Studies on β-methylated end-capped bithiophenes

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.