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PERSPECTIVE

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Transition metal complex assisted Csp3–F bond formation Xiaoxi Lin and Zhiqiang Weng* Fluoroorganic compounds have attracted significant attention in various fields, such as pharmaceutical, agricultural chemistry, and materials science, as a result of their unique physical, chemical, and physio-

Received 6th November 2014, Accepted 3rd December 2014

logical properties. Consequently, extensive efforts have been devoted to the site-specific synthesis of

DOI: 10.1039/c4dt03410d

organofluorine compounds. In recent years, transition-metal-mediated C–F bond formation has emerged as a powerful method for fabrication of these compounds. This Perspective mainly focuses on

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the most recent advances in transition-metal-assisted synthesis of alkyl fluorides.

1.

Introduction

It has been widely recognized that the introduction of fluorine atoms into a given molecule often modifies profoundly and uniquely its chemical stability, lipophilicity, and metabolic stability. As a consequence, organofluorine compounds are widely used in many fields, such as pharmaceuticals, agrochemicals, advanced materials, and polymers. In addition, fluorine plays a decisive role in medical imaging technologies. For instance, fluorine-18 is the most commonly used radioisotope for positron emission tomography (PET) and offers the

Department of Chemistry, Fuzhou University, Fuzhou, China, 350108. E-mail: [email protected]; Fax: +86 591 22866121

Xiaoxi Lin received her B.Sc. degree (2010) from Chongqing Normal University, and her M.Sc. degree (2013) from Fuzhou University under the supervision of Professor Zhiqiang Weng. In September 2013, she became a PhD student in Professor Zhiqiang Weng’s group working on transition-metal mediated synthesis of organofluorine compounds. Xiaoxi Lin

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advantages of high-resolution imaging, a relatively long halflife of 110 min, and minimal perturbation of radioligand binding.1–3 Fluorinated molecules now represent over 20% of approved medicines and 30–40% of commercially available agrochemicals.4–6 Although fluorine is among the most abundant elements in the earth’s crust, the occurrence of organofluorine compounds in nature is exceedingly rare, with about a dozen isolated to date.7 Therefore, the development of efficient methods for the preparation of organofluorine compounds continues to be an important goal of synthetic organic chemistry.8 Over the last decade, a number of transition-metal-based methods have been developed to synthesize aryl fluorides.9–13 These include Pd-catalyzed fluorination of aryl bromides and -triflates,14,15 arylboronic acid derivatives,16 and aromatic C–H bonds,17–20

Zhiqiang Weng received his B.Sc. (1994) and M.Sc. (1997) degrees from Fuzhou University, and his PhD degree (2003) from the National University of Singapore (NUS). He worked as a postdoc at NUS with Professor Dr T. S. Andy Hor (2003–2006), and then at the University of Illinois Urbana-Champaign (2006–2007) with Professor Dr John F. Hartwig. In 2008, he returned to NUS as a Research Fellow to Zhiqiang Weng start his independent research. In April 2010, he moved to Fuzhou University and became full professor. His research interests focus on the synthetic organometallic chemistry and fluorine chemistry.

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Perspective

Pd-mediated electrophilic fluorination of arylboronic acids,21,22 and Ni-mediated oxidative fluorination.23 Additional methods include Cu-mediated fluorination of aryl halides,24,25 -boronate esters,26 -trifluoroborates,27 and -stannanes,27 Cu-catalyzed fluorination of 2-pyridyl aryl bromides,28 and C–H bonds,29 and Ag-mediated and -catalyzed electrophilic fluorination of aryl stannanes,30,31 and C–H fluorination of pyridines and diazines.32,33 Finally, a variety of transition metal complexes have been utilized for catalytic allylic fluorination including Pd,34–39 Cu,40 Rh,41 and Ir.42,43 Despite these outstanding approaches, the methods for general and site-specific formation of aliphatic fluorides are rare and relatively underdeveloped.12,44–47 Generally, alkyl fluorides are prepared either by nucleophilic48 or electrophilic49 substitution reactions or by radical fluorination.45 The main strategy for the synthesis of such molecules is the nucleophilic substitution of alkyl sulfonates and halides with fluoride ions.50,51 However, these methods are variously limited by the use of fluoride ions (such as KF or CsF) with weak nucleophilicity in protic solvents or low solubility in aprotic solvents. Thus, generally vigorous reaction conditions are required for this reaction. To circumvent this problem, a variety of metal fluoride reagents such as KF/18crown-6,52 “spray-dried” KF,53 polymer supported fluoride,54 and calcium fluoride supported on alkali metal fluoride55 have been developed to enhance the nucleophilicity and solubility of fluoride ions in organic solvents and to accelerate this substitution reaction. Unfortunately, these processes were found to be less efficient than those using tetraalkylammonium fluorides. Because of its good solubility and reactivity, tetrabutylammonium fluoride has been the most commonly used reagent for the nucleophilic fluorination.56 Although these classic methods are quite effective for the synthesis of simple alkyl fluorides, they have certain drawbacks such as elimination of alkyl halides, the formation of alkyl sulfonates to alkenes, and the hydroxylation to alcohols because a “naked” fluoride can act not only as a nucleophile but also as a base.56 Therefore, a general approach through which substrates with a diverse array of substitution patterns can be reliably transformed into functionalized alkyl fluorides under mild reaction conditions is highly desirable. It is well known that fluorinated organometallic complexes provide a general route for the incorporation of the fluorine atom into organic molecules.57–60 Moreover, transition metal fluorides have been proposed as key intermediates in metal-mediated C–F bond formation.10,61 Thus, the development of transition-metal-catalyzed fluorination would provide a useful methodology for the preparation of fluorinated alkanes. It is encouraging to note that considerable efforts have been devoted recently to the preparation of these alkyl fluorides and several efficient and versatile methods have been developed. This Perspective focuses on the most recent advances in transition-metal-assisted Csp3–F bond formation.

