Hydroximoyl halide, nitrile oxide generation

Different procedures of this dehydrogenation are thoroughly discussed in the monograph (4). It is only necessary to note here that the process is carried out mainly as halogenation-dehydrohalogenation. The intermediate hydroximoyl halide is frequently not isolated (Scheme 1.3). The reaction is convenient for both the generation of unstable nitrile oxides (in the presence of a dipolarophile) and the preparation of stable nitrile oxides. It is usually carried out in a two-phase water-organic solvent system with methylene dichloride as the preferred solvent. 

The principal sources (Scheme 20) of nitrile oxides comprise hydroximoyl halides and oximes nitrolic acids and their precursors and primary nitroalkanes. In addition there is a range of miscellaneous methods of generation which can be applied in particular instances. 

Owing to their tendency to dimerize to furoxans (1,2,5-oxadiazole 2-oxides), nitrile oxides 5 are usually generated in situ, i.e., in the presence of suitable dipolarophiles such as alkenes, alkynes, etc., from stable precursors such as aldoximes 12 (X = H) or from primary nitroalkanes 13 (Scheme 2) [5,57-67]. Generation of nitrile oxides 5 from aldoximes 12 (X = H) involves either direct oxidation or halogenation of aldoximes 12 (X = H) to hydroximoyl halides 12 (X = Cl or Br) followed by dehydrohalogenation [5,57-67,79,80]. Alternatively, nitrile oxides 5 are conveniently generated via dehydration of primary nitroalkanes 13 [ 17,38,39,65,66,81-95]. This review covers the literature in the last 10-15 years pertaining to the chemistry of isoxazoHnes synthesized from primary nitroalkanes 13. 

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