Formation of unsymmetrical 1,4-dithiins from fused 1,2,3,4,5-pentathiepins: synthesis, structural, and computational study

Pentathiepinopyrroles reacted with methyl propiolate and triphenylphosphine to give regioselectively dithiinopyrroles in agreement with the electron distribution in the proposed reaction intermediate. Thieno[2,3-f][1,2,3,4,5]pentathiepin when treated with methyl propiolate and triphenylphosphine gave a pair of regioisomers where the higher yielding regioisomer contained the same mode of junction as in the case of pentathiepinopyrroles. X-ray crystal structures are provided for the thieno[2,3-b][1,4]dithiine carboxylate isomers. Quantum-chemical calculations B3LYP/6-31G(d) and B3LYP/6-311++G(d,p) have been carried out for better understanding of the reaction mechanisms; the index of synchronism of the Sh addition and the index of a relative difference in bond orders in transition states are in good agreement with the formation of the regioisomers by the reaction of unsymmetrical pentathiepins with alkynes containing one electron-withdrawing group.