An uncommon use of irradiated flavins: Bronsted acid catalysis

We present that thioacetalization of aldehydes can be induced by blue light irradiation in the presence of a catalytic amount of riboflavin tetraacetate (RFTA) under aerobic conditions. Several control experiments have suggested that the reaction is more likely to be catalyzed by acidic species generated in situ upon light irradiation. We have proposed that single electron transfer from a thiol (RSH) to the excited state of RFTA can take place to give a one-electron oxidized thiol (RSH+center dot) and the one-electron reduced RFTA (RFTA(-center dot)), which can be trapped by molecular oxygen to be stabilized as Bronsted acids including the protonated RFTA(-center dot) (RFTAH(center dot)). Finally, we have demonstrated that such acidic species can be prepared in advance as a solution and used as Bronsted acid catalysts for not only thioacetalization but also Mannich-type reactions.