Studies on β-methylated end-capped bithiophenes

Abstract

The reactivity of a variety of differently β-methylated oxidized α,α′-blocked (end-capped) bithiophenes has been studied. Electroanalytical methods such as cyclic voltammetry and studies with a rotating-ring disc electrode (RRDE) are employed, revealing two oxidation peaks. While the first electron transfer yields radical cations of increased stability as the number of β-methyl groups is increased, the second electron transfer is irreversible. For the first time, a dependence of the stability of radical cations (first oxidation step) on the degree of β-methylation is reported. Compounds with methylated 3-β-positions form more stable radical cations than 4-β-methylated ones. The stability of the oxidized end-capped bithiophene 6 can be explained by a reversible dimerization. The presence of β-substitution also exerts an influence on the reaction products of the chemical oxidation with FeCl₃·6H₂O, which have been isolated and purified by chromatography. In the case of bithiophene derivatives with methylated 3,3′-positions the main products are bithiophene-5-carbaldehydes; bithiophene derivatives with free 3-positions form bis(bithiophene)methanes. The reactive centers are α-methyl groups, not, as one might expect, free β-ring positions. In neither case has a β,β′-linkage between two thiophene rings been observed.