An ECEC Mechanism via an Electrophilic Co(II) Carbene in Reductive Cyclopropanation

Metal carbenes have long been recognized as a pivotal reactive intermediate in organometallic chemistry. Recently catalytic reductive processes with gem-dichloroalkanes have emerged as a promising strategy to generate nonstabilized donor carbenes for alkene cyclopropanation. However, the catalytic cycle remains challenging to study largely due to the inherent instability of metal carbene intermediates. In this study, we elucidated an ECEC mechanism for the (PNN)Co-catalyzed (PNN = bipyridyl phosphine) reductive cyclopropanation of alkenes through an integrated experimental-computational approach combining electroanalytical techniques and density functional theory (DFT) calculations. Initiated by ZnCl₂-assisted one-electron reduction of (PNN)Co^IICl₂ (1) to [(PNN)^•-]Co^IICl (2) featuring a reduced π-radical anion, activation of aryl gem-dichloromethanes by 2 proceeds via outer-sphere electron transfer (OSET) as determined by Hammett linear free-energy relationship (LFER) analysis, producing a cationic benzyl Co(III) species [(PNN)Co^IIICl(benzyl)]⁺ZnCl₃^- (A). Following the first EC, cyclic voltammetry studies, controlled potential electrolysis, and reactivity comparison experiments collectively established the second EC process involving one-electron reduction of A to a cationic Co(II) carbene [(PNN)Co^IICl(carbene)]⁺ZnCl₃^- (C) that is effective for the following carbene transfer. LFER analysis and computational data support the assignment of intermediate C as an electrophilic Co(II) carbene, rationalizing a concerted, nonradical addition pathway for cis-selective alkene cyclopropanation. This work provides a multielectron, multistep mechanistic framework, which could be valuable for developing sustainable catalytic transformations that employ base metals.