Recent Advances in Transition-Metal-Catalyzed Functionalization of

Review pubs.acs.org/CR

Recent Advances in Transition-Metal-Catalyzed Functionalization of Unstrained Carbon−Carbon Bonds Feng Chen,† Teng Wang,† and Ning Jiao*,†,‡ †

State Key Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical Sciences, Peking University, Xue Yuan Road 38, Beijing 100191, China ‡ State Key Laboratory of Organometallic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China 3.8. Cerium-Catalyzed Cleavage of CC Double Bonds 4. Carbon−Carbon Triple Bond Cleavage 4.1. Rhodium-Catalyzed Cleavage of CC Triple Bonds 4.2. Ruthenium-Catalyzed Cleavage of CC Triple Bonds 4.3. Palladium-Catalyzed Cleavage of CC Triple Bonds 4.4. Gold-Catalyzed Cleavage of CC Triple Bonds 4.5. Copper-Catalyzed Cleavage of CC Triple Bonds 4.6. Silver-Catalyzed Cleavage of CC Triple Bonds 5. Summary and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations References

CONTENTS 1. Introduction 2. Carbon−Carbon Single Bond Cleavage 2.1. Cleavage of C−Csp Bonds 2.1.1. Cleavage of CCN Bonds 2.1.2. Cleavage of CCC Bonds 2.2. Cleavage of C−Csp2 Bonds 2.2.1. Cleavage of C−C(O)O Bonds 2.2.2. Cleavage of C−C(O)H Bonds 2.2.3. Cleavage of C−C(O)C Bonds 2.2.4. Cleavage of C−C(O)Cl Bonds 2.2.5. Cleavage of C−C(O)N Bonds 2.2.6. Cleavage of C−C(O)S Bonds 2.2.7. Cleavage of C−Caryl Bonds 2.3. Cleavage of C−Csp3 Bonds 2.3.1. Cleavage of C−COH Bonds 2.3.2. Cleavage of C−COC Bonds 2.3.3. Cleavage of C−COO Bonds 2.3.4. Cleavage of C−CNC Bonds 2.3.5. Cleavage of C−CalkylC Bonds 2.3.6. Cleavage of C−C Bonds of Five-Membered Ring Hydrocarbons 3. Carbon−Carbon Double Bond Cleavage 3.1. Manganese-Catalyzed Cleavage of CC Double Bonds 3.2. Ruthenium-Catalyzed Cleavage of CC Double Bonds 3.3. Palladium-Catalyzed Cleavage of CC Double Bonds 3.4. Iron-Catalyzed Cleavage of CC Double Bonds 3.5. Gold-Catalyzed Cleavage of CC Double Bonds 3.6. Osmium-Catalyzed Cleavage of CC Double Bonds 3.7. Iridium-Catalyzed Cleavage of CC Double Bonds © 2014 American Chemical Society

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1. INTRODUCTION The carbon−carbon bond is the most widespread and fundamental bond existing in organic compounds. The reaction of this kind of bond is almost ubiquitous in live activities and industrial production. For instance, C−C bond cleavages are very common in the metabolism of carbohydrates and utilization of hydrocarbons in the oil industry. In addition, one of the worldwide preoccupations today is to find efficient ways to reduce the pressure of the energy crisis and environmental pollution. The processes of these bond cleavages are quite involved, either in the modification and utilization of the products of the oil industry or in the degradation of plastic articles that may cause “white pollution”.1 This means that the development of new methodologies in the chemistry of C−C bond cleavage is in urgent need and full of opportunities. In contrast to the highly developed C−C bond formations, development of selective C−C bond cleavage by a catalytic reaction system is typically considered to be an inert area.2 Owing to the thermodynamic stability of the C−C bond, two reasonable approaches have been taken into consideration: (1)

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Received: November 1, 2013 Published: July 25, 2014

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the literature. Among all these metals, only Ni, Fe, Rh, Pd, Cu, and Ir have been successfully developed for catalytic systems. 2.1.1.1. Nickel-Catalyzed Cleavage of CCN Bonds. Most of the catalytic C−CN bond cleavage reactions proceed via an oxidative addition mechanism. Because of the thermodynamic stability of the Csp3−CN bond and the susceptibility of the intermediate to β-hydride elimination, the activation of Csp3−CN bonds via the oxidative addition pathway is still difficult and uncommon. Ni0-catalyzed adiponitrile synthesis from 1,3-butadiene by hydrocyanation is shown in Scheme 2. A mixture of branched isomer 2 and linear

the use of reactive starting materials and (2) the generation of highly active intermediates.2,3 Hence, strained molecules (three- or four-membered ring compounds),4 chelation assistance,3 and some other means3 have been employed as the main strategies to induce C−C bond cleavage. Although significant progress has been made using these two strategies over the past few decades, some pending issues, such as reactivity, selectivity, and efficiencies in the transformations, still need to be resolved urgently, especially for unstrained carbon− carbon bonds. On the other hand, research topics mainly focus on stoichiometric reactions such as transition-metal insertion into carbon−carbon bonds via stable metallacycle formation. Currently chemists have developed a variety of catalytic systems for C−C bond functionalization. Among them, transition-metal catalysts such as ruthenium, rhodium, palladium, iron, copper, iridium, tungsten, rhenium, osmium, and gold have shown high efficiency in accomplishing many fascinating transformations via single, double, and triple C−C bond cleavage. So far, the activation of carbon−carbon bonds and their applications are still hot research areas in organic synthesis. As strained molecule systems,4 multiple bond metathesis5 and decarboxylation6 have been well reviewed. Herein, we will focus on the recent advances in transition-metal-catalyzed functionalization of unstrained carbon−carbon bonds to reveal the great achievements and potentials in this field. In this review, these reactions are classified by the class of carbon−carbon bonds and functional groups adjacent to these inert bonds.

Scheme 2

isomer 3 is produced by addition of HCN to 1,3-butadiene. The branched nitrile 2 could be converted into 3 by the C−CN bond cleavage reaction. A π-allylnickel cyanide complex is proposed as a key intermediate derived from Ni0-catalyzed C− CN bond oxidative addition.24 Aryl cyanides can serve as aryl halide equivalents in crosscouplings, although the C−CN bond dissociation energy is higher than that of the C−X bond of aryl halides (DAr−X increases in the order I < Br < Cl < CN < F).25 Miller reported NiCl2(PMe3)2-catalyzed cross-coupling reaction of benzonitriles with aryl Grignard reagents via the C− CN bond cleavage of benzonitriles.26 It is noted that NiCl2(PMe3)2 is air stable and commercially available. This methodology provides the corresponding unsymmetrical biaryl synthesis with high yield and selectivity (eq 1). Dankwardt and

2. CARBON−CARBON SINGLE BOND CLEAVAGE 2.1. Cleavage of C−Csp Bonds

2.1.1. Cleavage of CCN Bonds. C−CN bonds are very common and abundant in many organic molecules. Development of transition-metal-catalyzed C−CN bond cleavage is crucial for organic synthesis.7 The Rh-catalyzed decarbonylation of aroyl cyanides was reported by Bergman’s group.8a Green and co-workers reported the Pt-catalyzed alkenyl C−CN bond cleavage.8b Burmeister and co-workers reported C−CN bond cleavage via the oxidative addition process of tetrakis(triphenylphosphine)platinum(0) with 1,1,1tricyanoethane.8c After these results, a number of metals such as platinum,8b,c,9 palladium,10 nickel,11 copper,12 molybdenum,13 rhodium,8a,14 cobalt,15 iron,16 iridium,17 zinc,18 uranium,19 and silver20 have been used to activate C−CN bonds. Among these catalysts, two major mechanisms for C−CN bond activation are widely accepted: (a) oxidative addition of transition-metal catalysts such as Pt, Ni, Pd, Co, and Rh (Scheme 1a) and (b) deinsertion of silyl isocyanide for Fe and some Rh complexes (Scheme 1b). In addition, initial C−H activation,21 β-carbon elimination,22 and a photoredox process23 are also proposed in

co-workers developed the NiCl2(PMe3)2-catalyzed crosscoupling reactions of modified alkyl and alkenyl Grignard reagents with aryl and heteroaryl nitriles.27To avoid the direct addition of nucleophiles to the nitrile group, some additives such as LiO-t-Bu or PhSLi were added to the reaction prior to cross-coupling reaction by adjusting the reactivity of the Grignard reagents. The Ni-catalyzed coupling reaction of aryl cyanides could also be applied to alkynylation using alkynylzinc reagents as nucleophiles (Scheme 3a).28 Shi and co-workers demonstrated NiCl2(PCy3)2-catalyzed Suzuki−Miyaura coupling reaction of aryl nitriles with aryl/alkenyl boronic esters. This reaction reveals the potential ability of arylboroxine to reduce NiII to Ni0, which can trigger the oxidative addition of C−CN for further cross-coupling reactions (Scheme 3b).29 Wang and coworkers developed a simple and highly efficient process for the cross-coupling of (hetero)arenecarbonitriles with (hetero)arylmanganese reagents using NiCl2(PMe3)2 as the catalyst without any additives. This reaction is compatible with various functional groups and aromatic heterocycles under mild reaction conditions (Scheme 3c).10 In addition to C−C bond forming reactions, aryl cyanides can also act as suitable

Scheme 1

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Scheme 3

Scheme 5

bond to Ni0 giving a π-allylnickel intermediate is proposed as the key step for this C−C bond cleavage (Scheme 6).33

substrates for catalytic amination in the presence of Ni(CN)2· 4H2O and CsF (Scheme 3d).30 Transition-metal-catalyzed carbocyanation reactions involving C−CN single bond cleavages and direct addition to unsaturated bonds are highly atom economical in organic synthesis. Nakao, Hiyama, and co-workers demonstrated the pioneering example of arylcyanation of alkynes giving β-arylsubstituted alkenenitriles (Scheme 4).31 Most substituents on

Table 1

Scheme 4

entry

R1

R2

E:Z

yield (%)

1 2 3 4 5

H Me H t-Bu Ph

H H Me H H

83:17 85:15 >99:1 >99:1

78 55 69 49 86

Scheme 6

Recently, this group reported that the scope of substrates in this transformation can be expanded dramatically by utilizing a Lewis acid as a cocatalyst.34 Challenging substrates such as acetonitrile, (trimethylsilyl)acetonitrile, and propionitrile could be applied, affording the corresponding carbocyanation products (Table 2). The Lewis acid AlMe3 or AlMe2Cl is crucial for the activation of the cyano group through the acceleration of the reaction rate. Various aryl cyanides and alkenyl cyanides were also compatible in this reaction system (Schemes 7 and 8). Strained alkenes such as norborene and norbornadiene can also be employed in this cyanation transformation. The reactions proceed with high yield and high exo selectivity. An oxidative addition process is also proposed for this transformation (Scheme 9).35 The nickel-catalyzed asymmetric intramolecular arylcyanation of unactivated olefins via C−CN bond activation was achieved simultaneously by the groups of Jacobsen36 and Nakao.37 In Jacobsen’s strategy, Zn0 is crucial because it may prevent the isomerization of the substrate 10. (S,S,R,R)TangPhos (11) is the appropriate ligand, and corresponding

the β-aryl group are compatible to give the corresponding alkenenitriles in good yield. The less hindered and electron-rich PMe3 is the critical ligand in this transformation. It is interesting that pyridine derivatives are obtained using an Nheterocyclic carbene ligand.32 The mechanism involves oxidative addition of a C−CN bond to form NiII complex 7, which then inserts into an alkyne, leading to alkenylnickel 8. Then alkenylnickel 8 undergoes reductive elimination to form an arylcyanation product and regenerate the Ni0 catalyst (Scheme 5). Nakao and Hiyama first demonstrated that Ni(cod)2 can act as an efficient catalyst for allylcyanation of alkynes. Alkenenitriles with different functional groups could be obtained directly with regio- and stereoselectivity. The products are mainly (E)isomers; the E:Z ratio is up to 99:1. Both internal and terminal alkynes could undergo this carbocyanation reaction even on a gram scale. α-Siloxyallyl cyanide, which is readily available from acrolein and Me3SiCN, is a suitable substrate for this transformation (Table 1). The oxidative addition of a C−CN 8615


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Table 2

Scheme 10

Scheme 11

Scheme 7

They applied this method in the total synthesis of natural products (−)-esermethole (16) and analgesic (−)-eptazocine (19) in high ee (Scheme 12). Ferrocenylphosphane (R,R)-i-PrScheme 12 Scheme 8

a

Z:E = 84:16 for the substrate.

