Photochemistry of Polynuclear Aromatic Azides

Fusion of a benzene ring to another aromatic ring or rings changes the electronic structure of the modified nitrenes and related intermediates sufficiently enough to alter their chemistry, kinetics and thermodynamics. Thus photolysis of polynuclear aromatic azides in the presence of primary and secondary amines leads not to 3H-azepines, but to corresponding 

Much effort has been devoted to the study of the photochemistry of 1- and 2-naphthyl azides (79 and 80). The products obtained upon pyrolysis and photolysis of the naphthyl azides were reported in the 1970s and In 1974, the Suschitzky group  

Carroll et al. found that the yield of diamine product is sensitive to the photolysis time. A drastic reduction in the photolysis time leads to a much-improved yield of the diamines. It was also discovered that the yield of diamine products formed upon photolysis of the 1- and 2-naphthyl azides can also be significantly improved by the presence of (Me2NCH2)2 (or TMBDA) as co-solvent.  

Leyva and Platz demonstrated that reaction temperature plays an important role in the photochemistry of 79, as with the case of parent phenyl azide 47. Moderate yields of adducts were observed by simply lowering the temperature of the photolysis of 79 with DBA. 

The photoproduct studies mentioned above suggest the intermediacy of azirines in the photochemistry of 79 and 80. Additional evidence of these intermediates was provided by the observations of adducts in the photolysis of 80 with ethanethioL and with metha-nolic methoxide.  

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With the assumption that the undetected precursors to triplet 1- and 2-naphthylnitrenes are the azirines seen in the low temperature experiments, we can reach useful conclusions about the photochemistry of polynuclear aromatic azides. First, unlike phenyl azide where the closed-shell singlet intermediate formed in room temperature irradiations is dehydroazepine [46, 49, 69], the intermediates formed from both 1- and 2-naphthyl azide are azirines. The difference in the chemistry of 1- and 2-naphthyl azides is traced to a difference in the lifetime of the respective azirines. The azirine from 2-naphthyl azide survives at least 200 times longer than does the azirine formed from 1-naphthyl azide. The increase in lifetime permits the bimolecular trapping reaction by diethylamine to compete with isomerization to the triplet nitrene in the case of the 2-naphthyl but not the 1-naphthyl azides. 

Therefore, as in the case of parent phenyl azide 47 and its simple derivatives, the photochemistry of polynuclear aromatic azide, especially that of naphthyl azides 79 and 80, is now well understood. Specifically, the dynamics of the primary photophysical processes as well as the subsequent photochemical steps have been directly investigated using a variety of modem and conventional experimental techniques and compntational chemistry. It is clear now, that the difference between the photochemistry of phenyl azide (and its simple derivative) and polynuclear aromatic azide is caused mainly by the difference in the thermodynamics of the singlet nitrene rearrangement to azinine type species. 

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