Radical-Driven Imine Hydrogenation Promoted by the Cooperativity of Nickel and a Redox-Active Ligand

Abstract

In the case of N-alkylation reaction via borrowing hydrogen (BH) methods, imine hydrogenation is one of the critical steps. Mechanistically, the majority of imine hydrogenation, either by dihydrogen gas or by alcohol in case of BH methods, follow two-electron chemistry, where a metal-hydride intermediate plays a pivotal role. Herein we demonstrate a completely complementary protocol, where the hydrogenation is governed by a radical pathway. Such a pathway is adopted from the redox active nature of the azophenolate ligand backbone. The facile and reversible 2e⁻/2H⁺, azo/hydrazo redox couple in a nickel catalyst helps in storing the hydrogen from alcohol dehydrogenation. An imine substrate is reduced by one-electron from the azo motif with the assistance of the nickel center. The reduced imine drives a critical hydrogen atom transfer step from the hydrazo motif to conduct the hydrogenation. A set of kinetics experiments including Eyring analysis, radical inhibition experiment along with computational probation attest for the radical-promoted hydrogenation mechanism.