Palladium‐Catalyzed Oxidative Cyclization for the Synthesis of 2‐Alkylimidazo[5,1,2‐cd]indolizines

An effective palladium-catalyzed direct arylation of 2-alkyl-imidazo[1,2-a]pyridine with alkynes towards imidazo[5,1,2-cd]indolizines through double C–H functionalization was developed. The catalyst system is an ionic palladium(II) complex bearing amido-functionalized N-heterocyclic carbene and triphenylphosphine ligands. A mere 2.5 mol-% loading of palladium was sufficient for catalyzing the reactions. Copper acetate and n-tetrabutylammonium bromide were employed as oxidant and phase-transfer catalyst, respectively. Beside imidazo[1,2-a]pyridine, desirably, less electron-rich derivatives could be employed as coupling partners. Also a wide range of diarylalkynes was used. The structure of one 2-alkyl-imidazo[5,1,2-cd]indolizine was established by single-crystal X-ray diffraction analysis. UV and PL spectra of the compounds confirmed that some are highly fluorescent. The method provides an easy synthetic route for the preparation of these new fluorescent materials.