Modular Assembly of Ring-Fused and π‑Extended Phenanthroimidazoles via C–H Activation and Alkyne Annulation

π-Extension of 2-aryl-phenanthroimidazoles via rhodium­(III)-catalyzed C–H activation and alkyne annulation is developed. This method enables rapid, practical and modular assembly of diverse ring-fused phenanthroimidazoles, including an unusual rearrangement product using aryl-alkyl asymmetric alkyne and a thiophene fused product which could serve as a Fe3+ fluorescent probe. The feasibility of the one-pot synthesis and ruthenium­(II)-catalyzed versions of this reaction was also verified.