Photoswitchable Azoheterocycles via Coupling of Lithiated Imidazoles with Benzenediazonium Salts

In contrast to azobenzenes, heterocyclic azo compounds are less well investigated. Phenylazoimidazoles would be versatile as photodissociable ligands (PDLs) because imidazole is an important donor in coordination chemistry. Here, we present the synthesis of 4- and 5-phenylazoimidazoles via a novel azo-coupling method. 1,2-Protected imidazole is lithiated in the 5-position and coupled with benzenediazonium tetrafluoroborate. Several new phenylazoimidazoles were prepared. They exhibit an excellent switching behavior. Upon irradiation of the trans isomers with UV light, >95% of the cis forms are obtained. Upon heating, a complete transformation back to the trans configuration was achieved. Back switching with visible light, however, is incomplete.