Abstract
Quantum chemistry calculations have been performed to investigate the kinetics and mechanism of the cycloaddition reaction between cyclopentadiene and N-alkylmaleimides in the gas phase and different solvents. To investigate the effects of side-chain length on the cycloaddition reaction, the rate constants and kinetic parameters of the reaction between cyclopentadiene with N-methyl-, N-ethyl-, N-propyl- and N-butyl-maleimide were calculated. The results obtained indicate that the reaction in solvents is faster than in the gas phase. Moreover, the dipole moments of the transition states are larger than those of the reactants. Therefore, the reactions in the most polar solvent (water) are faster than in ethanol, n-hexane, 2,2,2-trifluoroethanol (TFE) and acetonitrile. Quantum mechanics-molecular mechanics (QM/MM) calculations on the reactions using the explicit solvent model for water and TFE indicate that hydrogen bond interactions of the solvents have a key role in the rate of the reaction and these are more important than the polarity of the solvent. Natural bond orbital analysis reveals that the charge transfer between the reactants in solvents is more than in the gas phase. Finally, HOMO–LUMO analysis indicates that solvents increase the reactivity of the reactants in comparison to the gas phase.