Electrochemical studies of the oxidation of sterically hindered pyrroles and thiophenes

The electrochemical oxidation of several alpha,alpha'-di-tert-butyl pyrroles and thiophenes has been investigated in acetonitrile. Reversible cyclic voltammograms for the oxidation of one-ring alpha,alpha'-di-tert-butyl thiophenes or pyrroles were obtained at low scan rates (0.05-5 V s(-1)), indicating a large stability of their cation-radicals. Chemical evolutions without polymerization of the cation-radical were observed at longer time scales. The formal potentials E degrees and electron exchange standard rate constants k(s) were determined and compared with preceding reports for the unsubstituted analogues. From the substituent variations, the E degrees values for the oxidation of the unsubstituted thiophene and bithiophene were estimated as +1.97 and +1.49 V(SCE) respectively.