Geometric Isomerism: The Simplest Illustrator of Orbital Symmetry Control of Molecular Stereochemistry

The thermal conversion of 1,3-butadiene to cyclobutene may occur in a conrotatory or a disrotatory fashion. In the former case, an axis of symmetry is maintained along the reaction coordinate while in the latter case a plane of symmetry is preserved during the conversion or reactants to products. This difference with respect to the existing symmetry elements becomes responsible for a difference in the symmetry labels of reactant and product orbitals. In turn, this becomes responsible for the existence of a barrier in the case of disrotation and the absence of a barrier in the case of conrotation at the level of Hückel MO theory. This is clearly revealed by the Longuet-Higgins-Abrahamson-Woodward-Hoffmann MO correlation diagrams1 for con- and dis-rotatory ring closure of 1,3-butadiene. The conrotatory ring closure of 1,3-butadiene is termed a symmetry “allowed” and the disrotatory ring closure of the same molecule is termed a “forbidden” reaction.