Nature of nitrenium:carboxylate ion pair intermediates in the hydrolysis of O-aroyl-N-acetyl-N-(2,6-dimethylphenyl)hydroxylamines

Abstract

O-Aroyl-N-acetyl-N-(2,6-dimethylphenyl)hydroxylamines [aroyl = benzoyl (1a), 3-nitrobenzoyl (1b), and pentafluorobenzoyl (1c)] are solvolysed in aqueous solutions by rate-limiting ionization to nitrenium : carboxylate ion pair intermediates. These in part collapse at the ortho position to give unstable 1,5-dimethyl-5-aroyloxy-6-acetyliminocyclohexa-l,3-dimes 2 that react further as described in the previous paper. The ion pairs from 1 also proceed directly to products of substitution para to the acetylamino group—4-aroyloxy-2,6-dimethylacetanilide 5, a product of internal return, and 4-hydroxy-2,6-dimethylacetanilide 6, a product of water addition. These same products also arise via ionization of 2. The ratio 5:6 obtained directly from 1 is significantly lower than that from 2, demonstrating that 1 and 2 do not ionize to exactly the same ion pairs. Experiments with 1a in the presence of bromide show that the yield of the cyclohexadiene is unaffected, while the yields of 5 and 6 are decreased, albeit to different amounts. Two new products, 4-bromo-2,6-dimethylacetanilide and 2,6-dimethylacetanilide, are observed in their places. Experiments with 1c in acid solutions demonstrate that the yield of cyclohexadiene can be decreased by H⁺, by protonation of the carboxylate ion in the ion pair. Using the H⁺ reaction as a clock, the lifetime of this ion pair, the initial ion pair in the ionization of 1, is calculated as ca. 10 ps. Thus this ion pair is too short-lived to react with external nucleophiles, and probably also with solvent. The trapping data for the p-ester 5 are shown to be inconsistent with a mechanism where a single ion pair serves as precursor, and this product is proposed to arise in part from a short-lived ion pair, and in part from a longer-lived one. The latter ion pair is probably also the species that gives rise to the p-phenol 6 by reaction with water. Using the bromide reaction as the clock, this ion pair is shown to have a lifetime of 0.25–0.50 ns. A number of mechanistic models incorporating these features are consistent with the experimental results, and two of these are discussed. Whatever the mechanism a minimum of three shortlived ion pair intermediates is required.