The mechanism and regioselectivity of 1,3-dipolar cycloaddition reactions of sulphur-centred 1,3-dipoles including thiocarbonyl S-imide (D1), thiocarbonyl S-oxide (D2) and thiocarbonyl S-sulphide (D3) with an electron-deficient dipolarophile, furan-2,3-dione (DPh), were studied in the light of some theoretical approaches, namely, activation energy, density functional theory (DFT) reactivity indices and Houk’s rule based on the frontier molecular orbital (FMO) theory at the B3LYP/6-311++G^∗∗ level. The present analysis reveals that the cycloaddition reactions under study can be classified in the normal electron demand category. An excellent agreement was observed between the kinetic results and the electronic approaches; in fact, maximum hardness principle (MHP), Chattaraj’s polar model, Houk’s rule and the Gazquez-Mendez rule confirm the resultant regioselectivity based on the calculated activation energies.

In this work, regioselectivity was investigated in the hetero-Diels–Alder reactions of styrenes (without substituent and with para-substituted groups)with 2-aza-1,3-butadiene from thermodynamic (Hammett study), electronic (HSAB criterion) and kinetic viewpoints. Furthermore, the effect of the presence of catalyst was inspected by the NBO analysis in addition to kinetic studies. The calculations were performed by the Gaussian 09 program at the B3LYP/6-311++G** level of theory in the gas phase. Thermodynamically, the stability difference between the two regioisomeric products is not significant, especially, in the reactions of styrenes with electron-releasing substituents. Kinetically, the obtained regioselectivity is in favor of formation of 4-regioisomers, especially, in the presence of BF3 catalyst (in agreement with the NBO results). On thebasis of the HSAB criterion, 5-regioisomers have preference, only in the reactions of styrenes with the strong electron-withdrawing substituents.