AN AB-INITIO STUDY OF FACIAL SELECTIVITY IN THE DIELS-ALDER REACTION

Facial selectivities in the Diels-Alder reactions of 5-substituted 1,3 -cyclopentadienes with a variety of dienophiles are predicted reliably at the ab initio HF/6-31G level. The ranges of activation energies f or syn addition are large relative to those for anti addition, which a re all similar to the activation energy for cyclopentadiene itself. Pa rtitioning the activation energy Into diene deformation, dienophile de formation, and diene-dienophile interaction energies shows that the ma jor factor in determining facial selectivity is in the energy required to deform the diene into its transition state geometry. Deformation o f the 5-fluoro-, 5-hydroxy- and 5-amino-1,3-cyclopentadienes into thei r syn transition state geometries is predicted to require less energy than deformation of cyclopentadiene itself, which is in accord with ex perimental observation of syn addition with these dienes. The first de finition of an ab initio steric factor is presented which correlates v ery well with syn activation energies. This indicates that facial sele ctivity with these dienes is primarily due to steric hindrance between the dienophile and the plane-nonsymmetric groups on the diene. Howeve r, we have also identified a significant lone pair-lone pair interacti on with the reacting nitrogens when the dienophile is 1,2,4-triazoline -3,5-dione.