Intramolecular Reaction of Silanol and Triarylborane: Boron−Aryl Bond Cleavage and Formation of a Si−O−B Heterocyle

o-(Hydroxydimethylsilyl)(dimesitylboryl)benzene (1) underwent intramolecular cyclization to give 5,5-dimesityl-2,2-dimethyl-3,4-benzo-1,2,5-oxasilaboracyclopentene (4) and mesitylene by boron−aryl bond cleavage. Whereas triethylamine as an additive accelerated the cyclization, DBU stabilized a silyloxyborate complex, 1,8-diazabicyclo[5.4.0]-7-undecenium 5,5-dimesityl-2,2-dimethyl-3,4-benzo-1,2,5-oxasilaboratacyclopentene, which was characterized by X-ray crystallographic analysis and HF calculations. The deuterium-labeling experiments revealed that the hydrogen migrated from the hydroxyl group in 1 to the leaving mesitylene.