Thiourea-Enhanced Flavin Photooxidation of Benzyl Alcohol

Upon irradiation, flavin oxidises 4‐methoxybenzyl alcohol to the corresponding aldehyde using aerial O2 as the terminal oxidant. We have observed that this reaction is significantly accelerated by the presence of thiourea. A series of thiourea‐functionalised flavins has been prepared from flavin isothiocyanates and their photocatalytic efficiencies have been monitored by NMR. The alcohol photooxidation proceeds rapidly and cleanly with high turnover numbers of up to 580, exceeding previously reported performances. A likely mechanistic rationale for the more than 30‐fold acceleration of the photo–redox reaction by thiourea has been derived from spectroscopic, electrochemical, and kinetic studies. Thus, thiourea acts as an electron‐transfer mediator for the initial photooxidation of 4‐methoxybenzyl alcohol by the excited flavins. This mechanism has similarities to electron‐relay mechanisms in flavoenzymes, for which cysteine sulfenic acid intermediates are proposed. The observation that thiourea mediates flavin photo–redox processes is valuable for the design of more sophisticated photocatalysts based on Nature's best redox chromophore. Improved photooxidation: The photooxidation of 4‐methoxybenzyl alcohol by flavin is significantly enhanced by the presence of thiourea, which acts as an electron mediator between the alcohol and the flavin chromophore (see scheme).