Efficient Palladium(II) Precatalysts Bearing 4, 5‐Dicyanoimidazol‐2‐ylidene for the Mizoroki–Heck Reaction. Issue 18 (20th May 2014)

• Record Type: Journal Article
• Authors: Baier, Heiko; Metzner, Philipp; Körzdörfer, Thomas; Kelling, Alexandra; Holdt, Hans‐Jürgen
• Is Part Of: European journal of inorganic chemistry. Issue 18(2014)
• Page Start: 2952
• Page End: 2960
• Publication Date: 2014-05-20
• Subjects: Homogeneous catalysis -- Palladium -- Cross coupling -- Carbene ligands; Chemistry, Inorganic -- Periodicals; Organometallic chemistry -- Periodicals; Bioinorganic chemistry -- Periodicals; Solid state chemistry -- Periodicals; 546
• DOI: 10.1002/ejic.201402040
• Languages: English
• ISSNs: 1434-1948

Abstract

Abstract: The new N‐heterocyclic carbene (NHC) complex [PdCl2 {(CN)2 IMes}(PPh3 )] (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 complex2 was formed by ligand exchange using the mononuclear precursor [PdCl2 (PPh3 )2 ] and the palladacycle3 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)2 IBzPic}] (6 ) ({(CN)2 IBzPic}: 1‐benzyl‐4, 5‐dicyano‐3‐picolylimidazol‐2‐ylidene) was prepared by in situ thermolysis of5 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 of2 and3 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 complexes2, 3 and6 . 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 of2, 3 and6 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. Abstract : The new precatalysts [PdCl2 {(CN)2 IMes}], [PdCl(dmba){(CN)2 IMes}] and [PdCl2 {(CN)2 IBzPic}] have been prepared as candidates for the Mizoroki–Heck reaction and used in catalytic screening for reactions between n ‐butyl acrylate and 27 aryl bromides.