Aromatic substitution in the gas phase. Ambident behavior of phenol toward t-C4H9+ cations.

The reactivity of phenol toward t-C4H9+ cations, obtained in the dilute gas state from the y-radiolysis of neopentane, is characterized by a typical ambident behavior. The competition between the n-type and the a-type nucleophilic centers of the substrate is kinetically biased in favor of 0-alkylation, leading to the predominant (ca. 8:l) formation of tert-butyl phenyl ether with respect to tert-butylphenols under conditions, i.e., high neopentane pressure and the presence of a gaseous base (NH3), which favor kinetic control of products. Ring-alkylated arenium ions appear, however, to be more stable than the isomeric oxonium ion, as indicated by the higher yields of tert-butylphenols under conditions of increasing thermodynamic control of products, reaching a 4.1 excess over the ether at the lowest pressure investigated (20 Torr). The results of competition experiments show that phenol reacts faster than toluene with gaseous t-C4H9+, the apparent kphenOl:ktoluene ratio ranging from 2.0 to 4.4. The isomeric composition of ring-alkylated products underlines the electrophilic character and the remarkable positional selectivity of the t-C4H9+ attack, yielding exclusively ortho- and para-substituted isomers, whose ratio depends on the pressure of the system and the presence of NH3. Thus, the ortho:para ratio, that reaches a 0.4 value under conditions of kinetic control of products, decreases as the pressure is lowered, down to the 0.04 value measured at 20 Torr. The mechanism of the gas-phase alkylation, and of the subsequent isomerization processes that appear of intermolecular nature, are discussed and compared with those of related alkylation reactions, both in the gas phase and in solution.