Mesityl-vinyl cation

The C NMR chemical shift of the para-carbon can be used as a probe to monitor the effect of different p-substituents on the n-conjugative charge delocalization. Comparing the para-carbon shift of a-mesityl vinyl cations Mes-C =CH(R) with different P-substituents (R = H, 180.0 ppm rbutyl, 178.5 ppm Si(iPr)3, 168.5 ppm) shows that P-Si-C hyperconjugation is more efficient than P-C-H or P-C-C hyperconjugation. 

Nucleophilic additions to mesitylphenylketene [Ph(Mes)C=C=0, Mes = 2,4,6-Me3C6H2] and the related vinyl cation, Ph(Mcs)C=d)Mcs, proceed as if the mesityl group was effectively smaller than the phenyl group.20 The effect is explained by calculations that show that the phenyl is coplanar with the carbon-carbon double bond, while the mesityl is twisted the in-plane nucleophilic attack prefers the mesityl side. 

The Cjg—Si bond fragmentation in cation 401 occurs already at temperatures where the corresponding 1-mesityl-, l-(p-tolyl)- and 1 -phenylvinyl-/l-(tri isopropyl Jsi lyl-substi tutcd vinyl cations 375, 389a and 393 are stable. The facile formation of the 1 (p-anisyl)vinyl cation 403 can be ascribed to a second protonation of 401 at the oxygen of the anisyl group. 

The vinyl cations were characterized by H, C, and Si NMR spectroscopy at low temperatures. Fig. 1 shows as a typical example the NMR spectrum of the l-mesityl-2-(triisopropyl)silyl vinyl cation 3. 

Silyl-substituted carbocations corresponding a-mesityl-substituted vinyl cation 372. 

---