Carbenium ions, a-silyl-substituted

Gas-phase studies of a-silyl substituted carbenium ions show that these intermediates exist only in a very flat potential well (5, 7, 8, 9 ). They undergo fast 1,2-H or -alkyl shifts, producing the more stable silicenium or p-silyl substituted carbenium ions. 

The simplest a-silyl-substituted carbenium ion H3SiCH2+ (1) is not a minimum on the potential energy surface and it rearranges without a barrier to the more stable (by 49 kcalmol at MP4/6-31G(d)//3-21G(d)) silylium ion H2Si+Me (2). For a-silylethyl cation (3) and a-silylvinyl cation (4) calculations at the correlated MP2(fu)/6-31G(d,p) level predict hydrogen bridged structures just as for the parent ethyl and vinyl cations . 3 is separated by a barrier of only 5.2 kcalmol" from the ethylsilylium ion 5, which is by 25.9 kcalmol more stable [at MP2(fu)/6-31G(d,p)]. Similarly, the activation barrier for the 1,2-H shift from the a-silylvinyl cation (4) to the isomeric 1-silaallyl cation (6) is... 

Silyl-substituted carbenium ions have attracted considerable experimental interest because they are believed to be intermediates in electrophilic additions to vinyl, ethynyl and aryl silanes, in solvolytic reactions and in cationic cyclization reactions1 -4,322. Despite this wide interest, knowledge concerning the effect of silyl substitution on the stabilities of carbenium ions was rather qualitative and only recently more quantitative data became available. Theoretical studies centered mainly around a- and / -silyl-substituted carbenium ions, but y-silyl effects have been also studied. 

The effect of a-silyl substitution on the stability of a carbenium ion was qualitatively unclear for a long time. Early solvolytic studies by the groups of liaborn36 and Cartledge37 suggest a destabilizing effect of a-silyl substitution compared with alkyl. The measurement and interpretation of the kinetic a-silicon effect in solvolysis reactions is, however, often complicated by the fact that steric and ground state effects may play an important role and that, in addition, the rates of ionization often involve a contribution from nucleophilic solvent assistance. 

The different reaction course followed by the silyl- and alkyl-substituted allenes is fully consistent with the hyperconjugation model, which predicts that a /i-silyl group stabilizes carbenium ions better than a /3-alkyl group, and with previous conclusions that a-silyl substitution in carbocations is destabilizing relative to a-methyl substitution. The larger space requirements of the trimethylsilyl groups compared with methyl groups may also contribute to the kinetic stability of 413. 

Apeloig, Y. Stanger, A. Are carbenium ions stabilized or destabilized by a-silyl substitution The solvolysis of 2-(trimethylsilyl)-2-adamantyl p-nitrobenzoate. J. Am. Chem. Soc. 1985, 107, 2806-2807. 

Stone and coworkers determined the /6-silicon effect in a-alkyl- and aryl-substituted carbenium ions and vinyl cations by measuring in a high-pressure mass spectrometer the thermodynamic data for the association of various alkenes and alkynes with trimethylsilylium ion. From their measured thamodynamic data they calculated, by using equations 13 and 14, the /S-silyl stabilization enagies hsted in Table 1. 

The presence of group 4 metals P to a carbenium ion has a dramatic effect on the stability of the carbenium ion, a remarkable phenomenon known as the P-effect.12 29, 30 The silicon P-effect was discovered in 1937 by Ushakov and Itenberg,31 who noted the exceptional reactivity of P-silyl-substituted halides R3SiCH2CH2X towards elimination (Scheme 1). The effect has since been the subject of many mechanistic32-36 and theoretical studies37-39 and several reviews.10-12,29... 

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