Carbocations Beckmann rearrangements

The reaction that normally occurs on treatment of a ketoxime with a Lewis or proton acid is the Beckmann rearrangement (18-17) fragmentations are considered side reactions, often called abnormal or second-order Beckmann rearrangements. Obviously, the substrates mentioned are much more susceptible to fragmentation than are ordinary ketoximes, since in each case an unshared pair is available to assist in removal of the group cleaving from the carbon. However, fragmentation is a side reaction even with ordinary ketoximes and, in cases where a particularly stable carbocation can be cleaved, may be the main reaction. ... 

A number of reactions have been explained on the basis of generation of carbocations. The examples include the Friedel-Crafts alkylation and arylation reactions. Besides pinacol-pinacolne rearrangement, Beckmann rearrangement and Wagner-Merwein rearrangement are other examples. 

How Does Carbocation Stability Control the Beckmann Rearrangement Reaction ... 

The focus of the next four chapters (Chapters 14-17) is mainly on the theoretical/computational aspects. Chapter 14 by T. S. Sorensen and E. C. F. Yang examines the involvement of p-hydrido cation intermediates in the context of the industrially important heptane to toluene dehydrocyclization process. Chapter 15 by P. M. Esteves et al. is devoted to theoretical studies of carbonium ions. Chapter 16 by G. L. Borosky and K. K. Laali presents a computational study on aza-PAH carbocations as models for the oxidized metabolites of Aza-PAHs. Chapter 17 by S. C. Ammal and H. Yamataka examines the borderline Beckmann rearrangement-fragmentation mechanism and explores the influence of carbocation stability on the reaction mechanism. 

Ammal SC, Yamataka H. How does carbocation stability control the beckmann rearrangement reaction In KK Laali, editor. Recent developments in carbocation and onium ion chemistry, ACS Symposium series, Vol. 965. New York American Chemical Society 2007, Chapter 17. 

Beckmann rearrangement of oximes to amides can deviate to fragmentation to form nitriles and carbocations, if the latter possess reasonable stability. Both 1-substituted-phenyl-2-propanones and 3-substituted-phenyl-2-butanones in aqueous solvents give both products, and calculations have been used to probe the mechanisms. In borderline cases, a dynamic path bifurcation from a single transition state is claimed. 

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