Three-Component Coupling of Arenes, Ethylene, and Alkynes Catalyzed by a Cationic Bis(phosphine) Cobalt Complex: Intercepting Metallacyclopentenes for C–H Functionalization

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CCDC 2122988 and 2122990–2122992 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

This article references 27 other publications.

1. 1

   Selected recent reviews on transition metal-catalyzed directed C(sp²)–H functionalization:

   (a) Chen, Z.; Wang, B.; Zhang, J.; Yu, W.; Liu, Z.; Zhang, Y. Transition Metal-catalyzed C–H Bond Functionalizations by the Use of Diverse Directing Groups. Org. Chem. Front. 2015, 2, 1107– 1295,  DOI: 10.1039/C5QO00004A

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   Transition metal-catalyzed C-H bond functionalizations by the use of diverse directing groups

   Chen, Zhengkai; Wang, Binjie; Zhang, Jitan; Yu, Wenlong; Liu, Zhanxiang; Zhang, Yuhong

   Organic Chemistry Frontiers (2015), 2 (9), 1107-1295CODEN: OCFRA8; ISSN:2052-4129. (Royal Society of Chemistry)

   A review. This review article gives an overview of the development of utilizing the functionalities as directing groups. The discussion is directed toward the use of different functional groups contg. nitrogen, oxygen, sulfur, phosphorus, silicon, π-chelation and bidentate systems as directing groups for construction of carbon-carbon and carbon-heteroatom bonds via C-H activation using various transition metal catalysts. The synthetic applications and mechanistic features of these transformations including arylation, olefination, alkylation, alkynylation, carbonylation, amination, halogenation and so on are discussed. The review is organized on the basis of the type of directing groups and the type of bond being formed or the catalyst.

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   (b) Huang, Z.; Lim, H. N.; Mo, F.; Young, M. C.; Dong, G. Transition Metal-catalyzed Ketone-directed or Mediated C–H Functionalization. Chem. Soc. Rev. 2015, 44, 7764– 7786,  DOI: 10.1039/C5CS00272A

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   Transition metal-catalyzed ketone-directed or mediated C-H functionalization

   Huang, Zhongxing; Lim, Hee Nam; Mo, Fanyang; Young, Michael C.; Dong, Guangbin

   Chemical Society Reviews (2015), 44 (21), 7764-7786CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)

   A review. Advancements in use of ketone carbonyls as directing groups, direct β-functionalization, and α-alkylation/alkenylation with unactivated olefins and alkynes has been reviewed.

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   (c) Dong, Z.; Ren, Z.; Thompson, S. J.; Xu, Y.; Dong, G. Transition-metal-catalyzed C–H Alkylation Using Alkenes. Chem. Rev. 2017, 117, 9333– 9403,  DOI: 10.1021/acs.chemrev.6b00574

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   Transition-Metal-Catalyzed C-H Alkylation Using Alkenes

   Dong, Zhe; Ren, Zhi; Thompson, Samuel J.; Xu, Yan; Dong, Guangbin

   Chemical Reviews (Washington, DC, United States) (2017), 117 (13), 9333-9403CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)

   A review. The transition metal-catalyzed alkylations of various carbon-hydrogen bonds (addn. of C-H bonds across olefins) using regular olefins or 1,3-dienes up to the May of 2016 are reviewed. According to the mode of activation, the review is divided into two sections: alkylation via C-H activation and alkylation via olefin activation.

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   (d) Hummel, J. R.; Boerth, J. A.; Ellman, J. A. Transition-metal-catalyzed C–H Bond Addition to Carbonyls, Imines, and Related Polarized π Bonds. Chem. Rev. 2017, 117, 9163– 9227,  DOI: 10.1021/acs.chemrev.6b00661

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   Transition-Metal-Catalyzed C-H Bond Addition to Carbonyls, Imines, and Related Polarized π Bonds

   Hummel, Joshua R.; Boerth, Jeffrey A.; Ellman, Jonathan A.

   Chemical Reviews (Washington, DC, United States) (2017), 117 (13), 9163-9227CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)

   The transition-metal-catalyzed addn. of C-H bonds to carbonyls, imines, and related polarized π bonds has emerged as a particularly efficient and powerful approach for the construction of an incredibly diverse array of heteroatom-substituted products. Readily available and stable inputs are typically employed, and reactions often proceed with very high functional group compatibility and without the prodn. of waste byproducts. Addnl., many transition-metal-catalyzed C-H bond addns. to polarized π bonds occur within cascade reaction sequences to provide rapid access to a diverse array of different heterocyclic as well as carbocyclic products. This review highlights the diversity of transformations that have been achieved, catalysts that have been used, and types of products that have been prepd. through the transition-metal-catalyzed addn. of C-H bonds to carbonyls, imines, and related polarized π bonds.

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   (e) Evano, G.; Theunissen, C. Beyond Friedel and Crafts: Directed Alkylation of C–H Bonds in Arenes. Angew. Chem., Int. Ed. 2019, 58, 7202– 7236,  DOI: 10.1002/anie.201806629

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   Beyond Friedel and Crafts: Directed Alkylation of C-H Bonds in Arenes

   Evano, Gwilherm; Theunissen, Cedric

   Angewandte Chemie, International Edition (2019), 58 (22), 7202-7236CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

   A review. Classical methods for the introduction of alkyl groups to arenes are mostly based on the Friedel-Crafts reaction, radical addns., metalation, or prefunctionalization of the arene: these methods, however, suffer from limitations in scope, efficiency, and selectivity. Moreover, they are based on the innate reactivity of the starting arene, favoring the alkylation at a certain position and rendering the introduction of alkyl chains at other positions much more challenging. This can be addressed by the use of a directing group that facilitates, in the presence of a metal catalyst, the regioselective alkylation of a C-H bond. These directed alkylations of C-H bonds in arenes will be comprehensively summarized in this Review.

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   (f) Achar, T. K.; Maiti, S.; Jana, S.; Maiti, D. Transition Metal Catalyzed Enantioselective C(sp²)–H Bond Functionalization. ACS Catal. 2020, 10, 13748– 13793,  DOI: 10.1021/acscatal.0c03743

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   Transition Metal Catalyzed Enantioselective C(sp2)-H Bond Functionalization

   Achar, Tapas Kumar; Maiti, Sudip; Jana, Sadhan; Maiti, Debabrata

   ACS Catalysis (2020), 10 (23), 13748-13793CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

   A review. Direct catalytic transformation of C-H bonds to new functionalities has provided a powerful strategy to synthesize complex mol. scaffolds in a straightforward way. Unstinting efforts of synthetic community have aided to overcome the longstanding major challenge of regioselectivity by introducing directing group concept. However, the full potential of the strategy cannot be realized unless the activated C-H bonds being stereochem. controlled. The enatioselective C-H bond functionalization could provide an imperative tool for the sustainable way of synthesizing chiral complex mol. scaffolds. Albeit the intrinsic challenges in achieving stereocontrol, the synthetic community has developed different tools in order to achieve stereoselective C-H bond functionalization. In this review, the remarkable recent advances in the emerging area of enantioselective C(sp2)-H bond functionalization has been discussed to highlight the challenges and opportunities, emphasizing on different techniques developed so far.

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   (g) Lam, N. Y. S.; Wu, K.; Yu, J.-Q. Advancing the Logic of Chemical Synthesis: C–H Activation as Strategic and Tactical Disconnections for C–C Bond Construction. Angew. Chem., Int. Ed. 2021, 133, 15901– 5924,  DOI: 10.1002/ange.202011901

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   (h) Ankade, S. B.; Shabade, A. B.; Soni, V.; Punji, B. Unactivated Alkyl Halides in Transition-metal-catalyzed C–H Bond Alkylation. ACS Catal. 2021, 11, 3268– 3292,  DOI: 10.1021/acscatal.0c05580

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   Unactivated Alkyl Halides in Transition-Metal-Catalyzed C-H Bond Alkylation

   Ankade, Shidheshwar B.; Shabade, Anand B.; Soni, Vineeta; Punji, Benudhar

   ACS Catalysis (2021), 11 (6), 3268-3292CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

   A review. Alkylation represents an important org. transformation in mol. science to develop privileged alkylated arenes and heteroarenes. Esp., the direct C-H bond alkylation using unactivated alkyl halides is a straightforward and attractive approach from both the step-economy and chemoselectivity perspectives. Substantial progress has been made in the direct alkylation using primary, secondary, and tertiary alkyl halides along with the methylation and fluoroalkylation. This review broadly summarizes the transition-metal-catalyzed alkylations of C-H bonds on various arenes and heteroarenes with unactivated alkyl halides until oct. 2020. On the basis of the substrates utilized for alkylation, the review is divided into two major sections: alkylation of arenes and alkylation of heteroarenes.

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   (i) Rej, S.; Das, A.; Chatani, N. Strategic Evolution in Transition Metal-catalyzed Directed C–H Bond Activation and Future Directions. Coord. Chem. Rev. 2021, 431, 213683,  DOI: 10.1016/j.ccr.2020.213683

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   Strategic evolution in transition metal-catalyzed directed C-H bond activation and future directions

   Rej, Supriya; Das, Amrita; Chatani, Naoto

   Coordination Chemistry Reviews (2021), 431 (), 213683CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)

   A review. Considering extraordinarily rapid progress in directed C-H bond functionalization reactions, the strategic evolution of directed C-H bond activation chem. Is summarized in this review. This review would be of particular interest to scientists who are interested in progress made in the area of directed C-H bond functionalization, which could stimulate new areas of research regarding this significant topic.

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2. 2

   Reviews on first-row transition metal-catalyzed C(sp²)–H functionalization:

   (a) Su, B.; Cao, Z.-C.; Shi, Z.-J. Exploration of Earth-abundant Transition Metals (Fe, Co, and Ni) as Catalysts in Unreactive Chemical Bond Activations. Acc. Chem. Res. 2015, 48, 886– 896,  DOI: 10.1021/ar500345f

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   2a

   Exploration of Earth-Abundant Transition Metals (Fe, Co, and Ni) as Catalysts in Unreactive Chemical Bond Activations

   Su, Bo; Cao, Zhi-Chao; Shi, Zhang-Jie

   Accounts of Chemical Research (2015), 48 (3), 886-896CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)

   A review. Activation of inert chem. bonds, such as C-H, C-O, C-C, and so on, is a very important area, to which has been drawn much attention by chemists for a long time and which is viewed as one of the most ideal ways to produce valuable chems. Under modern chem. bond activation logic, many conventionally viewed "inert" chem. bonds that were intact under traditional conditions can be reconsidered as novel functionalities, which not only avoids the tedious synthetic procedures for prefunctionalizations and the emission of undesirable wastes but also inspires chemists to create novel synthetic strategies in completely different manners. Although activation of "inert" chem. bonds using stoichiometric amts. of transition metals has been reported in the past, much more attractive and challenging catalytic transformations began to blossom decades ago. Compared with the broad application of late and noble transition metals in this field, the earth-abundant first-row transition-metals, such as Fe, Co, and Ni, have become much more attractive, due to their obvious advantages, including high abundance on earth, low price, low or no toxicity, and unique catalytic characteristics. In this account, the authors summarize their recent efforts toward Fe-, Co-, and Ni-catalyzed "inert" chem. bond activation. The research done by the authors unveiled the unique catalytic ability of iron catalysts in C-O bond activation of both carboxylates and benzyl alcs. in the presence of Grignard reagents. The benzylic C-H functionalization was also developed via Fe catalysis with different nucleophiles, including both electron-rich arenes and 1-aryl-vinyl acetates. Cobalt catalysts also showed their uniqueness in both arom. C-H activation and C-O activation in the presence of Grignard reagents. The authors reported the first cobalt-catalyzed sp2 C-H activation/arylation and alkylation of benzo[h]quinoline and phenylpyridine, in which a new catalytic pathway via an oxidative addn. process was demonstrated to be much preferable. Another interesting discovery made by the authors was the Co-catalyzed magnesiation of benzylic alcs. in the presence of different Grignard reagents, which proceeded via Co-mediated selective C-O bond activation. In C-O activation, Ni catalysts were found to be most powerful, showing the high efficacy in different kinds of couplings starting form "inert" O-based electrophiles. In addn., Ni catalysts exhibited their power in C-H and C-C activation, which have been proven by the authors and pioneers in this field. Notably, the developments indicated that the catalytic efficacy in cross coupling between aryl bromides and arenes under mild conditions was not the privilege of several noble metals; most of the transition metals exhibited credible catalytic ability, including Fe, Co, and Ni. The authors hope that their studies inspire more interest in the development of first row transition metal-catalyzed inert chem. bond functionalization.

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   (b) Pototschnig, G.; Maulide, N.; Schnürch, M. Direct Functionalization of C–H Bonds by Iron, Nickel, and Cobalt Catalysis. Chem. Eur. J. 2017, 23, 9206– 9232,  DOI: 10.1002/chem.201605657

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   Direct Functionalization of C-H Bonds by Iron, Nickel, and Cobalt Catalysis

   Pototschnig, Gerit; Maulide, Nuno; Schnuerch, Michael

   Chemistry - A European Journal (2017), 23 (39), 9206-9232CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)

   A review. Non-precious-metal-catalyzed reactions are of increasing importance in chem. due to the outstanding ecol. and economic properties of these metals. In the subfield of metal-catalyzed direct C-H functionalization reactions, recent years showed an increasing no. of publications dedicated to this topic. Nickel, cobalt, and last but not least iron, have started to enter a field which was long dominated by precious metals such as palladium, rhodium, ruthenium, and iridium. The present review article summarizes the development of iron-, nickel-, and cobalt-catalyzed C-H functionalization reactions until the end of 2016, and discusses the scope and limitations of these transformations.

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   (c) Gandeepan, P.; Müller, T.; Zell, D.; Cera, G.; Warratz, S.; Ackermann, L. 3d Transition Metals for C–H Activation. Chem. Rev. 2019, 119, 2192– 2452,  DOI: 10.1021/acs.chemrev.8b00507

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   3d Transition Metals for C-H Activation

   Gandeepan, Parthasarathy; Mueller, Thomas; Zell, Daniel; Cera, Gianpiero; Warratz, Svenja; Ackermann, Lutz

   Chemical Reviews (Washington, DC, United States) (2019), 119 (4), 2192-2452CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)

   A review. C-H activation has surfaced as an increasingly powerful tool for mol. sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018 has been reviewed.

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   (d) Loup, J.; Dhawa, U.; Pesciaioli, F.; Wencel-Delord, J.; Ackermann, L. Enantioselective C–H Activation with Earth-abundant 3d Transition Metals. Angew. Chem., Int. Ed. 2019, 58, 12803– 12818,  DOI: 10.1002/anie.201904214

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   2d

   Enantioselective C-H Activation with Earth-Abundant 3d Transition Metals

   Loup, Joachim; Dhawa, Uttam; Pesciaioli, Fabio; Wencel-Delord, Joanna; Ackermann, Lutz

   Angewandte Chemie, International Edition (2019), 58 (37), 12803-12818CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

   A review. Mol. syntheses largely rely on time- and labor-intensive prefunctionalization strategies. In contrast, C-H activation represents an increasingly powerful approach that avoids lengthy syntheses of prefunctionalized substrates, with great potential for drug discovery, the pharmaceutical industry, material sciences, and crop protection, among others. The enantioselective functionalization of omnipresent C-H bonds has emerged as a transformative tool for the step- and atom-economical generation of chiral mol. complexity. However, this rapidly growing research area remains dominated by noble transition metals, prominently featuring toxic palladium, iridium and rhodium catalysts. Indeed, despite significant achievements, the use of inexpensive and sustainable 3d metals in asym. C-H activations is still clearly in its infancy. Herein, we discuss the remarkable recent progress in enantioselective transformations via organometallic C-H activation by 3d base metals up to Apr. 2019.

