Cobalt-Catalyzed CâH Activation - ACS Catalysis (ACS Publications)

Review pubs.acs.org/acscatalysis

Cobalt-Catalyzed C−H Activation Marc Moselage,¶ Jie Li,¶ and Lutz Ackermann*

ACS Catal. 2016.6:498-525. Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/15/18. For personal use only.

Institut für Organische und Biomolekulare Chemie, Georg-August-Universität, Tammannstraße 2, 37077 Göttingen, Germany ABSTRACT: Catalytic C−H activation has emerged as a powerful tool for sustainable syntheses. In the recent years, notable success was achieved with the development of cobaltcatalyzed C−H functionalizations with either in situ generated or single-component cobalt-complexes under mild reaction conditions. Herein, recent progress in the field of organometallic cobalt-catalyzed C−H activation is reviewed until November 2015.

KEYWORDS: C−H activation, cobalt, C−C formation, C−Het formation, homogeneous catalysis

1. INTRODUCTION In the recent years, considerable advances have been achieved in transition-metal-catalyzed C−H functionalization, thereby characterizing this research area as an increasingly viable toolbox for selective C−C and C−X bond-forming reactions.1 Thus, C−H functionalization processes can significantly simplify the synthesis of pharmaceuticals, natural products, agrochemicals, polymers, and feedstock commodity chemicals.2 Until recently, the majority of catalyzed C−H functionalization was achieved employing precious second- and third-row transition metals.3 The development of catalysts based on the naturally more abundant and, hence, cost-efficient 3d transition metal complexes, represents an attractive alternative.4 As a result, the use of inexpensive first-row transition-metal catalysts for C−C transformations has gained considerable recent momentum. In this context, rather environmentally benign cobalt complexes bear great potential for applications in homogeneous catalysis. In 1941, Karash and Fields disclosed the cobalt-catalyzed homocoupling of Grignard reagents.5 Subsequent to these pioneering findings, 70 years of research has turned cobalt into one of the most promising metals for future use in homogeneous catalysis, with important applications among others in the hydroformylation,6 or the Phausen−Khand reaction.7 Given the success of precious 4d and 5d transition metals with d8 or d10 electron configuration, numerous studies have recently been undertaken to explore easily accessible cobalt complexes for C−H activation chemistry. Yet, a notable challenge is constituted by the fact that the electronic properties of the 3d transition metal cobalt significantly differ from those of 4d or 5d homologues in terms of electronegativity (Figure 1) and spin orbit couplings, among others.8 The reduced electronegativity of cobalt as compared to the homologous group 9 elements translates into more nucleophilic © 2015 American Chemical Society

Figure 1. Electronegativities of Group 8, 9, and 10 elements according to the Pauling scale.

organometallic cobalt intermediates which allow for unprecedented reaction pathways in transition-metal catalyzed C−H activations as well as significantly improved positional and chemo-selectivities. Herein, we summarize the progress in stoichiometric and catalytic organometallic C−H activation9 with cost-effective cobalt complexes until autumn 2015. C−H functionalizations by outer-sphere radical-type reaction manifolds are not covered herein.10

2. STOICHIOMETRIC C−H ACTIVATION As to the elementary step of C−H cobaltation, Klein and coworkers isolated the cyclometalated cobalt complex 2 by treating azobenzene (1) with [Co(CH3)(PMe3)4] (3) (Scheme 1).11 The cyclometalation process was proposed to proceed via coordination of the Lewis basic directing group with loss of one PMe3 ligand, along with oxidative addition of the cobalt(I) Received: October 18, 2015 Revised: November 21, 2015 Published: November 24, 2015 498

DOI: 10.1021/acscatal.5b02344 ACS Catal. 2016, 6, 498−525

Review

ACS Catalysis Scheme 1. Cyclocobaltated Compounds by Stochiometric C−H or C−F Cleavage Using [Co(CH3)(PMe3)4] (3)

instance, alkyl iodides or ethylene, led to a variety of cobalt complexes, while the C−Co was not cleaved here.

species into the C−H bond and reductive elimination of methane to give the cyclometalated species 2. In contrast, replacing the cobalt(I) complex by [Co(C2H4)(PMe3)3] did not lead to the desired cyclometalation. In subsequent studies, Klein and co-workers showed that various arenes 1−16, containing phosphorus,12,13 nitrogen,14 oxygen,15 or sulfur16 donor groups also form cyclocobaltated complexes 2−17. The cyclometalation was not restricted to the formation of fivemembered cobaltacycles, but the formation of six-membered complex 13 and even less favorable four-membered cobaltacycle 15 could be realized as well. When [Co(CH3)(PMe3)4] (3) was reacted with pentafluorobenzophenone (16), the site-selective cyclometalation was observed at the electron-deficient fluorinated arene, thus indicating the ortho-C−F cleavage being favored over the ortho C−H metalation.15 The formation of stoichiometrically metalated complexes was thereby well established. Despite these undisputed advances, further C−C, C−N, or C−X forming functionalizations remained rare in the following years. A notable exception was constituted by the transformation of the strained 4-membered cyclocobaltated complex 15 (Scheme 2).12 Under an atmosphere of ambient CO, insertion of CO into the C−Co bond yielded the acyl complex 18, albeit in rather low yield. When complex 18 was further exposed to CO, a cobalt carbonyl complex was formed, while the C−Co bond of the 5membered metallacycle remained intact.11 Several other manipulations of the cyclometalated complexes with, for

3. CATALYTIC C−H ACTIVATION 3.1. Early Examples. Murahashi elegantly unraveled a cobalt-catalyzed carbonylative cyclizations of azobenzene (1) or aldimines to indazolones 19 (Scheme 3),17,18 which in more Scheme 3. Cobalt-Catalyzed Carbonylative Cyclization of Azobenzene 1

general terms represent early examples of chelation-assisted C− H functionalization reactions. Though this cobalt-catalyzed process indicated the potential of cobalt complexes for C−H functionalizations, further applications were largely limited by the rather forcing reaction conditions, that is a reaction temperature of 190 °C at a pressure of 150 atm. 3.2. Addition Reactions. 3.2.1. Hydroarylation Reactions. The development of modern cobalt-catalyzed C−H functionalization remained somewhat dormant, until Kisch and coworkers reported, in 1994, the first cobalt-catalyzed hydroarylation of alkynes 20 using a cobalt(I) complex featuring a labile N2 ligand (Scheme 4).19 Here, the active catalyst was generated via substitution of the N2 ligand by the Lewis-basic azobenzene substrate 1. Alternatively, [CoH(H2)(PMe3)3] was also successfully employed as the precatalyst. Based on Kisch’s work, the development of cobalt-catalyzed hydroarylations of alkynes and olefins gained a significant momentum during the last five years, especially through the elegant work of Yoshikai. Thus, Yoshikai and co-workers devised a ternary catalytic system consisting of CoBr2, a

Scheme 2. Insertion of CO into a C−Co Bond

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work of Ackermann on C-2-selective C−H functionalizations on heteroarenes using a removable pyrimidyl directing group,21 the hydroarylation of alkynes 20 with indoles 24 and benzimidazoles 25 was realized by low-valent cobalt catalysis at ambient reaction temperature in a site-selective manner (Scheme 5b).22 Imines also turned out to be powerful directing groups. The additions of aryl ketimines 28 or aldimines 29 to internal alkynes were achieved by a catalytic system comprising of CoBr2, a phosphine ligand, a Grignard reagent, and pyridine.23 After acidic hydrolysis, diarylacetylenes 20 afforded the corresponding aromatic ketones 30a−d (Scheme 6a) and

Scheme 4. Cobalt-Catalyzed Hydroarylation of Tolane (20a)

Scheme 6. Cobalt-Catalyzed Hydroarylation with Aryl Ketimines 28 and Aldimines 29

phosphine ligand (PMePh2) and a stoichiometric reductant (MeMgCl), which efficiently catalyzed the hydroarylation of alkynes 20 with 2-arylpyridines 22 to yield the desired products 23 at lower catalyst loadings (Scheme 5a).20 On the basis of the Scheme 5. Cobalt-Catalyzed Hydroarylation of Alkynes 20

aldehydes 31 (Scheme 6b), while the hydrolysis of alkenylated imines 32 with alkyl substituents on the alkene led to a cyclization, furnishing benzofulvene derivatives 33 and 33′ (Scheme 6c).23,24 It is noteworthy that the hydroarylation of unsymmetrical dialkyl-substituted alkynes proceeded with synthetically useful levels of regio control. Furthermore, Petit and coworkers recently employed a low-valent cobalt catalyst for hydroarylations in the absence of Grignard reagents.24b The proposed catalytic cycle of the cobalt-catalyzed hydroarylations is initiated by an ill-defined low-valent cobalt complex, generated from the cobalt(II) precatalyst and an excess t-BuCH2MgBr (Scheme 7). Precoordination of the alkyne 20 to the active cobalt species is followed by oxidative addition of the ortho C−H bond in intermediate 34 to give the cobalt complex 35. Intramolecular hydrocobaltation of complex 500


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ACS Catalysis Scheme 7. Plausible Catalytic Cycle for the Cobalt-Catalyzed Hydroarylation of Internal Alkynes 20

Scheme 8. Additions of (a) Azoles 37 or (b) Thiazoles 38 to Alkynes 20

Scheme 9. Cobalt-Catalyzed Annulations with α,βUnsaturated Imines 42

35 and subsequent reductive elimination of the intermediate 36 furnish the desired product and regenerate the cobalt catalyst.23,25 In the case of unsymmertrical alkynes 20, the C−C bond formation takes predominantly place at the less hindered acetylenic site. This regioselectivity can be rationalized in terms of significant steric interactions exerted by the cobalt center within the elementary step of migratory insertion. The reason as to why stoichiometric amounts of Grignard reagents are required beyond the reduction of the cobalt precursor is as of yet not fully understood. Exploiting the differences in the inherent kinetic acidities is another viable approach to perform site-selective C−H functionalizations of heteroaromatic compounds. Thus, Yoshikai and co-workers developed a cobalt-catalyzed syn-addition of (benz)oxazoles 37 and thiazoles 38 onto internal alkynes 20 via C−H functionalization with high chemo-, site- and stereoselectivities (Scheme 8).26,27 Notably, the hydroarylation proceeded not only with arenes but also with olefins via vinylic C−H activation.28 Thus, dihydropyridine derivatives 41 were prepared by annulations of α,β-unsaturated imines 42 (Scheme 9). Related to similar reactions of this type by rhodium(I) catalysis,29,30 the mechanism was proposed to proceed by oxidative C−H activation/alkenylation followed by 6π-electrocyclization of the key azatriene intermediate.28 On the basis of the remarkably high efficacy of the alkyne hydroarylations, Nakamura and Yoshikai succeeded in developing analogous hydroarylations of generally more challenging alkenes 43, again using a low-valent cobalt-catalyst.31 Hence, Nakamura and co-workers reported on the hydroarylation of alkenes 43 with benzamides 44 (Scheme 10). Meanwhile, the Yoshikai group established hydroarylations of styrenes 43 giving selectively linear or branched adducts largely by the judicious choice of either a phosphine or a Nheterocyclic carbene (NHC) ligand (Figure 2), respectively.32 Thus, a catalytic system consisting of CoBr2, IMesHCl (46a) as a NHC precursor and neo-pentylmagnesium bromide afforded the linear alkylated products 47 with moderate to good selectivities, as depicted in Scheme 11a.32

Scheme 10. Cobalt-Catalyzed Alkene Hydroarylation with Benzamides 44

The selectivity also depended on the electronic features of the substrate. For example, when 4-trifluoromethylphenylpyridine (22a) was employed, both catalytic systems delivered the branched adduct as the major product 47aa′. Also the use of 2vinylnaphtalene (43b) significantly decreased the selectivity for the linear addition. Besides styrene derivatives 43, vinylsilanes 43c as well as alkyl-substituted alkenes 43d proved applicable for hydro501


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allowed for the branched-selective addition of arylpyridines as well as PMP-protected imines to styrenes 43 (Scheme 12a,b).25,32 The catalytic system showed a broad substrate Scheme 12. Branched-Selective Hydroarylation of Alkenes 43

Figure 2. NHC preligands utilized for cobalt-catalyzed C−H funtionalizations.

