Pericyclic Reactions: Introduction
Pericyclic Reactions: Aromaticity of Transition States
Essentially, pericyclic reactions can be treated in a qualitative theoretical manner using three methods:
- Conservation of orbital symmetry (correlation diagrams), Woodward-Hoffmann
- Frontier-orbital method (perturbational molecular orbital theory, orbital interaction diagrams), Fukui
- Aromaticity of transition states (topological methodology), Evans, Dewar, Zimmermann
Principally, all three approaches assure the same results. The third method, applying the principle of aromaticity of transition states, is the simplest and most descriptive one. According to the theoretical definition of pericyclic reactions, a cyclically delocalized system of electrons is formed in the transistion state which can be either aromatic or antiaromatic. This fact was recognized by Evans as early as 1938 in order to explain the observation that ethylene does not react with itself to produce cyclobutane but reacts with butadiene resulting in cyclohexene.
According to Hückel's rule, the transition state with four electrons leading to cyclobutane is antiaromatic and isoelectronic to the antiaromatic cyclobutadiene. The corresponding transition state producing cyclohexene with six electrons is aromatic and isoelectronic to benzene. The reaction preferrably will pass through the latter, stabilized transition state as compared to the former destabilized one. Although this simplified view is basically correct, the definition of aromaticity has to be expanded in order to be able to predict stereochemistry.