Origins of the Endo and Exo Selectivities in Cyclopropenone, Iminocyclopropene, and Triafulvene Diels–Alder Cycloadditions

The endo and exo stereoselectivities of Diels–Alder reactions of cyclopropenone, iminocyclopropene, and substituted triafulvenes with butadiene were rationalized using density functional theory calculations. When cyclopropenone is the dienophile, there is a 1.8 kcal/mol preference for the exo cycloaddition with butadiene, while the reaction of 3-difluoromethylene triafulvene with butadiene favors the endo cycloaddition by 2.8 kcal/mol. The influence of charge transfer and secondary orbital interactions on the stereoselectivity of Diels–Alder reactions involving triafulvenes and heteroanalogs is discussed. The predicted stereoselectivity correlates with both the charge and highest occupied molecular orbital (HOMO) coefficient at the C₃ carbon of the triafulvene motif.