Scheme 9 Forxap (15) is the best chiral ligand for the synthesis of (S)-14. (R,R)-Chiraphos is the most appropriate ligand for the synthesis of (R)-18 (Scheme 13).37 Recently, Ni(acac)2-catalyzed reductive decyanation of unactivated aryl and alkyl C−CN bonds with tetramethyldisiloxane (TMDSO) as the hydride source was developed by Maiti and co-workers. Especially, aliphatic nitriles (Calkyl−CN) which could undergo β-hydride elimination are compatible in this nickel-catalyzed system (Scheme 14).38 In addition, greener and milder hydrogen gas as the hydride source has also been developed successfully in this group.39

products 12 could be obtained in good yields with high enantioselectivities (Scheme 10). The proposed mechanism is shown in Scheme 11. The coordination of BPh3 to the nitrile moiety assists the oxidative addition process.36 Nakao and co-workers reported the Ni(cod)2- and AlMe2Clcocatalyzed intramolecular arylcyanation reaction of alkenes. 8616


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cyanoesterification and cyanocarbamoylation of alkynes catalyzed by the Ni(cod)2/B(C6F5)3 or Ni(cod)2/BPh3 system with high stereo- and regioselectivity.42 2.1.1.2. Iron-Catalyzed Cleavage of CCN Bonds. Ironcatalyzed C−CN bond activation reactions via deinsertion of silyl isocyanide are also well developed. Nakazawa et al. reported the CpFe(CO)Me-catalyzed photoreaction of acetonitrile with hydrosilane giving CH4 and silyl cyanide (Scheme 16).43 An active catalytic species of a silyliron complex

Scheme 13

Scheme 16

Scheme 14

(FeSiEt3) could insert into a cyano group to generate intermediate 30, followed by the deinsertion of silyl isocyanide to afford intermediate 31. 31 could decompose into complex 32 and silyl isocyanide 33. Methane could be formed via reaction of complex 32 with hydrosilane, and the active species could be regenerated simultaneously. 2.1.1.3. Rhodium-Catalyzed Cleavage of CCN Bonds. Chatani and co-workers reported the Rh-catalyzed C−CN bond cleavage reaction through deinserion of silyl isocyanide. By employing a silylrhodium complex, Bergman and Brookhart first reported this silicon-assisted mechanistic process.44 Chatani and co-workers successfully employed [RhCl(cod)]2 as an efficient catalyst to execute the reaction of benzyl, alkyl, and allyl cyanides with disilanes giving benzyl-, alkyl-, and allylsilanes and silyl cyanide via the cleavage of the C−CN and Si−Si bonds (Table 3). The nickel-based catalysts were inactive

The cleavage of C−C bonds connected with a carbonyl carbon is easier.3 In the same way, the oxidative addition of C− CN bonds in acyl cyanides or cyanoformates could occur easily. Some Rh8a and Pd40 catalysts have been successfully applied to the decarbonylation of acyl cyanides with terminal alkynes. Although acylcyanation almost proceeds using a Pd0 catalyst, Ni0 catalysts also have been developed for the CC double bond insertion of an allene moiety. Nakao and co-workers reported a cyanoesterification reaction of 1,2-dienes using the Ni(cod)2/PMe2Ph catalyst system. Various ethyl 2-(1-cyanoalk1-yl)acrylates could be produced from allenes with broad functional group tolerance. Compounds 22 are kinetic products whereas compounds 23 are thermodynamic products in this transformation. The oxidative addition of EtOC(O)CN to Ni0 occurs first for the kinetic process. Then σ-allylnickel intermediate 26 could be formed via the coordination of the terminal double bond of allene to the Ni center and the migration of EtOC(O) to the cumulative carbon. Finally, it undergoes reductive elimination to produce product 22 and regenerate the active Ni0 catalyst (Scheme 15).41 Recently, this group demonstrated that β-cyano-substituted acrylates and acrylamides could be obtained from the

Table 3

Scheme 15

isolated yield (%) entry

R

35

36

1 2 3 4

H 4-CF3 4-CO2Me 2-Me

52 59 35 63

18 18 12 17

for this reaction as they are usually applied to undergo facile oxidative addition of C−CN bonds. On the basis of related control experiments, the deinsertion of silyl isocyanide of η2iminoacyl complex 38 is proposed for this catalytic system (Scheme 17). The formation of enamine byproducts for benzyl cyanides also supports the key intermediate 39.45 Tobisu, Chatani, and co-workers established a silicon-assisted and catalytic reductive decyanation system for the removal of a cyano group using [RhCl(cod)]2 as the catalyst and hydrosilane 8617


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the synthesis of deuterium-labeled arenes with high regioselectivity.

Scheme 17

Chatani and co-workers recently reported the reaction of [RhCl(cod)]2-catalyzed borylation of aryl/vinyl/benzyl cyanide with a diboron reagent. Aryl cyanides easily underwent the reaction; even vinyl cyanide can give the borate in moderate to good yield. Unlike the previously reported Rh catalysts for functionalizations of a C−CN bond, a Rh−B complex is the reactive catalyst in this reaction (Scheme 19). In studying the Scheme 19

as a mild reductive agent (Scheme 18, Table 4). The substrate scope of this reaction is wide, involving aryl and alkyl cyanides. Scheme 18

a

reaction mechanism, they proposed an unconventional βcarbon elimination mechanism for this transformation. Initially, the [RhCl(cod)]2 reacts with a diboron reagent to generate a reactive Rh−B complex. Subsequently, 1,2-insertion of the Rh− B bond into the C−N bond gives an iminylrhodium intermediate. E/Z isomerization occurs to allow the R group to locate cis to the Rh center. β-Carbon elimination occurs to generate a Rh−R complex. Finally, the Rh−R complex reacts with the diboron reagent to yield the borylated product and regenerate the active Rh−B catalyst (Scheme 20).47 The mechanistic study of Rh−B- and Ir−B-complexcatalyzed C−CN bond activation by density functional theory

Run at 130 °C. bWithout P(O-i-Pr)3. cGC yield.

Table 4

entry 1 2 3 4

Ar 2-naphthyl (4-MeO)C6H4 (4-Me2N)C6H4 (4-EtO2C)C6H4

yield (%) a

99 74a 76a 84

entry

Ar

yield (%)

5 6 7 8

(2-PhO)C6H4 (2-Ph)C6H4 9-anthracenyl 1-Ts-3-indolyl

95 89 94b,c 81d

Scheme 20

a GC yield. bRun at 160 °C using [RhCI(cod)]2 (10 mol %) in the presence of P(O-i-Pr)3 (20 mol %). cToluene was used as the solvent. d Run at 100 °C without P(OBu)3.

Most aryl cyanides give corresponding arenes in high yield. βHydrogen-containing primary, secondary, and some tertiary alkyl cyanides are compatible. For example, tridecanenitrile is obtained in 78% yield, and the yield of 3-phenylpropanenitrile reaches up to 99%. These alkyl cyanides normally process βhydride elimination via the oxidative addition strategy.46 A cyano group could be replaced with a deuterium atom by using deuteriosilane (eq 2). Hence, this method should be applied to 8618


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calculations is reported by Fu and co-workers. The results indicated that the deinsertion mechanism is favored over the oxidative addition, initial C−H bond activation, and β-carbon elimination mechanisms in the examined reactions.48 Chatani and co-workers reported the RhI-catalyzed silylation reaction of aryl and alkenyl cyanides using disilane involving the cleavage of unreactive C−CN and Si−Si bonds. Notably, ferrocenyl cyanide gives the silane in 73% yield. This is the first example of catalytic C−CN bond activation for alkenyl cyanides via this RhI-catalyzed silylation reaction system (Scheme 21).45,49

Table 5

Scheme 21 in 80% yield, and 3,3-diphenylacrylonitrile gives the product in 41% yield. The mechanism of this reaction is shown in Scheme 23. Arylrhodium is afforded via deinsertion of silyl isocyanide. Scheme 23

a

Run for 60 h. b[Rh(cod)2]BF4 was used as the catalyst. c[Rh(OMe)(cod)]2 was used as the catalyst. dRun for 40 h. eA 10 mol % concentration of the catalyst was used. fGC yield.

Chatani and co-workers applied this silicon-assisted strategy to an intramolecular cyclization via the cleavage of C−CN bonds. Ar−Ar is obtained by the cross-coupling reaction between Ar−CN and Ar−Cl catalyzed by [RhCl(cod)]2 and P(4-CF3C6H4)3 (Scheme 22).45 Dibenzo[b,d]furan derivatives are produced in good yield by this method; even 9H-fluorene and 9H-carbazole can be obtained in moderate yield.

Then it could add to the CC double bond of vinylsilane to form an alkylrhodium complex, which could be converted to an alkenylated product with the regeneration of rhodium hydride by β-hydride elimination.50 2.1.1.4. Palladium-Catalyzed Cleavage of CCN Bonds. Nishihara and co-workers reported Pd(PPh3)4-catalyzed cyanoesterification of norbornenes with cyanoformates via NCPd-COOR (R = Me and Et) intermediates. The configuration of the 2-exo,3-exo product suggests that cyanoformate undergoes cis addition to the CC double bond via coordination of the palladium center on the less hindered face (Scheme 24).51 The R1 group could also be expanded to n-Pr, i-Pr, and n-Bu.52 Oxidative addition of a C−CN bond is the driving force for this palladium-catalyzed C−C bond cleavage (Scheme 25). Takemoto and co-workers developed Pd-catalyzed synthesis of four- to seven-membered functionalized lactams bearing a cyano group at the β-position via intramolecular cyanoamida-

Scheme 22

Scheme 24

The Mizoroki−Heck-type alkenylation of nitriles via the cleavage of C−CN bonds catalyzed by [RhCl(cod)]2 has been realized by Chatani and co-workers. Electron-rich substrates produce the alkenylation products with high selectivity. Alkenyl cyanides are also compatible with this reaction system (Table 5). For example, 4-methoxybenzonitrile gives the alkenylsilane 8619


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Scheme 25

Scheme 28

tion of alkynyl- and alkenylcyanoformamides. Various functional groups are compatible with moderate to excellent yields for this reaction (Scheme 26).53 For example, the sevenTakemoto and co-workers also developed the first intramolecular enantioselective cyanoamidation of olefins leading to various 3,3-disubstituted oxindoles 47. A combination of Pd(dba)2, a chiral phosphoramidite ligand 46, and N,Ndimethylpropyleneurea (DMPU) was employed in the optimal reaction conditions. The products could be obtained with high yields and ee values (Scheme 29).55 Douglas’s group reported a

Scheme 26

Scheme 29

similar reaction with excellent enantioselectivities which could be successfully applied in the syntheses of (−)-esermethole, (+)-horsfiline, and (−)-coerulescine (Scheme 30).56 Wang and co-workers realized Pd(OAc)2-catalyzed cyanation of aryl halides using benzyl cyanides as cyanation reagents including C−CN bond cleavage. Amino and hydroxyl groups are compatible with this reaction. A key intermediate, 48, could be generated via ligand exchange, and the benzyl halide is released from the complex by nucleophilic attack on the benzylic carbon with the halide along with C−CN bond cleavage (Scheme 31).57 2.1.1.5. Copper-Catalyzed Cleavage of CCN Bonds. Li and co-workers58a developed a Cu(OAc)2-catalyzed oxidative cyanation of aryl halides using commercially available acetonitrile as the cyanide source58 and Ag2O/air as the oxidation reagent (Scheme 32). They proposed a CuI/CuII/ CuIII mechanism for this transformation. Oxidative cleavage of the C−CN bond takes place in the presence of the oxidant Ag2O/O2 (Scheme 33).58a 2.1.1.6. Iridium-Catalyzed Cleavage of CCN Bonds. Photoredox catalysis is a relatively attractive and emerging field in organic synthesis.59 Recently, Ir(ppy)3-catalyzed benzylic amine formation via α-amine C−H arylation reaction of amines with aryl cyanides was reported by MacMillan’s group.23a The excited-state form [*Ir(ppy)3] could donate an electron to form

membered lactam substrate gives the alkenyl nitrile in 79% yield. The mechanism of this palladium-catalyzed C−CN bond cleavage is similar to the previous suggestion. Alkenylcyanoformamides could also be converted into cyclized adducts with quaternary carbon centers in high regioselectivity (Scheme 27).53,54 The oxidative addition of a C−CN bond to the catalyst Pd0, leading to PdII complex 42, occurs initially. The subsequent intramolecular carbopalladation of C(O)PdCN into the CC double bond could afford the intermediate 43, which finally produces the cyanoamidation product with the regeneration of the Pd0 catalyst (Scheme 28).53 Scheme 27

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Scheme 30

Scheme 31

Scheme 32

Scheme 33

the corresponding arene radical anion and IrIV(ppy)3, which could undergo a single-electron transfer event with amine to generate the amine radical cation. Subsequently, the amine radical cation could undergo deprotonation by NaOAc to give the α-amino radical. The radical−radical coupling reaction of the arene radical anion and α-amino radical could generate a tertiary nitrile which could undergo elimination of CN− to give the target product including a C−C bond cleavage process. In this study, both electron-deficient arenes and electron-deficient heteroaromatics were suitable in this transformation (Scheme 34).23a Very recently, MacMillan’s group realized the carbonyl βfunctionalization reaction of ketones or aldehydes via visible

light photoredox catalysis in combination with organocatalysis. The five-π-electron β-enaminyl radical could be generated from ketone or aldehyde in the presence of the catalyst *Ir(ppy)3 and organocatalyst amine. The β-enaminyl radical would 8621


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Scheme 34

Scheme 36

rapidly couple with the arene radical anion which is generated from the cyano-substituted aryl ring through a photoredox catalytic cycle. The elimination of CN− to give the target product leading to the C−C bond cleavage process is also proposed in this report (Scheme 35).23b Scheme 35

a

In DMF.

