Reactivity of organocobalt(IV) chelate complexes toward nucleophiles: diversity of mechanisms

Mechanistic Interrogation of Co/Ni-Dual Catalyzed Hydroarylation

“…Isolated organocobalt species have been shown to be susceptible to single-electron oxidation by outer-sphere chemical oxidants, followed by subsequent cobalt−carbon bond cleavage. 55 A high-valent nickel species, such as Ni-INT1 , could be invoked as a suitable outer-sphere oxidant of a metastable organocobalt species. While we cannot directly observe an aryl nickel(III) intermediate under our reaction conditions, analogies to the literature provided a frame of reference from which to consider Ni-INT1 as an oxidant and transmetalating species.…”

“…Electrochemical oxidations of nickel(II)(aryl)halide complexes result in irreversible oxidation waves ranging from +0.18 to +0.8 V (vs SCE), 43d,58 making it difficult to directly compare redox potentials to the reported oxidation potentials of organocobalt(III) complexes in the literature (+0.49 V vs SCE) 55f. Studies of isolated nickel(III)aryl organometallic complexes require stabilization with electron-rich amine ligands, resulting in quasi-reversible redox Ni(III)/Ni(II) redox waves ranging from onset potentials of −1.1 to −0.40 V (vs Fc/Fc+) in the literature.…”

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

“…Metastable organocobalt( iii ) species bearing porphyrin or salen ligands undergo reduction and oxidation to form unstable organocobalt( ii ) and organocobalt( iv ) species, respectively, which can undergo carbon–cobalt bond cleavage more readily. 126 In particular, organocobalt( iv ) complexes may undergo homolysis or heterolysis, depending on the identity of the ligand and/or surrounding nucleophiles. 127 Nucleophilic attack on the organocobalt( iv ) species at the carbon center may form the new carbon–heteroatom bond and regenerate the cobalt( ii ) precatalyst.…”

“… 127 Nucleophilic attack on the organocobalt( iv ) species at the carbon center may form the new carbon–heteroatom bond and regenerate the cobalt( ii ) precatalyst. 126 Several (salen)Co MHAT methods that form carbon–heteroatom bonds with nucleophilic coupling partners have revised the original carbon-centered radical oxidation model and now invoke these organocobalt( iv ) species. 18 , 31 An organocobalt( iv ) species could heterolyze to form a transient carbocation, or homolyze the Co–C bond to release a carbon radical that undergoes OLT from another cobalt complex, for example fluorine from a Co–F complex.…”

Cobalt‐Catalyzed Radical Hydroamination of Alkenes with N‐Fluorobenzenesulfonimides

“…Subsequently, a SET between the resulted alkylcobalt(III) complex D and Co III ‐NR 2 species B , illustrated as Figure 1 g, would occur to form the pivotal radical cationic alkyl Co IV intermediate E , i.e., with counter anion − NR 2 [44] . According to early Halpern and recent Pronin, Shigehisa and other's investigations, [29, 30, 44, 45] this radical cationic Co IV species, formally as [Co IV R − ], has been found to enable undergoing a stereochemical inversed displacement with various nucleophiles [44d,e,f] . On the basis of these reports, a similar S N 2‐like pathway, which is consistent with the Hammett analysis but unlike the actions of free carbon cation, [36] between this cationic alkyl Co IV species with nitrogen nucleophile could occur, affording the expected amination product with simultaneous release of the Co II for the next catalytic cycle.…”