Hyperconjugation summary

Summary 1-Ary 1-2-trialkyIsilyl-substituted vinyl cations are characterized in solution by NMR spectroscopy. The NMR chemical shift data reveal the stabilization of the positive charge by a (5-silyl substituent. The order of hyperconjugative stabilization of a positive charge by P-substituents is H < alkyl < silyl. The P-silyl effect is dependent on the electron demand of the carbocation and decreases with better electron donating a-substituents. NMR spectroscopy is a suitable tool to investigate the competition between 7i-resonance and CT-hyperconjugation in these type of carbocations. 

Summary The synthesis and characterization of a stable P-bis-silyl-substituted vinyl cation is reported. Its spectral properties and its experimental molecular structure clearly reveal the consequences of Si-C hyperconjugation. 

In summary, all the activating substituents and the weakly deactivating halogens are ortho-para directors (Table 19.1), whereas all substituents more deactivating than the halogens are meta directors. In other words, all substituents that donate electrons either by resonance or by hyperconjugation are ortho-para directors, whereas all substituents that cannot donate electrons are meta directors. 

In Summary Substituents that donate electrons by induction and hyperconjugation activate the benzene ring and direct electrophiles ortho and para their inductively electron-accepting counterparts deactivate the benzene ring and direct electrophiles to the meta positions. 

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