Azides nitrenes from

Scheme 6.6 Generation of nitrenes from azides and isocyanates.

The production of the nitrenes from a variety of sources has been reported, the most common of which are the sulphonyl azides. 

Starting materials other than sulphonyl azides have been used as possible sources of sulphonyl nitrenes. The decomposition of the triethyl-ammonium salt of iV- -nitrobenzenesulphonoxybenzenesulphonamide (26) in methanol, ethanol, and aniline gave products derived from a Lossen-type rearrangement 20> (Scheme 3). It was felt that the rearrangement did not involve a free sulphonyl nitrene since, when the decomposition was carried out in toluene-methylene chloride or in benzene, no products (benzenesulphonamides) of substitution of the aromatic solvent nucleus were found (as are usually found with sulphonyl nitrenes from the thermal decomposition of the corresponding azides). On the other... 

The most reliable method for generating nitrenes is the thermal or photochemical elimination of nitrogen from azides. An alternative method which is useful for indole and carbazole synthesis is the deoxygenation of aromatic nitro compounds with trivalent phosphorus compounds. Triethyl phosphite is the most commonly used reagent, though more reactive compounds may be useful in special cases (B-79MI30600). 

These reactions are important in synthetic organic chemistry, because they lead to highly reactive chemical species called carbenes and nitrenes. A carbene has two unpaired electrons on a carbon atom, a nitrene has two unpaired electrons on a nitrogen atom. They are formed through the elimination of N2 from azo-compounds, RN2, and from azides, RN3 (Figure 4.34b). 

Scheme 1.19 Sidewall functionalization of SWCNTs by [2+1]-cycloaddition of alkoxycarbonyl nitrenes obtained from azides.

The reaction of azides with 1,3-dienes affords vinyl aziridines (through nitrene or azide cycloaddition), which can be converted to dihydropyrroles. It is also possible that dihydropyrroles are directly produced from azidodienes. 

The addition of alkoxycarbonyl nitrenes, generated photochemically from azides, to substituted dihydropyrans and tri-O-acetyl-D-glycal57"59 in alcoholic solution gave the products of alcoholysis 30 and 31-33 of the intermediate aziridines by a one-pot procedure59. The attack of the nitrene takes place mainly on the less hindered face of the double bond, but complete control of the diastereoselectivity was not accomplished even the ring opening of the aziridine can follow both anti and syn stereochemistry. 

Common methods for generating nitrene intermediates are the photolysis and thermolysis of azides. Generation of nitrenes from acyl azides can only be effected photochemically thermolysis of an acyl azide gives the corresponding isocyanate. 

Eor a treatise on azides, which includes discussion of rearrangement reactions, see Scriven, E.F.V. Azides and Nitrenes, Academic Press, NY, 1984. For a review of rearrangements of alkyl and aryl azides, see Stevens, T.S. Watts, W.E. Selected Molecular Rearrangements, Van Nostrand-Reinhold, Princeton, NJ, 1973, pp. 45-52. For reviews of the formation of nitrenes from alkyl and aryl azides, see, in Lwowski, W. Nitrenes, Wiley, NY, 1970, the chapters by Lewis, E.D. Saunders, Jr., W.H. pp. 47-97, 47-78 and by Smith, P.A.S. pp. 99-162. 

Isocyanate dichlorides are obtained from azides and dichlorocarbene. The reaction proceeds at 0°, thus intermediates of type 87 rather than nitrenes are probably involved. -Octyl isocyanate dichloride (88) was prepared by this method from -octyl azide in 89% yield... 

The low reaction temperature of benzofuroxan formation from azides can be rationalized by assuming a concerted mechanism, rather than nitrene intermediates. Benzofuroxan formation is preferred over the products obtained usually from nitrenes. Heating of 2-azido-3-nitrobiphenyl (218) yielded exclusively the furoxzin 219 and not the carbazole 220 °. 

Tlie formation of free radical HN by decomposition of hydrazoic acid has been suggested by a number of authors since 1928 (see Vol. HI, p. 167). This was substantiated by experiments on the decomposition of HNj by the flasli photolysis of Thrush (Vol. Ill, p. 167). The formation of nitrene radicals from azides by flash photolysis was reported simultaneously and independently by Koto (56], Reiser et al. [57—59]. Reiser rationalized the reaction derived from flash pl otolysis of formation of azo compounds (5) ... 

Fig. 1 shows the spectral sensitivities of these azide compounds. The mechanism of the color reaction is not clear, but it is assumed that the various reaction products of nitrenes from photodecomposed azide compounds produce the mixed coloring matter. 

Nitrenes, 116-118, see also Nitrene (HN) aminonitrenes, 118 from azide photolysis, 116 phenylnitrene, 118 substituents on, 117... 

Tsuchia, T., Nitrene Formation by Photoextrusion of Nitrogen from Azides. In Horspool, W. M., Song, P. S. (eds), CRC Handbook of Organic Photochemistry and Photobiology, CRC Press, Boca Raton, FL, 1995, pp. 984 991. 

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