Solid State Electrochemical Oxidation Mechanisms Of Morin in Aqueous Media

Abstract

The mechanism of electrochemical oxidation of morin has been studied using cyclic, differential pulse and squarewave voltammetry techniques in aqueous electrolyte with solid, insoluble morin hydrate mechanically transferred to a glassy carbon electrode surface, over a wide pH range. The oxidation mechanism proceeds in sequential steps, related with the hydroxyl groups in the three aromatic rings and the oxidation is pH dependent over part of the pH range the oxidation potentials are shifted to lower values with increasing pH. Oxidation of the 2',4'dihydroxy moiety at the B ring of morin occurs first, at very low positive potentials, and is a one electron one proton reversible reaction. The hydroxyl groups oxidized at more positive potentials were shown to undergo an irreversible oxidation reaction.

Synthesis and electrochemical study of new 3-(hydroxyphenyl)benzo[f]coumarins

“…The electrochemistry of this selected series of synthesized coumarinnaphthalene-stilbene hybrids (6)(7)(8)(9)(10) showed that the difference in the position of the hydroxyl substituents have only a slightly effect upon the electroactivity of the selected coumarins.…”

“…The phenol group is irreversibly oxidised in compounds 6, 7, 8, and 10, and the catechol group is reversibly oxidised in compound 9. The oxidation products of the coumarin-naphthalene-stilbene hybrids (6)(7)(8)(9)(10) The monohydroxylated compounds 6-8, irreversible pH-dependent oxidation process, occurs with one electron and one proton transfer, following the phenol oxidation mechanism [43]. Compound 6, with the para position to the hydroxyl group occupied, formed a single oxidation product corresponding to a catechol moiety, Scheme 3.…”

“…To a solution of the corresponding 3-(methoxyphenyl)benzo[f]coumarin (1)(2)(3)(4)(5) (0.50 mmol) in acetic acid (5 mL) and acetic anhydride (5 mL) at 0 ºC, hydriodic acid 57% (10 mL) was added dropwise. The mixture was stirred, under reflux, for 3 h. The solvent was evaporated under vacuum and the dry residue was purified by CH 3 CN crystallization to give the corresponding 3-(hydroxyphenyl)benzo[f]coumarins (6)(7)(8)(9)(10) in yields between 60-67% in this last reaction, Scheme 2. …”

“…The dependence of the phenol derivatives oxidation potential on solution pH has been studied thoroughly for different classes of polyphenols [5][6][7][8][9][10][11], for the evaluation of the antioxidant capacity of several polyphenols and their mixtures [12], and for the understanding of their reaction mechanisms.…”

“…Their electrochemical oxidation behaviour was investigated for a wide range of solution conditions, using a glassy carbon electrode and cyclic, differential and square wave voltammetry, at different pH, and the results will play a crucial role in understanding coumarin derivatives antioxidant activity. (6)(7)(8)(9)(10) The aim was to study and compare one derivative with a catechol group (3',4'-OH derivative) and another one presenting no contiguous OH groups (3',5'-OH derivative). Both precursors (2',5'-dimethoxy and 3',5'-dimethoxy derivatives) being commercially available on Aldrich.…”