Photolysis nitrenes

Polyfluorination does seem to suppress rearrangements of alkyl azides and to extend the lifetimes of the corresponding singlet nitrenes. Photolysis of 5 in cyclohexane produces insertion adduct 6." ... 

Photolysis of anthranils 564 in methanol or amines gives 2-methoxy- or 2-ami no-3//-azcpincs 565 via ring expansion of intermediate nitrenes. Photolysis of 2-alkylindazoles probably also goes through a nitrene intermediate, which either abstracts hydrogen from the solvent to give 567 or ring expands to yield 566. 

Cleavage of Unsaturated Sulphides by Carbenes and Nitrenes Photolysis.—... 

Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

However, in some cases carboxylic acid-derived groups can participate in ring fission-reclosure reactions. Thus photolysis of 1,5-disubstituted tetrazole (399) gives nitrogen and appears to involve the amino-nitrene intermediate (400), which reacts further to give (401) (77AHC(21)323). 

Photochemical elimination of carbon dioxide from suitable precursors has given a variety of reactive intermediates at low temperatures where they are often stable and can be studied further. This approach has been utilized in attempts to generate new 1,3-dipolar species, and photolysis of (515) gave an azomethine nitrene intermediate (516) (see Section 4.03.6)... 

An intensely colored by-product of the photolysis reaction of methyl-2-azidobenzoate has been identified as the first known derivative of 3,3 -diazaheptafulvalene 70 (94LA1165). Its molecular mass was established by elemental analysis and mass spectroscopy as that of a formal nitrene dimer, whereas and NMR studies demonstrated the twofold symmetry as well as the existence of a cross-conjugated 14 7r-electron system in 70. Involving l-azido-2,3-dimethoxy-5,6-dimethoxycarbonylbenzene in thermal decomposition reactions, the azaheptafulvalene 71 could be isolated and characterized spectroscopically and by means of X-ray diffraction. Tliis unusual fulvalene can be regarded as a vinylogous derivative of azafulvalenes (96JHC1333) (Scheme 28). 

Photolysis of aryl azides in amine solution, with a tertiary amine as cosolvent to promote stabilization of the singlet nitrene, has met with some success. For example, the yield of 2-piperidino-3 W-azepme. obtained by the photolysis of phenyl azide in piperidine, is increased from 35 to 58% in the presence of A A /V. /V -tetramethylethylenediamine (TMLDA).180 Also, an improved yield (36 to 60 %) of A,(V-diethyl-3W-azepin-2-amine (38, R = Et) can be obtained by irradiating phenyl azide in triethylamine, rather than in dicthylaminc, solution.181 Photolysis (or thermolysis) of phenyl azide in TMEDA produces, in each case, 38 (R = Et) in 40% yield.181 In contrast, irradiation of phenyl azide in aniline with trimethylamine as cosolvent furnishes jV-phenyl-377-azepin-2-amine (32, R = Ph) in only low yield (2%).35... 

The unsubstituted nitrene NH has been generated by photolysis of or electric discharge through NH3, N2H4, or HN3. 

Aziridines can be prepared directly from double-bond compounds by photolysis or thermolysis of a mixture of the substrate and an azide. The reaction has been carried out with R = aryl, cyano, EtOOC, and RSO2, as well as other groups. The reaction can take place by at least two pathways. In one, the azide is converted to a nitrene, which adds to the double bond in a manner analogous to that of carbene addition (15-62). Reaction of NsONHC02Et/ CuO [Ns = A(/7-toluenesulfonyl-inimo)] and a conjugated ketone, for example, leads to the A-carboethoxy aziridine derivative.Calcium oxide has also been used to generate the nitrene.Other specialized reagents have also been used." ... 

Aziridines are important compounds due to their versatility as synthetic intermediates. In addition, aziridine rings are present in innumerable natural products and biologically active compounds. Nitrene addition to alkenes is one of the most well established methods for the synthesis of aziridines. Photolysis or thermolysis of azides are good ways to generate nitrenes. Nitrenes can also be prepared in situ from iodosobenzene diacetate and sulfonamides or the ethoxycarbonylnitrene from the A-sulfonyloxy precursor. 

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... 

Similar products are obtained from the photosensitized decomposition of the tertiary azides, suggesting that decomposition may result from the triplet azides under both direct and sensitized photolysis/461 Additional evidence for a discrete nitrene intermediate comes from the observation that this intermediate can be trapped by decomposition of the azides in the presence of good hydrogen donors such as tri- -butyItin hydride and jec-butyl mercaptan. Triarylamines result ... 

ESR spectra for a number of nitrene intermediates produced by photolysis at 77°K have been reported.<50) Analysis of these spectra has resulted in their assignment to triplet ground state nitrenes. 

Photolysis of bis(dimethylamino)phosphoryl azide 2071401 represents an entirely different entry to a metaphosphorimidate. If the reaction is performed in cyclohexane, it gives only 7 % of the amide 209 which can be rationalized as the insertion product of the intermediate nitrene 208 into a CH bond of cyclohexane. The major product component is a polymer. The assumption that it is polymeric aminometa-phosphorimidate 212 is substantiated indirectly by the nature of the principal product of photolysis of 207 in methanol. A 1,2-shift of a NMe2 moiety which... 

Photolysis of sulphonyl azides in dimethyl sulphoxide with 2537 A light gives IV-sulphonylsulphoximines 12 in 15—50% yield 5>. The reaction was formulated as going via a nitrene intermediate which was trapped by the nucleophilic solvent... 

Interestingly, no SO 2 was evolved in this reaction as in the photolysis of a-toluenesulphonyl azide. This could be explained on the basis of a cation-radical anion pair which collapses as in Eq, (8) to give a sulphon-amido radical, and no free nitrene is formed 21>. 

As will be discussed later, it is possible a4> that the thermolysis involves a metal-nitrene complex whereas the photolysis involves the free nitrene. The product distribution is not affected by the presence of a photosensitizer, but since ferrocene itself is both an efficient triplet quencher as well as a sensitizer 26,27) jt is very difficult to probe the spin state of ferrocenyl nitrene at the moment of reaction. The cycli-zation appears to be a singlet reaction since the yield of 27 in benzene solution is essentially unaffected by oxygen or the presence of hydro-quinone a5>. 

Photolysis (2537 A) of A-arylsulphonyldimethyl sulphoximines in aromatic hydrocarbon solvents did not produce arylsulphonyl nitrenes instead, aryl radicals were generated which arylated the solvent 43>. 

The success of this reaction was ascribed to the solubility of the chlorozinc intermediate, whereas other chloramine-T derivatives (e.g. the sodium salt) are insoluble. An alternative non-nitrene pathway was not eliminated from consideration. On the other hand, no aromatic substitution or addition, characteristic of a free sulphonyl nitrene (see below), took place on treatment of jV,lV-dichloromethanesulphonamides with zinc powder in benzene in the cold or on heating. The only product isolated was that of hydrogen-abstraction, methanesulphonamide 42>, which appears to be more characteristic of the behaviour of a sulphonyl nitrene-metal complex 36,37). Photolysis of iV.iV-dichloromethanesulphonamide, or dichloramine-B, or dichloramine-T in benzene solution led to the formation of some unsubstituted sulphonamide and some chlorobenzene but no product of addition of a nitrene to benzene 19>. 

This seems highly unlikely in view of the very mild reaction conditions. On the other hand, the photolysis of the isocyanate may yield the desired nitrene. 

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