Mechanistic Elucidation of a Radical‐Promoted Hydrogenation Relevant to Borrowing Hydrogen Catalysis

Abstract

AbstractIn the borrowing hydrogen catalysis, hydrogenation of an in situ generated imine or olefinic bond is a crucial step. There is a growing body of literature in olefinic hydrogenation promoted by metal hydride of Earth‐abundant metals, where radical mechanism is followed. This report presents a thorough study of the mechanistic details of a nickel catalyzed α‐alkylation of ketones with secondary alcohols and showcases that the olefinic hydrogenation of an enone happens, completely bypassing the involvement of a metal hydride. This pathway is radical promoted, where a single electron reduction of the substrate olefin and a subsequent hydrogen atom transfer step are most critical. A series of control reactions, detection of critical reaction intermediates, and radical probe experiments provide compelling proofs for such radical‐promoted olefinic hydrogenation. The experimental clues, further aided by DFT calculations altogether suggest the precise one‐electron chemistry where the involvement of metal‐hydride is not required. Notably, the redox non‐innocence of the azophenolate backbone, as well as imposed noninnocence of the substrate olefin, when bound to the catalyst molecule makes such mechanism feasible.