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

Solvent effects on the endo/exo selectivity of an asymmetric Diels-Alder re action in ambient and supercritical water are studied by means of a combine d electronic structure and liquid state theory. The target system is the cy cloaddition 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 fo r the equilibrium constant are in qualitative agreement with the experiment ally observed endo/exo selectivity. The relative rate constants show that t he endo reaction occurs preferentially in wide range of thermodynamic condi tions. Difference of the solvation free energy shows that endo/exo selectiv ity is enlarged in ambient water by hydrophobic effect and that it disappea rs completely in supercritical water. The theoretical results are analyzed in the light of the frontier orbital t heory in order to acquire physical insight of solvent effects on the stereo -selectivity. (C) 2000 Elsevier Science B.V. All rights reserved.