Synthesis and Electrochemical Reduction of Methyl 3-Halo-1-benzothiophene-2-carboxylates

Abstract

A preparative method of synthesis of the new methyl 3-iodo-1-benzothiophene-2-carboxylate was elaborated. Electrochemical behavior of methyl 3-chloro-, bromo- and iodo-1-benzothiophene-2-carboxylates 1-3, and of their reduction and dimer products 4, 5 in anhydrous dimethylformamide has been investigated at mercury and platinum electrodes using polarography, cyclic voltammetry and voltammetry on a rotating platinum disk electrode. The reduction in divided cells follows the ECE mechanism (electron - chemical step - electron), where the primary radical anion is split into a halide anion and neutral heterocyclic radical, which is immediately reduced by the second electron and protonated. The only reduction product is the methyl 1-benzothiophene-2-carboxylate (5); whereas the EDim mechanism (electron - dimer formation) leading to the dimeric species 4 was not observed under the above conditions. Reduction of 1-3 on platinum causes formation of a blocking film on the electrode. Sonication during electrolysis successfully reactivates the electrode.