Migratory Aptitudes in the Pinacol Rearrangement

Intermolecular migratory aptitudes in the pinacol rearrangement have been determined by comparing the rates of rearrangement of different pinacols, each with four identical substituents of the type 37. 

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

We have already mentioned that migratory aptitudes are dependent on the reaction and on the conditions under which the reaction is carried out. An example of the latter type of variation is that in the pinacol rearrangement of triphenylethylene glycol, the phenyl/hydrogen migration ratio may vary by a factor of 180 (from 7.33 to 0.41) when the catalyst is changed from concentrated sulfuric acid to HC1 in water/dioxane.48... 

Explain why the migratory aptitude of the o-anisyl group is only 0.3 in the pinacol rearrangement. 

The migratory aptitude of the substituents in the bicyclic triazoline (Scheme 163) is the same as in the pinacolic rearrangement. In addition to the aziridine, which is always minor, a ring-enlarged imine is formed when R1 =CH3.100... 

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). 

A recent review (Collins, 1960) of the pinacol rearrangement covers nearly all of the pertinent isotopic experiments that have been performed. One example of the use of two carbon-14 labels to relate the aldehyde-ketone and pinacol rearrangements will be discussed in detail here. The anomaly of the apparent reversal, in the aldehyde-ketone rearrangement, of the usual order of migratory aptitudes has been mentioned by Wheland (1949) and by Ingold (1953), and relates to the fiict that, for example, 2-methyl-2-phenylpropionaldehyde (67), in concentrated sul-... 

A number of experiments have been carried out to determine the relative migratory aptitude of groups in pinacol/pinacolone type rearrangements, and in general the relative ease of migration is found to be ... 

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