Catalytic CâC Bond Activations via Oxidative Addition to Transition

Review pubs.acs.org/CR

Catalytic C−C Bond Activations via Oxidative Addition to Transition Metals Laetitia Souillart and Nicolai Cramer* Laboratory of Asymmetric Catalysis and Synthesis, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne 1015, Switzerland ment of such selective catalytic C−C bond activations has made significant progress over the last three decades. However, the development of C−C σ-bond activations1−10 lags behind the more advanced field of selective C−H bond activations.11−19 This difference can be attributed to two main factors: (1) the CONTENTS higher inertness of C−C bonds, which are more encumbered 1. Introduction A and much less abundant than C−H bonds, and (2) the highly 2. Strain-Driven C−C Bond Activations B directed nature of C−C bonds, which have a less favorable 2.1. C−C Bond Activations of Three-Membered orbital directionality for an interaction with transition-metals Rings B than C−H bonds. Transition-metal-catalyzed C−C bond 2.1.1. C−C Bond Activations of Cyclopropanes B activations can be divided into three mechanistic categories 2.1.2. C−C Bond Activations of Alkylidenecy(Scheme 1). The C−C bond activation via direct oxidative clopropanes E addition (path A) is the reverse reaction pathway of the C−C 2.1.3. C−C Bond Activations of Vinylcycloprobond formation via reductive elimination. In many instances, panes J the reductive elimination is thermodynamically favored, as a 2.1.4. C−C Bond Activation of Cyclopropenes V relatively stable C−C bond in 1 (ca. 90 kcal/mol) is formed at 2.2. C−C Bond Activations of Four-Membered the expense of two weaker C−M bonds in 2 (ca. 30 kcal/ Rings W mol).20 A second main pathway is the β-carbon elimination 2.2.1. C−C Bond Activations of Biphenylenes X (path B). This pathway can be seen as the carbon analogue of 2.2.2. C−C Bond Activations of Cyclobutenethe common β-hydride elimination reaction of organometallics. diones and Benzocyclobutenediones Z Often, the formation of a strong CX bond (X = O) in 5 2.2.3. C−C Bond Activations of Cyclobutecontributes to the driving force of this process. In the case of nones and Benzocyclobutenones AA strained cyclic substrates, a ring-strain release facilitates the C− 2.2.4. C−C Bond Activations of CyclobutaC bond cleavage additionally. For acyclic substrates, the nones AD entropy increase generated by the formation of a stable 3. C−C Bond Activations of Unstrained Substrates AH byproduct participates to the success of the reaction. The third, 3.1. C−C Bond Activations Assisted by Chelation AH Scheme 1 3.1.1. Permanent Directing Groups AH 3.1.2. Temporary Directing Groups 3.2. Decarbonylation 3.3. C−CN Bond Activation 3.3.1. Carbocyanation of Alkynes 3.3.2. Carbocyanation of Alkenes 3.3.3. Coupling Reactions via C−CN Bond Activations 4. Summary and Outlook Author Information Corresponding Author Author Contributions Notes Biographies Acknowledgments References

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1. INTRODUCTION The cleavage and reorganization of carbon−carbon bonds is an industrial key technology for crude oil refining, carried out on a massive scale. In contrast, C−C bond cleavage reactions for fine chemical production catalyzed by homogeneous transitionmetal complexes are much rarer. Nevertheless, the develop© XXXX American Chemical Society

Special Issue: 2015 Frontiers in Organic Synthesis Received: March 6, 2015

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DOI: 10.1021/acs.chemrev.5b00138 Chem. Rev. XXXX, XXX, XXX−XXX

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Figure 1.

Retro-allylation (path C), proceeds via a six-membered transition state, and generates an allyl metal species 7. Transition-metal-mediated C−C bond activations were at first limited to the synthesis of organometallic complexes or stoichiometric transformations. This review focus exclusively on catalytic activations proceeding via oxidative addition to transition-metals (path A) which were reported in the literature from 2004 to the end of 2014. Transformations proceeding via β-carbon elimination (paths B and C) have been recently covered3,21,22 and fall out of the scope of this review. Both retroallylation and decarboxylative couplings consist of a specific case of β-carbon elimination and excellent recent reviews can be found.23−26 The review is organized according to the substrate classes involved in the C−C bond cleaving transformations. In the first part, strain-driven C−C bond activations of small ring systems such as cyclopropane and cyclobutane derivatives will be discussed. Due to the abovementioned thermodynamic and kinetic challenges, exploitation of the strain release provided by the C−C bond cleavage of small rings is one of the most important driving force to promote C−C bond activation. In the second part the cleavages of C−C bonds of unstrained molecules are reviewed. This includes chelation-assisted reactions, C−CN bond activations, and decarbonylation processes.

Scheme 2

pathways which will be discussed in the following chapter. Despite the energetically favorable release of ring-strain, an unsaturated tether further facilitates the activation of the cyclopropane and directs the metal toward the cleavable C−C bond. In this context, cyclopropyl ketones or imines 10, alkylidene cyclopropanes (ACPs) 11, vinylcyclopropanes (VCPs) 12, and cyclopropenes 13 have been the most versatile substrates. Besides their facilitating and directing effect on the C−C bond cleavage, the unsaturated tether can also participate in the postactivation transformation, thus broadening the range of reactions. 2.1.1. C−C Bond Activations of Cyclopropanes. The release of the ring-strain of cyclopropane (29 kcal/mol) facilitates the C−C bond cleavage. The isolation of the first metallacyclobutane intermediate resulting from the oxidative addition to a transition-metal complex (hexachloroplatinic acid) into the C−C bond of cyclopropane 8 dates from a report by Tipper in 1955 (Scheme 3a).29 The structure of platinacyclobutane complex 14 was elucidated by Chatt.30 The synthetic utility of this complex still remains limited, mainly because of the facile β-hydride elimination from the metallacyclobutane intermediate leading to 15. For substituted cyclopropanes, the three C−C bonds are not equivalent anymore. For a monosubstituted cyclopropane there are two possible reaction sites for the C−C bond cleavage (Scheme 3b). Generally, the oxidative addition of substituted cyclopropane 16 to transitionmetals results in the formation of metallacyclobutane 17 as a result of the cleavage of the more accessible distal C−C bond (path A). The cleavage of the more hindered proximal C−C bond, resulting in the formation of 18, requires most often the presence of a chelating group to direct the metal toward this bond (path B). A report by Chirik illustrates how the substitution on the cyclopropyl controls the regioselectivity of the C−C cleavage.31 Besides altering the regioselectivity, chelation assistance was also found to facilitate the oxidative addition, resulting in much faster transformations. For instance, Murakami reported a rhodium-catalyzed C−C bond activation of spiropentanes 19 to access cyclopentenones 23 (Scheme 4).32 Oxidative addition of the distal C4−C5 bond

2. STRAIN-DRIVEN C−C BOND ACTIVATIONS Small ring systems, in particular three- and four-membered rings play an important role in C−C bond activations as the release of ring strain facilitates the metal insertion (Figure 1). The strain energy of cyclopropane (29.0 kcal/mol) is similar to that of cyclobutane (26.3 kcal/mol) and much higher than the one of cyclopentane (7.4 kcal/mol).27 The ring strain does not only depend on the ring size but also on the substitution pattern. For instance, disubstituted cyclobutanes are stabilized by the Thorpe−Ingold effect and have a reduced strain compared to the parent cyclobutane. Exploitation of ring strain as enabling driving force for chemical transformations that otherwise do not proceed is a versatile and well appreciated synthetic tool. 2.1. C−C Bond Activations of Three-Membered Rings

Saturated three-membered rings are the cycloalkanes that exhibit the highest ring strain, thus rendering them appealing substrates for C−C bond cleaving processes. Since the development of transition-metal chemistry, great advances have been made in the C−C bond activation of cyclopropane derivatives leading to new types of ring-opening and cycloaddition reactions.28 The C−C bond activation of cyclopropane derivatives have also been implemented in the total synthesis of natural products. Generally, the strain-driven oxidative addition of the C−C bond of cyclopropane 8 to a transition-metal leads to the formation of metallacyclobutane 9 (Scheme 2). This intermediate was shown to open the way to different reaction B


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Scheme 3

obtained in good yields and the process was applied to a synthesis of (±)-β-cuparenone. Chung reported a rhodium(I)-catalyzed C−C bond activation of bis-cyclopropanes 24 (Scheme 5).33 This carbonylative [3 + 3+1]-cycloaddition gives access to 6,7-fused ring systems 28 in moderate to excellent yields of up to 98%. Precoordination of the rhodium(I)-catalyst to the double bond directs the metal toward the fused cyclopropane unit. Rhodacyclobutane derivative 25 is formed through oxidative addition. A subsequent β-carbon elimination of the second cyclopropane unit produces metallacycle 26. CO insertion into either of the two carbon−rhodium bonds gives 27 or 27′ and following reductive elimination provides the [3 + 3+1]cycloadduct 28. This rhodium-catalyzed [3 + 3+1]-cycloaddition was also applied to other biscyclopropanes such as 7cyclopropylbicyclo[4.1.0]hept-2-ene derivatives. Cyclopropyl ketones were shown as suitable substrate class for chelation-assisted C−C bond cleavage of cyclopropanes. For instance, Ogoshi34,35 and Montgomery36 independently reported nickel(0)-catalyzed dimerization of cyclopropylketones 29 leading to functionalized cyclopentanes 35 (Scheme 6a and 6b). Oxidative addition of the proximal C−C bond of cyclopropane to the nickel(0)-catalyst generates nickeladihydropyran 30. Subsequent β-hydride elimination and reductive elimination followed by tautomerization provides η2-coordinated enone-nickel complex 32. A second oxidative addition of another cyclopropyl ketone substrate yields nickeladihydropyran intermediate 33. Migratory insertion of the enone and

of spiropentane 19 to rhodium(I) forms spirocyclic rhoda(III)cyclobutane 20. Insertion of carbon monoxide generates Scheme 4

rhodacyclopentanone 21, and subsequent β-carbon elimination affords the six-membered rhodacycle 22. Reductive elimination delivers cyclopentanone bearing an exo-double bond, which isomerizes under the reaction conditions to cyclopentenone 23. Paraformaldehyde could be used instead of carbon monoxide as a convenient carbonyl source. The cyclopentanones were Scheme 5

C


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Scheme 6

Scheme 7

The catalytic cycle is initiated by oxidative addition of the proximal C−C bond of cyclopropyl ketone 39 to nickel(0) leading to suggested intermediate 41. Regioselective migratory insertion of alkyne 40, controlled by the differences in sterics and electronics of substituents R4 and R5, affords nickelacyclohexene species 42. Reductive elimination provides cyclopentene 43 and regenerates the catalyst. Montgomery extended the scope of the reaction of cyclopropyl ketones to the [3 + 2]-cycloadditions of cyclopropyl aldimines 44 (Scheme 8).39 The addition of titanium tert-butoxide increased the yields as well as shortened the Scheme 8

subsequent reductive elimination delivers cyclopentane 35. An additional substitution on the cyclopropyl ring results in a sluggish reaction and gives tetra-substituted cyclopentanes in low yields (24%). To overcome this, an improved process involving a crossed reaction between cyclopropyl ketones 36 and enones 37 allows for high substrate flexibility as well as increased molecular complexity of the products (Scheme 6c).36 Cycloadditions of cyclopropanes and alkynes as a two-carbon partners are known and for instance, Narasaka reported carbonylative rhodium-catalyzed [3 + 2+1]-cycloadditions of cyclopropanes with tethered alkynes.37 In extension of this previous report, Ogoshi disclosed an intermolecular nickelcatalyzed [3 + 2]-cycloaddition of cyclopropyl ketones 39 with alkynes 40 yielding cyclopentanes 43 (Scheme 7).38 The reaction did not proceed in the absence of the Lewis-acidic organoaluminum cocatalyst, which is believed to activate the substrate and stabilize the intermediates. A broad range of symmetrical, unsymmetrical internal and terminal alkynes are tolerated in this process. Moreover, a variety of substituted cyclopropyl ketones can be used, affording the cyclopentenes in moderate to good yields. The reaction of disubstituted cyclopropanes (R2 or R3 ≠ H) is more challenging and required the use of stoichiometric amounts of the cocatalyst.

reaction times. The scope exceeds that of the corresponding cycloaddition of cyclopropyl ketones and enones. Mechanistically, oxidative addition of cyclopropyl imine 44 to nickel(0) affords metalloenamine 46. Michael addition of 46 to enone 45 and reductive elimination gives cyclopentyl imine 48, which undergoes hydrolysis upon workup to deliver trans-cyclopentane 49. Tang and Shi showed a rhodium-catalyzed carbonylative synthesis of pyrrole derivatives from 3-alkynyl cyclopropyl imines 50 (Scheme 9).40 Oxidative addition of the proximal C− D


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alkyne moiety required the use of a urea directing group in order to achieve selective oxidative addition of the more hindered proximal C−C bond of the aminocyclopropane to the catalyst. Subsequent CO insertion resulted in the formation of rhoda(III)cyclopentanone 57. Migratory insertion of the alkyne followed by reductive elimination delivers products 59. This reactivity could be extended to the trapping of the rhodapentanone intermediates with tethered alkenes (Scheme 11).44 The weaker coordination of alkenes compared to alkynes allowed replacing the urea directing group by a carbamate in

C bond of cyclopropane 50 to rhodium(I)-catalyst forms rhodacyclobutane intermediate 51. Intramolecular nucleophilic Scheme 9

