Electrochemical reduction of 2-thiophenecarbonyl chloride at carbon and mercury cathodes in acetonitrile

Abstract

Cyclic voltammograms for the reduction of 2-thiophenecarbonyl chloride at glassy carbon and hanging mercury drop electrodes in acetonitrile containing 0.10 M tetraethylammonium perchlorate or tetra- n -butylammonium perchlorate exhibit two irreversible cathodic waves along with several small and poorly defined peaks; the first wave is attributable to electrolytic cleavage of the carbon-chlorine bond, and the second wave is caused by reduction of 2-thiophenecarboxaldehyde. Large-scale controlled-potential electrolyses of 2-thiophenecarbonyl chloride at potentials corresponding to the first cyclic voltammetric wave afford 1,2-di(2-thienyl)ethene-1,2-diol di(2-thiophenecarboxylate) as the major product, along with small amounts of 2-thiophenecarboxaldehyde. On the basis of the coulometric n values, the amount of aldehyde produced, and more importantly of experiments involving the use of deuterium oxide and acetonitrile- d 3 , one can conclude that 2-thiophenecarbonyl chloride undergoes principally a two-electron reduction to an acyl anion which (a) accepts a proton to give the corresponding aldehyde or (b) attacks the starting material to form a diketone (di(2-thienyl)ethanedione) which is subsequently reduced and then acylated to yield a tetramer (1,2-di(2-thienyl) ethene-1,2-diol di(2-thiophenecarboxylate)).