A comparative study of gas phase aromatic desilylation and detertbutylation by charged electrophiles

Aromatic desilylation and detertbutylation by CH5+, C2H5+, i-C3H7+, and t-C4H9+ ions have been studied in the gas phase by mass spectrometric and radiolytic techniques. The higher rate of desilylation than of dealkylation is traced to the step involving the formation of ipso arenium ions, protonated at the ring carbon bearing the SiMe3 or the CMe3 substituent. The latter has been shown by ab initio calculations at the SCF STO-3G level to selectively depress the basicity of the ipso position, hence the stability of the corresponding arenium ion relative to its protomers, e.g. the basicity of the ipso position of PhCMe3 is lower by ca. 11 kcal mol−1 than of the para position. The SiMe3 group has no comparable effect, very close basicities being computed for the para and the ipso position of PhSiMe3. The different effects of the CMe3 and SiMe3 groups are traced to electronic and steric factors.