GAS-PHASE HETEROAROMATIC SUBSTITUTION .9. SILYLATION OF SIMPLE 5-MEMBERED HETEROAROMATIC RINGS BY TRIMETHYLSILYL CATIONS

Trimethylsilyl cation, obtained in the gas phase from the Γ-radiolysis of CH4/(CH3)4Si mixtures, has been allowed to react with pyrrole, N-methylpyrrole, furan, and thiophene, both neat and in competition with toluene. Experiments have been carried out at pressures around 620-720 Torr and in the presence of variable concentrations of a gaseous base (NEt3 0-10 Torr). The mechanism of the silylation process and of the subsequent isomerization of the relevant ionic intermediates is discussed and the intrinsic substrate and positional selectivity of the (CH3)3Si+ ions evaluated. The poor substrate discrimination of (CH3)3Si+ as well as its tendency to attack preferentially the substrate positions with the highest net negative charge, i.e. the Cβ's of pyrrole (70%) and N-methylpyrrole (75%), the oxygen of furan, and the Cα's of thiophene (100%), characterize the reaction as dominated by extensive electrostatic interactions within the encounter pair. The pronounced site discrimination of (CH3)3Si+ toward pyrroles ranks it as a “hard” electrophile, but less “hard” than expected on the grounds of its calculated LUMO energy. A plausible explanation is found in the much larger 3p LUMO of (CH3)3Si+, if compared to the 2p LUMOs of alkylating carbocations. © 1990, American Chemical Society. All rights reserved.