Scheme 37

2.1.2. Cleavage of CCC Bonds. 2.1.2.1. PalladiumCatalyzed Cleavage of CCC Bonds. So far, cleavage of the CCC single bond has rarely been reported. Uemura and co-workers developed a novel Pd(acac)2-catalyzed oxidative reaction of tert-propargylic alcohols with several alkenes to afford ene−yne compounds via selective Csp3C C bond cleavage under an oxygen atmosphere. Most products are obtained in moderate yields; some of them have cis/trans isomers. A β-carbon elimination mechanism is proposed for this transformation (Scheme 36).60 2.1.2.2. Rhodium-Catalyzed Cleavage of CCC Bonds. Nishimura, Hayashi, and co-workers developed a new method of introducing alkynyl groups to the β-position of α,βunsaturated ketones with high enantioselectivity. It is realized by [Rh(OH)(cod)2]-catalyzed asymmetric 1,3-migration of alkynyl groups in alkynylalkenylcarbinols (Scheme 37). The absolute configuration of the alkynylation product (E)-7-(tertbutyldimethylsilyl)-5-methyl-1-phenylhept-1-en-6-yn-3-one could be achieved with different substrates [(E)- and (Z)propenyl isomers]. The catalytic cycle presumably involves the β-alkynyl elimination from an alkoxyrhodium intermediate as the key step of the reaction (Scheme 38).61 2.1.2.3. Gold-Catalyzed Cleavage of CCC Bonds. Jiao and co-workers demonstrated a direct C−C functionalization of aryl-substituted alkynes to amides catalyzed by PPh3AuCl using TMSN3 as the nitrogen source. A Csp2−Csp bond is cleaved with high selectivity in this transformation (Scheme 39). Alkenyl azide 50, which is formed in the presence of the active Au catalyst, is the key intermediate. Further protonation of 50 forms resonance azide cations 52 and 53, which could

Scheme 38

Scheme 39

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coupling reactions have been described by several groups.6 Compared to palladium catalysts in decarboxylative dehydrogenative cross-coupling reactions, copper catalysts are rarely reported.73 Decarboxylative Heck coupling reaction was reported by Meyer for the first time.74 Some improvements, such as an oxidant and a catalyst, have been reported to overcome the limitations of Meyer’s method.6 Homocoupling reactions of carboxylic acids are also reported.75 Moreover, besides that of Ar−COOH,71,6 the decarboxylative coupling of carbonyl−COOH6,76 and alkyl−COOH6,77a,b has also been significantly developed. In addition to C−C bond formation, C−X (F, Cl, Br),77 C− N,78 C−S,79 and C−P80 bonds could be formed through decarboxylative cross-coupling reactions.6 Carboxylic acids are suitable substrates to construct these C−heteroatom bonds with good functional group tolerance. 2.2.2. Cleavage of C−C(O)H Bonds. 2.2.2.1. PalladiumCatalyzed Cleavage of C−C(O)H Bonds. C−CHO bond cleavage with the release of a formyl group is one of the most important methodologies in organic synthesis. Palladiumcatalyzed decarbonylation of some aldehydes has been known since the 1950s.81 Generally, these decarbonylations of aldehydes need harsh reaction conditions such as a very high temperature. Recently, Maiti reported Pd(OAc)2-catalyzed decarbonylation of nonfunctional aryl aldehydes, functional aldehydes, heterocyclic aldehydes, alkanes, and alkenyl aldehydes. The scope of substrates is wide, and the reaction temperature is only 100−140 °C.82 They also developed the decarbonylation of aryl aldehydes under microwave heating conditions with a short reaction time. Especially for the substrate phenylglyoxal, only benzene is obtained under the microwave irradiation conditions (Scheme 42).83 Eulatachro-

transform to amide products through an acid-catalyzed rearrangement process. The migratory aptitude of aryl groups is higher than that of alkyl groups (Scheme 40).62 Scheme 40

2.2. Cleavage of C−Csp2 Bonds

2.2.1. Cleavage of C−C(O)O Bonds. Transition-metalcatalyzed decarboxylative cross-coupling reactions (C−COO bond cleavage) of stable carboxylic acids have become an intensive research area in organic synthesis, and great development has been achieved. They have been widely applied in the construction of carbon−carbon and carbon− heteroatom bonds with good selectivities and functional group tolerance.6 The decarboxylative reactions of carboxylic esters (C−COOC) are useful and important in organic synthesis and have also been well developed.6f As decarboxylation reactions (C−COOH and C−COOC) have been well reviewed,6 we do not discuss them in detail in this review, but just briefly give some introduction of the development in this specific area. The groups of Nilsson,63 Cohen,64 and Sheppard65 initially reported protodecarboxylation of aromatic carboxylic acids promoted by some copper complexes. Recent research shows that copper could catalyze the protodecarboxylation of aromatic acids and alkynyl carboxylic acids in the presence of nitrogen ligands.66 Silver,67 palladium,68 and rhodium69 are also applied to protodecarboxylation reactions. In addition, decarboxylative auration products could be obtained using gold catalysts.70 Decarboxylation of a carboxylic acid could generate an intermediate containing a C−metal bond. This organometallic intermediate can react with various C-precursors to construct new C−C bonds (Scheme 41). Goossen and co-workers

Scheme 42

mene, which is a natural product chromene, could be synthesized via this palladium-catalyzed decarbonylation reaction (Scheme 43). The hydroxymethyl (−CH2OH) groups could also be cleaved using this palladium catalyst via the aldehyde intermediate which is generated by the oxidation process.84

Scheme 41

Scheme 43 reported biaryl synthesis from ortho-substituted aromatic carboxylic acids with aryl halides catalyzed by the Pd/Cu system.71 Various biaryls, aryl esters, arylalkynes, and aryl ketones could be prepared through decarboxylative crosscoupling reactions.6 In addition, decarboxylative dehydrogenative cross-coupling reactions via C−H bond activation have also been disclosed. Palladium catalysts have been well developed for this transformation. Crabtree et al. reported the first examples of biaryl synthesis from aromatic acids with unactivated arenes via decarboxylative dehydrogenative crosscoupling reaction.72 Furthermore, a number of intramolecular and intermolecular decarboxylative dehydrogenative cross8623


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Larock and co-workers reported a new activation process of a C−CHO bond. The migration of palladium from an aryl position to an acyl position via a five-membered ring generates an acylpalladium species. The acylpalladium species could be converted into olefins via decarbonylation and β-hydride elimination (Scheme 44).85 However, the reaction yields are

versatile method for the rhodium-catalyzed decarbonylation of aldehydes by using the commercially available catalyst RhCl3· 3H2O with a (1,3-dicyclohexylphosphino)propane (dppp) ligand. This reaction tolerates a wide range of functional groups and could be applied to both aromatic and aliphatic aldehydes (Scheme 46).91

Scheme 44

Scheme 46

Carreira et al. reported a Tsuji−Wilkinson decarbonylation reaction of β,β-disubstituted aldehydes as an enantioselective approach to 1,1-diarylethanes in high yield and optical purity using the [Rh(cod)Cl]2 catalyst (Scheme 47).92

low, and only unfunctionalized aryl iodides with high catalyst loadings are suitable substrates for this transformation. Martin et al. report the development of a general and modified method for this transformation using aryl bromides as substrates and (1,3-dicyclohexylphosphino)propane bis(tetrafluoroborate) as a ligand with low palladium catalyst loadings.86 The reductive elimination of 56 via 1,4-palladium migration affords an acylpalladium intermediate 57. Then CO is extruded followed by reductive elimination to generate the target products (Scheme 45).

Scheme 47

Scheme 45

Madsen and co-workers reported a mechanistic study of the rhodium-catalyzed decarbonylation of aldehydes by density functional theory (DFT) calculations.93 They mentioned that the reaction mechanism involves four steps: (1) coordination of the aldehyde substrate to the metal complex, (2) oxidative addition of the aldehydic C−H bond to form a metal acyl complex (RhI → RhIII), (3) migratory extrusion of carbon monoxide, and (4) reductive elimination of the product (RhIII → RhI). A detailed DFT (B3LYP) study of the catalytic cycle suggests that elimination of CO is the rate-limiting step for this transformation. Li and co-workers discovered the first decarbonylative Hecktype reaction catalyzed by Rh(CO)2(acac) including conjugated addition of aldehyde with unsaturated carbonyl compounds (Table 6).94 Cinnamates are produced in moderate to high yield along with 3-arylpropanoates as the byproducts. A tentative mechanism is illustrated in Scheme 48. RhI(CO)2(acac) undergoes oxidative addition with the C−H bond of aldehydes to produce acylrhodium hydride 63, which decarbonylates to give arylrhodium hydride 64. An acrylate coordinates to the rhodium catalyst to generate complex 65. Subsequent insertion of CC into complex 65 generates intermediate 66, which undergoes β-hydride elimination to form the Heck-type product and active rhodium complex. On the other hand, intermediate 66 also undergoes reductive elimination to form the conjugate addition product and regenerates the active RhI catalyst.94 Recently, Li and co-workers presented the Rh(CO)2(acac)catalyzed synthesis of biaryls involving decarbonylation and C−

2.2.2.2. Rhodium-Catalyzed Cleavage of C−C(O)H Bonds. In 1965, Tsuji and Ohno described the decarbonylation of aldehydes with stoichiometric amounts of Wilkinson’s catalyst, Rh(PPh3)3Cl.87 They also reported the first example of a rhodium-catalyzed decarbonylation using Rh(CO)(PPh3)2Cl as the catalyst at very high temperatures.88 Some catalysts which are difficult to prepare such as Rh(dppp)2Cl89 and Rh(CO)(triphos)SbF690 were employed in decarbonylation of aldehydes in the past century. Madsen and co-workers established a 8624


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Table 6

(Scheme 52).98 The rate of this [IrCl(cod)]2-catalyzed decarbonylation reaction has a first-order dependence on the aldehyde concentration. [Ir(cod)Cl]2-catalyzed Pauson−Khand-type reactions using aldehydes as the CO source via decarbonylation are also reported using appropriate ligands.99 Guo and co-workers proposed a different pathway by theoretical investigation of the decarbonylation of aldehydes mediated by first-row transition metals Cr+, Co+, and 4Fe+. The mechanism starts with a C−C bond activation rather than a C−H bond activation.100 Paneque and co-workers developed IrIII metallacyclopentadiene-catalyzed decarbonylation of aliphatic aldehydes. Theoretical calculations reveal that the metallacycle is a shuttle for the transfer of a H atom in aldehyde. Various aliphatic aldehydes could undergo decarbonylation under photochemical (UV) irradiation conditions.101 2.2.2.4. Ruthenium-Catalyzed Cleavage of C−C(O)H Bonds. Li and co-workers developed [Ru(cod)Cl2]2-catalyzed olefination from aldehydes and alkynes via a decarbonylative addition. There is a strong electronic effect and high chemoselectivity for aromatic and aliphatic aldehydes in this reaction. Electron-rich aromatic substrates are more favorable in this reaction (Scheme 53). The monomer catalyst generated from the polymer catalyst coordinates with the alkyne, giving intermediate 76, which undergoes oxidative addition with the aldehyde to generate intermediate 77. Intermediate 78 could be obtained via a decarbonylative process. Finally, reductive elimination affords the decarbonylative addition product and regenerates the active ruthenium complex 75. A ruthenium carbonyl complex could be formed by an IR study of the reaction residue (Scheme 54).102 Shortly after their above report, Li and co-workers discovered a novel method for CC double bond formation, especially for aliphatic aldehydes and alkynes via a decarbonylative addition process.103 Different functionalized substrates were examined, and good to excellent yields were obtained. The ratio of E to Z

Scheme 48

H activation using a directing group. Both electron-rich and electron-withdrawing substituents of aromatic aldehydes could undergo this transformation successfully in moderate to good yields (Scheme 49). Oxidative addition of aldehyde to the RhI center generates species 72, which undergoes extrusion of CO to give intermediate 73. Intermediate 74 could be obtained through C−H bond activation. Finally, reductive elimination of intermediate 74 affords the target product and regenerates the active RhI catalyst (Scheme 50).95 Morimoto96 and Shibata97 independently reported Rhcatalyzed Pauson−Khand-type reactions using aldehydes as the CO source via decarbonylation (Scheme 51). Enantioselective reaction was also realized, where solvent-free conditions are essential for a high yield and ee value. 2.2.2.3. Iridium-Catalyzed Cleavage of C−C(O)H Bonds. The [IrCl(cod)]2-catalyzed decarbonylation of aldehydes with an easily accessible monodentate phosphine such as PPh3 or P(n-Bu)3 was developed by Tsuji. This efficient catalytic decarbonylation of aldehydes could proceed at lower temperatures (66−101 °C) and without any chemical scavenger of CO 8625


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Scheme 49

Scheme 50

Scheme 53

Scheme 54 Scheme 51

Scheme 52

is from 1:1.5 to 1.5:1 (Table 7). The copper catalyst can either promote the departure of CO from the ruthenium catalyst center or alter the nature of the phosphine ligand. A bulky ligand is necessary for this transformation to form a stabilized catalyst center while still maintaining free coordination sites for the decarbonylation and additions.103 The visible-light-mediated oxidative C−C bond cleavage of aldehydes using Ru(bpy)3Cl2 (bpy = 2,2′-bipyridine) as the 8626


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2.2.2.5. Manganese-Promoted Cleavage of C−C(O)H Bonds. Jiao and co-workers disclosed a novel Mn(OAc)3· 2H2O-promoted synthesis of formamides from amines and aldehydes through aerobic C−C bond cleavage of aldehydes with inert alkyl chain fragments as the leaving group. Molecular oxygen is employed as the oxidant and provides the oxygen atom of the products. This reaction proceeds via a radical process in the presence of the catalyst Mn(OAc)3·2H2O. A C− C bond is cleaved from an in situ generated peroxide intermediate (Scheme 57).105 Chi and co-workers developed

Table 7

Scheme 57

a metal-free transformation of chiral aldehydes into ketones using O2 as the sole oxidant.106 The C−C bond activation proceeds through in situ formed enamine intermediates. Various bioactive componds could be obtained from simple and readily available starting materials through this strategy (Scheme 58).105

photoredox catalyst has been reported recently by Xia’s group.104 The reaction could be conducted at room temperature and under an air atmosphere. α-Aryl, cyclic, and alkyl aldehydes are tolerated in this transformation (Scheme 55).

