Classical and non-classical secondary orbital interactions and Coulombic attraction in 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.