STUDIES ON BETA-METHYLATED END-CAPPED BITHIOPHENES

The reactivity of a variety of differently beta-methylated oxidized al pha,alpha'-blocked (end-capped) bithiophenes has been studied, Electro analytical methods such as cyclic voltammetry and studies with a rotat ing-ring disc electrode (RRDE) are employed, revealing two oxidation p eaks, While the first electron transfer yields radical cations of incr eased stability as the number of beta-methyl groups is increased, the second electron transfer is irreversible, For the first time, a depend ence of the stability of radical cations (first oxidation step) on the degree of beta-methylation is reported, Compounds with methylated 3-b eta-positions form more stable radical cations than 4-beta-methylated ones, The stability of the oxidized end-capped bithiophene 6 can be ex plained by a reversible dimerization. The presence of beta-substitutio n also exerts an influence on the reaction products of the chemical ox idation 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; bit hiophene derivatives with free 3-positions form bis(bithiophene)methan es, The reactive centers are alpha-methyl groups, not, as one might ex pect, free beta-ring positions, In neither case has a beta,beta'-linka ge between two thiophene rings been observed.