Pyrolysis, of phenyl azide

Phenylazide was first synthesized by Greiss in 1864. In 1912, Wolf studied the pyrolysis of phenyl azide in aniline. The product of this reaction, azepine (27), was identified by Huisgen and co-worker in 1958. Eight years later. Doering and Odum demonstrated that azepine (27) is formed upon photolysis of phenylazide in diethylamine and in 1977 Carroll et al. ° discovered the formation of 28 upon photolysis of phenyl azide in the presence of ethanethiol. 

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. 

One of the most ubiquitous reactions is the formation of azo compounds by the attack of nitrene on the azido group. This process accounts for the high overall quantum yield of nitrogen evolution in the gas phase photolysis of hydrazoic acid and of methyl azide It has also been demonstrated in the gas phase pyrolysis of phenyl azide and of or//jo-trifiuoromethyl azide where azobenzenes arc the major reaction products. Azo compounds are formed in near quantitative yield in the solution photolysis of / -methoxyphenyl azide and... 

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... 

Upon photolysis or pyrolysis of phenyl azide in the gas phase the nascent phenyl nitrene is formed with enough energy to overcome a substantial barrier and ring contract to form cyanocyclopentadiene, the lowest energy isomer of C6H5N [43], In fact laser photolysis of phenyl azide in the gas phase does not produce emission from triplet phenyl nitrene but from the cyanocyclopenadienyl radical instead. 

Egsgaard, H. and Carlsen, L. (1983) Techniques in gas-phase thermolyses (3)- gold plated filaments for Curie Point pyrolysis , J. Analyt. Appl. Pyrol., 5, 1-7. Egsgaard, H. and Carlsen, L. (1986) Techniques in gas-phase thermolyses (7)- direct surface precipitation in gas-phase Curie Point pyrolysis the pyrolysis of phenyl azide , J. Analyt. Appl. Pyrol., 10, 83-87. 

Photolysis and pyrolysis of phenyl azide (47) and most of its derivatives in hydrocarbons leads to polymeric tars instead of diagnostic insertion products and aziridines. A for-mnate exception is the formation of azepines in the presence of primary and secondary amines. The Huisgen group was the first to observe that thermolysis of 47 in the presence of aniline leads to extmsion of molecular nitrogen and formation of azepine 48a (Scheme 11.24). Eight years later. Doering and Odum demonstrated that azepine 48b is formed with a high yield (>70%) upon photolysis of 47 in diethylamine (Scheme 11.24). Aside from azepines, the formation of ortto-substituted aniline 49 (in a yield of 39%) was discovered upon photolysis of 47 in ethanethiol (Scheme 11.24). ... 

Scheme 11.24 Adduct formation upon photolysis and pyrolysis of phenyl azide"...

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-... 

Raasch, and Schaumann and co-workers, have studied the reactions of bis (trifluoromethyl)thioketene (83 R1 = CF3) with alkenes, with thioketones, and with carbodiimides or Schiff-base imines. The products were, respectively, thietans, 1,3-dithietans (84 R1 = CF3), and 1,3-thiazetidines (85 R1 = CF3) (from the carbodiimide).114,115 With phenyl azide the 1,2,3,4-thiatriazoline (86 R = CF3) is formed subsequent pyrolysis yields2,l-benzisothiazole.114... 

Nitration of various phenyl-49 and benzyl-phosphine60 oxides (59) has been described, and the P=0 group found to be mem-directing49 in the former case. Pyrolysis of the acyl azide (60)51 takes the course shown. 

The base-catalyzed condensation of a-azido esters and ketones with aromatic aldehydes has recently been developed as a new vinyl azide synthesis.42,43 The yields range from moderate to excellent in some cases. The thermal decomposition of ethyl a-azidocinnamate (87) in xylene gives only 2-ethoxycarbonylindole (88).44 The unstable 2-ethoxycarbonyl-3-phenyl-l-azirine could be detected if the thermolysis was carried out at a lower temperature. This fact indicates that the 1-azirine is probably an intermediate leading to the indole, although the intermediacy of the vinyl nitrene could not be established. This result is similar to that observed by Isomura et al. on the pyrolysis of terminal vinyl azides.27,28... 

