Rearrangement Reactions of Free Radicals

In a free radical, there is a third electron in the system that cannot occupy the same orbital as the other two electrons and must instead be in an antibonding level. As a result, the TS for migration is much less favorable than for the corresponding carbocation. The more facile migration of aryl and other unsaturated groups involves 

It has been possible to measure absolute rates and for rearrangement in a series of 2-substituted 2,2-dimethylethyl radicals. The rates at 25° C and the for several substituents are as indicated below. 

The rapid rearrangement of vinyl and acyl substituents can be explained as proceeding through cyclopropyl intermediates. 

Aryl migration is considerably slower because of the diminished susceptibility of the aryl ring to radical addition. Aryl migrations are promoted by steric crowding at the initial radical site. This trend is illustrated by data from the thermal decomposition 

Fukushima, T. Okuda, K. Matsu, N. Kombe, N. Sonada, and M. Komatsu, Synlett, 1265 

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The third section deals with structures and rearrangement reactions of free radicals. This field has seen great advances in recent years, especially because of the use of electron spin resonance techniques. 

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