Beckmann rearrangement mechanism

Theoretical studies on the Beckmann rearrangement mechanism over zeolite catalyst supported by experimental data have increased. The catalytic activity of the zeohte is determined by Brpnsted and Lewis acid sites created by protonation or activation by metallic cations. The reactivity of the acid sites is strongly influenced by the geometry and flexibility of the zeolite framework ". ... 

Coniey, R. T., Ghosh, S. "Abnormai" Beckmann rearrangements. Mechanisms of Molecular Migrations 1971,4, 197-308. 

Isoxazole derivatives are stable toward peracids231 232 but can be ozonolyzed. This, as is well known, enabled the O-benzoyloximes of a-diketones with a well established configuration to be obtained, which were used to investigate the Beckmann rearrangement mechanism.233,234... 

The intermediates (7) have been independently generated in aqueous solution. Note the similarity of this mechanism to those of insertion of CH2 (18-9) and of O (18-19). The three reactions are essentially analogous, both in products and in mechanism. Also note the similarity of the latter part of this mechanism to that of the Beckmann rearrangement (18-17). 

Oximes are also good substrates for allylsamarium bromide addition.25 The Beckmann rearrangement product 44 was produced from oxime 43 in good yields when the ratio of allysamarium bromide to oximes was more than 3 1 (Equation (8)). This type of product was also obtained when the other allylic organometallic compounds were used. The reaction mechanism was proposed as shown in Scheme 13. 

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. 

Triazines 347 were obtained from amidoximes 346 and ethyl orthoacetate (equation 151). The mechanism of formation of products 347 includes Beckmann rearrangement of amidoximes to carbodiimides, followed by reaction with amidoxime and orthoester. ... 

According to Limtrakul and colleagues the mechanism of the Beckmann rearrangement of the oxime molecule on the Br0nsted acid site of a zeolite proceeds according to the following steps outlined in equation 73. 

A similar mechanism to the previous ones was proposed by Deng, Shi and coworkers for the bis(2-oxo-3-oxazolidinyl)phosphinic chloride (BOP-Cl, 269) catalytic Beckmann rearrangement (equation 82). Addition of the Lewis acid zinc chloride improved the catalyst performance and amides were synthesized in excellent yields (92-99%). 

To explain the difficulties encountered in the reuse of sulfonyl chloride functionalized ionic liquids during Beckmann rearrangement, Deng and colleagues proposed a mechanism for rearrangement of cyclohexanone oxime (equation 97). 

A simplified mechanism for the Beckmann rearrangements and important related reactions is shown hi Scheme 9. Summarizing the mechanism section, the key step of the reaction is the migration of an a-carbon group to the electronically deficient nitrogen atom of the oxime. A nitrilium ion in some cases or an imidate in others are key intermediates in the reaction. Their destiny determines the course of the transformation. Basically, three different pathways may be possible and can be synthetically exploited ... 

The best evidence for this mechanism is that the azirine intermediate has been isolated.203 In contrast to the Beckmann rearrangement, this one is sterically indiscriminate 204 Both a syn and an anti ketoxime give the same product. The mechanism as shown above consists of three steps. However, it is possible that the first two steps are concerted, and it is also possible that what is shown as the second step is actually two steps loss of OTs to give a nitrene, and formation of the azirine. In the case of the dichloroamines, HC1 is first lost to... 

The most important route is the conversion of pyrimidines into 1,3,5-triazines. The first one-step transformation was effected by Taylor and Jefford (62JA3744) by heating the pyrimidine (179) with benzenesulfonyl chloride in pyridine (equation 106). The reaction may be considered as an example of an abnormal Beckmann rearrangement. The mechanism of the reaction of the 4-aminopyrimidine (180) is probably dependent on the nature of the 2-substituent (180, R). If R is an electron-releasing moiety, pathway B seems more likely (Scheme 109). The 4-hydroxypyrimidine (179 R = OH) behaves similarly. Many 2-cyano-1,3,5-triazines may be synthesized by this method. 

The acid-catalyzed conversion of ketoximes and aldoximes to amides or amines (the amide is often hydrolyzed to the corresponding amine under the reaction conditions) is known as the Beckmann rearrangement after its discoverer.147 The reaction and its widely accepted mechanism are shown in Equation 6.54. 

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