Vinyl azides, decomposition

In this chapter we will present a review of the literature through 1969 on the preparation and reactions of vinyl azides, then discuss briefly the conclusions which may be drawn concerning the mechanism of vinyl azide decomposition. A short section will also be included on the chemistry of azidoquinones. 

The mechanism of azirine formation has been discussed by L abbe.20 A priori, the three paths shown in Scheme 1 can be considered. From a comparison of the activation parameters27 for vinyl azide decomposition ( a 26-30 kcal/mol, AS - 3 to + 5 eu) with those for aryl azides (Ea 35-39, AS 18) it appears that the nitrene pathway a can be excluded, but a clear choice between paths b and c cannot be made at this time.20... 

A novel method for preparing amino heterocycles is illustrated by the preparation of 2-amino-5-methylthiophene (159). In this approach vinyl azides act as NH2 equivalents in reaction with aromatic or heteroaromatic lithium derivatives (82TL699). A further variant for the preparation of amino heterocycles is by azide reduction the latter compounds are obtained by reaction of lithio derivatives with p- toluenesulfonyl azide and decomposition of the resulting lithium salt with tetrasodium pyrophosphate (Scheme 66) (82JOC3177). 

Important synthetic paths to azirines and aziridines involve bond reorganization, or internal addition, of vinylnitrenes. Indeed, the vinylnitrene-azirine equilibrium has been demonstrated in the case of trans-2-methyl-3-phenyl-l-azirine, which at 110 °C racemizes 2000 times faster than it rearranges to 2-methylindole (80CC1252). Created in the Neber rearrangement or by decomposition of vinyl azides, the nitrene can cyclize to the p -carbon to give azirines (Scheme 4 Section 5.04.4.1). 

Formation of 2//-azirines by thermal decomposition of vinyl azides has been shown to exhibit small entropy of activation and insensitivity to solvent polarity acyclic vinyl azides decompose more readily than analogous cyclic ones and it is advantageous to have a hydrogen atom cis to the azido group ( -are more reactive than Z-isomers). These results and the linear correlation found for ring-substiment effects on decomposition of a-styryl azides are consistent with a nonconcerted mechanism in which elimination of nitrogen and cyclization into a three-membered ring proceeds synchronously. 

For the practical design of hypersurfaces, i.e. cuts through the (3n-6)dimensional hyperspace, some hints are outlined. The main purpose, however, is to illustrate the usefulness of hypersurface calculations especially for the detection, identification and characterization of unstable molecules. Examples chosen comprise the structure of RS-C=C-SR, the relative stability of thioacroleine isomers C,H S, the structural changes accompanying the oxidation of hydrazine and some of its derivatives, the isomerization of tetrahedrane to cyclobutadiene both thermally as well as on oxidation, the predicted existence of F SS and nonexistence of CI2SS or H2SS, and, finally, some aspects of the thermal decomposition of methyl and vinyl azides. 

Example VI Some aspects of the thermal decomposition of methyl and vinyl azides, H3C-N3 and H2C=CH-N3. 

Example VI Prediction and Detection of 2H-Azirine as Intermediate in the Thermal Decomposition of Vinyl Azide. Predictions concerning the hard-to-prove nonexistence of molecules are less convincing to the lay-colleague than those, which have stimulated successful experiments. Therefore, this last example has not only been selected because of its higher complexity, but rather due to the PE spectroscopic verification of the results anticipated by a preliminary hypersurface study (4,36). 

The question as to whether the thermal azirine formation proceeds through a vinyl nitrene intermediate or by a concerted mechanism is not as yet resolved. A nitrene intermediate seems most probable on the basis of its similarity with cyclopropene formation from alkenylcarbenesM>> and because either thermal or photochemical decomposition of vinyl azides yields die azirines (Table XI). 

Vinyl azide intramolecular cycloaddition is further illustrated by the formation of azidotriazoline 32 as a minor product in the thermolysis of the bisvinyl azide 31 (Scheme 41).200 An analogy is provided by the formation of 2,5-diphenylpyrrole from the slow decomposition of a-azidostyrene.202 Pyrrole formation is interpreted in terms of cycloaddition of the azide onto the electron-rich double bond of a second molecule to give a triazoline that loses nitrogen and rearranges to a pyrroline followed by hydrogen azide elimination (Section IV,D).203... 

