Phenyl azide, phenylnitrene

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. 

To add to the confusion, various groups reported that gas-phase photolysis of phenyl azide produced the absorption and emission spectra of triplet phenylni-trene. " These observations were reconciled by the work of Leyva et al. who discovered that the photochemistry of phenyl azide in the presence of diethylamine was very sensitive to temperature. Above 200 K, azepine 30 is formed, but <160 K, azobenzene, the product of triplet nitrene dimerization, is produced. The ketenimine can react with itself or with phenyl azide to produce a polymer, which can be converted into an electrically conducting material. Gritsan and Pritchina pointed out that at high-dilution ketenimine 30 can interconvert with singlet phenylnitrene which eventually relaxes to the lower energy triplet that subsequently dimerizes to form azobenzene. 

Photolysis of phenyl azide (32) produces singlet phenylnitrene (33s), but what happens next depends on temperature and phase. In the gas-phase 33s isomerizes to cyanocyclopentadiene 31, in solution at ambient temperature, it isomerizes to ke-tenimine 30 and in cryogenic matrices, singlet phenylnitrene isomerizes to triplet phenylnitrene (33t). 

The UV-vis spectrum of triplet phenylnitrene, obtained by brief photolysis of phenyl azide in glassy ether-pentane-alcohol is shown in Figure 11.2. 

In 1997, two groups simultaneously reported that LFP of phenyl azide 32 or compounds 34 and 35 produces a previously undetected transient with Lmax = 350 nm and a lifetime of 1 ns at ambient temperature.The transient decays at the same rate that cyclic ketenimine 30 is formed, implying that the newly detected transient is singlet phenylnitrene 33s. 

Laser flash photolysis (266 nm) of phenyl azide in pentane at 233 K produces a transient absorption spectrum with two sharp bands with maxima at 335 and 352 nm (Fig. 11.4). Spectrum 1 was measured, point by point, 2 ns after the laser pulse. In later work, the spectrum of 33s was reinvestigated and an additional very weak, long wavelength absorption band at 540 nm was observed (Spectrum 2). The transient spectrum of Figure 11.4 was assigned to singlet phenylnitrene in its lowest open-shell electronic configuration ( 2). 

Figure 11.4. Transient spectrum of singlet phenylnitrene produced upon LFP of phenyl azide. Spectrum 1 was recorded 2 ns after the laser pulse (266 nm, 35 ps) at 233 K. Long-wavelength band (2) was recorded with an optical multichanal analyzer at 150 K (with a 100-ns window immediately after the laser pulse, 249 nm, 12 ns). The computed positions and oscillator strengths (/, right-hand axes) of the absorption bands are depicted as solid vertical lines. For very small oscillator strength, the value multiplied by 10 is presented (/ X 10). [Reproduced with permission from N. R Gritsan, Z. Zhu, C. M. Hadad, and M. S. Platz, J. Am. Chem. Soc. 1999, 121, 1202. Copyright 1999 American Chemical Society.]...

The cyclic ketenimine 30 is the major trappable reactive intermediate present in solution when phenyl azide (at moderate concentrations) is decomposed photolyti-cally at 298 K. The rate of decay of singlet phenylnitrene 33s is equal to the rate of formation of the cychc ketenimine. The first step, cyclization to benzazirine (29) is rate determining, and is followed by fast electrocyclic ring opening to cychc ketenimine 30. 

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. 

By 1992 Schuster and Platz could write Scheme 1, which economically explained much of the photochemistry of phenyl azide. UV photolysis of PA produces singlet phenylnitrene and molecular nitrogen. In the gas phase, PN is born with excess vibrational energy and isomerizes over a barrier of >30kcal/mol to form cyanocyclopentadiene, the global minimum on the CsHsN surface." This species is also vibrationally excited and sheds a hydrogen atom to form radical 3 (Scheme 1), the species detected in gas-phase absorption and emission measurements. ... 

The key intermediate of Scheme 1 is singlet phenylnitrene the only intermediate which in 1992 had not been detected directly or chemically intercepted in the parent system. In 1997 our group" " and the Wirz group, simultaneously reported that laser flash photolysis of phenyl azide or phenyl isocyanate 5 produces a previously undetected transient with = 350 nm and a lifetime of ns at ambient temperature. 

Instead of intermolecular chemistry, phenylnitrene (1) rapidly undergoes intramolecular rearrangement. Substituted azepine (3) is the product of both thermolysis and photolysis of phenyl azide in the presence of amine. On the other hand, substituted aniline (4) is obtained when phenyl azide is photolyzed with ethanthiol. ... 

The parent phenylnitrene has been studied in detail.406 The thermal or photochemical decomposition of phenyl azide and most of its derivatives in solution results in the formation of intractable polymeric tars. Meaningful mechanistic studies became possible only when it was found that amines intercept the intermediate formed by thermal409 or photochemical410 decomposition of phenyl azide in solution that was responsible for polymerization. The current knowledge about the mechanism of phenyl azide photochemistry is summarized in Scheme 5.7. 

ESR parameters for triplet carbenes15 and nitrenes16 have been summarized, and it has been shown that phenylnitrene is produced predominantly (87-88%) in the singlet state by direct photolysis of phenyl azide in low-temperature matrices.17 The first spectroscopic observation of a singlet nitrene has been reported nanosecond-laser photolysis of 1-azidopyrene gives the S0 nitrene (Amax 450 nm) which has a lifetime of 22 nsec at room temperature (in benzene) and 34 nsec at 77 K in rigid solution. At room temperature it decays to the triplet ground state (Tj, Amax 415 nm) with a rate constant of about 4.4 x 107 sec. Tt is formed directly by biacetyl sensitized photolysis of the azide. The lifetime of the excited triplet (T2) was about 7 nsec. T dimerizes to azopyrene.18... 

Phenylnitrene and 2-pyridylcarbene interconvert in the gas phase, as evidenced by the formation of identical products from phenyl azides and 2-(diazomethyl)pyridines (Scheme 30).1 99,200 Although the carbene precursor 157 exists as a triazole in the solid state, the diazomethane valence tautomer... 

Phenylnitrene and its substituted derivatives tuidergo three principal reactions in the gas-phase [Eq. (53)] (1) dimerization to azobenzene (which can also derive from reaction between phenylnitrene and phenyl azide los.m), (2) H-abstraction giving aniline and (3) ring contraction to 1-cyano-cyclopentadiene Yields of these and other products are given in Table 19. 

Phenylnitrene and 2-pyridylcarbene interconvert in the gas phase, as evidenced by the formation of identical products from phenyl azides and... 

Intramolecular substitution by arylnitrenes has been known for a long time and the cyclization of o-azidobiphenyls to carbazoles is representative of an important series of nitrene reactions that yield heterocycles. Only fairly recently has an intermolecular counterpart of these reactions been observed. Originally when phenyl azide was thermolyzed in benzene only azobenzene (11%), aniline (18%), and tars were obtained, and no diphenylamine was found. Abramovitch considered that phenylnitrene is not sufficiently electrophilic to attack benzene, so he decomposed a series of aryl azides (bearing strongly electron-withdrawing substituents, NO2, CN, CF3) in solvents activated... 

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