Studies on β-methylated end-capped bithiophenes

The reactivity of a variety of differently beta-methylated oxidized alpha,alpha'-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 beta-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 beta-methylation is reported, Compounds with methylated 3-beta-positions form more stable radical cations than 4-beta-methylated ones, The stability of the oxidized end-capped bithiophene 6 can be explained by a reversible dimerization. The presence of beta-substitution also exerts an influence on the reaction products of the chemical oxidation with FeCl3 . 6H(2)O, 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 alpha-methyl groups, not, as one might expect, free beta-ring positions, In neither case has a beta,beta'-linkage between two thiophene rings been observed.