A Summary of Rules for Pericyclic Reactions

A simple summary of rules for pericyclic reactions is as follows ... 

Kenichi Fukui and Roald Hoffmann won the Nobel prize in 1981 (Woodward died in 1979 and so couldn t share this prize he had already won the Nobel prize in 1965 for his work on synthesis) for the application of orbital symmetry to pericyclic reactions. Theirs is an alternative description to the frontier orbital method we have used and you need to know a little about it. They considered a more fundamental correlation between the symmetry of all the orbitals in the starting materials and all the orbitals in the products. This is rather too complex for our consideration here, and we shall concentrate only on a summary of the conclusions—the Woodward-Hoffmann rules. The most important of these states ... 

In summary, the PMO method when extended to photochemical reactions of the G-type predicts a reversal of aromatic properties from ground state to excited state. This justifies the set of rules for photochemical pericyclic reactions given above. Dougherty s analysis also reveals dangers in the use of orbital and state correlation diagrams for assessing photo-pericyclic reactions because the breakdown in the BO approximation leads to a destruction of symmetry. 

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