Vinyl azides, pyrolysis

The vapor phase pyrolysis of vinyl azides leads to the formation of 2-Bub6tituted-l-flairinea 2 Eq. 87). 

Smolinsky developed the first general synthesis of 1-azirines by the vapor phase pyrolysis of vinyl azides (41).20,21 In addition to 50-60%... 

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... 

Terminal vinyl azides give, upon decomposition, vinylnitrenes which can stabilize by rearrangement to nitriles. The pyrolysis of jS-styryl azide (73) afforded in this manner phenylacetonitrile (74) (74%) Nitriles were also obtained from a-azidoketenes (75) gene-... 

Pyrazine derivatives have been obtained on pyrolysis of a-sub-stituted vinyl azides in ethanol. Thus l-azido-2-phenylpropene (305) produced 2,5-dimethyl-2,5-diphenyldihydropyrazine (306) (48%) on... 

The photodecomposition of vinyl azides is of interest as a potential synthetic route to the highly stre-ined azirine ring system. Smolinsky was the first to prepare compounds of this class by the pyrolysis of vinyl azides (39) and Hassner and Fowler have shown that the corresponding photo-reaction also produces 1-azirines (40) in excellent yield. Three possible mechanisms were proposed by these authors, one involving a nitrene intermediate 41. 

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,... 

Decomposition of 9-(l-azidoethylidene)fluorene (49) in refluxing benzene led to high yields of the spiroazirine 50 2. in contrast, pyrolysis of tlae terminal vinyl azide, 9-(azidomethylene)fluorene (51) in benzene led to only one identifiable product, 9-(A, A -fluorenylidene-aminomethylene)fluorene (52), which was isolated in 25% yield 2. 

Thermolysis of terminal vinyl azides has generally led to the isolation of products other than azirines. A number of j8-azido-styrene derivatives have, on pyrolysis, yielded indoles. Thus /S-azido-a-methylstyrene, (60) when boiled in mesitylene, produced an 80% yield of 3-methylindole (61) and 9% of a-phenylpropionitrile (62) Decomposition of the same compound in ethanol, however, gave only a trace of the indole the major product, isolated in about... 

A vinyl azide intermediate has been invoked to account for the formation of 3,5-dibromo-2-hydroxybenzonitrile (109) in the pyrolysis of 108 . The elimination of the elements of hydrazoic acid from 108 would give a terminal vinyl azide which might reasonably be expected to decompose and rearrange to the cyanide via an iminoketenc (equation 16). 

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). 

The iodo azide undergoes stereospecific trans dehydrohalogenation to give a vinyl azide, which on photolysis or pyrolysis gives the 2H-azirine. Reduction of the 2H-azirine with lithium aluminium hydride gives the m-aziridine in good yield. 

Vinylaziridine 7.6 Vinyl azide 5 (20 mmol) and trimethylsulfoxonium ylide (2 equiv) in DMSO was stirred at 20°C for 12 h. The reaction mixture wad diluted with Et20 (100 mL) and washed with water (5x100 mL). Evaporation of the solvent afforded triazoline 6 in 95% yield. Pyrolysis of 6 in refluxing PhMe (3 h) gave 7 in 65% yield alternatively flash vacuum pyrolysis (FVP) afforded 7 in 93% yield. 

The simplest member of the class of vinyl azides, H2C=CH-N3, has been known for about 100 years. However, it was not until the late 1960s that vinyl azides became an important and synthetically useful class of organic compound. The most interesting and important reaction of vinyl azides is the formation of azirines produced upon photolysis and thermolysis of vinyl azides. Smolinsky and Pryde first observed azirine formation, together with a small amount of keteneimine, by gas-phase pyrolysis of a-aryl-substituted vinyl azides. 

Unsubstituted 2/f-azirines were assumed to be intermediates in the decomposition of terminal vinyl azides, but for a long time they escaped detection. The thermal decomposition of terminal vinyl azides often results in the formation of nitriles and polymeric material without any detectable quantities of 2/f-azirines. Under certain conditions, however, it is possible to detect the 2/f-azirines spectroscopically and even to isolate them under controlled experimental conditions. The parent 2/f-azirine 284 was recently prepared by flash vacuum pyrolysis of vinyl azide at 400 C and was characterized by its pure rotational spectrum. The stability of this compound was somewhat greater than initially expected. On standing at room temperature, it decomposed to acetonitrile. 

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. 

Smolinsky and Pryde first observed azirine formation, together with small amount of ketenimin, by gas-phase pyrolysis of a-aryl substituted vinyl azides. 

Scheme 11.13 Pyrolysis of a-aryl substituted vinyl azides ...

The addition of HN3 to nonactivated alkenes with formation of the saturated azides requires drastic conditions and a catalyst. Olefinic double bonds activated by conjugated, electron-withdrawing groups like -CN, -NO2, or -C(0)R react easily [8, 9]. The photolysis [10] and the pyrolysis [7] of HN3 with alkenes yields nitriles, N2, and decomposition products. The reaction of HN3 with alkines usually leads to 1,2,3-triazoles. Single or double addition with formation of vinyl azides or saturated diazides are rare [8, 9]. Arenes and HN3 form aniline derivatives directly and efficiently in the presence of trifluoroacetic acid and the strongly acidic trifluoromethanesulfonic acid [11]. The yield is frequently low in the presence ofH2S04[1]. 

Synthesis from erythrose An efficient approach for the synthesis of (-l-)-trihy-droxyheliotridane (180) via a chiral erythrose derivative has been reported (Scheme 16). Wittig reaction of 2,3-(9-isopropylidene-L-erythrose (172) with Ph3P=CHCH=CHC02Et produced a 1 5 mixture of the ( , )-173 and (Z,A)-174 isomeric dienes, respectively. The diene 173 could be quantitatively obtained by isomerization of 174 with E. The diene 174 was converted to the azide 177, which upon boiling in benzene gave the vinyl aziridine 176. Pyrolysis of 176 furnished the pyrrolizidine 178. On the other hand, the diene 173 was... 

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