Alkyl azides, decomposition, thermally photolysis

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. 

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). 

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