DOI: 10.1002/asia.201403248

Focus Review

Reaction Mechanisms

Iodine-Catalyzed Oxidative Coupling Reactions Utilizing C H and X H as Nucleophiles Dong Liu[a] and Aiwen Lei*[a, b]

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Focus Review oxidative coupling reactions. This Focus Review gives a brief summary of recent development on iodine-catalyzed oxidative coupling reactions utilizing C H and X H as nucleophiles.

Abstract: In recent decades, iodine-catalyzed oxidative coupling reactions utilizing C H and X H as nucleophiles have received considerable attention because they represent more efficient, greener, more atom-economical, and milder bond-formation strategies over transition-metal-catalyzed

1. Introduction

tion reactions have been reported previously, only iodine-catalyzed intermolecular oxidative couplings utilizing C H and X H as nucleophiles are reviewed herein. Intramolecular oxidative coupling reactions utilizing iodine catalysis are not included herein.[5]

The cross-coupling reaction is a fundamental type of organic transformation because it opens up a brand-new area for the synthesis of numerous kinds of organic compounds, especially important natural products and biologically active molecules.[1] Generally, these types of transformations are based on bondformation reactions between nucleophiles and electrophiles. However, electrophiles are mostly generated from the corresponding C H or X H (X = N, O, S, etc.) nucleophiles. Thus, transition-metal-catalyzed oxidative coupling reactions utilizing C H and X H as nucleophiles have become more attractive and the topic of significant research efforts in recent years, owing to their atom economy and environmental sustainability.[2] However, a major drawback of these types of reactions is the inevitable residual metal in the final products, which greatly restricts their application in pharmaceutical synthesis. Recently, iodine-catalyzed oxidative coupling reactions have undergone rapid advances, which has solved the problem of residual metal in transition-metal catalysis.[3] Moreover, iodine catalysis has evident advantages over transition-metal catalysis, such as greater atom economy; greener, milder reaction conditions; and broader substrate compatibility, which makes them more powerful and promising in organic synthesis. Inspired and pioneered by hypervalent iodine chemistry,[4] various bond-formation methods utilizing iodide-catalyzed oxidative coupling reactions have been revealed in the past ten years. [3a–c, e–g] However, understanding of the exact role of iodine catalysts in those types of transformations is still rather limited. This Focus Review provides a brief summary of recent developments in iodine-catalyzed oxidative coupling reactions utilizing C H and X H as nucleophiles. Based on different roles of iodine catalysts proposed previously, the main part of this review is divided into three sections: 1) iodine as a radical initiator to decompose peroxides, 2) molecular or hypervalent iodine as the key species, and 3) X I bond-formation intermediates as key species. In each subsection, categories based on diverse bond-formation reactions are shown. Selected examples of substrate applicability are also listed. Because several reviews focusing on different iodine-catalyzed bond-forma-

2. Iodine as Radical Initiator to Decompose Peroxides In the first type of iodine-catalyzed oxidative coupling reactions, iodine was believed to act as only the radical initiator to decompose peroxides. In this section, reactions of this type are described. 2.1. C O Bond-Formation Reactions In 2011, Wan and co-workers reported a nBu4NI (TBAI)-catalyzed, metal-free oxidation for the synthesis of tert-butyl peresters.[6] In this reaction, H2O was utilized as the solvent and the reaction proceeded smoothly at 40 8C. Various kinds of substituted aldehydes could be transformed into the corresponding peresters in good yields [Scheme 1, Eq. (1)]. Notably, the au-

Scheme 1. TBAI-catalyzed C H oxidation of aldehydes. TBHP = tert-butyl peroxide.

thors combined the C H oxidation of aldehydes with the Kharasch–Sosnovsky reaction and the direct synthesis of allylic esters in one-pot from simple olefins and aldehydes [Scheme 1, Eq. (2)]. The C H oxidation reaction was believed to be initiated by TBAI to decompose TBHP into the tert-butoxyl and tert-butylperoxy radicals. The resulting tert-butoxyl radical traps hydrogen from aldehyde to form the acyl radical. Coupling of the acyl radical and the tert-butylperoxy radical affords the desired perester (Scheme 2). Later, other oxygen nucleophiles, such as N-hydroxyphthalimide (NHPI), N-hydroxysuccinimides, and even hexafluoroisopropyl alcohol (HFIP), could be utilized for cross-coupling with aldehydes and the TBAI/TBHP catalytic system [Eq. (3)].[7] Iodide is believed to decompose TBHP into tBuOC. Meanwhile,

[a] D. Liu, Prof. A. Lei College of Chemistry and Molecular Sciences Wuhan University, Wuhan 430072 (P.R. China) E-mail: [email protected] [b] Prof. A. Lei National Research Center for Carbohydrate Synthesis Jiangxi Normal University, Nanchang 330022 Jiangxi (P.R. China)

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Focus Review H nucleophiles could be cross-coupled smoothly to generate the corresponding allylic esters in good to excellent yields [Eq. (4)]. A mechanistic proposal revealed that iodide helped to decompose TBHP into tBuOC. The final product was derived from radical coupling between an allylic radical and an acyloxy radical.

Scheme 2. Proposed mechanism of TBAI-catalyzed oxidative coupling between aldehydes and TBHP.

Recently, a TBAI-catalyzed peroxidation of C H bonds adjacent to an amide nitrogen atom under ambient temperature was reported by Yu and Shen.[9] Different peroxides could be obtained in good yields through C O couplings [Eq. (5)]. Notably, the obtained peroxides could be further transformed into the corresponding a-substituted amides by utilizing Grignard reagents as the coupling partners [Eq. (6)]. A mechanistic hypothesis revealed that TBAI still served as a radical initiator, and the reaction proceeded through radical coupling between tBuOOC and nitrogen adjacent to the carbon radical.

the reaction of an oxygen nucleophile with an aldehyde formed an acetal species. Subsequent hydrogen abstraction from acetal species by tBuOC or tBuOOC resulted in a a-hydroxy carbon radical, followed by further single-electron oxidation by TBHP to generate the final product (Scheme 3). In addition to aldehydes, carboxylic acids have also been

Aiwen Lei (1973) obtained his Ph.D. (2000) under the supervision of Prof. Xiyan Lu at the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences (CAS). He then moved to Pennsylvania State University, USA, and worked with Prof. Xumu Zhang as a postdoctoral fellow. He joined Stanford University (2003), working with Prof. James P. Collman as a Research Associate. He then became a Full Professor (2005) at the College of Chemistry and Molecular Sciences, Wuhan University, P.R. China. His research focuses on novel approaches and understanding of bond-formation reactions.