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Dalton Transactions

2. Pioneering work with Co, Ir, and Ru complexes As early as 1994, Richmond and co-workers reported a cobaltocenium fluoride [Cp2Co]F (1, Cp = C5H5), which was synthesized from the reaction of Cp2Co with an excess of perfluorodecalin.62 The resulting cobaltocenium fluoride salt 1 provides a novel source of “naked” fluoride to react with organic halides to produce alkyl fluorides under strictly anhydrous conditions (Table 1). Bergman and co-workers described the reaction of Cp*Ir(PMe3)(Ph)(OTf ) (OTf = O2SOCF3) with TAS-F [(Me2N)3S+(Me3SiF2)−] to give aryliridium(III) fluorides Cp*Ir(PMe3)(Ph)F (2) in 1995.63 They also demonstrated that this complex undergoes fluorination with organic halides, such as PhCH2Br, Me3SiC1, MeCOCl, and CH2Cl2, to afford the corresponding alkyl fluorides in >90% yields (Scheme 1). In the year 2000, Togni and co-workers reported a fivecoordinate fluoro complex [RuF(dppp)2]PF6 (3), prepared by reacting [RuCl(dppp)2]PF6 with TlF (dppp = 1,3-bis(diphenylphosphino)propane). This 16-electron fluoro complex was next employed in the attempted fluorination with activated haloalkanes (Table 2). In this way, the reaction proceeds instantaneously and quantitatively with (E)-3-bromo-1,3-diphenylpropene and chlorotriphenylmethane to give the fluorinated organic derivatives. Notably, decreasing substitution at the halogen-bearing carbon atom resulted in lower conversion of substrates. Further work was carried out by the same group to develop catalytic fluorination by halide exchange with 16-electron

Table 1

Fluorination of alkyl halides with [Cp2Co]F (1)

Entry

Alkyl halide

Product

1 2

CH3I H3C(CH2)9I

CH3F H3C(CH2)9F + H3C(CH2)7CHvCH2

3

Scheme 1

Time (h)

Yield (%)

6 6

>95 64 24

1.5

>95

Fluorination of alkyl halides with Cp*Ir(PMe3)(Ph)F (2).

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Table 2

Entry

Perspective

Fluorination of alkyl halides with [RuF(dppp)2]PF6 (3)

Time

Conv. (%)

1

1 min

100

2

1d

75

3

18 h

70

90

4

1d

50

20

5

1 min

100

100

Fig. 1

Alkyl halide

Product

Selectivity (%) 90 —

Structures of complexes 4–6.

ruthenium(II) complexes [RuCl(dppe)2]PF6 (4; dppe = 1,2-bis(diphenylphosphino)ethane), [RuCl(chiraphos)2]PF6 (5; chiraphos = (S,S)-3,4-bis(diphenylphosphino)butane), and [RuCl(PNNP)]PF6 (6; PNNP = (1S,2S)-N,N′-bis[2-(diphenylphosphino)benzylidene]diaminocyclohexane) (Fig. 1) in 2001.64 Under their conditions, catalyst loading as low as 1 mol% has been successfully used for fluorination of the alkyl halides (CH3)3CX (X = Br, I), Ph2CHBr, and PhCH(Me)Br to the fluoro analogues in 31%–83% yields in the presence of TlF as the fluoride source. In spite of the narrow substrate scope, and the use of the expensive and toxic fluoride source TlF, this work represents a significant achievement in early attempts to synthesize alkyl fluorides via transition metal catalyzed nucleophilic fluorination.12

3. Copper(I) fluoride complexes In general, copper fluoride chemistry is underdeveloped relative to the chemistry of the other transition metals.65 There are

