Homo allyl migration

Thus in the rearrangement, in the transition state, there will be an overlap between the orbital of the atom or free radical (R) of the group which is migrating and the orbital of an allylic free radical. So there will be an overlap between the HOMO of one component and the HOMO of the other. 

The HOMO of an allylic radical depends on the number of carbons in the tt framework. The migrating group is passed from one end of the allylic radical to the other, and so it is the end carbons that we are concerned with. We see that... 

The Cope rearrangement has a nonpolar transition state (Fig. 20.51), but this time it is not a simple hydrogen atom that migrates but a whole allyl radical. An analysis of the symmetry of the orbitals involved shows why this reaction is a relatively facile thermal process but is not commonly observed on photochemical activation. As we break the C(l)—C(l) bond, the phases of the overlapping lobes must be the same. The HOMO of the allyl radical is 4>2, that information allows us to fill in the symmetries of the two allyl groups making up the transition state. 

Another type of equilibrium which has been studied by NMR spectroscopy is valence tautomerism. This is illustrated by the example of 3,4-homo-tropilidene (Figure 3.44) in which bond migration leads to the cyclopropane ring switching between two alternative positions. Thus C-8 changes from a cyclopropyl to an allylic carbon. By lowering the temperature to below — 40°Q it is possible to freeze the equilibrium sufficiently to record the spectrum of a single structure rather than that of the equilibrium mixture. 

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