Theoretical study of endo selectivity in the Diels-Alder reactions between butadienes and cyclopropene

The endo selectivity in the Diels-Alder reactions of some substituted butadienes with cyclopropene has been investigated theoretically. Electron delocalization between a diene and the dienophile has been presented in terms of pairs of interaction orbitals. In addition to the principal orbital interactions to form new σ bonds between the diene and the dienophile, the cyclopropene occupied interaction orbital shows significant amplitudes on the methylenic hydrogens to overlap in phase with the paired unoccupied interaction orbital of butadiene at the backbone C₂ and C₃ carbons. The contribution of this secondary orbital interaction to the stabilization of the transition state has been estimated numerically. It has been demonstrated that neither electron delocalization nor the electrostatic interaction interprets preference of the endo-addition over the exo-addition, but the sum of the two terms has been found to show a correlation with the difference in the barrier heights for the two modes of cycloadditions.