Dihydroboration of Alkyl Nitriles Catalyzed by an Osmium-Polyhydride: Scope, Kinetics, and Mechanism

Complex OsH6(PiPr3)2 is an efficient catalyst precursor for the addition of pinacolborane and catecholborane to the C–N triple bond of alkyl nitriles. In this way, a variety of N,N-diborylamines have been isolated and fully characterized, including 13 derivatives not described so far. The range of nitriles used is wide and comprises substrates having unfunctionalized linear and branched chains, and functionalized chains with methoxide, trifluoromethyl, aryl, pyridyl, benzoyl, or cyanide groups. Kinetic studies demonstrate that the overall process consists of two consecutive irreversible reactions: the catalytic metal-promoted monohydroboration of the nitrile to afford the borylimine and the metal-free stoichiometric hydroboration of the latter to give the diborylamine. The mechanism of the hydroboration has been established by combining the kinetic analysis of the catalysis, stoichiometric reactions, and DFT calculations. The rate-determining step of the catalysis is the insertion of the C–N triple bond of the nitrile into the Os–B bond of an osmium-σ-borane intermediate and is regiodirected by the nucleophilicity of the nitrogen atom and the electrophilicity of the carbon atom of the nitrile.