Scheme 11. Hydroarylation of Alkenes with Linear Selectivity

scope with good to excellent selectivities. When exploring aldimines 29 as the substrates, a substituent at the ortho position was necessary to prevent 2-fold hydroarylation (Scheme 12b).25,34 Interestingly, the double functionalization did not occur with ketimines 28 displaying increased steric bulk (Scheme 12c).25,35 Experiments with pentadeuterophenylpyridine revealed a H/ D exchange between the ortho-positions of the arene and the olefinic C−H bonds to take place prior to the C−C formation. A mechanism addressing these findings was proposed (Scheme 13),25 in which a reversible oxidative addition of the in situgenerated cobalt catalyst into the ortho-C−H bond provides intermediate 50. Thereafter, migratory insertion of the styrene into the Co−H bond leads to either the linear 51 or the branched intermediate 51′. Finally, irreversible reductive

arylations with linear selectivity. In the case of vinylsilanes 43c, 1,10-phenantroline turned out to be the ligand of choice for the reaction with ketimine 28e (Scheme 11b).33 The addition of Npyrimidylindole to vinylsilane was achieved in a similar fashion (not shown). When an aliphatic olefin, such as 43d, was used, the best yield was obtained with neocuproine (2,9-dimethyl1,10-phenantroline) as the ligand and Me3SiCH2MgCl as the stoichiometric reductant, which gave the alkylated product 48ed in 68% yield (Scheme 11c).33 In contrast to the linear-selective hydroarylation, a protocol for the preparation of branched products would arguably be more valuable in terms of stereoselectivity. A catalytic system based on CoBr2, a phosphine ligand and Me3SiCH2MgCl 502


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ACS Catalysis

whereas the SIMes analogue inverted the regioselectivity toward the formation of dihydropyrroloindole 54. Although the scope with homoallyl and bis(homoallyl) indoles was rather moderate, the study represented a proof of concept for a regiodivergent cobalt catalysis. When a methylene cyclohexenyl indole was submitted to the reaction conditions, the product 55a bearing a bicyclic [3.3.1]-moiety was isolated in 43% yield. The cyclization with homoallyl indoles delivered the six-membered ring compound as the major product 55b with just minor amounts of the seven-membered product. This observation illustrated that the regioselectivity was strongly influenced by the steric properties of the NHC ligand, while a dependence on the tether length was also of relevance. Implementing ketamine-assisted hydroarylations of alkenes, a tandem isomerization/hydroarylation sequence proved viable as well.36b Addition reactions with low-valent cobalt catalysts were not restricted to alkynes and alkenes.37 Indeed, aziridines 56 proved likewise applicable, yielding β-aryl amines 57 (Scheme 15a).37a

Scheme 13. Proposed Catalytic Cycles for the Branched- and Linear-Selective Hydroarylations of Styrenes 43a

Scheme 15. Alkyation of Aryl Pyridines with Aziridines 56 (a) and Aldimines (b)

elimination governs the regioselectivity and furnishes either the linear or branched alkylated arene. The first example of an intramolecular cobalt-catalyzed hydroarylation of alkenes via C−H activation at the C-2 position of indoles, was reported by Yoshikai and co-workers (Scheme 14).36a The reaction allowed for the direct transScheme 14. Cyclization of Indoles Bearing Olefin Tethers (rr: Regioisomeric Ratio)

The reaction displayed a broad substrate scope with excellent site- and regioselectivities in moderate to good yields. Moreover, reaction conditions were devised for powerful hydroarylations of aromatic aldimines 29 (Scheme 15b).37b In this context, it is noteworthy that Matsunaga/Kanai and co-workers developed unusual cobalt-catalyzed remote C-4alkylations38 of pyridines with styrenes, occurring in a branched-selective fashion (Scheme 16).39 Very recently, the first enantioselective hydroaralytion of styrenes 43 with indoles 60 was disclosed by Yoshikai.40 A chiral phophoramidate ligand 61 delivered the branched alkylated products in moderate to good yields with remarkable levels of enantioselectivity (Scheme 17). Among various monoand diphosphine ligands, the phosphoramidate 61 gave best results. Furthermore, yields and enantioselectivities strongly

formation of indole derivatives 52 and 53 into dihydropyrroloindoles 54 or tetrahydropyridoindoles 55 under mild reaction conditions. Interestingly, the ring size of the formed product did not only depend on the length of the tether, but was also controlled by the steric properties of the NHC ligand. Thus, the cobalt catalyst derived from the NHC IPr promoted the regioselective cyclizations to afford tetrahydropyridoindoles 55, 503


Review

ACS Catalysis Scheme 16. Cobalt-Catalyzed C-4-Selective Alkylation of Pyridine (58)

Scheme 18. Intramolecular and Intermolecular Hydroacylations

Scheme 17. Enantioselective Hydroarylation of Alkenes 43

Scheme 19. Hydroacylation of Aldehydes with 1,3-Dienes* depended on the substitution pattern on the N-position of the indole. While methyl, tosyl, and acetyl groups gave unsatisfactory yields and low enantioselectivities (not shown), the NBoc protected indoles 60a-c delivered optimal results. The substrate scope included a range of styrene derivatives 43 that gave the branched alkylated indoles 62 with good enantioselectivities. 3.2.2. Hydroacylation Reactions. The first cobalt-catalyzed hydroacylations were developed by Vinogradov41 (Scheme 18a) and later Brookhart.42,43 The latter protocol utilized the cobalt(I) complex 63 for the intermolecular hydroacylation of vinyl silane 43e with aromatic and aliphatic aldehydes 64 (Scheme 18b).42,43 The reaction proceeded under mild reaction conditions in excellent yields with low catalyst loading. Despite the enormous potential of this catalytic system at this time, the hydroacylation was limited to vinyl silanes. Attemps to perform hydroacylations with ethylene using [Cp*Co(C2H2)2] as the catalyst did not deliver the desired ketone. In contrast, recent progress in the field of cobalt-catalyted hydroacylation was achieved by Dong and Yoshikai.44−46 Hence, Dong and coworkers developed hydroacylations of aromatic and aliphatic aldehydes 64 with 1,3-dienes 66 (Scheme 19).44 This transformation turned out to be challenging with respect to regio- and diastereoselectivity. First, depending on the regioselectivity, the hydroacylation could occur at the C-3 position of the diene, as shown by Krische47 and Ryu48 for ruthenium catalysis, as well as at the C-1 position. Second, the position of the second double bond could lead to β,γ or γ,δ unsaturated ketones. Third, in the case of β,γ unsaturated systems, the configuration of the double bond had to be controlled. After optimizing the reaction conditions, a catalytic system consisting of the cobalt(II) complex 67 bearing a modified dppp ligand 68 and catalytic amounts of In and InBr3 to generate the catalytically active Co(I) species enabled the C-

[a]

[Co(dcpe)I2] (5.0 mol %), In (20 mol %), InBr3 (5.0 mol %), DCE/EtOAc (3:1), 50 °C, 20−24 h; dcpe = 1,2-bis(dicyclohexylphosphino)ethane. *Z/E selectivity >20:1, unless otherwise noted.

1-selective hydroacylation of aromatic aldehyds with 1,3-dienes in a highly selective 1,4-addition manner and very good Zselectivity.44 The broad scope with different aryl aldehydes 64 504


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ACS Catalysis Scheme 21. Hydroacylation of Alkenesa

and dienes 66 highlighted the synthetic utility of this catalyst. When using aliphatic aldehydes, the selectivity between β,γ and γ,δ unsaturated ketones significantly dropped. However, when the bis-aryl diphosphane ligand 68 was replaced by a more electron-donating bis-cyclohexyl diphosphane ligand, γ,δ unsaturated ketone 69aa′ was obtained in very good yields and high selectivity. As mechanistic experiments disfavor a more traditional C−H activation pathway, as earlier proposed by Kisch19 and Brookhart,41−43 it was suggested that this reaction proceeds via an oxidative cyclization of cobalt(I) species 70, yielding a πallyl complex 71, which then isomerizes either by 1,2- or 1,4addition to the five- or seven-membered cobaltacycle 72 or 72′, respectively. β-H elimination to 73 and 73′, followed by reductive elimination delivered either β,γ- or γ,δ- unsaturated ketones 69 and 69′ (Scheme 20). The proposed mechanism Scheme 20. Proposed Catalytic Cycles for the CobaltCatalyzed Hydroacylation of Dienes

a

Linear to branched (l/b) ratio >20:1, unless otherwise noted.

