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.
KeywordsTrans Isomer Orbital Symmetry Sigma Bond Symmetry Label Electronegative Ligand
Unable to display preview. Download preview PDF.
- (a).Hoffmann, R., Woodward, R.B., J. Am. Chem. Soc. 1965, 87, 395.Google Scholar
- (c).Hoffmann, R., Woodward, R.B., J. Am. Chem. Soc. 1965, 87, 2046.Google Scholar
- 2.Woodward, R.B., Hoffmann, R., “The Conservation of Orbital Symmetry”; Verlag Chemie: Weinheim, 1970.Google Scholar
- 3.a) Epiotis, N.D., Larson, J.R., Eaton, H., “Unified Valence Bond Theory of Electronic Structure” in Lecture Notes in Chemistry, Vol. 29, Springer-Verlag: New York and Berlin, 1982.Google Scholar
- (b).Epiotis, N.D., Larson, J.R., Israel J. Chem. 1983, 000.Google Scholar
- (d).Nascimento, C.C., Brinn, I.M., Z. Naturforsch 1978, 33a, 366.Google Scholar
- 7.See Chapter 8 of this work.Google Scholar