Selective B(4)-H Alkoxylation of o-Carboranes with Alcohols via Heteroleptic Cu(I)-Photocatalyzed 1,5-HAT-Enabled Boron Radical-Polar Crossover.

Carboranes are an important class of boron cluster compounds with broad applications in medicine, materials science, and catalysis. However, the site-selective functionalization of their B-H bonds remains a long-standing challenge owing to the nearly identical chemical environments and high bond dissociation energies (BDEs) of these bonds. Traditional approaches generally rely on precious metal catalysts, harsh reaction conditions, or substrate prefunctionalization, which significantly limit their synthetic utility. Herein, we report the first direct B-H/O-H cross-coupling of o-carboranes with alcohols, enabled by heteroleptic Cu(I) complexes under mild photocatalytic conditions. This protocol achieves highly regioselective B(4)-H alkoxylation with a broad substrate scope and excellent functional group tolerance, providing a facile route to diverse carborane-based building blocks. Mechanistic studies reveal that the benzoyl directing group effectively amplifies the difference in BDEs between B(3)-H and B(4)-H, thereby enabling selective activation of the B(4)-H bond via 1,5-hydrogen atom transfer (HAT). Notably, for the first time, we demonstrate that a boron radical-polar crossover process mediated by the heteroleptic copper catalyst converts boron radicals into highly electrophilic boron cation intermediates, which can be directly trapped by weakly nucleophilic alcohols to form B-O bonds. This work establishes a new and general strategy for carborane B-H functionalization via radical pathways.