Mechanism Switch between Radical‐Polar Crossover and Radical Buffering

Radical‐polar crossover is a classic concept that bridges one‐ and two‐electron chemistry. It has been widely used in Cr‐catalyzed carbonyl addition reactions to clarify the formation of alkyl chromium(III) intermediate and subsequent carbonyl insertion. Herein, we proposed an orthogonal bonding model, the radical buffering scenario, for Cr‐catalyzed carbonyl alkylation. This radical bonding model features the radical dissociation from the alkyl chromium(III) complex followed by the Cr(II)‐carbonyl‐coupled radical addition to form the C–C bond. The mechanism switch between the radical and polar bonding models is affected by the radical stability, radical nucleophilicity, radical size, and the presence of α‐heteroatom or α‐π bond. The collaborative computational and experimental studies have verified the reliability of the radical mechanism. More importantly, we demonstrated that this radical buffering scenario possesses a different stereoselectivity control model from that in the radical‐polar crossover scenario. A general enantioselectivity and diastereoselectivity control model derived from the multiple ligand‐radical interactions is thus established for CrCl2/bisoxazoline‐catalyzed asymmetric radical addition.