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 for syn addition are large relative to those for anti addition, which are all similar to the activation energy for cyclopentadiene itself. Partitioning the activation energy into diene deformation, dienophile deformation, and diene-dienophile interaction energies shows that the major factor in determining facial selectivity is in the energy required to deform the diene into its transition state geometry. Deformation of the 5-fluoro-, 5-hydroxy-, and 5-amino-1,3-cyclopentadienes into their syn transition state geometries is predicted to require less energy than deformation of cyclopentadiene itself, which is in accord with experimental observation of syn addition with these dienes. The first definition of an ab initio steric factor is presented which correlates very well with syn activation energies. This indicates that facial selectivity with these dienes is primarily due to steric hindrance between the dienophile and the plane-nonsymmetric groups on the diene. However, we have also identified a significant lone pair-lone pair interaction with the reacting nitrogens when the dienophile is 1,2,4-triazoline-3,5-dione.