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   (e) Woźniak, L.; Cramer, N. Enantioselective C–H Bond Functionalizations by 3d Transition-Metal Catalysts. Trends in Chemistry 2019, 1, 471– 484,  DOI: 10.1016/j.trechm.2019.03.013

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   2e

   Enantioselective C-H Bond Functionalizations by 3d Transition-Metal Catalysts

   Wozniak, Lukasz; Cramer, Nicolai

   Trends in Chemistry (2019), 1 (5), 471-484CODEN: TCRHBQ; ISSN:2589-5974. (Cell Press)

   A review. Direct catalytic modifications of carbon-hydrogen (C-H) bonds, ubiquitous in org. mols., represent a powerful strategy in org. synthesis. In the past decade, chemists have focused on the development of sustainable methods for functionalization of inert C-H bonds using cost-effective earth-abundant 3d transition-metal catalysts. To fully harness the potential of this technol., however, it is essential to control the stereoselectivity of the C-H functionalization processes. This review describes developments in the emerging area of enantioselective functionalization of C-H bonds by 3d transition-metal catalysts proceeding via inner-sphere C-H activation.

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   (f) Carvalho, R. L.; de Miranda, A. S.; Nunes, M. P.; Gomes, R. S.; Jardim, G. A. M.; da Silva Júnior, E. N. On the application of 3d metals for C–H activation toward bioactive compounds: The key step for the synthesis of silver bullets. Beilstein J. Org. Chem. 2021, 17, 1849– 1938,  DOI: 10.3762/bjoc.17.126

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   2f

   On the application of 3d metals for C-H activation toward bioactive compounds: the key step for the synthesis of silver bullets

   Carvalho, Renato L.; de Miranda, Amanda S.; Nunes, Mateus P.; Gomes, Roberto S.; Jardim, Guilherme A. M.; da Silva Junior, Eufranio N.

   Beilstein Journal of Organic Chemistry (2021), 17 (), 1849-1938CODEN: BJOCBH; ISSN:1860-5397. (Beilstein-Institut zur Foerderung der Chemischen Wissenschaften)

   A review. Several valuable biol. active mols. can be obtained through C-H activation processes. However, the use of expensive and not readily accessible catalysts complicates the process of pharmacol. application of these compds. A plausible way to overcome this issue is developing and using cheaper, more accessible, and equally effective catalysts. First-row transition (3d) metals have shown to be important catalysts in this matter. This review summarizes the use of 3d metal catalysts in C-H activation processes to obtain potentially (or proved) biol. active compds.

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3. 3

   Reviews on cobalt-catalyzed C(sp²)–H functionalization:

   (a) Gao, K.; Yoshikai, N. Low-valent Cobalt Catalysis: New Opportunities for C–H Functionalization. Acc. Chem. Res. 2014, 47, 1208– 1219,  DOI: 10.1021/ar400270x

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   3a

   Low-Valent Cobalt Catalysis: New Opportunities for C-H Functionalization

   Gao, Ke; Yoshikai, Naohiko

   Accounts of Chemical Research (2014), 47 (4), 1208-1219CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)

   A review. Rapid progress in the fields of organometallic chem. and homogeneous catalysis has made it possible for synthetic chemists to consider using ubiquitous yet unreactive C-H bonds as starting points to construct complex org. mols. However, a majority of the C-H functionalization reactions currently in use require noble transition metal catalysts and harsh reaction conditions, so researchers have placed a priority on the development of mild and cost-effective catalysts. Given this situation, we wondered whether earth-abundant first-row transition metals could emulate the reactivity of a noble transition metal catalyst and carry out similar C-H functionalization reactions at a lower cost and under milder conditions. We also wondered whether we could use first-row transition metals to achieve hitherto unknown, but useful, C-H functionalization reactions. This Account summarizes our research on the development of three different types of C-H functionalization reactions using low-valent cobalt catalysts: (1) hydroarylation of alkynes and olefins, (2) ortho C-H functionalization with electrophiles, and (3) addn. of arylzinc reagents to alkynes involving 1,4-cobalt migration. Although synthetic chemists have previously paid little attention to cobalt in designing catalytic C-H functionalization reactions, earlier studies, particularly those on stoichiometric cyclometalation, inspired us as we developed the hydroarylation and ortho C-H functionalization reactions. In these transformations, we combined a cobalt precatalyst, a ligand (such as phosphine or N-heterocyclic carbene (NHC)), and Grignard reagent to generate low-valent cobalt catalysts. These novel catalysts promoted a series of pyridine- and imine-directed hydroarylation reactions of alkynes and olefins at mild temps. Notably, we obsd. branched-selective addn. to styrenes, which highlights a distinct regioselectivity of the cobalt catalyst compared with typical rhodium and ruthenium catalysts. The combination of a cobalt-NHC catalyst and a Grignard reagent allows directed arom. C-H functionalizations with electrophiles such as aldimines, aryl chlorides, and alkyl chlorides or bromides. This second reaction has a particularly broad scope, allowing us to introduce secondary alkyl groups at the ortho position of aryl imines, a difficult reaction to carry out by other means. Serendipitously, we found that a cobalt-Xantphos complex catalyzed the third type of C-H functionalization: the addn. of an arylzinc reagent to an alkyne to afford ortho-alkenylarylzinc species through a 1,4-cobalt migration. This "migratory arylzincation" allowed us to quickly construct a diverse group of functionalized benzothiophenes and benzoselenophenes. Collectively, our studies of cobalt catalysis have provided cost-effective catalysts and milder conditions for existing C-H functionalizations and have led to some unprecedented, attractive chem. transformations.

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   (b) Gandeepan, P.; Cheng, C.-H. Cobalt Catalysis Involving π Components in Organic Synthesis. Acc. Chem. Res. 2015, 48, 1194– 1206,  DOI: 10.1021/ar500463r

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   3b

   Cobalt Catalysis Involving π Components in Organic Synthesis

   Gandeepan, Parthasarathy; Cheng, Chien-Hong

   Accounts of Chemical Research (2015), 48 (4), 1194-1206CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)

   A review. Over the last three decades, transition metal-catalyzed org. transformations have been shown to be extremely important in org. synthesis. However, most of the successful reactions are assocd. with noble metals, which are generally toxic, expensive, and less abundant. Therefore, the authors have focused on catalysis using the abundant first-row transition metals, specifically cobalt. In this Account, the authors demonstrate the potential of cobalt catalysis in org. synthesis as revealed by their research. The authors have developed many useful catalytic systems using cobalt complexes. Overall, they can be classified into several broad types of reactions, specifically [2+2+2] and [2+2] cycloaddns.; enyne reductive coupling; reductive [3+2] cycloaddn. of alkynes/allenes with enones; reductive coupling of alkyl iodides with alkenes; addn. of organoboronic acids to alkynes, alkenes, or aldehydes; carbocyclization of o-iodoaryl ketones/aldehydes with alkynes/electron-deficient alkenes; coupling of thiols with aryl and alkyl halides; enyne coupling; and C-H bond activation. Reactions relying on π components, specifically cycloaddn., reductive coupling, and enyne coupling, mostly afford products with excellent stereo- and regioselectivity and superior atom economy. The authors believe that these cobalt-catalyzed π-component coupling reactions proceed through five-membered cobaltacyclic intermediates formed by the oxidative cyclometalation of two coordinated π bonds of the substrates to the low-valent cobalt species. The high regio- and stereoselectivity of these reactions are achieved as a result of the electronic and steric effects of the π components. Mostly, electron-withdrawing groups and bulkier groups attached to the π bonds prefer to be placed near the cobalt center of the cobaltacycle. Most of these transformations proceed through low-valent cobalt complexes, which are conveniently generated in situ from air-stable Co(II) salts by Zn- or Mn-mediated redn. Overall, the authors have shown these reactions to be excellent substitutes for less desirable noble-metal systems. Recent successes in cobalt-catalyzed C-H activation have esp. advanced the applicability of cobalt in this field. In addn. to the more common low-valent-cobalt-catalyzed C-H activation reactions, an in situ-formed cobalt(III) five-membered complex with a 1,6-enyne effectively couples with arom. ketones and esters through ortho C-H activation, opening a new window in this research area. Interestingly, this reaction proceeds under milder reaction conditions with broad substrate scope. Furthermore, many of the reactions the authors have developed are highly enantioselective, including enantioselective reductive coupling of enones and alkynes, addn. of organoboronic acids to aldehydes, and the cyclization of 2-iodobenzoates with aldehydes. Overall, this Account demonstrates the versatility and utility of cobalt catalysis in org. synthesis.

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   (c) Moselage, M.; Li, J.; Ackermann, L. Cobalt-catalyzed C–H Activation. ACS Catal. 2016, 6, 498– 525,  DOI: 10.1021/acscatal.5b02344

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   Cobalt-Catalyzed C-H Activation

   Moselage, Marc; Li, Jie; Ackermann, Lutz

   ACS Catalysis (2016), 6 (2), 498-525CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

   A review. 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 cobalt-catalyzed 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 Nov. 2015.

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   (d) Yoshino, T.; Matsunaga, S. (Pentamethylcyclopentadienyl)cobalt(III)-catalyzed C–H Bond Functionalization: From Discovery to Unique Reactivity and Selectivity. Adv. Synth. Catal. 2017, 359, 1245– 1262,  DOI: 10.1002/adsc.201700042

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   3d

   (Pentamethylcyclopentadienyl)cobalt(III)-Catalyzed C-H Bond Functionalization: From Discovery to Unique Reactivity and Selectivity

   Yoshino, Tatsuhiko; Matsunaga, Shigeki

   Advanced Synthesis & Catalysis (2017), 359 (8), 1245-1262CODEN: ASCAF7; ISSN:1615-4150. (Wiley-VCH Verlag GmbH & Co. KGaA)

   A review. High-valent (pentamethylcyclopentadienyl)cobalt(III) [Cp*Co(III)] catalysts were found as inexpensive alternatives to (pentamethylcyclopentadienyl)rhodium(III) [Cp*Rh(III)] catalysts in the field of C-H bond functionalization, and applied to a variety of transformations. In this review, after the discovery and early examples of Cp*Co(III)-catalyzed C-H bond functionalization are summarized, the unique reactivity and selectivity of Cp*Co(III) and the differences between the cobalt and rhodium catalysis are intensively discussed. Such differences are assumed to be caused by the lower electronegativity, hard nature, and smaller ionic radius of cobalt.

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   (e) Usman, M.; Ren, Z.-H.; Wang, Y.-Y.; Guan, Z.-H. Recent Developments in Cobalt-catalyzed Carbon–Carbon and Carbon–Heteroatom Bond Formation via C–H Bond Functionalization. Synthesis 2017, 49, 1419– 1443,  DOI: 10.1055/s-0036-1589478

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   3e

   Recent Developments in Cobalt Catalyzed Carbon-Carbon and Carbon-Heteroatom Bond Formation via C-H Bond Functionalization

   Usman, Muhammad; Ren, Zhi-Hui; Wang, Yao-Yu; Guan, Zheng-Hui

   Synthesis (2017), 49 (7), 1419-1443CODEN: SYNTBF; ISSN:1437-210X. (Georg Thieme Verlag)

   A review. Cobalt catalysts have evolved to be seen as versatile eco-compatible and economical catalysts in org. synthesis in recent years. Cobalt-catalyzed reactions are undoubtedly a classic in synthetic chem. for the formation of carbon-carbon and carbon-heteroatom bonds. Another important aspect in this field is catalyst variants, such as low-valent and high-valent cobalt catalysts. This review summarizes the recent progress and synthetic utility of low-valent and high-valent cobalt catalysts towards C-H functionalization processes achieving C-C, C-O, C-N and C-B bond formation. Mechanistic insight is also discussed, with the goal of serving as a stepping stone for further development in this field. In addn., Csp3-H bond functionalization reactions provide many opportunities for novel synthesis approaches.

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   (f) Santhoshkumar, R.; Cheng, C.-H. Hydroarylations by Cobalt-catalyzed C–H activation. Beilstein J. Org. Chem. 2018, 14, 2266– 2288,  DOI: 10.3762/bjoc.14.202

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   Hydroarylations by cobalt-catalyzed C-H activation

   Santhoshkumar, Rajagopal; Cheng, Chien-Hong

   Beilstein Journal of Organic Chemistry (2018), 14 (), 2266-2288CODEN: BJOCBH; ISSN:1860-5397. (Beilstein-Institut zur Foerderung der Chemischen Wissenschaften)

   A review. The recent developments of Co-catalyzed hydroarylation reactions and their mechanistic studies were summarized.

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   (g) Planas, O.; Chirila, P. G.; Whiteoak, C. J.; Ribas, X. Current Mechanistic Understanding of Cobalt-catalyzed C–H Functionalization. Adv. Organomet. Chem. 2018, 69, 209– 282,  DOI: 10.1016/bs.adomc.2018.02.002

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   3g

   Current mechanistic understanding of cobalt-catalyzed C-H functionalization

   Planas, Oriol; Chirila, Paula G.; Whiteoak, Christopher J.; Ribas, Xavi

   Advances in Organometallic Chemistry (2018), 69 (), 209-282CODEN: AOMCAU; ISSN:0065-3055. (Academic Press)

   A review. This overview has demonstrated, using selected examples, how the rich redox chem. of cobalt has, and continues to, provide researchers with a variety of mechanistic pathways for the development of a diverse range of coupling protocols. Indeed, examples described within this overview have utilized low-valent oxidn. states, Go(0), all the way to high oxidn. states, Co(V).

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   (h) Ai, W.; Zhong, R.; Liu, X.; Liu, Q. Hydride Transfer Reactions Catalyzed by Cobalt Complexes. Chem. Rev. 2019, 119, 2876– 2953,  DOI: 10.1021/acs.chemrev.8b00404

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   3h

   Hydride Transfer Reactions Catalyzed by Cobalt Complexes

   Ai, Wenying; Zhong, Rui; Liu, Xufang; Liu, Qiang

   Chemical Reviews (Washington, DC, United States) (2019), 119 (4), 2876-2953CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)

   A review discusses the use of cobalt complexes as hydride transfer catalysts. The prepn. and properties of cobalt complexes, their use in hydrogenation, transfer hydrogenation, dehydrogenation, hydrogen borrowing, hydrofunctionalization, and olefin isomerization reactions, and the mechanisms of their reactions are discussed.

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   (i) Baccalini, A.; Vergura, S.; Dolui, P.; Zanoni, G.; Maiti, D. Recent Advances in Cobalt-catalysed C–H Functionalizations. Org. Biomol. Chem. 2019, 17, 10119– 10141,  DOI: 10.1039/C9OB01994D

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   3i

   Recent advances in cobalt-catalysed C-H functionalizations

   Baccalini, Alessio; Vergura, Stefania; Dolui, Pravas; Zanoni, Giuseppe; Maiti, Debabrata

   Organic & Biomolecular Chemistry (2019), 17 (48), 10119-10141CODEN: OBCRAK; ISSN:1477-0520. (Royal Society of Chemistry)

   Review. Ready availability, low cost and low toxicity of cobalt salts have redirected the attention of researchers away from noble metals, such as Pd, Rh, and Ir, towards Co in the field of C-H functionalization. In this context, the examples of Co-catalyzed functionalization have exponentially grown over the last few decades. This present review focuses on the most recent developments on Co-catalyzed C(sp2)-H and C(sp3)-H functionalizations. Included is also a comprehensive overview of enantioselective transformations.

