Aryl azides phenylnitrene

For most aryl azides, the rate constants of singlet nitrene decay and product formation (triplet nitrene and/or ketenimine) are the same. Thus, in all these phenyl-nitrenes cyclization to substituted benzazirines is the rate-limiting step of the process of isomerization to ketenimine, as is the case for the parent phenylnitrene. The only known exception, o-fluorophenylnitrene, will be discussed in the next section. 

The development of the chemistry of aryl nitrenes has occurred primarily over the last 20 years, even though initial forays were made nearly a century ago. This slow development can be attributed to the fact that photolysis or pyrolysis of typical aryl azides, such as phenylnitrene 1, produces polymeric tar (Reaction 5.1). This is in sharp contrast to the rich chemistry of phenylcarbene (2), which exhibits insertion into C-H bonds, addition to Jt-bonds, reaction with oxygen, and so on. ... 

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... 

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° - ... 

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