Ketoximes, ©-substituted Beckmann rearrangement

The rearrangement of oximes 1 under the influence of acidic reagents to yield A -substituted carboxylic amides 2, is called the Beckmann rearrangement. The reaction is usually applied to ketoximes aldoximes often are less reactive. 

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. 

Prolonged heating of Bi(OAc)3 wifh amines, N-substituted formamides, and alcohols affords moderate yields of amides, N-acetylformamides, and esters [203]. BiCU mediates the Beckmann rearrangement of ketoximes under microwave irradiation [204[, and Bi(NOj)3- il I,/) mediates fhe guanidylation of N-benzoylfhiour-eas (Scheme 14.100) [205]. 

Probably the best-known rearrangement in which an aryl group migrates from carbon to nitrogen is the Beckmann rearrangement. Here, ketoximes are converted to A-substituted amides when treated with an acidic reagent, such as sulfuric or polyphosphoric acid, phosphorus pen-tachloride or thionyl chloride (SOC ). In the context of this chapter, the amides are hydrolysed to liberate the amine. The mechanism in acid media is believed to proceed as illustrated in Scheme 8.3, where the 1,2-shift from carbon to nitrogen is noted as the key step. 

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). 

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