Scheme 22. Plausible Catalytic Cycle for Hydroacylation of Olefins 43

also provides a rationale for the observed Z-selectivity in case of a 1,4-addition, as the formation of the cobaltacycle 72 requires a cis-olefin geometry. A recent report by Yoshikai and co-workers documented the hydroacylation of simple unactivated alkenes.46 In this work, terminal alkenes 43 were hydroacylated with benzaldimines 74 bearing a picolyl-directing49 group (Scheme 21).46 Optimization of the cobalt source, ligand, catalytic reductant and solvent revealed a system consisting of CoBr2, the ferrocenyl bisphosphine ligand dippf 75 and Zn in acetonitrile to give the optimal results. The catalytic system proved suitable for aryl and alkyl alkenes 43 and allowed for the use of a large range of substituted benzaldimines 74 with very good yields and good linear selectivity under mild reaction conditions. Limitations occurred when using aryl benzaldimines 74 bearing an ortho substituent on the arene. Likewise, further substitution on the pyridine ring led to a significant decrease in efficacy, whereas the hydroacylation with aliphatic aldehydes could not be realized. The catalytic cycle of this reaction was proposed to commence by oxidative addition of the catalytically active cobalt(I) species 77 into the C−H bond of the aldimine 74 leading to intermediate 78 (Scheme 22). Migratory insertion of the olefin into the Co−H bond then occurs and leads to

intermediates 79. Reductive elimination finally delivers the product 80 and regenerates the active catalyst 77. Beyond the intermolecular hydroacylation, Yoshikai and coworkers discovered an enantioselective intramolecular hydroacylation with a chiral diphosphine ligand which was successfully applied to 2-acetoxybenzaldehydes 81 as well as 2-vinyl benzaldehydes 82.45 Related to the work of Dong for the enantioselective intramolecular hydroacylation using a chrial rhodium diphosphine complex,50 Yoshikai’s protocol for the cobalt-catalyzed intramolecular hydroacylation included CoCl2 as the cobalt source, (R,R)-Ph-DPE as the best chiral ligand and indium as the reductant of choice.45 When replacing indium by zinc or manganese, the enantioselectivities remained constant, but the yields dropped significantly. The scope was shown to include a variety of differently substituted 2-acetyl benzaldehydes (Scheme 23). A broad range of substituents on the arene 81 gave good yields of the desired products 83 with high enantioselectivities. Furthermore, different acyl groups were well tolerated by the catalyst. Limitations were found 505


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ACS Catalysis Scheme 23. Enantioselective Intramolecular Hydroacylation of Ketones 81

Scheme 25. C−H Arylation with Aryl Carbamates 85, Sulfamates 86, and Chlorides 87

when a substituent in the ortho position on one of the carbonyl groups was present, which shut down the reaction completely. For the enantioselective hydroacylation of 2-alkenylated benzaldehydes 82, slight changes in the catalytic system afforded enantioenriched 3-substituted indanones 84 (Scheme 24).45 Hence, CoBr2 was replaced by CoCl2, (R,R)-BDPP Scheme 24. Enantioselective Intramolecular Hydroacylation

proved to be the best chiral ligand and zinc was the reductant of choice allowing for the enantioselective hydroacylation of olefins. The alkene hydroacylation reaction proved to be rather sensitive and allowed only for a limited number of substituents on the vinyl group. The substitution pattern on the aromatic ring only tolerated the 4-fluoro group in compound 84a. Based on a kinetic isotope effect of 1.1 for the ketone hydroacylation the authors suggested the C−H activation to be facile, while the reductive elimination was proposed to be the rate-determining step. 3.3. C−H Arylations. Until 2012, low-valent cobalt catalysis was restricted to addition reactions, namely, hydroarylations and hydroacylations (vide supra). The first arylation reactions with organic electrophiles via C−H/C−O51 bond cleavage were reported by Ackermann and Song in 2012.52 The catalytic system consisting of Co(acac)2, IMesHCl as a NHC precursor and cyclohexylmagnesium chloride allowed for the arylation of arylpyridines 22 as well as 2-pyridyl and 2-pyrimidyl indoles 24 with phenol derived aryl carbamates 85 and sulfamates 86 at rather low reaction temperatures, as compared to related arylation methods (Scheme 25).53,54 In continuation of these studies, the Ackermann group extended the scope of electrophiles to include readily accessible aryl chlorides 87.55

The catalytic system displayed a broad substrate scope to include electron-rich and electron-deficient aryl and indolylpyridines. Double arylation was negligible and was only observed in trace amounts for a few substrates at elevated reaction temperatures. A remarkable feature of this reaction was the regioselectivity of the arylation with meta-substituted phenylpyridines. For most arylation reactions, the C−H functionaliza506


Review

ACS Catalysis tion took place at the sterically less hindered position, which hence was observed for products 88da-dd, with the exception of meta-fluorophenyl56 pyridines, as shown for product 88eb.52,55 For the cobalt-catalyzed C−H arylation, challenging aryl carbamates and sulfamates were also amenable substrates. Aryl chlorides led to the substitution at the sterically more hindered position when bearing a methoxy group (88fa, 88fc, 88fe, and 88ff in Scheme 25c). This secondary directing effect overcompensated even the high steric repulsion of 2-methyl and 2′-methoxy groups on the biaryl moiety of product 88ff with a relatively high yield of 76%. Related to the concept of cobalt-catalyzed C−H arylation with organic electrophiles, Yoshikai and co-workers concurrently unravelled the use of synthetically useful ketimines 28 as directing groups (Scheme 26).57 Hydrolysis of the ketimines yielded the corresponding ortho-arylated ketones 90. The reaction showed ample substrate scope with moderate to good yields.

Scheme 27. (a) C−H Arylation of Benzamides 44, (b) Synthesis of Biaryl Tetrazoles 93

Scheme 26. Imine-Directed Arylation with Aryl Chlorides 87

Scheme 28. C−H Arylation of Aryl Tetrazoles 91 by C−H/ C−O Bond Cleavage

Further progress in this field was achieved by the Ackermann group by establishing synthetically useful amides 44 as well as tetrazoles 91 as suitable directing groups.58 The ideal system for the arylation of benzamides 44 turned out to be Co(acac)2 as the cobalt source, a slightly modified NHC precursor ICyHCl (46d) and cyclohexylmagnesium chloride (Scheme 27). This reaction displayed a broad substrate scope for benzamides 44 and aryl chlorides 87. For further product diversification, these benzamides 92 could be treated with PCl5 and sodium azide to give biaryl tetrazoles 93, which are highly important motifs in drug design, such as in angiotensin-II-receptor blockers (ARBs) Valsartan, Losartan, or Candesartan.59 While the most common access to ARBs was until recently represented by palladium-catalyzed cross-coupling reactions,60 considerable progress has been made with the aid of ruthenium-catalyzed C−H bond arylation of aryl tetrazoles.59,61 The low-valent cobalt catalyst also proved to be suitable for the direct arylation of aryltetrazoles 91 with aryl carbamates 85, albeit as of yet with somewhat reduced efficiency (Scheme 28).

Despite considerable progress, the full mechanistic details of these arylation reactions have not been elucidated. A proposed catalytic cycle starts by cylocobaltation and features a SET-type mechanism (Scheme 29), as was suggested for the cobaltcatalyzed alkylation (vide inf ra, see section 3.5). Experiments with radical scavengers provided support for this hypothesis.55 3.4. C−H Alkenylations. Until very recently, alkenylated arenes were solely accessed through hydroarylation reactions of alkynes by cobalt catalysis (vide supra).19,22,23,26,27 However, this strategy inherently showed two major limitations. First, cycloalkenylated derivatives could not be prepared by this method. Second, the regioselectivity of the transformations was 507


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example of cobalt-catalyzed C−H functionalization on ferrocenes. The behavior of acyclic enol acetates 94 was found to be equally interesting. Hence, mixtures of diastereomers exclusively yielded the alkene with E configuration in a diastereoconvergent fashion as the sole product (Scheme 31).

Scheme 29. Plausible Catalytic Cycle for the CobaltCatalyzed C−H Arylation

Scheme 31. Stereo-Convergent Alkenylation with Acyclic Enol Acetates 94

It should be noted that this reaction was not restricted to enol acetates, but proved also applicable to enol phosphates 96, carbamates 97, and carbonates 98 (Scheme 32). Furthermore, purely sterically controlled. Thus, internal alkynes with substituents having close steric parameters resulted in a difficult to separate mixture of regioisomeric products. To circumvent these limitations, a general protocol for the direct alkenylation of alkenyl derivatives was desirable. In a recent contribution, Ackermann and co-workers presented the first direct alkenylation of (hetero)arenes with easily accessible enol esters (Scheme 30).62 CoI2, IPrHCl (46b) as a NHC

Scheme 32. Cobalt-Catalyzed Alkenylation with Enol (a) Phosphates 96, (b) Carbamates 97, and (c) Carbonates 98

Scheme 30. Cobalt-Catalyzed Alkenylation with Cyclic Alkenyl Acetates 94 by C−H/C−O Cleavage

[a]

46d (10 mol %) used at 60 °C.

it is noteworthy that direct C−H alkenylations of arenes 22 with enol acetates have as of yet only been achieved by ruthenium catalysis under more drastic reaction conditions.63,64 A series of competition experiments between enol acetates, carbamates, and phosphates illustrated that acetates and carbamates displayed close reactivities, while phosphates were

precursor and the base CyMgCl gave the best results for C−H alkenylations of indoles with cyclohexenyl acetate 94a. This method could be applied to numerous indoles 24 as well as enol acetates 94. Pleasantly, 2-pyridylferrocene was successfully alkenylated as well, giving the alkenyl ferrocene 95. While the yield was rather moderate in this case, it represented the first 508


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ACS Catalysis inherently less reactive. This order of reactivity was unexpected, since it was not in line with the corresponding bond dissociation energies. Mechanistic investigations suggested that an isomerization process took place and that the thusformed E isomer reacted preferentially. Reactions in the presence of radical scavengers, such as TEMPO, could not exclude a radical-type mechanism. However, it was suggested that the catalytic cycle initiates with the formation of cyclocobaltated complex 99 (vide supra), followed by coordination of the enol derivative to intermediate 100 and subsequent migratory insertion. β-Elimination delivers the product and reductive elimination regenerates the active catalyst (Scheme 33).

