A theoretical approach to the investigation of the photochemical internal cycloaddition in penta-1,4-diene and hexa-1,5-diene

Abstract

A theoretical calculation has been performed on the photochemical internal cycloaddition reaction of non-conjugated dienes by an ab initio SCF MO procedure. The ground-state geometries of the species in several rotamers were investigated and the geometries in the excited triplet state were analysed in the same way. This enabled clarification of the stable and/or preferred molecular geometries of penta-1,4-diene and hexa-1,5-diene in both ground and excited triplet states for the first time. Total energies along the reaction co-ordinates were followed systematically. From these results preferred mechanisms for the addition reactions were proposed. Previous experimental results are explained in the light of the present calculations, viz. the prevalent parallel compound from the 1,4-diene, the cross compound from the 1,5-diene, and the ratio of the parallel/cross products in each reaction species. The potential for internal rotation about the skeletal bond was calculated for some cases in connection with the facility of the reaction. Reaction orbitals were selectively extracted and analysed, and the factors governing the [2 + 2] internal cycloaddition reaction were considered. The calculations also clarified the position of initial closure in the molecule.