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Scheme 2

Synthesis of copper(I) fluoride complexes 7 and 8.

only a small number of examples of copper fluoride complexes found in the literature. These include (PPh3)3CuF,66 and N-heterocyclic carbene (NHC) ligated copper(I) fluoride complexes (NHC)Cu-F67–69 and bifluoride complexes (NHC)Cu-FHF.70 In 2013, Weng and co-workers reported the preparation of diimine-ligated copper(I) fluoride complexes from the reaction of CuOt-Bu with the phenanthroline ligand, and (HF)3·NEt3 in THF at rt. The fluoride complexes (Me2phen)2Cu(FHF) (7) and (t-Bu2Phen)Cu(F) (8) (Me2phen = neocuproine, t-Bu2Phen = 2,9-Di-tert-butyl-1,10-phenanthroline) were isolated in 92% and 49% yields based on Cu, respectively (Scheme 2).71 Consistent with the observations in (NHC)Cu-FHF,70 these complexes exist as either ionic or neutral forms in the solid state, depending on the steric bulkiness of the substituent groups of the phenanthroline ligands.

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Table 4 Fluorination of alkyl bromides by complex 7

Fig. 2

Table 3

Crystal structure (H atoms omitted) of 7 (top) and 8 (bottom).

Comparison of fluorination of 3-phenylpropyl bromide

Entry

[F−] source

Yielda (%)

1 2 3 4 5

Complex 7 Complex 8 KF (HF)3·NEt3 Bu4NF

92 21 6 0 27

a

Yields were determined by 19F NMR spectroscopy with PhOCF3 as the internal standard.

Complex 7 contains one cationic tetrahedral copper center ligated by two of the dative neocuproine ligands and one anionic [HF2]−, with the anion which carries the fluorine atoms not directly bonded with the Cu(I) center (Fig. 2). Complex 8 possesses a trigonal planar geometry with one bidentate phenanthroline ligand and one fluoride coordinated to the copper(I) center. Moreover, in 8 the Cu–F bond length (1.870(8) Å) is slightly longer than that of (IMes)CuF (1.820 Å)69 and significantly shorter than that of (PPh3)3CuF (2.062(6) Å).66 Table 3 shows a comparison of the yield for the fluorination of 3-phenylpropyl bromide with a variety of fluoride sources. Reaction of 7 with 3-phenylpropyl bromide (5 equiv.) in

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CH3CN at 110 °C for 15 h furnished (3-fluoropropyl)benzene in 92% yield and allylbenzene as a major side product. Meanwhile, the neutral form of complex 8, KF or (HF)3·NEt3 alone, and Bu4NF all led to poorer results in fluorination. The findings are consistent with previous studies on fluorination of mesylate with metal fluoride in polar aprotic solvents reported by Chi and co-workers.72 A number of structurally diverse alkyl bromides and benzyl bromides are reactive under these reaction conditions (Table 4). Alkyl bromides having potentially reactive functional groups, such as phenoxide, benzyl ether, thioether, amide, nitrile, hydroxyl and ester, are viable substrates as are more complex substrates, such as vitamin E derivative. In addition, unactivated secondary alkyl bromides bearing methoxyl, ester, ketone, hydroxyl, as well as amido groups can also be used for the reaction (Table 5). The fluorination proved to be efficient, chemoselective, easy to perform, and tolerant to a number of functional groups. Weng’s group further carried out some experiments to gain some insights into the plausible mechanism (Scheme 3). Treatment of 6-bromohexan-1-ol with 7 in the presence of a radical scavenger, cyclohexa-1,4-diene (CHD), produces 6-fluorohexan1-ol in 65% yield. Furthermore, reaction of 6-bromo-1-hexene with 7 affords 6-fluoro-1-hexene as the only product in 99% yield. This absence of the cyclization product and the lack of any influence on the reactivity with the addition of a radical scavenger suggest that a radical mechanism is less likely. In addition, the stereochemistry of the reaction with an enantiopure secondary tosylate was also investigated (Scheme 4). Fluorination of 1 with (2S,4R)-1-tert-butyl 2-methyl 4-(tosyloxy)-

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Dalton Transactions Fluorination of secondary alkyl bromides by complex 7

Table 6

Copper-catalyzed fluorination of alkyl triflates

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Table 5

Perspective

Scheme 3

Radical trapping experiments.

Scheme 4

Inversion of configuration experiments.

pyrrolidine-1,2-dicarboxylate, prepared from the corresponding chiral secondary alcohol, afforded the corresponding product (2S,4S)-1-tert-butyl 2-methyl 4-fluoropyrrolidine-1,2-dicarboxylate in 85% yield with complete inversion of the configuration. Finally, the authors proposed that the fluorination proceeds through an SN2-type displacement of the bromide leaving group with the copper fluoride reagent.

