Nitrenes azides, formation

Photolysis of aryl azides in acidic media leads to rapid protonation of the initially formed singlet nitrenes with formation of singlet nitrenium ions. Nitrenium ions are nitrogen-centered diradicals that are formally isoelectronic with carbenes. They have been in the focus of research for their role as ultimate carcinogen in the metaboHsm of certain aromatic amines." " Nitrenium ions can also be made by photolysis of 2,2-benzisoxazolium (anthranihum) salts" or by irradiation of N-aminopyridinium salts. 

The procedure described is essentially that of Shioiri and Yamada. Diphenyl phosphorazidate is a useful and versatile reagent in organic synthesis. It has been used for racemlzatlon-free peptide syntheses, thiol ester synthesis, a modified Curtius reaction, an esterification of a-substituted carboxylic acld, formation of diketoplperazines, alkyl azide synthesis, phosphorylation of alcohols and amines,and polymerization of amino acids and peptides. - Furthermore, diphenyl phosphorazidate acts as a nitrene source and as a 1,3-dipole.An example in the ring contraction of cyclic ketones to form cycloalkanecarboxylic acids is presented in the next procedure, this volume. 

Acyl azides can undergo photolytic cleavage and rearrangement upon irradiation at room temperature or below. In that case acyl nitrenes 8 have been identified by trapping reactions and might be reactive intermediates in the photo Curtius rearrangement. However there is also evidence that the formation of isocyanates upon irradiation proceeds by a concerted reaction as in the case of the thermal procedure, and that the acyl nitrenes are formed by an alternative and competing pathway " ... 

The concentration of alkyl azidoformate used in these decompositions is important since with an excess (0.5 molar equiv) of azide, thermolysis at 130°C results in the formation of 1 2 1 //-azepine (methoxycarbonyl)nitrene adducts as the major products.145,146... 

Formation of a Nitrene, The decomposition of acyl azides is one of several ways in which nitrenes are formed (see p. 253) ... 

Finally, in some cases diradical negative ions can even be generated directly npon ionization of appropriate precnrsors. For example, nitrene and carbene anions can be formed by El of organic azides, diazo-compounds, and diazirines, whereas Branman and co-workers have reported the formation of oxyallyl anions by El of flnorinated acetyl componnds (Eq. 5.12). ... 

The triplet state is usually the ground state for non-conjugated structures, but either species can be involved in reactions. The most common method for generating nitrene intermediates, analogous to formation of carbenes from diazo compounds, is by thermolysis or photolysis of azides.246... 

Evidence for the formation of alkyl and aryl radicals in some cases following loss of SO2 (Scheme 1) has been obtained. Thus, a small amount of M-pentane was formed in the decomposition of M-pentanesulphonyl azide in mineral oil ). Thermolysis of diphenyl sulphone-2-sulphonyl azide (8) in dodecane at 150 °C gave diphenyl sulphone 9 (27%) and diphenyl sulphone-2-sulphonamide 10 (9%) which arise by hydrogen abstraction by the aryl radical and sulphonyl nitrene, respectively. When this thermolysis was carried out in Freon E-4 at 150 °C, the products were diphenylene sulphone 77 (1.3%) (Pshorr-type cyclization product of the aryl radical) and 10 (1.5%) together with tars 16h Ferro-... 

On the other hand, thermolysis of ferrocenylsulpkonyl azide (14) in aliphatic solvents may lead to the predominant formation of the amide (16) 17>. A 48.4% yield of (16) was obtained from the thermolysis in cyclohexane while an 85.45% yield of 16 was formed in cyclohexene. Photolysis of 14 in these solvents led to lower yields of sulphonamide 32.2% in cyclohexane, 28.2% in cyclohexene. This suggests again that a metal-nitrene complex is an intermediate in the thermolysis of 14 since hydrogen-abstraction appears to be an important made of reaction for such sulphonyl nitrene-metal complexes. Thus, benzenesulphonamide was the main product (37%) in the copper-catalyzed decomposition of the azide in cyclohexane, and the yield was not decreased (in fact, it increased to 49%) in the presence of hydroquinone 34>. On the other hand, no toluene-sulphonamide was reported from the reaction of dichloramine-T and zinc in cyclohexane. 

