The Synthesis of Imidazo[1,2-f]phenanthridines, Phenanthro-[9,10-d]imidazoles, and Phenanthro[9′,10′:4,5]imidazo[1,2-f]-phenanthridines via Intramolecular Oxidative Aromatic Coupling

Abstract

A short and efficient access to phenanthro[9,10-d]imidazoles, imidazo[1,2-f]phenanthridines, and phenanthro[9′,10′:4,5]imidazo[1,2-f]phenanthridines was achieved by the action of [bis(trifluoroacetoxy)iodo]benzene (PIFA) on properly substituted tetraaryl­-imidazoles. By pre-installing suitable electron-donating groups, it is possible to control the site of intramolecular oxidative aromatic coupling. In particular, by placing 3,4-dimethoxyphenyl and 3-methoxyphenyl moieties in close proximity, it was possible to direct the reaction into forming two biaryl linkages leading eventually to the formation of phenanthro[9′,10′:4,5]imidazo[1,2-f]phenanthridines. Starting from bis-aldehydes that are derivatives of thieno[3,2-b]thiophene and fluorene enabled the synthesis of π-expanded imidazoles bearing 8-9 conjugated rings. By placing a dimethoxynaphthalene unit on the imidazole scaffold, we have directed the oxidative coupling reaction towards closing a five-membered ring with concomitant removal of methoxy group leading to formation of an α,β-unsaturated ketone. All resulting π-expanded imidazoles display blue emission, and the fluorescence quantum yields in some cases reaches 0.9.