Triplets aryl nitrenes

Nitrenes, the nitrogen analogs of carbenes, are too reactive for isolation under ordinary conditions. The ground state of NH, and of most nitrenes, is a triplet. Aryl nitrenes have been trapped at 77 K,186 the more reactive alkyl nitrenes in matrices at 4 K.187... 

The major intermolecular reaction of triplet aryl nitrenes in solution is hydrogen atom abstaction to form primary amines. For a photoaffinity reagent bound to a receptor, this would result in a failure to couple. However, it is possible that the intramolecular photochemistry of aryl azides is more relevant, and here numerous examples of insertion by triplets have been noted. Presumably, these are two step processes hydrogen atom abstraction, followed by radical coupling (cf. Figs. 2.1 and 2.3). 

Dahne, B., Bendig, J., and Stosser, R., About the thermal and photochemical reactivity of triplet aryl nitrenes, /. Prakt. Chem., 334, 707, 1992. 

Huron and Platz recently smdied the photochemistry of 13 in solution by LFP. The triplet state of 19 absorbs at 400 nm in 1,1,2-trifluorotrichloroethane with a lifetime of 1-2 ps. The triplet is formed within 10 ns of the laser pulse. Relaxation of the singlet to the triplet state of 19 is fast relative to the related process in aryl-nitrenes and is comparable to a carbenic process. As we will see later when we discuss intersystem crossing rates of singlet arylnitrenes, this difference is most likely due to the closed-shell electronic configuration of the singlet state of 19. 

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. 

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. 

Nitrenes,239 R—N, are the nitrogen analogs of carbenes, and most of what we have said about carbenes also applies to them. Nitrenes are too reactive for isolation under ordinary conditions. Alkyl nitrenes have been isolated by trapping in matrices at 4 K,24n while aryl nitrenes, which are less reactive, can be trapped at 77 K.241 The ground state of NH, and probably of most nitrenes,242 is a triplet, though nitrenes can be generated in both triplet and singlet states. In additions of EtOOC—N to C=C double bonds two species are involved,... 

Thermal decomposition of aryl azides under a nitrogen atmosphere leads to nitrenes which are able to substitute fluorine in polyfluorinated naphthalenes, albeit with a poor yield (4%). Since no substitution product was isolated when oxygen was present during this reaction, the intermediacy of a triplet nitrene was proposed, which is completely trapped by oxygen.207... 

Inue et al. claimed the first report of the infrared detection of an aryl nitroso oxide, made from the reaction of 4-nitrophenylnitrene with molecular oxygen.70 The singlet nitroso diradical is formed from the triplet nitrene and oxygen at a temperature of 50 K in argon or xenon matrices. The reaction does not occur at 40 K. 

The initial product of photolysis of aryl azides, such as (70), is a singlet nitrene that relaxes into the triplet form below 160 K.72 The reactivity below 160 K is... 

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