GAS-PHASE HETEROAROMATIC SUBSTITUTION .12. REACTION OF FREE TRIFLUOROMETHYL CATION WITH SIMPLE 5-MEMBERED HETEROARENES IN THE GAS-PHASE

The reactivity features of radiolytically (Co-60-gamma-rays) generated unsolvated trifluoromethyl cation CF3+ toward pyrrole, N-methylpyrrole, furan, and thiophene have been measured under conditions, i.e. 7 Torr Of C2H4 added, ensuring depletion of concomitant radicalic processes. Experiments have been carried out at atmospheric pressure and in the presence of variable concentrations of a gaseous base (NMe3, 0-5 Torr). The mechanism of the ionic trifluoromethylation process and of the subsequent isomerization of the relevant ionic intermediates is discussed and the intrinsic positional selectivity of CF3+ ions evaluated. The preference of CF3+ for the beta-carbons of pyrroles (ca. 56%) and furan (ca. 78%) and the alpha-carbons of thiophene (ca. 59%) conforms to a substitution route proceeding via the classical donor-acceptor S(E)2 mechanism, ruling out alternative substitution pathways, including the two-step process advanced in the case of gas-phase acylation of the same substrates. The selectivity features of the CF3+ ion toward pyrroles appear to fit into Klopman's reactivity model, since determined essentially, but not exclusively, by its LUMO energy. The pronounced preference of CF3+ toward the beta-carbons of furan is discussed and compared with the general tendency of other gaseous alkyl carbocations, irrespective of their LUMO energy, to attack predominantly the alpha-carbons of the same substrate. It is concluded that such a tendency is due to the establishment of electrostatic interactions between the n-electrons of the oxygen of furan and the hydrogens of the alkylating electrophile, a process which is prevented in the case of CF3+.