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Most recently, Lalic and co-workers reported an efficient copper-catalyzed fluorination of alkyl triflates with KF as a fluoride source.73 The reaction utilized 2 mol% of the copper catalyst [IPrCuOTf ] at 45 °C in 1,4-dioxane for one hour (Table 6). With as little as 10 mol% of the catalyst, full conversion could be achieved within 10 minutes at 45 °C. A wide range of functional groups including alkyl tosylates and alkyl bromides were tolerated under these reaction conditions. A reaction mechanism for this catalytic system was proposed which relies on the feasible conversion of [IPrCuOTf ] into the corresponding [IPrCuF] through a reaction with KF (Scheme 5). The subsequent fluorination of alkyl triflate by [IPrCuF] delivers the desired alkyl fluoride, and regenerates the catalytically competent [IPrCuOTf ]. The role of the copper catalyst is to serve as a phase-transfer catalyst which provides a soluble and nucleophilic source of fluoride from the rather insoluble KF. In 2012, Lectka and co-workers presented an example of catalytic aliphatic C–H bond fluorination.74 By the use of a multicomponent system of a copper(I) bisimine complex

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Perspective

Dalton Transactions Table 7

Polycomponent copper-catalyzed fluorination of alkanes

Entry

Product

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Scheme 5 Proposed mechanism for copper-catalyzed fluorination of alkyl triflates.

Fig. 3

Temp. (°C)

Yield (%)

1

3

25

75

2

3

0

40

3

2

81

72

4

1

81

66

5

0.5

81

41

6

0.5

81

47

7

2

25

52

8

1

81

33

9

2

81

63

10

1.5

81

56

11

5

81

62

12

3

81

55

13

24

25

28

14

3

25

47

Structures of (BPMED)CuI and (BPMED)CuI2−.

(BPMED)CuI (Fig. 3), KB(C6F5)4, KI, and N-hydroxyphthalimide (NHPI), various aliphatic substrates including cycloalkanes undergo Csp3–H bond fluorination with Selectfluor affording monofluorinated products (Table 7). The addition of KI could promote the formation of the active cuprate species (BPMED)CuI2− (Fig. 3). In addition, the addition of NHPI, known to form the phthalimide N-oxygen radical in situ in the presence of redox active metals, gave rise to the corresponding fluorinated product. However, in the presence of the radical scavenger TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl), the fluorination was completely inhibited, thus providing substantial evidence that the fluorination proceeds through a radical pathway. Although this process is an attractive strategy to realize alkyl fluorides, it suffers from poor site selectivity for straightchain alkanes, rather complex reaction mixtures, and moderate product yields in most cases.

4.

Time (h)

Silver complexes

In 2012, silver-catalyzed decarboxylative fluorination of aliphatic carboxylic acids was reported by Li and co-workers.75 Using AgNO3 as the catalyst and Selectfluor as the fluorine source, various carboxylic acids undergo decarboxylative fluorination to furnish alkyl fluorides in good yields (Table 8). Moreover, the mild reaction conditions of this novel catalytic process were shown to tolerate a variety of functional groups, including amide, ester, carbonyl, halide, ether, and aryl. These procedures can also find application in synthesis of more complex organic molecules, such as dehydrolithocholic acid derivatives. It was hypothesized that this silver-catalyzed fluorodecarboxylation proceeds via a free radical pathway. The proposed mechanism of this remarkable transformation involves initial oxidation of Ag(I) by Selectfluor, followed by single electron transfer (SET) from a carboxylate anion to the resulting Ag(III)–F intermediate to give divalent Ag(II)–F and a carboxyl radical. Thereafter, fast decarboxylation of the carboxyl radical generates the corresponding alkyl radical, which subsequently abstracts the fluorine atom from Ag(II)–F to furnish the desired

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Perspective

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Table 8 Silver-catalyzed decarboxylative fluorination of carboxylic acids

Scheme 6 Proposed mechanism for silver-catalyzed decarboxylative fluorination of carboxylic acids.

Scheme 7 alkenes.

