A competitive Diels-Alder/1, 3-dipolar cycloaddition reaction of1-H-imidazole 3-oxide toward sulfonyl methane. A DFT study on the energetic and regioselectivity

The dual diene/1,3-dipolar character of 1-H-imidazole 3-oxide, HIO 1, allows this compound toparticipate in a competitive Diels-Alder (DA)/1,3-dipolar cycloaddition (13DC) reaction toward C=Sdouble bond of the electro-deficient sulfonyl methane SFM 2. The B3LYP/6-311++G(d,p) calculatedrelative Gibbs free energies indicate that among the studied 13DC and DA reactions, former iscompletely preferred over the latter in the gas phase as well as in the presence of dichloromethane(DCM). This 13DC reaction takes place through a one-step mechanism to generate correspondingfive-membered cycloadduct via a quite C1-S5 regioselective fashion which is controlled bytheHOMO(HIO 1)- LUMO(SFM 2) interaction.The aromatic character of HIO 1 which is lost along thestudied cycloaddition reaction is responsible for the relatively high activation Gibbs free energies andthe endergonic character of these thermodynamically reversible reactions. The quite C1-S5regioselectivity, resulted from energetic studies, in the investigated 13DC reactioncan be explainedusingcalculated electrophilic and nucleophilic Parr functions at the reactive sites of reagents.