Scheme 11

attack of the imine on the activated alkyne and subsequent rearrangement generates pyrrole intermediate 52. Carbonylation of 52 yields acyl rhodium species 53, which undergoes reductive elimination to deliver 3-azabicyclo[3.2.0]hepta-1,4diene 54. Both aldimine and ketimine substituted cyclopropane substrates afford the pyrrole products in moderate to good yields. This transformation can be extended to other less strained cyclic or even acyclic substrates, such as cyclobutane alkynyl imines. Moreover, cyclopropyl imines were also found to be a versatile 5-atom synthon in the ruthenium-catalyzed carbonylative [5 + 1]-cycloaddition affording unsaturated sixmembered lactams.41 Rhodium-catalyzed hetero-[5 + 2]cycloadditions of cyclopropyl imines and alkynes yielding dihydroazepines were disclosed by Wender.42 Ring-expansions of heteroatom-substituted-cyclopropanes were exploited by Bower. In 2013, he reported the intramolecular rhodium(I)-catalyzed carbonylative [3 + 2+1]cycloadditions of amino-cyclopropane substrates 55 with alkynes giving rise to bicyclic products 59 (Scheme 10).43 A competitive coordination of the rhodium(I)-catalyst to the

cyclopropane 60. High levels of stereocontrol were achieved and the trans-fused-bicyclic products 61 are obtained in moderate to good yields. 2.1.2. C−C Bond Activations of Alkylidenecyclopropanes. Among the three-membered ring substrates utilized in transition-metal catalyzed C−C bond activations, alkylidenecyclopropanes (ACPs) and methylenecyclopropanes (MCPs) exhibit a higher strain energy than simple cyclopropanes (38.8 kcal/mol). Due to this increased ring strain and the presence of double bond able to additionally coordinate to the transitionmetal, ACPs and MCPs are more reactive toward C−C bond cleavages. In addition, the presence of the π-unsaturation allows for a broad variety of postactivation processes. Therefore, these substrates have been shown versatile for cycloaddition reactions28 and are depicted in the coming section. The reaction of MCPs (R = H) or ACPs (R ≠ H) 62 with transition-metal catalysts can occur via two different reaction pathways (Scheme 12). Direct oxidative addition of the distal (C1−C2) bond gives metallacyclobutane 63 to the metal atom, and addition into the proximal (C2−C3) bond provides intermediate 64 (path A). Nickel(0) catalysts such as [Ni(cod)2] were shown to favor the proximal ring-opening, whereas cleavage of the distal bond is preferred in the presence of phosphine or phosphite ligands.45 When the exo-methylene part of 62 reacts with an organometallic species, a carbometalation can form two regioisomeric products (path B). Both intermediates can further undergo a ring-opening reaction proceeding via β-carbon elimination. The Markovnikov adduct 65 gives allyl-metal species 66, whereas the antiMarkovnikov intermediate 67 forms homoallyl-metal intermediate 68. Reactivity control in the reactions of MCPs and ACPs is therefore critical to achieve chemoselective transformations. The reactions of ACPs following the addition/βcarbon elimination path B were recently reviewed45,46 and are beyond the scope of this review. ACPs are mostly used as three-carbon synthons for [3 + 2]cycloadditions with a variety of two-carbon π-systems. In 1970, pioneering work on nickel-catalyzed [3 + 2]-cycloadditions of ACPs with olefins was reported by Noyori.47,48 Since then, reports by Binger, Trost, and others were disclosed.49 In 2004,

Scheme 10

E


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Scheme 12

Scheme 13

Mascareñ as developed ruthenium-50 and palladium-catalyzed51,52 intramolecular [3 + 2]-cycloadditions of ACPs 69 with alkynes leading to bicyclo[3.3.0]octenes 73 (Scheme 13a). DFT studies were undertaken by Mascareñas and Cárdenas on the mechanism of the palladium-catalyzed reaction.53 They found that the most favorable reaction pathway involves an initial oxidative addition of the ACP distal C−C bond of 69 to the palladium(0) complex giving alkylidene-palladacyclobutane 70. A subsequent isomerization step leads to methylenepalladacyclobutane 71. Alkyne carbometalation generates palladacycle 72, and subsequent reductive elimination affords the bicyclo[3.3.0]octenes 73. Mascareñas further extended the scope of the [3 + 2]-cycloadditions by replacing the alkyne tether with alkenes54,55 and allenes56 leading to 5,5-fused rings

75 and 77 in high diastereoselectivities (Scheme 13b,c). Interestingly, both (Z)- and (E)-substrates 74 gave the cisproduct 75. Palladium-catalyzed intramolecular [3 + 2+2]-cycloadditions between an ACPs portion (the three-carbon unit) and tethered alkynes and alkenes (or a terminal alkyne) were developed by Mascareñas.57 The transformation of ACPs 78 provided access to 5,7,5 tricyclic skeletons 81 with moderate to excellent chemo- and diastereoselectivities (Scheme 14). Similarly to their previous report (see the formation of 72 in Scheme 13), oxidative addition and subsequent carbopalladation delivers intermediate 79. The introduction of an electron-withdrawing group at the alkene unit resulted in an improved chemoselectivity for the [3 + 2+2]-cycloaddition over the competing F


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found that, in the case of the rhodium catalyst, direct reductive elimination from 84 are energetically more demanding (31.4 and 43.3 kcal/mol). This is in agreement with the absence of [3 + 2]-cycloadducts. In contrast, for the palladium catalyst, reductive elimination and carbopalladation reaction are competitive (Scheme 14). Related improvements to intermolecular rhodium(I)-catalyzed [3 + 2+2]-cycloadditions were reported by Evans. Regioand diastereoselective intermolecular rhodium-catalyzed [3 + 2+2]-carbocyclization reactions of ACPs 87 with activated alkynes for the construction of the bicycloheptadienes 92 were reported (Scheme 16a). 59 The proposed mechanism is supported by the isolation and characterization of rhodacyclic

Scheme 14

Scheme 16

[3 + 2]-cycloaddition resulting of a direct reductive elimination from 79 to 82. The authors proposed that increasing the alkene ability to coordinate the metal favors the second carbopalladation from 79. The use of an unsubstituted hydrocarbon linker between the alkene and the alkyne units was prejudicial to the reactivity (16% yield). However, a fully carbon-based 5,7,5tricyclic structure could be obtained by introducing two malonate linking units. Interestingly, Mascareñas reported that the rhodium(I)catalyzed C−C bond activation of ACPs 83 is totally chemoselective for the [3 + 2+2]-cycloaddition leading to the formation of 5,7,5-fused tricyclic systems 86 (Scheme 15).58 Moreover, in contrast to the palladium catalyzed version, the rhodium-catalyzed process allows for the use of nonactivated di- and trisubstituted alkenes. DFT calculations were performed in order to account for the differences in reactivities. It was Scheme 15

intermediates.60 Carbon- and heteroatom-tethered ACPs 87 bearing mono- or disubstituted alkynes are tolerated in the reation. (E)-Olefins do react as well and afford the 5,7-cis-fused system 92 with regioselectivities of up to 19:1. However, the level of regiocontrol is significantly influenced by both the nature of the ACP and the alkyne substitution pattern. Moreover, only propargylic ketones are tolerated. To address this shortcoming, it was shown that the introduction of a triethoxysilyl group on ACP 93 improves the regioselective insertion of activated as well as unactivated alkynes with excellent regio- and diastereoselectivities (Scheme 16b).61 The utility of this process was further demonstrated by an application to the construction of the core structure of the sesquiterpene pyrovellerolactone (100) (Scheme 17).62 The same group extended the reactivity of ACPs to the [3 + 2+2]-cycloaddition with substituted allenes (Scheme 18).63 The reaction proceeds according to a similar mechanism as described above. Oxidative addition of ACP 101 to the G


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Scheme 17

Scheme 18

Scheme 19

rhodium catalyst gives rhodacyclobutane 103. Subsequent carborhodation of the tethered alkyne generates intermediate 104. Distal insertion of 1,1-disubstituted allene 102 forms rhodacycle 105 which undergoes reductive elimination delivering bicycloheptatriene 106. The process is tolerant to ACP substitution and different functional groups on the allene. In addition, the reaction permits the formation of 5,7- and 6,7bicyclic structures in high regioselectivities. Evans further disclosed a highly stereoselective rhodiumcatalyzed [3 + 2+1]-carbocyclization of ACPs 107 with carbon monoxide providing cis-fused bicyclohexenones 111 (Scheme 19).64 Mechanistically, the Rh−CO complex undergoes oxidative addition of the distal C−C bond of ACP 107 affording rhoda(III)cyclobutane 108. Subsequent carbometalation of the olefin tether gives cis-fused rhodacyclohexane 109. Migratory insertion of carbon monoxide leads to acyl-rhodium intermediate 110. Subsequent reductive elimination followed by double bond isomerization into conjugation delivers cyclohexenones 111. The authors demonstrated the potential for an enantioselective version using Foxap (L1) as the steering ligand. Under these conditions, product 111 was obtained in 75% yield and 89% ee. Mascareñ as reported a diastereoselective intramolecular palladium-catalyzed [4 + 3]-cycloaddition of ACPs 112 having a tethered electron-poor diene to access 5,7-fused bicyclic products 116 (Scheme 20).65 The catalytic cycle is initiated by oxidative insertion of the palladium(0)-catalyst into the distal C−C bond of ACP 112 to give palladacyclobutane 113. Palladacyclohexane 114 is then formed through carbopalladation. Even though the formation a five-membered ring by a direct reductive elimination is possible, a π-allylic rearrangement forming palladacyclooctane intermediate 115 is favored. Finally, reductive elimination delivers cis-fused bicyclic products 116 in moderate to good yields. The potential for the

Scheme 20

development of an asymmetric variant was demonstrated with phosphoramidite ligand L3. Under these conditions, products 116 are formed as single diastereoisomers in moderate enantioselectivities of up to 64% ee. H


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elation leading to 120, followed by migratory insertion of the alkene and reductive elimination from 121 afford bicyclic scaffold 122. The presence of an electron-withdrawing group R2 on the alkene is required for reactivity. Nonactivated alkenes, such as styrene or methyl-5-hexenoate, were found to be unreactive. In order to address the low reactivity of nonactivated alkenes, Mascareñas reported an intramolecular nickel(0)-catalyzed [3 + 2+2]-cycloaddition of ACPs 123 (Scheme 22a).67 The reaction mechanism is similar to the previous one. Initial oxidative addition of ACP to nickel(0) forms nickelacyclobutane intermediate 124. Subsequent migratory insertion of the alkyne leads to 125. Following migratory insertion of the tethered alkene and reductive elimination from 126 afford 6,7,5-tricyclic scaffold 127. The reaction is completely diastereoselective and retains the trans-stereochemistry of the alkene tether in 123. Besides alkenyl esters, other acceptors were tolerated (Scheme 22b). Electronically unactivated alkenes of allenes fail to undergo the reaction, but internal and terminal alkynes were found to react at room temperature in the presence of NHC ligands. Zhang disclosed a nickel-catalyzed intramolecular [3 + 2]cycloaddition of ACPs tethered to an aryl-alkynes leading to the formation of cyclopenta[a]indene derivatives 131 (Scheme 23).68 Directed by the alkyne, an oxidative addition of the proximal C−C σ-bond of ACP 128 to the nickel(0)-catalyst gives nickelacyclobutane species 129. Subsequent carbonickelation of the alkyne generates intermediate 130, which undergoes reductive elimination to deliver cyclopenta[a]indene 131. According to previous studies,66 the reductive elimination of intermediate 130 is energetically not favored. The authors propose that the formation of a larger conjugated system for this substrate class to facilitate reductive elimination. Substitution on the aromatic ring or at the ACP group R is possible

In contrast to the shown reports of distal bond activations, less examples of C−C bond cleavage via oxidative addition of the proximal bond of ACPs to the transition-metal are known. Mascareñas showed a nickel-catalyzed [3 + 2+2]-cycloaddition of alkyne-containing ACPs 117 with alkenes yielding 6,7-fused bicyclic systems 122 as a result of the formal cleavage of the ACP proximal bond (Scheme 21).66 Nickel coordination to the alkyne is believed to direct the C−C bond cleavage. DFT calculations combined with experimental data suggest that the Scheme 21

catalytic cycle involves the initial formation of 1-alkylidenenickelacyclobutane intermediate 119. Subsequent carbonickScheme 22

I


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formation of an allyl-metal bond of the ring-opened product constitutes a driving force. As a result, VCPs have been widely used in transition-metal catalyzed C−C bond activations, notably in cycloaddition reactions.28 In the presence of transition-metal catalysts, donor−acceptor vinylcyclopropanes 138 are ring-opened to zwitterionic π-allylmetal species 139 (Scheme 25a). The latter intermediates are involved in allylic substitution processes as well as in [3 + 2]-cycloaddition with various dipolarophiles, yielding 140 and 141, respectively. VCPs can as well be considered either as a three- or a fivecarbon synthon, depending on whether the vinyl substituent participates in the cycloaddition (five-carbon synthon) or not (three-carbon synthon). Transition-metal-catalyzed activation of VCPs 12 in combination with a broad variety of unsaturated acceptors has led to the development of new cycloaddition patterns which are discussed in the following section (Scheme 25b). As five-carbon synthons, VCPs engage in [5 + 1], [5 + 2], [5 + 2+1], and [5 + 1+2 + 1]-cycloadditions leading to the formation of six-, seven-, and eight-membered rings 143, 144, and 145 respectively, as well as fused ring systems 146. As three-carbon synthons, VCPs are mainly involved in [3 + 2] and [3 + 2+1] cycloadditions yielding five- and six-membered rings 148 and 149, respectively.70 These reactions have a high proven synthetic potential exemplified by their applications as the key scaffold-building step in natural product total syntheses. 2.1.3.1. VCPs as Five-Carbon Synthons. 2.1.3.1.1. Donor− Acceptor Substituted VCPs as Five-Carbon Synthons. Nucleophilic trapping of zwitterionic π-allylmetal species generated from ring-opening of donor−acceptor VCPs was first demonstrated in 1985 by Burgess.71 Different nucleophiles including malonates, 1,3-diketones and bis(phenylsufonyl) methane were used for this process. Since then, other nucleophiles have been reported to be effective for this reaction. For instance, Szabó showed a palladium(0)-catalyzed synthesis of allyl boronic acid derivatives 152 from donor− acceptor VCPs 150 and tetrahydroxydiboron (Scheme 26a).72 The reaction forms allyl boronic acids 151, which were subsequently reacted with aqueous KHF2 to obtain the more stable potassium trifluoro(allyl)borates 152 in high yields. Yorimitsu and Oshima further explored the borylative ringopening reaction of donor−acceptor VCPs.73 They reported a

