Alfa Chemistry is also a preferred partner for many universities and non-profit institutes. Tuesday, 24 April Beckmann Rearrangement. The Beckmann rearrangement, named after the German chemist Ernst Otto Beckmann —is an acid-catalyzed rearrangement of an oxime to an amide. Most commonly used catalysts are Conc. Aldoximes are less reactive than ketoximes.
Cyclic oximes yield lactams. Mechanism of Beckmann rearrangement:. Initially the -OH group of the oxime is protonated. Then 1,2 shift of alkyl group R 1 onto electron deficient nitrogen and the cleavage of N-O bond occurs simultaneously. Always the alkyl group which is 'anti' to the -OH group on nitrogen undergoes 1,2 shift which indicates the concerted nature of the beckmann rearrangement.
Where is it used? This reagent is useful in ring enlargement of cyclic ketone. A very good example is the industrial conversion of cyclohexanone to caprolactam, which is used in the manufacture of Nylon-6, involves Beckmann rearrangement. Mechanism of this rearrangement is given below. Also relative migratory aptitude comes in place when there are two different groups as shown below.
Above two isomers for the unsymmetrical oxime are possible. When these oximes are rearranged mixture of products are formed and the ratio in which they form is same as the isomer ratio in oxime.
Oximes derived from aldehydes are not good for Beckmann rearrangement because of the poor yields for primary amides. Tosyl chloride forms oxime tosylate which eliminates the stable tosylate anion. Newer Post Older Post Home.Beckmann rearrangement of ketoxime catalyzed by acidic ionic liquid- N -methyl-imidazolium hydrosulfate was studied.
The catalyst could be reused three cycles with the same efficiency. Finally, reactions of other ketoximes were also investigated. Over the past years, amide derivatives have received much attention owing to their broad range of applications in many fields such as the pharmaceutical industry, chemical biology, the agrochemical industry, engineering plastics, and so on [ 123456 ].
Various approaches have been developed for the synthesis of amide compounds including nucleophilic acyl substitution reactions with amines [ 7 ], Staudinger ligation [ 8 ], Schmidt reaction [ 9 ] and Beckmann rearrangement [ 10 ]. However, generations of large amounts of undesired by-products and corrosive phenomenon associated with common acid H 2 SO 4 and SOCl 2 based on liquid phase protocols provide a challenging task for chemists to develop alternative methods [ 1112 ].
Hypervalent Iodine-Mediated Beckmann Rearrangement of Ketoximes
A variety of alternative routes [ 13141516 ] based on organic and inorganic solid acids were developed. However, traditional methods often suffer from some drawbacks such as poor selectivity, harsh conditions, are not atom economic, or are not environmentally friendly. From the point of view of atom conversion efficiency, Beckmann rearrangement is a perfect way for construction of amides, in general sulfuric acid is most commonly used rearrangement catalyst in commercial production of amides.
However, it brings equipment corrosion and environmental pollution problems. Recently, ionic liquids [ 17 ] have emerged as potential green alternatives to organic solvents due to their unique properties of low volatility, high polarity, good thermal stability, and excellent solubility [ 181920 ].
Further, there are more potential capabilities as effective catalysts and reagents [ 13 ], as chemical transformations have also been explored. In order to develop a green pathway of amide synthesis, we report here a Beckmann rearrangement reaction catalyzed by N -methyl-imidazolium hydrosulfate [HMIm]HSO 4 [ 21 ] under solvent free conditions Scheme 1. When CuCl 2. The results are presented in Table 1. Reaction conditions: Benzophenone oxime 9. The effect of the amount of co-catalyst P 2 O 5 on reaction was investigated.
The results are presented in Figure 1. Effect of the amount of co-catalyst P 2 O 5 on Beckmann rearrangement of benzophenone oxime. The influence of the reaction temperature on the yield was investigated subsequently. The results are presented in Figure 2. Influence of the reaction temperature on Beckmann rearrangement of benzophenone oxime. The recycling performance of ionic liquid has the most benefits from the point of view of environmental protection.DOI : The procedure is mild and suitable for both aromatic and cycloaliphatic substrates affording the products in good to quantitative yields with short reaction times.
