Silicon-carbon hyperconjugation

Provided that the silicon-carbon bond can be coplanar with the vacant p orbital, the /J-silyl substituted carbocation should be stabilized by hyperconjugation, and this has been demonstrated by Kresge and coworkers47,49. 

The crucial influence on the reactivity pattern in both cases is the very high stabilization that silicon provides for carbocationic character at the p-carbon atom. This stabilization is attributed primarily to hyperconjugation with the C-Si bond (see Part A, Section 3.4.1).85... 

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 P-effect, the interaction of a P-carbon silicon a-bond with the vacant p-orbital at the C+ carbon, is called hyperconjugation i.e. the conjugation of the... 

Hyperconjugation should raise the bond order between the ipso and benzylic carbons, and lower the bond order between the benzylic carbon and the silicon atom. As the ipso-benzylic bond length decreases, the 13C-13C coupling constant should increase, and as the benzylic carbon-silicon bond length increases, the 13C-29Si coupling constant should... 

The trimethylsilyl substituent in 2-(trimethylsilyl)allyl cations is enforced into a perpendicular conformation and thus fi-C—Si hyperconjugative stabilization is essentially not possible. The carbocationic centre Cl (<513C = 295.0 ppm) and the terminal methylene carbon C3 of the allyl system (<513C = 166.6 ppm) in 348a are deshielded by 11.5 and 1.2 ppm, respectively, compared with the silicon-free analogue 348 (613C = Cl 283.5 C3... 

Treatment of aryl silane 10 with the iodonium source IC1 in an iododesilylation reaction yields compound 44, which proceeds by an ipso substitution mechanism.9 Activation towards electrophilic attack arises from stabilization of resonance form 45 by Si. Silicon is arranged / to the positive charge and the carbon-silicon bond can overlap with the empty Jt orbital (hyperconjugation). 

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