The Merger of Photoredox and Cobalt Catalysis

The merger of transition-metal catalysis and photoredox catalysis, so-called 'metallaphotoredox catalysis', has become a versatile strategy in organic synthesis.Due to the intrinsic compatibility of 3d-metal catalysis and photoredox catalysis, a combined cobalt/photoredox system promises high potential as an innovative new method in organic synthesis.While dehydrogenative transformations of organic molecules have been characteristic of cobalt/photoredox catalysis, more diverse sets of useful transformations involving organocobalt intermediates are now available.Compared with the intensively studied nickel/photoredox and copper/photoredox catalysis, the cobalt/photoredox system remains an unmatured area and is expected to constitute unique options in metallaphotoredox catalysis. In the past decade, synthetic chemists have discovered the outstanding generality and potential of visible-light-driven photoredox catalysis, which converts visible light into chemical energy, realizing numerous transformations of small molecules. The current state-of-the-art strategy in photoredox catalysis, combining photoredox and transition-metal catalysis, has received considerable attention in organometallic chemistry. In parallel with the rapid development of nickel/photoredox and copper/photoredox catalysis, cobalt/photoredox catalysis has emerged as a distinct new option in this area. This short review covers the general strategy, characteristics compared with other metallaphotoredox systems, and future perspectives of cobalt/photoredox catalysis. To our knowledge, this is the first review encompassing the general combination of cobalt and photoredox catalysis in synthetic chemistry. In the past decade, synthetic chemists have discovered the outstanding generality and potential of visible-light-driven photoredox catalysis, which converts visible light into chemical energy, realizing numerous transformations of small molecules. The current state-of-the-art strategy in photoredox catalysis, combining photoredox and transition-metal catalysis, has received considerable attention in organometallic chemistry. In parallel with the rapid development of nickel/photoredox and copper/photoredox catalysis, cobalt/photoredox catalysis has emerged as a distinct new option in this area. This short review covers the general strategy, characteristics compared with other metallaphotoredox systems, and future perspectives of cobalt/photoredox catalysis. To our knowledge, this is the first review encompassing the general combination of cobalt and photoredox catalysis in synthetic chemistry. Photochemistry historically represents a vital part of synthetic chemistry. 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The reduction of protons into molecular hydrogen is an indispensable transformation in artificial photosynthesis and, in that context, the combination of visible-light-driven photocatalysts and cobalt complexes has been intensively studied [39.Eckenhoff W.T. et al.Cobalt complexes as artificial hydrogenases for the reductive side of water splitting.Biochim. Biophys. Acta. 2013; 1827: 958-973Crossref PubMed Scopus (117) Google Scholar]. In addition, early contributions on biomimetic chemistry regarding vitamin B12 and its model compounds under photochemical conditions [40.Giedyk M. et al.Vitamin B12 catalysed reactions.Chem. Soc. Rev. 2015; 44: 3391-3404Crossref PubMed Google Scholar,41.Tahara K. et al.Learning from B12 enzymes: biomimetic and bioinspired catalysts for eco-friendly organic synthesis.Beilstein J. Org. Chem. 2018; 14: 2553-2567Crossref PubMed Scopus (6) Google Scholar] should also be regarded as pioneering work in cobalt/photoredox cooperative catalysis. Despite these precedents and recent developments in cobalt/photoredox cooperative catalysis, to the best of our knowledge there is still no review that focuses on this topic from the perspective of synthetic chemistry. This review aims to cover recent examples (typically those reported between 2014 and 2019) of the merger of cobalt and photoredox catalysis, focusing on: (i) dehydrogenative transformations [cross-coupling hydrogen evolution (CCHE) and acceptorless dehydrogenation]; and (ii) C–C- or C–heteroatom-bond-forming reactions in which organocobalt intermediates presumably play a vital role (e.g., hydrofunctionalization of alkenes, cycloaddition of alkynes, allylation/deallylation, reactions using stoichiometric oxidants or reductants). Despite the diversity of these precedents, they all share the general mechanistic strategies depicted in Figure 1C: the reduction of the cobalt catalyst by the reduced photocatalyst is responsible for low-valent cobalt or cobalt-hydride-mediated transformations. By contrast, one-electron oxidation of a cobalt complex would provide a high-valent cobalt complex that could subsequently mediate an oxidative transformation. The oxidative functionalization of C–H bonds to C–C or C–heteroatom bonds represents a desirable method for streamlining multistep syntheses of complex molecules, as it avoids the prefunctionalization steps that are usually associated with the preparation of reaction partners in classical cross-coupling reactions [42.Girard S.A. et al.The cross-dehydrogenative coupling of C–H bonds: a versatile strategy for C–C bond formations.Angew. Chem. Int. 