiomeric excess of 66%. Royo’s iron complexes with cyclopentadienyl-NHC chelating ligands [see for example (Cp*-NHC)FeCl, Figure 18, section 9.5] proved to be suitable catalysts for the hydrosilylation of benzaldehydes using various silanes as stoichiometric reductants.1400 The simple complex Fe[N(SiMe3)2]2 was also found to be an effective catalyst for the hydrosilylation of ketones and aldehydes. In some cases, catalyst loadings of 0.010.03% were sufficient to achieve an excellent conversion of the substrate in this reaction.1449 Guan and co-workers employed the hydridoiron complex [2,6-(iPr2PO)2C6H3]Fe(H)(PMe3)2, Figure 18, section 9.5) with phosphinite-based pincer ligands for these transformations and obtained the corresponding secondary and primary alcohols in high yields.1450 An iron NHC complex, generated in situ from iron(II) acetate, IPr 3 HCl (Figure 18, section 9.5), and n-butyllithium, has been successfully applied as catalyst for the hydrosilylation of ketones with polymethylhydrosiloxane (PMHS) as reductant.1451 Good to high isolated yields of the obtained secondary alcohols have been reported. 3340

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Figure 18. Iron complexes and ligands used for the hydrosilylation of carbonyl compounds and other reductions.

The group of Thiel used iron(II) acetate and octanoate together with various pyrazol-3-yl-pyridines, 4,40 -bipyrimidines, bipyrazoles, and a pyridylpyrimidine as catalytic system for the hydrosilylation of acetophenone.1452 The reaction could be

performed in alkane solution at 80 °C using polymethylhydrosiloxane (PMHS) as reductant. Ligands with electron-donating substituents led to higher yields than those with electron-withdrawing groups. 3341

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Scheme 768

Scheme 769

Togni and Fl€uckiger synthesized new ligands consisting of a pentasubstituted cyclopentadiene connected with a chiral diamine (see example in Figure 18, section 9.5) and applied them to the asymmetric hydrosilylation of acetophenone with phenylsilane or other silanes.1453 It was assumed that by addition of iron(II) or iron(III) salts chelating cyclopentadienyliron complexes are generated in situ. Fe(acac)2 was the most efficient catalyst precursor. A quantitative conversion and low to moderate enantioselectivities (up to 37% ee) were reported using 4 mol % Fe(acac)2 at room temperature. Darcel et al. utilized the iron complex (dppe)2Fe(H)2 as catalyst in combination with sodium tetraethoxyborate as cocatalyst.1454 The hydrosilylation reaction of several ketones and aldehydes with PMHS was carried out under visible light irradiation to give the corresponding alcohols in high to excellent yields. Low catalyst loadings of 1 mol % but a rather high temperature of 100 °C were used for this transformation.

REVIEW

Sun and co-workers applied an octahedral hydrido(silyl)iron(II) complex to the catalytic reduction of aldehydes and ketones.1455 Mild conditions could be applied when using neutral and cationic cyclopentadienyl(NHC)iron complexes (Guerchais’s complexes, Figure 18, section 9.5) as catalysts.1456 Thus, aldehydes could be completely converted within 3 h at 30 °C while ketones required 16 h at 70 °C for a good conversion. The cationic catalyst required activation by visible light. Further improvement was achieved by running the reaction under solvent-free conditions.1457 This procedure led to higher conversions at lower temperatures and tolerated a broader range of ketones as substrates. The phosphine analogues of these complexes, such as [CpFe(CO)2PPh3]PF6, [CpFe(CO)(PPh3)I], and others, have also been studied as catalysts for the hydrosilylation of aldehydes and ketones.1458 Compared to the corresponding NHCiron complexes, higher catalyst loadings were required to achieve a similar conversion (5 mol % instead of 1 mol %). However, the inexpensive polymethylhydrosiloxane (PMHS) could be employed as reductant. Cesar et al. used a zwitterionic cyclopentadienyl(NHC)iron complex for the catalytic hydrosilylation of aldehydes, ketones, and imines.1459 Plietker and co-workers introduced the nucleophilic iron complex TBAFe in combination with tricyclohexylphosphine as catalytic system for the hydrosilylation of aldehydes and ketones with inexpensive PMHS.47,1460 Even using low catalyst loadings (1 mol %) at reaction temperatures of 3050 °C, the corresponding alcohols were obtained in excellent isolated yields. No 1,4-reduction was observed with α,β-unsaturated ketones as substrates. In situ generated NHCiron complexes have also been employed as catalysts for a variety of aromatic aldehydes, aryl alkyl, heteroaryl alkyl, and dialkyl ketones.1461 A standard catalyst set consisted of iron(II) acetate, [HEMIM][OTf] as ligand precursor (Figure 18, section 9.5), and butyllithium. Polymethylhydrosiloxane (PMHS) was applied as an inexpensive reductant. An efficient hydrosilylation of aldimines and ketimines with phenylsilane has been developed by Sortais and Darcel (Scheme 770).1462 They employed the NHCiron complex Cp(IMes)Fe(CO)2I (Figure 18, section 9.5) as catalyst for this transformation and used neat conditions and visible light irradiation. Reaction of aldimines occurred at 30 °C, whereas ketimines required 100 °C for full conversion. The corresponding amines were obtained in high yields. Ohki, Tatsumi, and Glorius synthesized iron NHC complexes (IMes)2FeCl2 and trans-(IMes)2FeMe2 (Figure 18, section 9.5) and utilized them for the hydrosilylation of 2-acetonaphthone with triethoxysilane or diphenylsilane as reducing agents.1406 While (IMes)2FeCl2 led to no conversion, trans-(IMes)2FeMe2 turned out to be an efficient catalyst affording the alcohol in excellent yield when applied in only 0.1 mol %. Driess and Inoue synthesized the first electron-rich silyleneiron complexes.1463 One of them, [(dmpe)2Fe(r:Si(H)L)] (Figure 18, section 9.5), was also tested as catalyst for the hydrosilylation of various ketones. Using THF as solvent at 70 °C and 5 mol % catalyst, the corresponding alcohols were obtained in excellent yields (as determined by GCMS). Hydrosilylation of a broad scope of ketones, aldehydes, and also carboxylic acid esters with 1 equiv of phenylsilane at room temperature was achieved using an (N-phosphinoamidinate)iron precatalyst (Figure 18, section 9.5).1464 Low catalyst loadings of 0.0021 mol % were 3342

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Chemical Reviews Scheme 770

Scheme 771

sufficient to accomplish mostly quantitative conversion according to GC data. Royo et al. synthesized Fe(NHC)(CO)4 complexes and applied them to the hydrosilylation of benzaldehydes.1465 In particular, Fe(IMes)(CO)4 (Figure 18, section 9.5) proved to be an efficient catalyst for the reduction with phenylsilane at room temperature. 9.5.2. Reductive Amination and Amidation by Hydrosilylation. An overall reductive carbamation of aldehydes and ketones was achieved by a four-component procedure developed by Tian et al. (Scheme 771, eq a).1466 Thus, the carbonyl compounds were treated with benzyl chloroformate, hexamethyldisilazane, and triethylsilane in the presence of catalytic amounts of iron(II) sulfate heptahydrate in dichloromethane at room temperature. Under such mild conditions, the Cbzprotected benzylamines were obtained in high yields. Alternatively, acetals, ketals, and vinyl ethers were employed as carbonyl analogues (eq b). Reductive amination of aromatic and aliphatic aldehydes with anilines has been reported by Enthaler using simple iron(III) chloride as catalyst and PMHS as cheap hydride source (Scheme 772).1432 This method provided the corresponding secondary amines in excellent yields for a variety of examples. Copper(II) salts gave only very small amounts of the product indicating that traces of copper cannot be responsible for the catalytic activity of the iron salt. Mechanistic investigations suggested that the iron salt functions as Lewis acid activator of the intermediate imine. Darcel and co-workers employed their chelate phosphine pyridine iron complexes for the reductive amination of aromatic aldehydes with secondary alkylamines using PMHS as reducing agent (Scheme 773).1467 The transformation proceeds under mild conditions in dimethyl carbonate (DMC) as solvent at 40 °C under visible light irradiation and affords the corresponding tertiary amines in high yields.

REVIEW

Scheme 772

Scheme 773

Scheme 774

Scheme 775

An iron-catalyzed hydroamidation of carbon dioxide with phenylsilane and primary or secondary amines to a series of formamides has been reported by Cantat and co-workers (Scheme 774).1468 The reaction was conducted in THF at room temperature in the presence of catalytic amounts of Fe(acac)2 in combination with triphenylphosphine as ligand. Ketone and ester functionalities remained intact under these conditions. An increase of the reaction temperature to 100 °C led to further reduction and afforded the corresponding methylamines. 9.5.3. Hydrosilylation of Carboxylic Acids, Esters, and Chlorides. The first iron-catalyzed hydrosilylation of carboxylic esters, in particular alkanoates and 2-substituted acetates, has been developed by the group of Darcel (Scheme 775).1469 The substrates were treated with 4 equiv of phenylsilane in the presence of 5 mol % of the catalyst [CpFe(PCy3)(CO)2]BF4 under neat conditions at 100 °C and visible light activation. 3343

