Photodecomposition, of azides

In the absence of external perturbation, photodecomposition of azides must occur with overall conservation of spin. Within this restriction there are four possibilities for the fragmentation ... 

The idea that excitons are involved in photodecomposition of azides was previously suggested based on the effect of temperature and intensity on rates of emission of N2 gas [80]. However, information deduced about the mechanism of decomposition of the azide ion from studies of kinetics of gas evolution is... 

The method most commonly used to study photodecomposition of azides measures the rate of gas evolved from a sample under irradiation in a vacuum chamber. Typically, a powder in a 10" -10" -torr vacuum is exposed to UV radiation with intensities of lO -lO photons/cm sec. This energy input is sufficient to produce photochemical decomposition of azide molecules in the near-surface regions of the sample. The experimental rates of gas evolution are either calculated from pressure vs. time curves in a closed system or measured directly in a pumped system by relating the rate to the pressure rise. 

FIGURE 5.41 Reaction of nitrene formed by photodecomposition of azide, with cyclized polyisoprene (CPI) polymer. 

With regards to the hydrogen-abstraction method referred to in Table 3.13, it should be noted that triplet nitrene and benzoyl radicals can be formed by the photodecomposition of azides (Scheme 3.19), and of type I free radical photoinitiators (see Table 3.5), respectively. 

Scheme 3.19 Formation of singlet and triplet excited nitrene by photodecomposition of azides.

A.C. Forsythe, Photodecomposition of a Lead Azide in the Solid State , JPhysChem 71... 

The thermal decomposition reactions of KN3, T1N3, and AgN3 have been studied in the corresponding halide matrices [301]. The formation of NCCT from trapped C02 was described and labelling with ISN established that only a single end-N atom of the azide ion was involved in NCO formation. The photodecomposition of PbN6 and the effects of dopants have been followed [302] by the changes produced in the near and the far infrared. 

In contrast to diazido compounds [102] and [104], which throw off two azido groups and form silylene and germylene, photodecomposition of silyl azide [105] led to the generation of aminosilylene [107] via isomerization of initially formed nitrene [106] (Maier et al., 1989b). The IR spectrum of the... 

The photolysis of acyl azides has also been studied, and in some respects these appear to behave analogously. Pivaloyl azide, for example, adds to cyclohexene to give a 26% yield of an aziridine [Eq. (85)],321 and the assumption is that this addition again occurs via a nitrene. The photodecomposition of acetyl azide (295) in benzo-nitrile and phenylacetylene, on the other hand, affords322 2-methyl-5-phenyl-l,3,4-oxadiazole (296) and 2-methyl-5-phenyloxazole (297),... 

Ogilvie, J. F., and S. Cradock Spectroscopic Studies of the Photodecomposition of Silyl Azides in Argon Matrices near 4 K Detection of Iminosilicon, HNSi. Chem. Commun. 1966, 364—365. 

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 -... 

Recent experiments have established with a fair degree of certainty that 1,2-dehydroazepines are intermediates in the photodecomposition of aryl azides. The dehydroazepine (91), for example, has been detected at room temperature by time-resolved... 

The photodecomposition of vinyl azides is of interest as a potential synthetic route to the highly stre-ined azirine ring system. Smolinsky was the first to prepare compounds of this class by the pyrolysis of vinyl azides (39) and Hassner and Fowler have shown that the corresponding photo-reaction also produces 1-azirines (40) in excellent yield. Three possible mechanisms were proposed by these authors, one involving a nitrene intermediate 41. 

Irradiation of allyl azide produced the imine of acrolein but photolysis of l-azido-2-phenylprop-2-ene (63) in cyclohexane solution resulted in a small yield of the azacyclobutane (64) and 2-phenyl-propenalimine (65) . This is the first observation of an 1-azabi-cyclo [1.1.0]-butane ring in the products of photodecomposition of an allylic azide. 

The outstanding thermal reaction of the acyl azides is their rearrangement to isocyanates. This Curtius rearrangement occurs also in the photodecomposition of the acyl azides, but the yield of isocyanate is lower than in the corresponding thermal process, and other products appear which can be accounted for by a nitrene precursor. A considerable amount of work has been directed towards an elucidation of the mechanism of the photo-Curtius rearrangement and the bulk of the evidence seems to favour the view that it ° is a concerted process not involving a discrete nitrene intermediate. Nitrenes are nevertheless formed in the photolysis of acyl azides and the products of this alternative route will now be discussed. 

Ethyl azidoformate is a rigid azide in the Curtius classification However, in the photodecomposition of ethyl azido-... 

Smith and Brown were the first to study the photodecomposition of an aromatic azide and were able to synthesize a number of carbazole (119) derivatives by the photolysis of 2-azidobiphenyls (118). 

Further studies of the photochemistry of aryl azides have been reported. The photodecomposition of phenyl azide has been studied in an argon matrix at 12K the ring expanded product, didehydro-... 

Studies of the photodecomposition of azido derivatives of PCB have attracted interest because of their potential use as photolabels in intracellular distribution studies.Polyfluorinated aryl azides are also of current interest as new reagents for photoaffinity labelling. The major products of photolysis of methyl 4-azidotetrafluorobenzoate in cyclohexane or diethylamine, for example, arise by insertion, a property which enhances its use in labelling. Singlet pentafluorophenyl nitrene, formed in an analogous fashion on irradiation of pentafluorophenyl azide, can be trapped by toluene as the insertion products (79) and (80). °... 

The photodecomposition of aryl azides continues to attract most attention in this area although few additional details concerning the mechanism have been reported, particularly with reference to ring expansion via 1,2-didehydroazepines. Ring... 

The photodecomposition of acetyl azide in phenylacetylene similarly affords 2-methyl-5-phenyloxazole.167... 

Many photoreactions of aryl azides can be interpreted as arising via unstable ring-fused azirines. The formation of the 6,7-diaminobenzothiazoles (117) and the 6-amino-8//-thiazolo[5,4-c]azepines (118) on irradiation of the corresponding 6-azidobenzothiazoles (119) in diethylamine can be explained in this way, as shown in Scheme 14. An alternative explanation for the photodecomposition of phenyl azide (120) at low temperatures has recently been advanced and involves the formation of an intermediate l-azacyclohepta-l,2,4,6-tetraene (121). Further evidence for this pathway is to be found in a study of the photodecomposition of phenyl azide in acetic acid the azepin-2-one (122) is believed to arise in this way, as shown in Scheme 15. The formation of the same azepin-2-one on irradiation of phenyl azide in the presence of naked acetate anion has been reported the nature of the intermediate in this transformation is still open to question, but both benzazirine and l-azacyclohepta-l,2,4,6-tetraene intermediates are considered possible. In contrast, irradiation of the azide (123) in an argon matrix at lOK affords, on the basis of spectral evidence, the cyclic carbodi-imide (124). " 2-... 

L.J. Leyshon and A. Reiser, Sensitized photodecomposition of phenyl azide and naphthyl azide, Trans. Faraday Soc. 68, 1918 (1972). 

Of historic interest is a reaction in which the azide group was synthesized from hydrazine and nitrite in the presence of silver ions [19,98] (see p. 24). Most commonly, silver azide is prepared by mixing aqueous solutions of hydraz-oic acid or sodium azide with silver nitrate. The product is precipitated in fine crystalline form larger crystallites are obtained from more dilute reagents [200]. One author recommended the use of an excess of silver nitrate another believed this would enhance the photodecomposition of the product [202]. Of more significance is the recommendation to make the azide in the dark, or at least under red light, [203,204] and to wash the product completely ion free. 

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