Scheme 34. Direct Alkylation of Benzamides 44

Scheme 33. Proposed Catalytic Cycle for the CobaltCatalyzed C−H Alkenylation with Enol Esters

Scheme 35. C−H Alkylation with Alkyl Chlorides 101

3.5. C−H Alkylations. Protocols for the synthesis of alkylated arenes by cobalt catalysis were until recently only viable through hydroarylations of alkenes (vide supra). However, the hydroarylation was mostly restricted to styrenes 43 and examples for branched-selective hydroarylations continue to be rare. Therefore, a general protocol for direct alkylations with organic electrophiles, such as alkyl halides, was in high demand.65 A cobalt-catalyzed ortho-alkylation using alkyl chlorides 101 was reported by the group of Nakamura.66 Here, the alkylation of benzamides 44 was accomplished with various alkyl chlorides 101 (Scheme 34). The low-valent cobalt catalyst was generated in situ from Co(acac)2 and cyclohexylmagnesium chloride. Interestingly, the use of a σ-donating NHC or phosphine ligand was not required. When using substrates 44a with a substituent in the meta-position, the alkylation proceeded exclusively at the sterically less hindered position (45aa−45ad in Scheme 34). Moreover, meta- or orthosubstitution was essential to avoid double alkylation (45ba). Replacing alkyl chlorides 101 by alkyl bromides led to a significant decrease in the yield. The N−H proton on the benzamide was identified as being crucial, because a reaction with a tertiary benzamide did not deliver any product. Furthermore, replacing Co(acac)2 by Co(acac)3 or CoCl2 did

Scheme 36. Benzylation of Indole 24d with Phosphate 103a

not significantly affect the outcome of the catalytic reaction, which suggests that these cobalt precursors were converted into a comparable catalytically active species. Interestingly, t-BuCl 101d furnished the iso-butyl alkylated benzamide 45ad. This finding can be explained by an initial β-elimination. 509


Review

ACS Catalysis The synthetic utility of cobalt-catalyzed C−H alkylation by ligand acceleration was independently unraveled by Ackermann and Yoshikai. The Ackermann group established a general protocol for the site- and chemoselective alkylation of arylpyridines as well as N-pyridyl and pyrimidyl indoles with alkyl chlorides.55 Notably, the reaction did not proceed in the absence of a ligand. Among various ligands and preligands, the use of IPrHCl (46b), along with Co(acac)2 and CyMgCl, provided the best results (Scheme 35). The substrate scope included several decorated arylpyridines 22 as well as different alkyl chlorides 101. Moreover, indoles 24 with pyridiyl or pyrimidyl directing groups were site-selectively converted at the C-2 position. Alkylation at the inherently more reactive C-3 position of the indole was not observed. Note that sterically more congested derivatives with substituents at the C-3 or C-7 position were smoothly converted to products 102b and 102ce as well. Beside primary alkyl chlorides, this method was also successfully applied to secondary alkyl chlorides, yielding 47af and 102df. Competition experiments between arylpyridines and pyrimidyl indoles clearly showed indoles to react preferentially, likely due to the increased C−H acidity. Moreover, electron-deficient arenes, such as substrate 22a, turned out to feature a higher reactivity in this reaction compared to electron-rich derivatives, rendering a simple electrophilic C−H activation manifold less likely to be operative. Instead, the kinetic C−H acidity appears to be of key importance. Beyond C−H alkylations, the Ackermann group also developed the unprecedented cobalt-catalyzed benzylation in 2012 (Scheme 36).52 Thus, indoles were selectively benzylated with phosphates 103 at ambient reaction temperature. In addition, the Yoshikai group established direct alkylations of ketimines with primary and secondary alkyl halides 101 and 105 in independent studies.67 Here, NHC preligands 46e and 46f gave best results. The substrate scope involved primary and secondary alkyl chlorides 101 as well as alkyl bromides 105 (Scheme 37). Primary alkyl halides delivered significantly higher yields and even the steric hindrance of the neo-pentyl substituent did not interfere with the catalytic reaction, as illustrated for compound 48gg. Secondary alkyl halides were converted in moderate to good yields. Acyclic secondary alkyl halides, such as electrophile 103i, gave small amounts of isomerization products. In contrast, cycloalkyl halides furnished the desired products more efficiently. In addition to five-, six-, and seven-membered electrophiles 101l, 101f and 101m, strained four-membered analogue 101k could successfully be employed. However, alkylations with tertiary alkyl halides were not viable. The catalytic system was not restricted to aromatic imines 28, but was also subsequently adopted to arylpyridines.68 However, substituents on the aryl or pyridyl ring turned out to be mandatory to prevent double alkylation. Mechanistic studies by the groups of Ackermann52,55 and Yoshikai67,69 have been performed to gain insights into the catalysts mode of action. A simple electrophilic-type C−H activation mode could be ruled out, as electron-deficient arenes reacted preferentially, which is in good agreement with a deprotonative-type C−H metalation pathway. A potential reaction mode via β-elimination and hydroarylation was also ruled out. In the presence of the radical scavenger TEMPO a significant decrease of catalytic activity was observed.55 When enantiomerically pure secondary alkyl halides were employed, a significant racemization was observed.67 Moreover, experiments with typical radical clocks provided strong support for SET-

Scheme 37. Direct C−H Alkylation of Ketimines 28 with (a) Primary and (b) Secondary Alkyl Halides

type processes. These findings strongly suggested a radicalbased reaction mechanism to be operative. On the basis of these findings, a plausible catalytic cycle55,67 was proposed to initiate by cyclometalation of the arene yielding complex 106 (Scheme 38). The C−X bond activation is then suggested to proceed by a SET-type process to give cobalt-halide species 107 and the alkyl radical. Subsequent radical rebound generates intermediate 108, while reductive elimination and transScheme 38. Proposed Catalytic Cycle for the CobaltCatalyzed C−H Alkylation with Organic Electrophiles

510


Review

ACS Catalysis metalation delivers the product and regenerates the active cobalt species. 3.6. Catalysis with Cp*Co(III) Complexes. Until recently, cobalt-catalyzed C−H functionalization chemistry was dominated by low-valent cobalt complexes. These transformations somewhat suffered from the use of a Grignard reagent which limited the functional group tolerance. However, Brookhart’s studies on Cp*Co(I) complexes for hydroacylations indicated the unique potential of the Cp* ligand motif for cobaltcatalyzed C−H functionalizations. The development of welldefined cobalt(III) complexes70 as candidates for C−H activation reactions represented an advance in cobalt catalysis, being reminiscent of related rhodium(III) catalyzes. Thus, a series of analogues Cp*Co(III) has been prepared (Scheme 39),70,71 and recently been evaluated for catalytic C−H functionalizations.

Table 1. Catalytic Activity of Cobalt Complexes

entry

[Co] (mol %)

t [h]

yield [%]a

1 2 3 4 5 6 7 8

CoCl2 (10) CoCl2 (10) + AgPF6 (20) 109 (10) 110 (10) 111 (10) 112 (10) 113 (10) 116c (10) + AgPF6 (20)

12 12 20 20 20 20 20 20

0 0 39 11 traces 22 80b 48

a

Yield was determined by 1H NMR spectroscopy, bYield of isolated product. cThe tetra-chloro complex was used.

Scheme 39. High-Valent Cobalt(III) Complexes

cationic complex in situ generated from neutral complex 116 and a silver salt revealed the single-component sandwich complex to be more reactive (entries 7, and 8). The substrate scope of the optimized catalyst 113 included differently decorated imines. However, a decrease in performance was noted when using electron-rich imines. In addition to arylpyridines, synthetically more attractive indoles 24 proved to be suitable starting materials, with the C− H functionalization occurring solely at the C-2 position of the heterocycle by chelation assistance (Scheme 41).73 It was also found that the addition of catalytic amounts of KOAc improved the catalysts’ performance.

3.6.1. Hydroarylations. The direct addition of 2-arylpyridines to imines, and α,β-unsaturated enones using the cobalt(III) complex 113 was described by Matsunaga/ Kanai.72 For the addition to imines 117, best yields were obtained with complex 113 in 1,2-dichloroethane as the solvent (Scheme 40).

Scheme 41. Additions with 2-Pyrimidyl Indoles 24

Scheme 40. Cobalt(III)-Catalyzed Addition Reactions with Aldimines 117

It is noteworthy that further additives, such as silver salts or σ-donating ligands, were not required. Moreover, the simple cobalt salt CoCl2 did not allow for a catalytic reaction to occur (Table 1, entries 1 and 2). Probing different well-defined cobalt(III) complexes suggested the substitution pattern on the Cp-ring to be of key importance. Thus, sterically more congested complexes 109−111 also led to a decrease of catalytic efficacy (entries 3−5). Further, tetramethylcyclopentadienyl-cobalt complex 112 gave rather poor yields (entry 6). The direct comparison of the sandwich complex 113 with the

The versatile cobalt(III) catalyst proved applicable to a range of imines 117 as well as differently decorated 2-pyrimidyl indoles 24. The increased reactivity of the KOAc modified catalyst could be explained by a carboxylate-assisted C−H activation mechanism74 via the in situ formation of the cobalt acetate complex [Cp*Co(OAc)2]. 511


Review

ACS Catalysis The addition reaction was not restricted to imines 117 as the electrophiles. Indeed, similar observations were made for Michael acceptors including α,β-unsaturated enones and Nacylpyrroles 120, as depicted in Scheme 42.72 The catalyst

Scheme 43. Proposed Catalytic Cycle for the Cobalt(III)Catalyzed Addition (L = 2-Phpy)

Scheme 42. C−H Functionalization by Conjugate Addition to Michael Acceptors 120

Scheme 44. Cobalt(III)-Catalyzed Hydroarylation of Alkynes featured an excellent functional group tolerance with ample substrate scope, enabling the conversion of arenes displaying bromo (22g), dimethylamino (22i) or even free hydroxyl substituents (22h). A plausible mechanism was elaborated based on related rhodium(III)-catalyzed transformations,75,76 and is depicted in Scheme 43.72 Initially, the precatalyst 113 upon heating dissociates benzene and two arylpyridines 22 coordinate to complex 122. Afterward, the C−H activation takes place likely via concerted metalation-deprotonation-type (CMD) mechanism74,77 or an electrophilic-type metalation to form the cyclometalated intermediate 123. Thereafter, coordination of the imine 117 occurs to generate intermediate 124. Subsequently, insertion of the imine into the Co−C bond takes place, thereby forming the seven-membered cobaltacycle 125. Coordination of another arylpyridine 22 delivers 126 and finally proto-demetalation furnishes the product and regenerates the catalytically active species 123. While these studies focused on the addition onto double bonds, Matsunaga/Kanai and co-workers also devised related additions onto triple bonds. Based on rhodium-catalyzed transformations reported by the late Fagnou,78 a cobalt(III) catalyst emerged for the hydroarylation of alkynes with Ncarbamoylindoles 127, using the versatile cobalt-catalyst 113 and catalytic amounts of KOAc (Scheme 44).79 Here, both electron-donating and electron-withdrawing groups on the indole (127b and 127c) were well tolerated. As to the alkynes 20, both alkyl as well as aryl substitution patterns proved suitable, including challenging terminal alkynes 20d. Limitations were found in the case of sterically demanding alkynes and low regioselectivities were observed when using sterically close substituents on the alkyne (128af). These limitations were reminiscent of the ones observed for the low-valent cobalt catalysts (see section 3.2.1).22