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   (j) Carral-Menoyo, A.; Sotomayor, N.; Lete, E. Cp*Co(III)-catalyzed C–H Hydroarylation of Alkynes and Alkenes and Beyond: A Versatile Synthetic Tool. ACS Omega 2020, 5, 24974– 24993,  DOI: 10.1021/acsomega.0c03639

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   3j

   Cp*Co(III)-Catalyzed C-H Hydroarylation of Alkynes and Alkenes and Beyond: A Versatile Synthetic Tool

   Carral-Menoyo, Asier; Sotomayor, Nuria; Lete, Esther

   ACS Omega (2020), 5 (39), 24974-24993CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)

   A review. Cp*Co(III) complexes have been proven to possess unique reactivity compared, for example, to their Rh(III) counterparts, obtaining improved chemo- or regioselectivities, as well as yielding new reactivities. This perspective was focused on recent advances on the alkylation and alkenylation reactions of (hetero)arenes with alkenes and alkynes under Cp*Co(III) catalysis.

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   (k) Banjare, S. K.; Nanda, T.; Pati, B. V.; Biswal, P.; Ravikumar, P. C. O-Directed C–H Functionalization via Cobaltacycles: A Sustainable Approach for C–C and C–Heteroatom Bond Formations. Chem. Commun. 2021, 57, 3630– 3647,  DOI: 10.1039/D0CC08199J

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   3k

   O-Directed C-H functionalization via cobaltacycles: a sustainable approach for C-C and C-heteroatom bond formations

   Banjare, Shyam Kumar; Nanda, Tanmayee; Pati, Bedadyuti Vedvyas; Biswal, Pragati; Ravikumar, Ponneri Chandrababu

   Chemical Communications (Cambridge, United Kingdom) (2021), 57 (30), 3630-3647CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)

   This review focuses on providing comprehensive highlights of the recent advances in the field of cobalt-catalyzed C-H functionalization and related synthetic concepts, relying on these through oxygen atom coordination. In recent years, 3d transition metal (Fe, Co, Cu & Ni) catalyzed C-H functionalization reactions have received immense attention on account of its higher abundance and low cost, as compared to noble metals such as Ir, Rh, Ru and Pd. Among the first-row transition metals, cobalt is one of the extensively used metals for sustainable synthesis due to its unique reactivity towards the functionalization of inert C-H bonds. The functionalization of the inert C-H bond necessitates a proximal directing group. In this context, strongly coordinating nitrogen atom directed C-H functionalizations have been well explored. Nevertheless, strongly coordinating nitrogen-contg. scaffolds, such as pyridine, pyrimidine, and 8-aminoquinoline, have to be installed and removed in a sep. process. In contrast, C-H functionalizations through weakly coordinating atoms, such as oxygen, are largely underdeveloped. Since the oxygen atom is a part of many readily available functional groups, such as aldehydes, ketones, carboxylic acids, and esters, it could be used as directing groups for selective C-H functionalization reactions without any modification. Thus, the use of 3d transition metals, such as cobalt, along with weakly coordinating (oxygen) directing groups for C-H functionalization reactions are more sustainable, esp. for the large-scale(coating) prodn. of pharmaceuticals in industries. During the last decade, notable progress has been made using this concept. Nonetheless, almost all the reports are restricted to the formation of C-C and C-N bond. Therefore, there is a wide scope for developing this area for the formation of other bonds, such as C-X (halogens), C-B, C-S, and C-Se.

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   (l) Lukasevics, L.; Cizikovs, A.; Grigorjeva, L. C–H Bond Functionalization by High-valent Cobalt Catalysis: Current Progress, Challenges and Future Perspectives. Chem. Commun. 2021, 57, 10827– 10841,  DOI: 10.1039/D1CC04382J

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   3l

   C-H bond functionalization by high-valent cobalt catalysis: current progress, challenges and future perspectives

   Lukasevics, Lukass; Cizikovs, Aleksandrs; Grigorjeva, Liene

   Chemical Communications (Cambridge, United Kingdom) (2021), 57 (83), 10827-10841CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)

   A review. Over the last decade, high-valent cobalt catalysis has earned a place in the spotlight as a valuable tool for C-H activation and functionalization. Since the discovery of its unique reactivity, more and more attention has been directed towards the utilization of cobalt as an alternative to noble metal catalysts. In particular, Cp*Co(III) complexes, as well as simple Co(II) and Co(III) salts in combination with bidentate chelation assistance, have been extensively used for the development of novel transformations. In this review, authors have demonstrated the existing trends in the C-H functionalization methodol. using high-valent cobalt catalysis and highlighted the main challenges to overcome, as well as perspective directions, which need to be further developed in the future.

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4. 4

   Examples of cobalt-catalyzed late-stage C–H functionalization:

   (a) Lorion, M. M.; Kaplaneris, N.; Son, J.; Kuniyil, R.; Ackermann, L. Late-stage Peptide Diversification through Cobalt-catalyzed C–H Activation: Sequential Multicatalysis for Stapled Peptides. Angew. Chem., Int. Ed. 2019, 58, 1684– 1688,  DOI: 10.1002/anie.201811668

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   4a

   Late-stage peptide diversification through cobalt-catalyzed C-H activation: Sequential multicatalysis for stapled peptides

   Lorion, Melanie M.; Kaplaneris, Nikolaos; Son, Jongwoo; Kuniyil, Rositha; Ackermann, Lutz

   Angewandte Chemie, International Edition (2019), 58 (6), 1684-1688CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

   Bioorthogonal late-stage diversification of structurally complex peptides has enormous potential for drug discovery and mol. imaging. In recent years, transition-metal-catalyzed C-H activation has emerged as an increasingly viable tool for peptide modification. Despite major accomplishments, these strategies largely rely on expensive palladium catalysts. We herein report an unprecedented cobalt(III)-catalyzed peptide C-H activation, which enables the direct C-H functionalization of structurally complex peptides, and sets the stage for a multicatalytic C-H activation/alkene metathesis/hydrogenation strategy for the assembly of novel cyclic peptides.

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   (b) Friis, S. D.; Johansson, M. J.; Ackermann, L. Cobalt-catalysed C–H Methylation for Late-stage Drug Diversification. Nat. Chem. 2020, 12, 511– 519,  DOI: 10.1038/s41557-020-0475-7

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   Cobalt-catalysed C-H methylation for late-stage drug diversification

   Friis, Stig D.; Johansson, Magnus J.; Ackermann, Lutz

   Nature Chemistry (2020), 12 (6), 511-519CODEN: NCAHBB; ISSN:1755-4330. (Nature Research)

   The magic Me effect is well acknowledged in medicinal chem., but despite its significance, accessing such analogs via derivatization at a late stage remains a pivotal challenge. In an effort to mitigate this major limitation, the authors here present a strategy for the cobalt-catalyzed late-stage C-H methylation of structurally complex drug mols. Enabling broad applicability, the transformation relies on a boron-based Me source and takes advantage of inherently present functional groups to guide the C-H activation. The relative reactivity obsd. for distinct classes of functionalities were detd. and the sensitivity of the transformation towards a panel of common functional motifs was tested under various reaction conditions. Without the need for prefunctionalization or postdeprotection, a diverse array of marketed drug mols. and natural products could be methylated in a predictable manner. Subsequent physicochem. and biol. testing confirmed the magnitude with which this seemingly minor structural change can affect important drug properties.

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5. 5

   Well-defined cobalt precatalysts for directed C(sp²)–H functionalization:

   (a) Yoshino, T.; Ikemoto, H.; Matsunaga, S.; Kanai, M. A Cationic High-valent Cp*Co^III Complex for the Catalytic Generation of Nucleophilic Organometallic Species: Directed C–H Bond Activation. Angew. Chem., Int. Ed. 2013, 52, 2207– 2211,  DOI: 10.1002/anie.201209226

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   5a

   A Cationic High-Valent Cp*CoIII Complex for the Catalytic Generation of Nucleophilic Organometallic Species: Directed C-H Bond Activation

   Yoshino, Tatsuhiko; Ikemoto, Hideya; Matsunaga, Shigeki; Kanai, Motomu

   Angewandte Chemie, International Edition (2013), 52 (8), 2207-2211CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

   Five cationic high-valent cobalt(III) sandwich complexes were prepd. and their catalytic activity assessed for directed C-H activation and functionalization of 2-phenylpyridine derivs. The complex [Cp*CoIII(benzene)](PF6)2 exhibited the best reactivity, and was used to generate nucleophilic cyclometalated intermediates, which underwent nucleophilic addn. to N-sulfonylimines, α,β-unsatd. ketones, and α,β-unsatd. N-acylpyrroles. The transformation is atom economical, completely regioselective, and provides products in 57-91% yield.

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   (b) Klein, H.-F. Tetrakis(trimethylphosphane)cobalt(0): Preparation and Reactions. Angew. Chem., Int. Ed. Engl. 1971, 10, 343,  DOI: 10.1002/anie.197103431

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   5b

   Tetrakis(trimethylphosphane)cobalt(O). Preparation and reactions

   Klein, Hans Friedrich

   Angewandte Chemie, International Edition in English (1971), 10 (5), 343CODEN: ACIEAY; ISSN:0570-0833.

   Co(PPh3)4 was prepd. by the redn. of anhyd. CoX2 (X = halide) with Na amalgam in the presence of PPh3. Solns. of Co(PPh3)4 absorb NO to give diamagnetic CoNO(PPh3)4 and react with azobenzene (L) to give LCo(PPh3)2.

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   (c) Fallon, B. J.; Derat, E.; Amatore, M.; Aubert, C.; Chemla, F.; Ferreira, F.; Perez-Luna, A.; Petit, M. C–H Activation/Functionalization Catalyzed by Simple, Well-defined Low-valent Cobalt Complexes. J. Am. Chem. Soc. 2015, 137, 2448– 2451,  DOI: 10.1021/ja512728f

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   5c

   C-H Activation/Functionalization Catalyzed by Simple, Well-Defined Low-Valent Cobalt Complexes

   Fallon, Brendan J.; Derat, Etienne; Amatore, Muriel; Aubert, Corinne; Chemla, Fabrice; Ferreira, Franck; Perez-Luna, Alejandro; Petit, Marc

   Journal of the American Chemical Society (2015), 137 (7), 2448-2451CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

   A facile C-H activation and functionalization of arom. imines is presented using low-valent cobalt catalysts. Using Co(PMe3)4 as catalyst we have developed an efficient and simple protocol for the C-H/hydroarylation of alkynes with an anti selectivity. Deuterium-labeling expts., DFT calcns. coupled with the use of a well-defined catalyst have for the first time shed light on the elusive black box of cobalt catalyzed C-H functionalization.

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   (d) Yamamoto, A.; Miura, Y.; Ito, T.; Chen, H. L.; Iri, K.; Ozawa, F.; Miki, K.; Sei, T.; Tanaka, N.; Kasai, N. Preparation, X-ray Molecular Structure Determination, and Chemical Properties of Dinitrogen-coordinated Cobalt Complexes Containing Triphenylphosphine Ligands and Alkali Metal or Magnesium. Protonation of the Coordinated Dinitrogen to Ammonia and Hydrazine. Organometallics 1983, 2, 1429– 1436,  DOI: 10.1021/om50004a032

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   5d

   Preparation, x-ray molecular structure determination, and chemical properties of dinitrogen-coordinated cobalt complexes containing triphenylphosphine ligands and alkali metal or magnesium. Protonation of the coordinated dinitrogen to ammonia and hydrazine

   Yamamoto, Akio; Miura, Yoshikiyo; Ito, Takashi; Chen, Hui Lin; Iri, Kiyoshi; Ozawa, Fumiyuki; Miki, Kunio; Sei, Tsuyoshi; Tanaka, Nobuo; Kasai, Nobutami

   Organometallics (1983), 2 (10), 1429-36CODEN: ORGND7; ISSN:0276-7333.

   Treatment of CoH(N2)(PPh3)3 (I) with Et2Mg gave [Co(N2)(PPh3)3]2Mg(THF)4 (II). Subsequent reaction of II or direct reaction of I with LiBu gave [Co(N2)(PPh3)3]Li(Et2O)3 (III) and [Co(N2)(PPh3)3]Li(THF)3 (IV) depending on the solvent used. [Co(N2)(PPh3)3]Na(THF)3 (V) was also obtained by the reaction of I with Na metal. The mol. structures of III and IV were fully established by x-ray structural anal. These complexes are isomorphous and have a 3-fold symmetry with the N2 ligand bridging Co and Li on its both ends with the N-N bond length of 1.167(16) Å for III and 1.19(4) Å for IV. In contrast to I whose coordinated N2 ligand is incapable of reacting with protic acids, the ligated N2 in the electron-rich Co complexes II, III, IV, and V is attacked by concd. H2SO4 to afford N2H4 and NH3. These complexes provide the 1st examples of the conversion of dinitrogen coordinated to Co into N2H4 and NH3 on hydrolysis. The corresponding Fe analog having a ligating dinitrogen also was prepd. by the reaction of Fe(acac)3 (Hacac = acetylacetone) with MgEt2 in the presence of 2-6 equiv equiv of PPh3 under N. The complex was characterized as [FeEt(N2(PPh3)2]2Mg(THF)4 (VI). VI also affords N2H4 and NH3 on acidolysis.

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   (e) Suslick, B. A.; Tilley, T. D. Mechanistic Interrogation of Alkyne Hydroarylations Catalyzed by Highly Reduced, Single-component Cobalt Complexes. J. Am. Chem. Soc. 2020, 142, 11203– 11218,  DOI: 10.1021/jacs.0c04072

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   5e

   Mechanistic Interrogation of Alkyne Hydroarylations Catalyzed by Highly Reduced, Single-Component Cobalt Complexes

   Suslick, Benjamin A.; Tilley, T. Don

   Journal of the American Chemical Society (2020), 142 (25), 11203-11218CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

   Highly reactive catalysts for ortho-hydroarylations of alkynes have previously been reported to result from activation of CoBr2 by Grignard reagents, but the operative mechanism and identity of the active cobalt species have been undefined. A mechanistic anal. of a related system, involving hydroarylations of a (N-aryl)aryl ethanimine with diphenylacetylene, was performed using isolable reduced Co complexes. Studies of the stoichiometric reaction of Co(I) or Co(II) precursors with CyMgCl implicated catalyst initiation via a β-H elimination/deprotonation pathway. The resulting single-component Co(-I) complex is proposed as the direct pre-catalyst. Michaelis-Menten enzyme kinetic studies provide mechanistic details regarding the catalytic dependence on substrate. The (N-aryl)aryl ethanimine substrate exhibited satn.-like behavior, whereas alkyne demonstrated a complex dependency; rate inhibition and promotion depend on the relative concn. of alkyne to imine. Activation of the aryl C-H bond occurred only in the presence of coordinated alkyne, which suggests operation of a concerted metalation-deprotonation (CMD) mechanism. Small primary isotope effects are consistent with a rate-detg. C-H cleavage. Off-cycle olefin isomerization catalyzed by the same Co(-I) active species appears to be responsible for the obsd. Z-selectivity.

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6. 6

   Santhoshkumar, R.; Mannathan, S.; Cheng, C.-H. Cobalt-catalyzed Hydroarylative Cyclization of 1,6-Enynes with Aromatic Ketones and Esters via C–H Activation. Org. Lett. 2014, 16, 4208– 4211,  DOI: 10.1021/ol501904e

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   6

   Cobalt-Catalyzed Hydroarylative Cyclization of 1,6-Enynes with Aromatic Ketones and Esters via C-H Activation

   Santhoshkumar, Rajagopal; Mannathan, Subramaniyan; Cheng, Chien-Hong

   Organic Letters (2014), 16 (16), 4208-4211CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)

   A highly chemo- and stereoselective cobalt-catalyzed hydroarylative cyclization of 1,6-enynes with arom. ketones and esters to synthesize functionalized pyrrolidines and dihydrofurans is described. A mechanism involving cobaltacycle triggered C-H activation of arom. ketones and esters was proposed.

