Photoelimination of nitrogen

Compounds containing nitrogen-nitrogen double bonds exhibit extreme photoreactivity due to the stability of the photoelimination product, nitrogen gas. In fact, the photodecomposition of azo compounds occurs so readily that 

The photochemistry of the simplest member of the azo family, azomethane, has been extensively investigated in the vapor phase and in solution 

Photolysis in the gas phase leads to the quantitative production of nitrogen and methyl radicals. Photolysis in solution, however, results in a shift in the absorption spectrum to longer wavelengths due to the production of a new species, which is identified as the m-azomethane (the Irons configuration is the normal isomer). Similarly, irradiation of /ravu azoisopropane results in trans-cis isomerization to the cis isomer  

Irradiation of the cis isomer results in the isomerization with quantum yield, equal to 0.56. The fact that the quantum yields of cis and irons formation are similar and add to unity suggests the presence of a common intermediate, thought to be a twisted triplet state. Gas-phase photolysis of trons- 

Photolysis of azoisobutane at room temperature in solution yields 

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Photoelimination of nitrogen from diazoketones is complicated by Wolff rearrangement of the intermediate carbene, as shown below for diazoaceto-phenone<35) ... 

Azides are known to undergo photoelimination of nitrogen to produce univalent nitrogen intermediates (nitrenes). These electron-deficient intermediates can then undergo a variety of intermolecular and intramolecular... 

Evidence from nanosecond flash photolysis studies indicates that an oxa-ziridine precursor to the diazoketone is unlikely. Photoelimination of nitrogen is also observed in 4-methyl-1,2,3-benzotriazine 3-oxide and affords 3-methylanthranil a mechanism involving loss of nitrogen from an intermediate oxaziridine has been proposed.74... 

The photoelimination of nitrogen from azocycloalkanes is of interest both from the synthetic and mechanistic point of view. Acyclic azoalkanes appear to undergo elimination of nitrogen by a stepwise process involving an intermediate diazenyl radical, but the photoreactions observed in azocycloalkanes are to some extent dependent on ring size. 

Photoelimination of nitrogen from diazirines, for example, proceeds via carbene intermediates. 3-tert-Butyldiazirine (395) is converted into the cyclopropane 396 and the alkene 397 with the formation of the carbene insertion product being favored from the singlet state.328 3-Cyclopropyl-3-chlorodiazirine (398) has similarly been converted to the carbene 399 which undergoes both rearrangement to l-chlorocyclobutene(400)329 and addition... 

Photoelimination of nitrogen from 3,3,4,4-tetramethyldiazetine has been shown to proceed with a quantum efficiency of 0.52,332 but no systematic study of the photodecomposition of such compounds has been undertaken. The fused diazetine 403 is converted on irradiation into norbornadiene (404).333... 

The photoelimination of nitrogen from 1-pyrazolines is one of the most thoroughly investigated photoreactions and it has been used extensively in the synthesis of cyclopropane derivatives.334 Both stereospecific and non-stereospedfic processes have been observed and these are believed, at least in simple 1-pyrazolines, to correspond to singlet and triplet excited states, respectively. Two reaction pathways have been proposed in the azoalkane 405335 direct excitation via a thermally activated S, state affords the C6H6 isomers 406 to 409, whereas triplet-sensitized excitation results in a tem-... 

Photoelimination of nitrogen from 1-pyrazolines has also been employed in the synthesis of tricyclo[3.2.1.02,4]oct-6-ene,338 prismane,339 quadri-cyclane,340 snoutene ,341 and marasmic acid.342 The trimethylenemethanes 414 have been prepared by photolysis of azoalkanes 415 and characterized spectroscopically.343 Dimerization and cycloaddition to alkenes of these biradicals have been reported.344... 

Numerous reports of the photoelimination of nitrogen from spiro-3//-pyrazoles have been published.348 The diaza[2,2]spirenes 420 were converted into the benzocyclopropenes 421 by a pathway which has been shown not to involve intermediate indazoles.349 3f/-Indazoles undergo analogous photoreactions.350... 

Evidence for the formation of thiiren by photoelimination of nitrogen from 1,2,3-thiadiazoles has been described,351 and several thiirens, prepared in this way, have been identified in an argon matrix at 8K.352 Phenylthiazirine (422) appears to be an intermediate in the related transformation of 5-phenyl-l,2,3,4-thiatriazole (423) into benzonitrile sulfide (424),353 and further... 