Scheme 23

and indenes 131 are obtained in moderate to good yields. However, alkyl alkynes are not suitable substrates for this transformation. Ohashi and Ogoshi reported the selective formation of 1,2bis(exo-alkylidene)-cyclohexanes 136 as a result of a nickelcatalyzed [3 + 3]-cyclodimerization of electron deficient ACPs 132 (Scheme 24).69 Initial oxidative addition of the proximal C−C bond of ACP 132 to the nickel(0)-complex initiates the catalytic cycle. Subsequent dimerization of nickel cyclobutane 133 with ACP unit 132 provides intermediate 134. Reductive elimination delivers cycloadducts 135 in good yields with high selectivities for the (E,E) isomer 136. Higher reaction temperatures (100 °C) were required for the less reactive ketone derivative and the (E,Z) isomer 136 was isolated as a single product in 50% yield. The difference in diastereoselectivities was shown to be due to isomerization from the (E,E) to the (E,Z) isomer 136 via oxa-π-allyl complex 137. 2.1.3. C−C Bond Activations of Vinylcyclopropanes. Vinylcyclopropanes (VCPs) possess an unsaturated tether which directs the transition-metal toward the C−C bond to cleave. Moreover, the cleavage is energetically favorable as in addition of the release of ring-strain (ca. 28 kcal/mol), the Scheme 24

J


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Scheme 25

cases, the thiocarbonylation goes along with isomerization to the conjugated thioester in some extent. Besides palladium-catalyzed processes, Plietker studied the C−C bond activation of donor−acceptor VCPs 164 with iron complexes generating an intermediate allyl-Fe species 166 (Scheme 28).75 The carbanion part of allyl-Fe intermediate 166 acts as a base, deprotonating pronucleohile 165, which in turn attacks the allyl metal to give product 167. A variety of pronucleophiles such as malodinitrile, a cyanophenyl ester, cyanocyclopentanone, or a phenylazlactone are viable. The allylic product 167 are obtained in good to excellent yields and give good E-selectivity and regioselectivities, favoring the linear substitution products. Related ring-openings of methylene cycloalkane dicarboxylates were reported by Yorimitsu and Oshima.76 The authors proposed that coordination of dicarbonyl moiety of 168 to magnesium bromide assists the oxidative addition to nickel(0) generating nickel(II) complex 170 (Scheme 29). Transmetalation with zinc bromide followed by reductive elimination gives zinc enolate 171 and regenerates the nickel catalyst. Protonolysis of the resulting zinc enolate 171 provides arylated product 172. Kotora had previously showed similar C−C bond cleavage of methylene cycloalkane.77 In this case, the reaction was believed to proceed by a different mechanism, involving

nickel(0)-catalyzed reaction of VCPs 153 with bis(pinacolato)diboron yielding allylic boronates 158 (Scheme 26b). Oxidative addition of VCP to nickel complex affords π-allyl(oxa-πallyl)nickel species 155. Transmetalation occurs, yielding πallylnickel 156 and subsequent reductive elimination provides boron enolate 157 and regenerates the nickel catalyst. Protonolysis gives allylic boronate 158 in moderate to high yields with high E-selectivity. cis-VCP having only one electron withdrawing group (E1 = CO2t-Bu, E2 = H) react as well. In contrast, parent trans-VCP (E1 = H, E2 = CO2t-Bu) resulted in a reduced yield of 44%. The proposed mechanism accounts for this difference in reactivity as the trans-configuration does not allow for bidentate coordinate of the nickel catalyst to the carbonyl and vinyl moiety, thus hampering the following oxidative addition. Alper and Xiao described a palladium-catalyzed thiocarbonylative ring-opening of donor−acceptor VCPs with thiols and carbon monoxide (Scheme 27).74 Mechanistically, oxidative addition of VCP 159 to the palladium catalyst gives η3allylpalladium 161. CO insertion generates acylpalladium complex 162. Subsequent reductive elimination forms thioester 163 and regenerates the palladium catalyst. A range of aryl, heteroaryl, as well as alkyl thiols are tolerated, and unsaturated thioesters 163 are obtained in moderate to high yields. In most K


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Scheme 26

the ring-opening of a range of 5- and 6-membered methylcycloalkenes proceeds in moderate to excellent yields. 2.1.3.1.1. VCPs as Five-Carbon Synthons. VCP substrates having an additional olefin and alkyne substituent tethered at the β-position are extensively used in transition-metal catalyzed [5+n]-cycloadditions. There are two mechanisms for the transition-metal catalyzed [5 + 2]-cycloadditions of β-VCP derivatives 173 (Scheme 30). The first one involves the oxidation of a transition-metal catalyst leading to metallacyclohexene intermediate 174. Subsequent migratory insertion of the unsaturated moiety delivers metallacyclooctene intermediate 176 (path A). Reductive elimination from 176 delivers the seven-membered product 177. Theoretical studies showed that this pathway is preferred with rhodium catalysts and that the rate-determining step is the 2π-insertion to form metallacyclooctadiene 176.78−81 Alternatively, metallacyclopentane 175 can be generated by oxidative cyclization (path B). Subsequent β-carbon elimination results in the cyclopropane C−C bond cleavage and forms metallacyclooctene 176.

Scheme 27

hydronickelation of the olefin and a subsequent C−C bond cleaving event by an electrocyclic rearrangement. In both cases, Scheme 28

L


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Scheme 29

Scheme 30

Experimental and theoretical studies by Trost and Houk revealed that Path B is operative with ruthenium82,83 and nickel catalysts.84 C−C bond cleavages of VCPs proceeding via the βcarbon elimination pathway are beyond the scope of this review, and have been previously reviewed.85 Since their pioneering report in 1995,86 Wender extensively studied intramolecular rhodium(I)-catalyzed [5 + 2]-cycloadditions of VCPs with alkynes,86−88 alkenes,89,90 and allenes.91−93 In 2002, this group reported that the cationic rhodium(I) complex [{(C10H8)Rh(cod)}SbF6] is so far the most efficient catalyst for [5 + 2]-cycloaddition reactions.94 The alkyne substrates are converted to the corresponding cycloadducts in high yields (>90%). Only few asymmetric versions have been reported so far.90 In 2006, Wender reported enantioselective rhodium(I)-catalyzed intramolecular [5 + 2]cycloadditions using the cationic [Rh(Binap)SbF6] complex (Scheme 31).95 Substitution of the cyclopropane and the alkene moiety on 178, as well as modification of the tether are tolerated and the cycloheptenes 179 are obtained in excellent yields and selectivities of up to 99% ee. Replacing the alkene acceptor group by an alkyne resulted in reduced enantioselectivities ranging from 22 to 56% ee. The selectivity for the alkyne substrate class was later improved by Hayashi (Scheme 32).96 He reported rhodium(I)catalyzed asymmetric intramolecular [5 + 2]-cycloadditions of alkynyl-vinylcyclopropanes 180. The use of the noncoordinating BARF-counteranion was found to be critical to ensure high reactivity. Monodentate Feringa-type phosphoramidite ligand

Scheme 31

L5 enables the synthesis of cycloheptadienes 181 in moderate to high yields and enantioselectivities of up to 99% ee. A rhodium(I)-catalyzed [5 + 2+1]-cycloaddition of β-eneVCPs 182 with carbon monoxide for the construction of bicyclic cyclooctenones 186 was developed by Wender (Scheme 33).97 The initial oxidative addition of the VCP 182 to the rhodium(I)-catalyst gives rhodacyclohexene 183 and initiates the catalytic cycle. Migratory insertion of the tethered alkene leads to 184. Calculations indicated that formation of a seven-membered ring by a direct reductive elimination is not a favored process. Instead, CO insertion resulting in the generation of intermediate 185 occurs. Finally, a more favored acyl-carbon reductive elimination delivers the 5,8- or 6,8-cisfused rings 186 in moderate to high yields as single diastereoisomers. The reaction proceeds with a variety of substituted VCPs bearing different tethers and tethers lengths. M


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Scheme 32

Scheme 33

Scheme 34

Intermolecular rhodium(I)-catalyzed [5 + 2]-cycloadditions of VCPs with alkynes have been as well realized. An oxygen substitution on cyclopropyl part of the VCP is known to facilitate the intramolecular cycloaddition.80 In a pioneering report from 1998, Wender used siloxycyclopropanes for the intermolecular [5 + 2]-cycloaddition with alkynes.98 Later, different 1-alkoxy-substituted VCPs 187 were also used.99,100 The cationic rhodium(I) complex [{(C10H8)Rh(cod)}SbF6] (193) was found to be a highly efficient catalyst for the intermolecular [5 + 2]-cycloaddition of 1-alkoxy-VCPs 187 with alkynes 188 providing cycloheptenones 192 (Scheme 34a).101 Oxidative addition of VCPs proximal C−C bond to the rhodium catalyst generates rhodacycle 189. Subsequent migratory insertion of the external alkyne forms 190 which undergoes reductive elimination leading to enol ether 191. Seven-membered ketones 192 are obtained upon acidic workup. The products are formed in excellent yields (>90%) in the presence catalysts loadings as low as 0.5 mol % within minutes at room temperature. Notably, using this catalyst, even the less reactive 1-alkyl-VCP 194102 provides cycloheptadiene derivative 196 in quantitative yield, albeit requiring longer reaction times (48 h) (Scheme 34b). More recently, the [Rh(dnCOT)(MeCN)2]SbF6 catalyst (200) was shown to exhibit even better reactivity and yields in both inter- and intramolecular [5 + 2]-cycloadditions of VCPs 197 with alkynes 198 (Scheme 35).103 The reaction proceeds within minutes at room temperature, and is compatible with a wide range of functionalities. Moreover, rhodium(I)-complex

Scheme 35

N


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Scheme 36

200 was found to greatly enhance the regioselectivity81 when compared to previously reported catalysts. Wender described a method for accessing bicyclo[5.3.0]decanes 203 in which [{RhCl(CO)2}2] and AgSbF6 are used to catalyze [5 + 2]-cycloadditions between VCP 187 and enynones 201, as well as enabling a subsequent Nazarov cyclization of the intermediate dienone product 202 (Scheme 36).104 The process was also catalyzed by the cationic [{(C10H8)Rh(cod)}SbF6] complex (193), yielding the desired product in an improved 95% yield. In 2005, Wender reported a rhodium-catalyzed intermolecular [5 + 2]-cycloaddition of VCPs 187 with allenes as a twocarbon component.100,105 Allenes 204 having alkynyl, alkenyl, cyano, and cyanoalkyl substituents are tolerated, affording cycloheptanones 205 in moderate to high yields (Scheme 37). Notably, the cycloaddition of allene-ynes occurs exclusively at the terminal allene double bond. However, at least one methyl

Scheme 38

Scheme 37

substituent on the terminal allene double bond is necessary for the reaction to proceed. Terminally unsubstituted allenes are not reactive. DFT calculations allowed to rationalize the reactivity and selectivity of this transformation.106 The low reactivity of terminally mono- or unsubstituted allenes is associated with a competing allene dimerization process that irreversibly sequesters the rhodium catalyst. Computational studies also revealed the origin of chemoselectivity in the [5 + 2]-cycloaddition with allene-ynes. The allene’s terminal olefin is more reactive due to the enhanced d−π* backdonation. Moreover, insertion into the internal double bond of an allene-yne is disfavored as it would break π-conjugation. Finally, internal alkynes are more difficult to insert because of the steric repulsion from the substituents. Wender developed a four-component cycloaddition reaction of VCPs 206, terminal alkynes 207 and two CO units, yielding hydroxyindanones 215 in moderate to excellent yields (Scheme 38).107 The rhodium-catalyzed [5 + 1+2 + 1]-cycloaddition proceeds according to the following mechanism. Oxidative addition of VCPs C−C bond to the rhodium catalyst gives rhodacycle 208. A first CO insertion forms intermediate 209. Subsequent migratory insertion of the alkyne leads to the ninemembered rhodacycle 210. A second CO insertion and following reductive elimination generates the nine-membered

ring intermediate 212. Tautomerization to triene 213, and subsequent electrocyclization to 214 and aromatization-driven elimination provides 215. A rhodium(I)-catalyzed formal [5 + 1]/[2 + 2+1]-cycloaddition of 1-yne-VCPs 216 and two CO units was developed by Yu (Scheme 39).108 The process allows for the construction of 5,5,6-tricyclic scaffolds 222 bearing one or two adjacent bridgehead quaternary all-carbon stereocenters in moderate to excellent yields. Mechanistically, cleavage of the proximal C−C bond of the cyclopropane 216 via oxidative addition to rhodium(I) leads to π-allyl rhodium(III) intermediate 217. Alkyne insertion generates species 218. Carbon monoxide insertion into the Rh−C(sp2) bond gives rhodacycloheptenone 219. Migratory insertion of the alkene yields tricyclic rhodacyclohexane 220. A second CO insertion generates 221, which delivers 5,5,6-fused product 222 after reductive elimination. O