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Chemical Reviews Scheme 776

REVIEW

Scheme 778

Scheme 779 Scheme 777

Scheme 780

Following this protocol, almost complete conversions have been achieved in most cases and the corresponding alcohols could be isolated in high yields. Shortly thereafter, Beller and co-workers extended the scope of this reaction by a new procedure (Scheme 776).1470 Using iron(II) stearate in combination with ethylenediamine as catalytic system and inexpensive PMHS as reductant, a large number of aliphatic, aromatic, and heterocyclic substrates could be converted to the corresponding alcohols in high yields. The same transformation was also accomplished by Turculet, Stradiotto, and Sydora using their (N-phosphinoamidinate)iron precatalyst and phenylsilane as reductant (Figure 18, section 9.5).1464 This procedure enabled to perform the reaction at room temperature with very low catalyst loadings (0.01 1.0 mol %) and provided the alcohols in excellent yields (typically 9099%). Darcel and co-workers also established methods for the reduction of carboxylic acids based on iron catalysis (Scheme 777).1471 Notably, two different protocols have been reported. Using phenylsilane as reducing agent and (cod)Fe(CO)3 as catalyst in THF at room temperature with UV irradiation afforded the alcohols in high yields (eq a). In contrast, chemoselective reduction to the aldehydes was achieved when 1,1,3,3tetramethyldisiloxane (TMDS) was employed as reductant and (bda)Fe(CO)3 (bda = benzylideneacetone)1256 as catalyst in toluene at 50 °C without light activation (eq b). Beller and co-workers described the unusual reaction of carboxylic esters with TMDS in the presence of catalytic amounts of dodecacarbonyltriiron to afford the corresponding ethers (Scheme 778).1472 The utility of the method was demonstrated

by a large number of examples providing the ethers in good to high yields. Effective conditions for the selective reduction of esters to aldehydes have been reported by Darcel and co-workers (Scheme 779).1473 They treated the esters with diethylsilane in the presence of 1 mol % of the NHC iron complex (IMes)Fe(CO)4 (Figure 18, section 9.5) in toluene at room temperature under UV irradiation. A large number of different aromatic and aliphatic esters could be converted to the corresponding aldehydes in high yields. A mechanistic proposal (Scheme 780) includes oxidative addition of the hydrosilane to a coordinatively unsaturated iron complex, insertion of the carbonyl compound into the FeH or the FeSi bond, reductive elimination, and hydrolysis of the alkyl silyl acetal. The importance of this method has been emphasized by a recent highlight article.1474 In 2014, Tsuji and co-workers described a procedure for a Rosenmund-like reduction of carboxylic acid chlorides with phenylsilane to provide aldehydes in moderate yields.1475 The transformation was performed in toluene at 60120 °C using iron(II) oxide in combination with tris(2,4,6-trimethylphenyl)phosphine (TMPP) as catalytic system. 9.5.4. Hydrosilylation of Amides. Reduction of tertiary amides with PMHS has been performed in the presence of 3344

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Chemical Reviews Scheme 781

REVIEW

Scheme 783

Scheme 782

Scheme 784

catalytic amounts of docedacarbonyltriiron (Scheme 781).1476 Using this method, the group of Beller converted a broad range of substrates to tertiary amines in high to excellent yields. In a parallel work, Nagashima and co-workers also demonstrated that pentacarbonyliron or dodecacarbonyltriiron are efficient catalysts for the reduction of tertiary carboxamides to amines with TMDS under thermal (eq a) or photolytic (eq b) conditions (Scheme 782).1477 The photoassisted reaction proceeded at room temperature. Yields for both processes were generally good to excellent. The same authors employed a heptanuclear ironcarbonyl cluster [Fe3(CO)11(μ-H)]2Fe(DMF)4 as catalyst for the hydrosilylation of preferably tertiary carboxamides with 1,2-bis(dimethylsilyl)benzene.1478 This procedure led to shorter reaction times, lower catalyst loadings, and higher yields with electron-deficient and electron-rich benzamides as well as aliphatic amides as substrates. N-Methylbenzamide as representative of a secondary amide could also be converted in 80% yield. The hydrosilylation of carboxamides with phenylsilane was also achieved in the presence of the complex [CpFe(CO)2(IMes)]I (Figure 18, section 9.5).1479 The reaction was performed under solvent-free conditions and visible light irradiation. Tertiary and secondary amides were readily reduced to the corresponding amines in excellent yields. In contrast, primary amides were transformed into nitriles. The reduction of primary amides with diethoxymethylsilane was described by Beller et al. using two different iron catalysts consecutively (Scheme 783).1480 Thus, the primary carboxamides were dehydrated with the silane in the presence of triethylammonium hydridotriironundecacarbonylate [Et3NH][HFe3(CO)11] leading to nitrile intermediates. Subsequent reduction with the same reagent was achieved in the presence of catalytic amounts of iron(II) acetate in combination with a phenanthroline ligand. This protocol enabled the reduction of a variety of aromatic and aliphatic primary amides to the primary amines in good yields. Nakazawa and co-workers reported an iron-promoted desulfuration of N,N-dimethylthioformamide with triethylsilane.1481 In the presence of 10 mol % Cp(CO)2FeMe the desulfuration

took place under photoirradiation. The reaction was monitored by the formation of bis(trimethylsilyl) sulfide. A turnover number of 1.2 was determined which demonstrated that a stoichiometric conversion was hardly exceeded. The reaction proceeded via a carbene iron intermediate which could be isolated and characterized. An efficient protocol for the reduction of tertiary amides to the corresponding tertiary amines by iron-catalyzed hydrosilylation has been reported by Adolfsson and Buitrago (Scheme 784).1482 The method relied on an iron NHC catalyst generated in situ from iron(II) acetate, [Ph-HEMIM][OTf] (Figure 18, section 9.5), and n-butyllithium. A variety of aryl and heteroaryl amides could be converted to afford the amines in high yields. 9.6. Reduction of CdO with Other Reductants

Stoichiometric amounts of iron(II) chloride tetrahydrate in combination with catalytic amounts of 4,40 -di-tert-butylbiphenyl were required to effect the reduction of ketones, aldehydes, and imines with lithium powder as reductant.1483 The reaction proceeded under mild conditions in THF at room temperature and provided the corresponding alcohols and imines in high yields with mostly excellent diastereoselectivity. Beller and co-workers described an iron-catalyzed reduction of aldehydes to alcohols under water gas shift conditions (Scheme 785).1484 The cyclopentadienone precursor of Kn€olker’s iron complex has been employed as catalyst for this transformation. Aliphatic, aromatic, and α,β-unsaturated aldehydes could be converted in mostly excellent yields. The latter led to mixtures of allyl and fully reduced alkyl alcohols. 9.7. Reductive Amination

The group of Bhanage established a protocol for the direct reductive amination of aldehydes and cyclic ketones with primary and secondary amines in a biphasic system under moderate hydrogen pressure (Scheme 786).1485 An in situ generated iron(II)EDTA complex was found to be a useful catalyst for 3345

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Chemical Reviews Scheme 785

REVIEW

Scheme 788

Scheme 786

Scheme 787

Figure 19. TIPS-substituted (cyclopentadienone)iron complex.

this transformation. The utility of this method could be demonstrated by several examples affording the products in high yields. Only small amounts of alcohol byproducts were formed (59%). The same reaction can also be catalyzed by dodecacarbonyltriiron. Beller et al. performed this transformation under homogeneous conditions in toluene at 65 °C under a hydrogen pressure of 50 bar and obtained the corresponding alkylated amines in excellent yields (Scheme 787).1486 The substrate scope included primary and secondary anilines and aryl and alkyl ketones and aldehydes. Kn€olker’s iron complex1376 has been demonstrated to be an efficient catalyst for the reductive amination of aliphatic aldehydes and ketones with primary or secondary alkylamines under low hydrogen pressure (5 bar) (Scheme 788).1487 The catalyst was generated in situ from the corresponding cyclopentadienone iron complex upon treatment with trimethylamine N-oxide under a hydrogen atmosphere. The reaction was conducted in ethanol for aldehydes (eq a) or methanol for ketones (eq b) at 85 °C and provided the alkylated amines in high yields. Renaud and co-workers extended their studies on this reaction by the synthesis and application of various derivatives of Kn€olker’s iron complex.1488 The TIPS-substituted pyrrolidineannulated (cyclopentadienone)iron complex (Figure 19) was identified as most active and thus applied to the reductive amination of a series of aldehydes and ketones with primary and secondary, mostly benzylic amines. In most cases, it led to

Scheme 789

better yields than the “original” TMS-substituted congener (see Scheme 788). Computational modeling of the reaction revealed that the substituent α to the carbonyl group may prevent dimerization of the complex and thus loss of activity, whereas the annulated ring is responsible for the stabilization of the 16-electron intermediate. An iron-catalyzed reductive amination of aldehydes with primary and secondary alkyl and aryl amines using sodium borohydride as hydride source has been reported by Bandichhor and co-workers.1489 Iron(III) triflate was employed as catalyst. This transformation could be conducted at room temperature and afforded a variety of alkylated amines in very high yields. 9.8. Reductive Etherification

Oriyama and co-workers developed an iron-catalyzed procedure for the reductive etherification of ketones and aldehydes with alkoxytrimethylsilanes using triethylsilane as reductant (Scheme 789).1490 Iron(III) chloride was identified as an 3346

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Chemical Reviews Scheme 790

REVIEW

Scheme 793

Scheme 791

Scheme 794

Scheme 792

efficient catalyst for this transformation. The reaction proceeded under mild conditions in nitromethane at room temperature to afford a variety of benzyl ethers in mostly high to excellent yields. In a subsequent work, it was demonstrated that this type of transformation can also be conducted employing directly free alcohols instead of the alkoxysilanes (Scheme 790).1491 As in the previous application described above, iron(III) chloride was utilized as catalyst under identical conditions. The procedure led to the corresponding ethers in high yields. A photoinduced reductive etherification of aldehydes with dialkoxymethylsilanes has been reported by Argouarch et al. (Scheme 791).1492 They introduced the new η1-aryliron(II) complex Cp*Fe(CO)2(4-C6H4OMe) as catalyst for this transformation. The silane reagent delivered both the hydride equivalents and the alkyloxy group of the ether. 9.9. Hydrogenation of Heteroarenes

The hydrogenation of N-heterocycles in the presence of a bis(phosphino)amine (P,N,P) iron(II) pincer complex has been reported by Jones et al. (Scheme 792).1493 The reaction was performed under a hydrogen pressure of 510 atm in THF at 80 °C to provide the hydrogenated products in good to high yields. A trans-dihydride iron species was identified as active

catalyst in this process. Accordingly, dehydrogenative aromatization of N-heterocycles was achieved with the same precatalysts. The enantioselective hydrogenation of quinoxalines using a chiral Brønsted acid and Kn€olker’s catalyst1376 provided a series of tetrahydroquinoxalines in high yields and high enantioselectivities (Scheme 793).1431 9.10. Reduction of Alcohols

Benzylic alcohols can be reduced with polymethylhydrosiloxane (PMHS) catalyzed by iron(III) chloride (Scheme 794).1494 The reaction proceeded under mild conditions in 1,2dichloroethane (DCE) at room temperature and provided alkylarenes in high yields. It is most astonishing that esters, ketones, and even aldehydes (eq b) remain intact under these conditions. An unusual direct transformation of benzyl alcohols to the Grignard compounds has been achieved by treating them with hexylmagnesium chloride in the presence of catalytic amounts of iron(II) bromide and tricyclohexylphosphine as ligand (Scheme 795).1495 After quenching with ethanol, the corresponding methylarenes have been isolated in high yields. 9.11. Reduction of Ethers

Selective cleavage of aryl ether bonds in the presence of catalytic amounts of Fe(acac)3 and stoichiometric amounts of sodium tert-butoxide has been reported by Wang and co-workers using lithium aluminum hydride as reductant (Scheme 796, eq a).1496 In addition, the same catalyst system enabled the reduction of the 3347

