Beckmann fragmentation mechanism

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. 

Each migrating tertiary group must have lost contact with the amide fragment it started out with. Each molecule falls to bits to give a f-alkyl cation and a nitrile the Beckmann rearrangement now goes via a fragmentation mechanism. 

Another collection of related intermediates occurs in the Ritter reaction and the Beckmann fragmentation. The Ritter reaction involves the combination of a tertiary alcohol and a nitrile in acid solution and the proposed mechanism involves a series of intermediates. 

The experiments also provide stereochemical evidence that a carbocation is an intermediate in both reactions. Both starting materials are ns-decalins but the product is a frans-decalin. The carbocation intermediate has no stereochemistry and can react with the nitrile from either face. Axial attack is preferred and it gives the stable frcws-decal in. The formation of the carbocation is shown only by the Beckmann fragmentation formation from the alcohol by the S l mechanism is obvious. 

Scheme S.6 Proposed self-initiation mechanism during Rerkmann rearrangement. Scheme S.7 Beckmann fragmentation. 

The starting material is bicyclic, the product monocyclic, so we have broken a C-C bond the reaction is a fragmentation. The mechanism is straightforward once you know what happens to Beckmann rearrangements when the migrating group is tertiary—but hard to follow unless you number the atoms ... 

This is a good step because it is the normal Beckmann rearrangement. However, to get from this intermediate to the open-chain product took a fairly large number of creative steps. Application of Occam s razor suggests that the initial fragmentation occurs early in the mechanism. 

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