Azides, matrix photolysis

However, in 1978, Chapman and LeRoux discovered that photolysis of phenyl azide, matrix isolated in argon at 10 K, produces a persistent species with a strong vibrational band at 1880 10 cm . The carrier of this species was most reasonably assigned to ketenimine 30 rather than benzazitine 29 or triplet phenylnitrene. This result imphes that it is the ketenimine 30 and not benzazirine 29 that is trapped with amines to form the 37/-azepines (27) that had been isolated earher. It does, however, raise the question as to why two groups observed triplet phenylnitrene by low temperature spectroscopy while a third observed ketenimine 30. 

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. 

Matrix photolysis of several cyano-substituted phenyl azides with plane polarized light, combined with qualitative linear dichroism measurements in the IR spectra of the resulting matrices, has been used to deduce information about... 

Evidence for the formation of azirines of the type 162 by matrix photolysis of aryl azides is also beginning to appear.2203 In some cases it can be ascertained, however, that the seven-membered ring intermediates are more stable than the fused bicyclic azirines. Furthermore, in all cases uncovered to date it turns out that the seven-membered rings, when isolable, exist as cumulenes rather than carbenes. 

Mn (=0)TMP (28) was isolated and characterized 101). Infrared and Raman data have shown that the manganese-oxygen bond was unusually weak (i mi =o 574). The weakening of the Mn=0 rr-bond was ascribed to the high-spin electronic structure of 31 and the special nature of the resulting half-filled t2g orbital subset. Interestingly, an isoelectronic nitrido-iron(V) porphyrin recently prepared by low-temperature matrix photolysis of an iron(IIl) azide shows an unusually weak iron-nitrogen bond (876 cm ) 102). 

Matrix photolysis of 4-quinolyl azide gives 4-quinolylnitrene (Scheme 73) ... 

Arylnitrene triplet states have been observed by uv and esr when aryl azides are irradiated at low temperature in solid matrices. Flash photolysis of 1-azidoanthracene in ethanol at room temperature gives a short-lived intermediate (with a half-life of 3-10/nitrene generated by the matrix photolysis of the same azide at 77 When these solid matrices are allowed to warm up to room temperature the nitrene spectrum collapses and typical nitrene products vide supra) are obtained. The nature of the products formed by analysis of reaction mixtures is the main way used to adduce nitrene intermediacy on photolysis. 

Recently new work has appeared that concerns the relative importance of benzazirine and didehydroazepine (azacycloheptatetraene) intermediates in aryl azide decompositions. Dunkin and Thomson have published the first report of the direct observation of a benzazirine intermediate. They photolyzed a- (and P ) naphthyl azide in nitrogen or argon matrices at 12° K and observed, by ir, a naphthazirine 100 which collapsed to a more stable didehydrobenzazepine 101. Formation of the isomers 100a and 101a could not be precluded in the matrix photolysis study, but products consistent with their trapping in solution photolyses have never been detected. ... 

Argon matrix photolysis of 1- and 2-naphthyl azides 79 and 80 at 313 nm initially produced the singlet naphthyl nitrenes, 81 and 82. Relaxation to the corresponding lower energy, persistent triplet nitrenes 81 and 82 competes with cyclization to the azirines 83 and 84 which can also be formed photochemically from the triplet nitrenes (Figures 11.20 and 11.21). On prolonged irradiation, the triplet nitrenes 81 and 82 can be converted to the 7-membered cyclic ketenimines 85 and 86, respectively, as described earlier by Dunkin and Thomson. ... 

Low-temperature (matrix) photolysis (and also laser flash photolysis), eombined with high-level quantum-chemical calculations, are powerful tools for investigation of reaction mechanisms. A lot of information has been obtained by this method relative to the structure and further thermal and photochemical transformations of arylnitrenes, primary products of aiylazides, their rearranged intermediates (azacycloheptatetraenes). Some recent results on photolysis of azidopyridines and related heterocyclic azides at cryogenic temperatures are discussed below. 

Dunkin, I. R. and Thomson, P. C. R, Infrared evidence for tricycHc azirines and didehydrobenza-zepines in the matrix photolysis of azidonaphthalenes,/. Chem. Soc., Chem. Commun., 499,1980. Tomioka, H., Matsushita, X, Murata, S., and Koseki, S., Photochemistry of phenyl azides bearing 2-hydroxy and 2-amino groups studied by matrix-isolation spectroscopy generation and characterization of reactive o-quinoid compounds, Liebigs Ann., 1971,1996. 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Photolysis (254 nm) of this azide in an isopentane/methylcyclohexane matrix at 77 K produced an orange color, with the appearance of two bands at 272 nm and 474 nm in the UV/visible-spectrum. Irradiation in an argon matrix at 20 K gave the same results.3... 

Milligan and Jacox 44 have recently reported an elegant synthesis of CF2 in an argon matrix. Carbon atoms, produced from the photolysis of cyanogen azide, were allowed to react with molecular fluorine, and the presence of CF2 was demonstrated from infrared spectra. Use of radiation effective in photolyzing F2 produced CF3 from the reaction of the CF2 with atomic fluorine. 

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)... 

Confusion over the matrix and gas-phase optical spectroscopy of PN spilled over to the liquid phase. Initial flash photolysis experiments involving phenyl azide gave conflicting results, with different authors favoring the presence of triplet phenylnitrene, " benzazirine BZ, or cyclic ketenimine as the carrier of the transient spectra. 

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