Alkyl azides, decomposition, thermally

Thermal decompositions of alkyl azides are advantageously studied in millimole quantities using a PE spectroscopically controlled flow system under low pressure ( ), thereby reducing the hazards involved in handling these explosive compounds in bulk. Our investigations started with methyl azide, which splits off nitrogen unexpectedly only at temperatures above 500° C (37) ... 

These conclusions were anticipated by product studies. Alkyl azides are readily available and their thermal and photochemical decomposition reactions have been studied. In general, light and heat induced decomposition of methyl azide does not produce a MeN species that can be intercepted in respectable yields with a bimolecular trap. For example, attempts to trap MeN with cyclohexane as solvent produced only a 0.4% yield of adduct. Photolysis of CH3N3 or CD3N3 at cryogenic temperatures fails to produce an IR spectrum attributable to triplet methylnitrene. The IR spectrum of CH2=NH (or CD2=ND) is observed instead. " ... 

Photolysis of alkyl azides bearing pendant aryl groups does not lead to intramolecular trapping of a nitrene by the aryl group. However, intramolecular capture of the putative nitrene is observed upon pyrolysis of the azide precursor. These observations convinced Kyba and Abramovitch" ° that 1,2-migration is concerted with loss of nitrogen from the excited state of the azide, but that a free singlet nitrene is formed upon thermal decomposition. The chemistry of acyl azides will be shown later to exhibit the opposite pattern. 

Nitrene intermediates are formed in the thermal decomposition reactions of most alkyl azides, aryl azides, sulfonyl azides, and azidoformates. Some decompositions, however, must be regarded as proceeding by a synchronous mechanism. They are discussed in this section. 

When the more reactive sulfonyl isothiocyanates are used as 1,3-dipolarophiles 4-alkyl-5-sulfonylimino-A2-l,2,3,4-thiatriazolines are readily prepared at room temperature by reaction with alkyl azides.64,65 The thiatriazolines are obtained in 50-75% yield. Their structures are deduced from NMR, IR, and mass spectral data and degradation experiments. Thus thermal decomposition at moderate temperature (45°-80°) in inert solvents (dry toluene, CC14, acetone) furnished the corresponding carbodiimides [Eq. (22)]. These were identified by their... 

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 . 

This present treatment is subdivided into the five major modes of decomposition acid-catalysed, thermal, trivalent phosphorus compound-catalysed, photolytic and transition metal-catalysed decompositions. Each section is further subdivided into alkyl, aryl and sulphonyl azide decomposition. The nature of the products formed and how these are affected by the nature of the reactive intermediates is discussed. 

Some aspects of this topic have been reviewedThe thermal decomposition of alkyl azides was first demonstrated by Senior" , working with Stieglitz. He showed that benzophenone anils, nitrogen and tars were obtained by heating triarylmethyl azides at temperatures of about 200°. The effect of substituents on the nature of the products was considered also, but the analytical techniques available were quite crude. The work was repeated and extended more recently, and will be discussed later. 

The thermal decomposition of aryl azides can lead to a much wider variety of products than alkyl azides. One of the major reasons for... 

Scheme 37 Reaction of alkyl azides 189 with arylsulfonyl isothiocyanates 190 to yield 4-alkyl-5-arylsulfonylimino-l,2,3,4-thiatriazolines 191. Thermal decomposition of the latter results in the formation of sulfonylcarbodiimides 192 [139,140]...

The use of alkyl azides (Alk-Ns) or aryl azides (Ar-Ns) can yield the corresponding substituted 1,5- or 2,5-tetrazolene, 1,4-tetrazoUnones and/or the corresponding mer-captotetrazoles. Moreover, substituted 1,2,3-triazoles are feasible and also aziridines provided that the azide was thermally or photochemically converted by separation of the nitrogen or by decomposition of the resulting dihydro-l,2,3-tiiazole. 

Despite patient and exhaustive effort by many researchers, all attempts to isolate or trap a benzazirine intermediate (214) have so far failed, and unequivocal evidence for their participation in either the photolytic or thermal decomposition of aryl azides is still awaited. Evidence in favor of the proposed reaction pathway (Scheme 22) comes from the work of Sundberg and coworkers, who succeeded in identifying 3-alkyl-2-diethylamino-lff-azepines as oxygen-sensitive, metastable intermediates in the photolysis of o-alkylphenyl azides in diethylamine (72JA513). Later studies on the flash photolysis of aryl azides in dialkylamine solution provided kinetic data which not only confirmed the Iff- to 3/f-azepine tautomer-ism, but also strongly supported the involvement of a benzazirine intermediate (74JA7491). 

The alkyl- and aryl-substituted vinyl azides discussed above are all reasonably stable at room temperature. However, the introduction of strongly electron-withdrawing substituents on the double bond appears to facilitate thermal decomposition of these compounds. A number of azides thus substituted have been found to decompose at room temperature or belo. v. 

Mercury, Zinc, and Copper. The thermal decomposition of 2-thienylmercury thiocyanate, azide, acetate, and trifluoromethylsulphonate has been investigated. Thienylmercury derivatives have been cross-coupled with primary and secondary alkyl- and alkenyl-cuprate reagents. 2-Thienylzinc chloride has been coupled with iodobenzene and vinyl bromide, using Pd catalysis. ... 

---