Azepine, phenylnitrene

Singlet phenylnitrene, and hence /V,A -diethyl-3//-azcpin-2-amines, e. g. 102, can be generated by the thermolysis of A,-phenyl-Af,<9-bis(trimcthylsi]yl)hydroxylamine (100) in the presence of dialkylamines the reaction fails, however, with arylamines.210 Photofragmentation of the spiro oxaziridine 101 in diethylamine solution also produces the 3//-azepine 102,2,1 and an oxaziridine intermediate is probably involved in the formation, in low yield (1 %), of azepine 102 by the photolysis of A/,A( -diarylbenzoquinonc diimine A/,A/ -dioxides in benzene/die-thylamine solution.212... 

However, in 1978, Chapman and LeRoux discovered that photolysis of phenyl azide, matrix isolated in argon at 10 K, produces a persistent species with a strong vibrational band at 1880 10 cm . The carrier of this species was most reasonably assigned to ketenimine 30 rather than benzazitine 29 or triplet phenylnitrene. This result imphes that it is the ketenimine 30 and not benzazirine 29 that is trapped with amines to form the 37/-azepines (27) that had been isolated earher. It does, however, raise the question as to why two groups observed triplet phenylnitrene by low temperature spectroscopy while a third observed ketenimine 30. 

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. 

Secondly, Leyva et al found that the solution phase photochemistry of phenylazide (PA) was temperature dependent. Photolysis of PA in the presence of diethylamine at ambient temperature yields azepine 2, first prepared by Doering and Odum. Lowering the temperature suppresses the yield of 2 and encomages the formation of azo compound. Thus, high temperatures favor reactions of singlet state intermediates, whilst low temperatures favor reactions associated with triplet phenylnitrene. 

Instead of intermolecular chemistry, phenylnitrene (1) rapidly undergoes intramolecular rearrangement. Substituted azepine (3) is the product of both thermolysis and photolysis of phenyl azide in the presence of amine. On the other hand, substituted aniline (4) is obtained when phenyl azide is photolyzed with ethanthiol. ... 

Perhaps their most significant result is that the opposite trends in changes in the barrier heights for the two steps with increasing steric bulk infers that the intermediate azirine might be observable if a sufficiently bulky substituted phenylnitrene could be produced. Just such a molecule is 2,4,6-tri-t-butylphenylnitrene. Computations indicate a barrier of 4.1 kcal mol for the first step and 6.3 kcal moC for the second step. Laser flash photolysis detected the azirine intermediate 27 having a lifetime of 62 ns and a barrier to ring open to the azepine 28 of 7.4 0.2 kcal moL. ... 

The formation of phenylnitrene by thermolysis of 0,Af-bis(trimethylsilyl)-JV-phenylhydroxylamine at 100°C has an activation enthalpy of 27.7 kcal/mol and is supported by the formation of aniline, azobenzene, or an 85-95% yield of 2-diethylamino-3f/-azepine in the presence of diethylamine (Eq. 65).262... 

This is the highest reported yield of an azepine from phenylnitrene. 

Along with the substituent effect on the reactivity of singlet phenylnitrenes, the influence of substituents on the reactions of ketenimines with nucleophiles was also studied in detail. As in the case of unsubstituted ketenimine 51, its simple derivatives could be trapped by nucleophiles in solution. The primary products, corresponding IH-azepines, undergo subsequent isomerization to final products. Reaction of ketenimines with primary and secondary amines is the most studied of the reactions with nucleophiles. Rate constants of this reaction with DEA (Table 11.4) were measured for a series of substituted ketenimines using TRIR spectroscopy, as well as conventional LFP techniques. ... 

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