N-Heterocyclic Carbene Palladium Complexes Bearing Carboxylate Ligands and Their Catalytic Activity in the Hydroarylation of Alkynes

The synthesis and reactivity of palladium acetate/trifluoroacetate complexes stabilized by the presence of N-heterocyclic carbene (NHC) ligands are described. The structures and coordination characteristics of both (IPr)Pd(OAc)2 (1) and (IPr)Pd(OOCCF3)2(H2O) (2) were elucidated by spectroscopic and X-ray diffraction studies. The structure of 1 shows a novel coordination of the anions in a monomeric complex, with one acetate anion acting as a monodentate ligand while the second one coordinates through both oxygens. The NHC ligands in 1 and 2 are stable under acidic conditions. The complexes were used as precatalysts in the hydroarylation of alkynes. Using this simple protocol, a number of arenes react with various alkynes to produce stilbenes.