Cycloadditions of Aryl-Substituted 1,2,4-Triazines with 2-Cyclopropylidene-1,3-dimethylimidazolidine – Zwitterions as Discrete Intermediates

Cycloadditions of 2-cyclopropylidene-1,3-dimethylimidazolidine (1), a strong, electron-rich C-nucleophile, with a variety of aryl-substituted 1,2,4-triazines occur at temperatures between −100 and +100°, depending on the substitution pattern. At low temperatures, zwitterions, formed by nucleophilic attack of 1 on the triazines, could be detected spectroscopically and, in some cases, isolated. Two types of zwitterions were found: 1) those where the new bond was linked to C(5) of the triazine and which were formed in a reversible dead-end equilibrium, and 2) those where the new bond was linked either to C(3) or C(6). The latter exhibited the same regiochemistry as the final cycloadducts, and might be intermediates of a two-step Diels–Alder reaction. Energies and structural characteristics for stationary points in the reaction of monosubstituted triazines with 1 in the gas phase and in CH₂Cl₂ solution were calculated at the Becke3LYP/6-311+G(d,p)//Becke3LYP/6-31G(d) level of theory. Different reaction mechanisms are discussed on the basis of steric, electronic, and solvent effects.