Dipyridyl/pyridinium thieno[2,3-b]thiophenes as new atropisomeric systems. Synthesis, conformational analysis and energy minimization

Dipyridylthieno[2,3-b]thiophenes 2a and 2b exhibit syn/anti atropisomerism with energy barrier to rotation ΔG≠ in the range of 17-18 kcal/mol, whereas 2c and 2d exhibit free rotation at least up to −50 °C with ΔG≠≤10 kcal/mol.