Some mechanistic studies on electrochemical carboxylation of flavones to yield flavanone-2-carboxylic acids

Electrochemical carboxylation of flavones was successfully demonstrated. Constant current electrolysis of a DMF solution of flavones containing 0.1 M Bu4NBF4 in the presence of carbon dioxide by using a one-compartment cell equipped with a Pt cathode and an Mg anode at 0 °C resulted in fixation of carbon dioxide at the C2-position of flavones in a highly regioselective manner to give flavanone-2-carboxylic acids in moderate to good yields. From the results of some mechanistic examinations, we could conclude that flavanone-2-carboxylic acids were produced through exclusive mono-carboxylation of flavones at the C2-position and that dicarboxylation both at the C2- and C3-positions of flavones hardly proceeded. We report herein details of the electrochemical carboxylation of flavones to obtain flavanone-2-carboxylic acids, and we propose a reaction mechanism for the C2-selective carboxylation.