Application of intermolecular SCF-perturbation theory to the regioselectivity in the Diels-Alder reaction

Intermolecular perturbation theory in the density matrix formalism is applied to investigate the directional behaviour of an electron-donating (-CH₃) or an electron-accepting (-CN) group in 1- or 2-substituted butadienes in the Diels-Alder reaction with acrylonitrile. The calculated CNDO/2 perturbation energies are analysed in three different ways by considering: a) the different perturbation energies, b) the diatomic parts of the interaction energy and c) the HOMO-LUMO contribution to the second-order energy. The regioselectivity is due to a subtle balance of charge-transfer interactions and steric effects of the substituents on the diene and the dienophile. The changes of intra- and intermolecular diatomic energy contributions are correlated with the process of bond formation and bond weakening. The intermolecular perturbation energies are dominated by pairwise interactions between the terminal C-atoms and by the secondary Woodward-Hoffmann interaction. These three localized interactions determine the endo addition and reflect the orienting power of the substituents.