Classical and non-classical secondary orbital interactions and Coulombic attraction in the regiospecific dimerization of acrolein

Non-classical (bridging) and classical (Woodward–Hoffmann) secondary orbital interactions as well as a favourable electrostatic interaction are involved in the stabilization of the two lowest transition passes of the acrolein dimerization. The heteroatoms provide anchimeric assistance to the hetero Diels–Alder reaction through a neighbouring-group mechanism.