Photochemistry phenyl azide

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. 

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. 

AUcyl, cyano, acetyl and fluoro substituents in the ort/to-position do not change the mechanism of phenyl azide photochemistry influencing only the rate constants of elementary reactions ( kc. fe. ro. nuc)- At the same time, a number of photochemical and thermal cyclizations involving the orf/to-substituents are known for ortfto-substituted phenyl azides. The most interesting, important and well understood reaction of this type... 

An alternative explanation, based on a spectroscopic study involving an argon matrix, has been advanced to account for the singlet photochemistry of phenyl azide (457).381 The primary photoproduct is believed to be 1-azacyclohepta-l,2,4,6-tetraene (458). A separate but later study suggests that... 

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. 

Sundberg and co-workers studied the photochemistry of phenyl azide by conventional flash photolysis in 1974. They detected the transient UV absorption of ke-tenimine 30 and measured its absolute rate constant of reaction with diethylamine to form the IH azepine, which subsequently rearranges to Doering and Odum s 3H-azepine (27). ... 

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. ... 

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-... 

Investigation of the photochemistry of phenyl azide has been underway for nearly as long as the study of its thermal chemistry. In his 1959 review of carbenes and nitrenes, Kirmse [14] tells of earlier, unpublished work on the photolysis of phenyl azide carried out in Homer s laboratory. At first, photolysis was viewed simply as an additional approach to formation of nitrenes [12, 15, 16]. However, it was quickly realized that light-initiated decomposition of azides provides access to an important array of chemical and spectroscopic tools that permit detailed examination of important questions. In particular, photolysis of aryl azides permits examinations at room temperature or, specially, at low temperature in rigid media where normally reactive intermediates can be stable indefinitely. Furthermore, the use of fast, pulsed lasers as light sources allows the direct detection of shortlived intermediates and enables the detailed study of their reactions. In recent years, most inquiries into the chemistry of aryl azides have focused on application of the tools photolysis makes possible for characterization of the nature and role of reactive intermediates in their chemical transformations. 

The first triumph of photochemistry occurred in 1962 when Smolinsky et al. [17] reported photolyzing phenyl azide and measuring the EPR spectrum of phenyl nitrene in fluorolube at 77 K. This result established with certainty that the ground state of phenyl nitrene is a triplet and initiated three decades of debate about the relevancy of observations made at low temperature to the chemical reactions that occur at room temperature or above. 

In 1965, Reiser et al. [18] published the first in a series of seminal papers describing the photochemistry and optical spectroscopy of aryl azides in rigid media at low temperature. In particular, irradiation of phenyl azide in a glassy solution at 77 K. resulted in formation of an intermediate with absorption maxima 241, 303, and 368 nm. Thoughtful application of models and control experiments led to tentative assignment of these bands to the triplet state of phenyl nitrene. However, concern about the validity of this assignment is well illustrated by consideration of the chemistry and spectroscopy of 2-azidobiphenyl. 

The first serious study of the photochemistry of phenyl azide using a time-resolved method was performed by DeGraff et al. in 1974 [26], They observed the formation of a 1 //-azepine following flash photolysis of phenyl azide in the presence of secondary amines and proposed the following mechanistic scheme shown below. The 1 //-azepine isomerizes to the isolated 3/f-azirine on millisecond time scales. 

Subsequent studies using more sophisticated equipment confirmed the validity of their data but a new mechanistic picture emerged (see Sections IV and V). DeGraff et al. were also the first to study low temperature effects on the photochemistry of phenyl azide [26], They discovered that the absolute rate of formation of 1//-azepine 1 was essentially independent of temperature ( a = — 1.8 + 0.2 kcal/mol). 

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... 

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