Perturbation Theory in Pericyclic Reactions

There are three ways of looking at pericyclic reactions, all of which give the same results but each of which has its own contribution to make to the understanding of these processes. We shall examine each in turn. 

We have discussed in Section 10.3 the application of perturbation theory to processes in which two molecules come together. We saw there that the most important interactions will be between the HOMO of one molecule and the LUMO of the other. This method can serve as a useful guide in deciding whether there will be a stabilization as a pericyclic reaction begins to occur. The HOMO-LUMO approach was the first one that Woodward and Hoffmann used for ana- 

The symmetry (a) is shown at top center. At left are the butadiene 7r MO s, classified according to their symmetry with respect to o at right are the ethylene w MO s, also classified according to a. The HOMO of each molecule can interact with the LUMO of the other, and a stabilization occurs as they approach one another. 

Let us consider an ethylene and a butadiene approaching each other in the 

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