Vinyltriazolines are unique thermolysis leads to high yields of 1-vinylaziridines. This is true of fused-ring tetracyclic (Scheme 40)199,201 and tricyclic triazolines (Schemes 42 and 43)200 204 with vinylic substitution in the 1-position. 1-Vinyltriazoline with no substituents on the ring carbons furnishes 1-vinylaziridine without formation of pyrrolines.378 Thermal decomposition of vinyl azides in acrylic acid derivatives is a synthetic route for 2-substituted 1-vinylaziridines.470 Similarly, aziridine 1-oximes are the only products of thermolysis of triazoline 1-oximes.107... 

Three plausible mechanisms20 have been postulated for the thermal decomposition of vinyl azides (41) to 1-azirines (47). The azide can either (1) lose nitrogen to give the vinyl nitrene which then cyclizes to the 1-azirine, (2) lose nitrogen with simultaneous ring closure (45), or (3) first cyclize to triazole (46) which then loses nitrogen. To date little experimental evidence exists to favor any of these alternatives. 

The thermal decomposition of terminal vinyl azides was originally believed to give only nitriles or, in some cases, indoles. The presence of 3-monosubstituted 1-azirines, however, has been inferred in the photolytic decomposition of some terminal azides.22,29 30 The nitrile is thought to arise in a similar manner to the ketenimine by an analogous Curtius-type rearrangement. The ketenimine (53) derived from terminal azides is unstable and rearranges to the nitrile (54). 

The decomposition of vinyl azides offers an excellent method for the preparation of 1-azirines. The main limitation of this method is the availability of the prerequisite vinyl azide, which can, in some instances, be a severe one. 

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... 

Kurtz and Sheehter48 have observed similarly that irradiation of 3,4,5-triphenylisoxazole gives AT-phenylbenzoylphenylketenimine, 3-benzoyl-2,3-diphenyl-l-azirine, and 1,4,5-triphenyloxazole. The formation of the azirine and ketenimine from this reaction and from the decomposition of vinyl azides would suggest a similarity in mechanism. 

FIGURE 25. The heterogeneous gas-phase dehydrochlorination of /1-chloroethyazide on solid potassium rm-butoxide yielding vinyl azide, and the subsequent thermal decomposition of the latter to 27/-azirine. The double-oven apparatus is equipped with cold traps for the by-product tert-butyl alcohol and the end product 2/7-azirine. The PES ionization patterns of the pure compounds were used for optimization of the reaction conditions. Reproduced by permission of Verlag der Zeitschrift der Naturforschung from Reference 215... 

Pyridazines were obtained also by photolysis of 1-phenyl-l-vinyl azide in the presence of iron pentacarbonyl (3,6-di-phenylpyridazine was obtained in 1.1% yield) (78HCA589) or by thermal decomposition of an allenic hydrazonate (81JA7011). Acetylenic hydrazides can be transformed into pyridazines [84BSF(2)129], and thermal cyclization of dialkali metal salts of cu-hydroxyketone tosylhydrazones afforded pyridazines in moderate yield (85TL655). Propionyl phenylhydrazine, after reaction with 4-bromobutyronitrile, converts into a pyridazine (87SC1253). 

The decomposition of organic azides has been studied from a number of points of view, the objectives being mainly synthetic or mechanistic. The nature of the intermediates formed have received much attention. In this chapter we shall consider the decomposition of alkyl, aryl and sulphonyl azides under various conditions. Acyl and vinyl azides are considered elsewhere in this volume. 

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-... 

The overwhelming majority of vinyl azide reactions involve the expulsion of molecular nitrogen and subsequent reorganization of the remainder of the molecule. Such decompositions may be initiated thermally or photochemically. Azirines are frequently isolated from the reaction mixtures, but a great variety of other products, including nitriles, dihydropyrazines, indoles and isoxazoles have also been obtained. 

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. 

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