Gas-phase alkylation of phenyltrimethylsilanes. Using the trimethylsilyl group to probe proton shifts in gaseous arenium ions

Article abstract of DOI:10.1021/ja00016a003

The reactivity of (trimethylsilyl)benzene (TSB) and (trimethylsilyl)toluenes (TST) toward i-C₃H₇⁺ and (CH₃)₂F⁺ has been studied in the gas phase in the interval from 0.6 to 3040 Torr and from 37.5 to 100 °C by mass spectrometric and radiolytic techniques. The systematic investigation of the dependence of the relative rate of the alkylation and alkyldesilylation processes promoted by i-C₃H₇⁺ on the nature and the concentration of gaseous bases has allowed an evaluation of the rate constant of the isomerization via proton 1,2-shifts of the alkylated adducts from TSB into the corresponding ions protonated ipso to the SiMe₃ group. The results, i.e., k_i = 1.6 X 10⁹ s^-1 at 310 K and E_i? ≈ 9.5 kcal mol^-1, represent the first experimental evaluation of the kinetic parameters of proton 1,2-shifts within gaseous arenium ions, generally too fast to be monitored by mass spectrometric techniques, and are in excellent agreement with theoretical and solution-chemistry studies of the model C₆H₇⁺ ion. The selectivity of the alkylation of TSB and of m- and p-TST is discussed, particularly as regards direct ipso substitution and the steric hindrance to deprotonation of the ortho-isopropylated adducts.