Origin of the "endo rule" in Diels-Alder reactions.
Detailed density functional theory calculations definitively rationalize the preference for the endo cycloadduct (also known as endo rule) in text-book thermal Diels-Alder reactions involving maleic anhydride and cyclopentadiene or butadiene. This selectivity is mainly caused by an unfavorable steric arrangement in the transition-state region of the exo pathway which translates into a more destabilizing activation strain. In contrast with the widely accepted, orbital-interaction-based explanation for the endo rule, it is found that neither the orbital interactions nor the total interaction between the deformed reactants contributes to the endo selectivity.
Departamento de Química Orgánica I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Ciudad Universitaria, 28040, Madrid, Spain.
Israel
I
Fernández
F Matthias
FM
Bickelhaupt
Alder reaction; Diels; activation strain model; cycloaddition; density functional theory calculations; endo rule; reactivity