Gas-phase heteroaromatic substitution. 10. Reaction of free phenylium ion with simple five-membered heteroarenes in the gaseous and liquid phase

Abstract

Phenylium ion, obtained from the spontaneous β decay of 1,4-ditritiobenzene, has been allowed to react with pyrrole, N-methylpyrrole, furan, and thiophene, in both the gaseous and liquid phases. The differences between the reactivity pattern of phenylium ion in the two environments can be essentially reduced to significant ion-neutral electrostatic interaction in the gas phase and to the much greater efficiency of collisional stabilization in the condensed phase, allowing a larger fraction of the excited ionic intermediates, from the highly exothermic attack of phenylium ion on the aromatic substrate, to survive dissociation and isomerization. The mechanism of the phenylation process and of the subsequent isomerization of the relevant ionic intermediates is discussed and the intrinsic substrate and positional selectivity of the phenylium ion evaluated. While the limited substrate discrimination of phenylium ion fully agrees with its well-known exceedingly high reactivity, its pronounced affinity toward the α carbons of the selected heteroarenes does not conform with the relatively "hard" character of the reactant, expected on the grounds of its STO-3G calculated LUMO energy. The conceivable occurrence of an intimate entropy-favored two-step addition mechanism, involving a preliminary single-electron transfer (SET) from the heteroaromatic substrate to the ionic electrophile, which is thermochemically allowed only for phenylium and methyl cations and prevented for other alkylating electrophiles, is discussed. Key words: gas-phase ion chemistry, electrophilic heteroaromatic substitution, nuclear decay chemistry, phenylium ion, electron transfer.