Hydrogenation Involving Two Different Proton- and Hydride-Transferring Reagents through Metal–Ligand Cooperation: Mechanism and Scope

Abstract

Metal–ligand cooperation (MLC) allows cooperative action between active sites at both the metal and the ligand for transferring hydrogen to the substrate using hydride and proton transfer. Despite their utility in the development of green and sustainable synthetic transformations from a single source, these transfers using two different sources remain limited. Herein, we presented a bench-stable bifunctional 2,2′-bibenzimidazole (BiBzImH2)-based Ru(II)-para-cymene for selective and efficient hydrogenation of unsaturated carbonyl/nitro to saturated carbonyl/nitro using hydride and proton transfer from two different sources (silane for hydride and methanol for proton) via substrate-mediated interconvertible coordination modes (imino N → Ru and amido N–Ru) of the active catalyst. Remarkably, the generation of Ru–H and the presence of N–H of the coordinated BiBzImH2 are important for the generation of interconvertible coordination modes, which in turn is not operative without the N–H. The initial rate kinetics under standard reaction conditions showed a broken positive order in the substrate, first order in the catalyst, and first order in the hydrogen donor (TES). Mechanistic studies, evaluated from spectroscopic, kinetic, Hammett study, kinetic isotope effects (KIEs), and a few other controlled experiments, further reveal that both solvent-mediated proton transfer via the interconvertible coordination mode and hydride transfer between the substrate-coordinated intermediate and Ru–H might be involved in two separate rate-determining steps. The catalyst demonstrated good efficiency, selectivity (>98%), and functional group tolerance and displayed a broad scope with unsaturated ketones and β nitrostyrenes, affording their saturated keto and nitro products with excellent selectivity and emphasizing its potential synthetic utility.

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