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Chemical Reviews

REVIEW

Scheme 795

Scheme 797

Scheme 796

Scheme 798

Scheme 799 β-O-4-linkage of liginin model compounds under atmospheric hydrogen pressure affording phenols (eq b). The latter process may be of interest for the cleavage of lignin into synthetically useful small molecules. The reduction of benzyl methyl ethers to the corresponding alkylarenes could be achieved with triethylsilane in the presence of catalytic amounts of iron(III) chloride.80 The reaction was conducted under mild conditions in dichloromethane at room temperature. 9.12. Hydrodehalogenation

An early report on the reduction of aryl iodides with carbon monoxide catalyzed by in situ generated tetracarbonylhydridoferrate has been provided in 1988 by Brunet and co-workers.1497 The dechlorination of electron-rich chloroarenes with various Grignard reagents in the presence of catalytic amounts of iron(III) chloride has been achieved by Takahashi et al.1498 The utility of this reaction was demonstrated for p- and m-chloroanisoles, p-chlorotoluene, o-chloroaniline, 2-chloronaphthalene, and 3,4-dichlorotoluene. In most cases, the corresponding products were obtained in high yields. Novel three- and four-coordinate (fluoro)iron(II) ketiminate complexes have been isolated and applied as catalysts for the challenging hydrodefluorination of aromatic perfluoroarenes and fluoroalkenes with triethylsilane.381 The monohydrogenated products were formed as the main components in low to moderate yields. Accordingly, the turnover numbers were low also due to the high catalyst loading required. Efficient hydrodehalogenation of various aryl and heteroaryl halides (X = I, Br, Cl) has been reported by von Wangelin and co-workers (Scheme 797).1499 They used tert-butylmagnesium chloride as hydride source in the presence of catalytic amounts of iron(III) acetylacetonate and obtained the corresponding products in high yields for a series of substrates. Due to the mild reaction conditions, many functional groups were tolerated (e.g., fluoro, chloro, alkoxy, thioalkyl, nitrile, ester, and vinyl

substituents). Moreover, alkyl bromides could be defunctionalized under the same conditions as demonstrated by three examples. These conditions have also been applied to the selective monodebromination of 1,1-dibromocyclopropanes (Scheme 798, eq a).1500 Slightly higher temperatures and catalyst loadings enabled also the monodechlorination of 1,1-dichlorocyclopropanes (eq b). The hydrodebromination and the hydrodechlorination led to the corresponding products in high yields, and no allenic byproducts have been observed. The diastereoselectivity with respect to other cyclopropyl substituents was in most cases only low. Functional groups such as ester, carboxylic acid, amide, nitrile, keto, hydroxyl, and aromatic bromide did not react under these conditions. The hydrodehalogenation of 1,3-diaryl-3-halopropenes with benzylic alcohols as reductants has been achieved in the presence of catalytic amounts of iron(III) chloride hexahydrate (Scheme 799).1501 The reaction was carried out in toluene at 80 °C and provided the corresponding olefins in high yields. Allylic halides with different aryl substituents led to mixtures of double bond isomers. Several different functional groups such as nitriles, nitro, esters, and methoxy groups were tolerated. It is noteworthy that this reaction may also be of interest as a mild oxidation method for benzylic alcohols to benzaldehydes. 3348

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Chemical Reviews Scheme 800

REVIEW

Scheme 803

Scheme 804 Scheme 801

9.14. Other Reductions at Carbon

Scheme 802

9.13. Reductive CC Coupling

The ferrate complex (tBuO)3FeK proved to be a useful catalyst for the pinacol-type reductive homocoupling of benzaldehydes and acetophenones using magnesium as terminal reductant (Scheme 800).450,1502 Turnover numbers up to 20 could be achieved with this system. It should be noted that the electrontransfer system consisting of a metal “ate” complex and magnesium has also been applied to other organic reductive transformations such as the intramolecular biaryl coupling (cf. Scheme 225). Hayashi and co-workers employed Fe(acac)3 as catalyst for the pinacol coupling of aryl ketones in the presence of a phenyltitanium(IV) reagent (Scheme 801).1503 The iron catalyst triggers a disproportionation of the titanium compound leading to biphenyl and a low-valent titanium species which acts as reducing agent for the CC coupling. A series of aryl ketones could be converted to the pinacol compounds in good to high yields. A reductive cross coupling of alkenyl bromides and chlorides with alkyl and aryl Grignard reagents under hydrogen pressure could be achieved using an tetradentate bis(iminopyridine)iron(II) complex as precatalyst (Scheme 802).1346 Thus, products of a formal C(sp3)C(sp3) coupling were obtained in many cases in high yields.

Nakazawa and co-workers described a reduction of organonitriles with cleavage of the CCN bond, which was effected by triethylsilane in the presence of catalytic amounts of Cp(CO)2FeMe under photoirradiation (Scheme 803).1504,1505 This method might be of interest either for the defunctionalization of nitriles or the formation of silyl cyanides. The reaction has been studied by computational methods revealing a preferred reaction pathway via insertion of the CN group into the FeSi bond of a silyliron intermediate followed by CC bond cleavage.1506,1507 The same group achieved the desulfurization of secondary thioamides to imines using triethylsilane as reducing agent and an (η5-cyclopentadienyl)ironcarbene complex as catalyst.1508 The reduction of sulfonylhydrazones to sulfones in the presence of Grignard reagents has been achieved using iron(III) chloride as catalyst (Scheme 804).1509 This elegant method enabled the synthesis of a variety of synthetically valuable sulfones from aryl, alkyl, and α,β-unsaturated aldehydes and ketones in good yields. The mechanistic proposal includes the formation of a carbene iron complex upon base-induced decomposition of the hydrazones. 9.15. Reduction of Heteroatoms

9.15.1. Reduction of Nitro Groups. Anilines are key intermediates in many branches of the chemical industry. They are conveniently synthesized by reduction of nitroarenes. Ironmediated and iron-catalyzed reductions of aromatic nitro compounds, in particular with low-valent carbonyliron complexes, have been studied for a long time. In 1972, Landesberg described the reduction of nitroarenes with dodecacarbonyltriiron in the presence of methanol.1510 Several additional examples followed during the next three decades as summarized by Bolm in 2004.1 The contributions of the past decade are discussed below. 9.15.1.1. Hydrogenation. An efficient chemoselective hydrogenation of nitroarenes in a biphasic system has been developed by Chaudhari and co-workers.1511 They employed a watersoluble iron complex system consisting of iron(II) sulfate heptahydrate and EDTA-Na2 as catalyst for the reduction of various nitroarenes under hydrogen pressure (about 28 bar) at 150 °C to afford the corresponding anilines in excellent yields. Turnover frequencies up to 529 h1 have been achieved. Beller’s catalyst system consisting of Fe2O3 particles surrounded by a nitrogen-doped carbon layer, originally used for the reduction of nitroarenes with hydrazine hydrate 3349

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Chemical Reviews Scheme 805

REVIEW

Scheme 807

Scheme 806 Scheme 808

(cf. Scheme 808), proved to be also active for the hydrogenation of nitroarenes (Scheme 805).1329,1512 The particles were formed by deposition of phenanthrolineiron on carbon (Vulcan XC72R) and subsequent pyrolysis at higher temperatures (800 °C) under inert gas atmosphere. This catalyst was applied to the hydrogenation of a wide range of nitroarene substrates (more than 80 examples) under a hydrogen pressure of 50 bar in aqueous THF at 120 °C to afford anilines in excellent yields, also on gram scale. Another catalytic system for the hydrogenation of nitroarenes to anilines relied on a tetraphos iron complex (see ligand in Scheme 762).1513 The reaction was performed in tert-amyl alcohol at 120 °C under a hydrogen pressure of 20 bar. Trifluoroacetic acid was required as additive. A series of substrates has been converted in high to excellent yields. 9.15.1.2. Reduction with Silanes. Nitroarenes could also be reduced to anilines using 1,1,3,3-tetramethyldisiloxane (TMDS) as reducing agent in the presence of dodecacarbonyltriiron as catalyst (Scheme 806).1477 The transformation was conducted in toluene at 100 °C. It is worth noting that this reduction can be performed in the presence of amide groups which remain intact. This is surprising as these conditions have been applied to the reduction of amides in the same paper (cf. Scheme 782). Poisoning of the catalyst by the anilines generated may be a reason for this effect. It should be mentioned that this method is orthogonal to corresponding ruthenium- and platinumcatalyzed reductions of nitroarylamides by silanes which deliver nitrobenzylamines. Iron(II) bromide together with triphenylphosphine has been introduced as catalyst system for the reduction of nitroarenes with silanes, in particular phenylsilane.1514 A large number of nitroarene substrates has been treated in toluene at 110 °C under these conditions affording anilines in mostly good to excellent yields. At the same time, the group of Lemaire achieved the same transformation with Fe(acac)3 as catalyst and TMDS as reducing agent.1515,1516 The reaction could be performed in THF at 60 °C and led to the amines in excellent yields for a number of examples. Functional groups such as aryl halides, aldehydes, carboxylic acids, esters, and nitriles were tolerated. 9.15.1.3. Reduction with Hydrazine. Polymer-supported hydrazine hydrate has been reported to effect the reduction of nitroarenes to the corresponding aromatic amines in the presence of iron oxidehydroxide as catalyst.1517 The transformation was carried out in isopropanol solution at reflux and provided the aniline products in excellent yields.

