Chromium(VI) catalyzed oxidation of indole in aqueous acetic acid

Abstract

The catalytic activity of picolinic acid, 1,10-phenanthroline, oxalic acid, 2,2'-bipyridyl and ethylenediamine tetracetic acid have been investigated in the chromium(VI) oxidation of indole in aqueous acetic acid medium. The catalysts increase the electrophilic activity of the oxidant and the Cr(VI)-Cat complex is believed to be the reactive electrophile. The proposed reaction pathway involves the electrophilic attack of the Cr(VI)-Cat complex at the ethylenic bond of the indole ring to give an indoleninic intermediate. The reaction is routed through via a termolecular complex, Cr(VI)-Cat-IND, the decomposition of which in slow rate determining step is envisaged to explain the reactivity. In presence of the catalyst, indole is oxidized by chromium(VI) to give oxindole as the major product, under most conditions. The conversion is favoured in a medium of low dielectric constant. The failure to observe inhibition in the presence of acrylonitrile suggests the non-radical mechanistic pathway. The ratio, k(D2O)/k(H2O) ≈ 1, does not seem to indicate any significant solvent isotope effect. Cobalt(III) enhances the reactivity while pyridine and chromium(III) depress the reactivity. The relative catalytic activity for indole oxidation in the presence of different catalysts reveals that picolinic acid is the best catalyst under the experimental conditions while activity of EDTA is minimum. A detailed interpretation of the redox mechanism based on the available kinetic data has been proposed.