Orbital descriptions of -sigmatropic rearrangements

It is possible to give a frontier orbital description of a [3,3] -sigmatropic rearrangement but this is not a very satisfactory treatment because two reagents are not recognizing each other across space as they were in cycloadditions. There are three components in these reactions—two nonconjugated K bonds that do have to overlap across space and a O bond in the chain joining the two k bonds. 

The Woodward-Hoffmann rules give a more satisfying description and we shall follow the routine outlined for cycloadditions. Note that for stage 3, we can use the three-dimensional diagram we have already made. 

1 Draw the mechanism for the reaction (we shall stay with a familiar one) 

Choose the components. All the bonds taking part in the mechanism must be included and no others 

Make a three-dimensional drawing of the way the components come together for the reaction, putting in orbitals at the ends of the components (only ) 

4 Join up the components where new bonds are to be formed. Make sure you join orbitals that are going to form new bonds 

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