Singh et al. investigated three catalyst systems for the reduction of nitroarenes with hydrazine hydrate in aqueous ethanol at 120 °C (Scheme 807).1518 A combination of iron(II) phthalocyanine (for the structure, see Scheme 61) and iron(II) sulfate proved to be generally applicable for a variety of substrates. The method tolerated the presence of various functional groups and provided amines with excellent selectivity and in high yield. It is noteworthy that no additional acids or bases for catalyst preactivation or additional ligands were required. Reduction of nitroarenes with hydrazine hydrate was achieved using Fe3O4 nanoparticles as catalysts.1519 The reaction was conducted in ethanol at 80 °C, and for a number of examples the corresponding arylamines were obtained almost quantitatively. The catalyst could easily be reisolated due to its magnetic properties and reused up to at least nine times without loss of activity. Functional groups such as chloride, iodide, ester, amide, carbamate, and hydroxy were not affected under these conditions. The group of Beller reported an iron-catalyzed reduction of nitroarenes with hydrazine hydrate (Scheme 808).1520 The catalyst was generated by pyrolysis of an in situ generated Fe(OAc)2phenanthroline complex on carbon support. A large number of nitroarenes has been converted with this catalyst system in THF at 100 °C in excellent yields. No intermediates such as nitrosobenzenes, azoxybenzenes, azobenzenes, and hydrazobenzenes have been observed. The chemoselectivity in the presence of other reducible groups, such as aryl halides, alkenes, alkynes, nitriles, and esters, was also excellent. The reduction of nitroarenes with hydrazine hydrate could also be performed under iron oxide nanocrystal catalysis. These particles were formed in situ from Fe(acac)3 in the presence of hydrazine hydrate under microwave conditions.1521 Moreover, the reaction could be conducted in a continuous-flow reactor which led to the isolation of up to 12 g of the product. The nanosized magnetite particles aggregate and thus, using a magnet, can be easily removed after the reaction. Using a number of substrates, the corresponding products could be isolated in excellent yields for both operating procedures: the batch reaction and the continuous-flow technique. Xu, Su, and co-workers synthesized sub-10 nm γ-Fe2O3 polymer composites and used them as catalysts for the reduction of nitroarenes with hydrazine hydrate to afford almost 3350

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Chemical Reviews Scheme 809

Scheme 810

quantitative yields of arylamines.1522 The catalyst could be recycled and lost only a little of its activity after eight cycles. 9.15.1.4. Other Reducing Agents. The reducing system of Uchiyama, consisting of lithium trimethylferrate and magnesium [Me3FeLi (6.7 mol %)/Mg (16 equiv), cf. Scheme 225], has also been exploited for the reduction of 4-nitroanisole at room temperature to give 4-methoxyaniline in 86% yield.450 The transfer hydrogenation from isopropanol could be applied to the reduction of nitroarenes to anilines in the presence of potassium hydroxide as base and γ-Fe2O3 as catalyst.1523 An iron-catalyzed reduction of nitroarenes by transfer hydrogenation with formic acid has been reported by Beller et al.1524 They employed iron(II) tetrafluoroborate hexahydrate in combination with tetraphos (cf. Scheme 731) as ligand and were able to convert a number of nitroarenes in high to excellent yields. Thomas and co-workers established an operationally simple protocol for the reduction of nitroarenes with sodium borohydride using iron(III) triflate as a catalyst.1368 A plethora of differently substituted substrates could be reduced in ethanolic solution at room temperature to afford anilines in mostly good to high yields. 9.15.2. Reduction of Azides. Chirik’s bis(imino)pyridine iron complexes such as (iPrPDI)Fe(N2)2 (cf. Scheme 726) have been demonstrated to catalyze the hydrogenation of aryl azides to anilines (Scheme 809).1525 Imidoiron complexes which are proposed as catalytic intermediates could be isolated and characterized. The turnover frequency increases with increasing steric bulk of the azide. 9.15.3. Reduction of Sulfonamides. Reduction of sulfonamides to amines has been achieved using Me3FeLi as catalyst and magnesium as reducing agent (Scheme 810). High yields have been reported for three examples.450,1502 9.15.4. Reduction of Sulfoxides. Nonacarbonyldiiron was shown be a useful catalyst for the reduction of sulfoxides to sulfides using cheap silane reducing agents like PMHS or phenylsilane (Scheme 811).1526 The transformation was carried out in toluene at 100 °C and in many cases afforded the products

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Scheme 811

almost quantitativly. A variety of functional groups did not affect the yield of the sulfide; some of them, such as aldehyde, cyclic alkene, and epoxide, remained unchanged, whereas ketones, imines, and alkynes were also reduced under these conditions. The presence of ester and sulfone groups led to significantly decreased yields. A different system which also exhibited high catalytic activity for the reduction of sulfoxides with phenylsilane consisted of bidentate cyclopentadienyl-functionalized NHCiron(II) complexes such as (Cp-NHC)Fe(CO)I (Figure 18, section 9.5) in combination with silver tetrafluoroborate as additive.1527 The reaction was conducted in toluene at 100 °C and provided the corresponding sulfides in high yields for a number of aromatic and aliphatic substrates. 9.15.5. Reduction of Other Heteroatom-Containing Functional Groups. Thiolate- and methyldiazenido-bridged diiron complexes have been described as useful catalysts for the reduction of methylhydrazine with cobaltocene as reductant and lutidine tetraphenylborate as proton source.1528 The reaction proceeded in THF at room temperature and afforded methylamine and ammonia in high yields. Similar transformations with phenylhydrazine and 1,1-dimethylhydrazine provided along with ammonia, phenylamine, and dimethylamine, respectively.

10. OXIDATIONS The field of iron-catalyzed oxidation reactions is of enormous importance not only for synthetic organic chemistry but also for biochemistry and industrial applications. Many biological transformations rely on the selective enzymatic oxidation of unactivated positions in hydrocarbons and of various functional groups which to date cannot be achieved by synthetic procedures. Several review articles on iron-catalyzed oxidation reactions in biological systems are covering for example heme-containing oxygenases in general,1529 cytochrome P450 enzymes,1530,1531 mononuclear non-heme-containing enzymes,15321535 and nonheme diiron enzymes.1536,1537 A general review on iron-catalyzed reactions in biological systems and their applications in biomimetic organic reactions has been provided by M€uller and Br€oring.1538 For industrial applications the selective oxidation of nonactivated hydrocarbons from fossil sources, e.g., the oxidation of methane to methanol, is of importance in order to get access to reactive compounds as starting materials for the synthesis of more complex molecules. Iron-catalyzed oxidation reactions in synthetic chemistry have been investigated extensively during the last decades. After the appearance of Bolm’s review in 2004,1 the research in this area has increased tremendously. Due to the large number of contributions in this field, we would like to refer to recent review articles and highlight only a few selected examples. This holds true for the vast amount of research on iron-catalyzed oxidations of C(sp3)H bonds including allylic hydroxylation and amination.3,4,38,45,469,918,1530,1531,1537,15391555 Allylic oxygenations 3351

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Chemical Reviews and aminations with substituted reagents are covered as CH/heteroatom cross-coupling reactions in section 2.5.3 of this article. Analogously, review articles are available on the oxidation of alkenes (epoxidation, dihydroxylation, aminohyalkynes,3 droxylation),3,4,45,918,1530,1539,1545,1548,1551,15541558 469,1530,1555 3,1551,1555 1530 arenes, alcohols, and aldehydes. The oxidative esterification and amidation of aldehydes are covered in section 5.1.8 . Mechanistic investigations on aromatization reactions with cytochrome P450 enzymes have been included in a review by Meunier and Shaik1530 and with non-heme iron complexes with tetramethylcyclam ligands by Nam and de Visser.1555 Iron-catalyzed oxidation reactions of heteroatoms have also been part of several summarizing articles.3,45,918,1555 A general review on iron-catalyzed oxidations in organic synthesis has been provided by Bolm and co-workers.1547 Beyond the general areas of iron-catalyzed oxidations mentioned above, the following sections are dedicated to selected oxidation reactions which are related to organic synthesis.

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Scheme 812

10.1. Oxidation of C(sp3)H Bonds

The controlled oxidation of C(sp3)H bonds would provide a direct access to functionalized molecules in an atom-economic manner and thus has been a challenge for a long time. However, the large number of different C(sp3)H bonds in complex organic molecules and their relatively high bonding energies with only slight differences between them requires well-balanced conditions for selective oxidation reactions. Until recently, this has been achieved only by nature using complicated enzyme systems. Due to the obvious advantages of such transformations, the field has been investigated with increasing activity resulting in a large number of publications. The majority of them deal with the elucidation of the mechanism of natural systems and with the construction and characterization of synthetic analogues from a biochemical and inorganic background. As the focus of this article is on synthetic applications, the whole area has not been covered in full detail. However, a number of striking recent developments in this field during the past decade will be highlighted below. The oxidation of CH bonds by dioxygen activation with non-heme monoiron enzymes involves the iron(IV)oxo intermediates. M€unck, Nam, and Que obtained the first X-ray crystal structure of such a species, which was obtained from an iron(II)cyclam precursor upon treatment with iodosylbenzene.1559 In the natural system iron(IV)oxo porphyrin π-cation radicals are key intermediates of oxidation reactions with heme iron enzymes. Nam and Shaik demonstrated that electron-donating axial ligands increase the reactivity of such complexes in oxotransfer and hydrogen-atom abstraction reactions.1560 In an application, M€unck, Que, and co-workers described the shapeselective interception by hydrocarbons of the oxidant formed by oxygenation of an iron(II) α-ketocarboxylate complex.1561 White and co-workers employed the nitrogen-coordinated Fe(PDP) catalyst in combination with acetic acid as an additive and hydrogen peroxide as stoichiometric oxidant for the oxidation of C(sp3)H bonds (Scheme 812).1553,15621565 This mild oxidative system allowed for the selective and predictable oxidation of tertiary (eq a)1562 and secondary (eq b) C(sp3)H bonds.1563 The selectivity was governed by electronic, steric, and stereoelectronic effects which could be rationalized for many different systems. In general, tertiary CH bonds are more reactive than secondary ones. Consequently, secondary CH

bonds are oxidized to ketones via the intermediate alcohols. Positions adjacent to electron-withdrawing groups and sterically encumbered CH bonds are less reactive. Surprisingly, alkyl chains with a terminal electron-withdrawing group (EWG) provide the oxidation product with the keto group at the position most distant to the EWG group. The average yield for these oxidations is about 50% for a number of 26 examples (eq b). Using this highly selective system, even the oxidation of complex natural products could be achieved. For example, the tertiary C-10H bond of (+)-artemisinin was hydroxylated selectively with four additional tertiary CH bonds and four methylene groups being present, and moreover, the endoperoxide function stays intact (Scheme 813).1562 In a subsequent work, this pronounced selectivity could be altered in favor of the oxidation at C-9 using a modified catalyst with two 2,6-di(trifluoromethyl)phenyl substituents at the pyridine rings ([Fe-2] in Scheme 813). Thus, 9-oxo-artemisinin was obtained in a good yield overriding the inherent site selectivity of the substrate.1565 This change of selectivity was attributed to a restricted trajectory for the substrate approach disfavoring the tertiary CH bonds. The parent Fe(PDP) catalyst ([Fe-1] in Scheme 813) was also employed for the two-step methylene oxidation of ()-ambroxide via (+)-(3R)-sclareolide to (+)-2oxo-sclareolide in 37% overall yield.1563 M€unck, Que, and co-workers ascribed the beneficial effect of carboxylic acid for bioinspired non-heme iron catalyzed oxidations to the formation of a low-spin acylperoxoiron(III) intermediate.1566 According to DFT calculations, this complex is transformed into an iron(V) species which oxidizes the substrate without a barrier. Very strong oxidizing agents based on N-bridged diiron tetratert-butylphthalocyanine and N-bridged diiron tetraphenylporphyrin (Figure 20) have been prepared and characterized by M€ossbauer, EPR, XANES (X-ray absorption near-edge structures), and EXAFS (extended X-ray absorption fine structure) spectroscopies by Sorokin and co-workers.1567,1568 3352

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Scheme 813

Scheme 814

Figure 20. N-Bridged diiron tetra-tert-butylphthalocyanine and diiron tetraphenylporphyrin.