Scheme 3. Proposed mechanism of TBAI-catalyzed oxidative coupling between aldehydes and oxygen nucleophiles.

Dong Liu (1989) obtained his BS degree (2012) at Wuhan University. He joined the group of Prof. Aiwen Lei during the second year of his undergraduate study and started his Ph.D. studies in September 2012 at the same group. He is currently a third-year Ph.D. student and focuses on transition-metal-catalyzed cross-coupling reactions.

employed as nucleophiles to achieve oxidative C O bond-formation reactions. In 2012, Wan and co-workers described oxidative coupling of carboxylic acids with allylic C H nucleophiles by TBAI/TBHP catalytic system.[8] Notably, this reaction has a broad substrate applicability. A variety of aryl, heteroaryl, and alkyl carboxylic acids as well as different kinds of allylic C Chem. Asian J. 2015, 00, 0 – 0

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2.2. C N Bond-Formation Reactions In 2012, Wan and co-workers reported oxidative deacylative C N bond-formation reactions between aldehydes and formamides.[10] TBAI/TBHP is also the combination of catalyst/oxidant utilized. Different kinds of aldehydes and various substituted formamides could be tolerated in this reaction [Eq. (7)]. Iodine is believed to decompose TBHP to initiate radicals, and bond formations occur by cross-coupling of acyl and aminyl radicals generated from aldehydes and formamides respectively (Scheme 4).

Scheme 5. KI-catalyzed oxidative coupling between aldehydes and azoles. DCE = 1,2-dichloroethane.

best choice for achieving good reactivities. Different substituted benzoxazoles, even benzothiazole, could cross-couple smoothly with a series of formamides to give the desired products in 28–87 % yield [Eq. (8)]. Mechanistic assumption showed that TBAI still acted as a radical initiator to decompose TBHP to achieve further transformation.

2.3. C S Bond-Formation Reactions In 2012, Li and co-workers reported a TBAI-catalyzed oxidative allylic sulfonylation of a-methyl styrene derivatives with sulfonyl hydrazides.[13] Different substituted a-methyl styrenes could be tolerated to generate the desired C S bond-formation products in good yields [Eq. (9); Ts = tosyl]. Through the proposed mechanism, tBuOC or tBuOOC could be formed, as promoted by TBAI, which further reacted with sulfonylhydrazide to initiate formation of the sulfonyl radical. Addition of the sulfonyl radical to styrene and subsequent hydrogen abstraction afforded the final product.

Scheme 4. Proposed mechanism of iodine-catalyzed oxidative coupling between aldehydes and formamides.

Direct oxidative C N bond formation of aldehydes by employing nitrogen heterocycles was reported by Li and co-workers recently.[11] With KI as the catalyst and TBHP as the oxidant, a series of N-acylated azoles, such as pyrazoles, benzimidazoles, benzotriazoles, and indazoles, could be obtained in good to excellent yields (Scheme 5). Notably, this reaction can be expanded to the gram scale. Mechanistic research revealed that KI served as a radical initiator to decompose TBHP into tBuOC, and radical coupling between an acyl radical and an azole radical afforded the desired product. Benzoxazoles were also used as nucleophiles in iodine-catalyzed oxidative C N bond-formation reactions in 2014 by Wang’s group.[12] The combination of TBAI/TBHP is still the &

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2.4. Miscellaneous Bond-Formation Reactions In 2012, Mai and co-workers demonstrated C N and C O bond-formation reactions for the synthesis of a-ketoamides 4

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Focus Review from aryl methyl ketones with dialkylformamides.[14] The reaction proceeded smoothly in water with TBAI as the catalyst and TBHP as the oxidant [Eq. (10)]. A mechanistic hypothesis revealed that iodide assisted in the decomposition of TBHP to tBuOC, which trapped hydrogen from the ketone and amide to form radicals 2 and 3, respectively. Subsequent deacylation and radical coupling afforded the b-carbonyl amine, which was further oxidized to given the final product 1 (Scheme 6). Later,

A mechanistic study showed that the iodine catalyst acted as a radical initiator to decompose TBHP into tBuOC to achieve further transformations.

3. Molecular or Hypervalent Iodine as the Key Species In the second type of iodine-catalyzed oxidative coupling reactions, molecular iodine or in situ generated hypervalent iodine was believed to be the key reaction species. By the oxidation of the oxidant, the in situ generated hypervalent or molecular iodine plays a key role in the formation of the desired product. In this section, reactions in accordance with this type are described.

3.1. C C Bond-Formation Reactions In 2012, a TBAI-catalyzed C3-formylation of indoles with Nmethylaniline as the formyl source was demonstrated by Wang and co-workers.[18] By utilizing tert-butyl peroxybenzoate (TBPB) as the oxidant, different 3-formylindoles were obtained in good yields. Mechanistic assumption showed that iodine not only served as a radical initiator to decompose peroxide, but also acted as an effective oxidizer for the generation of iminium ions (Scheme 7). Interestingly, when N-methylaniline was replaced by 4-substituted-N,N-dimethylaniline, the simultaneous C3-formylation and N-aminomethylation of indoles selectively occurred with KI as the catalyst [Eq. (13); Piv = pivaloyl, DMSO = dimethyl sulfoxide].[19]

Scheme 6. Proposed mechanism of TBAI-catalyzed oxidative coupling between aryl methyl ketones and dialkylformamides.