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Soc. 2013; 135: 19052-19055Crossref PubMed Scopus (168) Google Scholar], it was quickly demonstrated that cobaloxime derivatives are able to play the same role under homogeneous conditions (vide infra). An early example of the intermolecular formation of C–C bonds by CCHE was reported by Wu and colleagues, who introduced a combination of the organophotocatalyst eosin Y and Co(dmgH)2Cl2 for the noble-metal-free CCHE between tetrahydroisoquinolines and indoles (Figure 2A ) [50.Zhong J-J. et al.Cross-coupling hydrogen evolution reaction in homogeneous solution without noble metals.Org. Lett. 2014; 16: 1988-1991Crossref PubMed Scopus (91) Google Scholar]. In their proposed catalytic cycle, photoexcited eosin Y is responsible for the one-electron oxidation of amines, while the second oxidation of the α-aminoalkyl radical intermediate is mediated by the cobaloxime cocatalyst. This successive two-electron oxidation leads to the formation of an iminium intermediate, which leads to the formation of a new C–C bond on reaction with a nucleophile. Finally, molecular hydrogen is generated by the two-electron reduction of cobaloxime followed by protonolysis. Through the similar photochemical oxidation of substrates, dehydrogenative functionalization of amino acids [51.Gao X-W. et al.Visible light catalysis assisted site-specific functionalization of amino acid derivatives by C–H bond activation without oxidant: cross-coupling hydrogen evolution reaction.ACS Catal. 2015; 5: 2391-2396Crossref Scopus (88) Google Scholar] and isochromanes [52.Xiang M. et al.Activation of C–H bonds through oxidant-free photoredox catalysis: cross-coupling hydrogen-evolution transformation of isochromans and β-keto esters.Chem. Eur. J. 2015; 21: 18080-18084Crossref PubMed Scopus (59) Google Scholar] was also possible. Furthermore, Luo and Wu and colleagues reported an asymmetric variant of the dehydrogenative coupling assisted by a chiral amine catalyst [53.Yang Q. et al.Visible-light-promoted asymmetric cross-dehydrogenative coupling of tertiary amines to ketones by synergistic multiple catalysis.Angew. Chem. Int. Ed. 2017; 56: 3694-3698Crossref PubMed Scopus (88) Google Scholar]. Cobaloxime/photoredox cooperative catalysis has been proven to be generally applicable to synthetically valuable C–C-bond-forming reactions including the synthesis of heterocycles [54.Wu C-J. et al.An oxidant-free strategy for indole synthesis via intramolecular C–C bond construction under visible light irradiation: cross-coupling hydrogen evolution reaction.ACS Catal. 2016; 6: 4635-4639Crossref Scopus (56) Google Scholar] and oxidative Heck [55.Hu X. et al.Photoinduced oxidative activation of electron-rich arenes: alkenylation with H2 evolution under external oxidant-free conditions.Chem. 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For example, Lei and colleagues reported dehydrogenative coupling between styrene derivatives and alkynes, which affords multisubstituted naphthalene rings in a single step (Figure 2B) [59.Zhang G. et al.Oxidative [4+2] annulation of styrenes with alkynes under external-oxidant-free conditions.Nat. Commun. 2018; 9: 1225Crossref PubMed Scopus (24) Google Scholar]. The high oxidation potential of an excited Acr+-Mes organophotocatalyst enables the oxidation of styrene derivatives. Then, the addition of alkynes to the resulting radical cation is responsible for the formation of the first C–C bond. The second C–C bond formation occurs by an intramolecular Friedel–Crafts-type reaction, followed by an aromatization through the release of two protons and an electron. This aromatization is facilitated by the simultaneous formation of cobalt hydride, whose protonation results in the release of hydrogen gas and the regeneration of the cobaloxime catalyst. Consecutive formations of C–C and C–N bonds were also demonstrated in the similar reaction design for the preparation of 3,4-dihydroisoquinoline derivatives [60.Hu X. et al.Selective oxidative [4+2] imine/alkene annulation with H2 liberation induced by photo-oxidation.Angew. Chem. Int. Ed. 2018; 57: 1286-1290Crossref PubMed Scopus (34) Google Scholar] or isoquinoline derivatives [61.Tian W-F. et al.Visible-light photoredox-catalyzed iminyl radical formation by N–H cleavage with hydrogen release and its application in synthesis of isoquinolines.Org. Lett. 2018; 20: 1421-1425Crossref PubMed Scopus (16) Google Scholar]. Since CCHE realizes oxidative transformations in the absence of chemical oxidants, its advantage is evident for the construction of C–heteroatom bonds, for which the starting materials or products are susceptible to undesirable oxidation. Lei and colleagues have demonstrated the intramolecular C–H thiolation of aromatic thioamides using a combined cobaloxime/photoredox catalytic system (Figure 2C) [62.Zhang G. et al.External oxidant-free oxidative cross-coupling: a photoredox cobalt-catalyzed aromatic C–H thiolation for constructing C–S bonds.J. Am. Chem. Soc. 