Synthesis, reactivity, and electronic structure of multifarious, five-membered heteroaryl and heteroaroyl azides

The aim of this account is to review our most important outcomes ― published during the last 30 years ― concerning the azido group that is linked to five-membered heteroaryl and heteroaroyl systems. The main focus of this manuscript is on the ‘azido transfer’ reaction to heteroaryl azides, and the peculiar thermal and chemical reactivities of these precursor species. In particular, the following topics are considered: (a) the ring cleavage of α-heteroaryl azides to form 4-cyano-1,3-heterodienes, (b) the reactivity of thermally generated β-nitrene, (c) the 1,3- dipolar cycloaddition of the azido group to various terminal or 1,2-disubstituted alkenes or alkynes to form nitrogen-containing biheterocycles, (d) the generation of electrophile nitrenium ions with Lewis acids, (e) the conversion of heteroaroyl azides into isocyanates, and (f) the 1,3- dipolar cycloadditions of heteroaroyl azides with ‘activated’ olefins. The concurrence and discordance of the thermal and chemical behaviour between heteroaryl azides, and the related substituted phenyl azides will be critically considered here.