the synthesis of a-ketoamides was also achieved by direct utilization of ethylarenes with dialkylformamides by Sun’s group in 2014 [Eq. (11)].[15] Aryl methyl ketones were believed to be generated first through the oxidation of ethylarenes to achieve further transformations similar to those reported by Qu et al.[14] Recently, toluene derivatives have also been used to couple with dialkylformamides to deliver amide products in good yields with the I2/TBHP catalytic system.[16]

In 2013, Itoh and co-workers reported a molecular iodine catalyzed dehydrogenative cross-coupling between two Csp3 H bonds by using hydrogen peroxide as the oxidant [Eq. (14)].[20] Different carbon nucleophiles, such as nitroalkanes, simple ketones, and 1,3-diketones, could be utilized for cross-coupling with tertiary amines to afford the corresponding C C bond-formation products in good yields. A mechanistic hypothesis proposed that in situ generated HOI acted as the key promoter for the formation of iminium ions (Scheme 8). Later, this reaction was also achieved by an aerobic pathway.[21]

Recently, other C C and C S bond-formation reactions were revealed by Wan and co-workers,[17] in which a three-component reaction of sulfonyl hydrazides with 1,3-diketones was demonstrated for the construction of fully substituted pyrazoles utilizing TBAI/TBHP. A series of fully substituted pyrazoles were synthesized by this method in excellent yields [Eq. (12)]. Chem. Asian J. 2015, 00, 0 – 0

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Scheme 7. Key role of molecular iodine in the generation of iminium ion intermediates. Bz = benzyl.

for the formation of isoquinolinium salt 5. Further generation of the 1,3-dipole and subsequent [3+2] cycloaddition and oxidative aromatization afforded the final product (Scheme 9). Soon afterwards, this reaction was also achieved by utilizing the KI/TBHP catalytic system.[2h]

Scheme 9. Proposed mechanism for iodine-catalyzed cascade 1,3-dipolar cycloaddition/aromatization.

Scheme 8. Key role of HOI for the generation of an iminium ion intermediate.

The first iodide-catalyzed oxidative biaryl cross-coupling between anilines and simple arenes was communicated by Kita and co-workers in 2013.[22] In this reaction, substituted 2,2’-diiodobiphenyls 4 were used as the catalyst and meta-chloroperbenzoic acid (m-CPBA) as the oxidant. The desired oxidative C C bond-formation products were obtained in good to excellent yields [Eq. (15); TFE = 2,2,2-trifluoroethanol]. However, the detailed reaction mechanism is still not clear, although it is possible that in situ generated hypervalent iodine species might act as the key intermediates to promote the reaction.

3.2. C O Bond-Formation Reactions In 2011, Wan and co-workers reported a TBAI-catalyzed dehydrogenative cross-coupling for C O bond formation between carboxylic acids and simple ethers.[24] With the promotion of TBHP, various carboxylic acids and simple ethers could be cross-coupled to afford the desired products in good to excellent yields [Eq. (17)]. In the proposed mechanism, the in situ generated molecular iodine served as the oxidant to oxidize the oxygen atom adjacent to the carbon radical to oxonium ion, followed by the subsequent nucleophilic addition of an anionic carboxylic acid to the oxonium ion to provide the final product (Scheme 10). Later, similar works from the groups of Yuan and Duan were reported, in which alcohols and ethers, phenylglyoxylic acids and ethers, as well as amides were used as the coupling partners.[25] In addition to ethers, benzylic C H bonds have also been employed for oxidative C O bond-formation reactions. In 2012, Yu and co-workers reported a TBAI-catalyzed oxidative coupling between carboxylic acids and benzylic substrates.[26]

In 2014, Gao and co-workers reported a molecular iodine catalyzed cascade 1,3-dipolar cycloaddition/aromatization for the synthesis of pyrrolo[2,1-a]isoquinolines with H2O2 as the terminal oxidant [Eq. (16); EWG = electron-withdrawing group, DMF = N,N-dimethylformamide].[23] For mechanistic elucidation, in situ generated HOI was believed to act as the key promoter &

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Focus Review

Scheme 10. Proposed mechanism of TBAI-catalyzed oxidative C O bond-formation reactions between carboxylic acids and ethers.

With TBHP as the oxidant, a series of benzylic esters could be synthesized in excellent yields [Eq. (18)]. Notably, this strategy could be used for the modification of a series of N-Boc-protected (Boc = tert-butyloxycarbonyl) amino acids. A mechanistic investigation showed that in situ generated hypervalent iodine species [Bu4N][IO] or [Bu4N][IO2] could oxidize benzylic C H bond to benzyl cation through two-step single-electron-transfer processes, followed by coupling of benzoate anion with benzyl cation to afford the final product (Scheme 11). The

Scheme 12. TBAI-catalyzed a-acyloxylation of ketones with benzylic alcohols.

3.3. C N Bond-Formation Reactions In 2010, Wang and co-workers revealed a simple synthesis of 2-phenylquinazolines by employing molecular iodine as the catalyst and TBHP as the oxidant.[29] Various 2-phenylquinazolines were prepared in good yields from 2-aminobenzophenones and benzylic amines through a tandem reaction process [Eq. (21)]. A tentative mechanism showed that the imine intermediate was first generated from the condensation of a ketone and amine, which was subsequently oxidized by molecular iodine to form a nitrogen-centered cation intermediate through a two-step single-electron transfer process. Final intramolecular cyclization and further oxidation afforded the desired product (Scheme 13). Inspired by this work, the same group then utilized a-amino acids, and even ammonia gas, as the nitrogen source to achieve the efficient synthesis of quinazolines from 2-aminobenzophenone derivatives [Eq. (22), conditions (1) and (2)].[30] With a combination of I2/TBHP and Nchlorosuccinimide (NIS)/TBHP as the catalytic system, respectively, different substituted quinazolines could be obtained in good to excellent yields.

