Origin of the endo Selectivity in the Diels–Alder Reaction between Cyclopentadiene and Maleic Anhydride

The Diels–Alder reactions of cyclopentadiene with maleic anhydride and with some other dienophile models in which the C=O bonds were replaced by less polarizable C=NH bonds and by C=CH 2 bonds were studied theoretically. Comparison of interactions in these systems disclosed a key to understanding the high reactivity of maleic anhydride and the endo selectivity in the reaction. It is not orbital interactions that favor the endo addition, but the electrostatic attractions introduced by the presence of highly polarized C=O bonds. The bonds also play important roles in depressing the unfavorable growth of overlap repulsion by storing a large amount of π‐type electron populations outside the region of interactions and in strengthening electron delocalization by providing a positive charge field to the reaction sites of the dienophile.