A theoretical study on a Diels-Alder reaction in ambient and supercritical water: viewing solvent effects through frontier orbitals

Solvent effects on the endo/exo selectivity of an asymmetric Diels-Alder reaction in ambient and supercritical water are studied by means of a combined electronic structure and liquid state theory. The target system is the cycloaddition of cyclopentadiene with methyl vinyl ketone. The rate constant and the equilibrium constant are obtained from the activation free energies and the free energy change of reaction for the two isomers. The results for the equilibrium constant are in qualitative agreement with the experimentally observed endo/exo selectivity. The relative rate constants show that the endo reaction occurs preferentially in wide range of thermodynamic conditions. Difference of the solvation free energy shows that endo/exo selectivity is enlarged in ambient water by hydrophobic effect and that it disappears completely in supercritical water. The theoretical results are analyzed in the light of the frontier orbital theory in order to acquire physical insight of solvent effects on the stereo-selectivity. (C) 2000 Elsevier Science B.V. All rights reserved.