Nitrenes with azobenzenes

Coordination chemistry reveals how two ArN species can be coupled into one azobenzene molecule. In the case of the reaction of Fe3(CO)12 with aromatic nitro compounds in benzene1161, formation of derivatives such as in structure 111 has been proven by X-ray diffraction. Azoxybenzene can be formed by reaction of nitrene with nitrosobenzene, formed by reduction of nitrobenzene. 

To add to the confusion, various groups reported that gas-phase photolysis of phenyl azide produced the absorption and emission spectra of triplet phenylni-trene. " These observations were reconciled by the work of Leyva et al. who discovered that the photochemistry of phenyl azide in the presence of diethylamine was very sensitive to temperature. Above 200 K, azepine 30 is formed, but <160 K, azobenzene, the product of triplet nitrene dimerization, is produced. The ketenimine can react with itself or with phenyl azide to produce a polymer, which can be converted into an electrically conducting material. Gritsan and Pritchina pointed out that at high-dilution ketenimine 30 can interconvert with singlet phenylnitrene which eventually relaxes to the lower energy triplet that subsequently dimerizes to form azobenzene. 

The vapor-phase pyrolysis of phenyl azide yields azobenzene in 72 % yield . The pyrolysis in aniline solution leads to the production of dibenzamil , identified as an azepine by modern techniques in benzene and p-xylene, the major products are azobenzene and aniline, presumably originating from reactions of PhN in hydrocarbon solvents the thermolysis produces aniline, alkyl-anilines, azobenzene and polymer . It has been shown that aniline and iV-alkylanilines arise from triplet state nitrene reactions with the solvent. In the gas-phase thermolysis of phenyl azide at low pressures aniline and azobenzene are the major products whereas at higher pressures the formation of l-cyano-1,3-cyclopentadiene (or 2-anilino-7H-azepine in the presence of aniline) predominates . It was deduced that aniline and azobenzene arise from triplet nitrene reactions and that at high pressures, a hot singlet nitrene is formed, which undergoes intramolecular insertion to form an azocyclopropene intermediate, viz. 

Phenyl azide and di-iron nonacarbonyl react rapidly in benzene at room temperature (as compared with thermolysis of PhNg alone which occurs at temperatures of 140-170° ). The principal product was the orange phenyl nitrene-complex (387) which decomposed spontaneously in solution to give the urea-based complex (388). Also obtained in low yield was the orange complex (389). The yield of azobenzene was reported to be negligible. C)n the other hand, when the decomposition was carried out in benzene under reflux in the presence of Fe3(CO)i2j a significant amount of azobenzene was found . ... 

Comparable results are obtained from the nanosecond time scale flash photolysis study of (4-nitro)phenyl azide reported by Liang and Schuster [45]. Irradiation of this azide leads to rapid formation of the triplet nitrene. This nitrene was identified by comparison of the observed spectrum with that recorded at low temperature, and by its observed dimerization to form the (4-nitro)azobenzene. The rate constant for the triplet nitrene dimerization is 1.0 x 109 M 1 s 1 a value approximately 10 times below the diffusion limit. This relatively small rate constant is consistent with the demands of spin statistics. Combination of two triplets to form a singlet product can be successful only one ninth of the time [64],... 

It is clear now that irradiation of phenyl azide at room temperature gives dehydroazepine. At high concentration of azide, the dehydroazepine polymerizes rapidly in competition with its slow isomerization to triplet phenyl nitrene. The major product formed from photolysis of phenyl azide under conditions where its quantum yield for disappearance is claimed to be greater than unity is poly-1,2-azepine [48], not azobenzene. Of course, the polymer does not elute from an HPLC, and analysis of reaction mixtures by chromatography will show only two components. 

In this account httle has been said about the spin states of the carbenes and nitrenes which react, and it hcis been tacitly assumed that rearrangement occurs from singlet states, whereas amine and azobenzene formation most probably occurs from triplets. Since we are dealing with thermal reactions, the law of spin conservation 88) would predict that the first-formed species are singlets. However, this is claimed not to be absolutely necessary, for the direct formation of triplets. 

Photolysis of 2-, 3- and 4-fluorophenylazide (39a,b,c) in aniline gives the products (40), (41) and (42) in the yields shown in Scheme 8. " With 2,6-difluoro- and 2,3,4,5,6-pentafluorophenyl azide, however, ring expansion to azepine products analogous to (42) is suppressed, and the corresponding unsymmetrical azobenzenes cf. 41) are formed in 50% and 55% yield, respectively. The azobenzenes are presumed to arise from insertion of a singlet nitrene into an N-H bond of aniline, followed by oxidation of the resulting hydrazine. 

N i trosobenzene reacts with Na2Cr9(C0) Q 9 azobenzene, while Bu NO gives Cr5 8u NH2), possi bl y v i a hydrogen abstraction by an intermediate nitrene compTex[50j ... 

The results of the mechanistic study on the [Rh(CO)4] catalysed reactions are summarised in Scheme 18. The scheme includes the reduction of azoxybenzene to azobenzene, which has been independently shown to occur easily in the absence of nitrobenzene [190], The formation of azoxybenzene is explained by the trapping of free nitrosobenzene by the intermediate nitrene complex. Since nitrosobenzene may also react with [Rh(CO)4] and reenter the... 

Abstract In the examples of the irreversible reactions, the original molecule is broken down on exposure to UV light and releases a part of the molecule to become a new prodnct. The reverse reaction cannot occur. The triplet carbenes are formed from the bis(2,4,6-triphenyl)diazomethane derivatives and the arylnitrenes are produced from the arylazide derivatives on exposure to UV hght The unstable carbenes and nitrenes were observed after the crystal was irradiated with UV light at 80 K. Moreover, the further reaction processes were observed from the intermediate nitrenes to the final products, azobenzene derivatives or five-membered ring derivatives. The acid-base complex formation between arylazides and amine derivatives is very effective to observe the reaction processes of nitrenes. 

The solution phase photochemistry of phenyl azide 47 is temperature dependent/ Photolysis of 47 in the presence of diethylamine at ambient temperature yields azepine 48b. Lowering the temperature suppresses the yield of 48b and below 160K, azobenzene, the product of triplet nitrene dimerization, is produced. Thus, high temperature favors reactions of singlet state intermediates whilst low temperatures favor reactions associated with triplet phenylnitrene. 

---