Synthesis of a Borylborenium Ion via Direct Dehydrocoupling of Two B–H Bonds

Abstract Dehydrocoupling of hydroboranes provides a highly efficient method for the synthesis of diboron species. While this process generally proceeds in the presence of transition metals or Lewis bases, direct dehydrocoupling of two hydroboranes to form a B–B bond is still unknown. Herein, we report that an in situ-generated dihydroborenium ion can readily undergo dehydrocoupling with an intramolecular o-carboranyl moiety to afford a borylborenium ion. Theoretical studies demonstrate that this seemingly symmetry-forbidden process is enabled by Lewis acid-mediated umpolung of the B–H bond of o-carboranyl moiety. The resulting borylborenium ion is a powerful electrophilic borylation reagent and readily borylates a number of unactivated arenes, such as dihaloarenes, at room temperature.