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7. 7

   Rhodium-catalyzed tandem cyclization-hydroarylation of 1,6-enynes and 1,6-diynes:

   (a) Tanaka, K.; Otake, Y.; Wada, A.; Noguchi, K.; Hirano, M. Cationic Rh(I)/Modified-BINAP-catalyzed Reactions of Carbonyl Compounds with 1,6-Diynes Leading to Dienones and ortho-Functionalized Aryl Ketones. Org. Lett. 2007, 9, 2203– 2206,  DOI: 10.1021/ol0707721

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   7a

   Cationic Rh(I)/Modified-BINAP-Catalyzed Reactions of Carbonyl Compounds with 1,6-Diynes Leading to Dienones and Ortho-Functionalized Aryl Ketones

   Tanaka, Ken; Otake, Yousuke; Wada, Azusa; Noguchi, Keiichi; Hirano, Masao

   Organic Letters (2007), 9 (11), 2203-2206CODEN: ORLEF7; ISSN:1523-7060. (American Chemical Society)

   A cationic rhodium(I)/H8-BINAP complex catalyzes a [2 + 2 + 2] cycloaddn. of both activated and unactivated carbonyl compds. R1COR2 (R1 = H, Me, EtO2C; R2 = Me, H2C:CH, Ph, EtO2C, PhC≡C) with 1,6-diynes R3C≡CCH2XCH2C≡CR3 [X = (MeO2C)2C, O, 4-MeC6H4SO2; R3 = Me, Et] leading to dienones I in high yields. On the other hand, unactivated aryl ketones, e.g. 4-R4C6H4COR5 (R4 = H, MeO; R5 = Me, Et, Me2CH, Ph), react with 1,6-diynes in the presence of a cationic rhodium(I)/Segphos complex to give ortho-functionalized aryl ketones, e.g. II, in high yields.

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   (b) Tsuchikama, K.; Kuwata, Y.; Tahara, Y.-K.; Yoshinami, Y.; Shibata, T. Rh-catalyzed Cyclization of Diynes and Enynes Initiated by Carbonyl-directed Activation of Aromatic and Vinylic C–H bonds. Org. Lett. 2007, 9, 3097– 3099,  DOI: 10.1021/ol0711669

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   7b

   Rh-Catalyzed Cyclization of Diynes and Enynes Initiated by Carbonyl-Directed Activation of Aromatic and Vinylic C-H Bonds

   Tsuchikama, Kyoji; Kuwata, Yusuke; Tahara, Yu-Ki; Yoshinami, Yusuke; Shibata, Takanori

   Organic Letters (2007), 9 (16), 3097-3099CODEN: ORLEF7; ISSN:1523-7060. (American Chemical Society)

   The Rh-catalyzed hydroarylative and hydrovinylative cyclization of diynes with aryl ketones or enones gave monocyclic 1,3-dienes, e.g. 87 % [2-[(1E)-1-((Z)-4-ethylidene-1-tosylpyrrolidin-3-ylidene)ethyl]phenyl](phenyl)methanone (1) from N,N-bis(2-butynyl)-4-methylbenzenesulfonamide and benzophenone. Enynes also underwent the same reaction and chiral products were obtained with high ee using a chiral Rh catalyst, e.g. 97 % (2E)-4-((Z)-4-ethylidene-1-tosylpyrrolidin-3-yl)-1,3-diphenylbut-2-en-1-one from N-allyl-N-(2-butynyl)-4-methylbenzenesulfonamide and (E)-1,3-diphenyl-2-propen-1-one. Carbonyl-directed activation of arom. and vinylic C-H bonds is likely the initial step in the present transformation. The crystal and mol. structures of 1 were detd. by x-ray crystallog.

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   (c) Tanaka, K.; Otake, Y.; Sagae, H.; Noguchi, K.; Hirano, M. Highly Regio-, Diastereo-, and Enantioselective [2 + 2+2]-Cycloaddition of 1,6-Enynes with Electron-deficient Ketones Catalyzed by a Cationic Rh^I/H₈-binap Complex. Angew. Chem., Int. Ed. 2008, 47, 1312– 316,  DOI: 10.1002/anie.200704758

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   7c

   Highly regio-, diastereo-, and enantioselective [2 + 2 + 2] cycloaddition of 1,6-enynes with electron-deficient ketones catalyzed by a cationic RhI/H8-binap complex

   Tanaka, Ken; Otake, Yousuke; Sagac, Hiromi; Noguchi, Keiichi; Hirano, Masao

   Angewandte Chemie, International Edition (2008), 47 (7), 1312-1316, S1312/1-S1312/26CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

   A cationic RhI/H8-binap complex catalyzed regio-, diastereo-, and enantioselective [2 + 2 + 2] cycloaddn. of 1,6-enynes with electron-deficient ketones to form fused dihydropyrans contg. two quaternary carbon centers, e.g., I, is reported. Electron-rich aryl ketones react with 1,6-enynes in the presence of the same catalyst to give ortho-functionalized aryl ketones with excellent regio- and enantioselectivity.

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8. 8

   Early stoichiometric studies on cobaltacyclopentadiene-mediated C(sp²)–H activation:

   (a) Yamazaki, H.; Wakatsuki, Y. Cobalt Metallocycles: III. Thermolysis of Cobaltacyclopentadiene Complexes. J. Organomet. Chem. 1978, 149, 377– 384,  DOI: 10.1016/S0022-328X(00)90403-0

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   8a

   Cobalt metallocycles. III. Thermolysis of cobaltacyclopentadiene complexes

   Yamazaki, Hiroshi; Wakatsuki, Yasuo

   Journal of Organometallic Chemistry (1978), 149 (3), 377-84CODEN: JORCAI; ISSN:0022-328X.

   Thermolysis of (η5-cyclopentadienyl)(triphenylphosphine)cobaltacyclopentadiene complexes gave (η5-cyclopentadienyl)(η4-cyclobutadiene)cobalt complexes in 18-80% yields. Similar treatment of benzyl-substituted cyclopentadienyl derivs. gave diene complexes, I (R-R1 = e.g. Ph), which were formed by addn. of the o-H of the benzyl group to the cobaltacyclopentadiene ring.

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   (b) Wakatsuki, Y.; Yamazaki, H. Cobalt Metallocycles: IV. Ring Opening of Cobaltacyclopentadienes by Addition of Si–H, S–H, N–H and C–H to the Diene Moiety. J. Organomet. Chem. 1978, 149, 385– 393,  DOI: 10.1016/S0022-328X(00)90404-2

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   8b

   Cobalt metallocycles. IV. Ring opening of cobaltacyclopentadienes by addition of silicon-hydrogen, sulfur-hydrogen, nitrogen-hydrogen and carbon-hydrogen to the diene moiety

   Wakatsuki, Yasuo; Yamazaki, Hiroshi

   Journal of Organometallic Chemistry (1978), 149 (3), 385-93CODEN: JORCAI; ISSN:0022-328X.

   Cobaltacyclopentadiene complexes I (R1, R2 = Ph, Me, CO2Me; Cp = cyclopentadienyl) reacted with RH (RH = Et3SiH, thiocresol, dimethyl- and ethylene-thiourea, pyrrole, thiophene) to give diene complexes, (η5-C5H5)(η4-HR1:CR2CR2:CR1R)Co, or uncomplexed, highly substituted butadiene derivs., HCR1:CR2CR2:CR1R. The reaction with thiourea proceeded catalytically in the presence of excess of diphenylacetylene although turnover of the catalyst was small.

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9. 9

   Experimental and theoretical studies remarking on cobaltacyclopentadiene-mediated C(sp²)–H activation:

   (a) Boese, R.; Harvey, D. F.; Malaska, M. J.; Vollhardt, K. P. C. [2 + 2 + 2] Cycloadditions of Alkynes to Furans and Thiophenes: A Cobalt-mediated “Enol Ether Walk. J. Am. Chem. Soc. 1994, 116, 11153– 11154,  DOI: 10.1021/ja00103a039

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   9a

   [2 + 2 + 2]Cycloadditions of Alkynes to Furans and Thiophenes: A Cobalt-Mediated "Enol Ether Walk"

   Boese, Roland; Harvey, Daniel F.; Malaska, Michael J.; Vollhardt, K. Peter C.

   Journal of the American Chemical Society (1994), 116 (24), 11153-4CODEN: JACSAT; ISSN:0002-7863.

   η5-Cyclopentadienylcobalt mediates the [2+2+2]cycloaddn. of two alkyne units to the 2,3-double bond in furans and thiophenes to give the corresponding complexed dihydrobenzoheterocycles, in turn capable of undergoing migration of the enol ether moiety along the periphery of the cyclohexadiene ligand. In some instances the cobalt-mediated alkyne addn. occurs by apparent C-H activation of the heterocyclopentadiene, resulting in the generation of butadienylated products.

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   (b) Pelissier, H.; Rodriguez, J.; Vollhardt, K. P. C. Cobalt-mediated [2 + 2+2] Cycloadditions of Pyrimidine Derivatives to Alkynes. Chem. Eur. J. 1999, 5, 3549– 3561,  DOI: 10.1002/(SICI)1521-3765(19991203)5:12<3549::AID-CHEM3549>3.0.CO;2-V

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   9b

   Cobalt-mediated [2+2+2] cycloadditions of pyrimidine derivatives to alkynes

   Pelissier, Helene; Rodriguez, Jean; Vollhardt, K. Peter C.

   Chemistry - A European Journal (1999), 5 (12), 3549-3561CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH)

   The scope and limitations of the Co-mediated [2+2+2] cycloaddn. of pyrimidine derivs. to alkynes was studied. The 5,6-double bond of these heterocyclic nuclei was found to participate in an entirely intermol. fashion to generate chemo- and stereoselectively novel, fused and substituted 5,6-dihydropyrimidine Co complexes, which upon oxidative demetalation liberate the corresponding new heterocyclic ligand (e.g. I). However, 1-alkynyl pyrimidines are suitable partners in the cocyclization with disubstituted alkynes, such as bis(trimethylsilyl)acetylene (BTMSA) or di-Me 2-butyne-1,4-dioate (DMAD), to allow the direct prepn. of hitherto unknown dihydropyrido[3,2-ij]quinazoline Co complexes (e.g. II). Effects of the substitution on the pyrimidine nucleus, the cocyclization partner, the complex auxiliary, and the reaction conditions were examd., and in some cases competing pathways that lead to [CpCo(cyclobutadienes)], cyclopentadienone complexes, and compds. that arise from a C-H activation-type reaction were obsd.

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   (c) Gandon, V.; Agenet, N.; Vollhardt, K. P. C.; Malacria, M.; Aubert, C. Cobalt-mediated Cyclic and Linear 2:1 Cooligomerization of Alkynes with Alkenes: A DFT Study. J. Am. Chem. Soc. 2006, 128, 8509– 8520,  DOI: 10.1021/ja060756j

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   9c

   Cobalt-Mediated Cyclic and Linear 2:1 Cooligomerization of Alkynes with Alkenes: A DFT Study

   Gandon, Vincent; Agenet, Nicolas; Vollhardt, K. Peter C.; Malacria, Max; Aubert, Corinne

   Journal of the American Chemical Society (2006), 128 (26), 8509-8520CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

   The mechanism of the Co-mediated [2 + 2 + 2] cycloaddn. of two alkynes to one alkene to give CpCo-complexed 1,3-cyclohexadienes (cyclic oligomerization) was studied by DFT computations. In contrast to the mechanism of alkyne cyclotrimerization, in which final alkyne inclusion into the common cobaltacyclopentadiene features a direct collapse pathway to the complexed arene, alkene incorporation proceeds via insertion into a Co-C σ-bond rather than inter- or intramol. [4 + 2] cycloaddn. The resulting seven-membered metallacycle is a key intermediate which leads to either a CpCo-complexed cyclohexadiene or hexatriene. The latter transformation, particularly favorable for ethene, accounts, in part, for the linear oligomerization obsd. occasionally in these reactions. With arom. double bonds, a C-H activation mechanism by the cobaltacyclopentadiene seems more advantageous in hexatriene product formation. Detailed studies of high- and low-spin potential energy surfaces are presented. The reactivity of triplet Co species was found kinetically disfavored over that of their singlet counterparts. Also, it could not account for the formation of CpCo-complexed hexatrienes. However, triplet Co complexes cannot be ruled out since all unsatd. species appearing in this study exhibit triplet ground states. Consequently, a reaction pathway that involves a mixing of both spin-state energy surfaces is also described (two-state reactivity). Support for such a pathway comes from the location of several low-lying min.-energy crossing points (MECPs) of the two surfaces.

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   (d) Aubert, C.; Gandon, V.; Geny, A.; Heckrodt, T. J.; Malacria, M.; Paredes, E.; Vollhardt, K. P. C. Cobalt-mediated [2 + 2+2] Cycloaddition versus C–H and N–H Activation of 2-Pyridones and Pyrazinones with Alkynes: A Theoretical Study. Chem. Eur. J. 2007, 13, 7466– 7478,  DOI: 10.1002/chem.200601822

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   9d

   Cobalt-mediated [2 + 2 + 2] cycloaddition versus C-H and N-H activation of 2-pyridones and pyrazinones with alkynes: a theoretical study

   Aubert, Corinne; Gandon, Vincent; Geny, Anais; Heckrodt, Thilo J.; Malacria, Max; Parcedes, Elisa; Vollhardt, K. Peter C.

   Chemistry - A European Journal (2007), 13 (26), 7466-7478CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)

   DFT computations have been executed aimed at illuminating the variety of pathways by which pyridones react with alkynes in the presence of [CpCoL2]: NH-2-pyridones furnish N-dienylated ligands (N - H activation pathway), N-methyl-2-pyridones are converted into ligated cyclohexadienes ([2 + 2 + 2] cocycloaddn. pathway), and N-alkynyl-2-pyridones may undergo either [2 + 2 + 2] cocycloaddn. or C-dienylation (C - H activation), depending on the length of the tether. The calcns. predict the formation of the exptl. obsd. products, including their regio- and stereochem. In addn., the unusual regiochem. outcome of the all-intramol. [2 + 2 + 2] cycloaddn. of N,N'-dipentynylpyrazinedione was rationalized by computation, which led to the discovery of a new mechanism.

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10. 10

    Santhoshkumar, R.; Mannathan, S.; Cheng, C.-H. Ligand-controlled Divergent C–H Functionalization of Aldehydes with Enynes by Cobalt Catalysts. J. Am. Chem. Soc. 2015, 137, 16116– 16120,  DOI: 10.1021/jacs.5b10447

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    10

    Ligand-Controlled Divergent C-H Functionalization of Aldehydes with Enynes by Cobalt Catalysts

    Santhoshkumar, Rajagopal; Mannathan, Subramaniyan; Cheng, Chien-Hong

    Journal of the American Chemical Society (2015), 137 (51), 16116-16120CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    We describe a highly step and atom economical cobalt-catalyzed cyclization of 1,6-enynes with aldehydes to synthesize functionalized pyrrolidines and dihydrofurans with high chemo- and stereoselectivity. The catalytic reaction plausibly proceeds via the cobaltacycle intermediate generated from the reaction of enyne substrate with cobalt catalyst, followed by switchable C-H functionalization of weakly coordinating aldehydes depending on the electronic nature of the ligand.