Silver-catalyzed

Scheme 8

Silver-catalyzed fluorination of alkylboronates.

phosphonofluorination

of

unactivated

their pinacol esters with the Selectfluor reagent furnished the desired alkyl fluorides with the wide functional group compatibility.

alkyl fluoride product, along with the regenerated Ag(I) catalyst (Scheme 6). Based on these results, Li and co-workers further developed silver-catalyzed radical phosphonofluorination of unactivated alkenes.76 Treatment of various unactivated alkenes with diethyl phosphate and Selectfluor in the presence of catalytic AgNO3 furnished fluorinated alkylphosphonates in good yields with good stereoselectivity (Scheme 7). These transformations also demonstrated excellent compatibility with a wide range of functional groups, e.g. unprotected and protected alcohols, protected amines, alkyl chlorides, ethers, sulfonates, esters, amides, ketones, and nitriles. Li and co-workers subsequently expanded upon these results in the silver-catalyzed radical fluorination of alkylboronates in acidic aqueous solution (Scheme 8).77 Under catalysis with AgNO3, the reactions of various alkylboronic acids or

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5. Gold(III) fluoride complexes The Csp3–F reductive elimination from alkylgold(III) fluoride complexes was reported in 2012 by the research group of Toste.78 A series of cis-F2AuIII(R)(IPr) complexes were prepared by oxidation of (IPr)AuIR complexes with XeF2 (Scheme 9). The reactivity of these alkylgold(III) fluoride complexes toward Csp3–F reductive elimination was investigated (Table 9). For alkyl groups bearing β-hydrogens, β-H elimination outcompeted Csp3–F reductive elimination, thus producing alkene and fluoroalkane mixtures. For strained cyclic alkyl groups and

Scheme 9

Synthesis of alkylgold(III) fluoride complexes.

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Table 9 Csp3–F complexes

Entry

reductive

elimination

R

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1 2

3

4

5

6

7

8

9

10

11

12

13

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from

alkylgold(III)

Product mixture None

fluoride

Scheme 10 Proposed mechanistic scheme for reaction of alkylgold(III) fluoride complexes.

most acyclic alkyl groups lacking β-hydrogens, the complexes underwent facile carbocation-like rearrangements to form tertiary rather than primary fluoroalkanes. The mechanism of this gold(III)-mediated Csp3–F reductive elimination was probed through a kinetics study of the decay of one cis-F2Au(R)(IPr) species, stereochemical analysis of reductive elimination with a chiral R group, and DFT analysis of relevant alkylgold(III) intermediates. These studies indicate that the reaction likely proceeds through transient cationic [(IPr)Au(F)(R)]+ intermediates with significant ionization of the Au–alkyl bonds (Scheme 10).

6. Palladium fluoride complexes Liu and co-workers reported an intramolecular oxidative aminofluorination of unactivated alkenes.79–81 They found that the regioselectivity of these palladium-catalyzed processes strongly depended on the protecting groups on the nitrogen atoms. For example, N-tosyl alkenes underwent endo-cyclization to yield fluorinated piperidines, while in the presence of a hexafluoroisopropyl alcohol additive the corresponding substrates containing urea protecting groups underwent exo-cyclization to afford the monofluoromethylated nitrogen-containing heterocycles (Table 10). It should be pointed out that this work represents the first example of oxidative fluorination of the C–Pd bond by using an inorganic fluoride salt and an oxidant. To gain mechanistic insight into the palladium-catalyzed aminofluorination process, the authors performed the reaction with a deuterium-labeled alkene under the standard conditions. These studies revealed that the reaction involved a PdIV(F) intermediate as shown in Scheme 11. An initial Pd(II)mediated trans-aminopalladation of the alkene with attack at the terminal carbon (6-endo) would give an Pd(II) intermediate. The subsequent oxidation of the Csp3–PdII intermediate using PhI(OPiv)2/AgF could afford the Csp3–PdIV(F) complex. The final reductive elimination from the Pd(IV) intermediate would yield the desired fluorinated products. In subsequent studies, Liu and co-workers developed a palladium-catalyzed intermolecular aminofluorination82 and fluoroesterification83 of styrenes using NFSI as the fluorine source (Scheme 12). Interestingly, the addition of carboxylic acids with weak nucleophilicity but strong acidity, such as CF3CO2H and CCl3CO2H, led to formation of the C–O bonds instead of the C–N bonds. The reaction afforded various vicinal fluoroamine and monofluorinated benzyl esters with remarkable regioselectivity, respectively. The mechanistic

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aminofluorination

of

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Table 10 Palladium-catalyzed alkenes

Perspective

Scheme 12 Palladium-catalyzed aminofluorination and fluoroesterification of styrenes.

Scheme 13

Scheme 11 Proposed mechanism for palladium-catalyzed aminofluorination of alkenes.

