Comments on the mechanism of one-electron oxidation of phenols: a fresh interpretation of oxidative coupling reactions of plant phenols

Abstract

Fieser's critical oxidation potentials can be used to predict the relative ease of 1 -electron abstraction from phenols, but the equation for Edt is pH dependent. For further oxidation to cations (ArO)+, is not pH dependent and so there should be an equipotential pH below which ArO-is metastable with respect to (ArO)+ and (Ar0)-. Oxidative C-0-C coupling and aryloxy-polymer formation is ascribed to reactions of ArO* radicals and many C-C coupling reactions to electrophilic attack on phenol molecules by (ArO)+ cations.It is suggested that in plants those'biochemical oxidations of phenols which lead to Aryl-O-Aryl structures probably involve the one-electron oxidation of aryloxy-anions to ArO* radicals which then dimerise, but that those oxidations which lead to aryl-aryl coupled products involve the electrophilic attack on phenol molecules by (ArO)+ cations. The diversity of the reaction paths which have now been established for biological oxidations of phenols in plants may be due to the involvement of two factors, (a) the oxidation potential of the reagent or active enzyme and ( 6 ) the pH of the reacting mixture, in the selection of the preferred oxidation route. (a)

Oxidation of Phenol in Acidic Aqueous Suspensions of Manganese Oxides

“…Other studies of the products of phenol oxidation at low pH (Waters, 1971;McDonald and Hamilton, 1973) similarly found C-C coupling and quinone formation favored over polymer formation by C-O coupling. This is attributed to phenoxy radicals remaining complexed to the metal ion during coupling (McDonald and Hamilton, 1973).…”

A spectroelectrochemical and chemical study on oxidation of hydroxycinnamic acids in aprotic medium

“…Both electrochemical [10][11][12][13] and chemical oxidation [10,[14][15][16] of these compounds have been studied by many authors, especially in aqueous medium at different pH, but the mechanisms involved in their oxidative process are not yet well understood, above all because the radical species likely formed during their oxidation (phenoxyl radicals and radical anions) are highly reactive and unstable and may be involved in complicated chemical reactions [15,17,18]. Their possible pro-oxidant activity and their metabolic fate in human body are still scarcely known, too, limiting the possible employment of these natural compounds in the preventive medicine as well as in the recognized pathology.…”

Reducing power: the measure of antioxidant activities of reductant compounds?

“…1,2 Many antioxidants are proven to undergo an electron-transfer mechanism upon exerting their antioxidant functions. [3][4][5][6][7][8][9] As a result, electrochemical assay has been employed to evaluate the activity of antioxidants since it can present direct and detailed information on the oxidation of reductant antioxidants. [10][11][12] Some workers have attempted to establish quantitative correlations between the antioxidant activities and redox potentials for reductant antioxidants, 10,[13][14][15] and suggested that the half-peak oxidation potential (E p/2 ) along with iron-chelating capacity could account almost completely for the antioxidant activities of flavonoids.…”