Development of a Reductive Photoredox System Using Riboflavin Analogues and Tris(bipyridine)ruthenium(II) Chloride

Abstract

As a naturally occurring cofactor with versatile redox activity, catalytic systems utilizing flavin analogues have long been of interest. However, there are limited examples of flavin utilized for reductive catalysis, either dark or photoredox. Optimization and mechanistic studies were undertaken to characterize racemic cyclization and deacetoxylation reactions that proceed in the photocatalytic coupling of flavin analogues with Ru(bpy)3Cl2 in aqueous buffer. This approach demonstrates a limited ability to catalyze the cyclization of 2’-allyl-acetophenone; and in a similar system, enables >80% yields of racemic deacetoxylation of a model substrate 2-acetoxy-2-methyl-1-tetralone. Preliminary mechanistic studies suggest single-electron transfer is likely affected by a species generated in the combination of the two photocatalysts that generates a stronger reductant than either photocatalyst alone. These experiments provide valuable groundwork for future experiments exploring the mechanisms of this unique dual-photocatalytic system, which is one of the first examples of the use of free riboflavin analogues for reductive photocatalysis.