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11. 11

    Whyte, A.; Torelli, A.; Mirabi, B.; Prieto, L.; Rodríguez, J. F.; Lautens, M. Cobalt-catalyzed Enantioselective Hydroarylation of 1,6-Enynes. J. Am. Chem. Soc. 2020, 142, 9510– 9517,  DOI: 10.1021/jacs.0c03246

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    11

    Cobalt-Catalyzed Enantioselective Hydroarylation of 1,6-Enynes

    Whyte, Andrew; Torelli, Alexa; Mirabi, Bijan; Prieto, Liher; Rodriguez, Jose F.; Lautens, Mark

    Journal of the American Chemical Society (2020), 142 (20), 9510-9517CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    An asym. hydroarylative cyclization of enynes RC≡CCH2YCH2CH=CH2 [R = n-Bu, Ph, 2-thienyl, etc.; Y = O, NTs, C(COOEt)2, thiophene-2-sulfonamido] involving a C-H bond cleavage is reported. The cobalt-catalyzed cascade generates three new bonds in an atom-economical fashion. The products I (Ar = 2-acetylphenyl, N-acetyl-1H-indol-2-yl, 2-(pyridin-2-yl)phenyl, etc.) were obtained in excellent yields and excellent enantioselectivities as single diastereo- and regioisomers. Preliminary mechanistic studies indicate that the reaction shows no intermol. C-H crossover. This work highlights the potential of cobalt catalysis in C-H bond functionalization and enantioselective domino reactivity.

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12. 12

    Herbort, J. H.; Lalisse, R. F.; Hadad, C. M.; RajanBabu, T. V. Cationic Cobalt(I) Catalysts for RegiodivergentHydroalkenylation of 1,6-Enynes: An Uncommon cis-β-C–H Activation Leads to Z-Selective Coupling of Acrylates. ACS Catal. 2021, 11, 9605– 9617,  DOI: 10.1021/acscatal.1c02530

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    12

    Cationic Co(I) Catalysts for Regiodivergent Hydroalkenylation of 1,6-Enynes: An Uncommon cis-α-C-H Activation Leads to Z-Selective Coupling of Acrylates

    Herbort, James H.; Lalisse, Remy F.; Hadad, Christopher M.; RajanBabu, T. V.

    ACS Catalysis (2021), 11 (15), 9605-9617CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    Two intermol. hydroalkenylation reactions of 1,6-enynes N(R)(CH2CH=CH2)CH2CCR1 (R = -N(Ts)-, -C(COOEt)2-, -O-, -N(Boc)-; R1 = 4-fluorophenyl, thiophen-3-yl, prop-1-en-2-yl, etc.) are presented which yield substituted 5-membered carbo- and -heterocycles I (R = -N(Ts)-, -C(COOEt)2-; R2 = H, Me; R3 = H, Me, n-Bu). This reactivity is enabled by a cationic bis-diphenylphosphinopropane (DPPP)CoI species which forms a cobaltacyclopentene intermediate by oxidative cyclization of the enyne. This key species interacts with alkenes in distinct fashion, depending on the identity of the coupling partner to give regiodivergent products I. Simple alkenes undergo insertion reactions to furnish 1,3-dienes whereby one of the alkenes is tetrasubstituted. The acerylates R4CH=C(R5)C(O)OR6 (R4 = H, Me, OMe; R5 = H, Me; R6 = Me, Bn, Cy, t-Bu) were employed as coupling partners, and the site of intermol. C-C formation shifts from the alkyne to the alkene motif of the enyne, yielding Z-substituted-acrylate derivs. II. Computational studies provide support for the exptl. observations and show that the turnover-limiting steps in both reactions are the interactions of the alkenes with the cobaltacyclopentene intermediate via either a 1,2-insertion in the case of ethylene, or an unexpected α-C-H activation in the case of most acrylates. Thus, the H syn to the ester is activated through the coordination of the acrylate carbonyl to the cobaltacycle intermediate, which explains the uncommon Z-selectivity and regiodivergence. Variable time normalization anal. (VTNA) of the kinetic data reveals a dependence upon the concn. of cobalt, acrylate, and activator. A KIE of 2.1 was obsd. with Me methacrylate in sep. flask expts., indicating that C-H cleavage is the turnover-limiting step in the catalytic cycle. Lastly, a Hammett study of aryl-substituted enynes yields a ρ value of -0.4, indicating that more electron-rich substituents accelerate the rate of the reaction.

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13. 13

    Other examples of cobalt-catalyzed tandem reactions using 1,6-enynes:

    (a) Xi, T.; Lu, Z. Cobalt-catalyzed Hydrosilylation/Cyclization of 1,6-Enynes. J. Org. Chem. 2016, 81, 8858– 8866,  DOI: 10.1021/acs.joc.6b01555

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    13a

    Cobalt-Catalyzed Hydrosilylation/Cyclization of 1,6-Enynes

    Xi, Tuo; Lu, Zhan

    Journal of Organic Chemistry (2016), 81 (19), 8858-8866CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)

    An iminopyridine cobalt dichloride complex, IP·CoCl2, was synthesized and demonstrated as an effective precatalyst for hydrosilylation/cyclization of 1,6-enynes with silanes. Various functional groups such as amine, free aniline, ester, ether, cyano, halide, trifluoromethyl, and heterocycle were tolerated to afford a variety of silicon-contg. compds. For example, (3-(allyloxy)prop-1-yn-1-yl)benzene reacted with diphenylsilane in the presence of IP·CoCl2 to give (Z)-((4-benzylidenetetrahydrofuran-3-yl)methyl)diphenylsilane in 81% yield. The reaction could be scaled up to afford products on the gram scale which could undergo further derivatizations. A primary mechanism was proposed based on anal. of side products and a deuterated expt.

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    (b) Xi, T.; Lu, Z. Cobalt-catalyzed Ligand-controlled Regioselective Hydroboration/Cyclization of 1,6-Enynes. ACS Catal. 2017, 7, 1181– 1185,  DOI: 10.1021/acscatal.6b02816

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    13b

    Cobalt-Catalyzed Ligand-Controlled Regioselective Hydroboration/Cyclization of 1,6-Enynes

    Xi, Tuo; Lu, Zhan

    ACS Catalysis (2017), 7 (2), 1181-1185CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    A ligand-controlled cobalt-catalyzed regioselective hydroboration/cyclization of 1,6-enynes with HBPin was developed by switching the size of the coordinated side arm to afford alkenylboronates and alkylboronates, resp. The gram-scale reactions could be easily conducted which is benefit for further derivatizations. A primary mechanism was proposed based on substrate-controlled expts. and deuterium expts.

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    (c) Yu, S.; Wu, C.; Ge, S. Cobalt-catalyzed Asymmetric Hydroboration/Cyclization of 1,6-Enynes with Pinacolborane. J. Am. Chem. Soc. 2017, 139, 6526– 6529,  DOI: 10.1021/jacs.7b01708

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    13c

    Cobalt-catalyzed asymmetric hydroboration/cyclization of 1,6-enynes with pinacolborane

    Yu, Songjie; Wu, Caizhi; Ge, Shaozhong

    Journal of the American Chemical Society (2017), 139 (19), 6526-6529CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    We report a cobalt-catalyzed asym. hydroboration/cyclization of 1,6-enynes RC≡CCH2YCH2CH:CH2 (1a, R = substituted Ph, thienyl, 1-indolylpropyl, alkoxyalkyl, acyloxyalkyl) and ArC≡CCR1R2XCH2CH:CH2 [1b, Ar = substituted Ph, 1-naphthyl; for 1a,b: Y, X = O, NTs, C(CO2R3)2] with catalysts generated from Co(acac)2 and chiral bisphosphine ligands and activated in situ by reaction with pinacolborane (HBpin), giving vinylboronates I (2a-x) and II (3b-s). A variety of oxygen-, nitrogen-, and carbon-tethered 1,6-enynes underwent this asym. transformation, yielding both alkyl- and vinyl-substituted boronate esters contg. chiral THF, cyclopentane, and pyrrolidine moieties with high to excellent enantioselectivities (86%-99% ee).

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    (d) Wang, G.; Khan, R.; Liu, H.; Shen, G.; Yang, F.; Chen, J.; Zhou, Y.; Fan, B. Cobalt-catalyzed Ligand-controlled Divergent Regioselective Reactions of 1,6-Enynes with Thiols. Organometallics 2020, 39, 2037– 2042,  DOI: 10.1021/acs.organomet.0c00179

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    13d

    Cobalt-Catalyzed Ligand-Controlled Divergent Regioselective Reactions of 1,6-Enynes with Thiols

    Wang, Gaowei; Khan, Ruhima; Liu, Haojie; Shen, Guoli; Yang, Fan; Chen, Jingchao; Zhou, Yongyun; Fan, Baomin

    Organometallics (2020), 39 (11), 2037-2042CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)

    An efficient Co-catalyzed method for the synthesis of carbocyclic organosulfur compds. from 1,6-enynes and thiols was developed. The significance of this methodol. is the ability to give three different cyclization products depending on the ligands used. The products were obtained in moderate to good yields with broad substrate scope.

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    (e) Whyte, A.; Bajohr, J.; Torelli, A.; Lautens, M. Enantioselective Cobalt-catalyzed Intermolecular Hydroacylation of 1,6-Enynes. Angew. Chem., Int. Ed. 2020, 59, 16409– 16413,  DOI: 10.1002/anie.202006716

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    13e

    Enantioselective Cobalt-Catalyzed Intermolecular Hydroacylation of 1,6-Enynes

    Whyte, Andrew; Bajohr, Jonathan; Torelli, Alexa; Lautens, Mark

    Angewandte Chemie, International Edition (2020), 59 (38), 16409-16413CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    The authors report a cobalt-catalyzed hydroacylation of 1,6-enynes with exogenous aldehydes in a domino sequence to construct enantioenriched ketones. The products were obtained in good yields with excellent regio-, diastereo-, and enantioselectivity. Furthermore, the chiral products served as valuable precursors to access complex spirocyclic scaffolds with three contiguous stereocenters. The asym. hydroacylation process exhibited no C-H crossover and no KIE, thus indicating that the C-H bond cleavage was not involved in the turnover-limiting step.

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    (f) You, Y.; Ge, S. Asymmetric Cobalt-catalyzed Regioselective Hydrosilylation/Cyclization of 1,6-Enynes. Angew. Chem., Int. Ed. 2021, 60, 12046– 12052,  DOI: 10.1002/anie.202100775

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    13f

    Asymmetric Cobalt-Catalyzed Regioselective Hydrosilylation/Cyclization of 1,6-Enynes

    You, Yang'en; Ge, Shaozhong

    Angewandte Chemie, International Edition (2021), 60 (21), 12046-12052CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    The authors report an enantioselective Co-catalyzed hydrosilylation/cyclization reaction of 1,6-enynes with secondary and tertiary hydrosilanes employing a catalyst generated in situ from the combination of Co(acac)2 and (R,Sp)-Josiphos. A wide range of O-, N-, and C-tethered 1,6-enynes reacted with Ph2SiH2, (EtO)3SiH, or (RO)2MeSiH to afford the corresponding chiral organosilane products in high yields and up to >99% ee. This Co-catalyzed hydrosilylation/cyclization also occurred with prochiral secondary hydrosilane PhMeSiH2 to yield chiral alkylsilanes contg. both C- and Si-stereogenic centers with excellent enantioselectivity, albeit with modest diastereoselectivity. The chiral organosilane products from this Co-catalyzed asym. hydrosilylation/cyclization could be converted to a variety of chiral five-membered heterocyclic compds. by stereospecific conversion of their C-Si and Si-H bonds without loss of enantiopurity.

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14. 14

    Examples of cobalt-catalyzed three-component coupling reactions:

    (a) Boerth, J. A.; Hummel, J. R.; Ellman, J. A. Highly Stereoselective Cobalt(III)-catalyzed Three-component C–H Bond Addition Cascade. Angew. Chem., Int. Ed. 2016, 55, 12650– 12654,  DOI: 10.1002/anie.201603831

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    14a

    Highly stereoselective cobalt(III)-catalyzed three-component C-H bond addition cascade

    Boerth, Jeffrey A.; Hummel, Joshua R.; Ellman, Jonathan A.

    Angewandte Chemie, International Edition (2016), 55 (41), 12650-12654CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Pyrazolyl benzenes undergo pyrazole group-directed cobalt(III)-catalyzed stereoselective three-component addn. of vinyl ketones CH2:CHCOR2 and aldehydes R3CHO, giving aryl β-hydroxyketones I (4a-s; R = H, Me, Br; R2 = Me, Et, Ph; R3 = Ph, MeOC6H4,, BrC6H4, MeO2CC6H4; CH:CHMe, 2-furyl, 3-thienyl) with >95:5 d.r. A highly stereoselective three-component C(sp2)-H bond addn. across alkene and polarized π-bonds is reported for which CoIII catalysis was shown to be much more effective than RhIII. The reaction proceeds at ambient temp. with both aryl and alkyl enones employed as efficient coupling partners. Moreover, the reaction exhibits extremely broad scope with respect to the aldehyde input; electron rich and poor arom., alkenyl, and branched and unbranched alkyl aldehydes all couple in good yield and with high diastereoselectivity. Multiple directing groups participate in this transformation, including pyrazole, pyridine, and imine functional groups. Both arom. and alkenyl C(sp2)-H bonds undergo the three-component addn. cascade, and the alkenyl addn. product can readily be converted into diastereomerically pure five-membered lactones. Addnl., the first asym. reactions with CoIII-catalyzed C-H functionalization are demonstrated with three-component C-H bond addn. cascades employing N-tert-butanesulfinyl imines. These examples represent the first transition metal catalyzed C-H bond addns. to N-tert-butanesulfinyl imines, which are versatile and extensively used intermediates for the asym. synthesis of amines.

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    (b) Boerth, J. A.; Maity, S.; Williams, S. K.; Mercado, B. Q.; Ellman, J. A. Selective and Synergistic Cobalt(III)-catalysed Three-component C–H Bond Addition to Dienes and Aldehydes. Nat. Catal. 2018, 1, 673– 679,  DOI: 10.1038/s41929-018-0123-4

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    14b

    Selective and synergistic cobalt(III)-catalysed three-component C-H bond addition to dienes and aldehydes

    Boerth, Jeffrey A.; Maity, Soham; Williams, Sarah K.; Mercado, Brandon Q.; Ellman, Jonathan A.

    Nature Catalysis (2018), 1 (9), 673-679CODEN: NCAACP; ISSN:2520-1158. (Nature Research)

    Two-component C-H bond addns. to a large variety of coupling partners have been developed with applications towards materials, natural product and drug synthesis. Sequential three-component C-H bond addn. across two different coupling partners potentially enables the convergent synthesis of complex mol. scaffolds from simple precursors. Here, we report three-component Co(III)-catalyzed C-H bond addns. to dienes and aldehydes that proceed with high regio- and stereoselectivity, resulting in two new carbon-carbon σ-bonds and four to six new stereocentres. The reaction relies on the synergistic reactivity of the diene and aldehyde, with neither undergoing C-H bond addn. alone. A detailed mechanism is supported by X-ray structural characterization of a Co(III)-allyl intermediate, obsd. transfer of stereochem. information, and kinetic isotope studies. The applicability of the method to biol. relevant mols. is exemplified by the rapid synthesis of the western fragment of the complex ionophore antibiotic lasalocid A.

