Hyperconjugative Aromaticity and Antiaromaticity Control the Reactivities and π-Facial Stereoselectivities of 5-Substituted Cyclopentadiene Diels–Alder Cycloadditions

摘要

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.