The source of the endo rule in the Diels−Alder reaction: Are secondary orbital interactions really necessary?

The endo preference in Diels−Alder reactions is usually attributed to the occurrence of attractive Secondary Orbital Interactions (SOI), whereas other interaction mechanisms (primary interactions, closed-shell repulsions, electrostatics) are assumed to be identical for both endo and exo approaches. However, analysis of the parallel approximation between s-trans butadiene and fumaronitrile shows that SOI is overcome by closed-shell repulsions. Furthermore, the study of several reactions (cyclopentadiene + maleic anhydride, cyclobutadiene + cyclobutenedione, cyclobutadiene + norbornadiene) indicates the absence of a net attraction for the atom pairs involved in SOI. As a conclusion, the endo preference is not due to the occurrence of an attractive interaction between the atom pairs involved in SOI and so this concept is unnecessary.This work was made possible by the generous financial support of the Comisión Interministerial de Ciencia y Tecnología (Project PPQ2002-04012).Peer reviewe