Because the electronegativity of cobalt is lower than the one reported for rhodium, the organometallic Cp*Co(III) intermediates generated by C−H activation should be more nucleophilic.79 Matsunaga/Kanai and co-workers envisioned that thereby a more electrophilic carbomyl moiety would lead to nucleophilic attack and, thus, directly furnish pyrroloindolones 129 in a one pot fashion. This hypothesis was put into practice with a better leaving group, namely the morpholino carbamoyl moiety (Scheme 45). Both electron-donating and electron-withdrawing substituents on the indole were tolerated (129eg and 129fg), and most reactions delivered a single regioisomer when using unsymmetrical alkynes 20.79 Although viable alkynes included bisaryl and aryl/alkyl substitution patterns, bisalkyl and terminal alkynes gave the alkenylated 512


Review

ACS Catalysis Scheme 45. Pyrroloindolones 129 via C−H Alkenylation/ Annulation

Scheme 47. Cobalt-Catalyzed C−H Activation/Annulation Cascade

Scheme 46. Cobalt(III)-Catalyzed Hydroarylation/ Annulation Reaction

Scheme 48. Experimental Evidence for the Reversibility of the C−H Functionalization products 128 instead. Furthermore, the attempted synthesis of pyrroloindolone 129 from alkenylated indole 128 under otherwise identical reaction conditions failed, indicating the proto-demetalation to be irreversible in nature. As to the reaction mechanism, experiments with isotopically labeled substrates showed that complex 113 catalyzed a H/Dexchange at the C-3 position of the indole. The same result was observed, when Sc(OTf)3 was used, demonstrating that this H/ D scrambling was due to the action of a Lewis-acid. In contrast, complex 113 modified with KOAc resulted in a selective H/D scrambling at the C-2 position which provided strong evidence for a carboxylate-assisted C−H activation to be operative.74 On the basis of the mechanistic findings, a possible catalytic cycle for the carboxylate-assisted hydroarylation/annulation sequence was put forward (Scheme 46).79 Dissociation of the benzene ligand from complex 113 as well as ligand exchange with acetate generates the neutral complex [Cp*Co(OAc)2] (130). Dissociation of an acetate anion delivers the catalytically active cationic complex [Cp*Co(OAc)]+ (131). This complex is coordinated by the carbamoyl group of the indole 127 and

acetate-assisted C−H activation delivers the cyclometalated complex 132. This is followed by insertion of the alkyne into the Co−C bond which gives the seven-membered cobaltacycle 133. Depending on the directing group, two reaction pathways are a priori possible. In the case of the N,N-dimethylaminocarbamoyl group, protodemetalation delivers the alkenylated indole 128 and regenerates the active species 131. If the carbamoyl group bears a better leaving group, annulation takes place by intramolecular nucleophilic attack to provide product 513


Review

ACS Catalysis Scheme 49. Proposed Mechanism of Cobalt(III)-Catalyzed Indazole Synthesis

Scheme 50. C−H Amidation of (Hetero)arenes

134 with release of the morpholinide, giving the desired pyrroloindolone 129 and the catalytically active species 131. As discussed in section 3.6.1, Cp*Co(III)-catalyzed reactions proceeded via cyclocobaltation and addition onto the multiple bonds. These additions were until recently restricted to alkynes and conjugated double bonds. Additions to aldehydes, yielding either alcohols or further annulation products were in contrast as of yet largely achieved by rhodium80 or rhenium81 catalysis. However, in a recent report the Ellman group disclosed a cobalt-catalyzed addition of C−H bonds to carbonyl compounds.82 The step-economical method involved the C−H functionalization of azobenzenes 1 as well as α,β-unsaturated oximes 135 (Scheme 47). Insertion of aromatic aldehydes and trapping of the thus formed alcohols by nucleophilic attack delivered indazoles 136 or furans 137, respectively. Best yields were achieved with the dimeric complex 116, AgB(C6F5)4 and AgOAc. On the downside, this catalytic system required two relatively costly silver salts as additives. To circumvent this problem, the Ellman group synthesized the novel cobalt complex [Cp*Co(PhH)][B(C6F5)4]2 (114) by salt metathesis from the sandwich complex 113 and KB(C6F5)4. With this precatalyst in hand, only catalytic amounts of acetic acid were required to perform the C−H cascade reaction, leading to indazoles 136 and furans 137. It is noteworthy that the products were usually obtained in higher yields as compared to those under the rhodium(III) catalysis regime.82 To confirm the postulated cascade reaction, the independently prepared addition product 138a was submitted to the optimized reaction conditions in the presence of aldehyde 64a.82 In addition to the annulated product 136a, the cross addition product 136aa was observed, indicating the reversible nature of the C−H functionalization process (Scheme 48). Based on these findings, a plausible catalytic cycle was put forward (Scheme 49). Dissociation of the benzene ligand delivers the active catalyst 139. Then, C−H metalation of the arene takes place by chelation assistance (140). Coordination (141) and migratory insertion form the seven-membered metallacycle 142, while protodemetalation yields the alkylated intermediate 138. Thereafter, intramolecular nucleophilic substitution and deprotonation furnish the indazole 136.

3.6.2. C−H Amidations. The synthetic utility of the cobalt(III) catalysis was extended to amidations of arenes.83−86 In 2014, Matsunaga/Kanai reported the cobalt(III)-catalyzed amidation of pyrimidyl-substituted indoles 24 with sulfonyl azides 143 (Scheme 50a).84 As the previously established sandwich complex 113 hardly gave any conversion, the catalytic activity of the neutral complex 115 was particularly noteworthy. The generation of the active cationic complex by the addition of a silver salt proved to be essential. With complex 115, AgSbF6 and KOAc as the optimal catalytic system, a wide range of indoles 24 and sulfonyl azides 143 could be directly functionalized with a high functional group tolerance. In a more recent contribution, Matsunaga/Kanai extended this strategy to include phosphoryl azides 144 for phosporamidation of indoles (Scheme 50b).86 Optimization studies revealed the sandwich complex 113 to be unreactive. Instead, the dimeric complex [Cp*CoI2]2 (116), modified with AgSbF6, proved to be ideal here, and furnished the products selectively. 514


Review

ACS Catalysis Cobalt(III)-catalyzed amidation reactions were not restricted to azides as the nitrogen source. Thus, Chang and co-workers discovered the cobalt-catalyzed amidation of arylpyridines with most user-friendly carbamates (Scheme 50c).85 Best results were obtained with N-Boc-protected carbamate 145 using the catalytic system derived complex 115 and AgSbF6. Using their optimized reaction conditions, a variety of arylpyridines with electron-donating or electron-withdrawing groups was successfully converted. It is noteworthy that the amidation of arenes bearing biologically valuable purine directing groups87,88 was also accomplished. A plausible catalytic cycle for these transformations is suggested to commence by the action of a cationic Cp*Co(III) species (Scheme 51). C−H cobaltation delivers the cyclo-

Scheme 52. C−H Cyanation of Arenes and Heteroarenes

Scheme 51. Proposed Catalytic Cycle for Cp*Co(III)Catalyzed Amidation

with excellent functional group tolerance and ample scope. Moreover, sterically congested C-3 substituted indoles 24b did not intervene with the catalytic transformation. It should be noted that the facile removal of the pyrimidyl group succeeded in a traceless fashion (Scheme 52b). In an independent study, the group of Glorius thereafter reported on an efficient system in which KOAc was replaced by NaOAc.93 The selectivities and yields were comparable to the Ackermann system. However, the scope was significantly expanded to include the chelation-assisted cyanation of alkenes 155 as well (Scheme 53).

metalated complex 149 and is proposed to proceed via a CMDtype74 mechanism or electrophilic-type metalation. After coordination of the azide or carbamate, migratory insertion takes place and the leaving group is released to form species 150. Finally, protodemetalation releases the products and regenerates the active catalyst. 3.6.3. C−H Cyanations. Benzonitriles are key structural motifs in natural products, pharmaceuticals, and agrochemicals.89 C−H cyanations were as of yet only achieved with 4d transition-metal complexes of ruthenium90 or rhodium.91 The Ackermann group reported the first Cp*Co(III)-catalyzed cyanation of (hetero)arenes using N-cyano-N-phenyl-p-toluenesulfonamid (151) as an easy to handle cyanating reagent (Scheme 52a).92 Optimization studies on the nature of the cobalt catalyst and additives revealed [Cp*Co(CO)I2] (115), AgSbF6 and KOAc to be ideal. The system proofed to be highly chemo- and regioselective. In case of meta-substituted arenes 22, the cyanation was observed to selectively take place at the sterically less hindered position, except for 3,4-difluororphenylpyridine 22j where a secondary directing group effect was noted. The method was not limited to aryl-substituted pyridines, but also proceeded well by chelation assistance with indoles, pyrazoles, and pyrroles. With the Cp*Co(III) catalyst, C-2-selective cyanations of indoles 24 were achieved

Scheme 53. C−H Cyanation of 2-Pyridylpropene (155)

The group of Chang established N-cyanosuccinimide (157) as an easily accessible and bench-stable electrophilic cyanating reagent.94 Optimization studies illustrated its improved performance as compared to N-cyanophthalimides. Studies directed toward the scope of arylpyridines 22 and indoles 24 revealed substrate 157 as being slightly less active. It is particularly noteworthy that the protocol proved applicable to the cyanation of 6-arylpurines 15887,88 (Scheme 54). To gain insights into the reaction mechanism, inter- and intramolecular competition experiments were performed and gave a KIE of 1.0−1.2,92,94 which indicated that the C−H cleavage was likely not involved in the rate-determining step. Furthermore, a Hammett-plot correlation indicated a change in the rate-determining step depending on the substitution on the 515


Review

ACS Catalysis Scheme 54. C−H Cyanation of 6-Arylpurines 158 with NCyanosuccinimide (157)

Scheme 56. Aminocarbonylation of Aryl Pyrazoles 162 with (a) Isocyanates 163, and (b) Acyl Azides 165

arene, as reported by Ackermann and Li.92 Based on these findings, a catalytic cycle (Scheme 55) involves initial reversible Scheme 55. Plausible Catalytic Cycle for the Cobalt(III)Catalyzed C−H Cyanation

withdrawing groups were smoothly converted. As isocyanates are frequently generated in situ from acyl azides by a Curtius rearrangement,96 the Ackermann group also devised a protocol for the aminocarbonylation using a broad range of acyl azides 165 (Scheme 56b).95 For this C−H transformation, a kinetic isotopic effect (KIE) of 1.4 within inter- and intramolecular reaction was determined, indicating the C−H metalation likely not to be ratedetermining. Furthermore, competition experiments showed that electron-rich arenes 162 and electron-deficient electrophiles 163 reacted preferentially, which again provides strong support for a rate-determining nucleophilic attack. In a concurrent study the Ellman group reported the aminocarbonylation of aryl pyrazoles 162 with isocyanates 163.97 Among different cobalt complexes, the sandwich complex 113 in combination with KOAc was identified as being optimal here. 3.6.5. C−H Halogenations. As cyanides can be regarded as pseudohalides, cobalt(III) catalysts were also explored for C−H halogenations. Thus, Glorius and co-workers disclosed versatile cobalt(III)-catalyzed brominations and iodinations with easily accessible N-iodosuccinimide (NIS) or N-bromophthalimide (NBP) 166, respectively, under reaction conditions being otherwise identical to those for the C−H cyanation (Scheme 57).93 Replacing the acetate base by pivalic acid furnished best results in most cases, which can be rationalized in terms of an

