Triplet nitrenes, aryl azides produce

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 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,... 

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. ... 

The majority of recent research in the photochemistry of azides and the generation of nitrenes has involved aryl rather than alkyl azides. Amongst the latter, however, is an interesting study of the photodissociation of methyl azide at wavelengths from 292 to 325 nm. The photodissociation dynamics of methyl azide are apparently complex, with the predominant pathway producing CH2=NH by concerted 1,2-H shift and N2 extrusion. Triplet methylnitrene (CH3N) was also observed by emission spectroscopy, but seems to arise by a minor, spin-forbidden pathway. 

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