Gas-phase acylation reactions. Acetylation of phenols by radiolytic acetylium (CH3CO+) ions

The gas-phase acetylation of representative phenols by CH3CO+ ions from the γ-radiolysis of CH3F/CO mixts. has been investigated in the pressure range from 380 to 760 Torr, and in the presence of a gaseous base (NH3). At the higher system pressures, the competition between the n-type and the π-type nucleophilic centers of the ambident substrates is kinetically controlled and biased in favor of O-acetylation, as demonstrated by the predominant (from ∼80 to ∼97%) formation of aryl acetates with respect to the isomeric hydroxyacetophenones. The isomeric compn. of the ring-acetylated products reflects the electrophilic character and the remarkable positional selectivity of the CH3CO+ ion, which yields predominantly o- and p-hydroxyacetophenones, in proportions depending on the total pressure and on the presence of NH3. In fact, O- as well as o-acetylated substrates, are more stable than the other isomeric ionic intermediates, as indicated by their increased yields at the lowest total pressure used (380 Torr), under conditions of increased thermodn. control of products. The results of competition expts., carried out in the presence of mesitylene as the ref. substrate, allow detn. of a gas-phase acetylation reactivity scale from p-xylene to 2,6-xylenol, this scale is characterized by values ranging over 3 orders of magnitude. Comparison of the gas-phase acetylation of CH3CO+ ions with related reactions occurring in soln. reveals no basic mechanistic differences, the gas-phase reactivity and selectivity of free acetylium ions displaying significant analogies with those of acetylating reactants generated in low-polarity media by the action of poorly co-ordinating Friedel-Crafts catalysts on suitable precursors.