Beckmann rearrangement promoter

Activated Fly ash , an industrial waste pollutant, is found to be a good catalyst in Beckmann rearrangement promoted by microwaves under solvent-free conditions. The amides are obtained from the corresponding ketoximes in high yields (75-94%). 

Harada, T., Ohno, T., Kobayashi, S., and Mukaiyama, T. 1991. The catalytic Friedel-Crafts acylation reaction and the catalytic Beckmann rearrangement promoted by gallium(Itl) or an antimony(V) cationic species. Synthesis 1216-1220. 

The pentachloride is most used. It will convert snlphonic acids to snlphonyl halides (12.276), alcohols to halides (12.277) and carboxylic acids to halides (12.278). Af-alkyl-substituted amides (12.279) and ketoximes (12.280) can be converted to nitriles and the Beckmann rearrangement promoted (12.281). 

Application in Green Chemistry As a Beckmann Rearrangement Promoter... 

A modification of this method, related to the Beckmann rearrangement, entails treatment of a ketoxime with one equivalent of CDI, then four to five equivalents of a reactive halide such as allyl bromide or methyl iodide (R3X) under reflux in acetonitrile for 0.5-1.5 h. Quatemization of the imidazole ring effectively promotes the reaction by increasing the electron-withdrawing effect. The target amides then are obtained by hydrolysis. High yields, neutral conditions, and a very simple procedure make this modification of the synthesis of amides by azolides a very useful alternative. 1243... 

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... 

The Beckmann rearrangement of oximes to produce amides is promoted by perrhenate ions under phase-transfer catalytic conditions, in the presence of trifluoro-methanesulphonic acid in nitromethane [6]. Under these conditions, the rearrangement reaction is frequently accompanied by the solvolysis of the oxime to the ketone. This can be obviated by the addition of hydroxylamine hydrochloride. No reaction occurs in the absence of the ammonium catalyst or with the O-acetyl oximes. 

Organoaluminum-promoted Beckmann rearrangement/methylation of cyclohexanone oxime mesylate, followed by allylation of ketimine 40a and Mannich cyclization of the intermediate iminium-allylsilane, provides piperidine 40b possessing cxo-unsaturation (08BKC1669). 

Alternatively, oximes may be converted to O-substituted oximes (typically O-tosyl oximes) making the rearrangement much easier. Sometimes, these oxime derivatives rearrange spontaneously under the condition of their formation and cannot be isolated. Usually, O -tosyl ketoximes rearrange smoothly with exclusive anti migration. Relative to the acid-promoted Beckmann rearrangement, the rearrangement of O -tosyl oximes is much milder and specific. 

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. 

Pathway 2 of Scheme 9 corresponds to one of the most interesting developments in the Beckmann rearrangement chemistry. By trapping of the electrophilic intermediate with a nucleophile (Nu ) other than water, an imine derivative 227 is produced that may be used for further transformations. Carbon or heteroatom nucleophiles have been used to trap the nitrilium intermediate. Reducing agents promote the amine formation. More than one nucleophile may be added (for example, two different Grignard reagents can be introduced at the electrophilic carbon atom). Some of the most used transformations are condensed in Scheme 11. 

Another synthetic route via the Beckmann rearrangement, which is promoted by organoaluminum reagent along with alkylation, involves a new stereoselective reduction of the imino group. The starting oxime sulfonate (228) was synthesized from cyclopentanone (226) in three steps Reaction of 226 with 1-undecene in the presence of silver oxide produced the a-undecylcyclopentanone (227) which on successive treatment with hydroxylamine and methanesulfonyl chlo-ride-triethylamine gave the mesylate (228). Treatment of the oxime mesylate... 

Alternative purification protocols are available. Zhaolin Sun of Lanzhou University reports (Tetrahedron Lett. 45 2681,2004) that the ionic liquid TISC was specifically designed to promote Beckmann rearrangement. TISC is not soluble in water, so the product caprolactam was easily removed from the ionic liquid by extraction with water. 

Activation of the Beckmann rearrangement of the enantiopure spirocyclic keto oximes (—)-(12) and (—)-(13) has been initiated with four acidic promoters.17 In two cases (PPE and PPSE), concerted [1,2]-shift of the anti carbon operates exclusively. This is not the case with PPA or Eaton s reagent, although optical activity is fully maintained in these ring expansions. 

Such condensation reactions are also promoted by certain trTvalent phosphorus compounds, e.g. triphenyl phosphite (2) or diphenyl ethylphosphonite (3), or to a lesser extent by pFosphonate esters, e.g. diphenyl n-butylphosphonate (3). "Bates reagent," p-oxobi s[tri s(cTi methyl ami no)phosphoni urn] bi s-tetra-f1uoroborate (2) may also be used to activate the carboxyl function towards amide bond formation during peptide synthesis (4) and to bring about the Beckmann rearrangement of ketoximes (F). 

Treatment of a wide variety of oxime sulfonates with several equivalents of alkyl-aluminum reagents in CH2CI2 resulted in formation of the imines, which were directly reduced with excess DIBAH to give the corresponding amines, as shown in Sch. 19. This organoaluminum-promoted Beckmann Rearrangement of oxime sulfonates has been successfully applied to the stereoselective synthesis of naturally occurring alkaloids, pumiliotoxin C, and solenopsin A and B, as illustrated in Sch. 20 [43]. 

Combination of silyl enol ethers with the organoaluminum-promoted Beckmann rearrangement of oxime sulfonates resulted in a novel reaction system that leads to the formation of enaminones [44]. Treatment of a mixture of anfr-2-methylcyclohexa-none oxime sulfonate (33) and 2-(trimethylsiloxy)-l-octene in dry CH2CI2 with Et2AlCl at -78 °C for 30 min, and at 20 °C for additional 1 h resulted in formation of the enaminone 34 in 90 % yield (Sch. 21). 

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