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    (c) Herraiz, A. G.; Cramer, N. Cobalt(III)-catalyzed Diastereo- and Enantioselective Three-component C–H Functionalization. ACS Catal. 2021, 11, 11938– 11944,  DOI: 10.1021/acscatal.1c03153

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    14c

    Cobalt(III)-Catalyzed Diastereo- and Enantioselective Three-Component C-H Functionalization

    Herraiz, Ana G.; Cramer, Nicolai

    ACS Catalysis (2021), 11 (19), 11938-11944CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    A diastereoselective and highly enantioselective three-component C-H functionalization catalyzed by an earth-abundant Co(III) complex equipped with a chiral cyclopentadienyl ligand (Cpx) has been described. The transformation provides a rapid access to substituted β-hydroxyketones I (R = 5-I, 3-F, 4-Me, etc.; R1 = C6H5, 4-BrC6H4, 2-naphthyl, etc.; R2 = CH=CHCH3, CH(CH3)2, cyclohexyl, etc.; R3 = H, Br; R4 = H; R3R4 = -CH=CHCH=CH-) and II using three readily accessible starting materials. The outlined reactivity of CpxCo(III) catalysis shows a higher and exploitable propensity for selective addns. across carbonyls in contrast to the chem. of Rh(III).

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    (d) Li, M.-H.; Si, X.-J.; Zhang, H.; Yang, D.; Niu, J.-L.; Song, M.-P. Directed Cobalt-catalyzed C–H Activation to Form C–C and C–O Bonds in One Pot via Three-component Coupling. Org. Lett. 2021, 23, 914– 919,  DOI: 10.1021/acs.orglett.0c04122

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    14d

    Directed Cobalt-Catalyzed C-H Activation to Form C-C and C-O Bonds in One Pot via Three-Component Coupling

    Li, Meng-Hui; Si, Xiao-Ju; Zhang, He; Yang, Dandan; Niu, Jun-Long; Song, Mao-Ping

    Organic Letters (2021), 23 (3), 914-919CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)

    Herein, an efficient cobalt-catalyzed three-component coupling of benzamides, diazo compds. and tert-Bu hydroperoxide was disclosed to construct C(sp2)-C(sp3) and C-O bonds in one-pot accompanied with C-H activation. This protocol featured low catalyst loading (4 mol %), the avoidance of additives and excellent functional group compatibility, providing three-component coupling adducts I [R = H, 5-Me, 4-I, etc.; R1 = H, Me, Bn, etc.] with high yields under mild conditions (up to 88%). Mechanism studies showed that the reaction may involved a radical process.

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15. 15

    Other recent examples of transition metal-catalyzed ortho-C(sp²)–H homoallylation:

    (a) Cera, G.; Haven, T.; Ackermann, L. Expedient Iron-catalyzed C–H Allylation/Alkylation by Triazole Assistance with Ample Scope. Angew. Chem., Int. Ed. 2016, 55, 1484– 1488,  DOI: 10.1002/anie.201509603

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    15a

    Expedient Iron-Catalyzed C-H Allylation/Alkylation by Triazole Assistance with Ample Scope

    Cera, Gianpiero; Haven, Tobias; Ackermann, Lutz

    Angewandte Chemie, International Edition (2016), 55 (4), 1484-1488CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Triazole assistance set the stage for a unified strategy for the iron-catalyzed C-H allylation of arenes, heteroarenes, and alkenes with ample scope. The versatile catalyst also proved competent for site-selective methylation, benzylation, and alkylation with challenging primary and secondary halides. Triazole-assisted C-H activation proceeded chemo-, site-, and diastereo-selectively, and the modular TAM directing group was readily removed in a traceless fashion under exceedingly mild reaction conditions.

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    (b) Ghorai, D.; Finger, L. H.; Zanoni, G.; Ackermann, L. Bimetallic Nickel Complexes for Aniline C–H Alkylations. ACS Catal. 2018, 8, 11657– 11662,  DOI: 10.1021/acscatal.8b03770

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    15b

    Bimetallic Nickel Complexes for Aniline C-H Alkylations

    Ghorai, Debasish; Finger, Lars H.; Zanoni, Giuseppe; Ackermann, Lutz

    ACS Catalysis (2018), 8 (12), 11657-11662CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    A set of bimetallic nickel(II)/nickel(II) complexes featuring paddle-wheel structures was synthesized and fully characterized. These homobimetallic nickel complexes were identified as powerful catalysts for challenging aniline C-H activations with primary and secondary β-hydrogen-contg. alkyl halides.

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    (c) Shen, Z.; Huang, H.; Zhu, C.; Warratz, S.; Ackermann, L. MnCl₂-catalyzed C–H Alkylation on Azine Heterocycles. Org. Lett. 2019, 21, 571– 574,  DOI: 10.1021/acs.orglett.8b03924

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    15c

    MnCl2-Catalyzed C-H Alkylation on Azine Heterocycles

    Shen, Zhigao; Huang, Huawen; Zhu, Cuiju; Warratz, Svenja; Ackermann, Lutz

    Organic Letters (2019), 21 (2), 571-574CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)

    Low-valent manganese-catalyzed C-H alkylation of pyridine derivs. with both primary and challenging secondary alkyl halides was established by amide assistance. The strategy provided expedient access to alkylated pyridines with wide functional group tolerance and ample scope through weak chelation. Mechanistic studies provided strong support for a rate-detg. C-H activation and a SET-type C-X scission.

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    (d) Kimura, N.; Katta, S.; Kitazawa, Y.; Kochi, T.; Kakiuchi, F. Iron-catalyzed ortho C–H Homoallylation of Aromatic Ketones with Methylenecyclopropanes. J. Am. Chem. Soc. 2021, 143, 4543– 4549,  DOI: 10.1021/jacs.1c00237

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    15d

    Iron-Catalyzed Ortho C-H Homoallylation of Aromatic Ketones with Methylenecyclopropanes

    Kimura, Naoki; Katta, Shiori; Kitazawa, Yoichi; Kochi, Takuya; Kakiuchi, Fumitoshi

    Journal of the American Chemical Society (2021), 143 (12), 4543-4549CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    We report here a C-H homoallylation reaction of arom. ketones with methylenecyclopropanes (MCPs) only using a catalytic amt. of Fe(PMe3)4. A variety of arom. ketones and MCPs are applicable to the reaction to form ortho-homoallylated arom. ketones selectively via regioselective scission of the three-membered rings. The homoallylated products are amenable to further elaborations, providing functionalized 1,2-dihydronaphthalenes. Of note, Fe(PMe3)4 is unstable at room temp. and may catch fire if exposed to air.

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16. 16

    Selected articles discussing metallacyclopentenes or metallacyclopentadienes derived from oxidative cyclization of two π components:

    (a) Jeganmohan, M.; Cheng, C.-H. Cobalt- and Nickel-catalyzed Regio- and Stereoselective Reductive Coupling of Alkynes, Allenes, and Alkenes with Alkenes. Chem. Eur. J. 2008, 14, 10876– 10886,  DOI: 10.1002/chem.200800904

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    16a

    Cobalt- and nickel-catalyzed regio- and stereoselective reductive coupling of alkynes, allenes, and alkenes with alkenes

    Jeganmohan, Masilamani; Cheng, Chien-Hong

    Chemistry - A European Journal (2008), 14 (35), 10876-10886CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)

    In this review, we focus on the cobalt- and nickel-catalyzed reductive coupling of alkynes, allenes, and alkenes with alkenes. Transition-metal-catalyzed coupling of two different C-C π components through a metallacycle intermediate is a highly atom economical method to construct C-C bonds in org. synthesis. The metal-catalyzed coupling of an alkene and alkyne generally gives an Alder-ene or reductive coupling product. These reductive coupling reactions provide convenient methods for the synthesis of various alkenes, dienes, functionalized alkanes, lactones, lactams, and cyclic alcs. in a highly regio- and stereoselective manner. A chemoselective formation of metallacyclopentene intermediate from the two different C-C π components and a low-valence metal species plays a key role for the high regio- and stereoselectivity of the catalytic reaction.

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    (b) Micalizio, G. C.; Mizoguchi, H. The Development of Alkoxide-directed Metallacycle-mediated Annulative Cross-coupling Chemistry. Isr. J. Chem. 2017, 57, 228– 238,  DOI: 10.1002/ijch.201600098

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    16b

    The Development of Alkoxide-Directed Metallacycle-Mediated Annulative Cross-Coupling Chemistry

    Micalizio, Glenn C.; Mizoguchi, Haruki

    Israel Journal of Chemistry (2017), 57 (3-4), 228-238CODEN: ISJCAT; ISSN:0021-2148. (Wiley-VCH Verlag GmbH & Co. KGaA)

    A review. Alkoxide-directed metallacycle-mediated cross-coupling is a rapidly growing area of reaction methodol. in org. chem. Over the last decade, developments have resulted in more than thirty new and highly selective intermol. (or "convergent") C-C bond-forming reactions that have established powerful retrosynthetic relationships in stereoselective synthesis. While early studies were focused on developing transformations that forge a single C-C bond by way of a functionalized and unsatd. metallacyclopentane intermediate, recent advances mark the ability to employ this organometallic intermediate in addnl. stereoselective transformations. Among these more advanced coupling processes, those that embrace the metallacycle in subsequent [4+2] chem. have resulted in the realization of a no. of highly selective annulative cross-coupling reactions that deliver densely functionalized and angularly substituted carbocycles. This review discusses the early development of this chem., recent advances in reaction methodol., and shares a glimpse of the power of these processes in natural product synthesis.

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    (c) Ma, W.; Yu, C.; Chen, T.; Xu, L.; Zhang, W.-X.; Xi, Z. Metallacyclopentadienes: Synthesis, Structure and Reactivity. Chem. Soc. Rev. 2017, 46, 1160– 1192,  DOI: 10.1039/C6CS00525J

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    16c

    Metallacyclopentadienes: synthesis, structure and reactivity

    Ma, Wangyang; Yu, Chao; Chen, Tianyang; Xu, Ling; Zhang, Wen-Xiong; Xi, Zhenfeng

    Chemical Society Reviews (2017), 46 (4), 1160-1192CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)

    A review. Metallacyclopentadienes, which possess two M-C(sp2) bonds and feature the structure of M(CR1:CR2CR3:CR4), are an important class of five-membered metallacycles. They are considered as both reactive intermediates in the stoichiometric and catalytic transformations of org. mols. and useful precursors to main group element compds., and have received considerable attention in organometallic chem., coordination chem. and synthetic org. chem. over the past six decades because of their unique metallacyclic structure. This review comprehensively presents the synthesis, structure and reactivity of the s-, p-, d- and f-block metallacyclopentadienes distributed in the whole periodic table. In addn., their application in synthetic org. chem. and polymer chem. is summarized. This review aims to be beneficial for the design and synthesis of novel metallacyclopentadienes, and for promoting the rapid development of metallacyclic chem.

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    (d) Kiyota, S.; Hirano, M. An Insight into Regioselectivity in the Transformation through a Ruthenacycle. New J. Chem. 2020, 44, 2129– 2145,  DOI: 10.1039/C9NJ04880D

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    16d

    An insight into regioselectivity in the transformation through a ruthenacycle

    Kiyota, Sayori; Hirano, Masafumi

    New Journal of Chemistry (2020), 44 (5), 2129-2145CODEN: NJCHE5; ISSN:1144-0546. (Royal Society of Chemistry)

    Ru(0)-catalyzed cross-dimerization of unsym. substituted internal alkynes with conjugated dienes yields two conjugated triene products depending on the regioselectivity of the C-C bond formation reaction via a ruthenacycle intermediate. The electronic and steric effects of alkynes are comprehensively evaluated based on Hammett's (σp) and Taft's (σ*, Es) substituent consts. An electron-withdrawing substituent favors the external position of the conjugated triene products. With unsym. 4,4'-disubstituted diaryl acetylenes, the logarithm plot for the regioisomer ratios of the products and differential Hammett's value Δσp between the substituents shows a linear relation with a pos. slope. This trend suggests that the electron-rich α-carbon in a ruthenacycle favors an electron-withdrawing group. This system is less sensitive to steric effects on the regioselectivity, but the sterically less bulky groups tend to prefer the external position.

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    (e) Roglans, A.; Pla-Quintana, A.; Solà, M. Mechanistic Studies of Transition-metal-catalyzed [2 + 2 + 2] Cycloaddition Reactions. Chem. Rev. 2021, 121, 1894– 1979,  DOI: 10.1021/acs.chemrev.0c00062

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    16e

    Mechanistic Studies of Transition-Metal-Catalyzed [2 + 2 + 2] Cycloaddition Reactions

    Roglans, Anna; Pla-Quintana, Anna; Sola, Miquel

    Chemical Reviews (Washington, DC, United States) (2021), 121 (3), 1894-1979CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)

    A review. The development of catalytic methodologies involving the formation of C-C bonds to enable the generation of cyclic systems constitutes a field of great relevance in synthetic org. chem. One paradigmatic process to accomplish this goal efficiently is the transition-metal-catalyzed [2 + 2 + 2] cycloaddn. reaction, since it permits the formation of a wide range of highly functionalized 6-membered carbo- and heterocyclic mols. in a single step with high efficiency and perfect atom economy. A key feature of these transformations is the mechanistic pathway that they follow, since a deep knowledge of this mechanism may enable us to understand and improve the efficiency of the reaction. This review covers the mechanistic aspects, studied both from theor. and exptl. points of view, of the transition-metal-catalyzed [2 + 2 + 2] cycloaddn. reaction involving all kinds of unsatd. substrates with metals such as Co, Ni, Ru, Rh, Ir, Pd, Zr, Ti, Ta, and Nb. A thorough overview is undertaken, from the seminal studies until the present day, of the key mechanistic aspects that influence the reactivity and selectivity of the reaction, comparing the involvement of different unsatd. substrates as well as the different transition metals used.

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17. 17

    Stoichiometric studies on Group 9 metallacyclopentene complexes:

    (a) O’Connor, J.; Closson, A.; Gantzel, P. Hydrotris(pyrazolyl)borate Metallacycles: Conversion of a Late-metal Metallacyclopentene to a stable Metallacyclopentadiene–Alkene Complex. J. Am. Chem. Soc. 2002, 124, 2434– 2435,  DOI: 10.1021/ja0255296

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    17a

    Hydrotris(pyrazolyl)borate Metallacycles: Conversion of a Late-Metal Metallacyclopentene to a Stable Metallacyclopentadiene-Alkene Complex

    O'Connor, Joseph M.; Closson, Adam; Gantzel, Peter

    Journal of the American Chemical Society (2002), 124 (11), 2434-2435CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    The bis(ethene) complex [(Tp)Ir(C2H4)2] (3) undergoes reaction with di-Me acetylenedicarboxylate (DMAD) in MeCN solvent at 60° to give the trispyrazolylborate metallacyclopent-2-ene complex [(Tp)Ir{CH2CH2C(CO2Me):C(CO2Me)}(NCMe)] (4). Spectroscopic anal. of a room-temp. reaction between 3 and DMAD in MeCN-d3 provides evidence for the formation of an η2-alkene/η2-alkyne intermediate on the path to 4. The reaction of 3 with DMAD in THF solvent gives the THF-ligated metallacyclopent-2-ene complex [(Tp)Ir{CH2CH2C(CO2Me):C(CO2Me)}(THF)] (5), which undergoes further reaction with DMAD at 60° in benzene to give [(Tp)Ir{C(CO2Me):C(CO2Me)C(CO2Me):C(CO2Me)}(η2-CH2:CH2)] (6). Complex 4 was structurally characterized by x-ray crystallog.

