4H-Azepin-4-ones from 4-azidophenols in low-temperature matrices

Abstract

In an attempt to develop a synthetic approach to 4H-azepin-4-ones in exceptionally mild conditions, 4-azidophenols have been photolysed in N₂ matrices at 12–14 K and subjected to flash vacuum pyrolysis (FVP) with subsequent trapping of the products in N₂ matrices. The reactions were followed by IR spectroscopy. The compounds studied were 4-azidophenol, which was prepared in a purer form than achieved by earlier workers, 4-azido-2-nitrophenol, the previously unknown 2,6-dichloro and 2,6-dibromo and 2-chloro-6-methyl derivatives of 4-azidophenol, and a heterocyclic analogue, 5-azido-8-hydroxyquinoline. In all cases, IR spectra indicated that the expected ring expansion to give azepinones occurred, while in some conditions intermediate hydroxyazacyclohepta-1,2,4,6-tetraenes could also be detected. For several of the azidophenols, photolyses and pyrolyses were attempted on a larger scale (50 mg or more) in a specially constructed low-temperature cell. These reactions were monitored by diffuse reflection IR spectroscopy (DRIFTS), which showed that the desired azepinones were formed at low temperature. Attempts to isolate the azepinones by warming to room temperature, however, resulted only in polymeric products. Preparative scale pyrolyses in conventional FVP equipment also gave polymers.