Metal Complexes of N‐Heterocyclic Carbenes—A New Structural Principle for Catalysts in Homogeneous Catalysis

Stabilized by “carbene” donor ligands, the Pd complex 1 catalyzes the Heck olefination of aryl halides unexpectedly efficiently and yet has long-term stability at elevated temperatures. The active Pd0 species can be generated during the Heck reaction or deliberately prepared by reduction of 1 with, for instance, hydrazine or sodium formate. Another similar catalyst can be synthesized in situ from Pd0 complexes and 1,3-dimethyldihydroimidazoline-2-ylidene.