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    (b) Bottari, G.; Santos, L. L.; Posadas, C. M.; Campos, J.; Mereiter, K.; Paneque, M. Reaction of [TpRh(C₂H₄)₂] with Dimethyl Acetylenedicarboxylate: Identification of Intermediates of the [2 + 2+2] Alkyne and Alkyne–Ethylene Cyclo(co)trimerizations. Chem. Eur. J. 2016, 22, 13715– 13723,  DOI: 10.1002/chem.201601927

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    17b

    Reaction of [TpRh(C2H4)2] with Dimethyl Acetylenedicarboxylate: Identification of Intermediates of the [2+2+2] Alkyne and Alkyne-Ethylene Cyclo(co)trimerizations

    Bottari, Giovanni; Santos, Laura L.; Posadas, Cristina M.; Campos, Jesus; Mereiter, Kurt; Paneque, Margarita

    Chemistry - A European Journal (2016), 22 (38), 13715-13723CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)

    The reaction between the bis(ethylene) complex [TpRh(C2H4)2], 1, (Tp = hydrotris(pyrazolyl)borate), and di-Me acetylenedicarboxylate (DMAD) was studied under different exptl. conditions. A mixt. of products was formed, in which TpRhI species were prevalent, whereas the presence of trapping agents, like water or acetonitrile, allowed for the stabilization and isolation of octahedral TpRhIII compds. An excess of DMAD gave rise to a small amt. of the [2+2+2] cyclotrimerization product hexamethyl mellitate (6). Although no catalytic application of 1 was achieved, mechanistic insights shed light on the formation of stable rhodium species representing the resting state of the catalytic cycle of rhodium-mediated [2+2+2] cyclo(co)trimerization reactions. Metallacyclopentene intermediate species, generated from the activation of one alkyne and one ethylene mol. from 1, and metallacyclopentadiene species, formed by oxidative coupling of two alkynes to the rhodium center, are crucial steps in the pathways leading to the final organometallic and org. products.

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18. 18

    (a) Hilt, G.; Treutwein, J. Cobalt-catalyzed Alder-Ene Reaction. Angew. Chem., Int. Ed. 2007, 46, 8500– 8502,  DOI: 10.1002/anie.200703180

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    18a

    Cobalt-catalyzed Alder-ene reaction

    Hilt, Gerhard; Treutwein, Jonas

    Angewandte Chemie, International Edition (2007), 46 (44), 8500-8502CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    An inexpensive cobalt-diphosphine catalyzed intermol. Alder-ene reaction of internal alkynes with terminal alkenes was reported. The products are functionalized 1,4-dienes which are obtained in good yields and with excellent chemo-, regio-, and stereoselectivities.

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    (b) Mannathan, S.; Cheng, C.-H. Cobalt-catalyzed Regio- and Stereoselective Intermolecular Enyne Coupling: an Efficient Route to 1,3-Diene Derivatives. Chem. Commun. 2010, 46, 1923– 1925,  DOI: 10.1039/B920071A

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    18b

    Cobalt-catalyzed regio- and stereoselective intermolecular enyne coupling: an efficient route to 1,3-diene derivatives

    Mannathan, Subramaniyan; Cheng, Chien-Hong

    Chemical Communications (Cambridge, United Kingdom) (2010), 46 (11), 1923-1925CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)

    The reaction of alkynes with vinyl arenes or vinyl tri-Me silane in the presence of a cobalt(ii) complex, Zn and ZnI2 in CH2Cl2 at rt to 50 °C provides 1,3-dienes in good to excellent yields.

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    (c) Hilt, G. Hydrovinylation Reactions – Atom-economic Transformations with Steadily Increasing Synthetic Potential. Eur. J. Org. Chem. 2012, 2012, 4441– 4451,  DOI: 10.1002/ejoc.201200212

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    18c

    Hydrovinylation reactions - atom-eco-nomic transformations with steadily increasing synthetic potential

    Hilt, Gerhard

    European Journal of Organic Chemistry (2012), 2012 (24), 4441-4451CODEN: EJOCFK; ISSN:1099-0690. (Wiley-VCH Verlag GmbH & Co. KGaA)

    A review. The intermol. carbon-carbon bond formation between two alkenes also known as 1,2-hydrovinylation reaction can be realized with different transition metal catalysts. The application of styrene derivs., norbornenes and other alkenes in asym. catalysis with a variety of chiral ligands leads to α-chiral alkene products in an atom-economic transformation. Accordingly, the 1,2-hydrovinylation is one of just a few asym. transformations which produce stereogenic centers in the absence of polarized functional groups. The 1,4-hydrovinylation of terminal alkenes and 1,3-dienes can be controlled by the electronic nature of the alkene starting material for the selective formation of linear or branched 1,4-dienes. These adducts can be used for the synthesis of 1,3- as well as 1,4-dicarbonyl derivs. upon ozonolysis of suitable intermediates. As an extension of the 1,4-hydrovinylation reaction a cobalt-catalyzed 1,4-hydrobutadienylation reaction is reported where two different 1,3-dienes react selectively for the formation of 1,3,6-trienes.

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    (d) Hirano, M. Recent Advances in the Catalytic Linear Cross-dimerizations. ACS Catal. 2019, 9, 1408– 1430,  DOI: 10.1021/acscatal.8b04676

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    18d

    Recent Advances in the Catalytic Linear Cross-Dimerizations

    Hirano, Masafumi

    ACS Catalysis (2019), 9 (2), 1408-1430CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    A review. Catalytic cross-dimerization is one of the powerful synthetic methods to produce linear mols. with high atom and step economy. Because this process involves a carbon-carbon-bond-forming reaction, enantioselective reactions have also been achieved. The most well-reviewed area in this field is probably hydrovinylation using ethylene, but the linear cross-dimerizations using substituted alkenes and alkynes have also been extensively developed. Not only do the products vary depending on these substrates, but they mostly differ from hydrovinylation in the mechanism. This Perspective presents a comprehensive summary on the basis of these substrates, including their brief historical background, mechanism, applications to the synthesis of biol. active compds. or contributions to the total synthesis, and the state-of-the-art advancements. The controlling factors in the chemo- and regioselectivities are also discussed.

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19. 19

    (a) Chao, K. C.; Rayabarapu, D. K.; Wang, C.-C.; Cheng, C.-H. Cross [2 + 2] Cycloaddition of Bicyclic Alkenes with Alkynes Mediated by Cobalt Complexes: a Facile Synthesis of Cyclobutene Derivatives. J. Org. Chem. 2001, 66, 8804– 8810,  DOI: 10.1021/jo010609y

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    19a

    Cross [2 + 2] Cycloaddition of Bicyclic Alkenes with Alkynes Mediated by Cobalt Complexes: A Facile Synthesis of Cyclobutene Derivatives

    Chao, Kuan Cheng; Rayabarapu, Dinesh Kumar; Wang, Chun-Chih; Cheng, Chien-Hong

    Journal of Organic Chemistry (2001), 66 (26), 8804-8810CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)

    Bicyclic alkenes undergo [2 + 2] cycloaddn. with PhC≡CPh, Me3SiC≡CH, HC≡CCMe2OH, Me3SiC≡CCO2Et, PhC≡CMe, EtC≡CEt, MeC≡CPr, and MeC≡CEt in the presence of Co(PPh3)2I2, PPh3, and Zn powder in toluene to afford the corresponding exo-cyclobutene derivs. in fair to excellent yields. The yield of this cycloaddn. is highly sensitive to the cobalt catalyst, solvent, ligand, and temp. used. A mechanism involving a metallacyclopentene intermediate is proposed to account for this cobalt-catalyzed cyclization.

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    (b) Buisine, O.; Aubert, C.; Malacria, M. Cobalt(I)-mediated Cycloisomerization of Enynes: Mechanistic Insights. Chem. Eur. J. 2001, 7, 3517– 3525,  DOI: 10.1002/1521-3765(20010817)7:16<3517::AID-CHEM3517>3.0.CO;2-V

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    19b

    Cobalt(I)-mediated cycloisomerization of enynes: mechanistic insights

    Buisine, Olivier; Aubert, Corinne; Malacria, Max

    Chemistry - A European Journal (2001), 7 (16), 3517-3525CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH)

    [CpCo(CO)2] catalyzes the cycloisomerization of 1,n-enynes to afford selectively five- and six-membered ring systems in high yields. The factors governing the cyclization were explored and the authors' have discovered that the reaction assocs. two different, but complementary, reactivities of the Co(I) complexes. By a judicious choice of the substitution of the enyne, it was also possible to isolate a cyclobutene that arises from a cobaltcyclopentene.

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    (c) Treutwein, J.; Hilt, G. Cobalt-catalyzed [2 + 2] Cycloaddition. Angew. Chem., Int. Ed. 2008, 47, 6811– 6813,  DOI: 10.1002/anie.200801778

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    19c

    Cobalt-catalyzed [2 + 2] cycloaddition

    Treutwein, Jonas; Hilt, Gerhard

    Angewandte Chemie, International Edition (2008), 47 (36), 6811-6813CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    A simple cobalt-diphosphine complex facilitates the synthesis of tricyclo[4.2.1.02,5]non-3-ene derivs., e.g., I, and other cyclobutene derivs. through the chemoselective transformation of strained five-membered unsatd. rings with internal alkynes. This atom-efficient, intermol. reaction generates polycyclic products in excellent yields and with excellent exo selectivity.

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    (d) Hilt, G.; Paul, A.; Treutwein, J. Cobalt Catalysis at the Crossroads: Cobalt-catalyzed Alder-Ene Reaction versus [2 + 2] Cycloaddition. Org. Lett. 2010, 12, 1536– 1539,  DOI: 10.1021/ol100266u

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    19d

    Cobalt Catalysis at the Crossroads: Cobalt-Catalyzed Alder-Ene Reaction versus [2 + 2] Cycloaddition

    Hilt, Gerhard; Paul, Anna; Treutwein, Jonas

    Organic Letters (2010), 12 (7), 1536-1539CODEN: ORLEF7; ISSN:1523-7060. (American Chemical Society)

    The application of bidentate phosphine ligands in cobalt-catalyzed transformations of cyclic alkenes such as cyclopentene and cycloheptene with internal alkynes led to a chemoselective Alder-ene or a [2 + 2] cycloaddn. reaction depending on the electronic nature of the alkyne and the bite angle of the ligand used.

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    (e) Nishimura, A.; Tamai, E.; Ohashi, M.; Ogoshi, S. Synthesis of Cyclobutenes and Allenes by Cobalt-catalyzed Cross-dimerization of Simple Alkenes with 1,3-Enynes. Chem. Eur. J. 2014, 20, 6613– 6617,  DOI: 10.1002/chem.201402218

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    19e

    Synthesis of Cyclobutenes and Allenes by Cobalt-Catalyzed Cross-Dimerization of Simple Alkenes with 1,3-Enynes

    Nishimura, Akira; Tamai, Eri; Ohashi, Masato; Ogoshi, Sensuke

    Chemistry - A European Journal (2014), 20 (22), 6613-6617CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Cobalt-catalyzed cross-dimerization of simple alkenes with 1,3-enynes is reported. A [2+2] cycloaddn. reaction occurred, with alkenes bearing no allylic hydrogen, by reductive elimination of a η3-butadienyl cobaltacycle. On the other hand, aliph. alkenes underwent 1,4-hydroallylation by means of exo-cyclic β-H elimination. These reactions can provide cyclobutenes, e.g., I, and allenes, e.g., II, that were previously difficult to access, from simple substrates in a highly chemo- and regioselective manner.

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    (f) Pagar, V. V.; RajanBabu, T. V. Tandem Catalysis for Asymmetric Coupling of Ethylene and Enynes to Functionalized Cyclobutanes. Science 2018, 361, 68– 72,  DOI: 10.1126/science.aat6205

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    19f

    Tandem catalysis for asymmetric coupling of ethylene and enynes to functionalized cyclobutanes

    Pagar, Vinayak Vishnu; RajanBabu, T. V.

    Science (Washington, DC, United States) (2018), 361 (6397), 68-72CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)

    Transformation of simple precursors into structurally complex cyclobutanes, present in many biol. important natural products and pharmaceuticals, is of considerable interest in medicinal chem. Starting from 1,3-enynes and ethylene, both exceptionally inexpensive starting materials, we report a cobalt-catalyzed route to vinylcyclobutenes, as well as the further enantioselective addn. of ethylene to these products to form complex cyclobutanes with all-carbon quaternary centers. These reactions can proceed in discrete stages or in a tandem fashion to achieve three highly selective carbon-carbon bond formations in one pot using a single chiral cobalt catalyst.

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    (g) Ding, W.; Yoshikai, N. Cobalt-catalyzed Intermolecular [2 + 2] Cycloaddition between Alkynes and Allenes. Angew. Chem., Int. Ed. 2019, 58, 2500– 2504,  DOI: 10.1002/anie.201813283

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    19g

    Cobalt-Catalyzed Intermolecular [2+2] Cycloaddition between Alkynes and Allenes

    Ding, Wei; Yoshikai, Naohiko

    Angewandte Chemie, International Edition (2019), 58 (8), 2500-2504CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    An intermol. [2+2] cycloaddn. reaction between an alkyne and an allene is reported. In the presence of a cobalt(I)/diphosphine catalyst, a near equimolar mixt. of the alkyne and allene is converted into a 3-alkylidenecyclobutene deriv. in good yield with high regioselectivity. The reaction tolerates a variety of internal alkynes and mono- or disubstituted allenes bearing various functional groups. The reaction is proposed to involve regioselective oxidative cyclization of the alkyne and allene to form a 4-alkylidenecobaltacyclopentene intermediate, with subsequent C-C reductive elimination.

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    (h) Parsutkar, M. M.; Pagar, V. V.; RajanBabu, T. V. Catalytic Enantioselective Synthesis of Cyclobutenes from Alkynes and Alkenyl Derivatives. J. Am. Chem. Soc. 2019, 141, 15367– 15377,  DOI: 10.1021/jacs.9b07885

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    19h

    Catalytic Enantioselective Synthesis of Cyclobutenes from Alkynes and Alkenyl Derivatives

    Parsutkar, Mahesh M.; Pagar, Vinayak Vishnu; RajanBabu, T. V.

    Journal of the American Chemical Society (2019), 141 (38), 15367-15377CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    In the presence of (phosphinoaryl)oxazoline cobalt(II) bromide complexes, sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, and activated zinc, alkynes such as 4-octyne and enynes such as (E)-MeCH:CHC≡Cc-Hex underwent regioselective and enantioselective [2+2]-cycloaddn. reactions with alkenes (alkenoates, alkenyl silanes and boranes, aryl alkenes, 2,3-dihydrofuran, and norbornene) to yield nonracemic cyclobutenes and alkenylcyclobutenes such as I (R = Me, F3CCH2, Et, t-Bu) and II [R = Me, F3CCH2, Et, t-Bu, (F3C)2CH] in 86-97% ee. Some of the novel observations made during these studies including a key role of a cationic Co(I)-intermediate, ligand and counterion effects on the reactions, can be expected to have broad implications in homogeneous catalysis beyond the highly valuable synthetic intermediates that are accessible by this route.

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20. 20

    Farmer, M. E.; Ehehalt, L. E.; Pabst, T. P.; Tudge, M. T.; Chirik, P. J. Well-Defined Cationic Cobalt(I) Precatalyst for Olefin-Alkyne [2 + 2] Cycloaddition and Olefin-Diene Hydrovinylation Reactions: Experimental Evidence for Metallacycle Intermediates. Organometallics 2021, 40, 3599– 3607,  DOI: 10.1021/acs.organomet.1c00473

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    20

    Well-Defined Cationic Cobalt(I) Precatalyst for Olefin-Alkyne [2 + 2] Cycloaddition and Olefin-Diene Hydrovinylation Reactions: Experimental Evidence for Metallacycle Intermediates

    Farmer, Marcus E.; Ehehalt, Lauren E.; Pabst, Tyler P.; Tudge, Matthew T.; Chirik, Paul J.