These complexes were demonstrated to be highly active for the oxidation of CH bonds in alkanes including methane.1569 Very recently, Nam and Latour presented high-spin iron(III)iodosylarene complexes with an N-methylated cyclam ligand which are highly reactive in alkane hydroxylation and sulfoxidation reactions.1570 For example, the metal-mediated oxidation of cumene provided 2-phenylpropan-2-ol (20% yield), α-methylstyrene (21%), and acetophenone (3%). The iron(III) iodosylarene complexes proved to be more active than the corresponding iron(IV) oxo complexes. Moreover, Comba et al. described the catalytic oxidation of alkanes in the presence of iron(II)bispidine complexes.1571 10.2. Oxidation of C(sp2)H Bonds

10.2.1. Oxidation of Alkenes. 10.2.1.1. Epoxidations. Iron(III) chloride hexahydrate in combination with pyridine2,6-dicarboxylic acid (H2pydic) and pyrrolidine was applied as an efficient catalyst system to the biomimetic epoxidation of terminal and 1,2-disubstituted aromatic olefins (Scheme 814).1572 Using hydrogen peroxide as oxidant, high to full conversions were achieved within 1 h at room temperature. The procedure is operationally simple and does not require the preparation of

complicated precatalysts. Subsequently, an asymmetric version of this process has been developed using chiral ethylenediamine ligands.1573 Che and co-workers employed an in situ generated iron(II)bis(4,40 ,400 -trichloro-2,20 :60 ,200 -terpyridine) complex (cf. Scheme 668) as catalyst for the epoxidation of a variety of alkenes with oxone.917 The transformation proceeded at room temperature and provided the corresponding epoxides in high yields (7196% for 16 examples). Afanasiev, Sorokin, and co-workers described the efficient epoxidation of olefins by hydrogen peroxide at room temperature in the presence of catalytic amounts of ironphthalocyanine complexes.1574 10.2.1.2. Dihydroxylations. Using catalytic amounts of iron(II) complexes with chiral tetradendate ligands containing a bipyrrolidine backbone with two α-methylpyridine pendant arms, an excellent example of an iron-catalyzed asymmetric cis-dihydroxylation of alkenes has been reported.1575 The reaction was performed at ambient temperature in acetonitrile as solvent using hydrogen peroxide as oxidant. Enantioselectivities of up to 97% could be achieved by this procedure. The group of Que presented two non-heme cis-β-configured iron complexes 1 and 2 with tetradentate N-donor ligands which catalyze the oxidation of alkenes by hydrogen peroxide in a versatile manner (Figure 21).1576 Without additive, the formation of cis-diols was observed, whereas addition of acetic acid afforded the corresponding epoxides. It was assumed that a nucleophilic oxidant is operating in the absence of acetic acid. 3353

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Scheme 815

Figure 21. Non-heme cis-β-configured iron complexes with tetradentate N-donor ligands (BPMCN and BQCN).

Addition of acetic acid gives rise to the formation of an epoxideselective electrophilic oxidant. Interestingly, the related topological cis-α-isomers 3 and 4 predominantly led to the epoxide oxidation product irrespective of the presence of acetic acid. 10.2.2. Oxidation of Arenes. N-Bridged diiron phthalocyanine complexes (cf. Figure 20) have been shown to catalyze the oxidation of benzene by hydrogen peroxide to afford phenol.1577 The reaction follows the NIH-shift pathway as supported by the product distribution resulting from 1,3,5-[D3]-benzene. For the first time, the intermediacy of benzene oxide in the course of the bioinspired oxidation of benzene could be observed. The efficiency of the process was demonstrated by turnover numbers of up to 66. The first example of a biomimetic iron complex that catalyzes the cis-dihydroxylation of an aromatic double bond has been presented by Que et al.1578 They applied a tris(2-pyridylmethyl)amine iron(II) complex for the dihydroxylation of naphthalene with hydrogen peroxide as oxidant. Turnover numbers up to 30 were achieved, thus demonstrating that the iron complex is a robust catalyst for this type of transformation. The groups of Rybal-Akimova and Que demonstrated the feasibility of aromatic hydroxylation promoted by iron complexes with tetradentate aminopyridine ligands.1579 Several substituted benzoic acids were treated with hydrogen peroxide in the presence of substoichiometric amounts of the iron catalyst at room temperature. The hydroxylation of the aromatic ring occurred in the vicinity of the carboxyl group. The regioselectivity was dependent on the electronic properties and the position of the additional substituents present in the substrate, thus leading to two reaction pathways: 3-substituted benzoic acids were preferentially ortho-hydroxylated to afford salicylates, whereas 2-substituted and, to a smaller extent, 4-substituted benzoic acids underwent an ipso-hydroxylation with concomitant decarboxylation to provide phenols. Further hydroxylations of aromatic compounds using hydrogen peroxide in the presence of catalytic amounts of non-heme iron(II) complexes were reported by Sorokin, Banse, and co-workers.1580

Later on, the group of Takehira used iron(III)-containing mesoporous silica (MCM-41) or an Fe/MgAl hydrotalcite catalyst, prepared by exploiting the “memory effect” of hydrotalcite, for this transformation.1582,1583 The latter proved to be more efficient and was applied to the conversion of a number of cyclic ketones to the corresponding lactones (Scheme 815). Koodali and co-workers employed an FeMCM-48 mesoporous material for the BaeyerVilliger reaction of cyclic ketones with oxygen in the presence of benzaldehydes and obtained the corresponding lactones in high yields.1584 The catalyst could be reused without loss of activity. Balaroui and co-workers employed AlFe-pillared clays as catalysts for this transformation.1585 Cyclohexanone was converted almost completely with a selectivity of about 80% for ε-caprolactone. A second system which has been applied is ironphthalocyanine supported on silica as heterogeneous catalyst. At a conversion of 61%, it provided 95% selectivity for ε-caprolactone. Highly ordered iron tetrakis(carboxyphenyl)porphyrin-bridged mesoporous organosilicas (Fe-TCPP-PMO) have been demonstrated by Park and co-workers to act as catalysts for the BaeyerVilliger oxidation of ketones with oxygen.1586 Ji and co-workers outlined a different reactivity of benzaldehyde and butyraldehyde in the meso-tetraphenylporphyrin chloride catalyzed aerobic BaeyerVilliger oxidation.1587,1588 Peroxy isobutyric acid formed from butyraldehyde was a less efficient oxidant compared to the high-valent iron porphyrin complex formed during the aerobic oxidation in the presence of benzaldehyde. The efficiency of the porphyrin iron(III) complexes for the BaeyerVilliger oxidation of ketones could be increased by addition of tin dioxide as cocatalyst.1589 10.4. Wacker Oxidation

An authentic Wacker oxidation using an iron species which catalyzes the addition of water to an alkene followed by β-hydride elimination has not been reported so far. Instead, iron(II) phthalocyanine has been introduced in combination with benzoquinone as redox-active components in a palladium-catalyzed Wacker oxidation.1590 The iron species was believed to catalyze the reoxidation of hydroquinone by oxygen as terminal oxidant in this multicomponent catalyst system. Che and co-workers described an iron-catalyzed oxidation of styrenes to phenylacetaldehydes (Scheme 816).1591 Unlike the classical Wacker oxidation, they employed iodosylbenzene as oxidant and source of the carbonyl oxygen. The iron(III) porphyrin complex [Fe(2,6-Cl2TPP)OTf] was utilized as catalyst for this transformation which, in contrast to the usual Wacker process, delivered regioselectively phenylacetaldehydes in high yields. Some reactions reminiscent of Wacker oxidation are presented in other sections of this review (cf. Schemes 847 and 498).

10.3. BaeyerVilliger Oxidation

An early work on iron-catalyzed BaeyerVilliger oxidation of ketones by oxygen in the presence of benzaldehydes was presented by Murahashi et al. using α-Fe2O3 as a catalyst.1581

10.5. CC Bond Forming Oxidation Reactions

Using a chiral iron(III)salan complex as catalyst, the group of Katsuki achieved an asymmetric oxidative dearomatization 3354

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Scheme 818

Scheme 819 Scheme 817

Scheme 820

(AOD) of 1,3-disubstituted 2-naphthols by reaction with nitromethane in the presence of air (Scheme 817).1592 This method allowed the construction of a quaternary stereogenic carbon center with high enantioselectivity. It represents an interesting entry into the chemistry of chiral cyclic enones starting from abundant aromatic precursors. 10.6. Oxidative CC Bond Cleavage

Pitchumani and Dhakshinamoorthy presented a method for the oxidative CdC bond cleavage in olefins with hydrogen peroxide using a clay-supported ironsalen complex (Scheme 818).1593 The reaction proceeded in acetonitrile at room temperature. The corresponding aldehydes could be obtained in moderate to high yields with turnover numbers of up to 60. The group of Zhang achieved an oxidative CC bond cleavage of 1,3-di(hetero)arylpropan-1,3-diones and 1-arylalkane1,3-diones under extrusion of carbon monoxide affording 1,2diketones (Scheme 819).1594 In this unusual method, iron(III) chloride was applied as catalyst and tert-butyl nitrite (TBN) as terminal oxidant under neat conditions. A range of 1,2-diketones could be synthesized in high yields. Jiao and co-workers described the unprecedented iron-catalyzed oxidation of diphenylmethanes to N-aryl benzamides (Scheme 820, eq a) and 1,3-diphenylpropenes to cinnamides

(eq b).1595 This transformation involves an iron-catalyzed oxidative CH and CC bond cleavage and is achieved by using 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) as oxidant, trimethylsilyl azide as nitrogen source, and iron(II) chloride as catalyst. Starting from appropriate cyclic systems (e.g., anthracen-9(10H)-one), the procedure did afford the corresponding ring-expanded lactams. Styrene derivatives were subjected to CdC bond cleavage conditions under formation of dimethyl acetals (Scheme 821).1596 This transformation was achieved by treatment with hydrogen peroxide in methanol in the presence of iron(II) sulfate heptahydrate as catalyst. A series of benzaldehyde dimethylacetal 3355

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Scheme 821

Scheme 824

Scheme 822

Scheme 825

Scheme 826 Scheme 823

derivatives could be synthesized in mostly high to excellent yields. Using PhI(OAc)2 as oxidant, acetalization without CdC bond cleavage was observed (Scheme 498).899 The selective oxidative cleavage of aromatic CC bonds is a common biosynthetic transformation with little precedence in synthetic chemistry. Based on preliminary work by Bugg using stoichiometric amounts of iron salts in combination with the 1,4,7-triazacyclononane ligand (TACN),1597,1598 Trauner and co-workers presented a catalytic system consisting of iron(II) bromide and TACN for the oxidative extradiol cleavage of catechols providing 2H-pyran acetals in synthetically useful yields (Scheme 822).1599 The utility of the method could be demonstrated by the synthesis of betanidin from L-DOPA. 10.7. Other Oxidations

Vishwakarma and Sawant established a protocol for the oxidation of arylboronic acids which led to phenols in very high yields (Scheme 823).1600 This transformation was achieved by atmospheric oxygen under solar irradiation in the presence of α-Fe2O3 as catalyst.

