HYDROBORATION WITH HALOBORANE TRIALKYLSILANE MIXTURES/

Trialkylsilanes or dialkylsilanes react rapidly with boron trichloride in the absence of ethereal solvents or other nucleophiles to form uns olvated dichloroborane. If no substrate is present, dichloroborane dis proportionates to trichloroborane and two geometric isomers of chlorob orane dimer, which in turn yield monochlorodiborane and, slowly but ir reversibly, diborane. All of the B-H compounds in the mixture except d iborane are highly active hydroborating agents. With alkenes in the pr esence of sufficient boron trichloride, the products are alkyldichloro boranes. These are free from detectable contamination by dialkylchloro boranes unless more than 1 mol of hydride is present. Similar hydrobor ation 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 tri alkylsilanes at 25 degrees C produce alkylmonochloroboranes, detected by B-11 NMR. 1,1-Bis(dichloroboryl)alkanes similarly yield 1,1-diboryl alkane dimers. An alkylmonochloroborane can hydroborate a second alken e to form a dialkylchloroborane. For this purpose, differing alkyl gro ups may be introduced in either order, regardless of their relative st eric properties. With 2 mol of trialkylsilane, alkyldichloroboranes ar e converted to alkylborane dimers. Boron tribromide and its bromoboran e derivatives behave similarly to the chloro compounds in the examples tested.