Ruthenium-Catalyzed Selective Hydroboration of Nitriles and Imines

Ruthenium-catalyzed hydroboration of nitriles and imines is attained using pinacolborane with unprecedented catalytic efficiency. Chemoselective hydroboration of nitriles over esters is also demonstrated. A simple [Ru(p-cymene)Cl₂]₂ complex (1) is used as a catalyst precursor, which upon reaction with pinacolborane in situ generates the monohydrido-bridged complex [{(η⁶-p-cymene)RuCl}₂(μ-H-μ-Cl)] 2. Further oxidative addition of pinacolborane to intermediate 2 leading to the formation of mononuclear ruthenium hydride species is suggested. Mass spectral analysis of the reaction mixture and independent experiments with phosphine-ligated ruthenium complexes indicated the involvement of mononuclear ruthenium intermediates in the catalytic cycle. Consecutive intramolecular 1,3-hydride transfers from the ruthenium center to coordinated nitrile and boronate imine ligands, leading to the reduction and resulting in the formation of diboronate amines, are proposed as a plausible reaction mechanism.