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mechanism was elucidated by DFT studies.110 Oxidative addition of the cyclopropyl proximal C−C bond forms nickelacyclobutane 224. Ring-opening generates nickelacyclohexene intermediate 225, which delivers the cyclopropene product after reductive elimination. The transition-metal-catalyzed [5 + 2]-cycloadditions have proven their synthetic potential by applications in natural product total syntheses as the key skeleton-building step. For instance, a diastereoselective rhodium(I)-catalyzed [5 + 2]cycloaddition was applied by Martin as key-step in the enantioselective syntheses of sesquiterpenes tremulenediol A (228) and tremulenolide A (229) (Scheme 41).111 Building on Wender’s [5 + 2+1]-cycloaddition,97 Yu developed a tandem cycloisomerization/aldol condensation to construct the linear triquinane core 232.112 This transformation was applied in concise diastereoselective syntheses of (±)-hirsutene (235), 112,113 (±)-1-desoxy-hypnophilin (236),112 and the formal synthesis of (±)-hirsutic acid (239)114 (Scheme 42). Moreover, a rhodium(I)-catalyzed [5 + 2+1]-cycloaddition is the key-step of Yu’s formal syntheses of (±)-asterisca-3(15),6diene (242) and (±)-pentalenene (243) (Scheme 43).115 Furthermore, the enantioselective total synthesis of (+)-asteriscanolide (246) was accomplished using VCP 244 to construct its bicyclic carbon framework.116,117 2.1.3.2. VCPs as a Three-Carbon Synthons. The neighboring unsaturated bond in VCP assists the selective cleavage by a coordination to the metal catalyst but does not necessarily engage in subsequent cycloaddition steps which results in a three-carbon synthon behavior if the VCPs. Donor−acceptor VCP derivatives were first found to act as three-carbon components by Tsuji in 1985.118 These metal-catalyzed nucleophilic-addition-type [3 + 2]-cycloadditions require the use of VCP 138 bearing one or two electron withdrawing groups in order to activate the cyclopropane ring for the C−C bond cleavage (Scheme 44a). A stepwise ionic mechanism involving zwitterionic π-allylmetal intermediates 139 and 247 was proposed. A second possible reaction pathway for VCPs 248 is depicted in Scheme 44b. Oxidative addition of VCP 248 to the transition metal gives intermediate 249, in which the opened cyclopropane ring and the unsaturated moiety bind to the metal center. Migratory insertion of the alkene/alkyne affords intermediate 250. Finally, reductive elimination delivers the [3 + 2]-cycloaddition product 251. Both reaction pathways are described in the following section. 2.1.3.2.1. Donor−Acceptor VCPs As a Three-Carbon Synthons. Pioneering studies by Tsuji showed that the zwitterionic πallylpalladium(II) species generated from C−C bond cleavage of donor−acceptor VCPs 252 undergoes formal [3 + 2]cycloadditions with acrylate or isocyanate electrophiles yielding

Scheme 39

Louie reported nickel(0)-NHC complexes as efficient catalysts for the rearrangement of VCPs 223 into cyclopentenes 226 (Scheme 40).109 Both 1,1- and 1,2-disubstituted VCPs as well as activated trisubstituted substrates are tolerated and afford the cyclopentenes in excellent yields. The reaction Scheme 40

Scheme 41

P


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Scheme 42

Scheme 43

Scheme 44

Q


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the corresponding cyclopentanes118 and γ-lactams.119 Since then, other dipolarophiles have been shown to be well suited for this transformation. For instance, Johnson demonstrated palladium-catalyzed [3 + 2]-cycloadditions of donor−acceptor VCPs 252 and aldehydes 253 for the synthesis of tetrahydrofuran derivatives 256 (Scheme 45).120 The process is initiated by ring opening of VCP 252, resulting in πallylpalladium(II) species 254. Subsequent aldol-type nucleo-

Trost further extended the reactivity to enantioselective [3 + 2]-cycloadditions between donor−acceptor VCPs 260 and Meldrum’s acid derived alkylidene acceptors 261 (Scheme 47).122 In this case, the use of Meldrum’s acid-substituted VCP 260 resulted in the best diastereoselectivities and chiral biphosphine L7 was identified as the most selective ligand. A range of aryl, heteroaryl and alkynyl acceptors are tolerated by the process, affording the substituted cyclopentanes 262 in moderate to good yields and with excellent enantio- and diastereoselectivities. In contrast, alkyl-substituted Meldrum’s acid alkylidenes failed. In 2012, Shi and Xu disclosed an enantioselective palladium(0)-catalyzed [3 + 2]-cycloaddition between donor−acceptor VCPs 263 and β,γ-unsaturated α-keto esters 264 (Scheme 48).123 The reaction proceeds in the presence of chiral imidazoline-phosphine ligand L8, giving functionalized cyclopentane cycloadducts 265 in good yields along with high diastereo- and enantioselectivities. Different aryl, heteroaryl and alkyl α-keto esters as well as a variety of VCPs are compatible for this transformation. Shi and Xu further applied this methodology to other types of activated unsaturated substrates.124 For instance, they reported enantioselective palladium(0)-catalyzed [3 + 2]cycloaddition of VCPs 266 and isatins 267 (Scheme 49). Chiral imidazoline-phosphine L8 was the ligand of choice for this process affording corresponding functionalized oxindolefused spirotetrahydrofurans 268 in good yields, diastereo- and enantioselectivities. While a variety of substituted isatins 267 are well suited for this transformation, a substitution on the vinyl moiety of VCP 266 (R2 ≠ H) was detrimental. Very recently, He and Liu reported enantioselective palladium(0)-catalyzed [3 + 2]-cycloadditions of dinitrile substituted VCPs 269 with nitroolefin acceptors 270 (Scheme 50).125 A range of aryl, heteroaryl and alkyl nitroolefins are compatible with the process, generating nitrocyclopentanes 271 bearing three consecutive chiral centers in good to excellent yields and enantioselectivities. The diastereomeric ratios remained modest; however, the two diastereoisomers could be separated by column chromatography. The reactivity of the corresponding VCP dicarboxylic acid methyl ester was investigated, but resulted in a dramatic decrease of the enantioselectivity (27% ee). Studies by Stoltz have strengthen the synthetic utility of this palladium(0)-catalyzed process which was used to assemble the

Scheme 45

philic attack of complex 254 on aldehyde 253 generates alkoxide 255. Ring closure via intramolecular nucleophilic trapping of the allyl species as well as concomitant displacement of the palladium(0) catalyst completes the catalytic cycle, delivering the tetrahydrofuran products 256. Whereas electronpoor aldehydes provide high yields of 256 in short reaction times, electron-rich aldehydes failed because of their lower electrophilicity. In 2011, Trost reported the first enantioselective [3 + 2]cycloaddition of donor−acceptor VCPs. His palladiumcatalyzed process provides access to functionalized chiral amino acid derivatives 259, simultaneously setting three stereogenic centers in the product (Scheme 46).121,122 The cycloadducts 259 are formed in good to excellent enantio- and diastereoselectivies with the Trost ligand (L6). The use of the bis(2,2,2-trifluoroethyl)malonyl VCPs 257 allows for higher yields and selectivities than other VCPs. A variety of substituted alkylidene azalactones partners 258 work well. Scheme 46

R


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Scheme 47

Scheme 48

Scheme 49

cyclopentane core of the melodinus alkaloids scandine (275) (Scheme 51).126 He and Liu showed that imines are suitable dipolarophiles for the palladium-catalyzed asymmetric [3 + 2]-cycloaddition of VCPs.127 The required α,β-unsaturated imine substrates 279 are generated in situ from aryl sulfonyl indole precursors 277 (Scheme 52). In the presence of the palladium(0) complex, C− C bond cleavage of VCP 276 occurs, generating 1,3-dipole species 278. The carbanion of 278 acts as a base and deprotonate indole derivative 277, releasing α,β-unsaturated imine 279. Nucleophilic conjugate attack of 1,3-dipole to imine 279 yields indole-tethered allylic palladium intermediate 280. Subsequent intramolecular nucleophilic cyclization delivers spiroindolenine product 281. Cycloadducts 281 are formed in high diastereo- and enantioselectivities.

Scheme 50

S


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Scheme 51

Scheme 52

Scheme 53

Besides the palladium-catalyzed processes described above, other transition-metal complexes were used to trigger the C−C bond cleavage of activated VCPs. For instance, Johnson reported nickel(0)-catalyzed rearrangements of 1-acyl-2-vinylcyclopropanes 282 to access dihydrofuran derivatives 284 (Scheme 53).128 Oxidative addition to nickel(0) gives πallylnickel(II) species 283 with a pendant enolate. Intramolecular trapping of the π-allyl intermediate by the tethered enolate delivers the dihydrofuran product 284 and regenerates the catalyst. Modifications of the keto-tether of VCP 282 with alkyl chains that contains alkene, aryl, cyano, and acetal functional groups are tolerated. In addition, terminal, di- and

trisubstituted olefins react as well, affording products 284 in high yields. In 2013, Kurahashi and Matsubara reported nickel-catalyzed intermolecular [3 + 2]-cycloaddition of vinylcyclopropanes 285 and imines 286 (Scheme 54).129 This transformation generates substituted pyrrolidines 287 in high yields, with excellent regioand diastereoselectivities. A variety of imines are tolerated by the process, including aryl, heteroaryl and alkyl imines. The authors demonstrated the potential for an asymmetric variant using iPr-Duphos L10 as chiral phosphine ligand, delivering the pyrrolidine cycloadduct in 83% yield and 56% ee. Plietker studied the C−C bond activation of donor−acceptor VCPs with iron complexes generating an intermediate allyl-Fe T


Chemical Reviews

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Scheme 54

Scheme 56

species 290 (Scheme 55).75 Reaction of allyl-Fe species 290 with activated olefins 289 or imines 293 delivers the corresponding cyclopentanes 291 or pyrrolidines 294. Different VCPs and dipolarophiles bearing nitrile, ester, amide or ketone functional groups are tolerated. The substituted carbo- and hetero cycles are formed in high yields albeit with low diastereoselectivities. All the above-mentioned reactions of donor−acceptor VCPs involve nucleophilic trapping at the donor site and electrophilic trapping at the acceptor site. In contrast, Johnson and Krische reported that exposure of such donor−acceptor VCPs to iridium catalyst 297 results in an umpolung of this polarity pattern, leading to a nucleophilic π-allyl intermediate.130 This key intermediate was shown to react with primary alcohols and aldehydes affording allylic alcohols 298 in good yields and diastereoselectivities as well as excellent enantioselectivities (Scheme 56). 2.1.3.2.2. Nonpolarized VCPs As a Three-Carbon Synthons. In 2008, Yu reported a rhodium(I)-catalyzed [3 + 2]-cycloaddition of olefin tethered VCPs 300 (Scheme 57).131 A cationic rhodium catalyst generated through the addition of silver triflate was found to have a better reactivity allowing running the reaction at lower temperatures. The cyclopropane C−C bond cleavage was found to proceed through oxidative addition of the proximal C−C bond leading to a π-allyl rhodium(III) intermediate 301.132 Following alkene insertion

Scheme 57

and subsequent reductive elimination from 302 delivers 5,5-cisfused product 303. Both a substitution on the VCP and the

Scheme 55

U


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Yu extended this principal reactivity to carbonylative [3 + 2+1]-cycloadditions of VCPs 312 (Scheme 60).135 Variation of the tether length of substrate 312 allowed for the formation of 5,6- and 6,6-bicyclic systems 313. A variety of tethers (nitrogen-, oxygen-, and malonate) could be used to synthesize hetero- and carbobicyclic fused-cyclohexanone and cyclohexenone products 313 in moderate to high yields. The process was applied as key-step for the construction of the bicyclic skeleton of the furanoid sesquiterpene α-agarofuran (316).135 2.1.4. C−C Bond Activation of Cyclopropenes. Compared to cyclopropanes, cyclopropenes have a higher strain energy (55.7 kcal/mol) and in consequence also exhibit a higher reactivity. Moreover, the double bond allows for coordination of transition-metals. The reaction of cyclopropenes 317 with transition-metals can occur via two fundamentally different reaction pathways (Scheme 61). The first pathway is the direct oxidative addition of cyclopropene C−C σ-bond to a transition-metal complex (path A). The transition-metal generally inserts into the less hindered C−C σbond of the cyclopropene resulting in the formation of metallacyclobutene 318 as the major compound (R3 < R4). The stoichiometric formation and isolation of platina-, nickela-, or colbaltacyclobutene complexes is well documented.136−138 In contrast, only few transition-metal-catalyzed reactions of cyclopropene derivatives have been reported.139,140 The other, more often encountered, reaction pathway of transition-metals with cyclopropenes 317 consist of a metalation of the C−C double bond leading to intermediates 320 and 321 (path B). This pathway is more common and has recently been reviewed.45 Cyclopropenones were used first as three-carbon components in cycloaddition reactions. For instance, Mitsudo reported ruthenium-catalyzed carbonylative dimerizations as well as crossed reactions with internal alkynes to give substituted pyranopyrandiones.141 In 2006, Wender disclosed a rhodium(I)-catalyzed [3 + 2]-cycloaddition of cyclopropenones 322 with alkynes 323 providing access to cyclopentadienones 326 (Scheme 62).142 Oxidative addition of the acyl-carbon bond of cyclopropenones 322 to the rhodium catalyst gives rhoda(III)cyclobutenones 324. Migratory insertion of the alkyne forms six-membered rhodacycles 325. Final reductive elimination delivers cyclopentadienones 326. The reaction proceeds with remarkably low catalyst loadings (0.1 mol %). Diaryl- and arylalkylcyclopropenones 322 can be used as a three-carbon component in combination with aryl-, heteroaryl-, and dialkyl-substituted alkynes affording products 326 as single regioisomers in moderate to high yields. Terminal alkynes and dimethyl acetylene dicarboxylate are not tolerated due to competing alkyne polymerization or decarbonylation processes. Wang reported rhodium(I)-catalyzed carbonylative carbocyclizations of cyclopropenes 327 with tethered olefins providing trans-fused cyclohexenones 331 in moderate to high yields (Scheme 63).143 An initial direct oxidative addition of the less hindered C−C σ-bond of cyclopropene 327 to rhodium(I)catalyst generates rhodacyclobutene intermediate 328. Subsequent carbon monoxide insertion gives intermediate 329. Migratory insertion of the tethered alkene leads to rhodacycle 330, which undergoes reductive elimination to deliver product 331 as single diastereoisomers. A substitution of the alkene moiety hampers the alkene insertion and results in lower yields.

