Theoretical study of thieno[3,4-d]thiepin and furo[3,4-d]thiepin as dienes in the diels-alder reaction

Theoretical study of thiophene and furan c-fused rings with unsaturated seven membered rings with oxygen or sulfur as dienes for Diels-Alder reactions was conducted with the semiempirical Austin Model 1 (AMI). Four approaches were considered: frontier orbital theory, resonance stability of the dienophiles determined through heat of hydrogenation, progress of the reaction through bond orders, and the estimation of the reaction barriers. It was determined that the most unreactive species with the highest resonance stability are sulfur heterocyclic compounds, while furan derivatives are the most reactive. Using o-quinonoid episulfide as an intermediate was rejected because the estimated activation energy for its formation is too high. The possibility of a three bond formation in the transition state of thieno3,4-d]thiepin reaction with ethylene was considered and rejected on the basis of AMI estimated activation energy. The S-methylated derivatives of thieno3,4-d]thiepin were predicted to react with less reactive dienophiles like ethylene under moderate reaction conditions. If thieno3,4-d]thiepin is not activated through S-methylation then a reactive dienophile and drastic reaction conditions are predicted to be necessary to force the cycloaddition reaction. The obtained results are in agreement with experimental evidence.