C−H metalation to deliver complex 149. Reversible coordination of the cyanating reagent forms intermediate 160 and migratory insertion furnishes complex 161. Finally, βelimination and protodemetalation delivers the cyanated arene and regenerates the active catalyst. 3.6.4. C−H Aminocarbonylations. Recently, the Ackermann group developed the cobalt(III)-catalyzed aminocarbonylation of aryl pyrazoles 162.95 The aminocarbonylation with isocyanates 163 as the electrophiles gave optimal results with [Cp*Co(CO)I2] (115) as the precatalyst, along with AgSbF6 and AgOPiv as the additives (Scheme 56a). The reaction showed a high functional group tolerance and remarkable siteselectivity with meta-substituted arenes 162. Furthermore, isocyanates bearing electron-donating as well as electron516


Review

ACS Catalysis Scheme 57. C−H Halogenation

Scheme 58. Cobalt-Catalyzed Allylation with (a) Allyl Carbonates 171, (b) Allyl Acetates 173, and (c) Allyl Alcohols 175

activation of the halogenation reagent by generation of a more reactive cobalt catalyst with a vacant coordination side. The method proved applicable to arylpyridines 22, acrylamides 167, as well as N-alkylbenzamides 44a−c, giving the desired products in moderate to good yields. 3.6.6. C−H Allylations. An allyl group can easily be transferred into a plethora of useful functionalities. Therefore, C−H allylations have been intensively studied in recent years, being dominated by 4d transition-metal catalysis.98 Efforts toward cobalt-catalyzed C−H allylation have been devoted by the groups of Glorius, Ackermann, and Matsunaga/ Kanai.93,99−101 In a contribution by Glorius, a catalytic system consisting of the complex [Cp*Co(CO)I2] (115), AgSbF6 and pivalic acid enabled the allylation of 2-pyrimidyl indoles 24 with allyl methyl carbonate (171) in a formal SN-type reaction (Scheme 58a).93 The substrate scope showed high functional group tolerance, delivering C-2-selective allylated indoles 172 in excellent yields. A remarkable feature of this catalytic system was represented by a low catalyst loading of 0.5 mol % of the cobalt complex and 1.25 mol % of the silver(I) salt additive. Furthermore, the versatility of the approach was highlighted by the allylation to benzamides 44 and acrylamides 167.99 Moreover, the allylation with 1- or 3-substituted allyl carbonates could be realized. However, 3-substituted allyl carbonates were converted in only moderate yields, as shown for 172ac. In independent studies, the Ackermann group showed that Cp*Co(III)-catalyzed C−H allylation was achieved with readily available allyl acetates (173) (Scheme 58b).100 The robustness of the user-friendly catalyst allowed for the use of a broad range of indole derivatives, being selectively allylated at the C-2 position with excellent functional group tolerance, such as amides 172fd and nitro 172gd groups. The catalytic system was not restricted to 2-pyrimidyl indoles but enabled reactions with pyrroles 174a,b and aryl pyri(mi)dines. Very recent progress was achieved by Matsunaga/Kanai using allyl alcohols 175 (Scheme 58c).101 Minor changes in the catalytic system had to be adjusted, in that the two silver salts AgOTf and AgOAc were mandatory. The reaction tolerated a broad range of indoles with substitution at various positions. Furthermore, allylic alcohols 175 with substitution at the 1- and 3-position turned out to be suitable substrates for C−H allylations with notable selectivities. According to previous reports on cobalt(III) catalysis, a plausible catalytic cycle is based on initial C−H cobaltation of the cationic cobalt species 131 (Scheme 59). This results in the formation of complex 149, which then undergoes coordination by the allyl alcohol derivative. Thereafter, migratory insertion of

5 mol % of 115, 40 mol % of AgSbF6, 20 mol % of PivOH, 80 °C, 16 h. [b]110 °C. [c]175 (10 equiv).

[a]

517


Review

ACS Catalysis Scheme 59. Proposed Catalytic Cycle for the CobaltCatalyzed Allylation

Scheme 60. C−H Alkynylation of Indoles 24

the allylic double bond into the Co−C bond yields key intermediate 180. An alternative concerted SN2′ mechanism is unlikely according to very recent DFT calculations.101 Finally, the product is released via β-oxygen elimination which is about 2.4 kcal/mol more favorable than a β-hydride elimination. 3.6.7. C−H Alkynylations. Until very recently, C−H alkynylations were not viable with cobalt catalysts, but mostly required more costly 4d transition metal complexes of palladium102 or rhodium.103 In independent studies, the groups of Shi and Ackermann devised two protocols for the cobalt(III)-catalyzed C2-selective C−H alkynylation of indoles (Scheme 60).104 The reaction discovered by Shi and coworkers comprised a hypervalent iodine reagent 181, Mg(OMe)2 and a catalytic system consisting of complex 115 and AgF (Scheme 60a).104a In contrast, Ackermann and co-workers devised the C−H alkynylation with synthetically useful silylbromoalkynes employing complex 116, along with KOAc (Scheme 60b).104b It is noteworthy that the later protocol was characterized by a considerable tolerance of functional groups, such as iodo or ketone substituents, under mild reaction conditions. 3.6.8. Isohypsic C−H/N−O Functionalizations. Very recently the cobalt-catalyzed synthesis of isoquinolines 183 from oximes 184 was independently discovered by Ackermann,105 Sundaraju,106 and Matsunaga/Kanai107 (Scheme 61). These catalytic systems are based on related cobalt(III) precatalyst 115, NaOAc and AbSbF6.105 The isohypsic, i.e. redox-neutral, reaction provided access to a wide range of isoquinolines 183 with high regio-selectivities.105 With respect to redox-neutral C−H/N−O cleavages, cobalt(III)-catalyzed amidations were also recently accomplished with 1,4,2-dioxazol-5-ones 185 as robust amidating reagents.108,109 While the previously described amidation methods (section 3.6.2) were restricted to the synthesis of phosporyl- and sulfonamides as well as carbamates,84−86 the use of heterocycles 185 allowed for the preparation of a variety of aryl as well as alkyl amides.109 The transformation exploited the power of complex [Cp*Co(MeCN)3][SbF6] (186), transforming arylpyridines 22, alkenylpyridines 155, as well as 6-aryl purines 158 (Scheme 62). 3.7. Oxidative C−H Functionalizations and Annulations. Since Daugulis initially devised directing groups based on 8-aminoquinoline (Q) for the palladium-catalyzed function-

[a]

Reaction performed at 80 °C.

Scheme 61. Isoquinoline Synthesis from Oximes 184 under Cobalt(III) Catalysis

[a]

15 min.

[b]

In HFIP.

alization of C−H bonds,110 the monoanionic, bidentate directing group has been extensively exploited for palladium-, ruthenium-, iron-, nickel-, or copper-catalyzed C−H functionalizations, among others.111,112 As discussed above, cobalt(III) complexes participated in C−H activation, followed by alkyne or alkene insertion. On the basis of these findings, Daugulis and co-workers reported on the cobalt-catalyzed oxidative alkyne annulation with the bidentate Q-auxiliary.113 The alkyne 518


Review

ACS Catalysis

converted. Alkyne annulations with a picolinaminde directing group 188 gave rather moderate yields with stoichiometric amounts of Co(OAc)2·4H2O (Scheme 63b), while the cleavage of the picolinamide occurred efficiently (Scheme 63c). Because it is well established that the bidentate Q-directing group stabilizes transition metals in high oxidation states, it was proposed that cyclometalation and oxidation furnish cobalt(III) intermediate 192. This species undergoes migratory insertion with the alkyne 20 to furnish intermediate 193 (Scheme 64).

Scheme 62. Cobalt(III)-Catalyzed Amidation with Dioxazolone 185aa

Scheme 64. Mechanistic Considerations for AminiquinolineDirected Alkyne Anuulation

a

186 (2.5 mol %).

Reductive elimination finally delivers the product 189 by elimination of Co(I). Alternative pathways via protodemetalation of 193 and oxidative cyclization are less likely to be operative. Subsequently, Daugulis developed the alkene annulation by benzamides 187 with the Q-directing group.114 Slight changes in the reaction conditions had to be adjusted to get access to a variety of dihydroisoquinolinones 194 (Scheme 65a). Furthermore, the 4-methoxy-substituted Q-directing group was cleaved in a traceless fashion with ceric(IV) ammonium nitrate (CAN), thereby furnishing the desired NH-free dihydroisoquinolinones 195 (Scheme 65b). While the system developed by Daugulis furnished sixmembered hetreocycles, ring-closure to yield five-membered

annulation with amide 187 led to best results when inexpensive Co(OAc)2·4H2O, sodium pivalate and Mn(OAc)2 were utilized (Scheme 63a). A broad range of benzamides 187 as well as internal and even terminal alkynes 20 were efficiently Scheme 63. Cobalt-Catalyzed Oxidative Alkyne Annulations

Scheme 65. Aminoquinoline-Directed Alkene Annulations

519


Review

ACS Catalysis

isoindolinones 196 was generated after intramolecular alkene hydroamidation. Based on the high catalytic efficacy of the cobalt catalyst in oxidative annulations of alkynes 20 and alkenes 197, Daugulis and co-workers disclosed the cobalt-catalyzed direct carbonylation of benzamdies 187 through bidentate-chelation assistance (Scheme 68).117 The C−H transformation occurred

isoindolin-1-ones 196 could not be accomplished until recently. Thus, Ackermann and Ma discovered a procedure for the cobalt-catalyzed oxidative alkenylation with electron-deficient alkenes that selectively delivered isoindolin-1-ones 196 (Scheme 66).115 Best results were obtained when Co(OAc)2 Scheme 66. Cobalt-Catalyzed Isoindolinone Synthesis