Scheme 11. Proposed mechanism of TBAI-catalyzed oxidative coupling between carboxylic acids and benzylic substrates.

groups of Wang and Patel reported two TBAI-catalyzed oxidative couplings of benzylic C H bonds with aldehydes and alkylbenzenes, respectively, in which iodine also acted as the key oxidizer for the generation of the benzyl cation [Eqs. (19) and (20)].[27] Recently, Cheng and co-workers communicated a TBAI-catalyzed a-acyloxylation of ketones with benzylic alcohols.[28] With TBHP as the oxidant and PhCN as the solvent, the reaction of alcohols and ketones proceeded smoothly to generate the desired products in good yields at 90 8C (Scheme 12). A mechanistic proposal showed that in situ generated hypervalent iodide species could oxidize ketones to give the a-carbonyl carbon radical or even cation, which served as key intermediates in this reaction.

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Focus Review ly aminated by using TBHP as the oxidant to give the corresponding products in good yields [Eq. (23)]. Moreover, aamino acids could also be used as the amine source to deliver the same products by decarboxylation. Afterwards, a TBAI-catalyzed synthesis of imidazo[1,2-a]pyridines from aminopyridines with nitroolefins was demonstrated by Li and co-workers [Eq. (24)].[33]

Scheme 13. Proposed mechanism of the iodine-catalyzed synthesis of 2-phenylquinazolines.

In 2012, Wang and co-workers revealed a molecular iodine catalyzed C N bond formation between alcohols and N-substituted formamides.[34] TBHP was utilized as the oxidant and a catalytic amount of NaOH was an effective additive [Eq. (25)]. A proposed mechanism showed that iodine not only decomposed TBHP to give tBuOC, but also oxidized alcohol into the corresponding aldehyde, which further released an acyl radical with the aid of tBuOC. Meanwhile, aminyl radical was generated from formamide by the hydrogen abstraction of tBuOC. Subsequent radical coupling afforded the final product (Scheme 15). This reaction was also achieved by Zhu and co-workers with TBAI/TBHP catalytic system.[35]

An efficient synthesis of benzimidazoles from o-phenylenediamines and benzaldehydes was demonstrated by Wei and Zhu in 2011.[31] TBAI was utilized as the catalyst and hydrogen peroxide served as the efficient oxidant. The reaction proceeded smoothly to generate the desired benzimidazoles in good to excellent yields at ambient temperature (Scheme 14). A plausible mechanism showed that the in situ generated hypervalent iodine species [Bu4N][IO] or [Bu4N][IO2] acted as the key promoter for the oxidative dehydrogenation of benzimidazoline intermediate to give the final product. Recently, Li and co-workers communicated a TBAI-catalyzed oxidative domino synthesis of imidazo[1,5-c]quinazolines from readily available 4-methylquinazolines and benzylamines.[32] Notably, only the 4-methyl group of quinazolines was selective-

Amides have also been employed to achieve direct imidation by Csp3 H functionalization. In 2012, Meng and co-workers revealed KI-catalyzed oxidative C N bond-formation reactions between imides and dimethylacetamides.[36] Differently substituted N,N-dimethylamides could cross-couple well with various imides to produce the desired products in good yields (Scheme 16). Mechanistic elucidation showed that in situ generated molecular iodine played a key role for the generation of an iminium ion from an amide. Subsequently, nucleophilic attack of an imide on an iminium ion gave the final product. Later, a TBAI-catalyzed oxidative imidation of ketones with imides for the synthesis of a-amino ketones was reported by Zhang and co-workers in 2014.[37] Different saccharin derivatives and other imides could cross-couple smoothly with various kinds of ketones to generate the corresponding a-amino ketones in good yields [Eq. (26)]. Mechanistic determination re-

Scheme 14. TBAI-catalyzed synthesis of benzimidazoles from o-phenylenediamines and benzaldehydes.

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Scheme 15. Plausible mechanism of iodine-catalyzed C N bond formation between alcohols and N-substituted formamides.

Scheme 17. Proposed mechanism of TBAI-catalyzed oxidative imidation of ketones with imides.

Scheme 16. KI-catalyzed oxidative C N bond formation between imides and dimethylacetamides. Scheme 18. TBAI-catalyzed oxidative C H amination of benzylic C H bonds.

vealed that in situ generated molecular iodine took charge of the formation of the imidyl radical from saccharin. Further addition of the imidyl radical to give the enol form of a ketone and final radical elimination delivered the desired product (Scheme 17). In 2013, Zhu and co-workers reported a TBAI-catalyzed oxidative C H amination of benzylic C H bonds.[38] TBHP was em-

Recently, Wang and co-workers also revealed a TBAI-catalyzed direct C H amination of allylic and benzylic Csp3 H bonds by utilizing simple anilines as the amine sources [Eq. (27)].[39] The reaction also proceeded through a nucleophilic reaction between anilines and allylic or benzylic cations, which were generated by the promotion of hypervalent iodine species.

ployed as the oxidant and various nitrogen heterocycles were utilized as the amine sources. Different kinds of benzylic C H bonds could be transformed into the desired amination products in good to excellent yields (Scheme 18). Mechanistic investigations revealed that in situ generated hypervalent iodine species played an important role for the generation of the benzyl cation from the benzylic C H bond. Further nucleophilic attack of an amine on a benzyl cation formed the final product. Chem. Asian J. 2015, 00, 0 – 0

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3.4. Miscellaneous Bond-Formation Reactions In 2011, Wang and co-workers demonstrated an iodine-catalyzed oxidative decarboxylative cyclization for the synthesis of pyridine derivatives from natural a-amino acids and aldehydes [Eq. (28); DMA = dimethylacetamide].[40] By utilizing TBHP as an effective oxidant, different multisubstituted pyridines could be synthesized in good yields through 9

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decarboxylative C C and C N bond-formation reaction. Mechanistic elucidation showed that molecular iodine played a key role in the decarboxylative cyclization step.

4. X I Bond-Formation Intermediates as Key Species

Scheme 19. Proposed mechanism of PhI-catalyzed oxidative a-acetoxylation of ketones.

In the third type of iodine-catalyzed oxidative coupling reactions, iodine is usually involved in bond-formation reactions during the reaction process. For different types of bond-formation reactions, different I X (X = C, N, O, S, etc.) bonds might be formed within the reaction process; these act as key reaction intermediates to enhance the desired products. In this section, iodine-catalyzed oxidative coupling reactions involving different I X bond-formation intermediates as the key species are discussed.

