Tetranuclear Palladium Complexes of Abnormal N‐Heterocyclic Carbene Ligands and their Catalytic Activities in Mizoroki‐Heck Coupling Reaction of Electron‐Rich Aryl Chlorides

Abstract

AbstractBased on the ligand scaffold of an imidazolyl/benziimidazolyl moiety and a N‐CH2C(=O)NHPh substituent, two series of ligand precursors with ortho hydroxy groups incorporated on the N‐phenyl rings were prepared. The structural fine tuning of the ligand scaffold allowed the synthesis of tetranuclear palladium complexes with abnormal N‐heterocyclic carbene (aNHC) ligands. For precursors with C2‐methyl blocking groups, pyridine‐assisted C−H bond activation led to the formation of mononuclear tridentate palladium aNHC complexes or tetranuclear complexes with tridentate CNO donors. Representative mononuclear and tetranuclearpalladium aNHC complexes were structurally characterized by X‐ray diffraction studies, revealing very short Pd−C bond distances. The tetranuclear palladium aNHC complexes were very effective in catalyzing Mizoroki‐Heck coupling reaction, and were capable of employing a range of aryl chlorides including deactivated substrates with low palladium loading of 0.2 mol%.magnified image