Chemical and photochemical oxidation of tetrahydrobetacarboline

Abstract

The photophysics and the mechanisms of the photochemical and chemical aromatization of 1,2,3,4-tetrahydro-7H-pyrido/3,4-b/indole (THBC) in 40% v/v methanol–water media have been investigated. The primary photophysical processes of THBC resemble those of indoles and related alkaloids. The photochemical oxidation has been carried out in the presence of atmospheric oxygen and light. The chemical oxidation has been studied using sodium peroxodisulphate (PDS) as the electrophilic agent. In both cases, strong acid media, i.e., sulphuric acid concentrations higher than 0.5 mol dm −3 are needed for the dehydroderivative (DH) to be formed. In the photochemical oxidation, the rate constants for the disappearance of THBC increase linearly with the concentration of acid and the intensity of the exciting radiation. However, the formation of DH only depends on the acidity of the media. In the chemical oxidation a similar behaviour is observed. In this case, the rate constants for the disappearance of THBC increase linearly with both PDS and acid concentrations, and the appearance of DH solely varies with the acid concentration. A two step mechanism is proposed for these oxidation reactions. In the first step, excited or ground state THBC reacts with ground state oxygen or PDS, respectively, to give an indolenine intermediate. This intermediate slowly rearranges, in a second acid catalysed step to yield DH.