Photooxidation of benzyl alcohol derivatives by oxygen, catalyzed by protonated flavin analogues

Protonated flavin analogues (3-methyl-10-phenylisoalloxazine, riboflavin, and 3-methyl-10-phenyl-5-deazaisoalloxazine) catalyze the photooxidation of benzyl alcohol by oxygen efficiently in the presence of perchloric acid in acetonitrile yielding benzaldehyde and hydrogen peroxide. In the absence of oxygen, a protonated flavin analogue is reduced by benzyl alcohol under irradiation of the visible light to yield a reduced flavin radical cation (half reduced) which is much stable towards oxygen than a fully reduced flavin. The quantum yields of the photooxidation of various benzyl alcohol derivatives catalyzed by each protonated flavin analogue were determined, showing a maximum with the change in the one-electron oxidation potentials of benzyl alcohol derivatives. No flavin-catalyzed photooxidation has been observed in the case of benzyl alcohol derivatives with strongly electron-withdrawing substituents (NO₂ and CN) or electron-donating substituents (OH and MeO). The reaction mechanism of photochemical reactions of protonated flavin analogues with benzyl alcohol derivatives in the absence and presence of oxygen is proposed to account for these results.