A Crystalline Boravinylidene from an Arrested Hydroboration Reaction

Boravinylidenes are intermediates in 1,1-hydroboration reactions of alkynes en route to vinyl boranes. Unlike transition metal-vinylidene complexes, boravinylidenes have hitherto, not been crystallographically characterized. Pairing bis(trimethylsilyl)ethyne with an extremely potent hydroboration reagent, bis(1-methyl-ortho-carboranyl)borane, enables crystallization and characterization of a boravinylidene. Solution multinuclear NMR experiments validate the boravinylidene and reveal it is in equilibrium with the starting materials. Density functional theory calculations indicate hyperconjugative interactions from the β-silyl groups and B-H bond stabilize this unusual species. Reaction of the feature compound with nitriles triggered an insertion and rearrangement to furnish an iminium-tethered borataallene.