Nitrene from phenyl azide photolysis

Leyva photolyzed azides 4 and 5 in EPA glass at 77 K assuming that only the triplet nitrene would be formed in the low temperature glass. As shown in Figures 5 and 6 the absorption, emission, and fluorescence excitation spectra obtained by brief photolysis of 4 and 5 are very similar to the low temperature spectra obtained from phenyl azide. This of course implies that brief photolysis of phenyl azide at 77 K produces only triplet phenyl nitrene as a UV-VIS active species. 

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... 

Photolysis of the aryl-alkyl azide CH CPhaNa showed that the migratory aptitudes of the methyl and phenyl groups were almost identical and this and the formation of triphenylmethyl amine from irradiation of triphenylmethyl azide in the presence of efficient hydrogen donors were taken to confirm the existence of discrete nitrene intermediates. Although the occurrence of a triplet-sensitized decomposition from alkyl azides and triphenylmethyl azides (the... 

Time-resolved IR studies of the photolysis of 2-(methoxycarbonyl)phenyl azide in solution at room temperature showed that the didehydroazepine (47) was the sole intermediate, at least on the ps time-scale. This contrasts with photolysis of the same compound in matrices at 10 K, where the nitrene, iminoketene (48) and azetinone (49) were observed as well as (47). Matrix photolysis of 2-hydroxy-phenyl azide gives at least three major products, all of which are photo-interconvertible. Two of these are identified as the EtZ mixture of iminodi-enones (50), while the third is the ring-opened compound (51), existing as a mixture of conformers. 2-Aminophenyl azide behaves in a similar manner. Rapid H-transfer from the ortho hydroxy or amino group to the nitrene centre in each case appears to suppress ring expansion completely. 

UV photolysis of the powdered crystals of several aryl azides has been found to give azo compounds, from dimerization of the nitrenes, mostly in yields of >91% The only exceptions noted were p-(A -methylacetamido)phenyl azide, which also gives a product from insertion of the nitrene into a C-H bond of the methyl group, and 2-azidobiphenyl, which gives comparable amounts of the azo product and carbazole. Monitoring these solid-state photoreactions by ESR revealed that the arylnitrenes have extremely long half-lives, compared with those of nitrenes in the gas-phase or solution, and clearly the crystalline... 

The ring enlargement is presumed to result from an intermediate nitrene, as in a similar reation observed on photolysis of phenyl azide in the presence of an amine. 

Sundberg and co-workers in 1974 reported results from flash photolysis studies of the reaction of phenyl azide with secondary amines [26]. Irradiation of the azide in hexane solution produced an intermediate absorbing strongly at 366 nm and having a lifetime of about 5 ms. This intermediate was found to react with amines to give, after tautomerization, the 3H-azepine that is obtained in preparative scale reactions. On the basis of these results, De Graff et al. [26] concluded that the detected intermediate is not a nitrene but is a closed-shell intermediate, either benzazirine or dehydroazepine. 

Upon photolysis or pyrolysis of phenyl azide in the gas phase the nascent phenyl nitrene is formed with enough energy to overcome a substantial barrier and ring contract to form cyanocyclopentadiene, the lowest energy isomer of C6H5N [43], In fact laser photolysis of phenyl azide in the gas phase does not produce emission from triplet phenyl nitrene but from the cyanocyclopenadienyl radical instead. 

When the triplet is an excited state, energy transfer occurs to form singlet oxygen. Ground state triplets react with oxygen by a spin-allowed process which, for carbenes in particular, produce carbonyl oxides [64], It seems that triplet nitrenes react with oxygen slowly. This will be discussed more fully later. Here we examine the products formed from reaction of photolysis of phenyl azide in the presence of oxygen. 

The conclusion seems inescapable that lowering the temperature changes the identity of the reactive intermediate formed on photolysis of phenyl azide from an electrophilic species that is trapped by amines, to triplet phenyl nitrene. This result requires that there is a branching point in the photochemistry of phenyl azide and that the rate constants of these two branches have very different Arrhenius parameters. Because triplet phenyl nitrene is... 

There is an old report in the literature that a weak florescence from triplet phenyl nitrene can be observed at 77 K although the spectrum was not reported [75a]. In 1986 Leyva et al. [69] reported a weak emission from triplet phenyl nitrene at 520 nm. The 77 K fluorescence exitation spectrum of this species is identical to the absorption spectrum of the intermediate produced by brief photolysis of phenyl azide. This experiment demonstrated that the UV-VIS spectrum of Figure 4 is due to a single species rather than to a mixture of C6H5N isomers. 

