HYDROBORATION WITH HALOBORANE/TRIALKYLSILANE MIXTURES

Trialkylsilanes or dialkylsilanes react rapidly with boron trichloride in the absence of ethereal solvents or other nucleophiles to form unsolvated dichloroborane. If no substrate is present, dichloroborane disproportionates to trichloroborane and two geometric isomers of chloroborane dimer, which in turn yield monochlorodiborane and, slowly but irreversibly, diborane. All of the B-H compounds in the mixture except diborane are highly active hydroborating agents. With alkenes in the presence of sufficient boron trichloride, the products are alkyldichloroboranes. These are free from detectable contamination by dialkylchloroboranes unless more than 1 mol of hydride is present. Similar hydroboration of terminal acetylenes can be controlled to yield either (E)-1-(dichloroboryl)alkenes or 1,1-bis(dichloroboryl)alkanes, each free from significant contamination by the other. Alkyldichloroboranes with trialkylsilanes at 25 degrees C produce alkylmonochloroboranes, detected by B-11 NMR. 1,1-Bis(dichloroboryl)alkanes similarly yield 1,1-diborylalkane dimers. An alkylmonochloroborane can hydroborate a second alkene to form a dialkylchloroborane. For this purpose, differing alkyl groups may be introduced in either order, regardless of their relative steric properties. With 2 mol of trialkylsilane, alkyldichloroboranes are converted to alkylborane dimers. Boron tribromide and its bromoborane derivatives behave similarly to the chloro compounds in the examples tested.