Molecular rearrangements Beckmann rearrangement

Acid catalysis by titanium silicate molecular sieves another area characterized by recent major progress. Whereas only two categories of acid-catalyzed reactions (the Beckmann rearrangement and MTBE synthesis) were included in the review by Notari in 1996 (33), the list has grown significantly since then. In view of the presence of weak Lewis acid sites on the surfaces of these catalysts, they can be used for reactions that require such weak acidity. 

The main intermediate of the rearrangement may be a nitrilium ion (225) in some cases or an imidate (226) in others. The resulting intermediate reacts with water to produce the amide (218) after tautomerization. If other nucleophiles (Nu ) are present, they can intercept the reactive intermediates (both inter- or intra-molecularly) and several different imino-substituted derivatives (227) can be formed. These rearrangement-addition reactions will be analysed later in this chapter as they can effectively broaden the scope of the Beckmann rearrangement reaction (Sections VI.D.2 and VI.E.2). 

The purpose of the present work is to study comparatively the activity of different acidic solids as catalysts in other "classical" type of molecular rearrangement as it is the conversion of oximes to amides (Beckmann rearrangement, egn.l), by adopting "dry media ... 

Caprolactam can be made by the Beckmann rearrangement of the oxime f of cyclohexanone. (Check that you can draw the mechanisms, of both these reactions and look at Chapters 14 and 37 if you find you can t.) Cyclohexanone used to be made by the oxidation of cyclohexane with molecular oxygen until the explosion at Fiixborough in Lincolnshire on 1 June 1974 that killed 28 people. Now cyclohexanone is made from phenol. 

A serious shortcoming of TS-1, in the context of fine chemicals manufacture, is the restriction to substrates that can be accommodated in the relatively small (5.lx5.5 A2) pores of this molecular sieve, e.g. cyclohexene is not epoxidised. This is not the case, however, with ketone ammoximation which involves in situ formation of hydroxylamine by titanium-catalysed oxidation of NH3 with H202. The NH2OH then reacts with the ketone in the bulk solution, which means that the reaction is, in principle, applicable to any ketone (or aldehyde). Indeed it was applied to the synthesis of the oxime of p-hydroxyacetophenone, which is converted, via Beckmann rearrangement, to the analgesic, paracetamol (Fig. 1.24) [75]. 

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

Suginome, H., Kaji, M., and Yamada, S., Photo Induced Molecular Transformations. 87. Regiospecihc Photo Beckmann Rearrangement of Steroidal a,P Unsaturated Ketone Oximes Synthesis of Some Steroidal Enamino Lactams, J. Chem. Soc., Perkin Trans. 1, 1988, 321 326. 

Yashima et al. compared the catalytic performance of H-ZSM-5, H-FER, H-MOR, Ca-A, H-B-MFI, and H-SAPO-5 in the Beckmann rearrangement of cyclohexanone oxime [28]. H-B-MFI was calcined at 603 K only and tetra-n-propylammo-nium remained in the pore. The conversions obtained with Ca-A (molecular sieve 5 A, 8MR) and H-B-MFI were low. As shown in Figure 3, however, the selectivity for e-caprolactam was higher over Ca-A, H-FER (10 MR) and H-B-MFI and lower over H-SAPO-5 (12-MR), H-ZSM-5, and H-MOR (12-MR), which could accommodate cyelohexanone oxime in their pores. It was concluded that the selective formation of e-caprolactam proceeded on the active sites on the external surface of zeolite crystallites rather than in the narrow space of the zeolite pore [28]. They even deduced that at higher reaction temperature cyclohexanone oxime would enter the pore, producing undesirable products such as cyclohexanone and 1-cyanopentene, which are smaller than e-caprolactam, because of the size effect. On the other hand, Curtin and Hodnett reported that caprolactam selectivity was lower over the zeolites with smaller pore diameters [29]. 

When ketoximes are treated with phosphorus pentachloride and then with water, they undergo a molecular transformation which is known as the Beckmann rearrangement. Benzophenoneoxime is converted in this way into benzanilide. The steps involved in the change are probably those indicated below —... 

Analogues of aluminosilicate zeolites, in which the place of aluminium is taken by other Bivalent cations such as B, Ga or Fe, are readily prepared. These give solids with closely similar structure but with weaker acid sites that are also more susceptible to removal of the heteroatom from the framework, for example in the presence of water or steam. Such weakly acidic solids can be selective catalysts for molecular rearrangements that give unwanted by-products in the presence of stronger add catalysts. The Beckmann rearrangement is such a reaction (see Section 8.6.1). 

In line with the idea of reducing the acidity of the zeolites in order to achieve high selectivity and a long catalyst life, the weakly acidic non-zeolitic molecular sieves, for example the medium-pore SAPO-11 or SAPO-41. were used for the Beckmann rearrangement [97]. Over SAPO-11. a 5 % strength solution of cyclohexanoneoxime in acetonitrile reacts at 350 -C. under atmospheric pressure and at a WHSV of 10.8 h to give e-caprolactam with 95 % selectivity and a conversion of 98 %. 

Another prominent route to develop 1,5-difunctionlized molecular skeletons is the oxidative cleavage of cyclopentene derivatives (Scheme 2.57). In Corey s terminology of retrosynthesis this is the reconnect operation (reconnection of the two functionalities). Other reactions that faU under this heading include the Baeyer-Villiger oxidation and the Beckmann rearrangement of cyclopentanones. 

The starting materials were low and medium molecular weight copolymers. Infrared spectra of the products from the Schmidt reaction show a high degree of conversion. The yields of the oximes and subsequent Beckmann rearrangements are also high. 

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