Hyperconjugative, Secondary Orbital, Electrostatic, and Steric Effects on the Reactivities and Endo and Exo Stereoselectivities of Cyclopropene Diels–Alder Reactions

The factors controlling the reactivities and stereoselectivities in the Diels-Alder reactions of substituted cyclopropenes with butadiene were explored with M06-2X density functional theory. Differences in reactivities result from differences in the hyperconjugative aromaticities and antiaromaticities of the cyclopropenes. When the 3-substituent is a σ-donor, the ground state is destabilized, and the reactivity is enhanced. Acceptors have the opposite effect. Electrostatic, secondary orbital, and steric effects are all found to influence stereoselectivities.