Figure 4.21 BASED can react with molecules after photoactivation to form crosslinks with nucleophilic groups, primarily amines. Exposure of its phenyl azide groups to UV light causes nitrene formation and ring expansion to the dehydroazepine intermediate. This group is highly reactive with amines. The cross-bridge of BASED is cleavable using a disulfide reducing agent.

Figure 5.16 Photoactivation of a phenyl azide group with UV light results in the formation of a short-lived nitrene. Nitrenes may undergo a number of reactions, including insertion into active carbon-hydrogen or nitrogen-hydrogen bonds and addition to points of unsaturation in carbon chains. The most likely route of reaction, however, is to ring-expand to a dehydroazepine intermediate. This group is highly reactive toward nucleophiles, especially amines.

This chapter has to do with reactions wherein the photochemical event is the breaking of a bond in a molecule. For a single bond this results in the formation of a pair of radicals or a diradical. For a double bond as in diazo compounds or in azides a carbene or a nitrene and nitrogen are formed. All these intermediates will then undergo further mono- or bi-molecular dark reactions or eventually recombine to ground state starting materials. 

Reaction of metal nitrosyls with azide ion proceeds with formation of N2 and N20 (56). This can be viewed as the result of a nitrene transfer reaction in analogy with the Curtius rearrangement (62) and its organome-tallic counterpart (63). 

The photochemical reaction of azide-functionalized tetrazole derivatives such as 38 leads to the formation of the 5-5 bicyclic ring system 40 (Scheme 5) in very moderate yields <1999JHC863>. This reaction is believed to proceed via the singlet nitrene intermediate 39. Attack at the aromatic substituent in ortho position leads to product 40 <1974JOG2546> by subsequent cyclization. This intermediate is deprotonated during the workup conditions to the mesoionic tricyclic derivative 41. 

Photolysis of sugar oximes produces iminolactones.178,179 These are the compounds proposed to arise from irradiation of glycosyl azides however, the mechanism leading to formation of iminolactones from these two starting-materials (azides and oximes) must be quite different (see Scheme 30). Photolysis of azides is considered to generate a nitrene, whereas photolysis of oximes produces an iminolactone that... 

In 1987, Nitta reported the formation of an unexpected vinylketene complex from the reaction of an azido-substituted cyclopropene with diiron nonacarbonyl.104 They had previously investigated the chemical behavior of the complexed nitrene intermediates that result from the reaction of organic azides and iron carbonyls113 and were interested in replicating the thermal isomerization of 3-azido-l,2,3-triphenylcyclopropene (163) into 4,5,6-triphenyl-l,2,3-triazine using a metal carbonyl-promoted re-... 

PI 2A. IR studies of N labeled azide la. We are concerned with providing further evidence that nitrene 2a was formed upon the photolysis of azide la. To this end, we plan to synthesize azide la as an isotope labeled in the N1 position (see Figure 15), obtain IR spectra before and after irradiation in an argon matrix, and compare the calculated shift for the C-N band in nitrene 2a with the experimental value. Because isotope shifts in IR bands can be calculated very accurately, this will be an excellent proof of the formation of a nitrene intermediate. (From Gudmundsdottir, 2001)... 

The fact that the ratio (55 45) of 1-methylcarbazole to 3-methylcarbazole was effectively the same when obtained either via the triethylphosphite (or trisdimethylaminophosphine) deoxygenation of 2-nitro-3 -methylbiphenyl or of 2-nitroso-3 -methylbiphenyl or via the thermal or photochemical decomposition of the corresponding azides is taken as strong circumstantial evidence that each process proceeds via a common intermediate-the nitrene. The electrophilic nature of such a nitrene was well illustrated by the formation of 275 and not 276 from the nitrene 277. ... 

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