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Synthesis and reactivity of Pd(IV) fluoride complexes.

studies indicate that the synthetic route may involve fluoropalladation of styrene as the key step for Csp3–F bond formation. In 2012, Sanford and co-workers reported the first example of a non-allylic Csp3–F bond-forming reductive elimination from a palladium(IV) center.84 The cyclometalated bipyridine PdIV fluoride complex 10 was synthesized by the oxidation of PdII complex 9 with NFTPT (N-fluoro-2,4,6-trimethylpyridinium triflate), as shown in Scheme 13. Facile displacement of the triflate ligand of 10 by pyridine, water, or fluoride generated cationic products 11 and 12, and difluoride complex 13, respectively. The reactivity of these PdIV fluoride complexes was then examined toward C–F bond-forming reductive elimination. Upon heating to 80 °C, both complexes 11 and 13 underwent elimination to give Csp3–F bond formation products 14 and 15 as the only product in 93% and 87% yields, respectively. The mechanism proposed for this reaction involves direct Csp3–F

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Table 11 Palladium-catalyzed hydrofluorination of alkenylarenes

Scheme 14

Synthesis and reactivity of Pd(II) fluoride complexes.

bond formation rather than SN2-type attack on the PdIV–alkyl bond. As early as 1998, Grushin reported that the PdII fluoride complex [Pd(F)(Ph)(PPh3)2] undergoes halide metathesis with CH2Cl2 to afford a mixture of CH2ClF and CH2F2.85 In 2012, Braun and co-workers reported a similar observation in a related PdII fluoride complex.86 Trans-[Pd(F)(4-C5NF4)-(iPr2PCH2CH2OCH3)2] 16 has been prepared by oxidative addition of pentafluoropyridine with [Pd(iPr2PCH2CH2OCH3)2] at room temperature. Complex 16 can act as a fluorinating agent and reacts with electrophilic compounds, such as EtOTf or 3-bromopropene, to yield the corresponding alkyl fluorides (Scheme 14). Most recently, Gouverneur and co-workers developed a palladium-catalyzed hydrofluorination of alkenylarenes through sequential H− and F+ addition.87 Vinylarenes and disubstituted alkenylarenes underwent hydrofluorination with Selectfluor in the presence of a catalytic amount of [Pd(PPh3)4] to give structurally diverse benzyl fluorides in moderate to good yield in a highly regioselective manner (Table 11). A number of functional groups were tolerated by these reaction conditions, including ether, amide, ester, sulfonamide, fluoro, bromo, alkyl, and trifluoromethyl groups. The proposed mechanism commences with oxidation of the precatalyst to give the electrophilic PdII species A, followed by silane activation to produce the PdII hydride species B (Scheme 15). The subsequent syn-hydropalladation of the alkenylarene affords the η3-benzyl complex C, which undergoes electrophilic fluorination with Selectfluor to afford the PdIVF dication D (or its η3-complex). Reductive elimination then occurs to form the desired benzylic fluoride, and regenerates A.

7. Iron and cobalt complexes The use of iron catalysts for fluorination is of particular interest as more cost-effective and sustainable alternatives to traditional precious metal catalysts. In 2012, an FeIII/NaBH4-mediated free radical hydrofluorination of unactivated alkenes using the Selectfluor reagent as a source of fluorine was reported by Boger and co-workers.88

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Scheme 15 Proposed catalytic cycle for palladium-catalyzed hydrofluorination of alkenylarenes.

The reaction proceeds under extremely mild reaction conditions (0 °C, 5 min, CH3CN/H2O). A number of structurally diverse unactivated terminal alkenes, di- and trisubstituted

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Table 12 alkenes

Perspective

Scheme 16

alkenes were reactive to afford alkyl fluorides with the exclusive Markovnikov addition regioselectivity under these reaction conditions (Table 12). In addition, a variety of functional groups, including unprotected and protected alcohols, protected and free amines, phenols, epoxides, ketals, acetals, carboxylic acids, carbamates, amides, esters, peptides, and carbohydrates, were tolerated. However, styrene substrates were shown to be ineffective for hydrofluorination under these conditions. The observed ratios of product diastereomers (1 : 1) for these reactions provided evidence that these hydrofluorinations likely occur by a free radical process. Moreover, hydrofluorination of a pendant alkene provided the cyclized fluorinated product and the reduced byproduct. In addition, the reaction of an alkyne substrate afforded only the monofluorinated product along with an alkene product, indicating that the alkene is an intermediate en route to the formation of

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Cobalt-catalyzed hydrofluorination of unactivated olefins.

