Organic azide, decomposition

Triazoles have been obtained via microwave-assisted [3-i-2] cycloaddition, under solvent-free conditions [54], starting from organic azides and acetylenic amides at 55 °C for 30 min (Scheme 23). The complete conversion of the reagents into AT-substituted-1,2,3-triazoles 69 was achieved without decomposition and side products. A control reaction carried out at the same temperature in an oil bath did not give the cycHc products, not even after 24 h of reaction time. 

See entry THERMOCHEMISTRY AND EXOTHERMIC DECOMPOSITION (REFERENCE 2) See Other ALDEHYDES, ORGANIC AZIDES... 

The preparation and chemistry of organic azides have been intensively investigated since about 1960 because of their importance in preparative heterocyclic chemistry and as a source of nitrenes [1]. Many nitrogen-containing heterocycles such as carbazoles, furoxans, azepines, A2-triazolines, triazoles [2], tetrazoles [3], aziridines [4, 5], and azirines have been obtained by either the addition or the decomposition of azides. 

Interesting material with which the mechanism of decomposition of organic azides was investigated is triphenylmethyl azide... 

To compare the reactivity of different classes of organic azides, the region is determined in which the rate constant for the decomposition reaction is of the order 10 to 10 min.-1 Table IIUI>-1M shows the results. The decomposition temperature decreases in the order alkyl and aryl azides > azidofor-mates and sulfonyl azides > acyl azides. The difference in reactivity between acid azides and alkyl or aryl azides can be explained easily in terms of resonance stabilization in the azide... 

The addition reactions of organic azides to unsaturated systems can occur by two mechanisms.1 If the reaction temperature is lower than the decomposition temperature of the azide, a 1,3-dipolar addition is observed. When, however, the reaction is carried out at the wavelength or at the decomposition temperature of the azide, the addition proceeds through an intermediate nitrene. 

Cushman, M. et al., J. Org. Chem., 1987, 52, 1517—1521 Heating the title azide at 100°C caused explosive decomposition. See related ORGANIC AZIDES, ORGANOMETALLICS... 

The thermal decomposition of most organic azides will lead to nitrene intermediates which, according to the spin conservation rule, must be formed initially in the singlet state (So). 

The thermolysis of methylazide was studied first by Ramsperger 21a> who found it to be a homogenous first-order reaction. Studies of thermolyses of several other organic azides in various solvents and at different concentrations also show first-order kinetics 22-28). This led to the conclusion that free nitrene intermediates are involved. There are however thermal decompositions of azides where nitrene intermediates are not involved. For example ortho-substituted arylazides like o-nitrophenylazide 2, undergo formation of benzofurazan 3 by a concerted mechanism according to l.c.21b). 

So nitrenes could not be trapped in this reaction by insertion into C—H bonds or olefinic double bonds. Decomposition temperatures of several azides and their corresponding rate constants (k) are listed in Table 1. Unlike most organic azides. 

Hydrazoic acid is conveniently prepared in situ by hydrolysis of azidotrimethylsilane over silica gel. Only a catalytic amount of trifluoromethanesulfonic acid is required to achieve the hydroazidation of cyclic alkenes the completion of the reaction is preferentially controlled by gas-chromatographic analysis, as prolonged reaction time causes the partial decomposition of the organic azide by the acid catalyst. From norbornene the exo-azide 5 was exclusively produced, but no diastereoselectivity was observed with 1,2-dimethylcyclohexene106. 

An important synthetic route to aziridines is the decomposition of organic azides to the correspondic nitrenes by loss of nitrogen. However, depending on the nature of both, the azide and the alkene and on the experimental conditions, the 1,3-cycloaddition of the azide group to the alkene double bond may occur to give a dihydro triazole, which can be converted in situ or after isolation to the aziridine. Furthermore, /3-functionalized amines are obtained from certain dihydro triazoles, or directly by reaction of azides with certain alkenes, and dihydropyrroles are obtained by reaction of azides with dienes, hence these topics will be treated separately (Section 7.2.9). 

Simple alkyl azides are quite labile even at room temperature and have a tendency to detonate on rapid heating for these reasons, the majority of kinetic studies have been confined to the solution phase. As with azocompounds, the common nitrogen elimination reaction is the consequence of the relative stability of the resulting, divalent RN radical, called a nitrene, and the high heat of formation of the N2 molecule. In some cases, particularly in the thermolysis of aryl azides, Nj elimination follows a concerted path nevertheless, nitrene formation is of more common occurrence in both the photolytic and thermal decompositions. Decomposition and addition reactions of organic azides have been recently reviewed . 

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. 

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