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.