Mechanistic Elucidation of an Alcohol Oxidation Reaction Promoted by a Nickel Azophenolate Complex

Abstract

Borrowing hydrogen (BH) reactions by base metals have attracted significant focus with a perspective in green and sustainable chemistry. In a BH reaction, dehydrogenation of the alcohol motifs is a fundamental step, where several well-established catalysts exhibit metal–ligand cooperativity. We showcase here that the redox noninnocence of the azophenolate backbone in an earth-abundant nickel catalyst drives the alcohol dehydrogenation by a radical pathway. Detailed mechanistic probation including saturation kinetics supports the binding of the substrate alcohol to the nickel center. Thermodynamic data derived from such analysis also proves the azophenolate hemilability to pave the further steps. An Eyring analysis quantifies the kinetic barrier associated with a crucial hydrogen atom transfer step that is driven by the azo radical. Furthermore, both Hammett correlation and linearity in Evans–Polanyi plot implicate that C–H abstraction by radical mechanism as the rate-determining step. The overall mechanism offered by the azophenolate catalyst closely resembles the galactose oxidase-based copper phenoxide chemistry.