Unusual head-to-tail coupling of alkyl benzoates by electroreduction

The electroreduction of alkyl benzoates in an alcoholic solvent gave unusual head-to-tail coupled products. Usual head-to-head coupled products derived from acyloin condensation could not be detected. The best result (73% yield) was obtained from methyl benzoate using an undivided cell with an Sn cathode in i-PrOH containing tetraalkylammonium salt as a supporting electrolyte. Using an undivided cell, the products cross-coupled with a solvent molecule were obtained as byproducts. The substitution at the para position of methyl benzoate considerably decreased the yields of the head-to-tail coupled products and increased those of the cross-coupled products. The possible mechanism of the head-to-tail coupling is the attack of anion radical, generated from methyl benzoate by one-electron transfer, to another methyl benzoate. The cross-coupled products were formed by the reaction with carbonyl compound anodically produced from a solvent molecule. The cross-coupling between methyl benzoate and aromatic aldehydes was also effected by the mixed electroreduction under the same conditions.