Mechanistic investigation: A redox-active azophenolate ligand supported Ni complex has been shown to be instrumental towards olefin hydrogenation following an unconventional pathway. Detailed mechanistic investigation reveals the ligand-radical promoted hydrogenation via discrete one electron pathway.

Abstract

In 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.