Triplet nitrenes, aryl azides

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. 

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... 

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... 

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). 

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,... 

Photolysis of phenyl and o-trifluoromethylphenyl azide in solid matrices led to the triplet nitrene as detected by e.s.r. The actual processes involved in aromatic azide photolyses have been the subject of much studyThe electronic spectra of the nitrenes were measured by photolysis of a number of aryl azides in organic matrices at 77°K . These species were stable indefinitely at this temperature, no change being observed in the spectra for hours. The photolysis of diazides at 77°K, whether conjugated (e.g. / -diazido-benzene) or not [e.g. bis (j6-azidophenyl) methane] proceeded in two distinct steps to the dinitrene. The second step was about two to three times as efficient as the first ( 2/ 1 2-3)... 

In 1965, Reiser et al. [18] published the first in a series of seminal papers describing the photochemistry and optical spectroscopy of aryl azides in rigid media at low temperature. In particular, irradiation of phenyl azide in a glassy solution at 77 K. resulted in formation of an intermediate with absorption maxima 241, 303, and 368 nm. Thoughtful application of models and control experiments led to tentative assignment of these bands to the triplet state of phenyl nitrene. However, concern about the validity of this assignment is well illustrated by consideration of the chemistry and spectroscopy of 2-azidobiphenyl. 

In principle, the application of time-resolved techniques permits identification of intermediates by monitoring their progress to the stable products of reaction. In 1973, Lehman and Berry [25] reported the first application of time-resolved photochemical methods to the study of aryl azides. Using conventional flash photolysis, they irradiated 2-azidobiphenyl in cyclohexane solution. Time-resolved absorption spectroscopy revealed an intermediate assigned as the triplet nitrene primarily on the basis of the similarity of its spectrum to that measured by Reiser [18] in low-temperature experiments. Lehman [25] monitored the rate of carbazole formation and found it to occur by a kinetically first-order process with a lifetime of 460 /is at room temperature. These findings led them to conclude that photolysis of 2-azidobiphenyl at room temperature leads rapidly to the triplet nitrene, and that this species is the precursor to carbazole [25], However, this point of view clearly is at odds with Swenton s triplet sensitization experiments [23],... 

The research results described in this introductory section present a somewhat confusing picture of aryl azide chemistry. In particular, it seems that different analytical methods lead to contradictory conclusions concerning the identity of reactive intermediates and their proper role in the chemical transformations of azides. Analysis at low temperature by EPR spectroscopy reveals a triplet nitrene, but IR spectroscopy requires a dehydroazepine. Irradiation at room temperature gives triplet nitrene-derived products unless a trap for a closed-shell intermediate (either a benzazirine or a dehydroazepine) is present in solution. The last ten years have witnessed remarkable progress in resolving these contradictions and questions. The remainder of this chapter is devoted to the presentation and analysis of this more recent work. 

Azobenzene formation signals reaction of the triplet nitrene, substituted 3H-azepines come from the trapping of dehydroazepines. Clearly substituents on aryl azides affect the formation and reactivity of these intermediates. The precise nature of the substituent effects was revealed by application of time-resolved absorption experiments that will be described later. However, from the perspective of product yields and synthetic applications, two noteworthy trends should be mentioned here. 

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). 

In 1951, Smith and Brown were the first to show that the photolysis of aryl azides led to carbazoles (Scheme 17, equation 1) [114], a reaction that also occurs upon heating. Other carbazoles so prepared are 36 to 39. This reaction has been reviewed [115-119], and has been of more interest mechanistically than synthetically, by the groups of Reiser [120, 121], Swenton [122-124], Berry [125], Sundberg [126, 127], Yabe [128-130], Meth-Cohn [131], and Spagnolo [132]. Sundberg provides an excellent summary of the possible mechanisms involved in the photolysis of 2-azidobiphenyl to carbazole [126, 127], and his own work indicates that a triplet nitrene may not be the sole or major carbazole precursor [127]. In any event, the photochemical transformation of aryl azides is a useful synthesis of carbazoles. Some additional examples are shown in Scheme 17 [114, 115, 124, 128, 129, 133]. In addition, Sauer and Engels obtained, as expected, a nearly 1 1 ratio of 1- and 3-methylcarbazoles (85%) on photolysis of 2-azido-3 -methylbiphenyl [134]. 

Many thermal reactions of aryl azides also take place photochemically and often photolysis is preferable to thermolysis as milder conditions may be employed, and the use of sensitizers allows the generation of the nitrene specifically in the singlet or triplet state ... 

Arylnitrene triplet states have been observed by uv and esr when aryl azides are irradiated at low temperature in solid matrices. Flash photolysis of 1-azidoanthracene in ethanol at room temperature gives a short-lived intermediate (with a half-life of 3-10/nitrene generated by the matrix photolysis of the same azide at 77 When these solid matrices are allowed to warm up to room temperature the nitrene spectrum collapses and typical nitrene products vide supra) are obtained. The nature of the products formed by analysis of reaction mixtures is the main way used to adduce nitrene intermediacy on photolysis. 

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