In 2006, Togo and Yamamoto reported the PhI-catalyzed a-tosyloxylation of ketones with p-toluenesulfonic acid.[44] By using m-CPBA as the effective oxidant, different substituted ketones, including aliphatic ketones, could be transformed into the desired tosyloxylated ketones in good yields [Eq. (31)]. Notably, poly(4-iodostyrene) also proved to be effective catalyst, albeit with a slightly lower efficiency. The reaction mechanism is similar to that reported by Ochiai and co-workers.[41]

4.1. C I Bond-Formation Reactions Among iodine-catalyzed oxidative coupling reactions involving C I bond formations, carbonyl a-C H activation involving C I bond formation is the most common one. Various nucleophiles have been employed to cross-couple with carbonyl C H bonds under iodine catalysis, in which iodine is believed to activate the carbonyl C H bond by direct iodination of an enol tautomerized from a carbonyl to form the a-iodocarbonyl compound [Eq. (29).

Considerable attention has been paid to this reaction since the report by Togo and Yamamoto.[44] Different kinds of iodine catalysts, including chiral iodine catalysts, have been designed and developed to achieve this reaction in good yields with acceptable enantioselectivities.[26, 45] Selected data are summarized in Table 1. In 2012, Wang and Zhang demonstrated an iodine-catalyzed oxidative coupling between acetophenones and amines for the synthesis of a-ketoamides.[46] TBHP was utilized as the oxidant and amine was used as the solvent. Different kinds of ketoamides were obtained in good yields at ambient temperature [Eq. (32)]. A mechanistic proposal showed that the a-iodoketone was first generated via three-membered iodonium intermediate 6 (Scheme 20). Subsequent intermolecular nucleophilic substitution of a-iodoketone with an amine could afford a-amino ketone intermediate 7. Final oxidation of the methylene unit of 7 delivered the desired product. Soon after, this reaction was also revealed by the groups of Wan and Prabhu.[47] Notably, Wang and co-workers subsequently employed formamides to achieve this C N bond-formation protocol in 2013, in

In 2005, Ochiai and co-workers reported the first iodobenzene-catalyzed oxidative a-acetoxylation of ketones with mCPBA as the oxidant [Eq. (30)].[41] The in situ generated hypervalent (diacyloxyiodo)benzene obtained by oxidation of mCPBA was believed to be the real catalytic species. Further electrophilic addition to obtain the enol tautomer from the corresponding ketone afforded the a-iodo(III)ketone intermediate. Finally, a SN2 substitution reaction with acetic acid delivered the desired product with the release of PhI (Scheme 19). This reaction was also achieved efficiently with the PhI/H2O2 catalytic system by Huang and co-workers in 2007.[42] Notably, a detailed mechanistic study of this reaction was reported by Wang and co-workers in 2012, in which electrospray ionization tandem mass spectrometry (ESI-MS/MS) was utilized as an efficient monitoring tool to clearly reveal the generation of hypervalent iodine species.[43] In addition to acetic acids, sulfonic acids have also been employed in the a-tosyloxylation of ketones catalyzed by iodine. &

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Focus Review Table 1. Different catalytic systems for iodine-catalyzed oxidative coupling between ketones and p-toluenesulfonic acid.

Entry

[I]

[O]

Yield and ee

Ref.

1

ionic-liquid (IL)-supported PhI

m-CPBA

39–83 % yield

[45a]

m-CPBA

up to 28 % ee

[45b, d]

m-CPBA

36–88 % yield

[45c]

4

m-CPBA

up to 26 % ee

[45e]

5

m-CPBA

42 % yield and 39 % ee

[45f]

m-CPBA

up to 96 % yield

[45g]

7

m-CPBA

up to 58 % ee

[45h]

8

m-CPBA

up to 49 % ee

[45i]

9

m-CPBA

67 % yield and 18 % ee

[45j]

10

m-CPBA

up to 50 % yield and 46 % ee

[45k]

2 3

6

PhI and Poly(4-iodostyrene)

I2

synthesized in excellent yields. A mechanistic study showed that both phenacyl iodine 8 and phenylglyoxal 9 were probable reaction intermediates. Moreover, monitoring of the reaction by 1H NMR spectroscopy clearly proved the intermediacy of phenacyl iodine and phenylglyoxal, and provided direct evidence of the C I bond-formation process for these types of reactions. In 2013, Wan and co-workers described a novel C N bondformation strategy for the construction of a-amino acid esters through decarboxylation.[50] TBAI was used as the catalyst and TBHP as an effective oxidant. Different kinds of a-amino acid esters could be synthesized in good yields from malonates and amines (Scheme 21). Mechanistic studies revealed that in situ generated IO could react with malonate to generate the iodinated malonate. Subsequent decarboxylation formed the a-iodoester, followed by nucleophilic substitution with an amine to deliver the final product.

which I2/TBHP was still the superior catalytic system and PhCOOH was used as the effective additive.[48] Recently, a similar oxidative C N coupling protocol was also been realized with methyl ketones and benzamidines hydrochloride as the substrates by utilizing iodine catalysis [Eq. (33)].[49] Notably, different kinds of a-ketoimides could be

Scheme 20. Proposed mechanism of iodine-catalyzed oxidative coupling between acetophenones and amines. Chem. Asian J. 2015, 00, 0 – 0

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A novel cascade C C/C N coupling strategy for the total synthesis of b-carboline and isoquinolinecontaining natural products and derivatives was revealed by Wu and co-workers in 2013.[51] By utilizing I2 as the catalyst and H2O2 as the oxidant, different substituted acetophenones and tryptamines could be transformed into the corresponding b-carbolines in good to excellent yields [Eq. (34)]. Notably, this strategy has also been successfully applied in the total synthesis of other natural products, such as fascaplysin and papaverin. In 2012, Zhu and co-workers reported TBAI-catalyzed oxidative cascade C N/C O bond-formation reactions for the synthesis of oxazole derivatives.[52] By employing TBHP as the oxidant, different 1,3-dicarbonyl compounds and benzylamines could cross 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Focus Review Another oxidative C O coupling reaction of carbonyl compounds with oxygen nucleophiles was communicated by Li and co-workers in 2012.[54] A TBAI-catalyzed oxidative coupling of b-ketoesters with carboxylic acids for the synthesis of a-carboxylic-b-ketoesters was reported with TBHP as the oxidant [Eq. (37)]. Various b-ketoesters and carboxylic acids could be tolerated. A plausible mechanism revealed that in situ generated [nBu4N][IO2] activated the b-ketoester to form a C I bond, followed by subsequent nucleophilic attack of a carboxylic acid to deliver the desired product.

