A density functional theory study of the regio- and stereoselectivity of the 1,3-dipolar cycloaddition of C-methyl substituted pyrazinium-3-olates with methyl acrylate and methyl methacrylate

A DFT [B3LYP/6-31G(d)] study was carried out on the 1,3-dipolar cycloaddition (13DC) reactions of multi C-methyl substituted pyrazinium-3-olates with methyl acrylate (MA) and methyl methacrylate (MMA). Thermodynamic and kinetic parameters of the possible endo/exo stereoisomeric and 6-ester/7-ester regioisomeric pathways have been determined. The skeleton rearrangement of the 6-exo [3 + 2] cycloadducts into the [4 + 2] adducts is also considered. The electrophilic, P k + , and nucleophilic, P k - , Parr functions are used to have better understanding of the regioselectivity of these 13DC reactions. In all cases the exo pathways are more favourable compared to the endo alternatives. Inclusion of methyl groups at the C-2 and C-5 positions of pyrazinium-3-olate shows that the 7-exo cycloadduct is predicted to be the major product followed by its 6-exo isomer. These theoretical results have been compared with the previously reported studies on 13DCs of unsubstituted and substituted pyrazinium-3-olates with MA and MMA. The activation energy of these 13DCs ranges from 42.5 to 98.2 kJ mol−1 in THF.