N—O bond fission

Ar—N=N—Ar). A number of possible reaction mechanisms have been suggested at different times and, at this time, the position is not settled. No heavy-atom KIE work has been reported for the acid catalysed reaction, but such experiments have been carried out for the photochemical reaction37 which gives the 2-hydroxy product and which is known to be intramolecular. There is an absence of a KIE when [15N,15N ] material is used, which at least reveals that if the proposed intermediate 36 is involved, then the rate-limiting step must be its formation and not its subsequent reaction since N—O bond fission cannot be part of the slow step. 

Unhindered 3,4-diacylfuroxans prefer ring opening by N—O bond fission of the oxadiazole ring with concomitant migration of an acyl group to give an a-(acyloximino)-p-ketonitrile oxide [type (b)] (103) (Scheme 6.8). 

With regard to the mechanism of decomposition, some authors (Taylor and Vesselovsky [28]) suggest N-O bond fission... 

Treatment of C-aryl oxaziridines (e.g., 109) with dilute acid causes C—O bond fission to the corresponding nitrone 108. However, similar treatment of C-alkyl oxaziridines (e.g., 112) leads to the 1,4-dicarbonyl compound (e.g., 113) by N—O bond fission.The mode of ring opening is influenced by the stabilization of the positive charge resulting from initial protonation on oxygen. 

Evidence for both C-O and N-O bond fission has been found following the irradiation of the pyridone derivatives (211). Irradiation in methanol purged with nitrogen and using wavelengths > 340 nm results in the formation of five products identified as the pyridone (212), the corresponding alcohol (213), the aldehyde (214), the ether (215) and the hydroxypyridone (216). The first three products arise by N-O bond homolysis while the remaining two are the results... 

The pyridinethiones (197) are a useful source of alkoxy radicals upon irradiation. The bond fission processes are brought about by irradiation at 350 nm and the resultant radicals have been demonstrated to react effectively with guanine. A study of the isomeric iV-hydroxypyridine-4-thione has also been reported. N-O bond fission has been studied in iV-phenylhydroxy-lamine. Derivatives of this type have been used frequently as sources of free radicals. For example, the Barton esters (198) have been used to form a-amido radicals. The kinetics of rotation around the C-N bond within these have been measured. Other studies have examined cyclisations within radicals produced from Barton esters. ... 

Lactams (298) can be formed in low yield by the photochemical ring contraction of the diones (299). This involves N-O bond fission followed by loss of COj and rebonding in the resultant 1,4-biradical. 

An extensive and a systematic photochemical studies had been done on 2-isoxazoline. Photolysis of 2-isoxazolines always result in a mixture of products, depending upon the nature of the substituents. Three possible different pathways of photolytic cleavage are reviewed <9iHC(49)l>. Ultraviolet irradiation of 5-unsubstituted 2-isoxazoline (127) in the presence of iron pentacarbonyl, resulted in ring fragmentation products (a ketone (128) and aldehyde (129)) as well as N—O bond fission products ((130) and (131)) (Scheme 21). The plausible mechanism involves N-donor complexation with photochemically generated pentacarbonyl, prior to N—O and C(3)—C(4) bond fissions <83CL51>. 

Whereas alkaline hydrolysis of (Z)-5-(ethoxycarbonylmethylene)-3-p-tolyl-5,6-dihydro-4//-l,2,4-oxadiazine (46 Ar = 4-MeC6H4) (see Section 6.14.9.2.2(i)) results in complete fragmentation of the molecule to yield 4-methylbenzamide, in hot dilute hydrochloric acid initial N—O bond fission is followed by ester hydrolysis and elimination of CO2 to give the amidoxime derivative (47) (Equation 2) <82CPB3987>. 

The steroidal dienol triflate 284 in pyridine photo-extrudes S02 and forms the 6/ -trifluoromethyl derivative 285 presumably via a free radical reaction path257. The anhydride 286 undergoes S—O bond fission to afford radicals. When the reaction is carried out in aromatic solvents such as benzonitrile or nitrobenzene, hydrogen abstraction reaction or addition to the aryl groups of the solvent takes place256. Cadogan and Rowley259 report that the irradiation of the tosyloxy compounds 287 results in N—O bond fission to afford phthalimido radicals. These react with solvent (arene) to afford... 

The enantioselective synthesis of antimalarial (+)-febrifugine and (+)-isofebrifugine alkaloids was achieved through the 1,3-DC of (S)-5-alkoxy-2,3,43-lctrahydropyridine-l-oxide 76 with allyl alcohol followed by hydrogcnolytic N-O bond fission and suitable elaboration of the product <01OL953>. 

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