Ester substituted bithiophenes. Abnormally low dihedral angle and rotation barrier due to dipolar stabilization

Abstract

Ab initio calculations [3−21G (*)] on dimethyl 2, 2′-bithiophene-4, 4′-dicar☐ylate ( 3), dimethyl 2, 2′-bithiophene-3, 4′-dicar☐ylate ( 4) and dimethyl 2, 2′-bithiophene-3, 3′-dicar☐ylate ( 5) have revealed that in 4 there is an abnormally small dihedral angle between the thiophene rings and an abnormally low barrier to rotation around the thiophene-thiophene single bond. This is due to a favorable coulombic interaction of the negative carbonyl oxygen and the positive sulfur of the adjacent ring which helps to flatten and bend the ground state and lower the rotation barrier. Ab inito calculations [3–21G (*)] show a favorable coulombic interaction between the negative carbonyl oxygen and the positive sulfur of the adjacent ring in dimethyl 2, 2′-bithiophene-3, 4′-dicar☐ylate ( 4), which helps to flatten the ground state considerably and substantially lower the thiophene-thiophene rotation barrier. [Display omitted]