Aromatic substitution in the gas phase. Predominant O-alkylation in the attack of tert-butyl(1+) ions to anisole.

The reaction of anisole with t-C4Hg+ ions, obtained in the dilute gas state from the y-radiolysis of neopentane, yields exclusively 0- and p-tert-butylanisole. Both the reactivity of anisole relative to toluene and the isomeric distribution of its alkylated products depend markedly on the composition, and especially on the pressure, of the gaseous reaction environment. The apparent kanisole:ktoluernaeti o increases from 0.8, in neat neopentane at 720 Torr, to 4-5 in neopentane at 20 Torr containing a few mol % of a gaseous base, e.g., NH3. Concurrently, the '/* ortho:para ratio decreases from 1.3 at 720 Torr to <0.2 at 20 Torr. These results, and the effects of gaseous additives, including NH3, (CH3)3N, and (CH3)2O, are consistent with a mechanism involving kinetically predominant 0-alkylation of anisole, and subsequent isomerization of the excited dialkylaryloxonium ions to thermodynamically more stable para-alkylated arenium ions. The substrate and positional selectivity of the gas-phase tert-butylation, and the mechanism of isomerization, involving intermolecular alkylation of the para position of anisole by the dialkylaryloxonium ion, are discussed and compared with the gas-phase data concerning tert-butylation of phenol, and with those of related alkylation reactions occurring in solution.