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.