Photoelimination of nitrogen from 1,2,3-triazolines has been widely used as a synthetic route to aziridines the reaction has been reviewed.355 Recent applications include the formation of a new valence isomer (425) of azepine from the triazoline 426,356 and conversion of the triazoline 427 into the aziridine 428, a process with potential as a synthetic route to mitomycins.357... 

Photoelimination of nitrogen from the tetrazole 429 results in the formation of the dihydropyrazole 430, presumably via intramolecular addition of the photochemically generated nitrile imine 431.358 Other examples of this type of behavior have been reported.359... 

The photoelimination of nitrogen from diazo compounds provides a simple and versatile route for the generation of carbenes, and in certain instances, insertion reactions of carbenes can be employed in the synthesis of heterocycles. Carbenes are believed to be involved at least in part in the photochemically induced conversion of N,N-diethyldiazoacetamide (439) into the y-lactam 440 and the /Mactam 441,365 and a similar approach has been successfully employed in the synthesis of a carbapen-2-em366 and of 7-methylcephalosporin analogues.367 Carbene insertion of a different type has been observed on irradiation of the 6-anilino-5-diazouracils 442 to give the indolo[2,3-d]pyrimidines 443.368 Ring contractions in heterocycles... 

Aryl and heteroaryl nitrene insertion processes have also been employed in the synthesis of heterocycles. The azide 459, for example, is converted on irradiation into the imidazole 460,383 and dihydro-10-thiaisoalloxazines are obtained in good yield on photoelimination of nitrogen from 6-(2-azido-phenylthio)uracils.384... 

Similarly, 2,3-diazabicyclo[2.2.2]oct-2-enes underwent photoelimination of nitrogen via singlet 1,4-diradicals15 to give bicyclic cyclobutanes (Houben-Weyl, Vol. 4/4, pp 40-44). However, the parent compound on photolysis at 185 nm gave bicyclo[2,2.0]hexane (11 a) in only 9% yield,10 while benzophenone sensitized or direct photolysis resulted in mixtures of bicy-clo[2.2.0]hexane and hexa-1,5-diene (27 73 or 42 58, respectively).16 In a separate study, a triplet 1,4-cyclohexadiyl was trapped with molecular oxygen.17... 

The photoelimination of nitrogen from azo compounds, diazo compounds, and azides is a topic that has been thoroughly reviewed elsewhere. These transformations are found in oxygen and sulfur heterocycles, but as they are not unique to these systems, they will not be discussed in this article. Examples can be found in a review on the photochemistry of nitrogen-containing heterocycles.1... 

The photoelimination of nitrogen from azoalkanes provides an easy and convenient method for the generation of alkyl radicals. No new results of any significance have been reported for such systems and attention has been devoted almost entirely to the study of cyclic azoalkanes. 

The pholodecomposition of diazirines is known to proceed via carbene intermediates. Chloromethoxycarbene, for example, has been identified by i.r. spectroscopy on irradiation of 3-chloro-3-methoxydiazirine in an argon matrix at 10 K. Detailed studies of the photoelimination of nitrogen from 3-benzyl-3-chlorodiazirine (1) have been reported. 1,2-Hydrogen migration in the carbene (2) formed on irradiation of (1) affords a mixture of (Z)- and (E)-/3-chlorostyrenes (3) and (4) the stereochemistry of the hydrogen shift is... 

In certain 1-pyrazolines in which photoelimination of nitrogen is relatively inefficient, the quantum yield for photodecomposition can be enhanced by an increase in temperature. In this way, the pyrazolines (15) were converted into the cyclopropanes (16) by irradiation in refluxing benzene or acetonitrile."... 

Diazabicyclo[2.2.2]oct-2-enes have been shown to undergo photoelimination of nitrogen in a similar fashion via singlet 1,4-biradicals.In a separate investigation, the azoalkane (32) was converted into the bicyclo[2.2.0]hexane (33) in 72% yield on irradiation, and an analogous transformation has been employed in the synthesis of compounds with a pentaprismane carbon skeleton. ... 

