Silyl group hyperconjugation with

The destabilizing effect of a silyl group compared with an alkyl group in trivalent carbocations was explained by the weaker hyperconjugation of the Si-R a-bond (R = alkyl) relative to a C-R cr-bond (R = H or alkyl) and by electrostatic repulsion between the adjacent positively charged cationic carbon and the electropositive silicon (10). 

The NMR spectroscopic data of the -sUyl-substituted a-mesitylvinyl cations corroborate the hyperconjugative charge-delocalizing ability of -silyl groups. Comparison with -alkyl- and -H-substituted a-mesitylvinyl cations gives a measure of the magnitude of the -silyl effect in these type of carbocations. The NMR spectroscopic data of the a-mesitylvinyl cations 372-375 and 378, 383-385 with various yS-substituents together with data for the sp -hybridized mesityl carbocations 386 and 387 are summarized in Table 15. 

As discussed in Section II. A, theoretical studies predicted that phospha-silenes with silyl substituents attached to phosphorus should have planar, trigonally coordinated silicon, with the Si—P w-bonds strengthened by the hyperconjugative influence of the silyl group.16 Recently, this was proved by a single-crystal X-ray structure determination of the derivative... 

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. 

The electron affinities of a number of a-silyl substituted silyl and carbon radicals were determined in photodetachment experiments and confirmed by data obtained from ab initio calculations. The authors conclude in this study that the stabilization a carbanion experiences through a-silyl substitution is approximately 14-20 kcalmol-1 per silyl group that of a silyl anion is approximately 6-14 kcal mol-1. The larger stabilization in the carbanionic systems is readily explained by stronger hyperconjugation of the anionic carbon center with the silyl groups as compared to that of the silyl anion with a silyl group. 

In p-silyl substituted benzyl cations with different a-aryl substituents the stabilizing effect of a P-silyl group depends on the electron donating ability of the a-aryl group. The better the a-aryl substituent can stabilize the positive charge, the lower is the need for P-o-hyperconjugative stabilization by the silyl group and the no-bond resonance structure becomes less important (Fig. 6). 

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