Recently, a synergistic pyrrolidine- and iodine-catalyzed domino aldol–Michael–dehydrogenative synthesis of flavones was revealed by Tilve and co-workers.[55] 2’-Hydroxyacetophenones and aryl aldehydes were utilized as the coupling partners. A series of substituted flavone derivatives could be synthesized in good to excellent yields upon promotion by pyrrolidine and iodine synergistic catalysis [Eq. (38)]. A proposed mechanism showed that the generation of an a-iodo ketone was probably involved as the key reaction intermediate.

Scheme 21. TBAI-catalyzed oxidative C N bond formation for the construction of a-amino acid esters.

couple efficiently to deliver the desired oxazoles in good yields [Eq. (35)]. Mechanistic elucidation showed that the in situ generated hypervalent iodine species [nBu4N][IO2] played an important role in the activation of the 1,3-dicarbonyl compound through C I bond formation.

Phosphates have also been used to perform oxidative C O couplings with carbonyl compounds under iodine catalysis. In 2012, Yan and co-workers reported an effective catalytic aphosphoryloxylation of ketones with iodobenzene as the catalyst and m-CPBA as an efficient oxidant [Eq. (39)].[56] Different ketones and phosphates could be tolerated. The reaction was believed to proceed through electrophilic addition of a hypervalent iodine reagent to an enolized ketone, and subsequent reductive elimination afforded the desired keto phosphates.

Later, Yu and co-workers reported a TBAI-catalyzed oxidative coupling of aminopyridines with b-ketoesters for the synthesis of imidazo[1,2-a]pyridines [Eq. (36)],[53] which resulted from a similar activation pattern of b-ketoesters by hypervalent iodine species to give the iodinated intermediate.

C I bond formation not only exists in carbonyl a-C H activation, but is also involved in alkene and alkyne reactions. Generally, the role of the iodine catalyst is to activate these &

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Focus Review types of double or triple bonds through coordination or electrophilic addition. Normally, iodonium ions were generated as the key reaction intermediates. In 2012, Zhdankin and co-workers communicated a TBAI-catalyzed aziridination of alkenes by utilizing m-CPBA as an effective oxidant and N-aminophthalimide (PhthNH2) as the nitrenium source.[57] Notably, various kinds of alkenes, including styrene derivatives, cyclic and acyclic internal alkenes, and even aliphatic alkenes, could be tolerated to generate the corresponding aziridination products in moderate to good yields [Eq. (40)]. Mechanistic elucidation showed that in situ generated HOI from TBAI could react with an alkene to give the iodonium ion, which was then opened by PhthNH2 to give the biodo-N-aminophthalimide, followed by further cyclization to afford the final product.

lecular ring closure, further cycloaddition with alkene and elimination of HI could afford the final product. The third type of C I bond formation in oxidative couplings is Cbenzylic I bond formation. In 2013, Fu and co-workers demonstrated a metal-free oxidative esterification of alcohols with toluene derivatives.[60] By employing TBAI/TBHP as the effective catalytic system, different kinds of benzyl alcohols and substituted tolyl arenes could be transformed into the corresponding ester products in good yields [Eq. (42)]. For the proposed mechanism, benzyl alcohol was believed to be oxidized to the benzoic acid anion by TBHP. Meanwhile, benzyl iodide was generated from toluene in the presence of iodine catalyst. Subsequent SN2 substitution between benzoic acid anion and benzyl iodide could produce the desired product.

Loh’s group reported an iodine-catalyzed oxidative a-amination of aldehydes in 2012 and 2013.[58] In the presence of sodium percarbonate (Na2CO3·1.5 H2O2) as an efficient oxidant and MeOH as a suitable nucleophile, different substituted amines and aldehydes, including arylaldehydes and aliphatic aldehydes, could be cross-coupled smoothly to generate the desired a-aminated acetals in good yields (Scheme 22). A mechanistic study revealed that the enamine intermediate was first formed from an amine and aldehyde during the reaction process. Electrophilic addition of hypoiodite to the enamine intermediate was probably involved. Afterwards, Nachtsheim and co-workers revealed a TBAI-catalyzed halocyclization/cycloaddition/elimination cascade reaction of o-alkynylphenyl carboxaldehydes in 2013.[59] A series of a-acyl-b-aryl-substituted naphthalenes could be synthesized in good to excellent yields by using Oxone as the optimized oxidant [Eq. (41)]. This reaction was believed to be initiated from activation of the C C triple bond by means of an electrophilic iodine catalyst to form an iodonium ion. Subsequent intermo-

Recently, Lei’s group achieved an oxidative sulfonylation reaction by utilizing alkenes as the nucleophiles and sulfonohydrazides as the sulfonyl sources with the KI/TBHP catalytic system.[61] Various a,b-unsaturated sulfones could be generated in good to excellent yields (Scheme 23). A plausible mechanism revealed that the sulfonyl radical was first generated from sulfonyl hydrazide through oxidation by TBHP. Then the addition of the sulfonyl radical to an olefin afforded a b-sulfonyl carbon-centered radical, which was further captured by I2 to afford an iodosulfonylation intermediate. Final elimination of HI gave the desired product (Scheme 24). 4.2. N I Bond-Formation Reactions N I bond-formation reactions are usually involved in iodinecatalyzed oxidative C N bond-formation reactions. Reactions usually go through the following processes: first, the nitrogen substrate reacts with the iodine catalyst to form the N I bond; the N I bond-formation intermediate can further undergo different transformations, including N I cleavage to generate the nitrogen-centered cation, insertion of the N I bond into multiple bonds or elimination of HI. In this subsection, detailed descriptions of reaction of this type are given. In 2012, Antonchick and co-workers reported an iodoarenecatalyzed intermolecular C H amination and hydrazination re-

Scheme 22. Iodine-catalyzed oxidative a-amination of aldehydes. Chem. Asian J. 2015, 00, 0 – 0

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Scheme 23. KI-catalyzed oxidative alkenylation of sulfonohydrazides.

