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We report the synthesis, characterization and reactivity of an air-stable, well-defined acenaphthoimidazolylidene palladium–BIAN–NHC chloro dimer complex, [Pd(BIAN–IPr)(μ-Cl)Cl] 2 . This rapidly activating catalyst merges the reactive properties of palladium chloro dimers, [Pd(NHC)(μ-Cl)Cl] 2 , with the attractive structural features of the BIAN framework. [Pd(BIAN–IPr)(μ-Cl)Cl] 2 is the most reactive Pd( ii )–NHC precatalyst discovered to date undergoing fast activation under both an inert atmosphere and aerobic conditions. The catalyst features bulky-yet-flexible sterics that render the C–H substituents closer to the metal center in combination with rapid dissociation to monomers and strong σ-donor properties. [Pd(BIAN–IPr)(μ-Cl)Cl] 2 should be considered as a catalyst for reactions using well-defined Pd( ii )–NHCs. 

Abstract Over the last 20 years, N-heterocyclic carbenes (NHCs) have emerged as a dominant direction in ligand development in transition metal catalysis. In particular, strong σ-donation in combination with tunable steric environment make NHCs to be among the most common ligands used for C–C and C–heteroatom bond formation. Herein, we report the study on steric and electronic properties of thiazol-2-ylidenes. We demonstrate that the thiazole heterocycle and enhanced π-electrophilicity result in a class of highly active carbene ligands for electrophilic cyclization reactions to form valuable oxazoline heterocycles. The evaluation of steric, electron-donating and π-accepting properties as well as structural characterization and coordination chemistry is presented. This mode of catalysis can be applied to late-stage drug functionalization to furnish attractive building blocks for medicinal chemistry. Considering the key role of N-heterocyclic ligands, we anticipate thatN-aryl thiazol-2-ylidenes will be of broad interest as ligands in modern chemical synthesis.