TRIMETHYLSILYL AND RELATED CATIONS IN SOLUTION

The silicon analogue of the tert-butyl cation, Me3Si+ (the trimethylsilyl cation), has been produced as its perchlorate in high-polarity, low-nucleophilicity solvents such as sulfolane and dichloromethane. Generation is carried out by treatment of trimethylsilane with trityl perchlorate. Ionicity is demonstrated (and covalency excluded) in dilute sulfolane by conductance, cryoscopic molecular weight, and multinuclear NMR experiments. The level of hydrolysis from residual water is established by direct observation of the H-1 resonances of Me3SiClO4 and of the hydrolysis product Me3SiOSiMe3. The water level is about 1 order of magnitude lower than that of substrate. Although the ions are free in dilute sulfolane, they are associated with the perchlorate anion in more concentrated solutions of sulfolane and at all concentrations in dichloromethane. Analogous experiments were carried out for the phenyldimethylsilyl (Me2PhSi+) and methyldiphenylsilyl (MePh2Si+) cations. The phenyldimethylsilyl cation also was prepared with the tetraphenylborate anion, which cannot be covalently bound to the cation.