Science of Synthesis

Amador, A. G.; Scholz, S. O.; Skubi, K. L.; Yoon, T. P., Science of Synthesis: Photocatalysis in Organic Synthesis, (2018) 1, 469.

The direct photolysis of azides is known to generate highly reactive nitrene intermediates with concomitant loss of dinitrogen. However, attempts to perform intermolecular aziridination using direct photolysis can give rise to complex product mixtures, exacerbated by the divergent reactivity of singlet and triplet nitrenes, which are both produced under these conditions. Alternatively, it is possible to utilize the iridium-based photosensitizer ‌4‌ {[Ir(ppy)2(dtbbpy)]PF6; ppy = 2-(2-pyridyl)phenyl; dtbbpy = 4,4′-di-tert-butyl-2,2′-bipyridyl} to sensitize 2,2,2-trichloroethoxycarbonyl azide and selectively access the triplet nitrene spin state for chemoselective aziridination of a variety of alkenes ‌3‌, providing aziridines ‌5‌ (‌Table 2‌).[‌10‌] The reaction has a broad scope and is tolerant of both aliphatic alkenes and styrenes. Notably, the alkene reaction partner can be used as the limiting reagent. However, when electron-rich alkenes are subjected to the reaction, the corresponding oxazoline product is observed. Although scaling in batch was found to be impractical, it is possible to scale the reaction efficiently using a continuous-flow reactor. The aziridine products have varying degrees of stability on silica gel. The most reproducible method of isolation of all aziridines products utilizes silica-9 (silica gel basified to pH 9).[‌11‌]

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References