Flash photolytic generation and study of ketene and carboxylic acid enol intermediates formed by the photolysis of diazonaphthoquinones in aqueous solution

Article abstract of DOI:10.1021/ja00076a019

Flash photolysis of each of five diazonaphthoquinones (1-diazo-2(1H)-oxonaphthalene-4-sulfonate, 2-diazo-1 (2H)-oxonaphthalene-4-sulfonate, 2-diazo-1(2H)-oxonaphthalene-5-sulfonate, phenyl2-diazo-1(2H)-oxonaphthalen-4-sulfonate, and phenyl 2-diazo-1(2H)-oxonaphthalene-5-sulfonate) in aqueous solution was found to produce two short-lived intermediates preceding the ultimate indenecarboxylic acid reaction products. Decay of the first of these intermediates is catalyzed weakly by hydroxide ion but not by dilute perchloric acid nor by acetic acid buffers, and its uncatalyzed reaction shows only weak solvent isotope effects; this serves to identify this intermediate as the ketene formed by photo-Wolff rearrangement of the diazonaphthoquinone. Decay of the second intermediate is catalyzed by perchloric acid in dilute acid solutions, with saturation of this catalysis occurring in more concentrated acid, and it shows general acid catalysis in acetic acid buffers. The perchloric acid catalyzed reaction gives an appreciable solvent isotope effect in the normal direction (k_H/k_D > 1), which increases in magnitude as this catalysis becomes saturated. This serves to identify this intermediate as the indenecarboxylic acid enol formed by hydration of the first intermediate. The form of acid catalysis by perchloric acid and the change in isotope effect indicates that this enol ketonizes through its enolate ion, with a shift of initial state from enol to enolate as the acidity of the medium is decreased; analysis of the kinetic data shows the enols to be rather strong acids, with pK_a = 0.4-1.3.