Singlet nitrenes, aryl azides

Photolysis of aryl azides in amine solution, with a tertiary amine as cosolvent to promote stabilization of the singlet nitrene, has met with some success. For example, the yield of 2-piperidino-3 W-azepme. obtained by the photolysis of phenyl azide in piperidine, is increased from 35 to 58% in the presence of A A /V. /V -tetramethylethylenediamine (TMLDA).180 Also, an improved yield (36 to 60 %) of A,(V-diethyl-3W-azepin-2-amine (38, R = Et) can be obtained by irradiating phenyl azide in triethylamine, rather than in dicthylaminc, solution.181 Photolysis (or thermolysis) of phenyl azide in TMEDA produces, in each case, 38 (R = Et) in 40% yield.181 In contrast, irradiation of phenyl azide in aniline with trimethylamine as cosolvent furnishes jV-phenyl-377-azepin-2-amine (32, R = Ph) in only low yield (2%).35... 

Photolysis of alkyl azides bearing pendant aryl groups does not lead to intramolecular trapping of a nitrene by the aryl group. However, intramolecular capture of the putative nitrene is observed upon pyrolysis of the azide precursor. These observations convinced Kyba and Abramovitch" ° that 1,2-migration is concerted with loss of nitrogen from the excited state of the azide, but that a free singlet nitrene is formed upon thermal decomposition. The chemistry of acyl azides will be shown later to exhibit the opposite pattern. 

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. 

The next development in direct detection of nitrenium ions came from McClelland et al. ° who applied the azide method to LFP measurements. This permitted the direct detection of those arylnitrenium ions implicated in carcinogenic DNA damage. McClelland s approach proved to be particularly useful in the study of 4-aryl and 4-alkoxy substituted phenylnitrenium ions. Apparently, the corresponding singlet nitrenes are sufficiently long lived to allow for protonation in aqueous solution. Several arylnitrenium ions studied by this route are described in Table 13.6. 

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... 

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,... 

Studies of the photodecomposition of azido derivatives of PCB have attracted interest because of their potential use as photolabels in intracellular distribution studies.Polyfluorinated aryl azides are also of current interest as new reagents for photoaffinity labelling. The major products of photolysis of methyl 4-azidotetrafluorobenzoate in cyclohexane or diethylamine, for example, arise by insertion, a property which enhances its use in labelling. Singlet pentafluorophenyl nitrene, formed in an analogous fashion on irradiation of pentafluorophenyl azide, can be trapped by toluene as the insertion products (79) and (80). °... 

The only other flash photolysis experiments reported before 1980 appeared in 1976 when Sumitani et al. [27] described the first application of pulsed lasers to the photochemistry of aryl azides. They found that irradiation of 1-pyrenyl azide on a nanosecond time scale gives two detectable intermediates. The first, with a 22 ns lifetime, was assigned to the singlet nitrene, and the second, with a much longer lifetime, was assigned to the... 

Dilution of toluene with the inert solvent methylene chloride was attempted in an effort to extend the singlet nitrene lifetime and enhance the yield of triplet nitrene [104]. Dilution, however, did not change the ratio of aryl C-H to benzyl C-H insertion products formed, instead the yield of all volatile products decreased at the expense of tar formation. Dilution with CH2C12 did not increase the yield of triplet nitrene derived products such as C6F5NH2 and decafluoroazobenzene, thus the yield of triplet phenyl nitrene is negligible (Table 7) in methylene chloride. The results can be understood with the aid of Scheme 10, which is identical to the mechanistic hypotheses written for parent phenyl azide (Scheme 7). 

Apparently, 139 is not formed by thermal rearrangement of the anthranil (see Section III,C,5,a) but directly from the azide, presumably by singlet nitrene attack at the pyridine ring nitrogen. Zwitterion 139 becomes the major product when the decomposition is carried out at higher temperature (215°C). Analogous by-products have also been noted in other azide decompositions. For example, low yields of 3-aryl-4-quinolones accompany 3-(j3-styryl)anthranil formation189 (see also Section III,C,5,a). 

The consensus of opinion on the formation of azepines from aryl azides suggests that an intermediate singlet nitrene is in equilibrium with a didehydroazepine, which itself may be in equilibrium with a azepinocarbene. This has received further support form the investigations of Murata and co-workers, who have isolated both the azomethine ylide 1 and the... 

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