Scheme 58

Scheme 55

2.2.2.6. Vanadium-Catalyzed Cleavage of C−C(O)H Bonds. Maiti and co-workers developed the first vanadiummediated decarbonylative halogenation reaction starting from the divanadium oxoperoxo complex. Various substituents such as −OH, −OMe, −Br, −NH2, and −NO2 are tolerated in this reaction (Scheme 59). A concerted decarbonylative chlorinaScheme 59 The aldehyde could react with a secondary amine to form enamine 83, which could be oxidized to cation radical 84. The species [O2]•−, which is generated from Ru+ and O2, could react with cation radical 84 to form a four-membered ring, 85. The product could be obtained through cleavage of the C−C bond of this intermediate (Scheme 56).104 Scheme 56

tion reaction is proposed for this transformation. Hydrogen peroxide (H2O2) is crucial for the (re)generation of vanadium oxoperoxo species [V5+(O2−)(O22−)]+ (Scheme 60).107 2.2.3. Cleavage of C−C(O)C Bonds. 2.2.3.1. RhodiumCatalyzed Cleavage of C−C(O)C Bonds. Jun and co-workers have pioneeringly developed rhodium-catalyzed activation of C−C bonds adjacent to carbonyl groups by applying a new 8627


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although the yields are still low under the current conditions (Scheme 63).110

Scheme 60

Scheme 63

concept called metal−organic cooperative catalysis (MOCC). The catalytic cycle includes installation of a 2-aminopyridyl group into ketones and the cleavage of C−C bonds by oxidative addition (Scheme 61).108 Scheme 61 Johnson and co-workers accomplished in-depth mechanistic investigations on Rh-catalyzed intramolecular carboacylation of alkenes, especially in the C−C bond activation step. This transformation could be realized in the presence of the RhCl(PPh3)3 or [Rh(C2H4)2Cl]2 catalyst. Mechanistic studies such as determination of the rate law and kinetic isotope effects were conducted. The C−C bond activation process for species with minimal alkene substitution is the rate-limiting step for each catalyst (Scheme 64).111 However, alkene insertion becomes the rate-limiting step for more sterically encumbered substrates. Shi and co-workers significantly developed a [(CO)2Rh(acac)]-catalyzed decarbonylation process with the assistance of N-containing directing groups such as pyridyl, quinolinyl, pyrazolyl, and oxazolyl (Scheme 65). The proposed reaction

The atomic economy in the utilization of unstrained C−C σbond cleavage to build complex molecules still needs improvement for chemists. Douglas and co-workers disclosed a new rhodium-catalyzed intramolecular alkene carboacylation reaction via C−C bond activation directed by quinoline. This reaction allows for the construction of all-carbon quaternary centers leading to carbocyclic and heterocyclic compounds in good to excellent yields (Scheme 62).109 Douglas and co-workers also first realized the activation of an unstrained C−C single bond and subsequent intermolecular carboacylation of an olefin to form two new C−C σ-bonds,

Scheme 64 Scheme 62

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Scheme 65

Scheme 67

Scheme 68

pathway is shown in Scheme 66. The octahedral acylrhodium(III) species 92 or 93 could be formed via oxidative cleavage of Scheme 66

the C−C bond with the square-planar RhI complex directed by the pyridine group. Then reverse migratory insertion and reductive elimination produce the desired product with regeneration of the active RhI catalyst.112 Wang and co-workers described a Rh(PPh3)Cl-catalyzed method for ketone synthesis by exchange of a methyl or aryl group of a ketone to another aryl group of arylboronic acids via a C−C bond cleavage process (Scheme 67). Various arylboronic acids are compatible in the reaction. The reaction mechanism is proposed in Scheme 68. First, a five-membered cycloacylrhodium(III) intermediate, 98, could be formed from a RhI complex with an acetyl C−C bond via oxidative addition. Transmetalation of 98 with arylboronic acid generates intermediate 99. Subsequent reductive elimination of 99 and a phosphine ligand insertion lead to active RhI intermediate 100, which could be oxidized to RhIII complex 101 in the presence of CuI/O2. RhIII complex 102 could be generated via another transmetalation of species 101 with arylboronic acid. Finally, reductive elimination of 102 leads to the formation product and regenerates the active RhI complex.113 Arisawa, Yamaguchi, and co-workers developed a RhH(CO)(PPh3)3-catalyzed acyl transfer reaction between thioesters/aryl

esters and benzyl ketones in the presence of 1,2-bis(diphenylphosphino)benzene (dppBz). The C(O)−C bond of the ketone is cleaved via oxidative addition, leading to a C(O)−Rh−C intermediate, which undergoes an exchange reaction giving a new C′(O)−Rh−C species. Finally, the product is formed by reductive elimination with the regeneration of the active catalyst (Scheme 69).114 Scheme 69

Direct transition-metal-catalyzed decarbonylation of stable ketones via double C−C bond cleavage to construct new C−C bonds has rarely been reported. Brookhart and co-workers discovered the (1,2,4-tri-tert-butylcyclopentadienyl)rhodium bis(ethylene)-catalyzed decarbonylation of diaryl ketones giving biphenyl products. The reaction proceeds with a high catalyst 8629


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loading, whereas the conversion is low and the substrate scope is limited (Scheme 70).115

pathways which are different in the timing of reductive elimination.117

Scheme 70

Scheme 72

[Rh(cod)Cl]2-catalyzed Csp2−Csp σ-bond cleavage of diynones to conjugated diynes through decarbonylation without relying on any ring strain or auxiliary directing group is reported by Dong et al. The oxidative addition of RhI into the Csp2−Csp bond, leading to acylrhodium(III) acetylide, is the key step for this C−C bond activation (Table 8).116

Subsequently, Kuninobu, Takai, and co-workers reported rhenium- and manganese-catalyzed transformations via the insertion of alkynes into a nonstrained C−C single bond of cyclic and acyclic β-keto esters.118 However, the products were obtained as mixtures of regio- and stereoisomers when acyclic β-keto esters were used.118b,d Very recently, they realized the regio- and stereoselective insertion of terminal alkynes into an unstrained C−C single bond between the α- and β-positions of the β-keto sulfones successfully catalyzed by Re2(CO)10 (Scheme 73).119

Table 8

R1

R2

yield (%)

R1

R2

yield (%)

4-OMeC6H4 4-CIC6H4 C6H11 PMBOC2H4

4-OMeC6H4 4-CIC6H4 C6H11 PMBOC2H4

81 53 57 20

Ph Ph Ph Ph

4-OMeC6H4 2,4,6-(CH3)3C6H2 1-naphthyl PMBOC2H4

60 83 68 51

Scheme 73

2.2.3.2. Rhenium-Catalyzed Cleavage of C−C(O)C Bonds. Kuninobu, Takai, and co-workers reported [ReBr(CO)3(thf)]2catalyzed insertion of terminal acetylenes into a C−C single bond adjacent to a carbonyl group of nonstrained cyclic compounds using isocyanide as a ligand (Scheme 71). Notably, even 9- and 10-membered rings can be formed in high yields. The proposed reaction mechanism is shown in Scheme 72. The C−C bond is cleaved by a retro-aldol reaction in two possible Scheme 71

2.2.3.3. Manganese-Catalyzed Cleavage of C−C(O)C Bonds. An efficient method to prepare aromatic carboxylic acids and cyclopropanecarboxylic acids has been developed by Minisci et al. through the oxidation of aryl alkyl and cyclopropyl alkyl ketones with C−C bond cleavage catalyzed by Mn(NO3)2 in combination with Co(NO3)2 or Cu(NO3)2 in an O2 atmosphere. A free radical redox chain mechanism for this transformation is proposed (Scheme 74).120 Kuninobu, Takai, and co-workers developed the [{HMn(CO)4}3]-catalyzed synthesis of amides from ketones and carbodiimide through the cleavage of unstrained C−C single bonds of ketones. Isocyanates could also be used instead of carbodiimides (Scheme 75). Various N-arylbenzamides are obtained in high yields. The proposed mechanism is shown in 8630


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Scheme 74

Scheme 77

amines could also be used as nucleophiles with the generation of the corresponding carboxylic acids and amides (Scheme 78).122

Scheme 75

Scheme 78

Scheme 76. The ketone tautomerizes into an enol which undergoes nucleophilic addition to carbodiimide activated by 2.2.3.5. Iron-Catalyzed Cleavage of C−C(O)C Bonds. Jana and co-workers demonstrated an Fe(OTf)3-catalyzed C−C single bond cleavage of 1,3-diketones through a retro-Claisen condensation reaction. This new methodology provides a concise and efficient synthesis of various substituted methyl ketones and structurally diverse esters (Scheme 79).123 The mechanism of this transformation is similar to that of the above indium-catalyzed cleavage of C−C(O)C bonds (Scheme 78). 1,2-Diketones are of great importance in organic synthesis. Zhang and co-workers developed an FeCl3-catalyzed synthesis of 1,2-diketones from 1,3-diketones through selective C−C

Scheme 76

Scheme 79

[{HMn(CO)4}3]. Intermolecular nucleophilic cyclization produces azetidin-2-imine 104, which could be converted into an amide via a C−C single bond cleavage of the four-membered ring.121 2.2.3.4. Indium-Catalyzed Cleavage of C−C(O)C Bonds. Kuninobu, Takai, and co-workers developed an In(OTf)3catalyzed synthesis of esters from 1,3-dicarbonyl compounds and alcohols by deacylation reaction (Scheme 77). In(OTf)3 can act as a Lewis acid to coordinate 1,3-dicarbonyl compounds. Nucleophilic attack of an alcohol on a carbonyl group of a 1,3-diketone and subsequently a C−C bond cleavage occur via retro-aldol-type reaction. Furthermore, water and 8631


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bond cleavage using tert-butyl nitrite (TBN) as the oxidant in solvent-free conditions (Scheme 80). Isotopic labeling studies

Scheme 82

Scheme 80

show that the central carbon of 1,3-diketones is lost during the reaction. A triketone which is generated from a 1,3-diketone with TBN via a radical pathway could be converted into a 1,2diketone by 1,2-Wagner−Meerwein-type rearrangement with the assistance of the catalyst FeCl3.124 Furthermore, Yuan and co-workers reported a similar reaction based on an iodinecatalyzed oxidative cleavage reaction to form a diaryl 1,2diketone from a diaryl 1,3-diketone under metal-free conditions.125 2.2.3.6. Copper-Catalyzed Cleavage of C−C(O)C Bonds. Chiba and co-workers reported a Cu(OAc)2-catalyzed synthesis of nitriles from α-azidocarbonyl compounds under an oxygen atmosphere via C−C bond cleavage of a transient iminylcopper intermediate.126 Many aryl and alkyl nitriles are produced in high yields by this transformation (Scheme 81). α-Azidocar-

Scheme 83

Scheme 81

Scheme 84

bonyl compounds are converted into iminylcopper species through denitrogenation. The iminylcopper species could transform into CuIII species 109 by the oxidation of O2. The corresponding nitrile would be obtained through intramolecular C−C bond cleavage. CuII species could be formed by protonation of acylperoxy copper 110 (Scheme 82). An isotopic labeling experiment shows that one of the oxygen atoms of O2 is incorporated into the β-carbon fragment, giving a carboxylic acid (eq 3).126 Lei and co-workers disclosed an efficient arylation of βdiketones including a C−C activation process. Various α-aryl ketones could be efficiently synthesized from β-diketones and aryl halides smoothly catalyzed by CuI with K3PO4·3H2O (Scheme 83). A putative pathway is listed in Scheme 84. CuIII intermediate 112 could be formed from ArX via an oxidative

addition process. Subsequently, the C−C bond is cleaved in the presence of H2O, and intermediate 113 is generated with the release of KOAc. Reductive elimination occurs, and CuI intermediate 114 is produced. Finally, the desired α-aryl ketone 8632


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is obtained from intermediate 114, and the CuI intermediate 111 species is regenerated.127 Jiao and co-workers reported the first example of oxidative esterification reaction of 1,3-diones with alcohols through C(CO)−C σ-bond cleavage under the CuBr/O2 system (Scheme 85). Oxygen is the efficient oxidant and reagent for

Scheme 88

Scheme 85

Zhou and co-workers demonstrated a CuI-catalyzed synthesis of acridones via intramolecular cyclization including a C(O)−CH3 bond cleavage process using air as the oxidant. Acridin-9(10H)-ones are obtained in good to high yields. Many substituents on the aromatic rings are tolerable in the reaction. 13 C labeling experiments show that only about 86% of the carbon atom of the carbonyl originates from the substrate. They were not able to speculate on a reasonable mechanistic pathway for this transformation. The authors proposed that a copper-catalyzed intramolecular Friedel−Crafts-type reaction pathway is disfavored (Scheme 89).131

this C−C bond cleavage. The hemiacetal intermediate undergoes further oxidative fragmentation to give the desired product and byproduct.128 Xi and co-workers demonstrated a new method for the synthesis of 3-substituted isocoumarins from 2-halobenzoic acids and 1,3-diketones catalyzed by CuI in DMF with K3PO4 at 90−120 °C including a deacylation process (Scheme 86). A

Scheme 89

Scheme 86

four-membered ring which is formed through the Hurtley reaction followed by an intramolecular addition reaction undergoes C−C and C−O bond cleavage to give the desired product (Scheme 87).129 Johnson and co-workers reported a Cu(NO3)2·3H2Ocatalyzed preparation of β-stereogenic α-keto esters from substituted acetoacetate esters through aerobic deacylation. Various functional groups such as esters, ketones, ketals, and nitro groups are tolerant of this transformation (Scheme 88).130

Fu and co-workers developed a relevant aerobic synthesis of acridone derivatives from 1-[2-(arylamino)aryl]ethanones catalyzed by Cu(O 2CCF3)2. They proposed a possible mechanism for this reaction which is shown in Scheme 90. Intermediate 1-arylindoline-2,3-dione (116) could be formed by the oxidation of 1-[2-(arylamino)aryl]ethanones with the elimination of H2O. Intermediate 116 could be transformed into intermediate 117 with pyridine and H2O under heating conditions. Intermediate 118 could be generated via Friedel− Crafts reaction of 117, and it could be converted into intermediate 119 via a dehydration process. The C−C bond is cleaved by decarboxylation of intermediate 119, giving intermediate 120, which undergoes an oxidation reaction to produce intermediate 121. Finally, acridone could be obtained by isomerization.132