11. VARIOUS CYCLIZATION AND ANNULATION REACTIONS An intramolecular annulation of an alkyne to a 1-arylethanone unit has been reported by Li and co-workers (Scheme 824).1601 This iron-catalyzed transformation apparently consists of a key [4 + 2] cycloaddition step followed by 1,4-elimination of a phenol fragment. The utility of this protocol was confirmed by the synthesis of several naphthalene-1-ols. An overall redox-neutral cyclizing hydroxyalkylation of 1,6dienes in the presence of catalytic amounts of phthalocyanine iron [Fe(Pc)] has been reported by Ishibashi et al. (Scheme 825).1602

Scheme 827

Sodium borohydride was employed as hydride source. The hydroxy function originates from oxygen. The reaction follows a radical pathway and provides five-membered carbo- and heterocyclic compounds in mostly good yields as mixtures of cis/trans isomers. A related cyclizing haloalkylation required stoichiometric amounts of iron(III) halide as mediator and halide source. The group of Tian treated N-benzylsulfonamides with internal alkynes under iron(III) chloride catalysis and obtained substituted indenes (Scheme 826).1603 The reaction includes CN cleavage under formation of a benzylic cation. Subsequent addition to the alkyne and electrophilic ring closure affords the indene skeleton. A large number of substrates could be converted in mostly good to high yields. A spirocyclization of 3-[2-(alkylamino)benzylidene]indolin2-ones has been achieved by Yuan and co-workers using iron(III) chloride as Lewis acid catalyst (Scheme 827).1604 Most likely, the transformation proceeds via a 1,5-hydride transfer from the α-position of the amine to the β-position of the α,β-unsaturated carbonyl moiety followed by ring closure. This procedure leads to spirooxindole tetrahydroquinolines in excellent yields. Palladium(II) chloride in combination with iron(II) chloride was shown to be an active catalyst system for a cyclizing 3356

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Chemical Reviews Scheme 828

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Scheme 831

Scheme 829

Scheme 830

trimerization of alkynyl ketones to 4H-cyclopenta[c]furans (Scheme 828).1605 This domino process includes a palladiumcatalyzed cycloaddition and an iron-catalyzed acyl rearrangement. A broad range of acylalkynes could be employed providing the cyclopentene-annulated furans in good yields. Zhu and co-workers described an iron-catalyzed benzannulation of 2-(2-oxoethyl)benzaldehydes with alkynes to form naphthalene derivatives (Scheme 829).1606 The reaction was performed in dichloroethane at room temperature using iron(III) chloride as catalyst. In general, good to high yields could be achieved. Mechanistically, a [4 + 2] cycloaddition of a 3H-isochromen-3-ol intermediate with the alkyne is the key step. Subsequent cycloreversion releases a molecule of carboxylic acid and the aromatized product. An iron-catalyzed redox condensation of o-nitroanilines and phenethylamines has been exploited for the synthesis of a series of quinoxalines (Scheme 830).1607 Iron(III) chloride hexahydrate and sulfur were added in catalytic amounts to generate iron sulfide as catalytically active species. Substituted nitroanilines led to mixtures of isomers with little regiocontrol. This transformation represents an example for the bioinspired cubane-type Fe4S4 cluster catalyzed reactions which have been summarized in a review by Seino and Hidai.1608 Cheng and co-workers developed an iron-catalyzed synthesis of quinolines by reaction of N-aryl aldimines with styrenes (Scheme 831, eq a).1609 The reaction was performed in a sealed tube using nitromethane as solvent at 110 °C in the presence of catalytic amounts of iron(III) chloride. An oxygen atmosphere was required to achieve the final aromatization. More than 25 different 2,4-disubstituted quinolines were obtained in high yields. Moreover, the procedure could be simplified by replacement of the aldimine substrate with the corresponding aniline and aldehyde precursors (eq b). The resulting threecomponent protocol was almost equally efficient under the same conditions.

Scheme 832

12. DOMINO REACTIONS This section deals with multiple transformations proceeding in one step under identical conditions in which the subsequent transformation takes place at a functional group generated in the previous reaction.1610,1611 Sequential one-pot reactions are excluded. If one of the reactions appears to be clearly the key step, the whole process will be outlined in the corresponding section. For example, this applies to procedures consisting of basic transformation combined with a reductive, an oxidative, or an isomerization step (see, for example, oxidative esterification of aldehydes in Scheme 558, which has been classified as carbonyl reaction). In a few cases, however, the oxidative, reductive, or isomerization step will be regarded as more important, and consequently such processes will appear there. This applies for example for reductive amination (sections 9.5.2 and 9.7), etherification (section 9.8), and the domino processes consisting of allyl alcoholcarbonyl isomerization/aldol reactions in section 7.1.1. The domino Grignard formation/cross coupling procedure as highlighted by F€urstner252 is introduced as direct CH/CX coupling in this paper in section 2.4.3. Zhan et al. developed an iron-catalyzed protocol for a domino propargylation/cycloisomerization starting from propargylic alcohols or acetates and 1,3-dicarbonyl compounds (Scheme 832).1612 Iron(III) chloride functioned as dual catalyst for both reaction steps. With this method in hand, the authors synthesized a large number of substituted furans in mostly high yields. An iron-catalyzed domino benzylation (propargylation)/ cyclization has been reported by the group of Wang (Scheme 833).1613 They treated 2-hydroxymethylanilines with 3357

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Scheme 835

Scheme 836 Scheme 834

Scheme 837

β-keto esters in the presence of equimolar amounts of zinc chloride and catalytic amounts of iron(III) chloride to afford 3-quinolinecarboxylic esters in high yields (eq a). In addition, 4-alkynyl-3-quinolinecarboxylic esters were obtained from the corresponding o-nitrophenyl propargyl alcohol substrates which, prior to the cyclization, were reduced to the amino congeners by tin chloride dihydrate (eq b). Nanoparticles of Fe3O4 were shown to catalyze a domino reaction of 2-halobenzaldehydes with β-enaminones providing 1,4-dihydroquinoline derivatives (Scheme 834).1614 The transformation includes a BaylisHillman-type reaction of the enaminone with the halobenzaldehyde followed by nucleophilic addition of the second enaminone and a final ring-closing nanoFe3O4-catalyzed CN cross coupling to the product. Jiang and co-workers established an iron-catalyzed procedure for the synthesis of 2-furancarbaldehydes by reaction of electrondeficient alkynes with 2-yn-1-ols (Scheme 835).1615 In a first step, the substrates react in a Michael addition to form the corresponding propargyl vinyl ethers. A subsequent iron-catalyzed domino cyclization/rearrangement/oxidation affords the furancarbaldehydes in good yields. 2,5-Dihydro-1H-pyrroles and pyrrolidines have been synthesized by a domino imination/aza-Cope/Mannich reaction starting from aldehydes and 2-hydroxyhomopropargyl- or homoallylamines, respectively.1616 This transformation was conducted preferably with stoichiometric amounts of iron(III) chloride and trimethylsilyl chloride as additive. Using catalytic amounts of the iron salt led to slightly lower yields of the cyclization products. The cyclization of 2,3-allenoates and subsequent allylation with allyl bromides was achieved under FeCl3/PdCl2 cocatalysis

in one step and provided a concise access to allyl butenolides (Scheme 836).1617 The cyclization was catalyzed by the iron species leading to a η1-alkenyliron intermediate which undergoes transmetalation with the palladium species. Subsequent formal C(sp2)C(sp3) coupling was supposed to proceed via carbopalladation followed by debromopalladation. Previously, the group of Li reported the reaction of 1,3-dicarbonyl compounds with N,N-dimethylaniline (cf. Scheme 274).538 In an extension of this earlier study, they treated triethylamine with 1,3-dicarbonyl compounds in the presence of nonacarbonyldiiron as catalyst and tert-butyl hydroperoxide as oxidant (Scheme 837).1618 Surprisingly, this reaction led to β-1,3-dicarbonyl-substituted aldehydes by a three-component domino reaction. A plausible mechanistic pathway starts with the oxidation of the amine to an iminium species which is in equilibrium with the corresponding enamine. Condensation of the two isomers leads to α,β-unsaturated imines or aldehydes which function as Michael acceptors for the 1,3-dicarbonyl nucleophiles. The alkylated 1,3-dicarbonyl compounds were obtained in good yields. 2-Aryl-1,1-dibromoalkenes have been converted to α-acetoxy aryl ketones upon treatment with potassium acetate in the presence of catalytic amounts of iron(III) chloride, acetylacetone (CH2Ac2) as ligand, and DBU as base (Scheme 838).1619 Mechanistically, this transformation may proceed via dehydrobromination to the bromoalkyne, activation by the iron catalyst, Michael-type addition of acetate to give the 1-acetoxy-2bromoalkene, and a subsequent “transesterification” by an 3358

