Electro-organic reactions : Part 27. The mechanism of cathodic cleavage of activated esters; oxalates, squarates and oxamates

Abstract

Esters of oxalic acid, 3,4-dihydroxy-3-cyclobutene-1,2-dione (squaric acid), and oxamic acid, are reduced cathodically at modest potentials. In aprotic solvent, and on the cyclic voltammetric time scale, the esters are cleaved to the corresponding alkane. For oxalates, the mechanism of cathodic cleavage was investigated thoroughly by voltammetry, coulometry, and detailed product analysis. On the time scale of controlled potential electrolysis the rapid electrogenerated base-catalysed hydrolysis of the esters by adventitious water competes with cathodic cleavage. Similarly, rapid base-catalysed transesterification involving oxalates and added alcohols is observed which provides a practical method of reductively cleaving alcohols to alkanes by co-electrolysis of a mixture of alcohol and readily available oxalate (e.g. diethyloxalate). The leaving group in such cathodic fragmentation is the half-ester anion and the efficiency of reaction depends on the stability of the other, radical, fragment.