Carbocations migratory aptitudes

The carbocations generated by non-Kolbe electrolysis can rearrange by alkyl, phenyl or oxygen migration. The migratory aptitudes of different alkyl groups have been studied in the rearrangement of a-hydroxy carboxylic acids (Eq. 34) [323]. 

In order to answer the question about inherent migratory aptitudes, the obvious type of substrate to use (in the pinacol rearrangement) is RR C—CRR , since the same carbocation... 

In reactions of this type, it is possible to form more than one initial carbocation if the starting material is not symmetrical. In this situation, the more stable carbocation is usually formed in step 1. Once this initial carbocation has been formed, the course of step 2 is more difficult to predict because it depends on the propensity of one group to migrate in preference to another (migratory aptitude), which often depends on reaction conditions. 

Of the two OH groups, the one which forms the more stable carbocation is protonated preferentially. This factor takes precedence over the migratory aptitude factor. Further evidence for carbenium ion formation in the pinacol rearrangement has been obtained by oxygen-exchange experiments. Partial rearrangement of pinacol to pinacolone has been carried out in acidic solutions containing H20 (Scheme 2.27). 

From these studies, it can be concluded that it is not a simple matter to define the general substituent migratory aptitudes in chloiin and bacteriochlorin systems, because distant conjugated peripheral substituents can dramatically affect the stability of carbocation intermediates and hence the products. The facility of the rearrangement of various substituents depends not only on the intrinsic nature of the migratory group but also on the electronic and steric factors present elsewhere on the tetrapyrrolic systems. 

The product is formed via the more stable carbocation intermediate, with the final product determined by the migratory aptitude of the substituents at the neighboring alcohol-bearing carbon. 

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