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 during the 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+•) and the one-electron reduced RFTA (RFTA–•), which can be trapped by molecular oxygen to be stabilized as Bronsted acids including the protonated RFTA–• (RFTAH•). 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 the thioacetalization but also Mannich-type reactions.