Nitrenes from azide photolysis

Nitrenes, 116-118, see also Nitrene (HN) aminonitrenes, 118 from azide photolysis, 116 phenylnitrene, 118 substituents on, 117... 

Dimethyl-5,6-dihydrophenanthridine (29) is one of the minor products obtained from the thermolysis of l-(biphenyl-2-yl)-l-methylethyl azide (26). This appears to be the first authentic example of an intramolecular substitution by an alkyl nitrene (27). The formation of 29 was also observed in the photolysis of 26, but evidence was brought forward suggesting that 29 arises here from the photolysis of the imine (28) 28 also formed 29 on treatment with polyphosphoric acid (PPA).62... 

Common methods for generating nitrene intermediates are the photolysis and thermolysis of azides. Generation of nitrenes from acyl azides can only be effected photochemically thermolysis of an acyl azide gives the corresponding isocyanate. 

Tlie formation of free radical HN by decomposition of hydrazoic acid has been suggested by a number of authors since 1928 (see Vol. HI, p. 167). This was substantiated by experiments on the decomposition of HNj by the flasli photolysis of Thrush (Vol. Ill, p. 167). The formation of nitrene radicals from azides by flash photolysis was reported simultaneously and independently by Koto (56], Reiser et al. [57—59]. Reiser rationalized the reaction derived from flash pl otolysis of formation of azo compounds (5) ... 

Sundberg and co-workers in 1974 reported results from flash photolysis studies of the reaction of phenyl azide with secondary amines [26]. Irradiation of the azide in hexane solution produced an intermediate absorbing strongly at 366 nm and having a lifetime of about 5 ms. This intermediate was found to react with amines to give, after tautomerization, the 3H-azepine that is obtained in preparative scale reactions. On the basis of these results, De Graff et al. [26] concluded that the detected intermediate is not a nitrene but is a closed-shell intermediate, either benzazirine or dehydroazepine. 

It is conceivable that the excess vibrational energy imparted to phenyl azide from UV photolysis, combined with the exothermicity of the ensuing isomerization of singlet phenyl nitrene, may produce cyanocyclopentadiene with enough excess vibrational energy to fragment a CH bond to form the fluorescent radical 7, detected by absorption and emission spectroscopy. 

Azide anion is a good nucleophile. Many azide reactions proceed via formation of an active intermediate, nitrene, analogous to carbene. This results from the photolysis of hydrazoic acid ... 

An experiment in rare gas matrix at a temperature of a few K is by no means comparable to an experiment in a frozen solvent. Likewise, the IR spectrum is much richer of lines and thus gives much more structural information. This is apparent when the much better resolved spectrum of an intermediate, a nitrene, obtained by photolysis of azide 26 [38] in an argon matrix at 12 K is compared to that in ethanol at 90 K and the wealth of data that are obtained from the IR spectmm of this species is considered. Furthermore, it easy to check for their correspondence to the calculated spectmm. The secondary conversion of the first formed intermediate is also conveniently followed under these conditions (Figs. 6.17 and 6.18) (Scheme 6.12). 

The formation of highly reactive intermediates from the photolysis of aromatic azides favor their use for surface modification and the preparation of substrates for solid phase reactions. Thus, azide photochemistry has been exploited for facile modification of graphitic surfaces and N-5-azidonitrobenzoyloysuccinimide undergoes photoaddition in a solid state reaction. A review on the covalent functionalization of graphene includes examples via photogenerated nitrenes. ... 

Nitrene addition reactions have somewhat more scope when the reactions are carried out by azide photolysis for example, the acyinitrene pivaloylnitrene adds in moderate yield and with good stereoselectivity to cj>alkenes when it is generated from pivaloyl azide by photolysis in dichloromethane." Similarly p-cyanobenzoylnitrene, when generated by photolysis of the azide 21 in the presence of 2,5-dihydrofuian, gives the aziridine 22 in moderate yield (Scheme 6.11)." This nitrene and other p-substituted benzoylnitrenes also react with Cgo to give fulleroaziridines such as 23." ... 

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. ... 

Thermal decomposition of ( >-azidophenylthio)uracil (299) furnishes a 2 1 mixture of the dihydro-5-thia- (300) and dihydro-10-thia-isoalloxazines (301). The formation of the former product is analogous to the preparation of pheno-thiazines from o-azidodiphenyl sulphides, and presumably involves rearrangement via a Spiro-intermediate. However, the direct insertion of the nitrene intermediate derived from azide (299) into the uracil ring to give the 1,4-benzothiazine system (301) is unique. More surprising is the production of the unrearranged 10-thiaisoalloxazine (301) (70%) as the only product from the photolysis of (299) in methanol solution. 

Aryl substituents also activate benzyHc C-H bonds adjacent to a nitrene center. An interesting study on this topic has dealt with the atropoisomeric 3,5-dimethyl-2-(9-fluorenyl)phenylazides 49 and 52. ° The low-temperature photochemistry (77 K) of these rotameric azides proved to be fundamentally different Photolysis of 49 resulted in the formation of nitrene 50 in addition to azanorcaradiene 51, while photolysis of 52 gave nitrene 53 in addition to iminoquinone methide 54. This subsequently underwent intramolecular cycloaddition to yield dihydroindenocridine 55. Laser flash photolysis showed that the formation of iminoquinone methide 54 from azide 52 occurred in less than 10 ns. Deuteration of the 9-position of the fluorene chromophore resulted in a significantly diminished yield of 55. LFP of the rotameric azide 49 yielded the rate constant for the formation of azanorcaradiene 51 which was determined as k = 7.1 x 10 ... 

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