Scheme 68. C−H Carbonylation of Benzamides 187

at ambient reaction temperature and afforded the desired phthalimides 200. A wide range of functional groups, such as halogen, nitrile, ester, and cyano substituents, was well tolerated under the optimized reaction conditions. Intramolecular competition experiments with meta-substituted arene exhibited high levels of site-selectivity control. The directing group was easily removed by treatment with methanolic ammonia, delivering the parent phthalimide in high yield. Recently, Song and co-workers developed the cobaltcatalyzed C−H alkoxylation of benzamindes 201 bearing a 2aminopyridine-1-oxide moiety with simple alcohols 202.118 The reaction proceeded under mild reaction conditions using Co(OAc)2·4H2O as the catalyst and Ag2O as the oxidant of choice (Scheme 69a). With the optimized conditions in hand, various benzamides 201 as well as different alcohols 202 were

and AgOPiv were employed in a 4:1 mixture of PEG400 and TFE as the reaction medium. The use of PEG400 had previously been exploited for ruthenium- and palladiumcatalyzed C−H arylations.116 Differently substituted Nquinolinyl benzamides 187 and acrylates 197 furnished a broad range of isoindolin-1-ones 196.115 Mechanistic studies with deuterium-labeled arenes unraveled the C−H cobaltation as an irreversible elementary step and a KIE of 1.4 and 1.6 for the intermolecular and intramolecular competition experiment suggested that the C−H metalation is likely kinetically relevant. On the basis of these mechanistic studies, a plausible catalytic cycle was proposed to proceed by a carboxylate-assisted C−H cobaltation to furnish complex 198 (Scheme 67). Subsequent migratory insertion and β-hydride elimination furnished the alkenylated species 199. Finally,

Scheme 69. (a) Cobalt-Catalyzed Alkoxylation of Benzamides 201, (b) Directing Group Removal

Scheme 67. Proposed Catalytic Cycle for the Oxidative C−H Alkenylation

520


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ACS Catalysis efficiently transformed. Furthermore, this strategy enabled selective alkoxylation of acrylamides, while the 2-aminopyridine-1-oxide directing group was easily removed to afford the benzoic acid 204 (Scheme 69b). EPR spectroscopy studies and control experiments with TEMPO as radical scavenger suggested a radical-based reaction pathway likely to be operative.118 DFT calculations by Wei and Niu provided further strong support for an intermolecular SETtype mechanism, and set the stage for a novel catalyst design.119 3.8. Organometallic C−H Activation by Cobalt Carbenoids. Very recently, Glorius and Wang independently described the effective C−H functionalization with α-diazo esters 205 as carbene precursors under cobalt(III) catalysis.120 When using arylpyridines and benzo[h]quinolone 22 an interesting class of diverse conjugated polycyclic hetereocarbons 206 could be accessed (Scheme 70).

Scheme 71. Proposed Reaction Mechanism for the Cp*Co(III)-Catalyzed C−H Activation/Annulation Reaction

Scheme 70. Cobalt(III)-Catalyzed Annulation with Diazocompounds 205

valuable reagents for the C−H alkylation of azoles 37/38.121 Best catalytic performance was achieved here when using CoBr2 as the cobalt source, phenantroline (phen) as the ligand, and LiOtBu as the base (Scheme 72). Scheme 72. C−H Alkylations with Tosylhydrazons 210

Interestingly, when the Cp*Co(III) complex was replaced by the analogous Cp*Rh(III) complex, the corresponding alkylated arenes were obtained, highlighting the unique features of cobalt(III) catalysis. The difference in chemo-selectivity can be rationalized by the lower electronegativity of cobalt as compared to rhodium. Hence, in both cases, the orthoalkylation is supposed to occur first. However, the cobalt(III) catalyst is also acting as Lewis-acid, therefore, enabling the nucleophilic addition. Based on this rationale, a catalytic cycle was proposed to commence with C−H metalation (Scheme 71), followed by metal-carbene formation with the diazo compound. Migratory insertion to intermediate 208 and protodemetalation delivers the alkylated arene 209. Lewisacid activation of the carbonyl group by the cobalt(III) species sets the stage for the nucleophilic attack of the pyridine nitrogen to give the polycyclic compound 206. This approach draws inspiration from the previous use of carbene precursors for cobalt-catalyzed C−H functionalization. Thus, Miura and co-workers exploited tosylhydrazons 210 as

[a]

With NaOtBu and DMF instead of LiOtBu and 1,4-dioxane.

4. CONCLUSION During the past few years, organometallic cobalt-catalyzed C− H activation has emerged as an increasingly viable tool for the step-economical functionalization and assembly of organic molecules. Thus, the development of low-valent cobalt-catalysis set the stage for C−H arylations, alkenylations, benzylations, and alkylations of arenes as well as alkenes under rather mild reaction conditions. In addition to hydroarylations and hydroacylations, C−H functionalizations with organic electrophiles have proven particularly instrumental for the development of step-economical syntheses. Hence, these inexpensive catalysts proved generally applicable and enabled applications to the synthesis of block-buster drug scaffolds, among others. 521


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(priority program SPP 1807) and the CSC (fellowship to J.L.) is gratefully acknowledged.

Here, a significant ligand rate acceleration was achieved, most notably with monodentate N-heterocyclic carbene or phosphine ligands that provide kinetic stabilization through a bulky substitution pattern. The vast majority of low-valent cobaltcatalyzed C−H activations thus far called for stoichiometric amounts of Grignard reagents, partly because of the reductive in situ generation of the catalytically active cobalt catalysts, with notable exceptions by Vinogradov,41 Kisch,19 Brookhart,42,43 and Petit24b in addition reactions. In contrast, the discovery of high-valent cobalt(II) and cobalt(III) catalysis recently set the stage for versatile C−H functionalization processes. A significant impetus was gained by the identification of a Cp*derived ligand design for cobalt(I) and cobalt(III) catalysts by Brookhart and Matsunaga/Kanai thatto some extentmimic the previously used, more expensive122 rhodium and iridium catalysts. However, based on inter alia the different electronic properties between iridium and rhodium versus cobalt, this strategy enabled improved selectivities and entirely novel chemical transformations, which were largely based on the higher nucleophilicity of organometallic cobalt(III) species, along with their Lewis-acidic properties. For instance, the positional and chemo selectivities in direct allylations,101 and annulations120 were found considerably improved, while the catalytic efficacy in C−H amidations108 or annulations od aldehydes82 proceeded best with cobalt(III) catalysts. Generally, the high-valent cobalt catalyst precursors are air-stable and offer an excellent functional group tolerance at rather low catalyst loading. While in cobalt(III)-catalyzed C−H functionalizations silver(I) salts are often used as stoichiometric oxidants or cocatalytic additives, strategies for their replacement by less expensive reagents have emerged. The cobalt(III)catalyzed C−H activation mostly occurs by facile carboxylateassisted C−H metalation, and proved amenable to removable directing group strategies. The outstanding potential of cobalt catalysis for and beyond C−H functionalizations is further reflected by the very recent application to the transformation of C(sp3)−H bonds,123 as well as to the catalytic activation of C− C bonds.124 Thus far, the majority of cobalt-catalyzed C−H activations utilized relatively high catalyst loadings. Thus, future efforts should inter alia be directed toward the design of more effective and robust catalysts. In addition, in low-valent cobaltcatalyzed C−H functionalization chemistry it is desirable to establish a generally applicable replacement for the omnipresent Grignard reagents.19,24b,42,43 Considering the sustainable nature of C−H functionalization technologies, along with the cost-effective nature of cobalt catalysis, further exciting developments are expected in this rapidly evolving research area.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+49) 551-39-6777. Tel: (+49)551-39-3202. Author Contributions ¶

Both authors contributed equally (M.M. and J.L.).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support by the European Research Council under the European Community’s Seventh Framework Program (FP 2007−2013)/ ERC Grant agreement no. 307535, the DFG 522


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Review

ACS Catalysis

(95) Li, J.; Ackermann, L. Angew. Chem., Int. Ed. 2015, 54, 8551− 8554. (96) Curtius, T. Ber. Dtsch. Chem. Ges. 1890, 23, 3023−3033. (97) Hummel, J. R.; Ellman, J. A. Org. Lett. 2015, 17, 2400−2403. (98) Begouin, J.-M.; Klein, J. M. N.; Weickmann, D.; Plietker, B. In Transition Metal Catalyzed Enantioselective Allylic Substitution in Organic Synthesis; Kazmaier, U., Ed.; Springer: Berlin Heidelberg, 2012; Vol. 38, p 269−320. (99) Gensch, T.; Vásquez-Céspedes, S.; Yu, D.-G.; Glorius, F. Org. Lett. 2015, 17, 3714−3717. (100) Moselage, M.; Sauermann, N.; Koeller, J.; Liu, W.; Gelman, D.; Ackermann, L. Synlett 2015, 26, 1596−1600. (101) Suzuki, Y.; Sun, B.; Sakata, K.; Yoshino, T.; Matsunaga, S.; Kanai, M. Angew. Chem., Int. Ed. 2015, 54, 9944−9947. (102) Tolnai, G. L.; Ganss, S.; Brand, J. P.; Waser, J. Org. Lett. 2013, 15, 112−115. (103) Xie, F.; Qi, Z.; Yu, S.; Li, X. J. Am. Chem. Soc. 2014, 136, 4780−4787. (104) (a) Zhang, Z.-Z.; Liu, B.; Wang, C.-Y.; Shi, B.-F. Org. Lett. 2015, 17, 4094−4097. (b) Sauermann, N.; Gonzalez, M.; Ackermann, L. Org. Lett. 2015, 17, 5316−5319. (105) Wang, H.; Koeller, J.; Liu, W.; Ackermann, L. Chem. - Eur. J. 2015, 21, 15525−15528. (106) Sen, M.; Kalsi, D.; Sundararaju, B. Chem. - Eur. J. 2015, 21, 15529−15533. (107) Sun, B.; Yoshino, T.; Kanai, M.; Matsunaga, S. Angew. Chem., Int. Ed. 2015, 54, 12968−12972. (108) Park, J.; Chang, S. Angew. Chem., Int. Ed. 2015, 54, 14103− 14107. (109) Liang, Y.; Liang, Y.-F.; Tang, C.; Yuan, Y.; Jiao, N. Chem. - Eur. J. 2015, 21, 16395−16399. (110) Zaitsev, V. G.; Shabashov, D.; Daugulis, O. J. Am. Chem. Soc. 2005, 127, 13154−13155. (111) For selected examples, see the following. Nickel: (a) Aihara, Y.; Chatani, N. J. Am. Chem. Soc. 2014, 136, 898−901. (b) Song, W.; Lackner, S.; Ackermann, L. Angew. Chem., Int. Ed. 2014, 53, 2477− 2480. Iron: (c) Asako, S.; Ilies, L.; Nakamura, E. J. Am. Chem. Soc. 2013, 135, 17755−17757. (d) Gu, Q.; Al Mamari, H. H.; Graczyk, K.; Diers, E.; Ackermann, L. Angew. Chem., Int. Ed. 2014, 53, 3868−3871. Ruthenium: (e) Rouquet, G.; Chatani, N. Chem. Sci. 2013, 4, 2201− 2208. Palladium: (f) Zhang, S.-Y.; He, G.; Nack, W. A.; Zhao, Y.; Li, Q.; Chen, G. J. Am. Chem. Soc. 2013, 135, 2124−2127. (g) He, G.; Chen, G. Angew. Chem., Int. Ed. 2011, 50, 5192−5196. Copper: (h) Nishino, M.; Hirano, K.; Satoh, T.; Miura, M. Angew. Chem., Int. Ed. 2013, 52, 4457−4461 and references cited therein. (112) Rouquet, G.; Chatani, N. Angew. Chem., Int. Ed. 2013, 52, 11726−11743. (113) Grigorjeva, L.; Daugulis, O. Angew. Chem., Int. Ed. 2014, 53, 10209−10212. (114) (a) Grigorjeva, L.; Daugulis, O. Org. Lett. 2014, 16, 4684− 4687. For a related study, see: (b) Suzuki, Y.; Sun, B.; Yoshino, T.; Kanai, M.; Matsunaga, S. Tetrahedron 2015, 71, 4552−4556. (115) Ma, W.; Ackermann, L. ACS Catal. 2015, 5, 2822−2825. (116) (a) Ackermann, L.; Vicente, R. Org. Lett. 2009, 11, 4922−4925. See also: (b) Kavitha, N.; Sukumar, G.; Kumar, V. P.; Mainkar, P. S.; Chandrasekhar, S. Tetrahedron Lett. 2013, 54, 4198−4201. A review: (c) Colacino, E.; Martinez, J.; Lamaty, F.; Patrikeeva, L. S.; Khemchyan, L. L.; Ananikov, V. P.; Beletskaya, I. P. Coord. Chem. Rev. 2012, 256, 2893−2920. (117) (a) Grigorjeva, L.; Daugulis, O. Org. Lett. 2014, 16, 4688− 4690. (b) For a recent related report, see: Liu, X.-G.; Zhang, S.-S.; Jiang, C.-Y.; Wu, J.-Q.; Li, Q.; Wang, H. Org. Lett. 2015, 17, 5404− 5407. (118) Zhang, L. B.; Hao, X. Q.; Zhang, S. K.; Liu, Z. J.; Zheng, X. X.; Gong, J. F.; Niu, J. L.; Song, M. P. Angew. Chem., Int. Ed. 2015, 54, 272−275. (119) Guo, X.-K.; Zhang, L.-B.; Wei, D.; Niu, J.-L. Chem. Sci. 2015, 6, 7059−7071.