the monofluorinated product. This result also supported the radical nature of these transformations. One year later, Shigehisa, Hiroya, and co-workers developed a complementary strategy for hydrofluorination of unactivated olefins under cobalt catalysis (Scheme 16).89 Various monoand α,α′-disubstituted olefins underwent hydrofluorination in the presence of a catalytic amount of (salen)Co to give alkyl fluorides in good yields with an exclusive Markovnikov selectivity. Furthermore, a variety of functional groups were tolerated including ethers, alcohols, silyl ethers, esters, amides, tosylates, nitro, bromo, and thiophene. A preliminary mechanistic experiment provides support for the radical intermediates proposed in this reaction. In 2013, Lectka and co-workers developed an Fe(II)-catalyzed benzylic fluorination.90 This reaction, which provides a mild route to various monofluorinated benzylic compounds, proceeds with commercially available iron(II) acetylacetonate and Selectfluor in good to excellent yields and selectivity (Table 13). Moreover, this protocol is especially attractive as it allows for the preparation of β-fluorinated products of 3-aryl ketones that are difficult to generate with existing methods. In 2014, Xu and co-workers described an iron(II)-catalyzed diastereoselective olefin aminofluorination reaction.91 Various functionalized hydroxylamines underwent aminofluorination with Et3N·3HF to yield the desired syn- or anti-fluoro oxazolidinone products using 10 mol% Fe(BF4)2·6H2O at r.t. in CH2Cl2 (Scheme 17). An iron nitrenoid was proposed as key reactive intermediate in this stereoconvergent process. Based on previous work on the chiral (salen)M (M = Cr, Co) complex-mediated fluoride ring opening of epoxides,92,93 Doyle and co-workers reported (R,R)-(salen)Co- and aminecocatalyzed enantioselective fluorination of epoxides.94 The use of benzoyl fluoride and HFIP (1,1,1,3,3,3-hexafluoroisopropanol) as a latent source of HF led to the highest yields and ee values of the desired products. Under reaction conditions, five-, six-, seven-, and eight-membered cyclic epoxides could be successfully converted to fluorohydrins in 85–95% ee, whereas acyclic substrates and those containing more Lewis basic groups reacted with lower selectivity (Table 14).

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Table 13

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Enantioselective fluoride ring opening of meso epoxides

Scheme 18

Proposed homobimetallic mechanism.

aliphatic and styrenyl epoxides. However, other epoxides possessing Lewis basic nitrogens or α-branching were not effective. Scheme 17

Iron-catalyzed olefin aminofluorination.

8. On the basis of detailed mechanistic investigations of this process, the authors proposed that a cobalt bifluoride acts as an active nucleophilic fluorinating agent to transfer fluoride to an epoxide substrate that is activated by another (salen)Co species (Scheme 18).95 The high enantioinduction of these epoxide-opening reactions was attributed to the interaction between two chiral (salen)Co species in the transition state. Related to this observation, Doyle and co-workers applied similar reaction conditions for enantioselective (salen)Co- and Ti(NMe2)4-catalyzed synthesis of trans-β-fluoroamine products through ring opening of aziridines.96 This methodology was later applied to the enantioselective radiosynthesis of [18F]fluorohydrins.97 Using [18F](salen)CoF species in MTBE (methyl t-butyl ether) at 50 °C for 20 min, various racemic terminal epoxides were generally radiolabeled in 23–68% RCY (radio-chemical yields) and >90% ee (Table 15). In addition, the reaction is compatible with ethers,

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Manganese fluoride complexes

Catalytic conversion of unactivated Csp3–H bonds to Csp3–F bonds is one of the most challenging topics in organic synthesis. In 2012, Groves and co-workers reported that a manganese porphyrin complex (Fig. 4) catalyzed alkyl fluorination using silver fluoride/tetrabutylammonium fluoride trihydrate under mild conditions in conjunction with stoichiometric oxidation by iodosylbenzene (Table 16).98 Various simple alkanes and substituted alkanes, as well as larger natural-product molecules, underwent oxidative aliphatic C–H fluorination to give monofluorinated products in the presence of catalytic amounts of the bulky manganese porphyrin Mn(TMP)Cl (17). The authors observed that the reactivity and selectivity of the fluorination reaction depended on the nature of the substituents on the alkanes. As a result, treatment of camphor having the electron-withdrawing carbonyl group led to a significantly diminished reactivity under the standard fluorination conditions.

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Table 15 Enantioselective radiosynthesis of [18F]fluorohydrins (RCY)

Fig. 4

Perspective Table 16 Manganese porphyrin complex catalyzed alkyl fluorination

Structures of Mn(TMP)Cl (17).

A reaction mechanism for the manganese-catalyzed C–H fluorination of alkanes has been proposed (Scheme 19). Mn(TMP)Cl (17) could initially react with AgF/TBAF to form the MnIII–F complex I. Subsequent oxidation of I leads to the highly reactive Mn(V)-oxo species II, which then abstracts a H atom from the alkane to afford a C-centered radical and a HO-MnIV–F rebound species III. The resulting radical should then abstract a fluorine atom from the complex [MnIV(TMP)F2] (IV) to form a fluorinated product. The intermediate IV was isolated and structurally characterized. The reaction of IV with 1 equiv. of cyclooctane formed cyclooctyl fluoride in 43% yield, thus implying that the isolated trans-difluoromanganese complex IV is an intermediate in the fluorination pathway. The same group further developed a facile, no-carrieradded, 18F labeling method for late stage benzylic C–H fluorination.99 By using Mn(salen)OTs (18) (Fig. 5) in the presence of PhIO (iodosylbenzene) as the oxidant, a variety of organic molecules and known drugs were labelled with 18F with RCY ranging from 20% to 72% within 10 min. Representative examples of this fluorination are reported in Table 17. In general, substrates bearing electron-donating groups undergo efficient 18F labelling, probably due to the electrophilic nature of the hydrogen-abstracting oxomanganese(V) intermediate. Some functionalities, such as esters, amides, imides, ketones, alkynes, ethers, cyanides, heterocycles, carbamates, and aryl and aliphatic halides, were tolerated under these conditions. In addition, the dry-down free procedure

