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.