A variety of alkenylsilanes underwent transmetallation with BBr3 to afford the corresponding alkenyldibromoboranes in solution. Similarly, the transmetallation of alkynylsilanes with BCl3 afforded alkynyldichloroboranes. A number of the alkenyl-and alkynylboranes are novel and useful Diels—Alder dienophiles. An ionic mechanism for the transmetallation is proposed.