alkene is tolerated and the [3 + 2]-cycloadducts are obtained as single diastereoisomers in moderate to good yields. The process is limited to trans-VCP-enes and the related cis-compounds are found to undergo instead [5 + 2]-cycloadditions. α-Substituted VCPs 304 were found to act as three-carbon synthon as well, allowing for the construction of bicyclic scaffolds 305 (Scheme 58).133 Both 5,6- and 5,7-cis-fused rings were obtained in good yields and excellent diastereoselectivities. Yu developed rhodium(I)-catalyzed [3 + 2]-cycloadditions of VCPs 306 affording cyclopentane- and cyclopentene-containScheme 58

ing bicyclic structures 309 (Scheme 59a).134 The cationic complex [{Rh(dppp)}SbF6] was found to be the best catalyst, affording a variety of 5,5-cis-fused structures 309 with quaternary stereogenic centers at the bridgehead position. Both terminal and internal 1-yne VCPs yielded cyclopentenes 309. Allene-substituted VCPs 310 gave a mixture of cycloadducts 311 with both exo and endo double bond isomers (Scheme 59b). Scheme 59

V


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Scheme 60

Scheme 61

Scheme 63

Scheme 62

hexenones 338 are then formed upon hydrolysis, whereas an oxidative treatment generates phenols 339. 2.2. C−C Bond Activations of Four-Membered Rings

When compared with cyclopropanes, the related fourmembered rings exhibits similar ring strain (26.3 kcal/mol for cyclobutanes vs 29.0 kcal/mol for cyclopropanes). Therefore, cyclobutane derivatives also occupy a privileged role in transition-metal catalyzed C−C σ-bond activations. Generally, strain-driven oxidative addition of the C−C bond of cyclobutane 340 to a transition-metal leads to the formation of metallacyclopentane 341 (Scheme 65). This metallacycle can be used as reactive intermediate for different reaction pathways to close catalytic cycles which are described in the following chapter. In addition to highly strained biphenylenes 342,145

Phenols 333 are obtained when cyclopropenes 332 bearing tethered alkynes are used. Recently, Wang studied rhodium-catalyzed rearrangements of cyclopropenes 334 containing an attached silylated cyclopropanol unit (Scheme 64).144 Mechanistically, rhodacyclobutene 335 is formed though oxidative addition of the C−C σbond of the cyclopropene. A strain-releasing β-carbon elimination affords rhodacycloheptadiene 336. Subsequent reductive elimination forms silyl enol ether 337. CycloW


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Scheme 64

2.2.1. C−C Bond Activations of Biphenylenes. The C− C σ-bond strength for biphenylene (65.4 kcal/mol) is much weaker than that of the C−C bond in biphenyl (114.4 kcal/ mol), making it an attractive target for C−C bond cleavage.147 Besides a reduction in ring strain, the driving force for the C−C bond cleavage is the formation of two relatively strong metal− C(sp2) bonds at the expense of a weak C−C bond. Thus, biphenylene has a long history and a rich chemistry with transition-metal-promoted C−C bond cleavages.145 The aryl− aryl C−C bond is cleaved to give metallacyclic complex 349 (Scheme 66). In 1985, Eisch reported the stoichiometric reaction of biphenylene with nickel(0) complexes to give C−C insertion complexes.148 Intermediates 349 were then used for (a) CO insertion yielding fluorenone 350, (b) insertion of symmetrical alkynes giving phenanthrenes 351, (c) oxidation leading to dibenzofuran 352, or (d) hydrogenolysis providing biphenyl 353. Stoichiometric reactions of biphenylene giving dibenzometallocyclopentadienes 349 with other transitionmetals such as Rh,149−151 Co,150 Pt,152,153 Ir,154,155 and Fe156,157 are known as well. The first catalytic example of dimerization of biphenylene accessing symmetrical tetraphenylene 354 in the presence of catalytic amounts of [{Ni(cod)(PMe3)2}] was reported by Vollhardt.158 The nickel(0)-catalyzed trapping of metallacyclic intermediate 349 with acetylenes has been further studied by Jones159−161 and Radius.162,163 Cleavage of the biphenylene C−C bond by the reactive nickel complex generates the nickel(II) intermediate 356 (Scheme 67). Insertion of the alkyne into the nickel−carbon bond leads to the nickela(II)cycle 357. Reductive elimination delivers phenanthrene 358 and complexation of another alkyne regenerates the nickel(0) complex. Whereas such reactions are usually performed at temperatures ranging from 70 to 100 °C, the reaction of diphenylacetylene with biphenylene 342 occurred at room temperature using the complex 359.162,163 Jones showed that

Scheme 65

four-membered rings bearing a ketone moiety such as cyclobutenediones/benzocyclobutenediones 343, cyclobutenones/benzocyclobutenones 344, and cyclobutanones 345 are versatile substrates.146 Compared to an unsubstituted cyclobutane, the acyl-carbon bond in these substrates is more susceptible toward C−C cleavage as the arising acyl-metal bond is stronger than an alkyl-metal bond. In addition to facilitating the C−C bond cleavage, they enable a precoordination of the metal to the carbonyl group. Cyclobutanone derivatives 346 can be regarded either as a four-carbon synthon or a threecarbon synthon (by extrusion of CO). X


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Scheme 66

Scheme 67

Scheme 68

mediate 349 generated from biphenylene C−C cleavage was never used for further transformations.154,155 In 2012, Kotora and Roithova reported the iridium-catalyzed synthesis of substituted phenanthrenes 361 by the reaction of biphenylene 342 with various alkynes (Scheme 68).165 The reaction tolerates the presence of sterically demanding substituents on the alkyne, for instance a ferrocenyl group. Moreover, the authors also described the trapping of intermediate 349 with

several symmetrical and unsymmetrical internal alkynes could be used in reaction with complex 360. In contrast, no insertion products were observed for electron-poor as well as for terminal alkynes. To address this shortcoming, Jones disclosed a rhodium(I)catalyzed formal [4 + 2]-cycloaddition of biphenylene with alkynes.164 In this case, terminal alkynes such as phenylacetylene were competent. Until recently, iridacyclic interY


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nitriles. This rhodium-catalyzed reaction of biphenylenes 342 with nitriles provides phenanthridines 362 in moderate to high yields (Scheme 68).165 The biphenylene substrate was exploited by Shibata for catalytic enantioselective construction of axial chiral biphenyl derivatives 366 (Scheme 69).166 In detail, they showed that a

Scheme 70

Scheme 69

combination of [{Ir(cod)Cl}2] and Me-BPE (L11) as chiral ligand was the most efficient catalytic system, yielding phenanthrene derivatives 366 in moderate to good yields and enantioselectivities up to 95% ee. The use of polarized alkynes 363 was required in order to achieve high enantioselectivities. Substituents R1 on the ortho-position of the aryl group were found to drastically influence the enantioselectivity. In the case of a 2-methoxyphenyl group, the enantiomeric excess of cycloadduct 366 was very low (9% ee). In contrast, the enantiomeric excess of cycloadduct 366 bearing a 2trifluoromethylphenyl group exceeded 90% ee. Both aryl portions of the biphenylenes can be differentially functionalized. For instance, Jones reported a palladiumcatalyzed reaction of biphenylene 342 with olefins resulting in the formation of biphenyl derivatives 371 (Scheme 70).167 Protonolysis of intermediate 368 with weak acids such as pcresol give aryl(aryloxy)palladium species 369. Subsequent Heck-type reaction delivers the biphenyls 371. Suzuki-type additions of arylboronic acids 372 leading to biaryls 373 are also described. Methyl ketones 374 and benzylic nitriles 376 possessing weakly acidic C−H bonds were also found to add across palladium(II) species 368 affording biaryls 375 and 377, respectively. The tetraphenylene synthesis by a homodimerization of two biphenylene molecules is known since the pioneering work of Eisch,148 Gallagher reported the first synthesis of unsymmetrical mixed heterocyclic tetraphenylenes 380 in moderated yields (Scheme 71).168 It was found that the oxidative addition of the aryl-bromide bond of 378 to a palladium(0) catalyst is faster than the reaction with biphenylene 342, suggesting

palladacycle 379 as an intermediate. Slow reaction might account for the formation of significant amounts of the C−Br reduction product and thus account for the moderate yields of the desired compounds 380. 2.2.2. C−C Bond Activations of Cyclobutenediones and Benzocyclobutenediones. As above-mentioned, fourmembered rings bearing a ketone moiety exhibit higher reactivity toward C−C bond activations. The formation of a stronger acyl-metal bond (when compared to an alkyl-metal bond) as a result of the oxidative addition of the acyl-carbon bond of these substrates to a transition-metal facilitates the C− C bond cleavage. Cyclobutenediones/benzocyclobutenediones 343 were the first four-membered ketone derivatives studied for C−C bond activations. Pioneering work by Kemmit in 1973 showed platinum-mediated C−C bond cleavages of benzocyclobutenediones.169,170 Further stoichiometric reactions with Z


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Scheme 71

other transition-metals were reported,171,172 and the synthesis of quinones has been thoroughly studied by Liebeskind.173−180 The reaction of cyclobutenediones/benzocyclobutenediones 343 with transition-metal catalysts can occur via two different reaction pathways (Scheme 72). Treatment of cyclobutene-

Scheme 73

Scheme 72

diones with platinum, ruthenium, or rhodium complexes leads to unsymmetrical cleavage of the four-membered ring giving 381 (path A). Alternatively, thermodynamically more stable metal complex 382 can be formed as a result of an insertion of the metal between the two carbonyl groups in cyclobutenediones (path B). In the case of rhodium, the initially formed kinetic product (path A) was shown to isomerize to the thermodynamically favored intermediate 382. The first catalytic example, a ruthenium-catalyzed C−C bond activation of cyclobutenediones to access cyclopentanones via a decarbonylative alkene insertion was reported by Mitsudo in 2000.181 Yamamoto reported in 2006 a rhodium(I)-catalyzed decarbonylative alkene insertion into cyclobutenediones 383 leading to azabicycloalkenones 389 (Scheme 73).182 This transformation is initiated by oxidative addition of cyclobutenedione 383 to rhodium(I), resulting in the formation of rhoda(III)cyclopentenedione 385. Subsequent decarbonylation gave rhoda(III)cyclobutenone 386. Migratory insertion of the olefin tether delivered, after reductive elimination from 388, azabicycloalkenones 389 in moderate to good yields. 2.2.3. C−C Bond Activations of Cyclobutenones and Benzocyclobutenones. In addition to cyclobutenediones, cyclobutenones and benzocyclobutenones have been used as substrates to induce catalytic C−C bond activations involving the direct oxidative addition to transition-metals.146 Because of the different nature of their acyl-carbon bonds (one C(sp2)-acyl and one C(sp3)-acyl), the selectivity for the C−C bond cleavage site is an additional challenge (Scheme 74). Cyclobutenones can undergo thermal electrocyclic ring-opening to vinyl ketenes 390, so that the cleavage of the C1−C4 bond

leading to intermediate 391 is often preferred (path A). However, the cleavage of the C 1 −C 2 bond can be thermodynamically preferred due to the formation of a stronger C(sp2)−[M] bond of 393 compared to C(sp3)−[M] bond of 391 (path B). In addition, due to the possibility for CO extrusion yielding metallacyclobutene 392, cyclobutenone derivatives can serve both a four-carbon synthon as well as a three-carbon synthon. Besides thermal electrocyclic ring opening,183 pioneering work on the stoichiometric transition-metal-catalyzed C−C bond activation of cyclobutenones was reported by Liebeskind.184−186 The reactivity comprised nickel(0)-catalyzed synthesis of phenols via alkyne insertion into the generated nickelacyclopentenone.187 Moreover, they disclosed a method for the catalytic C−C bond activation of cyclobutenones in 1993.188 In 2004, Kondo and Mitsudo showed a ruthenium and rhodium-catalyzed stereoselective synthesis of 2-pyranones by ring-opening dimerization of cyclobutenones (Scheme 75).189 In this case, treatment of cyclobutenones 394 with 5 mol % [{RuCl2(CO)3}2] in toluene at 110 °C for 12 h gave 6-alkenyl2-pyranones 398, in high yields with good (Z)-selectivity. Interestingly, changing the catalyst to [{RhCl(CO)2}2] resulted in a reversal of stereoselectivity to give (E)-6-alkenyl-2pyranone 398 as the only formed product in high yields. Mechanistically, regioselective ring-opening of cyclobutenone AA


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Scheme 74

Scheme 75

none intermediate 402. Under an argon atmosphere, decarbonylation is favored giving rhodacyclohexene 404, which subsequently reductively eliminated affording the corresponding cyclopentene 405. Under high carbon monoxide pressure, decarbonylation is suppressed and direct reductive elimination delivered cyclohexenone 403. The same group disclosed a related rhodium(I)-catalyzed synthesis of substituted phenols 412 from the C−C bond cleavage of cyclobutenones 406 and regioselective insertion of alkenes 407 (Scheme 77).190 The use of tricyclohexylphosphine as the ligand suppressed the formation of the previously reported dimerization product 398 (Scheme 75).189 Oxidative addition of the cyclobutenone C−C bond to the rhodium(I)complex leads to intermediate 408. Subsequent regioselective formal [4 + 2]-cycloaddition reaction with electron-deficient alkene 407 gives cyclohexenone 409. During the course of the reaction, dehydrogenation and subsequent isomerization from 410 or 411 delivers substituted phenols 412. In 2012, Dong reported a rhodium(I)-catalyzed regioselective olefin carboacylation reaction of benzocyclobutenones 413 (Scheme 78).191 The process is initiated by oxidative insertion into the aryl-acyl C−C bond of benzocyclobutanone 413 generating rhodacyclopentenone 414. The observed regiose-

394 by the metal center gives metallacyclopentene 395, which is equivalent to a η4-vinylketene intermediate 396. A formal hetero-Diels−Alder reaction with another equivalent of vinyl ketene generates intermediate 397. Successive isomerization of 397 delivers the corresponding 2-pyranone 398. No isomerization between (Z)-398 and (E)-398 was observed under the reaction conditions. Although initial studies by Liebeskind showed that rhodacyclopentenones obtained by the stoichiometric reaction of Wilkinson’s catalyst with cyclobutenones were resilient toward alkyne insertion,185 Kondo and Mitsudo reported on rhodium(I)-catalyzed decarbonylative coupling and direct coupling of cyclobutenones 394 with reactive alkenes such as norbornene 399 (Scheme 76).189 Running the reaction under 30 atm. of CO, a direct coupling with the alkene leads to cyclohexenones 403 in high yields. In contrast, when the reaction is executed under an argon atmosphere, decarbonylative couplings with norbornene 399 delivered cyclopentenes 405 in high yields. This behavior can be accounted for as follows. The initial oxidative addition of the cyclobutenone C− C bond to the rhodium(I)-complex forms rhoda(III)cyclopentenone 401. Coordination and subsequent stereoselective insertion of norbornene 399 gives rhodacyclohepteAB


