Phenyl azide thermolysis

Perfluorotetramethylthiadiphosphanorbornadiene and bis(trifluoromethyl) thiadiphosphole can be prepared by thermolysis of an adduct of methanol and hexakis(trifluoromethyl)-l,4-diphosphabarrelene with sulfur [113] (equation 23) Pyrolysis of the adduct of hexafluorinated Dewar benzene and phenyl azide results in ring expansion giving azepine, which photochemically yields an intramolecular 2-1-2 adduct, a good dienophile for the Diels-Alder reaction [114, //5] (equation 24) Thermolysis of fluonnated derivatives of 1,5-diazabicyclo-... 

Pioneering studies have shown that the yield of iV-phenyl-3//-azepin-2-amine (32, R = Ph) from the thermolysis of phenyl azide in aniline increases as the ratio of azide to aniline decreases, and in dilute solution with an azide to aniline ratio of 1 200 a 54% yield of the 3//-azepine can be achieved.34 The reaction is successful with other arylamines, but the procedure is of limited preparative value as large volumes of amine are required and only moderate yields of 3H-azepines are obtained. 

Likewise, thermolysis of 4-azidophenyl methyl ketone in methanol yields 5-acetyl-2-methoxy-3//-azepine (60%), compared to only an 8% yield from the photolytic reaction.78 119 The thermolysis of phenyl azide in refluxing cyclohexanol yields no 3H-azepine, only diphenyl-diazene (10%) and aniline (30%).74 In contrast, thermolysis of methyl 2-azidobenzoate in cyclohexanol furnishes a mixture of methyl 2-(cyclohexyloxy)-3//-azepine-3-carboxylate (20 % bp 127°C/0.1 Torr) and methyl 2-aminobenzoate (60%). Thermolysis of the azido ester in methanol under nitrogen in an autoclave at 150 C yields a 7 10 mixture (by 1HNMR spectroscopy) of the amino ester and methyl 2-methoxy-3//-azepine-3-carboxylate, which proved to be difficult to separate, and much tar.74 The acidic medium179 is probably responsible for the failure of methyl 2-azidoberjzoate to yield a 3//-azepine when thermolyzed in 3-methoxyphenol aniline (40%) is the major product.74... 

Photolysis of aryl azides in amine solution, with a tertiary amine as cosolvent to promote stabilization of the singlet nitrene, has met with some success. For example, the yield of 2-piperidino-3 W-azepme. obtained by the photolysis of phenyl azide in piperidine, is increased from 35 to 58% in the presence of A A /V. /V -tetramethylethylenediamine (TMLDA).180 Also, an improved yield (36 to 60 %) of A,(V-diethyl-3W-azepin-2-amine (38, R = Et) can be obtained by irradiating phenyl azide in triethylamine, rather than in dicthylaminc, solution.181 Photolysis (or thermolysis) of phenyl azide in TMEDA produces, in each case, 38 (R = Et) in 40% yield.181 In contrast, irradiation of phenyl azide in aniline with trimethylamine as cosolvent furnishes jV-phenyl-377-azepin-2-amine (32, R = Ph) in only low yield (2%).35... 

Historically the first fully unsaturated azepine was obtained by Wolff in 1912 by the decomposition of phenyl azide in aniline. However, the actual structure of this product, named dibenzamil , remained unknown until 1955, when Huisgen and coworkers showed it to be an anilinoazepine, originally formulated as the 7-anilino-2H-azepine, but later corrected, mainly on the basis of HNMR evidence, to the 2-anilino 3H-tautomer (221 R1=PhNH R2 = H). Subsequently, the thermolysis and photolysis of aryl azides in primary, secondary and, in certain instances, tertiary amine solution has become a standard method for the preparation of 2-amino-3/f-azepines (79AG(E)900,8lAHC(28)23l). 

The strained double bonds in Dewar benzene and thiophene make them good dipolarophiles in 1,3-cycloadditions. The hexamethyl,148 1,3,5-trimethyl, and perfluoro-1,3-dimethyl122 Dewar benzenes all yield a monoadduct by reaction with phenyl azide. The hexafluoro derivative, however, gives, depending on the conditions, either a monoadduct (19) or a mixture of mono-and bisadducts accompanied by an aziridine resulting from thermolysis of the triazoline ring system (Scheme 19).146... 

The results have been corroborated by further studies of o-(allyloxy)phenyl azide and 14 derivatives substituted on the allyl group. The allyloxy azide in Scheme 39 underwent complete conversion to triazoline at 35°C in 3 weeks as indicated by NMR.197 Thermolysis of o-allylphenyl azides, on the other hand, required high temperatures of the order of 155-200°C, apparently suggesting nitrene insertion reactions.197 The higher rate of decomposition of ortho-substituted phenyl azides as compared to the corresponding meta or para isomers, noted primarily in those systems where the ortho substituents have... 

By contrast, phenyl azide and tetramethoxyethylene yield only the products of the triazoline thermolysis.274,275... 

The reaction of phenyl azide with 1,2-dibenzoylethylene is more complex the various thermolysis products (Section IV,A,3,c and D,2,g) clearly indicate the intermediacy of a 4,5-dibenzoyltriazoline.306... 

