Nitriles benzofurazans

The thermal or photolytic fragmentation of furazans to nitriles and nitrile Af-oxides has been reported (73JOC1054, 75JOC2880). The irradiation of dimethylfurazan (419) in the presence of cyclopentene, and benzofurazan (420) in the presence of dimethyl acety-lenedicarboxylate, gave isoxazoline (421) and isoxazole (422), respectively, in good yields. The thermolysis of acenaphtho[l,2-c]furazan (423) in the presence of phenylacetylene gave isoxazole (424) in 55% yield. 

Contrary to other Af-oxide containing heterocycles, Bfx and Fx are not formed by direct oxidation of the parents systems, benzofurazan (Bfz) and furazan (Fz), respectively. Even though each kind of heterocycle possesses individual synthetic procedures, some common approaches should be mentioned, i.e., thermo or photochemical intramolecular cychzation of 1,2-azidonitro derivatives and oxidative cychzation of 1,2-dioximes. Some other synthetic procedures, such as oxidation of o-nitroanUines to Bfx or dimerization of nitrile oxides and dehydration of a-nitrooximes to Fx, have been depicted. The following sections provide the most recent descriptions. 

Catalysed oxidation of non-activated haloalkanes by hypochlorite provides an attractive low-cost and convenient procedure for their conversion into carbonyl compounds [6] primary haloalkanes produce carboxylic acids and secondary haloalkanes are converted into ketones (Table 10.12). Secondary amines are oxidized to ketones under analogous conditions, whereas primary amines yield nitriles (Table 10.13) [1,2], o-Nitroanilines are oxidized to benzofurazan-1-oxides [15]. 

Benzofurazans are thermally more stable but can be cleaved photolytically. For example, benzo-furazan itself in benzene affords cyanoisocyanate (17) and azepine (18), the latter being formed by reaction of the solvent with the putative intermediate acylnitrene (19) (Scheme 5) further supporting evidence for the proposed pathway is provided by trapping the nitrile oxide precursor with dimethyl acetylenedicarboxylate and isolation of the methylurethane derivative of the isocyanate <75JOC2880>. Photolysis of diphenylfurazan yields benzonitrile, diphenylfuroxan and 3,5-diphenyl-1,2,4-oxadiazole. 

Benzofurazans show greater thermal stability but may be cleaved photochemically. Irradiation of benzofurazan in benzene and in methanol gives the azepine (26) and the urethane (27), respectively in the presence of triethyl phosphite (Z,Z)-l,4-dicyanobuta-1,3-diene is formed. The proposed mechanism (Scheme 3) involves nitrile oxide, oxazirene, acyl nitrene and isocyanate intermediates, and is supported by spectrophotometric studies (76HCA2727) and by trapping of the nitrile oxide as its isoxazole cycloadduct with DMAD (75JOC2880). 

Oxadiazoles undergo thermal and photochemical ring cleavage at the 0(1)N(2) and C(3)C(4) bonds to yield nitrile and nitrile oxide fragments, and products derived therefrom. Thus, diphenylfurazan 31 (X — O) decomposes under flash vacuum pyrolysis conditions (600 C, 103mmHg) affording benzonitrile and benzonitrile oxide in nearly quantitative yields. Benzofurazans are thermally more stable but can be cleaved photolytically. 

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