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Scheme 841

Scheme 842 Scheme 839

Scheme 843

Scheme 840

acetate ion to release the α-bromoketone. Finally, the bromide is substituted by another acetate nucleophile. The cyclization of two molecules of phenylacetylene with one molecule of a 1,2-diaminoethene derivative led to the formation of diazepines. Maleki demonstrated that this transformation is efficiently catalyzed by the magnetically recoverable silicasupported superparamagnetic Fe3O4 nanoparticles (Scheme 839, for a similar reaction with ketones as C2-building blocks see Scheme 657).1620 Moreover, the protocol could be extended to a three-component reaction by subsequent addition of cyclohexyl isocyanide which provided the corresponding carboxamidofunctionalized diazepines. In situ generated iron sulfide was shown to catalyze the reaction of 2-amino- or 2-hydroxynitrobenzenes with methylheteroarenes providing a variety of 2-heteroarylbenzimidazoles and 2-heteroarylbenzoxazoles in high yields (Scheme 840).1621 The reaction was conducted under neat conditions at 150 °C and involves a reduction of the nitro group and concomitant oxidation of the methyl group. Similarly, the iron-catalyzed reaction of 2-nitrophenol with benzyl alcohols afforded benzoxazoles in high yields (Scheme 841).1622 1,10 -Bis(diphenylphosphino)ferrocene (dppf) has been identified as the most efficient catalyst for this transformation which includes alcohol oxidation, nitro reduction, ring closing

condensation, and aromatization. No additional reducing agent was required, but an excess of benzylic alcohol was needed. The groups of Li and Deng described an oxidative arylation of azoles with aromatic aldehydes (Scheme 842).1623 Iron(II) sulfate heptahydrate was applied as catalyst. The transformation proceeded in water/diglyme at 150 °C under oxygen atmosphere. Good to high yields could be achieved for a series of examples. Substituents such as methoxy, fluoro, chloro, bromo, and nitro groups were tolerated under these conditions. It is noteworthy that the C-2 carbon of the azole product originates from the aldehyde as could be demonstrated by 13C-labeling. This fact may be explained by an initial hydrolysis of the benzothiazole to the 2-aminothiophenole. Subsequent reaction with the aldehyde to the imine, cyclization, and oxidation would lead to the product. In a subsequent endeavor, Deng et al. revealed a reaction of benzothiazoles with aryl alkyl ketones leading to 2-acylbenzothiazoles (Scheme 843).1624 Employing iron(III) chloride [Fe(tcpp)Cl] hexahydrate as catalyst in combination with tricyclohexylphosphonium tetrafluoroborate, a variety of benzothiazoles could be acylated with different aryl ketones in good to high yields. The reaction was conducted in a DMSO/ water mixture in oxygen atmosphere at 120 °C. The mechanism of this transformation is similar to the one discussed above. The requisite 2-oxophenylacetaldehyde is formed by oxidation of the acetophenone. The same aroylation of benzothiazoles with aryl methyl ketones could be achieved in the presence of catalytic amounts of iron(III) chloride hexahydrate and potassium peroxodisulfate as oxidant.1625 The aroylated benzothiazoles were obtained as mixtures with the arylated derivatives. In addition, Yu and Chen also demonstrated that the arylation of benzothiazoles with benzylic alcohols under the same conditions afforded 2-arylbenzothiazoles (Scheme 844). 3359

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Chemical Reviews Scheme 844

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Scheme 847

Scheme 845

Scheme 846

Bhanage and co-workers achieved the arylation of benzothiazoles with styrenes using a similar approach (Scheme 845).1626 Iron(III) nitrate nonahydrate was introduced as catalyst. The C-2 carbon of the benzothiazole product originates from styrene. Thus, as in the cases described above, an initial cleavage of the benzothiazole to a 2-aminothiophenol occurs. Moreover, the styrene double bond is cleaved oxidatively to generate the corresponding benzaldehyde which undergoes a ring closure with the 2-aminothiophenol to give the resulting benzothiazole by final oxidation under oxygen atmosphere. Homoallylic azides became readily available by an ironcatalyzed three-component one-pot reaction of aromatic or heteroaromatic aldehydes, allyltrimethylsilane, and trimethylsilyl azide (Scheme 846).1627 In a first step, the aldehyde is allylated followed by nucleophilic displacement of the hydroxy group (or its silylated and/or iron complexed equivalent) by azide. Ghorai and Pramanik demonstrated the utility of this procedure by conversion of a series of aldehydes to the homoallylic azides. Manche~ no and co-workers developed a domino oxidative CN/CC coupling protocol for the synthesis of dihydroquinazolines by self-condensation of N-phenyl glycinates (Scheme 847, eq a).1124 Similarly, quinolines could be obtained by reaction of N-phenyl glycinates with olefins (eq b).1123,1124 Moreover, the method could be developed to a three-component reaction employing anilines, ethyl glyoxylate, and olefins to afford quinolines (eq c).1124 These iron(III) chloride catalyzed domino processes include oxidation of the amine to an imine (does not apply to eq c), reaction with a nucleophile (second molecule of amine or olefin, respectively), oxidation, intramolecular Friedel Crafts-type reaction (for eq a), or alternatively concerted [4 + 2] cycloaddition (for eqs b and c), and further oxidation. A TEMPO oxammonium salt (T+BF4) was used as mild and

Scheme 848

nontoxic oxidant. This interesting procedure provided access to a variety of dihydroquinazolines and quinolines. Multiple cross-dehydrogenative coupling has been realized by reaction of arylamines with N-substituted lactams in the presence of tert-butyl hydroperoxide as oxidant (Scheme 848).1628 In the course of this iron-catalyzed three-component transformation, two CC bonds and one CN bond were formed and one CN bond was cleaved. The resulting ring-fused tetrahydroquinolines were accessible in moderate yields. A three-component domino process involving methyl diazoacetate (MDA), an unsaturated phosphorus ylide, and 2-bromoacetophenone has been reported by Tang and co-workers (Scheme 849).1629 This tetra(p-chlorophenyl)porphyrin iron chloride [Fe(tcpp)Cl] catalyzed procedure afforded cyclopentadienes in good to high yields. In the course of the transformation multiple CC bond cleavages and reorganizations occur. A complex mechanism has been proposed including three different cyclopropane derivatives. One of them and a vinylphosphonium intermediate could be isolated and converted to the product. In analogy to a silver-catalyzed reaction of thioamides with propargylic alcohols, substituted thiazoles have also been obtained by reaction of propargylic alcohols with carboxamides in the presence of catalytic amounts of iron(III) chloride and 3360

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Chemical Reviews Scheme 849

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Scheme 851

Scheme 850

Scheme 852

subsequent treatment with Lawesson’s reagent in a one-pot procedure.1630 A ring opening of 1,1-dibromocyclopropanes was achieved in the presence of catalytic amounts of zinc bromide and substoichiometric amounts of iron powder (Scheme 850).1631 By loss of hydrogen bromide, 1,3-dienes were formed as intermediates which were treated with aldehydes (eq a) or imines (eq b). This in situ hetero-DielsAlder reaction led to 2H-3,6-dihydropyrans and 1,2,3,6-tetrahydropyridines in moderate to high yields. Reddy and co-workers established a protocol for a threecomponent coupling of 4-phenylurazole with various benzaldehydes and dimedone or cyclohexane-1,3-dione (Scheme 851, eq a).1632 The authors performed this reaction in acetonitrile at reflux using iron(III) chloride as catalyst. A sequence of aldol condensation, two conjugate additions, and dehydration provided the corresponding triazolo[1,2-a]indazoletriones in high yields. Using isatin instead of the aldehyde, spirotriazolo[1,2-a]indazoletetraones were accessible (eq b). An overall enantioselective hydroalkylation of allylic alcohols with β-keto esters has been described by Rodriguez and Quintard employing a tricarbonyl(η4-cyclopentadienone)iron complex reported by Kn€olker1203 and a chiral pyrrolidine ligand (Scheme 852).1633 Additionally, trimethylamine N-oxide was required to activate the catalyst. The transformation provided alkylated β-keto esters, which are in equilibrium with the cyclic hemiacetal form. The proposed mechanism combines two catalytic cycles, a socalled borrowing-hydrogen catalysis and an iminium activation (Scheme 853). In the first cycle the allyl alcohol is oxidized to an

α,β-unsaturated aldehyde. Subsequent iminium activation by the chiral pyrrolidine allows for a Michael addition of the carbon nucleophile in a diastereoselective fashion. The resulting enamine is hydrolyzed to the enantioenriched β-alkylated aldehyde. This species enters the first catalytic redox cycle where it is hydrogenated by the (hydrido)iron complex releasing the corresponding alcohol. Rodriguez and Quintard extended their method by an acyl transfer from carbon to the hydroxy group via a retroClaisen reaction of the cyclic hemiacetals to provide acylated 3-alkylpentanol units (Scheme 854).1634 The acylated γ-chiral alcohols were obtained in high yields and high enantioselectivities. The utility of this method was demonstrated by the synthesis of several key fragments of biologically active products. Based on their earlier results on the oxidative dearomatization of 2-naphthols (cf. Scheme 817), Katsuki et al. established a domino oxidation/Michael addition/oxidation/nucleophilic dearomatization process (Scheme 855).1635 This reaction of 1-methyl-2-naphthols with phenols was catalyzed by a chiral ironsalan complex and provided spirocyclic (2H)-dihydrobenzofurans via quinone methide intermediates in mostly high yields and high enantioselectivities. An isomerization/FriedelCrafts-type cyclization/dehydration process has been described starting from 2-[(indoline-3-ylidene(methyl)]benzaldehyde derivatives affording benzo[b]carbazoles (Scheme 856).1636 This transformation was achieved in the presence of iron(III) chloride as catalyst in 1,2-dichloroethane at 3361

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Scheme 853

Scheme 854

Scheme 856

Scheme 857

Scheme 855 Scheme 858

room temperature. A number of substrates could be converted in high to excellent yields.

