Flavin Analogue-Metal Ion Complexes Acting as Efficient Photocatalysts in the Oxidation of p-Methylbenzyl Alcohol by Oxygen under Irradiation with Visible Light

Flavin analogues (3-methyl-10-phenylisoalloxazine and 3-methyl-10-phenyl-5-deazaisoalloxazines) have been found to catalyze the photooxidation of p-methylbenzyl alcohol by oxygen efficiently in the presence of Mg²⁺ or Zn²⁺ ion in acetonitrile to yield p-methylbenzaldehyde and hydrogen peroxide under irradiation of the visible light of X > 360 nm. The Mg²⁺ or Zn²⁺ion forms a 1:1 complex with flavin analogues in acetonitrile. Such a complex formation between flavin analogues and the metal ion not only increases the oxidizing ability of the excited states of flavin analogues as indicated by the significant acceleration in the fluorescence quenching rate of flavin analogues with electron donors (methyl- and methoxy-substituted benzenes) by the complex formation but also stabilizes flavin analogues against irradiation with visible light to prevent the photodegradation of flavin analogues. The photocatalytic oxidation of p-methylbenzyl alcohol by oxygen is shown to proceed mainly via the singlet-excited states of flavin analogue-metal ion complexes acting as efficient photocatalysts.