    Organometallics (2021), 40 (21), 3599-3607CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)

    The synthesis and characterization of the cationic cobalt(I) arene complex, [(dppf)Co(η6-C7H8)][BArF4] (dppf = 1,1'-bis(diphenylphosphino)ferrocene; BArF4 = B[(3,5-(CF3)2)C6H3]4) from an air-stable cobalt precursor is described. Dissoln. in benzene-d6 or THF resulted in rapid arene substitution and generated [(dppf)Co(η6-C6H6)][BArF4] or [(dppf)Co(THF)2][BArF4]. The latter compd. was characterized by a combination of x-ray diffraction and magnetometry and established an S = 1 cobalt(I) deriv. The isolated bis(phosphine)cobalt complexes were evaluated as precatalysts for carbon-carbon bond-forming reactions. The [2 + 2] cycloaddn. of internal alkynes and olefins was obsd. with cobalt precatalyst loadings of 0.25 mol % with high chemoselectivity. The catalytic method was compatible with Lewis basic functional groups, an advantage over in situ-generated catalysts that rely on excess trialkyl aluminum activators. The cationic bis(phosphine)cobalt arene complex was also an effective catalyst precursor for the hydrovinylation of isoprene with ethylene. In both C-C bond-forming reactions, the corresponding cobalt(0) complex, [(dppf)Co(COD)] (COD = 1,5-cyclooctadiene), was inactive, providing strong evidence of the role of cobalt(I) during catalysis. In both catalytic reactions, deuterium crossover expts. provide exptl. evidence of the role of metallacyclic intermediates during turnover.

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21. 21

    (a) Friedfeld, M. R.; Zhong, H.; Ruck, R. T.; Shevlin, M.; Chirik, P. J. Cobalt-catalyzed Asymmetric Hydrogenation of Enamides Enabled by Single-electron Reduction. Science 2018, 360, 888– 893,  DOI: 10.1126/science.aar6117

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    21a

    Cobalt-catalyzed asymmetric hydrogenation of enamides enabled by single-electron reduction

    Friedfeld, Max R.; Zhong, Hongyu; Ruck, Rebecca T.; Shevlin, Michael; Chirik, Paul J.

    Science (Washington, DC, United States) (2018), 360 (6391), 888-893CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)

    Cobalt catalysts are identified by high-throughput screening for the enantioselective hydrogenation of enamides using zinc as an activator rather than previous activating agents such as Me3SiCH2MgCl which are air-sensitive and incompatible with protic solvents. In the presence of 0.08 mol% CoCl2 and 0.084 mol% of the bis(phospholane) ligand I, (oxopyrrolidinyl)butenamide II underwent enantioselective hydrogenation with zinc as an activator to yield levetiracetam III in 97% yield and 98.2% ee on 200 g scale. Potential catalyst intermediates and precursors for hydrogenation reactions using I and a bis(phospholanyl)benzene ligand were prepd. and characterized; the cobalt (II) catalyst precursor underwent ligand displacement by methanol, while the cobalt(I) complex generated on redn. with zinc more stably bound I.

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    (b) Zhong, H.; Friedfeld, M. R.; Camacho-Bunquin, J.; Sohn, H.; Yang, C.; Delferro, M.; Chirik, P. J. Exploring the Alcohol Stability of Bis(phosphine) Cobalt DialkylPrecatalysts in Asymmetric Alkene Hydrogenation. Organometallics 2019, 38, 149– 156,  DOI: 10.1021/acs.organomet.8b00516

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    21b

    Exploring the Alcohol Stability of Bis(phosphine) Cobalt Dialkyl Precatalysts in Asymmetric Alkene Hydrogenation

    Zhong, Hongyu; Friedfeld, Max R.; Camacho-Bunquin, Jeffrey; Sohn, Hyuntae; Yang, Ce; Delferro, Massimiliano; Chirik, Paul J.

    Organometallics (2019), 38 (1), 149-156CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)

    Co complexes bearing enantiopure, bidentate bis(phosphine) ligands exhibit extraordinary activity and stereoselectivity for the hydrogenation of enamides. Optimal performance was obsd. in polar protic solvents such as MeOH, an industrially preferred green solvent but a medium that is often a poison for reduced Earth abundant metals. The interaction of the low spin Co(II) dialkyl complex, (R,R)-(iPr-DuPhos)Co(CH2SiMe3)2 with alcs. including: 4-methoxyphenol, pinacol, and MeOH was studied. With the alcs. lacking β-hydrogens, Co bis(alkoxide) complexes were isolated and structurally characterized. With MeOH, protonolysis of the alkyl ligands was again obsd. followed by dehydrogenation of the alc. and [(R,R)-(iPr-DuPhos)Co]2(μ-CO)2 was isolated. Both solid-state and soln. EXAFS studies were conducted to establish the spectroscopic signatures of bis(phosphine)cobalt(II) and Co(0) complexes relevant to catalytic hydrogenation and also to probe the role of phosphine dissocn. in MeOH.

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    (c) Zhong, H.; Friedfeld, M. R.; Chirik, P. J. Syntheses and Catalytic Hydrogenation Performance of Cationic Bis(phosphine) Cobalt(I) Diene and Arene Compounds. Angew. Chem., Int. Ed. 2019, 58, 9194– 9198,  DOI: 10.1002/anie.201903766

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    21c

    Syntheses and Catalytic Hydrogenation Performance of Cationic Bis(phosphine) Cobalt(I) Diene and Arene Compounds

    Zhong, Hongyu; Friedfeld, Max R.; Chirik, Paul J.

    Angewandte Chemie, International Edition (2019), 58 (27), 9194-9198CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Chloride abstraction from [(R,R)-(iPrDuPhos)Co(μ-Cl)]2 with NaBArF4 (BArF4=B[(3,5-(CF3)2)C6H3]4) in the presence of dienes, such as 1,5-cyclooctadiene (COD) or norbornadiene (NBD), yielded long sought-after cationic bis(phosphine) cobalt complexes, [(R,R)-(iPrDuPhos)Co(η2,η2-diene)][BArF4]. The COD complex proved substitutionally labile undergoing diene substitution with THF, NBD, or arenes. The resulting 18-electron, cationic cobalt(I) arene complexes, as well as the [(R,R)-(iPrDuPhos)Co(diene)][BArF4] derivs., proved to be highly active and enantioselective precatalysts for asym. alkene hydrogenation. A cobalt-substrate complex, [(R,R)-(iPrDuPhos)Co(MAA)][BArF4] (MAA=methyl 2-acetamidoacrylate) was crystallog. characterized as the opposite diastereomer to that expected for productive hydrogenation demonstrating a Curtin-Hammett kinetic regime similar to rhodium catalysis.

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    (d) Zhong, H.; Shevlin, M.; Chirik, P. J. Cobalt-catalyzed Asymmetric Hydrogenation of α,β-Unsaturated Carboxylic Acids by Homolytic H₂ Cleavage. J. Am. Chem. Soc. 2020, 142, 5272– 5281,  DOI: 10.1021/jacs.9b13876

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    21d

    Cobalt-Catalyzed Asymmetric Hydrogenation of α,β-Unsaturated Carboxylic Acids by Homolytic H2 Cleavage

    Zhong, Hongyu; Shevlin, Michael; Chirik, Paul J.

    Journal of the American Chemical Society (2020), 142 (11), 5272-5281CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    The asym. hydrogenation of α,β-unsatd. carboxylic acids using readily prepd. bis(phosphine) cobalt(0) 1,5-cyclooctadiene precatalysts is described. Di-, tri-, and tetra-substituted acrylic acid derivs. with various substitution patterns as well as dehydro-α-amino acid derivs. were hydrogenated with high yields and enantioselectivities, affording chiral carboxylic acids including Naproxen, (S)-Flurbiprofen, and a D-DOPA precursor. Turnover nos. of up to 200 were routinely obtained. Compatibility with common org. functional groups was obsd. with the reduced cobalt(0) precatalysts, and protic solvents such as methanol and isopropanol were identified as optimal. A series of bis(phosphine) cobalt(II) bis(pivalate) complexes, which bear structural similarity to state-of-the-art ruthenium(II) catalysts, were synthesized, characterized, and proved catalytically competent. X-band EPR expts. revealed bis(phosphine)cobalt(II) bis(carboxylate)s were generated in catalytic reactions and were identified as catalyst resting states. Isolation and characterization of a cobalt(II)-substrate complex from a stoichiometric reaction suggests that alkene insertion into the cobalt hydride occurred in the presence of free carboxylic acid, producing the same alkane enantiomer as that from the catalytic reaction. Deuterium labeling studies established homolytic H2 (or D2) activation by Co(0) and cis addn. of H2 (or D2) across alkene double bonds, reminiscent of rhodium(I) catalysts but distinct from ruthenium(II) and nickel(II) carboxylates that operate by heterolytic H2 cleavage pathways.

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    (e) MacNeil, C. S.; Zhong, H.; Pabst, T. P.; Shevlin, M.; Chirik, P. J. Cationic Bis(phosphine) Cobalt(I) Arene Complexes as Precatalystsfor the Asymmetric Synthesis of Sitagliptin. Submitted for publication

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22. 22

    For a comparison of the ¹H NMR spectra of the crude reaction mixtures for entries 1–4 from Table 1, see the Supporting Information.

    There is no corresponding record for this reference.
23. 23

    Previously reported cationic (dcype)cobalt(I) η⁶-arene complexes:

    (a) Grossheimann, G.; Holle, S.; Jolly, P. W. η⁶-Arene–Cobalt(I) Complexes. J. Organomet. Chem. 1998, 568, 205– 211,  DOI: 10.1016/S0022-328X(98)00839-0

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    23a

    η6-Arene-cobalt(I) complexes

    Grossheimann, G.; Holle, S.; Jolly, P. W.

    Journal of Organometallic Chemistry (1998), 568 (1-2), 205-211CODEN: JORCAI; ISSN:0022-328X. (Elsevier Science S.A.)

    (η6-Arene)Co(I) complexes stabilized by bisphosphines, e.g. [(η6-MeC6H5)Co(Pr2iPC2H4PPr2i)]+BF4-, have been prepd. by reacting (η3-cyclooctenyl)Co(bisphosphine) species with HBF4 in the presence of an arene. The (η6-C6H6)Co(I) compds. can also be prepd. by hydrogen abstraction from the corresponding (η5-cyclohexadienyl)Co(I) complex or by hydrogenation of (η3-cyclooctenyl)Co(II) species in the presence of benzene. Facile arene-exchange occurs upon treatment of these compds. with a second arene. In contrast, (η3-cyclohexenyl)Co(I) and (η5-cycloheptadienyl)Co(I) complexes are oxidized by HBF4 in the presence of an alkene to give (η3-cyclohexenyl)Co(II) and (η5-cycloheptadienyl)Co(II) species: the former have been characterized as their diamagnetic NO adducts and the latter by a crystal structure detn.

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    (b) Boyd, T. M.; Tegner, B. E.; Tizzard, G. J.; Martinez-Martinez, A. J.; Neale, S. E.; Hayward, M. A.; Coles, S. J.; Macgregor, S. A.; Weller, A. S. A Structurally Characterized Cobalt(I) σ-Alkane Complex. Angew. Chem., Int. Ed. 2020, 59, 6177– 6181,  DOI: 10.1002/anie.201914940

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    23b

    A Structurally Characterized Cobalt(I) σ-Alkane Complex

    Boyd, Timothy M.; Tegner, Bengt E.; Tizzard, Graham J.; Martinez-Martinez, Antonio J.; Neale, Samuel E.; Hayward, Michael A.; Coles, Simon J.; MacGregor, Stuart A.; Weller, Andrew S.

    Angewandte Chemie, International Edition (2020), 59 (15), 6177-6181CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    A Co σ-alkane complex, [Co(Cy2P(CH2)4PCy2)(norbornane)][BArF4], was synthesized by a single-crystal to single-crystal solid/gas hydrogenation from a norbornadiene precursor, and its structure was detd. by x-ray crystallog. Magnetic data show this complex to be a triplet. Periodic DFT and electronic structure analyses revealed weak C-H→Co σ-interactions, augmented by dispersive stabilization between the alkane ligand and the anion microenvironment. The calcns. are most consistent with a η1:η1-alkane binding mode.

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24. 24

    Zhu, D.; Janssen, F. F. B. J.; Budzelaar, P. H. M. (Py)₂Co(CH₂SiMe₃)₂ as an Easily Accessible Source of “CoR₂. Organometallics 2010, 29, 1897– 1908,  DOI: 10.1021/om901045s

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    24

    (Py)2Co(CH2SiMe3)2 As an easily accessible source of "CoR2"

    Zhu, Di; Janssen, Femke F. B. J.; Budzelaar, Peter H. M.

    Organometallics (2010), 29 (8), 1897-1908CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)

    Cobalt(II) dialkyl complex with hemilabile pyridine ligands, [(Py)2CoR2] (R = CH2SiMe3) is easily prepd. from (Py)4CoCl2 and RLi; the complex undergoes facile ligand exchange with tridentate 2,6-pyridinediketimines and 2,6-bis(2-oxazolinyl)pyridine, producing Co(I) and Co(II) alkyl complexes. The complex [(Py)2CoR2] is fairly stable at room temp. and serves as a convenient source of CoR2 for transfer to other ligands. Unfortunately, (Py)2CoR2 was obtained only as an oil, but the structure of the related complex [(Py)2CoR'2] (R' = CH2CMe2Ph) could be confirmed by a single-crystal x-ray diffraction study. Transfer of the CoR2 fragment from [(Py)2CoR2] or (TMEDA)CoR2 to diiminepyridine-type ligands 2,6-(R1N:CR2)C5H3N (1-6; R1 = 2,6-Me2C6H3, mesityl, Ph, CH2Ph, 2,6-iPr2C6H3; R2 = Me, CF3) was studied as a function of ligand steric and electronic properties. Reaction with N,N'-bis(2,6-dimethylphenyl) and N,N'-dimesityl 2,6-diacetylpyridinediketimines (1, 2, resp.) produced diamagnetic monoalkyl complexes; the structure of (1)CoR was confirmed by x-ray diffraction. With the less shielding N,N'-diphenyl and N,N'-dibenzyl (4) 2,6-diacetylpyridinediketimine ligands, 1H NMR indicated formation of diamagnetic CoI alkyl species, but they were not stable enough to allow isolation. Fluorinated ligand N,N'-dimesityl 2,6-bis(trifluoroacetyl)pyridinediketimine (5) appears to be less reactive and, despite its supposedly stronger π-acceptor character, also does not lead to formation of a stable CoI alkyl complex. With 2,6-bis(4,4-dimethyloxazolin-2-yl)pyridine (PyBOX) ligand 6, high-spin dialkyl complex (6)CoR2 was obsd. by 1H NMR. Based on these observations and DFT calcns., a mechanism is proposed for formation of diiminepyridine CoI alkyls that involves formation of a high-spin κ2-complex, spin flip to give a low-spin κ3-complex, and irreversible loss of an alkyl radical.

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25. 25

    For a summary of low-yielding or unsuccessful substrates, see the Supporting Information.

    There is no corresponding record for this reference.
26. 26

    The X-ray crystal structure of 3ac can be found in the Supporting Information.

    There is no corresponding record for this reference.
27. 27

    H/D exchange between ethylene and 2a-d₅ was not mediated by 1 in the absence of alkyne.

    There is no corresponding record for this reference.