Solvent-free Beckmann rearrangement

The direct conversion of ketones 1 to secondary amides 3 from a solvent-free Beckmann rearrangement was accomplished by heating the ketones with hydroxylamine hy(frochloride and anhydrous oxalic acid at 100 °C for 4-12... 

The quest for a solvent-free deprotection procedure has led to the use of relatively benign reagent, ammonium persulfate on silica, for regeneration of carbonyl compounds (Scheme 6.10) [48]. Neat oximes are simply mixed with solid supported reagent and the contents are irradiated in a MW oven to regenerate free aldehydes or ketones in a process that is applicable to both, aldoximes and ketoximes. The critical role of surface needs to be emphasized since the same reagent supported on clay surface delivers predominantly the Beckmann rearrangement products, the amides [49]. 

Due to its Lewis acidic properties, the use of chloral (trichloroacetaldehyde) in the Beckmann rearrangement was investigated . When a variety of ketoximes is admixed with chloral hydrate and the mixture is heated at low pressure in nitrogen atmosphere, the Beckmann rearrangement afforded the corresponding amides in excellent yields (73-98%). The transformation occurs under neutral, relatively mild and solvent-free conditions. 

Under solvent-free conditions, one-step Beckmann rearrangement of a variety of ketones and aldehydes proceeded in the presence of alumina sulfuric acid 285 (equation 93). Good selectivities were also obtained in the rearrangement of aldoximes to primary amides using zinc oxide as catalyst " (equation 93). 

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%). 

Sharghi, H., Hosseini, M. Solvent-free and one-step Beckmann rearrangement of ketones and aldehydes by zinc oxide. Synthesis 2002, 1057-1060. 

Chandrasekhar, S., Gopaiaiah, K. Ketones to amides via a formal Beckmann rearrangement in one pot a solvent-free reaction promoted by anhydrous oxalic acid. Possible analogy with the Schmidt reaction. Tetrahedron Lett. 2003,44, 7437-7439. 

A simple montmorillonite KIO clay surface is one among numerous acidic supports that have been explored for Beckmann rearrangement of oximes (Scheme 8.42) [54]. The conditions are not, ho vever, adaptable for aldoximes that are readily dehydrated to the corresponding nitriles under solvent-free conditions. 

Eshghi and Hassankhani (2007) studied the Beckmann rearrangement using sulfuric acid immobilized on the surface of silica gel (Scheme 5.57). Under these conditions, high conversion and excellent selectivity were found in the Beckmann rearrangement of cyclohexanone oxime. Treatment of carbamates and oxazolidi-nones with anhydrides in the presence of SSA under solvent-free conditions at room temperature results in the formation of V-acyl carbamates and oxazolidinones... 

Moghaddam et reported that the Beckmann rearrangement of oximes 2 (R = alkyl or aryl and = aryl) in the presence of AlCls-ZnCb mixture supported on silica gel under microwave irradiation in solvent-free conditions furnished the corresponding amides 3 in excellent yields. 

The cyclic ketoximes 19 and 21 underwent Beckmann rearrangement in the presence of FeCl3 to afford the amides 20 and 22, respectively, under solvent free conditions. ... 

The solvent-free one step Beckmann rearrangement of ketones 1 and aldehydes in the presence of zinc oxide afforded the corresponding amides 3 (R = alkyl, aryl, cycloalkyl = H, cycloalky 1). Symmetrical ketones furnished the amides in good to excellent yields. In the case of unsymmetrical ketones the reaction was selective and one of the two possible amides was formed. Aromatic and aliphatic aldehydes were converted to the corresponding primary amides. 

Alumina sulfuric acid (ASA) was found to be an effective catalyst for the solvent-free one pot Beckmann rearrangement of several alkyl and aryl aldehydes and ketones 1. It has been reported that the electron rich aldehydes and ketones require shorter reaction times than the electron poor aldehydes. Cyclic ketones require longer reaction time than the aryl ketones ascribable to the steric factors. ... 

Phosphoric acid is an efficient and mild reagent for conversion of ketoximes to amide or lactam by Beckmann rearrangement (Baneijee and Mitra, 2005). The reactions are carried out in solvent-free condition under microwave irradiation. It gives good to excellent yields (73-82%). 

Potassium dihydrogen phosphate (KH2PO4) was found to be a new and efficient medium for the Beckmann rearrangement in solvent-free conditions under micro-wave irradiation (Niralwad et al, 2013). It was observed that the yield of the products increases with reduction in time as the power of microwaves was increased. The yield was 93% in 25 min for power 600 watts. 

Bagheri and Karimkoshteh (2013) used nano silica-H SO as an efficient and mild catalyst for the Beckmann rearrangement of ketoximes to amides. The reactions were carried out in solvent-free conditions under microwave irradiation (600 W) within 50-120 sec in good yields. 

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