RADIOLYTIC SILYLATION OF ALKENES AND ALKYNES BY GASEOUS R3SI- STEREOCHEMICAL EVIDENCE FOR THE BETA-SILYL EFFECT( IONS )

Carbocation intermediates stabilized by a beta silyl group have been c haracterized, 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 p ressure 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-b utene shows a high degree of retention of configuration, as expected i f 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 beta sil yl group and the vacant p orbital, should be inferred. The charged int ermediates from the silylation of alkenes and alkynes are found to be unreactive toward conceivable isomerizations to more stable species, s uch as the ones bearing the positive charge on silicon. Stereoelectron ic factors affect the deprotonation of the silylated intermediates, wh ich may involve loss of the proton from either the alpha or the gamma position with respect to the silylated carbon. A comparison of the rea ctivity of alkenes and alkynes in the cationic silylation reaction is presented.