Organocobalt Complexes as Sources of Carbon-Centered Radicals for Organic and Polymer Chemistries

Abstract

Organocobalt(III) complexes (R-Co III ), defined as cobalt complexes featuring a carbon−cobalt bond, are largely used to produce carbon-centered radicals by homolytic cleavage of their C−Co bond under mild conditions. They are key compounds in cutting-edge developments in the fields of organic chemistry, biochemistry, medical research, radical reactions, and organometallic chemistry. This is the first Review of the use of R-Co III in both organic and polymer chemistries. Although pioneering works in organic synthesis have largely contributed to the implementation of R-Co III in polymer design, the two fields have evolved independently, with many breakthroughs on both sides. The main motivation of this Review is to confront both fields to stimulate cross-fertilization. It notably describes the most important synthetic pathways for R-Co III , the influence of the ligand structure and the environment of the complex on the C−Co bond strength, the modes of formation of the radicals, and the most relevant R-Co III -promoted radical reactions, with a focus on the main reaction mechanisms. CONTENTS 6.4. Polymerizations 6938 7. Conclusions 6945 Author Information 6946 Corresponding Author 6946 ORCID 6946 Notes 6946 Biographies 6946 Acknowledgments 6946 References 6946

Catalytic hydrogen atom transfer to alkenes: a roadmap for metal hydrides and radicals

“…7A). 89 Because of this supporting literature, alkyl-cobalt complexes have been hypothesized frequently as intermediates, beginning with seminal mechanistic work by Nishinaga on the Drago hydration. 90 Only recently, however, was a MHAT-relevant sec-alkyl cobalt salen complex characterized by 1 H NMR and implicated in a turnover-limiting transmetalation step to a nickel cocatalyst.…”

Switchable Polymerization Triggered by Fast and Quantitative Insertion of Carbon Monoxide into Cobalt–Oxygen Bonds

“…Relative to the polar Co−O bonds, Co−C bonds tend to break homolytically to generate carbon‐based radicals as is known to occur, for instance, in the biological action of vitamin B12 . This process may be promoted by thermal treatment or by light irradiation . Recently, cobalt complexes have been used to ensure the well‐controlled polymerization of a wide variety of the challenging nonconjugated monomers through the strategy known as organometallic mediated controlled radical polymerization (OMRP) (Scheme B) .…”

“…[8] This process may be promoted by thermal treatment or by light irradiation. [9] Recently,c obalt complexes have been used to ensure the well-controlled polymerization of aw ide variety of the challenging nonconjugated monomers through the strategy known as organometallic mediated controlled radical polymerization (OMRP) (Scheme 1B). [10] Especially,F ua nd Peng et al demonstrated that an alkoxycarbonyl cobalt Salen complex (Salen is as pecific Salcyl ligand with a trans-cyclohex-diyl tether,Scheme 2) could serve as the photoinitiator as well as the mediator for visible light controlled OMRP.…”

A Widely Applicable Dual-Catalytic System for Cross-Electrophile Coupling

“…[21] In turn, these high valent Co III complexes can undergo homolysis of the Co III -Alk bond, which produces an alkyl radical and regenerates Co Pc . [35] Further support for the proposal that Co Pc is capable of generating alkyl radicals in the presence of TDAE was obtained by performing an analogous radical trapping experiment using 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as the radical acceptor instead of (dtbbpy)Ni II (o-tol)I. [36] In a similar fashion to our experiment with (dtbbpy)Ni II (o-tol)I, trapping of the benzyl radical by TEMPO is only observed in the presence of excess TDAE and catalytic amounts of Co Pc (see SI).…”