H)-Oxazolones are formed by the spontaneous cyclization of /3-oxo isocyanates (equation 134). Similarly, o-hydroxyphenyl isocyanate, produced by the Curtius rearrangement of the azide of salicylic acid or by the action of sodium hypochlorite on salicylamide, forms benzoxazolone (equation 135). An analogous reaction is the formation of IV-phenyl-benzoxazolone by the action of thionyl chloride on the hydroxamic acid shown in equation (136) (78TL2325). Pyrolysis of aryl azidoformates affords benzoxazolones by nitrene insertion (equation 137) (81CC241). 

The pyrolysis of both (E)- and (Z)-isomers of 2-[(3-phenyl-2-propenyl)oxy]phenyl azide gave /rans-disubstituted bicyclic aziridines 1 in good yield with minor amounts of 2/7-1,4-benzoxazi-nes, from decomposition of the intermediate dihydrotriazoles114 115. However, substitution of methyl for a phenyl group on the double bond resulted in loss of diastereoselectivity, since a mixture of both cis- and /ra/ts-isomers in the same ratio were obtained from both (E)- and (Z)-isomers115. The cis stereochemistry was retained when the double bond was part of a cycle (five- or six-membered)115. In the presence of an allylic substituent, the stereochemistry of the aziridine stereocenters relative to the pre-existing stereocenter (substrate-induced diastereoselectivity) was not assessed115. 

As noted before, the products most frequently obtained from decomposition of vinyl azides are 2//-azirines, particularly when the azido group is not located on a terminal carbon. Thus vapour phase pyrolysis of 1-phenyl or l-(o-tolyl)vinyl azide (34) gave azirines,... 

Gas-phase pyrolysis of 1,2,3-triazoIe produced vinyl azide and its decomposition products (83JA7681). 4-Diazo-l,2,3-triazole (20) (prepared by dia-zotizing 4-aminotriazole in water), when refluxed in benzene, gave 4-phenyl-1,2,3-triazole (53% yield) and a mixture of cyanobicycloheptatrienes (15%). Photolysis proceeded similarly, except that the latter products preponderated (82TL5115). 

Pyrolysis of pyridine derivatives is a method for allylic and benzylic deamination the preparation of nitriles from aldehydes, and the preparation of isocyanates from acid chlorides or hydrazides Flash vacuum pyrolysis continues to yield interesting reactions and products. Recently, it has been reported that quite sensitive acetylene derivatives can be obtained by this method from 4-alkylideneisoxazol-5(4H)-ones by ring degradation. On the other hand, 2 carbamyl azide molecules cyclize under these conditions to form l,2,4-triazolidine-3,5-dione 1,2-ylids . 2H-Cyclohepta[b]furan-2-ones have been obtained by ring expansion of phenyl propiolates... 

In 1961, Smolinsky reported on vapor phase pyrolysis of a-azidostyrene (52, Scheme 5.8), which furnished 3-phenyl-2//-azirine as the main product and provided the first example of the synthesis of such strained heterocycles from vinyl azides. By analyzing the IR data of the pyrolysates produced from 52, it was shown one year later that N-phenylketenimine is formed as a side product. By utilizing IR and NMR spectroscopy at low temperature, Wentrup and coworkers have smdied recently the detailed structures of another azirine-ketenimine pair, generated by thermal or photochanical decomposition of an enazide. The transformation of vinyl azides into 2//-azirines is currently the most frequently used access to these heterocycles. The manifold chemistry of azirines was reviewed several times," " and most of the aspects of their synthesis from alkenyl azides are summarized in Chapter 6 (Gilchrist Alwes). Therefore, only some additional certain points are included here. 

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