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Scheme 76

Scheme 78

minimize the formation of the byproduct arising from decarbonylation. The process tolerates the insertion of terminal, di- or trisubstituted olefins. The addition of Lewis acid cocatalysts such as zinc(II) chloride was found to enhance the reactivity of more challenging substrates. Later, an asymmetric version of this transformation was developed using DTBM-Segphos (L12) as the chiral ligand to control the migratory insertion of the olefin tether (Scheme 79).192 Fused-tetralones 416 are obtained in good yields and excellent enantioselectivities ranging from 92−99% ee. The obtained tricyclic carbon skeleton could be exploited for the synthesis of cycloinumakiol 419 (Scheme 80).193 From a retrosynthetic point of view, the authors planned to access the cycloinumakiol core structure 419 via catalytic intramolecular carboacylation from benzocyclobutenone 417. To achieve this, benzocyclobutenone 417 was efficiently rearranged into tetracyclic core 418 using [{Rh(CO)2Cl}2]/P(C6F5)3 as the catalytic system. A concise synthesis of the proposed structure of cycloinumakiol was successfully achieved. A further extension of this C−C activation process involves the deliberate decarbonylation of intermediate 393. In this case, the transformation consists of a rhodium(I)-catalyzed decarbonylative spirocyclization of olefins and cyclobutenones 420 (Scheme 81).194 Again, the reaction is initiated by oxidative addition of the C1−C2 bond of benzocyclobutenone 420 to rhodium(I). Migratory insertion of the tethered olefin into rhoda(III)cyclopentanone 421 gave the seven-membered metallacycle 422. Complementary to the previously shown reductive elimination, a β-hydride elimination followed by CO extrusion gave rise to intermediate 423. Finally, reductive elimination and isomerization delivered the spirocyclic products 424. Employment of the electron poor rhodium(I) precursor [{Rh(CO)2Cl}2] in combination with tripentafluorophenyl phosphine ligands slowed the reductive elimination down and favored the β-hydride elimination pathway. The interception of rhodacyclic intermediate 393 could be extended to the insertion of alkynes. In this respect, rhodium(I)-catalyzed direct or decarbonylative alkyne-benzocyclobutenone couplings were reported (Scheme 82).195 Similar to the previous report, the tethered alkyne serves as a directing group leading to oxidative insertion into the aryl-acyl C−C bond of benzocyclobutanone 425 generating rhodacyclopentenone 426. Insertion of the alkyne and subsequent

Scheme 77

lectivity of the C−C cleavage has been explained by the formation of a more stable C(sp2)−[Rh] bond in 414. A synselective migratory insertion of the tethered alkene moiety then led to rhodacycloheptanone 415. Intermediate 415 underwent reductive elimination affording fused tetralones 416. The use of a ligand with a large bite angle such as dppb was shown to AC


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Scheme 79

Scheme 80

Scheme 81

reductive elimination affords β-naphthols 428. Dppp was found to be the most efficient ligand in promoting a fast reductive elimination. Complementarily, the decarbonylative process leading to indenes 430 was best performed in an open flask system, refluxing under argon atmosphere. DTBM-Segphos (L12) exhibited the highest reactivity for this decarbonylative process. 2.2.4. C−C Bond Activations of Cyclobutanones. Among four-membered rings containing a carbonyl group, simple cyclobutanones have been shown to be highly versatile for reactions involving undirected direct oxidative additions to transition-metals.146 The C−C bond activation of cyclobutanone 345 allows for the selective formation of reactive five-membered metallacycles 431 that can be exploited for a range of downstream reactions in catalytic transformations as depicted in the following section (Scheme 83). Moreover, given the additional possibility for decarbonylation leading to metallacyclobutane 432, cyclobutanones can be regarded as a

three- or four-carbon synthon, leading to distinct following transformations. Pioneering work by Ito and Murakami in 1994 showed that cyclobutanones are suitable substrates for rhodium(I)-catalyzed C−C bond activations (Scheme 84).196 The rhodium catalyst inserts into the less hindered acyl-carbon bond of cyclobutanone 433 leading to rhoda-cyclopentanone 434 (Scheme 84a). Under 50 atm. of hydrogen atmosphere, reductive ring cleavage occurs delivering alcohol 435 in 87% yield. The formation of cyclobutanol 436 was not observed, indicating that the C−C bond cleavage proceeds faster than the cyclobutanone hydrogenation under the reaction conditions. They also described a stoichiometric decarbonylative ring contraction of cyclobutanones 437 reaction leading to cyclopropanes 440 in the presence of Wilkinson’s catalyst (Scheme 84b). Later on, a catalytic version was developed by changing the catalyst system to [{Rh(cod)dppb}BF4] delivering cyclopropane 440 in quantitative yield.197 AD


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Scheme 82

Murakami disclosed a sequence of two consecutive C−C bond activations of spirocyclobutanone 445 combining both main C−C bond cleavage pathways (Scheme 86).199 First, oxidative addition of the acyl-carbon bond of spirocyclobutanone 445 to rhodium(I) forms rhodacycle 446. Second, cleavage of the adjacent cyclobutane through β-carbon elimination leads to metallacycle 447. Subsequent reductive elimination and isomerization of the exo-double bond into conjugation affords cyclohexanone 448. For substrates with nonequivalent C−C bonds, β-carbon cleavage of the less hindered C−C bond of cyclobutanone is favored. In 2002, Murakami reported the intramolecular interception of rhodium(III) cyclopentanone 452 (R1 = H) by a tethered olefin (Scheme 87a).200 Subsequent reductive elimination from 453 gave access to benzobicycloheptanone 450. In 2014, Cramer showed the feasibility for zwitterionic rhodium(I) complexes 451 to undergo enantioselective oxidative addition of the enantiotopic acyl-carbon bond of 3-styryl cyclobutanone 449 (Scheme 87b).201 This type of zwitterionic rhodium(I) complex allows for an enhanced reaction rate and therefore a significantly lower reaction temperature than analogous cationic rhodium complexes. The moderate levels of enantioselectivity were later improved using DTBM-Segphos L12 as steering ligand (Scheme 87c).202 Coordination of the rhodium(I) catalyst to both the olefin tether and the carbonyl of the cyclobutanone is believed to be responsible for a relatively rigid transition state and accounts for the high levels of enantioselectivity. Migratory insertion of the tethered olefin, followed by reductive elimination from 453 delivers benzobi-

Scheme 83

Murakami also reported a chemoselective rhodium(I)catalyzed activation of cyclobutanone C−C bonds over aldehyde C−H bonds (Scheme 85).198 Heating cyclobutanone 441 in m-xylene at 150 °C in the presence of a catalytic amount of [{RhCl(cod)(NHC)}] resulted in selective decarbonylation leading to cyclopropane 444 in high yields (82−92%). In contrast, aliphatic or aromatic aldehydes, esters, as well as less strained ketones such as 4-phenylcyclohexanone were unreactive toward decarbonylation, even using stoichiometric amounts of the rhodium complex. Interestingly, Wilkinson’s catalyst exhibits the opposite chemoselectivity and only the aldehyde group was decarbonylated, leaving the cyclobutanone unit intact. No chemoselectivity was observed using [{RhCl(cod)dppp}] which led to decarbonylation of both the ketone and aldehyde. AE


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Scheme 84

cycloheptanone 450. A range of mono-, di-, and trisubstituted olefins are tolerated. The products are formed in excellent yields and excellent enantioselectivities of up to 99.6% ee despite a high reaction temperature of 130 °C. They have extended the trapping possibility of the intermediate rhoda(III)cyclopentanone to carbonyl groups (Scheme 88).203 Similarly, enantioselective oxidative addition of one of the two enantiotopic acyl-carbon bonds of the cyclobutanone 454 leads to rhoda(III)cyclopentanone species 455. Subsequent migratory insertion of the carbonyl generates acyl rhodium species 456, which, in turn undergoes a C−O bond-forming reductive elimination to form the ester linkage delivering benzo[c]oxepinone 457 in high yields and enantioselectivities of up to 99.6% ee. In 2014, Dong disclosed a rhodium-catalyzed formal [4 + 2]cycloaddition reaction using 2-amino-3-picoline as a temporary directing group in order to facilitate oxidative addition of the cyclobutanone acyl-carbon bond (Scheme 89).204 This work will be described in more details in section 3.1.2. The activation of 2-o-styryl cyclobutanones 460 gives access to unsaturated dihydrobenzo[8]annulenones 463 and 464 (Scheme 90).205 Precoordination of rhodium(I) to the vinyl tether directs the metal insertion toward the more substituted bond, generating selectively rhoda(III)cyclopentanone 461. Subsequent migratory insertion of the vinyl moiety into the rhodium-acyl carbon bond generates intermediate 462. A nonselective β-hydride elimination of Ha or Hb, followed by reductive elimination delivers a mixture of enone 463 and ketone 464. α,β-Unsaturated ketone 463 is formed predominantly. The reaction is sensitive toward steric bulk, as 2,2disubstituted cyclobutanones or higher substituted olefins were not reactive. In 2012, Murakami reported a palladium(0)-catalyzed C−C bond activation of cyclobutanones 465 bearing a disilane tether allowing access to silaindane skeleton 469 in good yields (Scheme 91).206 During this transformation, both the Si−Si σbond and the C−C σ-bond of the cyclobutanone are cleaved. The authors suggested the following mechanism: first, an

Scheme 85

Scheme 86

AF


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Scheme 87

Scheme 88

Scheme 89

oxidative addition of the silicon−silicon bond of 465 to the palladium(0) species generates bis-silylpalladium(II) species 466, thus locating the palladium center in close proximity to the cyclobutanone unit. A second oxidative addition of the carbon-acyl bond of the cyclobutanone to the palladium(II) is

proposed giving rise to bicyclic palladium(IV) intermediate 467. A first reductive elimination of the C(sp3)-Si bond forms intermediate 468. A second reductive elimination from 468 affords acylsilane product 469 regenerating the palladium(0) catalyst. In a related report, Murakami reported a palladium(0)catalyzed ligand-controlled regiodivergent rearrangement of silacyclobutane containing cyclobutanones 470 (Scheme 92).207 When a bulky ligand such as diadamantanylbutylphosphine was used, silabicyclo[5.2.1]-decane 475 is obtained in good yields. In contrast, using the small trimethylphosphine as the ligand, aldehyde 476 is exclusively formed. Mechanistically, palladium(0) oxidatively adds first into the strained C−Si bond of the silacyclobutane moiety AG


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Scheme 90

Scheme 91

decarbonylation of unstrained ketones 479 are known as well (Scheme 93b). Besides a directed chelation, the C−CN bond activation of nitriles 483 leading to complex 485, a precoordination of the metal center to the reactive nitrile group in 484 helps the C−C bond cleavage (Scheme 93c).210

generating silapalladacycle 471. The palladium(II) center, which is brought into proximity of the cyclobutanone, stereoselectively undergoes another oxidative addition of the acyl-carbon bond furnishing suggested palladium(IV) intermediate 472. Subsequent reductive elimination forming the C(sp3)−Si bond gives rise to the cis-fused bicyclic palladium(II) intermediate 473. At this branching point, the sterically demanding ligand facilitates a reductive elimination producing 475 to relieve the steric congestion around the palladium center. The use of the less bulky ligand allows for agostic interactions between the β-hydrogen and the palladium center of intermediate 473. The resulting β-hydride elimination generates acylpalladium hydride 474, which in turn reductively eliminates to ring-opened aldehyde 476 (Scheme 92).

