Photochemistry of Phenyl Azide

The photochemistry of phenyl azide and its simple derivatives have received the most attention in the literature. The results of early studies were summarized in a number of reviews. Over the last decade, modem time-resolved spectroscopic techniques and high level ab initio calculations have been successfully applied and reveal the detailed description of aryl azide photochemistry. This progress was analyzed in recent reviews. Femtosecond time resolved methods have been recently employed to study the primary photophysical and photochemical processes upon excitation of aryl azides. The precise details by which aryl azide excited states decompose to produce singlet arylnitrenes and how rapidly the seminal nitrenes lose heat to solvent and undergo unimolecular transformations were detailed. As a result of the application of modem experimental and theoretical techniques, phenylnitrene (PhN) - the primary intermediate of phenyl azide photolysis, is now one of the best characterized of all known organic nitrenes. 5 -2° -  

In this section we will briefly consider the most important early results which oeated the basis for the interpretation of the more recent studies. The largest part of this section will be devoted to the experimental and theoretical discoveries of the last decade. 

The high dilution of solutions of phenyl azide suppresses polymer formation and azobenzene forms instead. This indicates that singlet intermediates (50 and/or 51) serve as a reservoir for triplet phenylnitrenes ( 52), which either undergo dimerization or react 

it was demonstrated that photolysis of the dilute hydrocarbon solutions ( 10 M) of simple derivatives of 47 in the presence of oxygen gives the corresponding nitro- and nitrosobenzenes with a yield of -80%. The latter are also the products of triplet arylnitrenes reactions.  

The solution phase photochemistry of phenyl azide 47 is temperature dependent/ Photolysis of 47 in the presence of diethylamine at ambient temperature yields azepine 48b. Lowering the temperature suppresses the yield of 48b and below 160K, azobenzene, the product of triplet nitrene dimerization, is produced. Thus, high temperature favors reactions of singlet state intermediates whilst low temperatures favor reactions associated with triplet phenylnitrene. 

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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. ... 

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 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... 

A summary of the available spectroscopic information relating to the photochemistry of phenyl azide is presented in Scheme 7, along with the... 

The discovery of the temperature dependence of the photochemistry of phenyl azide prompted Leyva and Platz [85] to reexamine the photochemistry of 1-naphthyl azide. As mentioned previously photolysis of 1-naphthyl azide at 298 K in the presence of diethylamine fails to produce an azepine adduct, instead only a trace of diamine 9 is observed along with small amounts of azonaphthalene [51, 57, 91], The major product is 1-naphthy-lamine. Carroll et al. [51] improved the yield of diamine adduct with piperidine by adding N, N, N, N tetramethylethylenediamine (TMEDA) to complex with singlet 1-naphthyl nitrene. TMEDA did not improve the yield of diethylamine adduct, however. 

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