The reactions of indoles in aqueous solution initiated by one-electron oxidation

Abstract

By pulse or gamma radiolysis in the presence of Br^–, 1-methylindole (1b) is oxidized to its radical cation (2b), which adds to (1b) to form a radical dimer (8b). Species (2b) also reacts with OH^– to yield the pseudo-base, proposed to be the C₂-OH adduct (4b). Identical end-product yields and spectral comparison reveal that the addition of OH˙ radicals to OH towards the C₂ position appears to apply to the other indoles as well. The neutral indolyl radicals (3) disappear both by coupling and dismutation. Discrimination between these models is governed buy the subtituent. A methyl group at the C₃ position favours C₃–C₃′ coupling. Molecular oxygen adds to neutral indolyl radicals at the C₃ position. The measured and estimated rates of O₂ addition relate to the redox potentials of the indolyl radicals. The reaction between O₂^–˙ and the indolyl radical cation (2) produces (1) and O₂. On the other hand, O₂^–˙ couples to (3) to yield C₃-OOH hydroperoxides (14). The latter decompose into C₂–C₃ opened amides (13) with concomitant chemiluminescence.