Carbenes are known intermediates in the photodecomposition of diazo-compounds. Insertions of singlet methylene, generated by photoelimination of nitrogen from diazomethane, into 2,3-dimethyl-2,3-epoxybutane and into acetonitrile have been described. Methylene and other photochemically generated carbenes (43) add efficiently to 2-acetoxyacrylonitrile to give the corresponding 1-acetoxy-l-cyanocyclopropanes (44). The formation of methylidyne radicals by irradiation of diazomethane in the presence of hydrogen, however, does not seem to involve an intermediate carbene. ... 

In contrast with fluorenylidene, singlet-triplet equilibrium in the carbene derived by photoelimination of nitrogen from the diazodihydroboraanthracene (57) is slow. Direct irradiation affords singlet carbene which reacts with propan-2-ol to give the ether (58), whereas 2-acetonaphthone-sensitized irradiation yields triplet carbene which undergoes non-stereospecific addition to alkenes. Products obtained by irradiation of 3-diazo-2,5-diphenylpyrrole can arise via the intermediacy of singlet and triplet carbenes. Decomposition of the pyrrole (59) in the presence of benzene, for example, yields the 2H-cycloocta[c]pyiTole (60) as the major product. 

The generation and reactions of arylnitrenes continue to attract attention. Evidence for a photoinitiated autocatalytic chain mechanism in the photodecomposition of phenyl azide has been reported, and the reaction of photochemically generated phenyl nitrene with oxygen has been reexamined. Irradiation of p-azidoaniline in aqueous solution yields triplet p-aminophenyl nitrene, which on reaction with water is converted into the highly reactive p-benzoquinone diimine. Both m- and p-nitrophenyl azides, on photoelimination of nitrogen at 77 K, afford the corresponding nitrenes, whereas o-nitrophenyl azide (93) is converted without the intermediacy of a nitrene into the benzofurazan (94). 4,4 -... 

Aryl and heteroaryl azides are known to undergo ring expansion to azepines on photoelimination of nitrogen in the presence of nucleophiles. Evidence for benzazirine and 1-azacycloheptatetraene intermediates has previously been described and both species continue to be invoked to explain transformations of this type. The presence and nature of substituents appears to influence the course of the reaction, with electron-withdrawing substituents promoting azepine formation. o-Substituted aryl azides (102) are converted on irradiation in methanol-tetrahydrofuran into the 3-substituted 2-methoxy-3H-azepines (103). A pathway via the benzazirine (104) is proposed in Scheme 4, although... 

In contrast with these ring expansion processes, photoelimination of nitrogen from ethyl o-azidobenzimidate (114) is followed by cyclization with the formation of 3-ethoxy-IH-indazole (115), and irradiation of the 2-azidopyrazines (116) yields the ring-contracted 1-cyanoimidazoles (117), presumably via the nitrenes (118). Products derived by insertion of (p-toluenesulphonyl)nitrene are obtained on irradiation of p-toluenesulphonyl azide in p-xylene or cyclohexane. "... 

Interest has been restricted this year to studies of photoelimination of nitrogen from arenediazonium salts and attempts to characterize the resulting aryl cations. Nanosecond laser photolysis techniques have been used to examine the photodecomposition of diazonium tetrafluoroborates. Aryl cations could not directly be detected, but evidence from bleaching experiments indicates that, in water, the phenyl cation has a lifetime of about 500 ps. The isolation of the ether (131) as the sole product of photodecomposition of the diazonium salt (132) in methanol demonstrates that rearrangement of the intermediate aryl cation does not occur. Both heterolytic and homolytic pathways have been described for photodecompositions of arenediazonium salts complexed with crown ethers. A useful synthetic application of this conversion is the formation in 53% yield of ethyl 2,4-difluoroimidazole-5-carboxylate (133) on irradiation of the diazonium fluoroborate (134). ... 

More attention has been paid to photoelimination of nitrogen from cyclic azoalkanes. Diazirine is a special case and undergoes decomposition via an identifiable carbene, with evidence in some cases for the intermediacy of a linear diazoalkane. The infrared spectra of phenylbromocarbene and phenyltrifluoromethy1-carbene, generated by photolysis of the corresponding diazirines... 

Carbene rearrangement products and insertion products are formed on photoelimination of nitrogen from a phenylmethyl-... 

The photoelimination of nitrogen from 1-pyrazolines provides a valuable route to 1, 3-biradicals and products derived therefrom. The high yield conversion of the pyrazoline (7) into... 

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