Scheme 87

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Scheme 90

Bi, Liu, and co-workers reported the chemoselective oxidative cleavage of the C(CO)−CH3 bond of various aromatic and aliphatic methyl ketones to yield the corresponding aldehydes under the CuI/O2 catalytic oxidation system. This oxidative cleavage process could efficiently terminate at the aldehyde stage. The sequence of reaction involving αoxygenation/hydration/1,2-hydride shift/C−C bond cleavage is proposed by the authors. Control experiments show that another product which could be generated from the C−C bond cleavage is HCOOH. Finally, HCOOH could decompose into H2 and CO2 (Scheme 91).133

Table 9

Scheme 91

Li and co-workers successfully developed a Cu/Fecocatalyzed C−C σ-bond cleavage reaction of 2-substituted amino-1-phenylethanones (Table 9). The two new oxygen atoms in the product originate from O2 according to control experiments. Intermediate 128 could be formed from intermediate 126 and imine intermediate 127. Subsequently, intermediate 128 undergoes C−C σ-bond cleavage reaction and reductive elimination, leading to intermediate 129, which could be converted into the desired product via O−O bond cleavage reaction and reductive elimination (Scheme 92).134 2.2.3.7. Palladium-Catalyzed Cleavage of C−C(O)C Bonds. Catalytic Tsuji−Trost allylation is a ubiquitous method for the allylation of active methylene compounds.135 Tunge reported the development of deacylative allylation (DaA) of nitroacetone derivatives by a retro-Claisen condensation. Deacylative allylation realizes the selective monoallylation of nitronates and could also be used to control synthesis of unsymmetrical 1,6-dienes via a tandem three-component coupling reaction (Scheme 93).136 Many ketone pronucleophiles could also undergo intermolecular deacylative allylation using this reaction system.137 2.2.3.8. Nickel-Catalyzed Cleavage of C−C(O)C Bonds. Nickel-catalyzed β-alkyl eliminations of ketones are rarely reported. Cheng et al. reported that o-iodoaryl alkyl ketones

Scheme 92

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Palladium-catalyzed reactions of aroyl chlorides with disilanes giving arylsilanes via decarbonylation were first reported by Yamamoto142 and improved by Rich.143 The Tanaka group reported that pentafluorobenzoyl chlorides react with hexamethyldisilane under the PdCl2(PhCN)2/P(OEt)3 system, giving pentafluorophenyltrimethylsilane with high selectivity (eq 4).144 The PdII precatalyst is reduced to the catalytically active Pd0 species, which undergoes oxidative addition of pentafluorobenzoyl chloride in this reaction.

Scheme 93

could react with bicyclic alkenes to form polycyclic ketone derivatives (Scheme 94). 1-Alkylcyclopentenol is a key intermediate, and the C−C bond between the alkyl and the β-carbon is cleaved in the presence of the catalyst Ni(dppe)Br2 (Scheme 95).138

Müller and co-workers disclosed the synthesis of alkynones by PdCl2(PPh3)2/CuI-catalyzed decarbonylative alkynylation of the heteroarylglyoxylyl chlorides with terminal alkynes. Oxidative addition, decarbonylative elimination, transmetalation, and reductive elimination are involved in this transformation. The C(O)−C(O) single bond of oxalyl chloride is cleaved with the release of CO (Scheme 96).145 Recently, this group also developed a one-pot four-component process for the synthesis of pyrimidyl- and pyrazolylazulenes including the similar decarbonylative process.146

Scheme 94

Scheme 96 Scheme 95

2.2.4. Cleavage of C−C(O)Cl Bonds. 2.2.4.1. PalladiumCatalyzed Cleavage of C−C(O)Cl Bonds. Decarbonylations of acid chlorides are less common in the literature and are usually employed in Friedel−Crafts acylation reactions.81h Blaser and Spencer reported early significant examples of the palladium-catalyzed decarbonylative Mizoroki−Heck-type reaction using aroyl chlorides with amine bases in less polar solvents.139 Tsuji and co-workers reported arylation of dienes from aroyl chlorides by decarbonylation.140 Recently, Miura and co-workers developed Mizoroki−Heck-type arylation of alkenes in the PdCl2(PhCN)2/(PhCH2)Bu3NCl system using aroyl chlorides under base-free conditions.141

CO is released via palladium-catalyzed decarbonylation of acid chlorides, which provides a new safe and storable source of CO gas. Lindhardt and Skrydstrup successfully applied this method to aminocarbonylations.147 2.2.4.2. Rhodium-Catalyzed Cleavage of C−C(O)Cl Bonds. Rhodium-catalyzed cleavage of the C−C(O)Cl bond of acyl halides to yield olefins was reported by Tsuji in 1966.148 Miura and co-workers developed [RhCl(cod)]2/PPh3-catalyzed synthesis of vinyl chloride derivatives from aroyl chlorides and terminal alkynes via decarbonylation with regio- and stereo8635


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selectively.149 The diarylethenes are formed as the (E)-isomer. They also realized Mizoroki−Heck-type arylation of alkenes from benzoyl chlorides catalyzed by [RhCl(C2H4)2]2 without the addition of any base and phosphine ligand. The substrate aroyl chloride undergoes oxidative addition with RhICl species to generate intermediate [ArCORhIIICl2], which could be converted into [ArRhIIICl2] by decarbonylation and then react with alkenes (Scheme 97).150

mechanism is proposed in Scheme 99. Oxidative addition of RhI species to acid chloride gives an aroylmetal complex, [RCORhIIICl2], which undergoes decarbonylation to form an arylrhodium(III) intermediate. This intermediate reacts with arenes to form intermediate complex 130 via intramolecular ortho-chelating assistance, which is converted into the target product by reductive elimination.151 Bergman and Ellman developed the direct ortho-arylation of quinolines catalyzed by [RhCl(CO)2]2 with aroyl chlorides via a decarbonylation pathway.152 Electron-rich aroyl chlorides react better than electron-poor aroyl chlorides in this transformation (Scheme 100). A possible mechanism is

Scheme 97

Scheme 100

Recently, [Rh(cod)Cl]2-catalyzed regioselective functionalization of arenes or N-heteroaromatic C−H bonds with acid chlorides via decarbonylation under phosphine-free conditions was realized by Yu and co-workers (Scheme 98). A possible Scheme 98

Scheme 101

proposed in Scheme 101. Nitrogen coordination of the 2substituted pyridine to [RhCl(CO)2]2 provides adduct 131, which undergoes ortho-C−H activation, giving 132. 132 may tautomerize to N-heterocyclic carbene complex 133. Oxidative addition and decarbonylation of aroyl chlorides generate 136, which is converted into the product via reductive elimination. Unfavorable steric interactions in the N-bound complex increase the equilibrium ratio of complexes 132 and 133. This may rationalize that ortho-substituted pyridine is needed.152 2.2.4.3. Iridium-Catalyzed Cleavage of C−C(O)Cl Bonds. In 2002, Miura and co-workers developed [IrCl(cod)]2-catalyzed substituted naphthalene and anthracene synthesis from aroyl chlorides with internal alkynes (Scheme 102). Oxidative addition, decarbonylation, and C−H activation processes with the Ir catalyst are proposed for this reaction (Scheme 103).153

Scheme 99

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Pd(OAc)2/phosphine ligand including the decarbamoylation process.155 Wang and co-workers developed a highly chemoand regioselective arylation of azoles, thiazoles, and oxadiazoles with arylamides in the Pd(OAc)2/Phen/K2S2O8 reaction system.156 N-substituted groups, such as MeO, Me, NH2, nC4H9, and t-C4H9, attached to N-substituted benzamides are compatible in this reaction system. N,N-Diethylbenzamide and benzamide are not active, and only trace products are obtained (Schemes 105 and 106). The Pd0/ PdII process is proposed for

Scheme 102

Scheme 105

Scheme 103

Scheme 106

this transformation. N-Methoxyarylamide undergoes decarbonylation to generate ArPdX species and release CO via the insertion of palladium into an amide C−N bond (Scheme 107).156 Once the ligand dicyclohexyl(2′,6′-diisopropoxy-[1,1′-biphenyl]-2-yl)phosphine (RuPhos) was selected, aroyl chlorides reacted with teminal alkynes, giving the corresponding addition products (Z)-vinyl chlorides with decarbonylation. The stoichiometric experiments show that oxidative addition of aroyl chloride followed by a fast decarbonylation and an insertion of alkyne might afford the decarbonylation product (Scheme 104).154 2.2.5. Cleavage of C−C(O)N Bonds. 2.2.5.1. PalladiumCatalyzed Cleavage of C−C(O)NR Bonds. Acid amides are extremely stable, cheap, and readily available, and they have been used as decarbonylative coupling reagents recently. In 2002, Miura and co-workers reported that secondary 2thiophenecarboxamides could undergo triarylation with the

Scheme 107

Scheme 104

2.2.5.2. Nickel-Catalyzed Cleavage of C−C(O)NR Bonds. Matsubara, Kurahashi, and co-workers developed a new synthetic method for preparation of isoquinolones by Ni(cod)2-catalyzed reaction of alkynes with N-arylphthalimides (Scheme 108). It is noteworthy that N-phenylquinolimide could give a single isomer in this reaction. This regioselectivity of this substrate could be rationalized by nucleophilic attack of Ni0 on the more electrophilic carbonyl group. The C−N bond easily undergoes nucleophilic attack of the Ni0 species, which undergoes decarbonylation and intermolecular addition to 8637


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Ni(acac)2 via a decarbonylative process to produce orthosubstituted benzamides (Scheme 111).159

Scheme 108

Scheme 111

2.2.6. Cleavage of C−C(O)S Bonds. 2.2.6.1. PalladiumCatalyzed Cleavage of C−C(O)S Bonds. In 2007, Liebeskind and co-workers found that enamides can be obtained as byproducts using Pd(PPh3)4/CuTc from amino thioesters and organoboronic reagents.160 Skrydstrup and co-workers reported enamide synthesis from aminothioesters via [(η3-allyl)PdCl]2catalyzed decarbonylation and a β-hydride elimination process (Scheme 112).161 This method could be applied to various amino thioesters that contain aromatic chains, heterogroups, and alkyl groups. 2.2.6.2. Platinum-Catalyzed Cleavage of C−C(O)SR Bonds. Thioesters could also undergo arylthiolation to internal alkynes catalyzed by Pt(PPh3)4 via decarbonylation. This reaction is realized by Kuniyasu and Kambe with an oxygen-containing group in alkynes at the proper position. Various aryl(styryl)sulfanes are obtained in high yields (Scheme 113). The mechanism for this Pt-catalyzed decarbonylative arylthiolation of internal alkynes is similar to that previously introduced for the reaction of aroyl chlorides with terminal alkynes.162 2.2.7. Cleavage of C−Caryl Bonds. 2.2.7.1. PalladiumCatalyzed Cleavage of C−Caryl Bonds. Youn and co-workers developed a Pd(O2CCF3)2-catalyzed transformation of 2alkenylphenyl β-keto esters and 1,3-diketones to 1-naphthols with the generation of arylpalladium(II) species which could be captured with olefins. A unique process of C−C bond activation through a novel aromatization-driven syn β-carbon elimination is included in this catalytic system. The results show that the β-carbon elimination is expected to occur for M−C− C−Caryl species, while a trace yield was obtained for M−C−C− Calkyl species (Scheme 114).163

alkynes, giving the seven-membered nickelacycle (Scheme 109).157 Scheme 109

Matsubara, Kurahashi, and co-workers also achieved a decarbonylative cycloaddition of phthalimides with 1,3-dienes, giving 3-vinyldihydroisoquinolone catalyzed by Ni(cod)2 and PMe3 with excellent regio- and chemoselectivity (Scheme 110).158 Johnson and co-workers developed cross-coupling reactions of phthalimides with diorganozinc reagents in the presence of

2.3. Cleavage of C−Csp3 Bonds

2.3.1. Cleavage of C−COH Bonds. 2.3.1.1. RutheniumCatalyzed Cleavage of C−COH Bonds. Catalytic C−C bond cleavages via β-carbon elimination of [M]−O−C−C species have become a hot research area. Kondo, Mitsudo, and coworkers developed the first deallylation of tertiary homoallyl alcohols via selective cleavage of a C−C bond using RuCl2(PPh)3 as the catalyst under CO (10 atm).164 An (alkoxy)ruthenium intermediate undergoes β-alkyl elimination, which is the driving force for C−C bond cleavage in this catalytic system. CO may play the role of an effective π-acid to promote the reductive elimination to generate the product (Scheme 115).164 2.3.1.2. Palladium-Catalyzed Cleavage of C−COH Bonds. The palladium-catalyzed C−COH bond activation of acyclic alcohols and cyclanols has been well developed and reviewed.165 Miura and co-workers found that α,α-disubstituted arylmethanols could react with aryl bromides to produce biaryls via cleavage of the Csp2−Csp3 bonds catalyzed by Pd(OAc)2. This reaction is the first catalytic C−C bond formation of biaryls including the β-carbon elimination process (Table

Scheme 110

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Scheme 112

10).166 The detailed studies of this reaction revealed that a bulky phosphine ligand such as PPh3 or PCy3 is crucial for the

Scheme 113

Table 10

Scheme 114

R1, R2 (amt, mmol)

R3 (amt, mmol)

L

time (h)

Me, H (0.6) H, Me (1.5) H, CO2Et (0.8) H, H (0.6)

H (0.5) H (0.5) H (0.5) OMe (0.5)

PPh3 PCy3 PPh3 PPh3

4 8 4 2.5

yield (%) 90 94 94 98

(72) (82) (91) (82)

