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

Abstract

Abstract The new N‐heterocyclic carbene (NHC) complex [PdCl 2 {(CN) 2 IMes}(PPh 3 )] ( 2 ) ({(CN) 2 IMes}: 4,5‐dicyano‐1,3‐dimesitylimidazol‐2‐ylidene) and the NHC palladacycle [PdCl(dmba){(CN) 2 IMes}] ( 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 [PdCl 2 (PPh 3 ) 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 [PdCl 2 {(CN) 2 IBzPic}] ( 6 ) ({(CN) 2 IBzPic}: 1‐benzyl‐4,5‐dicyano‐3‐picolylimidazol‐2‐ylidene) was prepared by in situ thermolysis of 5 in the presence of [PdCl 2 (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) 2 IBzPic was compared with (CN) 2 IMes 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.