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.