C−C bond cleavage.167 1,1,2,2-Tetraarylethanes would be obtained by a 2:4 coupling under the palladium catalytic system when excess bromobenzenes are used.168 The palladium intermediate which is generated from β-carbon elimination could also be converted into hydroarylation products through reaction with unsaturated compounds such as alkynes and α,βunsaturated ketone compounds.169 Johnson provided detailed research on the catalytic mechanism of this transformation. The results show that the ortho-substituted aryl ring of triarylmethanols undergoes β-aryl elimination. Furthermore, electron-deficient or electron-rich aryl rings of triarylmethanols undergo C−C cleavage more readily than a neutral substituent (Scheme 116).170 Yorimitsu and Oshima et al. developed powerful allylations of aryl halides with tertiary homoallyl alcohols catalyzed by Pd(OAc)2/(p-tolyl)3 via a retroallylation process. The retroallylation would proceed in a concerted fashion via a conformationally regulated six-membered cyclic transition state with high stereo- and regiospecific selectivity (Scheme 117).171 When tricyclohexylphosphine is used as a ligand, a wider variety of aryl halides could perform arylative ringopening reactions with unstrained cyclic homoallyl alcohols.172 Yorimitsu and Oshima et al. also reported Pd(CO2CF3)2catalyzed substitution reactions of 2-pyridylmethyl groups which are generated from 2-(2-pyridyl)ethanol derivatives via β-carbon elimination with aryl and alkenyl chlorides (Schemes

Scheme 115

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Scheme 116

Scheme 118

118 and 119). Nitrogen atom containing groups are directing groups and are essential for this Csp3−Csp3 bond activation process (eqs 5 and 6).173

Scheme 119

Yorimitsu and Oshima et al. also found that various 2benzylpyridine N-oxide derivatives could be obtained from 2(2-hydroxylalkyl)pyridine N-oxide derivatives with aryl bromides. β-Csp3 elimination is proposed via a four-membered transition state for this transformation (Scheme 120).174 Oh and co-workers developed unusual Pd(PPh3)4-catalyzed arylative fragmentations of acyclic 3-allen-1-ols with aryl halides to give arylated dienes. The π-allylpalladium intermediates are converted into arylated dienes through β-C elimination, leading to C−C bond cleavage (Scheme 121).175 Tamaru and co-workers disclosed a new and efficient Pd(PPh3)4-catalyzed synthesis of ω-dienyl aldehydes 137 from diols 138 via a β-carbon elimination process. A series of cycloalkanols, ranging from cyclobutanol to cyclodecanol with the exception of cyclohexanol, undergo the ring-opening reaction (Scheme 122).176 Chiba and co-workers reported PdCl2(dppf)-catalyzed ringexpansion reactions of cyclic 2-azido alcohols to form azaheterocycles such as pyridine, isoquinoline, and γ-carboline derivatives.177a C−C bond cleavage and C−N bond formation are involved in this reaction (Scheme 123). Palladium(II) alcoholate 139 undergoes β-carbon elimination, leading to C− C bond cleavage (Scheme 124).177a A palladium-catalyzed ringopening reaction of norbornene-derived tertiary alcohols was successfully achieved by Cramer’s group.177b 2.3.1.3. Rhodium-Catalyzed Cleavage of C−COH Bonds. Tertiary alcohols are common substrates for catalytic selective C−C bond cleavage via a β-carbon elimination mechanism. In comparison, secondary alcohols have rarely been reported for

Scheme 120

catalytic C−C bond cleavage. In 2001, Jun and co-workers developed Rh(PPh3)3Cl-catalyzed C−C bond activation of secondary alcohols giving an alkyl-group-exchanged product in the presence of 2-amino-3-picoline and K2CO3. Oxidation of alcohol through hydrogen transfer and chelation-assisted C−C bond activation are proposed for this transformation (Scheme 125).178

Scheme 117

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affording the corresponding secondary alcohols in situ via retroallylation (Scheme 126).179 Nishimura and Hayashi developed a [{Rh(OH)(cod)}2]catalyzed conjugate arylation of α,β-unsaturated carbonyl compounds with trisubstituted arylmethanols. Most β-aryl ketones are obtained in excellent yields. β-Aryl elimination from an alkoxorhodium intermediate is a key step for this catalytic cycle. Chiral diene ligands such as (S,S)-Bn-bod enabled this arylation to proceed with high enantioselectivity (Scheme 127).180 Shintani, Hayashi, and co-workers demonstrated that the [{Rh(OH)(cod)}2]/(R)-H8-binap catalytic system could be applied to the kinetic resolution of tertiary homoallyl alcohols (Table 11). Alkoxorhodium species 144 generated from (S)143 preferentially undergoes retroallylation to produce acetophenone and allylrhodium species 145, which could be protonated through ligand exchange with (S)-143 to regenerate intermediate 144. Therefore, (R)-143 is successfully resolved by this method (Scheme 128).181 Shi and co-workers reported an interesting [Cp*RhCl2]2catalyzed alkenylation of secondary diarylmethanols with terminal alkenes directed by a pyridinyl group via chelation assistance (Scheme 129). The five-membered rhodacycle generated via a β-carbon elimination process is the key step for this transformation (Scheme 130).182 [Cp*Rh(CH3CN)3][SbF6]2 is also applied to catalyze oxidative arylation of secondary alcohols with arylsilanes via a pyridinyl-directed C− C single bond activation process in this group. AgF plays dual roles as the oxidant and the activating reagent in this reaction. This arylation is initiated from RhIII-catalyzed C−C cleavage, and a five-membered rhodacycle intermediate is also proposed (Scheme 131).183 Shi’s group also realized the direct transformation of secondary alcohols, tertiary alcohols, and even primary benzyl/allylic alcohols into amines or other alcohols by nucleophilic addition of a rhodacycle intermediate to imines and aldehydes via a C−C bond cleavage/protonation/C−H activation/addition or C−C bond cleavage/addition process (Scheme 132).184 Reductive C−C bond cleavage is important but still challenging for chemists.185 Shi et al. demonstrate the first successful example of the reductive C−C bond cleavage of unstrained 1,1-biarylmethanols catalyzed by [Cp*Rh(CH3CN)3][SbF6]2 with H2 as the reducing agent.186 Pyridinyl or pyrazolyl groups are directing groups, and various functional groups are tolerated in this reductive system (Scheme 133). The C−C bond is cleaved via β-carbon elimination with the assistance of RhIII, giving a five-membered rhodacycle intermediate and a molecular aldehyde. A rhodium(III) hydride species could be generated from the five-membered rhodacycle intermediate with H2 and could reduce the aldehyde to the corresponding alcohol (Scheme 134).186 Very recently, the Rh-catalyzed photocatalytic dehydrogenation of the primary alcohol accompanied by decarbonylation of aldehyde was realized by the Sadow group. [ToMRh(CO)2] (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) is an effective catalyst to couple the two steps of this C−C single bond cleavage. Both aryl and aliphatic alcohols are decarbonylated successfully in this catalytic condition. Unfortunately, the COOMe, NO2, and Cl groups were not tolerated. The Sadow group disclosed that CO dissociation in [ToMRh(CO)2] is required for alcohol dehydrogenation (Scheme 135).187

Scheme 121

Scheme 122

Scheme 123

Scheme 124

Yorimitsu and Oshima et al. reported allylrhodium species generated from homoallyl alcohols could react with aldehydes, 8641


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Scheme 125

Scheme 126

Scheme 127

Table 11

2.3.1.4. Iridium-Catalyzed Cleavage of C−COH Bonds. Obora, Ishii, and co-workers developed a new method to synthesize α,ω-diarylalkanes from easily accessible ω-arylalkanols in the presence of the [(Cp*IrCl2)2]/[{IrCl(cod)}2]/dppe combined catalyst system (Scheme 136). Aldehyde 149 and an iridium hydride complex could be formed by hydrogen transfer from the alcohol to the iridium catalyst. The oxidative addition of a C(O)−H bond to the Ir complex, followed by migration of CO and β-hydrogen elimination, led to the intermediate 1,3diphenyl-1-propene (150). Subsequent hydrogenation of compound 150 by the iridium hydride complex could produce the desired products (Scheme 137).188 2.3.1.5. Copper-Catalyzed Cleavage of C−COH Bonds. Recently, Yorimitsu, Oshima, and co-workers developed a [Cu(IPr)Cl]-catalyzed retroallylation of homoallyl alcohols and allylation reaction of carbonyl compounds. This reaction system could also be applied to the synthesis of allenic amines or homopropargylamines from allenylation and propargylation of imines with excellent selectivity. These products are useful precursors for the synthesis of some valuable five-membered

azacycles such as pyrroline and pyrrole derivatives. Coppercatalyzed C−C bond cleavage through retroallylation is the key step for this reaction (Scheme 138).189 2.3.2. Cleavage of C−COC Bonds. 2.3.2.1. CopperCatalyzed Cleavage of C−COC Bonds. There are only a few examples of catalytic cleavage of inert C−C bonds using oxygen or air as the ultimate stoichiometric oxidant. Liu and coworkers reported the first example of Cu2O-catalyzed aerobic cleavage of the Csp3−Csp3 bond in ethers by using O2 as the terminal oxidant (Scheme 139).190 A plausible mechanism for this C−C bond cleavage reaction is depicted in Scheme 140. 8642


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Scheme 128

Scheme 131

Scheme 132

Scheme 129 Scheme 133

Scheme 130 Scheme 134

Radical intermediate 153 would be formed by hydrogen abstraction with a copper(II) peroxide radical which is generated from CuI/O2. Oxidation of radical intermediate 153 with O2 could produce peroxide radical 154, which could be converted into another peroxide radical intermediate, 155, and product 156. The intermediate 155 undergoes a series of

electron transfers, and the C−H and C−C bonds in glycol ethers could undergo homolysis to generate product 157. The competing kinetic isotope effect (KIE) experiment indicates that the C−H bond cleavage should be the rate-determining step of this transformation.190 8643


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Scheme 135

Scheme 139

Scheme 136

be the suitable peroxide in this reaction. Various 2-phenylpyridine derivatives and acetanilides are effective for this transformation (Table 12). A methyl group of the peroxide could be cleaved by the palladium catalyst through β-methyl elimination followed by the insertion of palladium into the weaker O−O bond (Scheme 141).191 2.3.4. Cleavage of C−CNC Bonds. 2.3.4.1. RutheniumCatalyzed Cleavage of C−CNC Bonds. Photocatalysis and its synthetic applications have become a hot research area in organic chemistry, and great developments have already been demonstrated. Li, Wang, and co-workers realized the C−C bond cleavage of 1,2-diamine in a photocatalytic system. Reactive intermediates iminium ions and radicals are formed. 1,2-Diamine 158 could be converted into the radical cation species 159 catalyzed by [Ru-(bpy)3]Cl2·6H2O/O2 via the {Ru2+−[Ru2+]*−Ru1+} redox cycle with an electron abstracted from the nitrogen lone pair. Then the iminium ion 160 and αamino radical 161 could be generated from species 159 through reorganization of electron densities and cleavage of the C−C bond. The nucleophile 162 could react with iminium ion 160 to give the aza-Henry product 163 in 85% yield. In addition, αamino radical 161 is detected by photopolymerization of 2hydroxyethyl acrylate (Scheme 142).192 2.3.5. Cleavage of C−CalkylC Bonds. 2.3.5.1. RhodiumCatalyzed Cleavage of C−CalkylC Bonds. The discovery of new catalytic processes to cleave unstrained and unactivated C−C bonds still remains a major challenge for chemists. Milstein and co-workers demonstrated the catalytic cleavage of the Caryl− Csp3 bond using [Rh(coe)2Cl]2 (coe = cyclooctene) with a P− C−P pincer-type ligand under H2 pressure or silane.185i Kotora et al. reported the unactivated C−C bond could be cleaved in allylmalonates and related compounds catalyzed by Fe, Ru, Co, Rh, Ni, and Pd in the presence of alkylaluminums.193 The allyl moiety is removed with high selectivity for this kind of reaction (Scheme 143). A proposed mechanism is shown in Scheme 144. The species 166 could be formed from hydride 169 with allylmalonate in the presence of triethylaluminum. A sequence of C−C, C−Rh, and C−O bond cleavage and Rh−O bond formation through a six-membered transition state produces rhodium enolate 167. Finally, the intermediate 167 undergoes transmetalation with triethylaluminum to generate aluminum enolate 168 and ethylrhodium species, which could be converted into active intermediate 169.193

Scheme 137

Scheme 138

2.3.3. Cleavage of C−COO Bonds. 2.3.3.1. PalladiumCatalyzed Cleavage of C−COO Bonds. Li et al. found that peroxide compounds could undergo C−C bond cleavage catalyzed by a palladium catalyst when they are used as methylating reagents in direct methylation of aryl C−H bond reactions. (Peroxybis(propane-2,2-diyl))dibenzene is verified to 8644


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Scheme 140

Table 12

Scheme 141

RhII(ttp), which could be generated from RhIII(ttp)I or RhIII(ttp)Me through intermediate RhIII(ttp)OH at high temperature, is the active catalyst. RhII(ttp) reacts with [2.2]paracyclophane to give intermediate 170, which could be converted into the product via hydrolysis (Scheme 145).194 2.3.5.2. Nickel-Catalyzed Cleavage of C−CalkylC Bonds. Recently, Kotora et al. realized the highly selective C−C bond cleavage of unactivated methylenecycloalkenes with five- and six-membered rings catalyzed by NiCl2(PPh3)2.195 Most desired products are obtained in excellent yields (Scheme 146). The “Ni−H” species, which is generated in situ from NiCl2(PPh3)2/ Et3Al, is the key intermediate for this intramolecular deallylation (Scheme 147).195 Very recently, Ikeda et al. reported a NiCl2(PPh3)2-catalyzed facile C−C bond cleavage proceeded via a domino reaction of enynes and enones through syn β-elimination of the 1,3dicarbonyl part in the presence of Zn/ZnCl2 (Figure 1). Ni0 is the active catalyst and could be generated from NiCl2(PPh3)2 reduced by Zn powder. The substrate scope shows that the presence of two carbonyl groups in the leaving group is essential for this transformation. But-3-en-2-one, ethyl vinyl ketone, 3-methylbut-3-en-2-one, and crotonaldehyde are suitable substrates in this reaction (Table 13).196 2.3.5.3. Iron-Catalyzed Cleavage of C−CalkylC Bonds. Li and co-workers reported an efficient FeCl3-catalyzed C−C bond cleavage reaction in which the 1,3-dicarbonyl unit is a new and useful leaving group. A variety of aromatic compounds, alkenes, and alkynes could react with diones to give the corresponding products. The authors proposed that the Lewis acid catalyst FeCl3 may promote the cleavage of the C−C bond through the coordination of iron with the 1,3dicarbonyl unit in this transformation. A large dipole moment and bulky substituents on the 1,3-dicarbonyl group facilitate this reaction (Scheme 148).197 Azides have been widely used as aminating reagents in organic synthesis. Jiao et al. developed some new approaches

Chan et al. reported RhIII(ttp)I- or RhIII(ttp)Me-catalyzed hydrogenation of [2.2]paracyclophane via C−C σ-bond cleavage with water as the hydrogen source at 200 °C (eq 7).