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Scheme 19 Proposed mechanism for manganese porphyrin complex catalyzed C–H fluorination.

indicates the significant potential of this method for PET imaging applications. Scheme 20 illustrates the proposed catalytic cycle for the manganese salen complex catalyzed 18F labeling of benzylic C–H bonds. In contrast to 19F chemistry wherein a transdifluoromanganese(IV) complex was shown to be the reactive

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Fig. 5

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Structures of Mn(salen)OTs (18).

Table 17 Mn(salen)OTs (18) catalyzed direct C–H bonds (RCY)

18

F labeling of aliphatic

Scheme 20 Proposed mechanism for Mn(salen)OTs (18) catalyzed labeling of benzylic C–H bonds.

Table 18

18

F

Vanadium-catalyzed Csp3–H fluorination

fluorinating agent,98 the [18F]fluorine transfer is proposed to proceed directly through an 18F–MnIV–OH intermediate. A preliminary computational study using density functional theory (DFT) provided support for the [18F]fluorine transfer reactivity of the 18F–MnIV–OH intermediate. The observation of the enantioselectivity of the resulting 18F-labeled product was consistent with theoretical investigations.

9. Vanadium complexes Vanadium-complexes are well-known for catalysis of C–H oxygenation and olefin halogenation.100–102 In 2014, Chen and co-workers developed a vanadium-catalyzed Csp3–H fluorination with Selectfluor.103 Treatment of alkanes with Selectfluor in the presence of catalytic V2O3 in CH3CN at 23 °C afforded the corresponding alkyl fluorides in moderate to good yields (Table 18). The heterogeneous catalyst can be easily separated from the reaction mixture by simple filtration. Notably, the reaction was compatible with substrates bearing a carbonyl group in the α-position. Furthermore, when

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compared to the manganese-porphyrin catalyst system,98 the vanadium catalyst system promoted more efficiency and selectivity in the fluorination of sesquiterpenoid sclareolide. However, in substrates having an aromatic ester group, low

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Dalton Transactions

yield of the desired product was obtained. Likewise, poor yield of benzylic fluorides was observed in electron-rich benzylic substrates. The overall process provides an improvement in aliphatic C–H fluorination at non-benzylic positions compared to previous methods.98 A KIE of kH/kD = 4 was experimentally determined, indicating that C–H abstraction is the rate-limiting step. The reaction may involve alkyl radical species which are not oxidized to a carbocation before being trapped by a fluoride. Moreover, the authors favor a mechanism involving a vanadium(II/III) or (III/IV) cycle in this system.

10. Conclusions Over the last few years, impressive progress relating to the formation of alkyl fluorides has been made. Among the various synthetic approaches to these substances, those that utilize the transition-metal-assisted fluorination have attracted increasing interest, owing to their exceptional reactivity and high site selectivity. Significant breakthroughs have been realized, which include oxidative aliphatic C–H fluorination with the fluoride ion under manganese catalysis, Csp3–F bond-forming reductive elimination from PdIV fluoride complexes, and metal-catalyzed Csp3–F bond formation through radical pathways. These reactions provide the most promising and powerful methods for construction of alkyl fluorides from poorly activated substrates. In addition, these transformations showed good compatibility with a wide range of functional groups. The developed strategy has the potential to find widespread application of this chemistry for biomolecules and synthetic building blocks, as well as molecular imaging probes. The future developments in this field may encompass, for example, the use of inexpensive metal fluoride as the fluorine source, improved mechanistic understanding of these transformations, and rational development of more reactive and selective catalysts for Csp3–H fluorination.

Acknowledgements Financial support from the National Natural Science Foundation of China (NSFC) (grant number 21072030 and 21372044), the Research Fund for the Doctoral Program of Higher Education of China (grant number 20123514110003), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, P. R. China (grant number 2012-1707), the Science Foundation of the Fujian Province, China (grant number 2013J01040), and Fuzhou University (grant numbers 022318, 022494) is gratefully acknowledged.

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Transition metal complex assisted Csp3-F bond formation.

Fluoroorganic compounds have attracted significant attention in various fields, such as pharmaceutical, agricultural chemistry, and materials science,...
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