Radiolytic Silylation of Alkenes and Alkynes by Gaseous Me3Si+Ions. Stereochemical Evidence for the B-Silyl Effect.

Carbocation intermediates stabilized by a beta silyl group have been characterized, using the silylation of alkenes by R3Si+ ions as a route of formation. Neutral silylated products have been obtained from the reaction of R3Si+ ions, generated in a gaseous medium at atmospheric pressure by a radiolytic technique, with selected alkenes, alkynes, and allene, thereby indicating the occurrence of electrophilic silylation. Notable features of the charged silylated intermediates emerge from the isomeric product distribution. The silylation of cis- and trans-2-butene shows a high degree of retention of configuration, as expected if a bridged species (I) were the reaction intermediate. Alternatively, the intermediacy of an open structure (II), whereby C-C bond rotation is inhibited by the hyperconjugative interaction between the â silyl group and the vacant p orbital, should be inferred. The charged intermediates from the silylation of alkenes and alkynes are found to be unreactive toward conceivable isomerizations to more stable species, such as the ones bearing the positive charge on silicon. Stereoelectronic factors affect the deprotonation of the silylated intermediates, which may involve loss of the proton from either the alfa or the gamma position with respect to the silylated carbon. A comparison of the reactivity of alkenes and alkynes in the cationic silylation reaction is presented.