In the pinacol rearrangement

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... 

Rearrangements Involving Diazonium Ions. Aminomethyl carbinols yield ketones when treated with nitrous acid. The reaction proceeds by formation and rearrangement of diazonium ions. The diazotization reaction generates the same type of (J-hydroxycarbocalion that is involved in the pinacol rearrangement. 

In the mildest of these reactions the hydroxylamine is refluxed in ether over phosphorus pentachloride and the same order of migration aptitudes is observed as in the pinacol rearrangement. The range in... 

Stereochemical evidence confirms that neither alkyl nor hydride provides anchimeric assistance in the pinacol rearrangement. Compounds 9 and 10 both... 

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. 

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... 

Although these types of bridged phenonium ions are accepted intermediates in a number of reactions, they do not appear to be involved in the pinacol rearrangement (see Schubert, W. M. LeFevre, P. H. J. Am. Chem. Soc. 1972, 94, 1639). Stabilization of the carbocation 4-10 by resonance with the ojgrgen substituent may be a factor in determining the preference for phenyl migration over phenonium ion formation in the pinacol rearrangement. 

The evidence for carbocation intermediates in the pinacol rearrangement is compelling for some substrates and conditions. Bunton" found that pinacol itself that was recovered from O-enriched aqueous sulfuric acid had incorporated an appreciable amount of solvent oxygen. Under these conditions the reaction goes to completion, i.e. the incorporation cannot be due to reversible formation of diol from pina-... 

High activity and selectivity in the pinacol rearrangement of simple diols were observed in subsequent studies. Y zeolites (HY [27], LaHY [26,28], CaHY [28], NaY, and NaHY [29,30]) had the highest activity, and they were also more selective than X zeolites [29,30]. Though HZSM-5 was less active, its activity remained constant whereas substantial deactivation was observed for Y zeolites [28]. This is explained by the higher resistance of HZSM-5 to coke formation. 

Collins (1955), using two different carbon-14 labels, was able to demonstrate the contributions of four reaction paths in the pinacol rearrangements of 1,1,2-triphenylethylene glycol (125) to benzhydryl phenyl ketone (126) and triphenylacetaldehyde (127) and, in addition, to calculate the contribution of each path under several different conditions from his radiochemical experiments. In a separate series of experiments, Collins and co-workers (1959) applied the same techniques to the rearrangement of l,l,2-triphenylethylene-2-d (125a). The ratios kjk and kjkj were then evaluated from which... 

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). 

Which word or phrase best describes the stereochemistry of the product formed in the pinacol rearrangement of the diol shown ... 

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