Catalysts for Beckmann Rearrangements

Beckmann rearrangement. The antimony(V) salt 1 is an effective catalyst for Beckmann rearrangement of ketoxime trimcthylsilyl ethers to amides or lactams. ... 

Silica supported dichlorophosphate has been found to be an efficient, recoverable and reusable catalyst for Beckmann rearrangement of a variety of ketoximes and dehydration of various aldoximes in tetrahydrofuran under microwave irradiation (Li and Lu, 2008). This protocol is also advantageous because of high conversion, high selectivity, short reaction time, no environmental pollution and simple work-up procedure. 

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). 

Salts of o-halogeno-N-heterocyclics, such as 2-fluoro-l-methylpyri-dinium tosylate, and related compounds have been used successfully in a variety of reactions, particularly as agents for esterification, lactoni-zation, and the preparation of amides and thiolic esters They have been used also for the preparation of iodides from alcohols and acid fluorides from the acids as well as for Beckmann rearrangements and reductions, e.g. of a-hydroxyketones to ketones . Various condensed heterocyclic salts have been prepared from l-acylmethyl-2-chloropyridinium salts 2-Dialkylaminopyridinium salts have been used as phase transfer catalysts... 

Zeolites have also been described as efficient catalysts for acylation,11 for the preparation of acetals,12 and proved to be useful for acetal hydrolysis13 or intramolecular lactonization of hydroxyalkanoic acids,14 to name a few examples of their application. A number of isomerizations and skeletal rearrangements promoted by these porous materials have also been reported. From these, we can underline two important industrial processes such as the isomerization of xylenes,2 and the Beckmann rearrangement of cyclohexanone oxime to e-caprolactam,15 which is an intermediate for polyamide manufacture. Other applications include the conversion of n-butane to isobutane,16 Fries rearrangement of phenyl esters,17 or the rearrangement of epoxides to carbonyl compounds.18... 

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 common name caprolactam comes from the original name for the Ce carboxylic acid, caproic acid. Caprolactam is the cyclic amide (lactam) of 6-aminocaproic acid. Its manufacture is from cyclohexanone, made usually from cyclohexane (58%), but also available from phenol (42%). Some of the cyclohexanol in cyclohexanone/cyclohexanol mixtures can be converted to cyclohexanone by a ZnO catalyst at 400°C. Then the cyclohexanone is converted into the oxime with hydroxylamine. The oxime undergoes a very famous acid-catalyzed reaction called the Beckmann rearrangement to give caprolactam. Sulfuric acid at 100-120°C is common but phosphoric acid is also used, since after treatment with ammonia the by-product becomes... 

The previous referred inconveniences have prompted an increasing interest in the development of alternative, essentially neutral and more environmental-friendly catalysts to promote the rearrangement of O-unsubstituted oximes. The development of highly efficient and selective transformations and also of processes for catalyst recovery and its reuse are the aim of some of the more recent studies. Much of this work is being done in industry to improve current production processes and is the subject of new patent applications. During the last two decades environment concerns have led to the development of green, simple and cost-effective catalytic systems for the Beckmann rearrangement. 

Zeolites and other mesoporous materials are excellent catalysts for industrial and laboratory applications. Favourable characteristics are their capacity to immobihze homogenous catalysts rendering them heterogeneous, their thermal stability, and the ease of separation from the reaction products and reuse in hquid- and gas-phase conditions. The pore size and Brpnsted and Lewis acidic properties are determinant for their use as catalyst in the Beckmann rearrangement. Recently, a review on the use of zeolites and mesoporous materials in the Beckmann rearrangement was published. ... 

Rare-earth exchanged [Ce ", La ", Sm"" and RE (RE = La/Ce/Pr/Nd)] Na-Y zeolites, K-10 montmorillonite clay and amorphous silica-alumina have also been employed as solid acid catalysts for the vapour-phase Beckmann rearrangement of salicylaldoxime 245 to benzoxazole 248 (equation 74) and of cinnamaldoxime to isoquinoline . Under appropriate reaction conditions on zeolites, salicyl aldoxime 245 undergoes E-Z isomerization followed by Beckmann rearrangement and leads to the formation of benzoxazole 248 as the major product. Fragmentation product 247 and primary amide 246 are formed as minor compounds. When catalysts with both Br0nsted and Lewis acidity were used, a correlation between the amount of Br0nsted acid sites and benzoxazole 248 yields was observed. 

Acidic zeolites, K-10 clay and silica are highly active and selective catalysts for the dehydration/Beckmann rearrangement reactions of aldoxhnes (benzaldoxime and 4-methoxybenzaldoxime) for the synthesis of nitriles and amides . 

Sulfamic acid (+H3NS03 ) has been proved to be an efficient and green catalyst for liquid Beckmann rearrangement of ketoxime in anhydrous acetonitrile. Due to its intrinsic zwitterionic property, the use of a base for the neutralization is avoided and wastes can be reduced. 

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%). 

Indium trifluoromethanesulfonate was found to be an effective high-yielding catalyst for the facile dehydration of aldoximes to nitriles and Beckmann rearrangement of ketoximes to anilides. ... 

The industrial e-caprolactam processes with cyclohexanone oxime as intermediate product were recently reviewed . The catalytic gas-phase Beckmann rearrangement has great industrial interest. Since the process proposed by DuPont in 1938 the investigation on catalytic gas-phase Beckmann rearrangement has been investigated, and a large variety of catalysts have been tested for the reaction. 

As the Beckmann rearrangement is believed to be a typical acid-catalysed reaction, many researchers have reported the relationship between the vapour phase reaction catalysis and the acidity of the catalysts tested on non-zeolitic catalysts - i2s- i3i. 318-334 and on zeolitic catalysts Another interesting point for the heterogeneous gas-phase Beckmann rearrangement is the location of the reaction on the catalyst and different studies have been published ° . The outer surface of the catalyst particle seems to be the most probable place for the Beckmann rearrangement supported by the traces of reagents, and notable amounts of by-products found only in the outer layers of the zeolite crystal. Development of new and more efficient catalysts have also been reported " . ... 

Recently, the Sumitomo Chemical Co., Ltd. developed the vapour-phase Beckmann rearrangement process for the production of 8-caprolactam. In the process, cyclohexanone oxime is rearranged to e-caprolactam by using a zeolite as a catalyst instead of sulfuric acid. EniChem in Italy developed the ammoximation process that involves the direct production of cyclohexanone oxime without producing any ammonium sulfate. The Sumitomo Chemical Co., Ltd. commercialized the combined process of vapour-phase Beckmann rearrangement and ammoximation in 2003 ". 

There is, as is well known, a close similarity between the crystalline and porous structures of silicalite-1 and silicalite-2. The same similarity therefore exists between TS-1 and TS-2, and it appears logical that they should have very similar catalytic properties. TS-2 has been evaluated as a catalyst for many different reactions, such as Beckmann rearrangement of cyclohexanone oxime with vapor-phase reactants H202 oxidation of phenol, anisole, benzene, toluene, n-hexane, and cyclohexane and ammoximation of cyclohexanone. As described in detail in Section V.C.3, differences that had been claimed between the catalytic properties of TS-1 and those of TS-2 have not been substantiated. Later investigations have shown that, when all the relevant parameters are identical, the catalytic activities of TS-1 and TS-2 are also identical. The small differences in the crystalline structure between the two materials have no influence on their catalytic properties (Tuel et al., 1993a). 

---