THEORETICAL INVESTIGATIONS ON THE REGIOCHEMISTRY AND STEREOCHEMISTRY OF THE PHOTOCHEMICAL [2+2]-CYCLOADDITION OF PROPENE

Energy hypersurfaces for the lowest singlet states of the [2 + 2] cycloaddition of two ethylene molecules and of two propene molecules in all possible face-to-face orientations were calculated by the semiempirical MNDOC-CI method. A rhomboidal distortion of the pericyclic geometries results in conical intersections. Geometries and energies of the conical intersections were determined and show that for the propene dimerization the head-to-tail approach is favored over the head-to-head approach. Two different ground-state reaction paths emanate from the conical intersections leading to a cyclobutane structure and to a 1,4-biradical, respectively. The larger the rhomboidal distortion at the conical intersection the more pronounced is the 1,3-interaction that yields the 1,4-biradical. Electronic effects on the geometry at the conical intersection are discussed on the basis of the 2-electron-2-orbital model and suggest the rhomboidal distortion to be less pronounced for more electronegative substituents, yielding 1,3-disubstituted cyclobutanes via a concerted pathway.