Scheme 25. Iodoarene-catalyzed intermolecular C H amination and hydrazination reaction.

Scheme 24. Proposed mechanism for the KI-catalyzed oxidative alkenylation of sulfonohydrazides.

communicated a novel iodine-catalyzed oxidative C H amination of benzoxazoles.[64] TBAI was the optimized catalyst, and H2O2 and TBHP proved to be efficient oxidants. Various substituted 2-aminobenzoxazoles could be produced in good yields [Eq. (44)]. Mechanistic analysis revealed that TBAI could react with H2O2 in the presence of AcOH to generate AcOI, which further reacted with an amine to produce the iodinated amine. Subsequent N I bond insertion into the C=N bond of benzoxazole generated another N I-bonded intermediate. Final elimination of HI in the presence of base delivered the desired product. Afterwards, the same group expanded this oxidative C N bond-formation protocol to the use of simple aliphatic amines, even ammonia, in 2012.[65]

action by employing AcOOH as the optimized oxidant.[62] By using simple arenes as one coupling partner, different kinds of aryl-substituted amides and hydrazines could be synthesized in good yields (Scheme 25). The proposed mechanism showed that the in situ generated hypervalent iodine catalyst could react with an amide to form the N I bond, which further cleaved to the generate nitrogen-centered cation, followed by electrophilic addition to give the arene, and final elimination to afford the desired carbazole product. Recently, Antonchick and Manna reported an organocatalytic oxidative annulation of benzamide derivatives with alkynes.[63] PhI was used as the effective catalyst and AcOOH as the optimal oxidant. A series of different substituted isoquinolones could be synthesized in good yields at room temperature within 1 h [Eq. (43)]. The reaction was believed to be initiated by N I bond formation between the substrate and in situ generated PhI(OAc)2. Subsequent disproportionation of the N Ibonded intermediate gave a nitrogen cation and regenerated PhI. The nitrogen cation was then captured by the alkyne and a further Friedel–Crafts process resulted in the desired isoquinolone. In addition to the formation of a nitrogen-centered cation, insertion into multiple bonds or elimination of HI is also common process for the further transformation of the N I bonded intermediate. In 2011, Nachtsheim and co-workers &

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Focus Review Elimination of HI is the third type of transformation of N Ibonded intermediates. In 2011, Bai and co-workers reported a facile synthesis of 2-imidazolines from aldehydes and diamines with NaI as the catalyst and H2O2 as the optimal oxidant [Eq. (45)].[66] MgSO4 was employed as an efficient additive that acted as the desiccant to remove water generated during the reaction process. In the proposed mechanism, intermediate 12 was first formed by cyclization of Schiff base intermediate 11. Intermediate 12 then reacted with in situ generated I2 from the oxidation of NaI by H2O2 to form the key N I-bonded intermediate 13, which further underwent elimination of HI to give the desired imidazoline. HI was then neutralized by NaOH generated from reaction of NaI with H2O2 to regenerate the NaI catalyst and water. Finally, water was removed by MgSO4, so that the whole catalytic process could operate effectively (Scheme 26).

In 2013, Singh and co-workers reported an iodine-catalyzed three-component reaction for the synthesis of highly functionalized 1,3,5-trisubstituted pyrazoles in aqueous medium.[68] Aldehydes, arylhydrazines, and alkynes were employed as efficient coupling partners. Different trisubstituted pyrazoles could be obtained in good yields [Eq. (47)]. Mechanistic determination revealed that an N I bonded intermediate was possibly involved. Almost at the same time, this three-component reaction protocol for the synthesis of pyrazoles was realized with aldehydes, arylhydrazines, and malanonitrile by the same group [Eq. (48)].[69]

Later, Gao and Wei communicated an efficient oxidative cyclization of acid hydrazides to form 2,5-disubstituted 1,3,4-oxadiazoles with TBAI as the catalyst and TBHP as the oxidant [Eq. (46); DABCO = 1,4-diazabicyclo[2.2.2]octane],[67] in which N I bond formation and subsequent elimination of HI processes were probably involved. N-Acylhydrazone 14 was probably the reaction intermediate because it could be transformed into the desired oxadiazoles in good yields.

Yoshimura and co-workers reported a novel synthesis of isoxazolines from aldoximes and alkenes by using KI as the catalyst and Oxone as the terminal oxidant.[70] Different substituted isoxazolines could be efficiently synthesized in good yields at room temperature [Eq. (49)]. Hypoiodite was believed to be generated in situ from KI and Oxone, which reacted with an aldoxime to form an iodonium ion as a key intermediate. Further reaction with an alkene gave the final product. Soon after, this reaction was also achieved with the iodoarene/Oxone catalytic system,[71] in which a nitrile oxide was proposed to be generated as a key intermediate to undergo [3+2] cycloaddition with an alkene to deliver the final product. Wang and Ji reported an interesting reaction of iodine-catalyzed chemoselective amination of indoles in 2013, in which two equivalents of anilines reacted with indoles to afford N-C3 linked pyrrolidinoindolines.[72] TBHP was utilized as an effective oxidant to enhance the formation of the desired products in good yields [Eq. (50)]. A detailed mechanism is still not yet clear, although the formation of an N I bonded intermediate is probably involved.

