Transient bis(carboranyl)boryl anions

The reduction of bromoboranes with two ortho-carborane substituents results in the generation of transient boryl anions. DFT computations reveal that their ground state is significantly different from previously reported examples as an unusual linear singlet engendered by negative hyperconjugation stabilization by the carboranes. The substitution on the ortho-carbon of the carborane is influential on the fate of the anion with intramolecular C-H insertion and cyclization to generate five-membered boracycles occurring with phenyl or trimethylsilyl groups in this position. Gratifyingly, methyl substitution enables intermolecular in situ boryl anion reactivity as a nucleophile to generate a copper-boryl complex and in a cycloaddition with tetrachloro-o-benzoquinone.