Radiolytic Silylation of Alkenes and Alkynes by Gaseous R₃Si⁺ Ions. Stereochemical Evidence for the β-Silyl Effect

Abstract

Carbocation intermediates stabilized by a β silyl group have been characterized, using the silylation of alkenes by R 3 Si + ions as a route of formation. Neutral silylated products have been obtained from the reaction of R 3 Si + 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 β 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 R or the γ position with respect to the silylated carbon. A comparison of the reactivity of alkenes and alkynes in the cationic silylation reaction is presented.

Nucleogenic silylium cations and their analogs

“…To the best of our knowledge only the silylation of ethylene with the Me 3 Si þ ions, produced by a radiolytic technique, was reported [28]. In our studies of the interaction of ethylsilylium ions generated in situ with ethylene the tritiated vinylsilane was detected, however, the yield of this compound does not exceed 10%.…”

Factors Controlling the Alkyne Prins Cyclization: The Stability of Dihydropyranyl Cations

“…The origin of this effect is more commonly attributed to the strongly stabilizing interaction between the CSi bond orbital and a developing or fully formed empty p orbital of the carbenium ion at the β‐position to the silicon atom 19. The structure of the postulated intermediate species could be described as either a bridged structure I , in which the silicon atom exploits the ability to expand its coordination, or a β‐silylated carbenium ion II (Scheme ) 20…”

“…[19] The structure of the postulated intermediate species could be described as either a bridged structure I, in which the silicon atom exploits the ability to expand its coordination, or a bsilylated carbenium ion II (Scheme 8). [20] Vinyl cations are especially well suited for the study of bhyperconjugation. The C + =C b bond is shorter than a single bond and the s bond of a b-substituent to the C b atom is in the plane of the vacant 2p orbital on the C + atom, thus allowing maximum overlap for hyperconjugation.…”

Communication: Infrared spectroscopy of protonated allyl-trimethylsilane: Evidence for the β-silyl effect

“…[1][2][3] The β-silyl effect on carbocations has been extensively studied, both experimentally and computationally. [4][5][6][7][8][9][10] Carbocations bearing a β-silyl group are significantly stabilized relative to unsilylated analogues. The stabilization is markedly orientation-dependent, due to the hyperconjugative interaction between the Si−C β σ -bond and the formally empty 2p(C α + ) orbital as represented in structure A in Figure 1.…”