3.1. C−C Bond Activations Assisted by Chelation

Chelation-assisted C−C bond activations, operating through the coordination between metal complexes and organic substrates, has proven to be a fruitful strategy. In transformations of this type, the participation of the properly positioned chelating ligand located close to the targeted C−C bond of 477 results in the energy releasing formation of stabilized metallacyclic complexes 478 through the coordination-directed C−C bond cleavage. The use of permanent directing groups such as 8-acylquinoline 486 or pincer-type ligands 487, as well as transient directing groups such as imine 488, installed on the fly via condensation of a ketone with 2amino-3-picoline, were reported and are discussed in the following section. 3.1.1. Permanent Directing Groups. A first example of chelation-assisted C−C bond activation was reported in 1981 by Suggs and Jun (Scheme 95).211 A rhodium(I) complex coordinates to the nitrogen atom of 8-quinolinyl acetynyl ketone 489, directing oxidative addition of the metal toward the α-keto C−C bond and forming stable 5-membered rhodacyclic complex. A reaction of 8-quinolinyl ketone 489 with [{Rh-

3. C−C BOND ACTIVATIONS OF UNSTRAINED SUBSTRATES Besides strain-driven transformations, other facilitating strategies have been devised to enforce C−C bond activations (Scheme 93). In this respect, a chelation-assisted oxidative addition is a commonly used strategy for the C−C bond activation of unstrained molecules.208,209 Appropriate directing groups such as pincer-type ligands or nitrogen-containing groups, bring the metal center closer to the targeted C−C bond (Scheme 93a). Such chelation-assistance is also used to enhance the decarbonylation of unstrained ketones by carbon monoxide extrusion. However, examples of undirected AH


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Scheme 92

Scheme 93

version was developed employing higher reaction temperatures (100 °C), longer reaction times (48 h) and high ethylene pressure (6 atm).213 The exchange reaction with other alkenes was not successful. Limited to the reaction with ethylene, this process remained dormant for more than 20 years. In 2009, Douglas reported additional examples of C−C bond activation using 8-acylquinolines as a permanent directing group (Scheme 96).215 They developed a rhodium(I)-catalyzed olefin carboacylation of

(C2H4)2Cl}2] in the presence of an excess of pyridine gave the dipyridyl acyl rhodium(III) complexes 490.212 The formation of complex 490 is not observed for 8-quinolinyl alkyl ketones bearing β-hydrogen atoms (491).213 In this case, 8-quinolinyl ethyl ketone 494 is formed. Here, β-hydride elimination from rhodium(III) complex 492 delivered 1-butene and acyl rhodium(III) complex 493. Migratory insertion of an ethylene ligand across the metal hydride and subsequent reductive elimination affords 8-quinolinyl ethyl ketone 494.214 A catalytic AI


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oxygen linker by a nitrogen atom in 495 required the use of Wilkinson’s catalyst. The electron-donation from the amino group in the substrate renders the ketone less electrophilic and thus slows down the C−C bond activation step. However, the presence of a heteroatom linker is not required and dihydroindene 496 was obtained in 93% yield. Extension of the tether length allowed for the synthesis of dihydrobenzopyran 496 in 81% yield. Mechanistic studies on this transformation were performed by Johnson216,217 demonstrating that a kinetic isotope effect is observed with each catalyst: [{RhCl(PPh3)3}] or [{Rh(C2H4)Cl}2], implying the C−C bond activation as rate-limiting step for substrates with minimal alkene substitution (R = Me) (Scheme 97). In contrast, alkene insertion becomes rate-

Scheme 94

Scheme 97 Scheme 95

determining for more sterically encumbered olefins (R ≠ Me). Measurement of the energy requirement for the C−C bond activation revealed a ΔH value of ca. 28 kcal/mol for both catalysts. Kinetic studies with Wilkinson’s catalyst showed a rate law with a zero-order dependence on the substrate and firstorder dependence on the catalyst. In fact, high concentrations of 501 resulted in some product inhibition, suggesting rhodium complex 498 as the resting state. In contrast, in the presence of [{Rh(C2H4)Cl}2], the reaction exhibited first-order dependence on both the substrate and the catalyst, leading to an overall second order rate law. Consequently, no product inhibition was observed, indicating a resting state with an unbound rhodium catalyst. Electron-donating substituents on the aromatic linker were shown to increase the reaction rate, presumably by stabilizing the rhodium(III) intermediate 499. In addition to the intramolecular carboacylation of alkenes,215 Douglas developed an intermolecular version (Scheme 98).218 The authors chose 8-acylquinoline 503 and

Scheme 96

substrates 495 with tethered olefins leading to dihydrobenzofurans 496 in good to excellent yields. The process is tolerant toward modification of the olefin tether. By changing the catalyst to [{Rh(cod)2}OTf], an allyl ether moiety was also compatible affording 496 in 25% yield. Replacement of the AJ


Chemical Reviews

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rhodium(III) complex 510 by copper mediated consumption of oxygen. A second transmetalation gives 511 and subsequent reductive elimination delivers coupling product 512. 3.1.2. Temporary Directing Groups. Despite facilitating the C−C bond activation tendency, chelation-assisted C−C bond activations require the use of specific substrates with attached directing groups. The use of temporary directing groups which can be easily installed and removed is very attractive, offering an expansion of the reaction scope to substrates without the need of pre- or postfunctionalization steps. Suggs investigated the activation of aldehyde C−H bonds via temporary installation of a 2-aminopyridyl group.220,221 The active imine species is in situ generated requiring only catalytic amounts of the 2-aminopyridyl ligand.222 He later applied this “organic cofactor” process for the C−C bond activations of alkyl ketones 513 (Scheme 100).223 In the presence of Wilkinson’s catalyst, ketones 513 are converted into ketones 522 in a formal σ-bond metathesis reaction. Condensation of ketone 513 with 2-amino-3-picoline 514 forms imine 515, the substrate with the required installed temporary directing group. Rhodium coordination to the pyridyl nitrogen atom and subsequent C−C bond cleavage generates 5-membered rhodium(III) complex 516. β-hydride elimination liberates alkene 517 and leads to rhodium hydride intermediate 518. Coordination and insertion of olefin 519 affords complex 520. Reductive elimination forms the new C−C bond and closed the catalytic cycle. Finally, hydrolysis of imine 521 releases the 2aminopyridine cofactor and delivers ketone 522. The authors reported that the reaction rate increased dramatically by heating under microwave irradiation and employing solvent-free conditions (Scheme 101).224 Addition of catalytic amounts of cyclohexylamine further accelerated the reaction. For example, the reaction of benzylacetone with norbornene in the presence of 5 mol % of Wilkinson’s catalyst, 20 mol % of 2-amino-3-picoline 514 and 20 mol % of cyclohexylamine gives 85% of ketone 525 by heating to 200 °C under microwave irradiation for 5 min. In contrast, using conventional heating, only 16% of product 525 is formed under otherwise identical reaction conditions. In addition, a rhodium(I)-catalyzed ring-contraction of cycloalkanones were disclosed by Jun.225 Besides aldehydes and ketones, the C−C bond activation of sec-alcohols226 and allylamines227,228 were reported as well. In 2014, Dong disclosed a formal [4 + 2]-cycloaddition reaction using 2-amino-3-picoline 514 as a temporary directing group in order to facilitate oxidative addition of the cyclobutanone acyl-carbon bond (Scheme 102).204 Condensation of amine 514 with cyclobutanones 458 forms imines 527. As competent substrates 527 can now coordinate to the rhodium(I) catalyst which in turn oxidatively inserts into the C−C bond leading to 528. Subsequent migratory insertion of the tethered olefin gives intermediates 529. Reductive elimination provides bicyclic imines 530 and regenerates the rhodium(I) catalyst. Finally, imine hydrolysis delivers the bicyclic ketone products 459. Furthermore, the potential for an asymmetric version is shown and chiral phosphoramidite ligands provide preliminary levels of enantioselectivity of 37% ee.

Scheme 98

norbornene 399 as model substrates which both have no βhydrogen atoms available for β-hydride elimination. For instance, heating the reagents with [{Rh(cod)2}OTf] at 100 °C in THF, the C−C bond activation products 505 were obtained in moderated yields. In 2012, Wang coupled the chelation-assisted C−C bond activation with a C−C bond forming cross-coupling reaction.219 In the presence of Wilkinson’s catalyst, copper iodide and potassium carbonate under aerobic conditions, the reaction of 8-acylquinoline 506 with an aryl boronic acid resulted in the formation of the cross-coupled product 512 in moderate to high yields (Scheme 99). This coupling was successful with aryl Scheme 99

boronic acids containing electron-withdrawing or donating groups. Any substitution at the ortho-position was not tolerated. Halide-containing boronic acids were also well suited for this transformation. Mechanistically, the catalytic cycle is initiated by a directed C−C bond activation of the acetyl group in 506 giving rhoda(III)cycle 507. A first transmetalation with the aryl boronic acid leads to intermediate 508. Subsequent reductive elimination removing the methyl group from the catalyst affords rhodium(I) complex 509, which is reoxidized to

3.2. Decarbonylation

Compared to strained-ketone counterparts, examples of transition-metal-catalyzed CO extrusion from unstrained ketone substrates are scarce, (see section 3). Seminal work AK


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Scheme 100

Scheme 101

metal center generates intermediate 539. Carbonyl extrusion gives complex 540 and following reductive elimination delivers the decarbonylated product 541. For ketone substrates having β-hydrogens, dehydrogenation to enone 543 occurs prior to C−C bond cleavage (Scheme 104b). In 2013, Dong disclosed a rhodium(I)-catalyzed C−C bond activation of diynones 545 to synthesize conjugated diynes 548 through decarbonylative CO extrusion (Scheme 105).236 A variety of symmetrical and unsymmetrical aryl-substituted diynones undergo a catalytic decarbonylation process to give diynes 548 in moderate to high yields. Alkenyl and alkylsubstituted diynones can be used as well, although lower yields are observed for conjugated diynes bearing an alkyl substituent due to substrate decomposition.

on the stoichiometric decarbonylation of ketones using Wilkinson’s catalyst were reported in 1965.229 For particular ketones such as acyl cyanides230,231 and α- or β-diketones,232 catalytic palladium- and rhodium-catalyzed decarbonylations were disclosed. In addition to these highly activated substrates, ketone derivatives possessing a chelating group are as wellknown to undergo decarbonylations reactions. For instance, Murai reported alkyl phenyl ketones bearing an oxazoline or pyridine directing group on the phenyl group to enable ruthenium-catalyzed decarbonylative C−C bond cleavage.233 Recently, Sun and Shi reported rhodium(I)-catalyzed decarbonylation processes through the assistance of an N-containing directing group such as pyridine, quinolone, oxazole, or pyrazole (Scheme 103).234 The process tolerates a range of transferable groups such as alkenyl, alkyl, aryl, and heteroaryls giving the CO-extruded products 535 in moderate to excellent yields with 5 mol % of [{Rh(CO)2(acac)}]. The first undirected rhodium-catalyzed decarbonylative C−C bond cleavage of unstrained substrates was reported by Brookhart in 2004 (Scheme 104a).235 Albeit with a high catalyst loading of 25 mol %, benzo- and acetophenone derivatives were decarbonylated using a rhodium(I) catalyst that as a bulky Cp ligand (537). Complexation of the rhodium catalyst with the substrate leads to η2-enone complex 538. Subsequent oxidative addition of the acyl-carbon bond to the

3.3. C−CN Bond Activation

Despite the high bond dissociation energies of C−CN bonds (>100 kcal/mol), transition-metal complexes have been proven competent to cleave C−CN bonds. In 1971, DuPont described the oxidative addition of benzonitriles to nickel(0) to occur at ambient temperature.237 They later applied nickel(0)-catalyzed C−CN bond activations to the isomerization of 2-methyl-3butenenitrile for the synthesis of adiponitrile. Two pathways for C−CN bond activation are commonly accepted (Scheme 106). The metal coordinates first to the nitrile moiety in a η1- or η2fashion leading to metal complexes 549 or 550. Most of the AL


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Scheme 102

553 across alkynes 554 giving selectively (Z)-β-aryl-substituted alkenenitriles 559 (Scheme 107).239,240 A combination of [Ni(cod)2] and the electron rich trimethylphosphine ligand is required to obtain the desired alkenes in high yields. A variety of symmetrical and unsymmetrical alkynes as well as arylcyanides are compatible with the transformation. Subsequently, DFT studies investigating the reaction mechanism were performed.241 Initial coordination of the nickel catalyst to the nitrile set the stage for the oxidative addition of the aryl cyanide C−CN bond delivering nickel(II) intermediate 556. It was found that the oxidative addition is the rate-determining step. Minimization of the sterical interaction between the alkyne substituent R3 and the aryl group on the catalyst accounts for the observed regioselectivities. Subsequent migratory insertion of the alkyne into the nickel-aryl bond affords alkenyl nickel(II) species 558. Reductive elimination delivers the arylcyanation product 559. Besides arylcyanides, Nakao and Hiyama further reported on the nickel-catalyzed allylcyanation of alkynes with allylcyanides leading to (Z)-acrylonitriles 564 using an electron-poor triaryl phosphine ligand (Scheme 108).242,243 In this case, oxidative addition of the C−CN bond of allyl cyanide 560 to the metal catalyst affords π-allylnickel intermediate 562. A selective migration of the primary allylic carbon atom to the less hindered end of alkyne 561 forms alkenyl-nickel(II) intermediate 563. Final reductive elimination delivers the cisallylcyanation product 564. As before, a variety of alkynes, including terminal alkynes, are well suited for this transformation. It was found that the Lewis acid cocatalysts significantly facilitate nickel-catalyzed carbocyanations.244 Lewis acids are reported to enable oxidative additions of the C−CN bond to nickel catalysts by their coordinating to the cyanide nitrogen atom. The use of catalytic amounts of AlMe3 or AlMe2Cl

Scheme 103

catalytic C−CN bond activations are initiated by η 2coordination. A direct oxidative addition leading to C−CN cleaved metal complex 551 occurs with transition-metals such as Ni, Pd, Pt, or Rh (path A). Alternatively, some iron or rhodium silyl complexes result in the formation of silylnitrile complexes 552 (path B). This silicon-assisted mechanism was first reported by Bergman and Brookhart.238 3.3.1. Carbocyanation of Alkynes. Carbocyanation is a reaction involving transition-metal-catalyzed C−CN bond activation and subsequent addition of the in situ generated metal-cyanide across unsaturated bonds. Nakao and Hiyama demonstrated nickel(0)-catalyzed additions of aryl cyanides AM


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Scheme 104

Lewis acid catalysis, arylcyanation products 567 are generally obtained in yields over 90% and even labile aryl halides as well as sterically more demanding substrates are tolerated by using this improved procedure (Scheme 109). Moreover, not only aryl cyanides 565 but as well alkenyl-,244 benzyl-,245 and alkylcyanides244 (568, 573, and 571, respectively) are suitable substrates. Even the Lewis acid cocatalysts significantly promote the carbocyanation reaction, a reaction of propionitrile results in low yields of the ethylcyanation product due to competing β-hydride elimination.244 This undesired β-hydride elimination pathway could be suppressed with bulky monodentate ligands such as Buchwald’s S-Phos ligand.246 Alkyl nitriles 575 bearing a heteroatom at the γ-position were shown to add stereo- and regioselectively across alkynes 576 to give acrylonitriles 580 (Scheme 110).247 Notably, no hydrocyanation products formed via competing β-hydride elimination are formed. A coordination of the heteroatom to the nickel center forms five-membered chelate 577, preventing this undesired β-hydride elimination.

