Two-electron electrochemical oxidation of quercetin and kaempferol changesonly the flavonoid C-ring

Bulk electrolysis of the antioxidant flavonoids quercetin and kaempferol in acetonitrile both yield a single oxidation product in two-electron process es. The oxidation products are more polar than their parent compounds, with an increased molecular weight of 16g/mol, and were identified as 2-(3,4-di hydroxybenzoyl)-2,4,6-trihydroxy-3 (2H)-benzofuranone and 2-(4-hydroxybenzo yl)-2,4,6-trihydroxy-3(2H)-benzofuranone for quercetin and kaempferol, resp ectively. Two-electron oxidation of the parent flavonoid is suggested to yi eld a 3,4-flavandione with unchanged substitution pattern in the A- and B-r ing, which may rearrange to form the substituted 3(2H)-benzofuranone throug h the chalcan-trione ring-chain tautomer. The acidity of the 3-OH group is suggested to determine the fate of the flavonoid phenoxyl radical originall y formed by one-electron oxidation, as no well-defined oxidation product of luteolin (lacking the 3-OH group) could be isolated despite rather similar half-peak potentials: E-p/2 = 0.97 V, 0.98 V and 1.17 V vs. NHE for querce tin, kaempferol and luteolin, respectively, as measured by cyclic voltammet ry in acetonitrile.