Aryl azides, reactions produce nitrenes

Laser flash photolysis of a series of fluorinated aryl azides produces the transient spectra of the corresponding singlet nitrenes. ° With the exception of singlet o-fiuorophenylnitrene (39s), the rate of decay of the singlet nitrene was equal to the rate of formation of the reaction products, for example, didehydroazepines and triplet nitrenes. Values of fejsc and the Arrhenius parameters for azirine formation are summarized in Table 11.5. 

The photochemistry of aryl azides is quite complex, suggesting that the nitrene 14 may not be the only reactive intermediate and that insertion reactions may not be the only route to form photoconjugates.Although aryl nitrenes are much less susceptible to rearrangements than acyl nitrenes, they may still occur and lead to the formation of reactive intermediates such as azepines, which may go on to react with nucleophiles.[911 141 Addition of nitrenes to double bonds will generate azirines, while dimerization will produce azobenzenesJ11 Aryl azides are stable to most of the procedures used in the course of peptide synthesis except for reduction reactions. Non-photochemical reduction of aryl azides to the primary amines by thiols has been reported by Staros et al.[15]... 

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. ... 

The photolysis of aryl azides in low-temperature matrices yields triplet (ground) state nitrenes which have been identified by and absorption spectroscopy. Dinitrenes and trinitrenes have also been reported in the solid-state photolysis of di- and triazides. Quantum yields of photolysis of some aromatic azides are listed in Table 21 and it appears that nitrenes are produced in solution, at room temperature, as well. The lifetimes of some aromatic nitrenes and the absolute rates of some of their reactions have been measured . Some interesting features of photolytic azide decompositions will now be briefly described. 

Chemiluminescence has been used to measure the relative yields of excited ketones formed from self reaction of alkoxyl and alkylperoxyl radical pairs . In the photochemistry of aryl azides a dehydroazepine is detected by time resolved infra red spectroscopy and flash photolysis at room temperature . Singlet and triplet nitrenes and dehydroazepenes have also been detected in the photochemistry of 3- and 4-nitrophenyl azides . Picosecond and nanosecond laser photolysis of p-nitrophenyl acetate in aqueous media produces a triplet state of the -nitrobenzylanion and CO2 after cleavage of the rnr triplet. Absorption, emission, and reaction kinetics of dimethylsilylene produced by flash photolyses of dodecamethylcycloherasilane is another interesting study 2,... 

Bis(aryl)azide rubber resist is a common negative photoresist. In a photoreaction, the evolution of nitrogen from the acylazide produces a highly reactive intermediate, nitrene. Through a series of reactions the nitrene intermediate leads to the formation of polymer by cross-linking the resin. The unreacted negative photoresist can then be dissolved in organic solvent, and xylene is commonly used. 

The reactions of nitrenes produced in the photoelimination reactions of aryl azides. 

H)-Oxazolones are formed by the spontaneous cyclization of /3-oxo isocyanates (equation 134). Similarly, o-hydroxyphenyl isocyanate, produced by the Curtius rearrangement of the azide of salicylic acid or by the action of sodium hypochlorite on salicylamide, forms benzoxazolone (equation 135). An analogous reaction is the formation of IV-phenyl-benzoxazolone by the action of thionyl chloride on the hydroxamic acid shown in equation (136) (78TL2325). Pyrolysis of aryl azidoformates affords benzoxazolones by nitrene insertion (equation 137) (81CC241). 

In contrast to carbonyl azides, photolysis and thermolysis of azidoformates (RO-CO-N3, R=Alkyl, Aryl) yield mainly products derived from capmre of the nitrenes (RO-CO-jvj) 108,142 146 Carbethoxy azide 42 has been studied most extensively. Formation of products characteristic of reactions of carbethoxynitrene M3 have been observed by thermolysis and photolysis of azide 42 " and by -elimination of arysulfonate ion from N-(p-nitrobenzenesulfonyloxy) urethane. " The reaction of M3 with cis and trans-4-methyl-2-pentene was studied as a function of alkene concentration. At large alkene concentrations, aziridination is stereospecific, but upon dilution of the alkene, the stereospecificity is lost. The triplet nitrene M3 also reacts with the olefins, but non-stereospecifically, presumably through intermediate biradical formation (Scheme 11.21). These results are completely analogous to studies of carbenes in which a stereospedlic singlet intermediate is produced initially, and subsequently relaxes to a less selective, lower eno gy triplet intermediate. ... 

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