Scheme 105

significantly accelerates the reaction and the catalyst loading could be lowered to 1 mol % nickel. By this cooperative nickel/ Scheme 106

AN


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Scheme 107

Scheme 108

Scheme 109

The substrate scope was further extended to alkynyl cyanides 581 yielding conjugated enynes 583 (Scheme 111).248,249 In this case, triphenylborane was found to be the Lewis acid of choice, giving products 583 in high yields. Besides alkyne acceptors, alkynyl cyanide 585 was shown to add across terminal allenes 586 (Scheme 112).248,249 For alkylsubstituted allenes, a selective reaction at the internal double bond delivers enynes 590. In contrast, a silylallene exhibits the opposite regioselectivity giving 591. The transformation follows a similar mechanism initiated by a BPh3-assisted oxidative addition of the C(sp)-CN bond of nitrile 585 to nickel(0). The oxidative addition of C−CN bonds adjacent to carbonyl groups such as acyl cyanides, cyanoformates or cyanoformamides generally proceeds more easily. Oxidative addition of the C(O)−CN bond to transition-metal catalysts gives rise to intermediate 593 (Scheme 113). Due to their propensity to undergo decarbonylation to 594,231 complexes arising from acyl nitriles are less stable than their ester or carbamoyl congeners generated from cyanoformates or cyanoformamides. On a

productive reaction pathway, alkyne migratory insertion and subsequent reductive elimination of 595 gives cyanoesterification or cyanocarbamoylation products 596. In order to suppress the decarbonylation pathway, intramolecular transformations with reactive alkyne acceptors were investigated first. In this respect, Nakao and Hiyama used their nickel/Lewis acid cooperative catalysis for alkyne cyanoesterifications and cyanocarbamoylations.250 Moreover, Takemoto disclosed a palladium(0)-catalyzed cyanocarbamoylation of AO


Chemical Reviews

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Scheme 110

Scheme 111

temperatures under microwave irradiation for short reaction times in Lewis basic solvents. Higher yields were observed for electron-rich, sterically unencumbered alkynes. To account for these findings, the authors propose the migratory insertion of the alkyne leading to 601, rather than the C−CN bond activation, as the rate-determining step. Nakao and Hiyama reported intermolecular cyanoesterifications of allenes 604 using a dual nickel(0)/Lewis acid catalytic system (Scheme 115).254,255 As before, nickel(0) oxidative addition of the C−CN bond initiates the reaction and gives intermediate 605. Coordination of the allene at its terminal double bond, followed by formate group transfer at the central allene carbon atom provides π-allylnickel species 607. Reductive elimination delivers the product 608. 3.3.2. Carbocyanation of Alkenes. Besides the shown examples with alkyne acceptors, alkenes were found to engage in cyanoesterification and cyanocarbamoylation reactions. For instance, Nishihara reported intermolecular palladium(II)catalyzed cyanoesterifications of strained reactive alkenes 610 such as norbornene or norbornadiene (Scheme 117).256−258 Modification of the ester moiety of 609 with a variety of alkyl groups is tolerated and the functionalized norbornene derivatives 613 are obtained in moderate to high yields with high exo-selectivity. However, the process is limited to activated norbornenes and other less reactive olefin acceptors fail. The reactivity was later extended to the nickel(0)-catalyzed arylcyanation of norbornene (Scheme 122).259

Scheme 112

alkynyl cyanoformamides 597 (Scheme 114).251,252 The reaction proceeds exclusively in a 5-exo cyclization mode with high (Z)-selectivity of the corresponding lactam products 598. In addition, six- and seven-membered lactams could also be obtained in high yields. Douglas reported intramolecular palladium(0)-catalyzed cyanoesterifications of alkynes 599 providing butenolides 602 in moderate to high yields (Scheme 115).253 The undesired decarbonylation event could be minimized by heating at high AP


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Scheme 113

Scheme 114

Scheme 117

Scheme 115

of this transformation (Scheme 118).260,261 Good enantioselectivities of up to 86% ee are obtained with Feringa ligand L5. A range of substituents on the vinyl as well as on the aromatic core are compatible. However, without the rigidifying aryl backbone, the transformation is significantly less efficient (15% and 27% ee). The utility of this process was demonstrated in synthetic studies toward a synthesis of vincorine (618).262 Hiyama and Nakao263,264 and Jacobsen265 independently developed an asymmetric nickel-catalyzed intramolecular carbocyanation of olefin providing indanes 620 in high yields and enantioselectivities of over 90% ee with Foxap L13 and TangPHOS L14, respectively (Scheme 119). The Lewis acid cocatalyst had a great influence on the reaction yields in both cases. This asymmetric carbocyanation process was applied to the total synthesis of (−)-esermethole (623) as well as to the formal synthesis of (−)-eptazocine (626) (Scheme 120).263 In the latter case, the enantioselective formation of the sixmembered intermediate 625 was best performed with ChiraPhos L9 as chiral ligand. Due to the strong tendency for decarbonylation, carbocyanations with acyl nitriles are problematic. To work around this issue, Douglas reported a formal palladium-catalyzed intramolecular acylcyanation of alkenes using α-iminonitriles 627 as acylnitrile surrogates (Scheme 121).266 Iminonitriles do not undergo decarbonylation during the reaction and the carbonyl group is revealed upon hydrolytic workup of imine products 628. This process tolerates different substituents on the arene, as well as various 1,1-dialkyl substituted olefins and the substituted indanones 629 are obtained in moderate to high yields. 3.3.3. Coupling Reactions via C−CN Bond Activations. Aryl cyanides can also be regarded as aryl pseudo halides in cross-coupling reactions, even though the C−CN bond dissociation energy (>100 kcal/mol) is much higher than for aryl chlorides. For instance, C−CN bonds can be reduced to C−H bonds by reactions with metal hydrides, thus making the cyano group a removable directing group. Nickel-catalyzed

Scheme 116

While intermolecular carbocyanations of alkynes and highly reactive alkene derivatives proceed efficiently, less reactive olefin acceptors require intramolecular processes. Takemoto reported a palladium(0)-catalyzed intramolecular cyanoamidation of olefins yielding 3,3-disubstituted oxindoles 617.251,252 Later on, the same group disclosed an enantioselective version AQ


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Scheme 118

Scheme 119

Scheme 120

hydride source. Initial oxidative addition of C−CN bonds to the nickel complex forms 632. Transmetalation with the hydride source yields nickel-hydride 635 and subsequent reductive elimination delivers defunctionalized product 631.

reductive C−CN bond cleavages of aryl and aliphatic cyanides 630 were reported by Maiti affording the decyanated compounds 631 in good yields (Scheme 122). Either tetramethyldisiloxane267 or hydrogen gas268 were used as the AR


Chemical Reviews

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Scheme 121

Scheme 122

oxidative addition of the C−CN bond to rhodium leading to intermediate 648. Intercepting the intermediate 648 with disilane 644 regenerates the active silyl-rhodium complex 646 and forms silylated product 640 or arylated compound 642 together with silyl isocyanide, which isomerizes to thermodynamically more stable silyl cyanide 649. Moreover, Chatani and Tobisu demonstrated that rhodium intermediates 654 formed by C−CN bond cleavage with silyl rhodium intermediate 646 are susceptible toward trapping with vinyl silanes 651 to give Heck-type products 652 (Scheme 124).273 Mechanistically, an addition of intermediate 654 to vinylsilane forms alkyl rhodium complex 656 which undergoes subsequent β-hydride elimination delivering the alkenylated product 652 and a rhodium-hydride species 657. Reaction of 657 with disilane 644 regenerates the active catalyst 646. Orthogonal and iterative functionalizations enabled the synthesis of elaborated arenes. The transmetalations described above can be extended to other organometallic reactants in order to broaden the range of the cross-coupling reactions. Pioneering studies performed by Miller showed the nickel-catalyzed cross-coupling of aryl-, alkyl, and alkenyl-Grignard reagents with aryl nitrile derivatives afforded the corresponding coupled-products in good yields.274−276 The reactivity of the Grignard reagent could be tamed by the addition of either LiOt-Bu or PhSLi in order to prevent premature its direct addition to the nitrile. Crosscoupling with nitrogen-based nucleophiles affording aniline derivatives are also described.276 The reactivity was further extended to the alkynylation of aryl cyanides yielding internal alkynes with alkynyl zinc reagents.277 More recently, a nickel-

Relatively high catalyst loadings are required, presumably because of the formation of catalytically inactive [Ni(PCy3)2(CN)2]. As illustrated previously, trimethylaluminum species can facilitate the desired C−CN bond activation. Under these reaction conditions, a wide range of functional groups are tolerated, including allyl cyanides with β-hydrogen atoms. Chatani disclosed rhodium-catalyzed hydrodecyanation reactions using hydrosilane 633 as the reducing agent (Scheme 121).269 A variety of aryl and alkyl cyanides, including tertiary alkyl cyanides 630 could be reduced. It is believed that the generation of silyl metal species 636 initiates the catalytic cycle. Nitrile coordination leads to iminoacylmetal intermediate 637. Rearrangement to silylnitrile 638 followed by extrusion of silyl isocyanide delivers product 631. An iron-catalyzed procedure was developed by Nakazawa.270 In this case, photolysis is required to initially activate the iron catalyst. Besides hydrogenative defunctionalizations reactions, replacements of the nitrile functionality by silyl groups were reported. A rhodium-catalyzed silylative decyanation of nitriles with disilanes was developed by Chatani and Tobisu.271,272 C− CN Bonds in aryl, alkenyl, allyl, and benzyl cyanides 639 bearing a variety of functional groups are exchanged for silyl groups in moderate to excellent yields. With certain substrates intramolecular arylations is realized instead.272 In this respect, the aryl-rhodium intermediate generated upon cleavage of the aryl−CN bond of 641 is trapped intramolecularly with aryl halides to give fluorenes, dibenzofurans or carbazoles 642 (Scheme 123). Mechanistically, the reaction of the rhodium precatalyst with disilane 644 generates silyl-rhodium species 646. Formation of the η2-iminoacyl complex 647 is followed by AS


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metrical biaryl and styrenyl derivatives 661 in moderate to good yields.

Scheme 123

Scheme 125

The nickel-catalyzed phosphorylation of aryl nitriles 662 was explored by Yang (Scheme 126).279 This protocol allows for Scheme 126

the preparation of substituted monodentate phosphine ligands 664 using [NiCl2(PPh3)2] as the catalyst and diphenyl(trimethylsilyl)phosphane 663 as the phosphide source. The development of formal cycloadditions via sequential C− CN bond activation followed by another C−C bond activation was reported by Kurahashi and Matsubara.280,281 In details, the transformation consists of a nickel-catalyzed reaction of arylcarboxybenzonitriles 665 and alkynes 666 (Scheme 127). Scheme 124 Scheme 127

Oxidative addition of the aryl−CN bond to nickel catalyst forms intermediate 667. Subsequent C−C bond cleavage of the acyl-aryl bond leads to the formation of five-membered nickelacycle 668 together with arylcyanide. Migratory insertion of the alkyne gives intermediate 669 which undergoes reductive elimination delivering product 667. Coumarins and quinolones 670 are accessed in high yields in the presence of [Ni(cod)2]

catalyzed Suzuki-Miyaura coupling using boronic esters 660 was disclosed by Shi (Scheme 125).278 Different aryl- and alkenyl-boronic esters are tolerated, affording the unsymAT


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and PBn3 using MAD (methyl aluminum bis(2,6-ditert-butyl-4methyl phenoxide)) as Lewis acid (Scheme 127). Unsymmetrical alkyne acceptors give the desired product in moderate to excellent regioselectivities.

Laetitia Souillart graduated as chemical engineer from the ENSC Lille, France in 2010. She obtained her Ph.D. at the EPF Lausanne (EPFL) under the guidance of Prof. Nicolai Cramer. Her research interest mainly focused on the development of enantioselective rhodiumcatalyzed C−C bond activations of strained rings. Currently she is a postdoctoral fellow in the group of Prof. Alois Fürstner in the Max Planck Institute für Kohlenforschung, Germany.

4. SUMMARY AND OUTLOOK Selective cleavages of carbon−carbon bonds catalyzed by transition-metal complexes have been shown to be increasingly versatile tools for organic synthesis allowing for complementary synthetic strategies. Yet it remains a challenging task due to the weak interactions of the sterically and directionally constraint C−C bond with the d orbitals of transition metals. In this review, recent advances in catalytic activations proceeding via oxidative addition of C−C bonds to transition-metals were described. In particular, the clever exploitation of the strain release provided by the C−C bond cleavage of small rings is one of the most important driving force to promote C−C bond activations. As a result, numerous examples of transition-metal catalyzed C−C bond activations of three- and four-membered ring systems have been reported. These strained rings have been shown to engage in a variety of new ring-opening rearrangements and cycloaddition reactions leading to valuable structures. As such, they were explored as attractive synthetic strategies for complex molecule synthesis. Besides strain-driven transformations, other facilitating strategies to enforce the C−C bond activation of unstrained molecules have been developed as well. While the variety of different transformations is less abundant, they concentrate on chelation-assisted reactions using appropriate permanent or transient directing groups. In particular, the cleavage and subsequent functionalization of the C−CN bonds and decarbonylation processes operating by an excision of carbon monoxide from ketone derivatives have witnessed a large progress. In the future, transformations induced by C−C bond cleavages open many opportunities for the design and discovery of new reactivities.

Nicolai Cramer studied chemistry at the University of Stuttgart, Germany. He stayed at the same institution and earned his Ph.D. degree in 2005 under the guidance of Professor Sabine Laschat. After a short research stage at Osaka University, Japan, he joined the group of Professor Barry M. Trost at Stanford University as a Feodor-Lynen postdoctoral fellow in 2006. From 2007 to 2010, he worked on his habilitation at the ETH Zurich, Switzerland associated with the chair of Professor Erick M. Carreira and received his venia legendi. In 2010, he joint EPF Lausanne, Switzerland as Assistant Professor and was promoted to Associate Professor in 2013. His general interests encompass enantioselective metal-catalyzed transformations and their implementation for the synthesis of biologically active molecules. A key focus of his research is the development of asymmetric C−H and C−C bond functionalizations enabled by designed and tailored ligands.

AUTHOR INFORMATION Corresponding Author

ACKNOWLEDGMENTS This work is supported by the European Research Council under the European Community’s Seventh Framework Program (FP7 2007−2013)/ERC Grant Agreement No. 257891.

*E-mail: Nicolai.cramer@epfl.ch. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

REFERENCES

The authors declare no competing financial interests.

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