4.3. XHeteroatom I Bond-Formation Reactions

Scheme 26. Plausible mechanism for the NaI-catalyzed synthesis of 2-imidazolines from aldehydes and diamines. Chem. Asian J. 2015, 00, 0 – 0

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In iodine-catalyzed oxidative coupling reactions involving I X bond formation, there are other heteroatom-iodine bond-formation processes apart from C I and N I bond-formation reactions, such as I O, I halogen, I S, and I P bond formation.  2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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In 2013, Kitamura and co-workers revealed an iodoarene-catalyzed fluorination of 1,3-dicarbonyl compounds.[73] m-CPBA proved to be an optimal oxidant and aqueous HF acted as an efficient fluorine source. Different substituted 2-fluoro-1,3-dicarbonyl compounds could be obtained in good yields [Eq. (51)]. Mechanistic analysis showed that iodoarene was first converted into (difluoroiodo)arene by reaction with HF, which further reacted with enols of the 1,3-dicarbonyl compounds to give the desired fluorination product, along with regeneration of the iodoarene catalyst.

Scheme 27. Iodine-catalyzed regioselective sulfenylation of imidazoheterocycles in PEG400.

An iodine-catalyzed synthesis of substituted pyrroles from aamino carbonyl compounds and aldehydes was reported by Yan and co-workers in 2014.[76] Various 1,3,4-triarylpyrroles could be generated in good yields in the presence of an equivalent amount of ZnCl2 and molecular sieves [M.S.; Eq. (53)]. The detailed reaction mechanism is still not clear. However, iodine-promoted elimination of water would possibly account for generation of the final product.

In 2007, Kirihara and co-workers reported a mild iodine-catalyzed oxidative homocoupling of thiols to give disulfides.[74] By employing NaI/H2O2 as the catalytic system, different disulfides could be synthesized in excellent yields at ambient temperature [Eq. (52)]. Mechanistic elucidation showed that NaI was first oxidized to give I2 by H2O2, which further reacted with a thiol to afford the iodosulfonium intermediate. Subsequent coupling with another thiol delivered the final product and regenerated iodide ion.

Prabhu and Dhineshkumar communicated a molecular iodine catalyzed hetero-dehydrogenative cross-coupling reaction of phosphites with different nucleophiles in 2013.[77] Different nucleophiles, such as amines, alcohols, and sulfoximines, were employed in this reaction to achieve direct phosphorylation in the presence of molecular iodine as the catalyst and H2O2 as the sole oxidant. Various N P and O P bond-formation products could be obtained in good yields at room temperature [Eq. (54)]. The tentative mechanism postulated that an I(I) P bond-formation intermediate was probably involved in the reaction process.

Recently, Hiebel and Berteina-Raboin revealed an iodine-catalyzed regioselective sulfenylation of imidazoheterocycles in polyethylene glycol 400 (PEG400).[75] By utilizing I2 as the catalyst and H2O2 as an effective oxidant, different 3-arylthioimidazoheterocycles were obtained in moderate to good yields (Scheme 27). Mechanistic studies revealed that the S I bondformation species PhS I served as a key reaction intermediate, which reacted with the heterocyclic compound through electrophilic attack to afford an imidazolenium intermediate. Finally, elimination of HI formed the desired product. &

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5. Summary and Outlook We briefly summarized recent development in iodine-catalyzed oxidative coupling reactions that utilized C H and X H as nucleophiles. The categories were based on different roles of iodine catalysts proposed previously. Normally, KI, TBAI, I2, and 16

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Focus Review different iodoarenes were the most utilized catalysts in these transformations, and TBHP, m-CPBA, H2O2, and AcOOH were the most commonly used oxidants. To date, various kinds of C H and X H (X = C, N, O, S, P, etc.) nucleophiles have been employed in oxidative cross-coupling reactions under iodine catalysis to construct different useful molecules, including biologically active compounds and pharmaceutical derivatives. Notably, the reaction conditions are generally mild and substrate compatibility is also good. Moreover, iodine catalysis avoids the residual heavy metals in the final products; thus making it a powerful tool for pharmaceutical synthesis. Although great progress has been made in this field, challenges still remain. A promising direction in this field is the development of iodine-catalyzed, highly enantioselective oxidative coupling reactions. Notably, some progress has been made in iodine-catalyzed enantioselective intramolecular oxidative coupling reactions.[5a–c, e, g–j] We believe that enantioselective intermolecular reactions could also be a promising development. Moreover, developing additional synthetic methods for useful compounds under iodine catalysis is still in urgent demand. In addition, the mechanistic studies of these reactions are far from adequate. The role of iodine catalysts remains largely unclear, except for rare examples with direct evidence of the reaction mechanism. Therefore, mechanistic studies of these types of reactions will be a very hot topic in the future.

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[5]

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Acknowledgements

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This work was supported by the 973 Program (2011CB808600, 2012CB725302), the National Natural Science Foundation of China (21390400, 21025206, 21272180, and 21302148), the Research Fund for the Doctoral Program of Higher Education of China (20120141130002), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1030). The Program of Introducing Talents of Discipline to Universities of China (111 Program) is also appreciated.

[13] [14] [15] [16] [17] [18]

Keywords: cross-coupling · iodine · nucleophiles · oxidation · reaction mechanisms

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Received: November 1, 2014 Revised: December 17, 2014 Published online on && &&, 0000

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Focus Review

FOCUS REVIEW Reaction Mechanisms

Roles of iodine: Recent developments in iodine-catalyzed oxidative coupling reactions utilizing C H and X H as nucleophiles are summarized based on the different roles of iodine catalysts (see figure). Various reaction mechanisms are also discussed, in which different iodine species act as key intermediates of different reactions. Selected examples of substrate applicability are also listed and described.

Chem. Asian J. 2015, 00, 0 – 0

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Dong Liu, Aiwen Lei* && – && Iodine-Catalyzed Oxidative Coupling Reactions Utilizing C H and X H as Nucleophiles

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Iodine-catalyzed oxidative coupling reactions utilizing C - H and X - H as nucleophiles.

In recent decades, iodine-catalyzed oxidative coupling reactions utilizing C - H and X - H as nucleophiles have received considerable attention becaus...
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