Cope rearrangement labeling experiments

FIGURE 20.49 The degenerate Cope rearrangement, a [3,3] shift, can be revealed by a labeling experiment. 

FIGURE 20.50 A labeling experiment shows that the Cope rearrangement does not involve formation of a pair of free allyl radicals because the radical recombination products are not all formed. 

There are many things that are important about this work. First of all, it is a prototypal example of the way labeling experiments can be used to dig out the details of a reaction mechanism. The second reason is more subtle, but perhaps more important. As we will see in Section 20.7, the study of the Cope rearrangement leads directly to an extraordinarily intellectually creative idea. 

Many stereochemical labeling experiments are described in this chapter. A careful labeling experiment is able to probe deeply into the details of a reaction mechanism. Examples include W. R Roth s determination of the suprafadal nature of the [1,5] shift of hydrogen and Doering and Roth s experiments on the structure of the transition state for the [3,3] sigmatropic shift known as the Cope rearrangement. 

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