Trimethylsilyl cation, obtained in the gas phase from the y-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 beta C of pyrrole (70%) and N-methylpyrrole (75%), the oxygen of furan, and the alfa C of thiophene (loo%), 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),Si+, if compared to the 2p LUMOs of alkylating carbocations.
Gas Phase Heteroaromatic Substitution. Silylation of Simple Five-Membered Heteroaromatic Rings by Trimethylsilyl Cations / Crestoni, Maria Elisa; Fornarini, Simonetta; Speranza, Maurizio. - In: JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. - ISSN 0002-7863. - STAMPA. - 112:(1990), pp. 6929-6935. [10.1021/ja00175a029]