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Scheme 142

Scheme 143

Scheme 145

Scheme 144

Scheme 146

oxidation with the catalyst FeCl2 and oxidant DDQ. The diarylmethyl cation 174 could react with the azide reagent to generate key intermediate 175, which would be oxidized to the intermediate 177 in this oxidative system. The trans group of intermediate 177 could migrate from the carbon to the nitrogen atom via a C−C bond cleavage process, leading to the intermediate 178. Subsequent nucleophilic attack by H2O leads to the desired amide product. When the azide reagent is changed from TMSN3 to alkyl azides under similar FeCl2 catalytic conditions, N-alkylanilines, which are widely used in organic synthesis, are obtained through the cleavage of the Caryl−Csp3 bond.199 The substrate scope of this transformation is broad. Diarylmethanes and

for inert C−C single bond cleavage using azides as useful reagents and iron or copper salts as catalysts under oxidative conditions.198 Benzyl hydrocarbons could be converted into the corresponding amides through C−H and C−C bond cleavage which is catalyzed by FeCl2 with DDQ as the oxidant. Lactams could be obtained using this strategy via a ring-expansion process (Scheme 149).198 Additionally, (E)-1,3-diarylpropenes could also be converted into the corresponding acrylamides. A tentative mechanism for this transformation is proposed in Scheme 150. Initially, substrate 171 could be converted into the corresponding diarylmethyl cation 174 via single electron transfer (SET) 8646


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Scheme 147

Scheme 148

Figure 1.

Table 13

Scheme 149

for this transformation involving release of nitrogen and migration of the aryl group from the carbon to the nitrogen atom (Scheme 151).199 2.3.5.4. Copper-Catalyzed Cleavage of C−CalkylC Bonds. When the catalyst CuI is used without the additive H2O in the presence of 4 Å molecular sieves under similar reaction conditions, Jiao’s group found that the intermediate 178 could react with residual azide reagent TMSN 3, giving 1,5disubstituted tetrazoles with high selectivity. Diarylmethanes and (E)-1,3-diarylpropenes are appropriate substrates in this reaction (Scheme 152).200 2.3.5.5. Palladium-Catalyzed Cleavage of C−CalkylC Bonds. Fillion and co-workers developed Pd/C-catalyzed reductive cleavage of unstrained C−C σ-bonds of benzyl Meldrum’s acids into tertiary benzylic stereocenter products. The reductive cleavage of enantioenriched benzylic quaternary centers proceeds through an SN2 pathway. The Meldrum’s acid moiety could be displaced either by a palladium hydride or by a Pd0 directly to generate a benzylic organopalladium intermediate

alkylarenes are compatible with this protocol. Significantly, this chemistry developed by Jiao and co-workers may provide a new strategy for degradation of polystyrene which may cause “white pollution”. A Schmidt-type rearrangement process is proposed 8647


Chemical Reviews

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Scheme 150

Scheme 153

leaving group in this reaction through C−C bond cleavage in the presence of the Pd0 catalyst (Scheme 154).202 Scheme 154

Scheme 151 Gryko and co-workers reported the Pd(OAc)2-catalyzed synthesis of vitamin B12 derived cobryketone 185 via Csp3− Csp3 bond cleavage with the release of a ketone. The process through oxidation of Csp3−H into Csp3−OH followed by βcarbon elimination to give the product is proposed for this transformation (Scheme 155).203 2.3.6. Cleavage of C−C Bonds of Five-Membered Ring Hydrocarbons. Highly strained cyclopropanes and cyclobutanes and their derivatives have been well recognized as useful and powerful acyclic building blocks for C−C bond cleavage catalyzed by transition metals.4 Compared to that of the small-sized strained rings, utilization of the five-membered rings via C−C bond cleavage has rarely been reported. C−C bond cleavage of the cyclopentadienyl ligand has been reported by Rosenthal,204 Crowe,205 Stryker,206 Kempe,207 and Takahashi and Xi.208 2.3.6.1. Rhodium-Catalyzed Cleavage of C−C Bonds of Five-Membered Ring Hydrocarbons. Mukai and co-workers reported a RhCl(PPh3)3-catalyzed C−C bond cleavage of the simple unactivated cyclopentane ring of allenylcyclopentanes giving nine-membered carbocycles (Table 14). The bicyclo[7.4.0]tridecatriene derivatives could be obtained from in situ generated 8-rhodabicyclo[4.3.0]nona-1,6-diene intermediate 187, which undergoes [7 + 2] cycloaddition through βcarbon elimination (Scheme 156).209

Scheme 152

3. CARBON−CARBON DOUBLE BOND CLEAVAGE The cleavage of a CC bond is one of the primary reactions in organic chemistry. Olefin metathesis reactions5 and enyne metathesis reactions210 (skeletal reorganization of enynes) which include CC bond cleavage are very useful in organic synthesis. They have been deeply developed and well reviewed. On the other hand, oxidative cleavage represents another class of CC bond activation. There are mainly four kinds of methodologies for CC bond oxidative cleavage according to

which could be converted into the product through protonation. Control experiments show that the hydrogen atom incorporated into the product originated strictly from the solvent MeOH (Scheme 153).201 Lambert et al. developed Pd(PPh3)4-catalyzed deallylation of allylpentakis(p-acetylphenyl)cyclopentadiene with a nucleophile. The substituted cyclopentadienyl moiety is a facile 8648


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Scheme 155

and (4) reactions using transition-metal catalysts, which will be summarized in this review. Additionally, some ring-expansion reactions of CC bonds of aryl cycles catalyzed by transitionmetal catalysts are also reported in this field.

Table 14

3.1. Manganese-Catalyzed Cleavage of CC Double Bonds R1

R2

X

time (h)

yield (%)

SO2Ph SO2Ph SO2Ph SO2Ph P(O)(OEt)2 n-Bu SO2Ph SO2Ph

Me Me Me Me Me Me n-Bu CH2OBn

C(SO2Ph)2 C(MOM)2 CH2 NTs C(CO2Me)2 C(CO2Me)2 C(CO2Me)2 C(CO2Me)2

0.2 2 4 0.2 4 1 3 20

85 85 53 60 69 71 47 40

Jugé and co-workers reported a catalytic CC bond cleavage of various aliphatic or functionalized olefins under an oxygen atmosphere to aldehydes and ketones promoted by thiophenol and related derivatives at room temperature. The CC bond cleavage probably occurred via the decomposition of the βhydroperoxy sulfide catalyzed by manganese salts or corresponding complexes. β-Hydroperoxy sulfide is formed in situ from thiyl radical addition to the double bond, followed by oxygen trapping (Scheme 157).216 Liu and co-workers developed a manganese−porphyrin catalytic system to cleave the CC bond of olefins to produce aldehydes using NaIO4 as the oxidant. Various olefins undergo oxidative cleavage into the corresponding carbonyl compounds in excellent yields. The manganese-based catalyst is easy to prepare and less toxic than traditional OsO4. The authors propose a mechanism in which C−C bond cleavage occurs from 1,2-diol which is formed from oxidation of olefins (Scheme 158).217

Scheme 156

3.2. Ruthenium-Catalyzed Cleavage of CC Double Bonds

Neumann and co-workers found that [cis-Ru(II)(dmp)2(H2O)2]2+ (dmp = 2,9-dimethylphenanthroline) is an appropriate catalyst for alkene CC bond cleavage with hydrogen peroxide to form aldehydes with high regioselectivity.218 Primary alkenes could be oxidized to the corresponding aldehydes efficiently. Secondary alkenes were much less reactive, leading to regioselective oxidation of substrates at the terminal position. Primary allylic alcohols could be oxidized to the corresponding allylic aldehydes. Tabatabaeian and co-workers developed an effective catalytic system using RuCl3·nH2O and NaIO4 for the oxidative cleavage of the CC bond in aromatic compounds to give carbonyl compounds via ultrasonic irradiation at room temperature in short times (Scheme 159).219

the literature: (1) The traditional methods are ozonolysis211 and the Lemieux−Johnson protocol.212 Other methods are (2) nonmetal reactions using equivalent oxidants,213 especially using oxygen as the oxidant,214 (3) photochemical methods,215 Scheme 157

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Scheme 158

Scheme 160

product is the key step for C−C bond cleavage (Scheme 161).222 Scheme 161 Scheme 159

Jiao and co-workers demonstrated Pd(OAc)2-catalyzed ringexpansion reaction of indoles with alkynes, leading to polysubstituted tetrahydroquinoline products with high selectivity (Scheme 162).223 The C−C bond cleavage process in which the five-membered ring intermediate may rearrange into the more stable ring-fused intermediate is proposed. Li and co-workers described a novel PdCl2-catalyzed oxidative cleavage and cyclization of biaryl-2-amines with alkenes into phenanthridines. The NHTs group and NHBn group are appropriate amino-directing groups in this transformation (Scheme 163). Intermediate 189 is formed via Hecktype reaction. It undergoes a cyclization reaction giving

3.3. Palladium-Catalyzed Cleavage of CC Double Bonds

In 2008, Jiang and co-workers observed that 1-methoxy-1,2diphenylethene could be cleaved into methyl benzoate via C C bond cleavage catalyzed by Pd(OAc)2 in the presence of ZnCl2·2H2O under O2 (8 atm) (Scheme 160).220 Recently, they developed a general procedure for the Pd(OAc)2-catalyzed oxidative cleavage of olefins with PTSA (p-toluenesulfonic acid monohydrate) as an additive and O2 as the sole oxidant. Various terminal and internal olefins, especially linear aliphatic alkenes, could be efficiently converted into the corresponding aldehydes or ketones. 1,2-Diol is a key intermediate which could be transformed into the dioxopalladium(II) intermediate, followed by C−C bond cleavage, giving an aldehyde under an oxygen atmosphere.221 Zhang et al. described PdCl2-catalyzed C-3 acylation of indolizines with α,β-unsaturated carboxylic acids via C−H bond and CC double bond cleavage using K2CrO4 as the oxidant. Indolizin-3-ones are obtained in moderate yields. The cleavage of the in situ formed 1,2-diol into the corresponding carbonyl

Scheme 162

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Scheme 163

Scheme 165

Scheme 166

intermediate 193, which could be transformed into phenanthridines via C−C bond cleavage (Scheme 164).224 Scheme 164

3.5. Gold-Catalyzed Cleavage of CC Double Bonds

Shi and co-workers developed a novel method to oxidize olefins into ketones or aldehydes using the AuCl/neocuproine/TBHP system under mild conditions in water. Various functional groups can be tolerated in this transformation. This reaction may proceed via epoxidation and a subsequent oxidation to produce the target products (Scheme 167).227 Scheme 167

3.4. Iron-Catalyzed Cleavage of CC Double Bonds

Pitchumani and co-workers developed CC bond cleavage reactions of various olefins and chalcones catalyzed by an iron− salen complex using hydrogen peroxide as the terminal oxidant in an aqueous medium. The oxidation products are aldehydes and their derivatives (Scheme 165).225 Garcia and co-workers realized aerobic oxidation of styrene to benzaldehyde using NHPI/Fe(BTC) (BTC = 1,3,5benzenetricarboxylate) as the heterogeneous catalyst.226 Electron spin resonance (ESR) spectra support that the PINO radical is generated from the NHPI/Fe(BTC)/O2 system. The PINO radical reacts with the CC bond to generate a new carbon-centered radical which is trapped by molecular oxygen, giving a peroxyl radical. A dioxetane intermediate214a could be formed by intramolecular reaction, and benzaldehyde is obtained by [2 + 2] cycloreversion of aryldioxetane (Scheme 166).226

3.6. Osmium-Catalyzed Cleavage of CC Double Bonds

Borhan and co-workers reported an OsO4-catalyzed oxidative cleavage of olefins into carboxylic acids using Oxone as the cooxidant without the intermediacy of 1,2-diols. The authors proposed the intermediacy of an osmate ester which undergoes the cleavage in this transformation (Scheme 168).228 Goswami and co-workers also introduced a pentacoordinated oxoamidoosmium(VI) complex catalyzed oxidative cleavage of C−C bonds of various alkenes by TBHP giving the corresponding carboxylic acids or ketones at room temperature (Scheme 169).229 8651


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Scheme 168

Scheme 171

Scheme 169 carbonyl products. The catalyst could be recycled without any loss in activity and selectivity.231

4. CARBON−CARBON TRIPLE BOND CLEAVAGE The cleavage of a CC triple bond is very difficult to realize owing to its large bond dissociation energy (>200 kcal mol−1) in organic molecules. Despite the fact that stoichiometric reactions of CC triple bond cleavage have been studied intensively, catalytic reactions are rarely reported. Since alkyne metathesis232 and enyne metathesis (skeletal reorganization of enynes)210 have been well reviewed recently, they will not be included in this review.