Enamine formation occurs by the thermolysis of diazo compounds (Scheme 150)67 109,278 284 288,304 332 453 454 via a carbene-like intermediate.284 332 When R1 = Ph, it enters into competition with hydrogen migration,284,332 and the electrophilic character of the carbene enhances the migration of the dimethylaminophenyl more than the phenyl.332 When triazoline synthesis is carried out at temperatures higher than that at which thermolysis of diazo compounds occurs, enamines are obtained exclusively, as in the addition of phenyl azide to cinnamic nitriles and ketones, with phenyl migration dominating in the nitrile.284 Enamine is also formed quantitatively in the reaction of ethyl diazoacetate with benzylideneaniline at 110°C.455... 

Enamine-aziridine formation is also influenced by reaction conditions. Whereas phenyl azide addition to cinnamonitrile at 90°C results exclusively in the enamine,284 thermolysis of the independently synthesized l,5-diaryl-4-cyanotriazoline (Scheme 90) yields a mixture of the aziridine and enamine from phenyl migration.332... 

Thermolysis of these products generates the face-capped nitrenes Os3(/i-H)2(/i3-NR)(CO)9 24). The substituted clusters Os3(CO),L react with phenyl azide to give the products Os3(CON3Ph)(CO),oL, whose structures (15, L = py or MeCN) have been established by diffraction methods (25). 

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. ... 

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. 

The decomposition is first order in nitrobenzene and tetrahydronaphthalene and the Arrhenius parameters obtained with various solvents for phenyl azide and some derivatives are collected in Table 16. As seen from the data, meta substitution has no efiect on the rate, and it was concluded that the rate-determining step is N2 evolution and not ring closure to form azepines (in aniline solution). In indene, Smith observed that para substituents increased the rate of Nj evolution by about eight times however, with both meta and para substituents the amine yields varied widely and in some cases the Nj yields were not quantitative with respect to the amount of azide decomposed. Benzenes and substituted benzenes have been observed in small amounts and it appears that a minor decomposition pathway would involve azide radical loss . On the other hand, Waters reports that the thermolysis of phenyl azides proceeds by different mechanisms, depending on the medium ... 

KINETIC PARAMETERS FOR THE THERMOLYSIS OF SUBSTITUTED PHENYL AZIDES... 

Additional support for the postulate of these two reaction paths comes from the thermolysis of cyclohexyl azide in ethyl benzoate and indene, where = 47.5 and 34.6 kcal.mole", and AS = 32.2 and 17.4 eu, respectively ". (The enormously high value of 32.2 eu may be due to some experimental error.) Ring enlargement has been observed in the thermolysis of phenyl azide in aniline solutions " " . 

The ring enlargement observed in the thermolysis of phenyl azide in aniline solutions also occurs in the photolysis at 325-366 m/i in amine solutions, viz. 

Thermolysis of benzoyl peroxide in a degassed solution of phenyl azide in benzene at 80° led to no decomposition of the azide over two... 

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 . ... 

Another synthesis of azepines from azides involved the photolysis or thermolysis of aryl azides in the presence of nucleophiles. The photolysis of phenyl azide in diethylamine yielded (34%) 2-diethylamino-3/f-azepine (311) °. In the same manner 2-substituted azepines were obtained from phenyl azide and liquid ammonia, aniline and hydrogen sulphide . 

Phenyl azide can yield benzotriazole on irradiation , but more generally products of self-insertion or of attack on the solvent are found to lead to azepines and related compounds " - . Photolysis of o-azidobiphenyls formed carbazoles by intramolecular cyclization similar to that found in thermolysis and an analogous intramolecular cyclization involving, e.g. a nitro group could occur (reaction 74) in direct photolysis but not apparently in photosensitized reactions when only azobiphenyl was formed in appreciable yield The... 

Triphenylbismuth dichloride (3) reacts with sodium azide and sodium cyanide to afford triphenylbismuth diazide and triphenylbismuth dicyanide. Thermolysis of these compounds leads to phenyl azide or to benzonitrile, respectively. ... 

Thermolysis of phenyl azide leads to loss of nitrogen with formation of phenyl nitrene. This conclusion is well supported by experiment. Walker and Waters [6] studied the pyrolysis of phenyl azide at 130°C and confirmed the observations of Smith and Hall [7] from 2-substituted azido biphenyls that thermolysis of aryl azides, lacking ortho-substituents, in inert solvents leads... 

The thermolysis of phenyl azide in amine-containing solution results in isolation of 2-substituted 3H-azepines. This reaction was discovered by Wolff [1] in 1912, but the structure of the product was not elucidated until 1958 when Huisgen et al. [11] again studied this reaction. Later, Doering and Odum [12] reported that similar ring-expanded products arise from photolysis of phenyl azide in the presence of secondary amines. 

Reports of the products formed from photolysis of phenyl azide in unreactive solvents at or near room temperature presented a confusing picture until it was discovered that the result obtained depends critically on the concentration of the azide and the power of the light source. In essence, only two products are obtained from the irradiation of phenyl azide in oxygen free, inert solvents such as hexane or acetonitrile tarry polymer and azobenzene. Very little, if any, of the anilines observed in high-temperature thermolysis of the azide are obtained. 

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