Anion effect of 5-ethylisoalloxazinium salts on flavin-catalyzed oxidations with H2O2

Abstract

The anion effect of the 5-ethylisoalloxazinium salts on their catalytic activity was investigated for the oxidation of sulfide with aq H2O2 in order to design more practical and safer flavinium salt catalysts that can replace the conventional perchlorate salt. The anions forming stronger conjugated acids were found to accelerate the catalytic sulfoxidation. The riboflavin-derived isoalloxazinium triflate salt was chosen as a readily accessible flavin with an efficient catalytic activity. In fact, the isoalloxazinium triflate 1b is an excellent organocatalyst for the chemoselective oxidations of a sulfide to a sulfoxide, Bayer–Villiger reaction of a cyclobutanone to a γ-butyrolactone, Dakin reaction of an arylaldehyde to a phenol, and oxidation of an aldehyde to a carboxylic acid using aq H2O2 as a terminal oxidant under mild conditions.