A Crystalline Boravinylidene from an Arrested Hydroboration Reaction

Boravinylidenes are postulated intermediates in 1,1-hydroboration and 1,1-carbaboration reactions of alkynes en route to vinyl boranes. Unlike transition metal-vinylidene complexes, boravinylidenes have hitherto, not been crystallographically characterized. Pairing 1,2-bis(trimethylsilyl)ethyne with an extremely potent hydroboration reagent, bis(1-methyl-ortho-carboranyl)borane, enabled crystallization and characterization of a previously speculated boravinylidene that features a vinyl cation with an adjacent borohydride. In solution, multinuclear NMR experiments validate the boravinylidene in equilibrium with the starting materials. Density functional theory calculations suggest the bulk of the bis(carboranyl)boryl unit and hyperconjugative interactions from the beta-silyl groups and the BH bond facilitate the stabilization of this unusual species. Reaction of the feature compound with nitriles triggered an insertion into one of the C–Si bonds and a hydride shift to furnish an iminium-tethered borataallene.