Journal Name: Letters in Organic Chemistry. Volume 14Issue 2 Journal Home. Methods: A suspension of acetophenone oxime 0. The combined organic extracts were dried over Na2SO4, filtered, and concentrated. Results: In a continuation of our studies on the use of the Mukaiyama reagent in organic transformations, we became interested in evaluating it as a reagent in the presence of triethylamine as a base for the conversion of ketoximes into corresponding N-substituted amides under mild conditions.
To optimize the reaction conditions and find the best base and solvent using benzophenone oxime as a model substrate, a few experiments were carried out with Mukaiyama reagent and various bases and solvents at room temperature. The optimum conditions of reaction involved benzophenone oxime 1 mmolMukaiyama reagent 1. Under the optimized conditions, a series of N-substituted amides were studied to establish the scope and limitations of this method. A wide range of substituted ketoximes derived from various aromatic, cycloaliphatic, and heterocyclic ketoximes gave desired products in good to excellent yields.
Conclusion: We have disclosed a mild procedure for obtaining amides from the corresponding ketoximes via Beckmann rearrangement using Mukaiyama reagent. Among the attractive features of this protocol are its use of inexpensive and commercially available reagent, mild reaction conditions, simplicity, general applicability, relatively short reaction time, high yield and good selectivity.
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Show More. No Downloads. Views Total views. Actions Shares.The Beckmann rearrangementnamed after the German chemist Ernst Otto Beckmann —is a rearrangement of an oxime functional group to substituted amides. Cyclic oximes and haloimines yield lactams. The Beckmann rearrangement is often catalyzed by acid, however other reagents have been known to promote the rearrangement. These include tosyl chloridethionyl chloridephosphorus pentachloridephosphorus pentoxidetriethylaminesodium hydroxidetrimethylsilyl iodide among others.
Certain conditions have been known to racemize the oxime geometry, leading to the formation of both regioisomers. The rearrangement of aldoximes occurs with stereospecificity in the gas phase and without stereospecificity in the solution phase. A few methodologies allow for the rearrangement of aldoximes to primary amides, but fragmentation commonly competes in these systems. Nitrone rearrangement also occurs without stereospecificity; the regioisomer formed has the amide nitrogen substituted with the group possessing the greatest migratory aptitude.
The archetypal Beckmann rearrangement  is the conversion of cyclohexanone to caprolactam via the oxime. Caprolactam is the feedstock in the production of Nylon 6. The Beckmann solution consists of acetic acidhydrochloric acid and acetic anhydrideand was widely used to catalyze the rearrangement. Other acids, such as sulfuric acidpolyphosphoric acidand hydrogen fluoride have all been used. Sulfuric acid is the most commonly used acid for commercial lactam production due to its formation of an ammonium sulfate by-product when neutralized with ammonia.
Ammonium sulfate is a common agricultural fertilizer providing nitrogen and sulfur. The most common reaction mechanism of the Beckmann rearrangement consists generally of an alkyl migration anti-periplanar to the expulsion of a leaving group to form a nitrilium ion. This is followed by solvolysis to an imidate and then tautomerization to the amide: . This nitrilium ion has been known to be intercepted by other nucleophiles, including the leaving group from the oxime.
Presumably after the phenyl group migrates and expels the cyanateit then attacks the nitrillium ion formed. In carbon tetrachloride the isocyanate can be isolated, whereas in ethanol the urethane is formed after solvolysis of the isocyanate. One computational study has established the mechanism accounting for solvent molecules and substituents. The oxygen atom in the hydroxyl group is stabilized by three acetic acid molecules.
In the next step the electrophilic carbon atom in the nitrilium ion is attacked by water and a proton is donated back to acetic acid. In the transition state leading to the imidate, the water oxygen atom is coordinated to 4 other atoms. In the third step, an isomerization step protonates the nitrogen atom leading to the amide.These metrics are regularly updated to reflect usage leading up to the last few days.
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