3.7. Iridium-Catalyzed Cleavage of CC Double Bonds

Rueping and co-workers developed an [Ir(ppy)2(dtb-bpy)]PF6catalyzed addition/aromatization/C−C bond cleavage cascade reaction of α,β-unsaturated ketones via photoredox reaction giving indole-3-carbaldehyde derivatives (Scheme 170).230

4.1. Rhodium-Catalyzed Cleavage of CC Triple Bonds

Jun and co-workers developed a CC triple bond cleavage reaction of alkyne under a catalytic system consisting of Rh(PPh3 ) 3Cl/PhCOOH/allylamine derivative/cyclohexylamine. This tandem reaction includes the following steps: hydroiminoacylation of alkyne with an allylamine derivative by the Rh(I) catalyst, a conjugate addition of cyclohexylamine into the resulting α,β-unsaturated ketimine, and a retro-Mannichtype fragmentation giving the corresponding aldimine and ketimine (Scheme 172).233 When an aldehyde and 2-amino-3picoline were used instead of the allylamine derivative in this transformation, a similar result could be obtained.234 Tanaka and co-workers described the formation of laddertype molecules via a [Rh(cod)2]BF4-catalyzed [2 + 1 + 2 + 1] cycloaddition involving CC triple bond cleavage from triyne compounds. To release the steric hindrance, the rhodacyclopentadiene 195 may be converted to rhodacyclopentadiene 197, which leads to the ladder-type molecules 198 (Scheme 173).235

Scheme 170

4.2. Ruthenium-Catalyzed Cleavage of CC Triple Bonds

Retro-Mannich fragmentation is a very powerful method for cleaving α,β-unsaturated ketimine. Yamamoto and co-workers reported Ru3(CO)12 or Pd(NO3)2 catalyzed the reaction of diynes with o-aminophenols, giving the corresponding 2substituted benzoxazoles, along with ketones after hydrolysis. This transformation includes CC triple bond cleavage via retro-Mannich fragmentation (Scheme 174).236 Liu and co-workers utilized an efficient TpRu(PPh3)(CH3CN)2PF6-catalyzed cleavage of the CC triple bond of propargyl alcohols via an allenylidene intermediate into alkene and carbon monoxide, the carbon atom of which is derived from the terminal carbon of the alkyne moiety (Scheme 175).237 The authors proposed a plausible mechanism for this reaction. The proton migration of ruthenium π-alkyne complex 199 affords ruthenium η1-alkynyl hydride species 200. Then

Control experiments indicate that the aldehyde oxygen originates from the superoxide anion radical, O2•−, generated by the reaction of dissolved oxygen with Ir(II). The cyclic peroxide intermediate is generated from an α-radical carbonyl intermediate. It undergoes cleavage of the C−C bond to produce indole-3-carbaldehyde (Scheme 171).230 3.8. Cerium-Catalyzed Cleavage of CC Double Bonds

Pitchumani and co-workers reported a novel reaction in which CAN-supported K10-montmorillonite clay catalyzed selective oxidation of olefins and chalcones and their derivatives to the corresponding carbonyl compounds. CeIV ion is effective for the selective molecular oxygen oxidation of the CC double bond giving site-specific dioxetane, which could be cleaved into 8652


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Scheme 172

Scheme 173

Scheme 174

formed via decarbonylation. Species 203 undergoes reductive elimination to regenerate the active ruthenium catalyst and release an alkene and carbon monoxide (Scheme 176).237 Propargyl ethers could also be converted into the corresponding ketones, ethylene, carbon monoxide, and hydrogen via an allenylidene intermediate in the presence of TpRuPPh3(CH3CN)2PF6. The mechanism is similar to that previously introduced (Scheme 177).238 Saá and co-workers realized the intermolecular synthesis of cycloalkenes from terminal alkynals catalyzed by [CpRu(CH3CN)3]PF6 (Scheme 178). Vinyl Ru species 205 is

Scheme 175

species 200 undergoes ionization to form ruthenium allenylidenium species 201 assisted by LiOTf. The Cα carbon of species 201 is attacked by LiOH to form ruthenium acyl species 202. Subsequent vinylruthenium hydride species 203 is 8653


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Scheme 176

The intermediate 208 is formed by hydroalkoxylation catalyzed by Pd(OAc)2, which is subsequently attacked by activated molecular oxygen to generate a cyclic peroxide intermediate, 209. The C−C bond of intermediate 209 is cleaved to give an ester and aldehyde. The aldehyde undergoes further oxidation and esterification into another ester (Scheme 181).

Scheme 177

Scheme 178

Scheme 181

afforded by nucleophilic addition to ruthenium(II) vinylidene species 204. Aldol-type condensation would then give the acylruthenium hydride 206. The terminal carbon of the alkyne is lost via decarbonylation followed by reductive elimination to afford the corresponding cycloalkenes (Scheme 179).239 7Unsubstituted 1,6-diynes could also undergo a similar reaction under the same conditions, giving exo-alkylidenecyclopentanes (Scheme 180).240

4.4. Gold-Catalyzed Cleavage of CC Triple Bonds

Liu and co-workers developed a new method to cleave CC triple bonds in (Z)-enynols using AuCl(PPh3) as the catalyst with molecular oxygen.241 Furan-2(5H)-one derivatives could be produced in good yields. This transformation includes gold(I)-catalyzed cyclization and oxidative cleavage of a dihydrofuran intermediate into butenolides. Control experiments showed that a radical species is involved, and oxidative cleavage of the CC double bond to carbonyl compounds is an important step in this transformation (Scheme 182).241 The AuCl(PPh3)-catalyzed oxidative transformation of 3aryl-3-alkoxy-1-alkynes to alkyl aryloates using molecular oxygen was developed by Liu’s group.242 C−H, C−C, and CC bonds are cleaved simultaneously in this case (Scheme

Scheme 179

Scheme 182

Scheme 180

4.3. Palladium-Catalyzed Cleavage of CC Triple Bonds

Jiang and co-workers demonstrated a ZnCl2·2H2O-promoted and Pd(OAc)2-catalyzed cleavage reaction of CC triple bonds of alkynes to give carboxylic esters using molecular oxygen as the sole oxidant and various alcohols as solvents.220 8654


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183). Control experiments reveal that gold-containing enol ethers are the active species for the activation of oxygen (Scheme 184).242

Scheme 185

Scheme 183

4.5. Copper-Catalyzed Cleavage of CC Triple Bonds

Asao and Yamamoto reported an unprecedented [4 + 2] benzannulation from o-alkynylbenzaldehydes or enynals with alkynes catalyzed by Cu(OTf)2 in the presence of Brønsted acid giving debenzoylated naphthalenes or benzenes (Scheme 185).243 Intermediate 214 could be obtained by protonolysis of the C−Cu bond of intermediate 213 with a Brønsted acid (HA), followed by the attack of the anion on the carbon of RCO. Then intermediate 214 undergoes the retro-Diels−Alder reaction, giving 215 and 216 (Scheme 186).243

Scheme 186

4.6. Silver-Catalyzed Cleavage of CC Triple Bonds

Jiao and co-workers demonstrated Ag2CO3-catalyzed nitrogenation of alkynes into nitriles through CC bond cleavage. Both aryl and aliphatic alkynes could be converted into the corresponding nitriles directly (Scheme 187). Vinyl azide 219 generated from alkyne catalyzed by silver is the key intermediate in this transformation. Vinyl azide 219 cyclizes with azide to form the unstable intermediate 220, which undergoes a fast rearrangement process, giving the nitrile. HN3 and CH2N2 are released and could be detected (Scheme 188).244 Soon afterward, Yanada and co-workers also reported a direct transformation of alkynes to nitriles via CC triple bond cleavage reaction with NIS and TMSN3 in the absence of a metal catalyst.245

Scheme 187

5. SUMMARY AND OUTLOOK Selective cleavage of the carbon−carbon bond catalyzed by transition-metal complexes is a useful protocol in organic synthesis. It could provide new synthetic methods which are difficult to achieve via traditional methods, although many challenges still remain for chemists. In this review, we introduced some recent advances in transition-metal complex catalyzed cleavage of unstrained carbon−carbon bonds, including C−C single bonds, CC double bonds, and CC triple bonds. Many examples of unstrained C−C single bond cleavage catalyzed by various transition-metal complexes have been reported. Generally, functional groups adjacent to these bonds

are necessary for these transformations. For instance, C−C single bonds such as C−CN, C−C(O)H, C−C(O)C, C− C(O)Cl, C−C(O)NR, C−C(O)SR, C−C−OR, etc. are usually chosen for C−C single bond cleavage reactions. Simple C−C

Scheme 184

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Biographies

Scheme 188

Feng Chen was born in Jiangsu, People’s Republic of China, in 1985. He received his B.S. and M.S. degrees in 2007 and 2010, respectively, from Yangzhou University. In 2013, he received his Ph.D. degree from Peking University under the supervision of Prof. Ning Jiao. Currently, he is a postdoctoral fellow in Matthias Beller’s group at the Leibniz Institute for Catalysis (Germany). His research interests include activations of inert bonds, redox reactions, and heterogeneous catalysis.

bonds without functional groups adjacent to them are still rarely developed as their development requires more attention and effort. In the developed methods, oxidative addition, βcarbon elimination, decarbonylation, and retroallyation are common mechanisms for transition-metal complex catalyzed C−C single bond functionalization. Nevertheless, more quantitative kinetic data and detailed mechanisms for these transformations are still needed. Olefin metathesis and enyne metathesis reactions are very important methods for CC double bond cleavage, and they have been well developed. Additionally, oxidative cleavage is another class of CC bond activation process. Many transition-metal complexes have been developed for this kind of reaction using appropriate oxidants, especially molecular oxygen. Exploration of methodologies which could avoid the use of toxic metals and stoichiometric amounts of traditional oxidants with a broad range of substrates is still required. Alkyne metathesis and enyne metathesis reactions are common methods for CC triple bond cleavage. In addition, tandem reactions including initial transformation of the CC triple bond into a CC double bond and the subsequent fragmentation reaction via CC double bond cleavage are also reliable. Transition-metal catalysts such as Rh, Ru, Pd, Au, Cu, and Ag have been developed to catalyze CC triple bond cleavage and functionalization. Furthermore, new and concise reaction catalytic systems and detailed mechanistic studies for CC triple bond cleavage should be further developed. Finally, as discussed in this review, various mechanisms are proposed for cleavage of carbon−carbon bonds. The further development of new carbon−carbon bond cleavage reactions in this field will be expected to explore new catalytic systems with inexpensive transition-metal catalysts, under mild conditions, with green and sustainable oxidants, and with a wide substrate scope. The application of these methods in organic synthesis is also desirable. Moreover, detailed mechanisms of these reactions should be investigated, which would be useful for the design of new catalytic reaction systems. We hope this review will stimulate the advancement of studies in this fascinating and nascent field.

Teng Wang was born in 1984 in Beijing, China. He received his Ph.D. degree (2013) from Tianjin University with Prof. Jun-An Ma. Since 2010, he has been an exchange student in Prof. Ning Jiao’s group in the State Key Laboratory of Natural and Biomimetic Drugs, Peking University. Currently, he is a postdoctoral fellow in Thomas Hoye’s group (department of chemistry, University of Minnesota Twin Cities). His research interests include developing nitrogenation strategies via inert bond activations.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 8656


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TMDSO tetramethyldisiloxane TMS trimethylsilyl

Ning Jiao received his Ph.D. degree (2004, with Prof. Shengming Ma) at the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences. He spent 2004−2006 as an Alexander von Humboldt postdoctoral fellow with Prof. Manfred T. Reetz at the Max Planck Institute für Kohlenforschung. In 2007, he joined the faculty at Peking University as an Associate Professor and was promoted to Full Professor in 2010. His current research efforts are focused on (1) the development of green and efficient synthetic methodologies through the single electron transfer (SET) process, (2) aerobic oxidation, oxygenation, and nitrogenation reactions, and (3) the activation of inert chemical bonds.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grants 21325206 and 21172006), National Young Top-notch Talent Support Program, and Ph.D. Programs Foundation of the Ministry of Education of China (Grant 20120001110013) is greatly appreciated. ABBREVIATIONS 9-PhBBN 9-Phenyl-9-borabicyclo[3.3.1]nonane Ac acetyl acac acetylacetonyl Ar aryl (substituted aromatic ring) BINAP 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl Bn benzyl Boc t-butoxycarbonyl bpy 2,2′-bipyridine Cbz benzyloxycarbonyl cod 1,5-cyclooctadiene Cp cyclopentadienyl Cy cyclohexyl DABCO 1,4-diazabicyclo[2.2.2]octane dba dibenzylideneacetone DCE 1,1-dichloroethane DDQ 2,3-dichloro-5,6-dicyano-1,4-benzoquinone DIPEA diisopropylethylamine DME 1,2-dimethoxyethane DMF N,N-dimethylformamide DMPU N,N-dimethyl propylene urea DMSO dimethylsulfoxide dppBz 1,2-bis(diphenylphosphino)benzene dppe 1,2-bis(diphenylphosphino)ethane dppf 1,1′-bis(diphenylphosphino)ferrocene dppp 1,3-bis(diphenylphosphino)propane ee enantiomeric excess Et ethyl p-Anis 4-methoxyphenyl Phen 1,10-phenanthroline Piv pivaloyl ppy 2-phenylpyridine Pr propyl PTSA p-toluenesulfonic acid Py pyridine r.t. room temperature TBHP tert-butyl hydroperoxide TBN tert-butyl nitrite TBP tert-butyl peroxide TBSO 3-tert-butyldimethylsilyloxy TEMPO 2,2,6,6-tetramethyl-1-piperidinyloxy free radical Tf trifluoromethanesulfonyl THF tetrahydrofuran 8657


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