Sonogashira-type cross-coupling reactions catalyzed by copper complexes of pincer N-heterocyclic carbenes

Abstract

Copper complexes of N-heterocyclic carbenes (NHC) have recently received great attention in catalysis, however, the application scope has been limited to neutral complexes bearing monodentate ligands. Herein, we report the synthesis and full characterization of a cationic Cu-pincer bis(NHC) complex with bulky tert-butyl wingtips that serves as catalyst for the coupling of aryl iodides and phenylacetylene, alongside its analogs with small alkyl substituents. Unlike other copper catalysts that require an inert atmosphere to prevent alkyne homocoupling, a competing side reaction to Sonogashira reaction, the Cu-pincer bis(NHC) complexes provide good to excellent cross-coupling yields in air. Interestingly, the reaction under argon affords substantially reduced cross-coupling, indicative of the oxygen involvement and its facilitating effect in the mechanism. In our controlled studies, the air-assisted cross-coupling reaction came to a complete stop when using a radical trap, suggesting the presence of a radical in the mechanism. Examining the reaction of Cu-pincer bis(NHC) complexes with oxygen via UV–vis spectroscopy reveals a dark green chromophore formation at −40 °C with strong absorbance (ε > 400 M−1cm−1) at 460 nm and 500 nm, likely due to a high-valent copper-oxygen adduct.