Five-membered cyclic silylboranes, 2-(diisopropylamino)-1,1-diorgano-1-sila-2-boracyclopentanes, were synthesized via intramolecular reductive B-Si bond formation with 3-[chloro(diisopropylamino)boryl]-1-(chlorodiorganosilyl)-propane. The cyclic silylboranes were thermally stable and could be distilled under a vacuum. The reaction of the cyclic silylboranes with alcohols resulted in a cleavage of the