It now seems incontestable that irradiation of phenyl azide at room temperature gives dehydroazepine. Once formed, dehydroazepine can react with itself and/or phenyl azide to give tarry polymer or with nucleophiles to give substituted 3f/-azepines. The rate of reaction of dehydroazepines with amines depends dramatically on substitution the 5-acetyl substituted compound reacts 10,000 times faster than does the 5-methoxy substituted dehydroazepine. At low concentration of phenyl azide, dehydroazepine itself has a lifetime of approximately 5 ms and, presumably, isomerizes to phenyl nitrene. We will have more to say about this point later. The achievement of positive structural identification of the reactive intermediate formed in the room temperature photolysis of phenyl azide permits the detailed characterization of phenyl azide photochemistry. Further consideration of this analysis will be aided by examination of the results from time-resolved experiments for other aryl azides. 

Kobayashi et al. [86] studied (4-dimethylamino)phenyl azide by means of laser flash photolysis on a picosecond time scale. They found that the triplet nitrene is formed from an unobserved precursor that has a lifetime of approximately 120ps. By consideration of model compounds, these workers suggested that the precursor is the singlet nitrene, but this conclusion must await confirmation by alternative experiments. However, in this work Kobayashi was able to show that triplet (4-dimethylamino)phenyl nitrene reacts to form the (4-dimethylamino)azobenzene by a kinetically second-order process. 

Comparable results are obtained from the nanosecond time scale flash photolysis study of (4-nitro)phenyl azide reported by Liang and Schuster [45]. Irradiation of this azide leads to rapid formation of the triplet nitrene. This nitrene was identified by comparison of the observed spectrum with that recorded at low temperature, and by its observed dimerization to form the (4-nitro)azobenzene. The rate constant for the triplet nitrene dimerization is 1.0 x 109 M 1 s 1 a value approximately 10 times below the diffusion limit. This relatively small rate constant is consistent with the demands of spin statistics. Combination of two triplets to form a singlet product can be successful only one ninth of the time [64],... 

It is conceivable that the excess vibrational energy imparted to phenyl azide from UV photolysis, combined with the exothermicity of the ensuing isomerization of singlet phenyl nitrene, may produce cyanocyclopentadiene with enough excess vibrational energy to fragment a CH bond to form the fluorescent radical 7, detected by absorption and emission spectroscopy. 

Schrock and Schuster [46] have demonstrated that the quantum yield to loss of nitrogen from triplet phenyl azide is rather low. The fact that a strong transient spectrum of the brominated triplet phenyl nitrene is observed following laser flash photolysis thereby implies that the heavy atom effect has catalyzed intersystem crossing from the singlet to the triplet state of the nitrene rather than of the azide excited state. 

It is clear now that irradiation of phenyl azide at room temperature gives dehydroazepine. At high concentration of azide, the dehydroazepine polymerizes rapidly in competition with its slow isomerization to triplet phenyl nitrene. The major product formed from photolysis of phenyl azide under conditions where its quantum yield for disappearance is claimed to be greater than unity is poly-1,2-azepine [48], not azobenzene. Of course, the polymer does not elute from an HPLC, and analysis of reaction mixtures by chromatography will show only two components. 

If the photoinitiated autocatalytic chain decomposition outlined in Scheme 12 operates at all, it should be specially important in the photolysis of (4-nitro)phenyl azide where formation of the dehydroazepine is a minor path and the triplet nitrene is formed rapidly. Indeed, Waddell and co-workers [110] report that the quantum yield for disappearance of this azide is 434 when a 0.1 M solution in acetonitrile is irradiated. However, attempts to reproduce this experiment reveal a quantum yield for disappearance of (4-nitro)phenyl azide in acetonitrile solution of 0.7 that is independent of starting azide concentration from 0.001 to 0.1 M [45]. 

Thus, in the late 1980s a series of intermediates produced by the photolysis of phenyl azide had been directly observed ( 52 in matrices and low temperature glasses, 51 in matrices and liquids, and 55 in the gas-phase). However, the results obtained in solution and inert gas matrices differ substantially from those obtained in low temperature glasses. In glasses, triplet nitrene 52 is the major product, whereas ketenimine 51 is the major product in solution at ambient temperature and often in the inert gas matrices at -10 K. Formation of 51 in inert gas matrices was explained by the slow vibrational relaxation of the hot singlet nitrene 52 in these matrices, which competes with fast isomerization to 51/ °... 

The matrix photochemistry of phenyl nitrenes becomes more complex when substituents are present that can take an active part in the reactions. For instance, irradiation of azide 79 in Ar matrices gave a product (80) from nitrene insertion into an adjacent CH bond as well as the ring-expanded ketenimine 81. The nitrene center can also react by attack on oxygen. Thus, the photolysis of 2-(methoxycarbo-nyljphenyl azide (82) in Ar matrices gave rise to at least five major products nitrene 83, cycKc ketenimine 84, the two geometrical isomers of ketene-oxime 86, and N-methoxyazetinone 87 (Scheme 10). Products... 

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