Theoretical study of the Diels-Alder reaction between the S-methylthiophenium ion and ethene

The capability and activation of thiophene as a diene for the Diels-Alder reactions has been investigated by ab initio methods. The reactants and transition structures are optimized with RHF/3-21 + G*, RHF/ 6-31 + G* and MP2/6-31 + G*. Energies are evaluated with the same level of theory and by single point calculations employing MP2/6-31 + G*//RHF/6-31 + G* and MP3/6-31 + G*//MP2/6-31 + G* methods. The ab initio calculated geometries are compared with experimental data, where available. The relative reactivity of buta-1,3-diene, thiophene and the S-methylthiophenium ion were estimated by comparing their FMO energy gap with ethene as the dienophile. The predicted activation energies are in full agreement with the qualitative determination of reactivity and suggest that thiophene is not a suitable diene for the Diels-Alder reaction. For the S-methylthlophenium ion, the predicted activation energy may be reached under normal reaction conditions. Thus S-methylthiophenium should be a suitable diene for the Diels-Alder reactions.