13. MISCELLANEOUS Iron(III) tosylate hexahydrate has been demonstrated to be a suitable catalyst for the cleavage of tert-butyldimethylsilyl

(TBDMS), triethylsilyl (TES), and triisopropylsilyl (TIPS) ethers (Scheme 857).1637 The reaction was conducted in methanol at room temperature and provided deprotected alkyl alcohols in high yields. Interestingly, phenolic TBDMS ethers, TBDPS ethers, and Boc groups were not affected. Takai and Kuninobu described an iron(III) chloride catalyzed reaction of ethyl diazoacetate with tertiary amines to give glycine derivatives (Scheme 858).1638 This unusual transformation involves cleavage of a CN bond of the amine rather than insertion of the carbene into a CH bond adjacent to the nitrogen atom. The acylation of indoles with N-benzylanilines under oxidative conditions has been reported using iron(II) chloride as catalyst (Scheme 859).1639 tert-Butyl hydroperoxide and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) were required 3362

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Chemical Reviews Scheme 859

Scheme 860

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Scheme 862

Scheme 863

Scheme 861 Scheme 864

as oxidants and pivalic acid as additive. The reaction proceeded in DMSO at 2550 °C and provided 3-acylindoles in high yields. The iron-catalyzed reaction of α,β-unsaturated esters with aldehydes and tert-butyl hydroperoxide afforded α-peroxo-γ-oxo esters (Scheme 860).1640 A variety of organic peroxides became accessible by this method which has been utilized as key step for the total synthesis of (()-clavilactone A and B and the proposed (()-clavilactone D.1641 A nonclassical Polonovski reaction for the demethylation of opiate N-oxide hydrochlorides has been presented by Scammells and Kok using iron powder in combination with iron(III) chloride as catalyst (Scheme 861).1642 The reaction was conducted in an isopropanol/chloroform mixture mostly at room temperature. Four opiate N-oxide hydrochlorides could be demethylated in high yields. The two-step N-oxidation/ demethylation could also be combined to a one-pot process with equal efficiency. In a subsequent work the authors demonstrated that this process can be performed under continuous flow conditions.1643 The synthesis of vinyl-substituted heteroarenes has been described by an iron-catalyzed reaction of 2-methylpyridines and -pyrazines with N-methylcarboxamides under oxidative conditions (Scheme 862).1644 The methyl transfer was achieved by dehydrogenative cross coupling and subsequent amide

elimination. This method led to the synthesis of a series of vinylated pyridines and pyrazines in high yields. Holland et al. described the synthesis of unsymmetrical carbodiimides from isocyanides and organoazides by a catalytic nitrene transfer using iron(I)isocyanide complexes which were generated in situ.1645 Sodeoka and co-workers described an iron-catalyzed trifluoromethylation of allylic alcohols with concomitant 1,2-migration of an aryl group (Scheme 863).1646 This transformation was achieved using Togni’s reagent II as trifluoromethyl source and iron(II) acetate as catalyst in the presence of potassium carbonate as base. A number of 1,1-diaryl-allyl alcohols with identical or different aryl groups could be converted in high yields. In the latter case a pronounced selectivity for the migration of the aryl groups was observed. The iron-catalyzed coupling of indoles in the presence of sodium nitrite led to the formation of (E)-2,30 -biindol-3-one oximes (Scheme 864).1647 The utility of this procedure was demonstrated by the transformation of several functionalized indoles in good to high yields. The arylation of phenols with arylboronic acids under oxidative conditions has been reported by Maiti and co-workers to provide arylated quinones in good yields (Scheme 865).1648 Iron(III) sulfate was applied as catalyst for this transformation. Oxidative conditions were realized by addition of potassium peroxodisulfate and an oxygen atmosphere. The transformation most likely follows a radical pathway. Zhong and Han established an iron-catalyzed procedure for a carbonylative Suzuki reaction (Scheme 866).1649 They treated 3363

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Chemical Reviews Scheme 865

Scheme 866

Scheme 867

aryl iodides with phenylboronic acids in the presence of catalytic amounts of iron(II) and iron(III) chlorides in a polyether at 100 °C under CO atmosphere. The resulting diaryl ketones could be isolated in high yields. An allylic CH amination of substituted 1,3-dienes with arylhydroxylamines has been developed by Srivastava and Murru (Scheme 867).1650 Using an iron azobenzene dioxide catalyst, they were able to aminate a variety of dienes and trienes with allylic CH groups to the corresponding aminomethyldienes or aminomethyltrienes in good yields. The reaction probably proceeds via a heteroene reaction of activated nitrosoarenes with the allyl moiety. Hetero-DielsAlder adducts were formed as byproducts. An iron-squarate-based three-dimensional metalorganic framework was shown to catalyze the transformation of tetrazines to oxadiazoles.1651 The transformation presumably proceeds via nucleophilic attack of iron-ligated hydroxide at a carbon atom of the tetrazine with ring opening and loss of dinitrogen. Ring closure and aromatization finally lead to the formation of the oxadiazoles.

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14. CONCLUSIONS AND OUTLOOK Around the turn of the millennium, a renaissance began in using iron as a catalytically active metal for organic synthesis and tremendous progress in this field has been achieved. A large variety of highly intriguing specific iron-catalyzed transformations with an enormous potential for future organic synthesis are described in the present review. Numerous methods have been elaborated to replace rare, expensive, and often toxic noble metal catalysts by the much more abundant iron. The iron-catalyzed allylic substitutions complement their palladium-catalyzed counterparts. Some of the novel procedures have become wellestablished in organic synthesis. Prominent examples include the extensively investigated iron-catalyzed cross-coupling reactions. Thus, efficient procedures became available to couple alkenyl, aryl, but also alkyl electrophiles with organometallic reagents. Their practicability has been proven by application to several total syntheses of complex natural products. Many of these reactions have become standard methods in the toolbox of organic chemists. For the future, it is expected that they will gain ground against the cross-coupling reactions catalyzed by palladium or other precious metals. During the past two decades CH bond activations have come into focus for organic chemists. Oxidative CC bond forming reactions by cross-dehydrogenative couplings are very attractive as they allow a straightforward construction of new carbon skeletons without prefunctionalization of the substrates. But also the functionalization of nonactivated CH bonds with heteroatom-containing groups is a useful extension of classical reaction principles. The area is still dominated by palladium, rhodium, ruthenium, copper, and iridium catalysts, but very promising iron-catalyzed variants have been developed as outlined in the sections 2.4.4 and 2.5.3. For example, the activation of sp3-carbon atoms adjacent to nitrogen and oxygen atoms for CC bond forming reactions, via oxidation to iminium and oxonium species or radical processes, may be developed to a versatile tool for the construction of complex molecules. The iron-catalyzed insertion of carbene and nitrene analogues into unactivated CH bonds may become selective enough to be applied in advanced organic synthesis, as already demonstrated by the synthesis of N-heterocycles via amination of CH bonds with azides. Moreover, it has been demonstrated that iron complexes show high activities and pronounced selectivity for the oligomerization and homo- and copolymerization of olefins, as well as for the controlled atom transfer radical polymerization (ATRP). These reactions are of interest for industrial applications and have even been tested under industrial conditions in pilot plants. The ironporphyrin catalyzed cyclopropanation with in situ generated diazomethane constitutes a valuable procedure which is avoiding the preliminary formation and isolation of reactive and hazardous diazomethane by working in a biphasic system with careful fine-tuning of the catalyst. This principle may be applied to other toxic or labile intermediates which are generated in aqueous solution and react with the substrate in the organic phase. Other areas in which iron has become competitive with noble metal catalysts are hydrogenations, especially asymmetric transfer hydrogenations, and hydrosilylation reactions for the reduction or functionalization of olefins, carbonyl compounds, and nitroarenes. The enantioselective hydroalkylation of allylic alcohols via a hydrogen-borrowing mechanism exploits the reversible 3364

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Chemical Reviews hydrogen transfer ability of iron catalysts. It is very likely that in future this principle will be seen in other domino processes as well. The bioinspired oxidation by iron-catalyzed CH bond activation, which has been covered only briefly in the present review, has a great potential for future synthetic applications. In particular some of these reactions, for example the hydroxylation of nonactivated aliphatic CH bonds and arenes, have only little or no precedence in classical synthetic organic chemistry. Besides the fascinating developments mentioned above, there is a range of further highly promising iron-catalyzed transformations described in the present article. Further improvements of known iron-catalyzed reactions and new discoveries will fill the drawbacks which partly still exist compared to the more established noble-metal-catalyzed processes used in organic synthesis. Considering all aspects, especially including availability, price, toxicity, and environmental concerns, it is expected that in future applications of iron-catalyzed transformations will go even beyond those of noble metal catalysts.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] The authors declare no competing financial interest.

BIOGRAPHIES

Ingmar Bauer was born in Freiberg, Germany, in 1967. He studied chemistry at the Technische Universit€at Dresden (19871992). From 1991 to 1992, he moved as a DAAD fellow to Aston University Birmingham, U.K., and joined the research group of Dr. S. Al-Malaika for his Diploma thesis on polyolefin stabilization. He received his Ph.D. in the research group of Dozent Dr. W. D. Habicher at the Technische Universit€at Dresden with a work on the synthesis of phosphorous acid derivatives and their application in polymer stabilization (19921997). From 1998 to 2005, he continued in the group of Dozent Dr. W. D. Habicher as a research associate. During that period he focused on macrocyclic and supramolecular chemistry with organophosphorus compounds. In 2005, he joined the research group of Prof. H.-J. Kn€olker at the Technische Universit€at Dresden as a permanent staff chemist. His current research interests include organoiron chemistry and the synthesis of natural products and biologically active derivatives.

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Hans-Joachim Kn€olker was born in Rehren, Germany, in 1958. He studied chemistry at the Universities of G€ottingen and Hannover (19781983) and received his Ph.D. in the research group of Prof. E. Winterfeldt at the University of Hannover (19831985). Following postdoctoral studies with Prof. K. P. C. Vollhardt at the University of California in Berkeley (1986), he returned to the University of Hannover for his habilitation (19871990). He was Full Professor of Organic Chemistry at the University of Karlsruhe (19912001) and Chairman of the Chemistry Department at the University of Karlsruhe (19951997). In the year 2000, he was a visiting scientist in India at the invitation of the Indian National Science Academy (INSA). Since 2001, he has been Professor of Organic Chemistry at the Technische Universit€at Dresden. In 2006, he was elected as an Ordinary Member of the Saxon Academy of Sciences and he became a fellow of the Royal Society of Chemistry. He was a visiting scientist in Japan from October to November 1998 (University of Tsukuba) and from October to November 2007 (Kyushu University) as fellow of the Japan Society for the Promotion of Science (JSPS). Since 2011, he has been the editor-in-chief of the book series The Alkaloids: Chemistry and Biology. His research interests include the development of novel synthetic methodologies, organometallic chemistry, natural product synthesis, biomolecular chemistry, and medicinal chemistry.

ACKNOWLEDGMENT We are grateful to the Division of Chemistry and the Environment and the Committee on Chemistry Research Funding (CCRF) of the International Union of Pure and Applied Chemistry (IUPAC) for the financial support of our project “Green and Sustainable Catalysts for Synthesis of Organic Building Blocks” in the frame of the international call on “Novel Molecular and Supramolecular Theory and Synthesis Approaches for Sustainable Catalysis” (DFG Grant KN 240/19-1). We wish to thank Raphael Fritsche, Dr. Arndt W. Schmidt, and Ulrike Schmidt for helpful discussions and for proofreading of the manuscript. REFERENCES (1) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. Rev. 2004, 104, 6217. (2) Necas, D.; Kotora, M. Chem. Listy 2006, 100, 967. (3) Bauer, E. B. Curr. Org. Chem. 2008, 12, 1341. 3365

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Iron catalysis in organic synthesis.

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