UCLA

The reactivities and π-facial stereoselectivities of Diels-Alder reactions of 5-substituted cyclopentadienes were studied using density functional theory. Burnell and co-workers previously showed that the π-facial selectivities result from the energies required to distort the reactants into the transition state geometries. We have discovered the origins of these distortions. C₅-X σ-donors predistort the cyclopentadiene into an envelope conformation that maximizes the stabilizing hyperconjugative interaction between the C₅-X σ-bond and the diene π-system. This envelope conformation geometrically resembles the anti transition state. To minimize the destabilizing effect of negative hyperconjugation, C₅-X σ-acceptors predistort in the opposite direction toward an envelope geometry that resembles the syn transition state. We now show how hyperconjugative effects of the C₅-X substituent influence the stereoselectivities and have developed a unified model rationalizing the stereoselectivities and reactivities of 5-substituted cyclopentadiene Diels-Alder reactions.