Theoretical Study of the Mechanism of an Inverse-Demand Diels–Alder Reaction

Abstract

Quantum mechanical calculations (AM1, PM3, ab initio HF/3-21G, DFT(B3LYP/6-31G*) and MP2//(B3LYP/6-31G*) have been used to study the inverse-demand synchronous concerted Diels–Alder reactions between dimethyl-1,2,4,5-tetrazine-3,6-dicarboxylate (diene) and a variety of dienophiles (ethylene, cyclopentadiene, 1-hexene, cyclohexene). All the molecular structures (reactants, transition states, intermediates and adducts) were optimized using the semi-empirical AM1 method. The calculated energies and volumes showed that the cycloaddition reaction followed a mechanism involving the formation of an intermediate, elimination of N2, and a 1,3-hydrogen shift adduct. The reaction energies of the systems were obtained by using semi-empirical AM1 calculations and showed good agreement with the experimental data. In contrast, calculations of the reaction energies using PM3, HF/3-21G, DFT (B3LYP/6-31G*) and MP2//(B3LYP/6-31G* were in poor agreement with the experimental data. Compared to the experimental data, the activation energies were overestimated using AM1, PM3 and HF/3-21G, while they were underestimated using DFT (B3LYP/6-31G*) and MP2//(B3LYP/6-31G*).