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

Abstract

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.

Synthesis and site selective C–H functionalization of imidazo-[1,2-a]pyridines

“…An ionic Pd(II) complex with an amido-functionalized N-heterocyclic carbene and PPh 3 ligands was used in the catalytic system (Scheme 67). 131 Scheme 61 C3 amination.…”

Recent Developments in Transition‐Metal‐Catalyzed Regioselective Functionalization of Imidazo[1, 2‐a]pyridine

“…al . in 2016 carried out dual functionalization using this same Pd catalyst complex again [42] . Here they had used 2‐alkylimidazo[1, 2‐ a ]pyridine as the substrate rather than 2‐phenylimidazo[1, 2‐ a ]pyridine, keeping all other conditions the same (Scheme 12).…”

Indolizines and Their Hetero/Benzo Derivatives in Reactions of [8+2] Cycloaddition

“…Imidazo[1,2-a]pyridines and imidazo[1,2-a]pyrimidines readily reacted with diaryl acetylenes in presence of catalyst [ 92 ], Scheme 34 , Table 16 .…”