Nitrenes water

Preparation from Nitrene Intermediates. A convenient, small-scale method for the conversion of carboxyhc acid derivatives into isocyanates involves electron sextet rearrangements, such as the ones described by Hofmann and Curtius (12). For example, treatment of ben2amide [55-21-0] with halogens leads to an A/-haloamide (2) which, in the presence of base, forms a nitrene intermediate (3). The nitrene intermediate undergoes rapid rearrangement to yield an isocyanate. Ureas can also be formed in the process if water is present (18,19). 

Fig. 18 Transient absorption spectra detected 60 ns (1) and 350 ns (2) after LFP of n-propyl-4-azido-2,3,4,6-tetrafluorobenzamide (15g) in water-acetonitrile mixture (1 4) at room temperature in the presence of 0.057 M of HCl. Insert dependencies of the rate constants of the decay of singlet nitrenes 16g (1) and 16f (2) on the concentration of HCl.

The various Nb and Ta nitrenes were converted to oxo derivatives by an excess of carboxylic compounds.291,296 This seems characteristic of the nucleophilic reactivity of nitrenes, but water is sometimes required (equation 25). 

The reduction of aromatic nitro compounds to the corresponding amines was catalyzed by [Ru3(CO)i2] in combination with aliphatic amine cocatalysts (95). A mixture of diglyme and water was used as a solvent, turnover frequencies were about 5000 h-1, and a CO partial pressure of 20-50 atm was necessary. The reaction is highly selective for aromatic amines. It was speculated that the reaction proceeds via an intramolecular hydrogen transfer in a hydrido-metal-nitrene intermediate without prior formation of H2 in the water gas shift reaction. 

Photochemical a-addition of a nitrene to 1-isocyano sugars leads to the carbodiimide which adds water yielding glycosylurea or malonic acid to form potential glycosyl-barbiturate synthons [57] (Scheme 29). 

Oximes generally have a high barrier to inversion, and accordingly this reaction is envisioned to proceed by protonation of the oxime hydroxyl, followed by migration of the alkyl substituent trans to nitrogen. The N-O bond is simultaneously cleaved with the expulsion of water, so that formation of a free nitrene is avoided. 

A picosecond Raman spectroscopic examination of the reactions of 2-fluorenyl-nitrene and 4-methoxyphenylnitrene with water revealed singlet nitrenium ions.69 Evidence was presented to suggest that the nitrenes were produced during the photolysis of the corresponding azides. The 4-methoxyphenylnitrene decayed much faster than the 2-fluorenylnitrene. 

As stated in the introduction, chloramine-T (where T denotes three crystalline water molecules) is a commonly used nitrene precursor, which is commercially available and costs less than do most other nitrene sources. The benefit of a silver salt in nitrene transfer reactions with chloramine-T is surprisingly simple. Because silver chloride is insoluble in most solvents, substoichiometric amounts of silver salts (like silver nitrate) can be used to remove the chloride from chloramine to facilitate the release of a free nitrene radical, which can aziridinate olefins. Since the amount of silver is near stoichiometric, it should not be called silver-based catalysis, although turnover numbers (TONs) higher than 1 have been observed in some cases. 

Cytochrome P-450 can perform both intramolecular and intermolecular nitrene insertions with 106 in water solution [181]. However, the intermediate metallo-nitrene (108) also hydrolyzed to some extent so the enzyme performed a hydroxylation as well as an amidation. Although we have also performed an intermolecular amidation of a steroid, using such a metalloporphyrin reaction with 106 to form 112 [182], we have not yet extended it to the kinds of directed functionalizations described above with either benzophenones or chlorinations. 

Photochemical irradiation of 4-hydrazinoisoxazolo[5,4- ]pyridines affords pyrazolo[4,3-c]py-ridines via cleavage of the N—O bond to give nitrene intermediates which then evolve into the final products by an intramolecular attack of the hydrazine group <88H(27)1899>. For example, irradiation of 6-chloro-4-hydrazino-3-methylisoxazolo[5,4- ]pyridine (592) with a water-cooled low-pressure mercury immersion lamp (6W) gave l-amino-6-chloro-3-methylpyrazolo[4,3-c]pyridin-4-one (593) (60%) (Scheme 76). 

Nitrenes, like carbenes, are immensely reactive and electrophilic, and the same Wolff-style migration takes place to give an isocyanate. The substituent R migrates from carbon to the electron-deficient nitrogen atom of the nitrene. Isocyanates are unstable to hydrolysis attack by water on the carbonyl group gives a carbamic acid which decomposes to an amine. 

Nitrene, NH, has been suggested as an intermediate in the photolysis of aqueous hydrazoic acid (285). The only detected reaction of this proposed species was insertion into water to form NH2OH. 

The generation and reactions of arylnitrenes continue to attract attention. Evidence for a photoinitiated autocatalytic chain mechanism in the photodecomposition of phenyl azide has been reported, and the reaction of photochemically generated phenyl nitrene with oxygen has been reexamined. Irradiation of p-azidoaniline in aqueous solution yields triplet p-aminophenyl nitrene, which on reaction with water is converted into the highly reactive p-benzoquinone diimine. Both m- and p-nitrophenyl azides, on photoelimination of nitrogen at 77 K, afford the corresponding nitrenes, whereas o-nitrophenyl azide (93) is converted without the intermediacy of a nitrene into the benzofurazan (94). 4,4 -... 

Friedlander - assumed that the phenols arose by way of a rearrangement of a hydroxylamine which was formed by the reaction of water with a nitrene. Bamberger compared the behaviour of arylhydroxylamines and aryl azides under similar conditions and concluded that hydroxylamines are not intermediates in the acid-catalysed decompositions of aryl azides. He proposed instead that with phenyl azide, for example, the initially formed nitrene was... 

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