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.