(67) (a) Gao, K.; Yoshikai, N. J. Am. Chem. Soc. 2013, 135, 9279− 9282. (b) For related benzylations, see: Xu, W.; Paira, R.; Yoshikai, N. Org. Lett. 2015, 17, 4192−4195. (68) Gao, K.; Yamakawa, T.; Yoshikai, N. Synthesis 2014, 46, 2024− 2039. (69) Gao, K.; Paira, R.; Yoshikai, N. Adv. Synth. Catal. 2014, 356, 1486−1490. (70) Koelle, U.; Fuss, B.; Rajasekharan, M. V.; Ramakrishna, B. L.; Ammeter, J. H.; Boehm, M. C. J. Am. Chem. Soc. 1984, 106, 4152− 4160. (71) Li, W.; Weng, L.-H.; Jin, G.-X. Inorg. Chem. Commun. 2004, 7, 1174−1177. (72) Yoshino, T.; Ikemoto, H.; Matsunaga, S.; Kanai, M. Angew. Chem., Int. Ed. 2013, 52, 2207−2211. (73) Yoshino, T.; Ikemoto, H.; Matsunaga, S.; Kanai, M. Chem. - Eur. J. 2013, 19, 9142−9146. (74) Ackermann, L. Chem. Rev. 2011, 111, 1315−1345. (75) Li, Y.; Zhang, X.-S.; Li, H.; Wang, W.-H.; Chen, K.; Li, B.-J.; Shi, Z.-J. Chem. Sci. 2012, 3, 1634−1639. (76) Tauchert, M. E.; Incarvito, C. D.; Rheingold, A. L.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc. 2012, 134, 1482−1485. (77) Lapointe, D.; Fagnou, K. Chem. Lett. 2010, 39, 1118−1126. (78) Schipper, D. J.; Hutchinson, M.; Fagnou, K. J. Am. Chem. Soc. 2010, 132, 6910−6911. (79) Ikemoto, H.; Yoshino, T.; Sakata, K.; Matsunaga, S.; Kanai, M. J. Am. Chem. Soc. 2014, 136, 5424−5431. (80) (a) For indazole synthesis, see: Lian, Y.; Bergman, R. G.; Lavis, L. D.; Ellman, J. A. J. Am. Chem. Soc. 2013, 135, 7122−7125. (b) For furan synthesis, see: Lian, Y.; Huber, T.; Hesp, K. D.; Bergman, R. G.; Ellman, J. A. Angew. Chem., Int. Ed. 2013, 52, 629−633. For phthalide synthesis, see: (c) Lian, Y.; Bergman, R. G.; Ellman, J. A. Chem. Sci. 2012, 3, 3088−3091. (d) Shi, X.; Li, C.-J. Adv. Synth. Catal. 2012, 354, 2933−2938. (81) For the rhenium-catalyzed synthesis of isobenzofurans, see: Kuninobu, Y.; Nishina, Y.; Nakagawa, C.; Takai, K. J. Am. Chem. Soc. 2006, 128, 12376−12377. (82) Hummel, J. R.; Ellman, J. A. J. Am. Chem. Soc. 2015, 137, 490− 498. (83) Park, S. H.; Kwak, J.; Shin, K.; Ryu, J.; Park, Y.; Chang, S. J. Am. Chem. Soc. 2014, 136, 2492−2502. (84) Sun, B.; Yoshino, T.; Matsunaga, S.; Kanai, M. Adv. Synth. Catal. 2014, 356, 1491−1495. (85) Patel, P.; Chang, S. ACS Catal. 2015, 5, 853−858. (86) Sun, B.; Yoshino, T.; Matsunaga, S.; Kanai, M. Chem. Commun. 2015, 51, 4659−4661. (87) (a) Hocek, M.; Naus, P.; Pohl, R.; Votruba, I.; Furman, P. A.; Tharnish, P. M.; Otto, M. J. J. Med. Chem. 2005, 48, 5869−5873. (b) Bakkestuen, A. K.; Gundersen, L.-L.; Utenova, B.-T. J. Med. Chem. 2005, 48, 2710−2723. (c) Gundersen, L.-L.; Nissen-Meyer, J.; Spilsberg, B. J. Med. Chem. 2002, 45, 1383−1386. (d) Hocek, M.; Holy, A.; Votruba, I.; Dvorakova, H. J. Med. Chem. 2000, 43, 1817− 1825. (88) For a recent review, see: Gayakhe, V.; Sanghvi, Y. S.; Fairlamb, I. J. S.; Kapdi, A. R. Chem. Commun. 2015, 51, 11944−11960. (89) Anbarasan, P.; Schareina, T.; Beller, M. Chem. Soc. Rev. 2011, 40, 5049−5067. (90) Liu, W.; Ackermann, L. Chem. Commun. 2014, 50, 1878−1881. (91) (a) Han, J.; Pan, C.; Jia, X.; Zhu, C. Org. Biomol. Chem. 2014, 12, 8603−8606. (b) Gu, L.-J.; Jin, C.; Wang, R.; Ding, H.-Y. ChemCatChem 2014, 6, 1225−1228. (c) Chaitanya, M.; Yadagiri, D.; Anbarasan, P. Org. Lett. 2013, 15, 4960−4963. (d) Gong, T.-J.; Xiao, B.; Cheng, W.-M.; Su, W.; Xu, J.; Liu, Z.-J.; Liu, L.; Fu, Y. J. Am. Chem. Soc. 2013, 135, 10630−10633. (92) Li, J.; Ackermann, L. Angew. Chem., Int. Ed. 2015, 54, 3635− 3638. (93) Yu, D. G.; Gensch, T.; de Azambuja, F.; Vasquez-Cespedes, S.; Glorius, F. J. Am. Chem. Soc. 2014, 136, 17722−17725. (94) Pawar, A. B.; Chang, S. Org. Lett. 2015, 17, 660−663. 524


Review

ACS Catalysis (120) (a) Zhao, D.; Kim, J. H.; Stegemann, L.; Strassert, C. A.; Glorius, F. Angew. Chem., Int. Ed. 2015, 54, 4508−4511. For a related recent report, see: (b) Liu, X.-G.; Zhang, S. S.; Wu, J.-Q.; Li, Q.; Wang, H. Tetrahedron Lett. 2015, 56, 4093−4095. (121) Yao, T.; Hirano, K.; Satoh, T.; Miura, M. Angew. Chem., Int. Ed. 2012, 51, 775−779. (122) In November 2015, the prices for cobalt, rhodium and iridium were 0.65, 708 and 520 US $ per troy oz., respectively, see: http:// taxfreegold.co.uk/preciousmetalpricesusdollars.html and http://www. lme.com/en-gb/metals/minor-metals/cobalt/. (123) (a) Zhang, J.; Chen, H.; Lin, C.; Liu, Z.; Wang, C.; Zhang, Y. J. Am. Chem. Soc. 2015, 137, 12990−12996. (b) Wu, X.; Yang, K.; Zhao, Y.; Sun, H.; Li, G.; Ge, H. Nat. Commun. 2015, 6, 6462. (124) (a) Ozkal, E.; Cacherat, B.; Morandi, B. ACS Catal. 2015, 5, 6458−6462. For further very recent contributions, see: (b) Zhang, G.; Liu, C.; Yi, H.; Meng, Q.; Bian, C.; Chen, H.; Jian, J.-X.; Wu, L.-Z.; Lei, A. J. Am. Chem. Soc. 2015, 137, 9273−9280. (c) Beck, R.; Flörke, U.; Klein, H.-F. Organometallics 2015, 34, 1454−1464. (d) Yang, K.; Chen, X.; Wang, Y.; Li, W.; Kadi, A. A.; Fun, H.-K.; Sun, H.; Zhang, Y.; Li, G.; Lu, H. J. Org. Chem. 2015, 80, 11065−11072. (e) Planas, O.; Whiteoak, C. J.; Company, A.; Ribas, X. Adv. Synth. Catal. 2016, DOI: 10.1002/adsc.201500690. (f) Fallon, B. J.; Garsi, J.-B.; Derat, E.; Amatore, M.; Aubert, C.; Petit, M. ACS Catal. 2015, 5, 7493−7497. (g) Kalsi, D.; Sundararaju, B. Org. Lett. 2015, 17, 6118−6121.

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