Electrochemical oxidation of synthetic amino-substituted benzamides with potential antioxidant activity

Abstract Amino-substituted benzamide derivatives are very attractive compounds due to their capacity to act as powerful antioxidants by scavenging free radicals. Knowledge on electrochemical oxidation mechanisms plays an important role in understanding the free radical scavenging activity of antioxidants. In this paper, the electrochemical oxidation of four amino-substituted derivatives [N-(4-aminophenyl)-2-hydroxybenzamide (1), N-(4-aminophenyl)-2-hydroxy-4-methoxybenzamide (2), N-(4-aminophenyl)-2-methoxybenzamide (3), N-(4-aminophenyl)-3,4,5-trihydroxybenzamide (4)] in an aqueous buffer solution was studied by means of cyclic and square-wave voltammetry, and probable reaction mechanisms have been proposed. In compounds 1–3, the primary amino group was found to be the main electroactive centre undergoing a complex, pH-dependent oxidation process with the transfer of two electrons and two protons. This reaction occurs in one step or in two separate steps, depending on the pH of the medium. A possible explanation for this electrochemical behaviour is discussed. The final products of initial electrochemical oxidation of amino-substituted benzamides 1–3 are quinonediimine derivatives that undergo further chemical transformation into electroactive forms. Oxidation of compound 4 proceeds through two sequential reactions; the first anodic reaction is the oxidation of the trihydroxyphenyl group to the corresponding o-quinone, followed by the oxidation of a primary amino group. Considering the oxidation potentials, and bearing in mind that lower/less positive potential implies better radical scavenging activity, the order of antioxidant activity of the here studied amino-substituted benzamides is: 4 ˃ 2 ˃ 1 ˃ 3.