Efficient Palladium(II) Precatalysts Bearing 4,5-Dicyanoimidazol-2-ylidene for the Mizoroki–Heck Reaction



The new N-heterocyclic carbene (NHC) complex [PdCl2{(CN)2IMes}(PPh3)] (2) ({(CN)2IMes}: 4,5-dicyano-1,3-dimesitylimidazol-2-ylidene) and the NHC palladacycle [PdCl(dmba){(CN)2IMes}] (3) (dmba: N,N-dimethylbenzylamine) have been synthesized by thermolysis of 4,5-dicyano-1,3-dimesityl-2-(pentafluorophenyl)imidazoline (1) in the presence of suitable palladium(II) precursors. The acyclic complex 2 was formed by ligand exchange using the mononuclear precursor [PdCl2(PPh3)2] and the palladacycle 3 was formed by cleavage of the dinuclear chloro-bridged precursor [Pd(μ-Cl)(dmba)]2. The new NHC precursor 1-benzyl-4,5-dicyano-2-(pentafluorophenyl)-3-picolylimidazoline (5) was formed by condensation of pentafluorobenzaldehyde with N-benzyl-N′-picolyldiaminomaleonitrile (4). The NHC palladacycle [PdCl2{(CN)2IBzPic}] (6) ({(CN)2IBzPic}: 1-benzyl-4,5-dicyano-3-picolylimidazol-2-ylidene) was prepared by in situ thermolysis of 5 in the presence of [PdCl2(PhCN)2]. The three palladium(II) complexes were characterized by NMR and IR spectroscopy, mass spectrometry and elemental analysis. In addition, the molecular structures of 2 and 3 were determined by X-ray diffraction. The π-acidity of (CN)2IBzPic was compared with (CN)2IMes and perviously reported π-acidic imidazol-2-ylidenes by NBO analysis. The Mizoroki–Heck (MH) reactions of various aryl halides with n-butyl acrylate were performed in the presence of complexes 2, 3 and 6. The new precatalysts showed high activity in the MH reactions giving good-to-excellent product yields with 0.1 mol-% precatalyst. The nature of the catalytically active species of 2, 3 and 6 was investigated by poisoning experiments with mercury and transmission electron microscopy. It was found that palladium nanoparticles formed from the precatalysts were involved in the catalytic process.