The electrochemical reduction of coumarin and some substituted coumarins at a mercury cathode in the presence of tertiary amines known to lower the hydrogen overpotential at the pH used has been examined. Coumarin alone yields a dimeric reduction product. In the presence of amines the yield of this dimeric product is decreased and some 3,4-dihydrocoumarin is formed. The dihydrocoumarin is considered to arise from hydrogen transfer between the radical R3ṄH derived by reduction of the trialkyl ammonium ion and the radical which is the initial product of the reduction of coumarin. Electrochemical reduction of 4-methylcoumarin and 5-methoxy-4-methylcoumarin in the presence of optically active amines affords optically active dihydrocoumarins. The maximum optical yield achieved for 3,4-dihydro-3-methylcoumarin is 17%.
