Kinetics and mechanism of oxidation pathways of some catecholamines with periodic acid

Abstract

The kinetics and mechanism of oxidation of adrenalin, L-dopa, and dopamine to the corresponding open-chain o-benzoquinones with periodic acid have been investigated in the pH range 0–7, at different temperatures. The reactions are of the first order in each reagent and are strongly dependent on pH. Possible mechanisms are discussed with reference to previous work on oxidation with periodic acid and the variation of the rate as a function of pH has been explained in terms of differently protonated reagent couples. At pH >4, the substituted o-benzoquinones rearranged with cyclization of the side-chain into the corresponding leuco-aminochromes (5,6-dihydroxy-2,3-dihydroindoles), which are then rapidly oxidized to aminochromes (2,3-dihydroindole-5,6-quinones). The rate law is, in the pH range 4–8, in the form k_obs=k_cK′₁/(K′₁+[H⁺]), where K′₁ is the acid dissociation constant of the ammonium group of the side-chain, thus predicting that for pH >9 only the cyclization rates are nearly independent of pH. The cyclization rates have been found to decrease significantly in the order adrenalin L-dopa > dopamine.