2015; 137: 9273-9280Crossref PubMed Scopus (194) Google Scholar]. The products were obtained in lower yield when chemical oxidants were used, presumably because thioamides readily decompose by these reagents. From a mechanistic perspective, the one-electron oxidation of a thioamide by the excited Ru(bpy)32+ photoredox catalyst provides access to a thiyl radical, which undergoes addition to an aromatic ring. The oxidation of the resulting radical intermediate by the reduced cobaloxime affords the benzothiazole product, while hydrogen gas is released by protonolysis of the cobalt hydride. The authors proposed that precise tuning of the proton-transfer steps by basic additives contributes to high catalytic turnovers. Wu, Tung, and colleagues reported another impressive example of CCHE; namely, the C–H amination and hydroxylation of benzene (Figure 2D) [63.Zheng Y-W. et al.Photocatalytic hydrogen-evolution cross-couplings: benzene C–H amination and hydroxylation.J. Am. Chem. Soc. 2016; 138: 10080-10083Crossref PubMed Scopus (124) Google Scholar]. The UV-light-driven, highly oxidizing photocatalyst QuCN+ oxidizes benzene to afford a benzene radical cation. The addition of a heteroatom nucleophile to the intermediate and the release of a proton delivers a cyclohexadienyl radical intermediate, which is presumably oxidized by the reduced cobaloxime to afford the products with a new C–N or C–O bond. Protonation of the resulting cobalt hydride regenerates the cobaloxime cocatalyst under concomitant release of hydrogen gas. It is proposed that overoxidation of the products is inhibited by a rapid electron back transfer between the product and the photocatalyst. Other challenging dehydrogenative bond-forming reactions (e.g., C–N [64.Yi H. et al.Photocatalytic dehydrogenative cross-coupling of alkenes with alcohols or azoles without external oxidant.Angew. Chem. Int. Ed. 2017; 56: 1120-1124Crossref PubMed Scopus (79) Google Scholar, 65.Niu L. et al.Photo-induced oxidant-free oxidative C–H/N–H cross-coupling between arenes and azoles.Nat. Commun. 2017; 8: 14226Crossref PubMed Scopus (94) Google Scholar, 66.Chen H. et al.External oxidant-free regioselective cross dehydrogenative coupling of 2-arylimidazoheterocycles and azoles with H2 evolution via photoredox catalysis.Adv. Synth. Catal. 2018; 360: 3220-3227Crossref Scopus (16) Google Scholar], C–O [64.Yi H. et al.Photocatalytic dehydrogenative cross-coupling of alkenes with alcohols or azoles without external oxidant.Angew. Chem. Int. 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Ed. 2019; 58: 10941-10945Crossref PubMed Scopus (6) Google Scholar]) by CCHE have also been reported, providing further evidence for the versatility of this strategy to construct C–heteroatom bonds. Desaturation of a C–C or C–heteroatom single bond to a double bond is a fundamental process for the preparation of fine chemicals and other industrial applications. Among these transformations, acceptorless dehydrogenation is considered to be ideal because chemical oxidants are not required, and useful hydrogen gas is the sole byproduct [48.Cartwright K.C. et al.Cobaloxime-catalyzed hydrogen evolution in photoredox-facilitated small-molecule functionalization.Eur. J. Org. Chem. 2019; (Published online October 10, 2019. https://doi.org/10.1002/ejoc.201901170)Crossref Scopus (0) Google Scholar]. However, catalytic acceptorless dehydrogenation under thermal conditions generally requires high reaction temperatures because the release of hydrogen gas together with the desaturation is thermodynamically unfavorable in terms of enthalpy. To realize acceptorless dehydrogenation of alkanes under milder conditions, Sorensen and colleagues have used a cooperative base-metal catalytic system that consists of a tetra-n-butylammonium decatungstate photocatalyst and cobaloxime (Figure 3A ) [75.West J.G. et al.Acceptorless dehydrogenation of small molecules through cooperative base metal catalysis.Nat. Commun. 2015; 6: 10093Crossref PubMed Scopus (49) Google Scholar]. Notably, the acceptorless dehydrogenation of unactivated alkanes proceeds at room temperature under irradiation with UV light. The authors proposed that the excited decatungstate photocatalyst abstracts a hydrogen atom from the alkane, which affords a radical intermediate and the partially hydrogenated photocatalyst. Then, a second hydrogen-atom transfer from the radical intermediate to the cobaloxime affords an alkene and cobalt hydride. The release of hydrogen gas by the reaction between the reduced photocatalyst and the cobalt hydride regenerates the two base-metal catalysts. The same cooperative base-metal catalysts are also applicable to the dehydroformylation of aliphatic aldehydes (Figure 3B) [76.Abrams D.J. et al.Toward a mild dehydroformylation using base-metal catalysis.Chem. Sci. 2017; 8: 1954-1959Crossref PubMed Google Scholar]. In this case, the photochemical cleavage of the C–H bond is followed by a decarbonylation, which leads to an sp3-hybridized carbon-centered radical. This radical intermediate engages with the cobaloxime catalyst to afford an alkene and molecular hydrogen. Li and colleagues have demonstrated that the merger of