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

Abstract

The new N-heterocyclic carbene (NHC) complex [PdCl 2 {(CN) 2 IMes}(PPh 3 )] (2) ({(CN) 2 IMes}: 4,5-dicyano-1,3dimesitylimidazol-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-4,5dicyano-2-(pentafluorophenyl)-3-picolylimidazoline (5) was formed by condensation of pentafluorobenzaldehyde with Nbenzyl-NЈ-picolyldiaminomaleonitrile (4). The NHC palladacycle [PdCl 2 {(CN) 2 IBzPic}] (6) ({(CN) 2 IBzPic}: 1-benzyl-4,5dicyano-3-picolylimidazol-2-ylidene) was prepared by in situ [a] 2952 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.Scheme 2. Preparation of NHC palladium(II) complexes 2 and 3.

PEPPSI‐Effect on Suzuki–Miyaura Reactions Using 4,5‐Dicyano‐1,3‐dimesitylimidazol‐2‐ylidene‐Palladium Complexes: A Comparison between trans‐Ligands

“…It is known from recent studies concerning the Mizoroki–Heck reaction that the catalytically active species derived from 4 and 5 are palladium nanoparticles 11. To obtain information about the nature of the catalytically active species under milder conditions of the Suzuki–Miyaura reaction, poisoning experiments were performed, using elemental Hg (10000 equiv., based upon the precatalyst loading).…”

“…Recently, we presented palladium precatalysts 4 and 5 bearing the π‐acceptor NHC 4,5‐dicyano‐1,3‐dimesitylimidazol‐2‐ylidene {(CN) 2 IMes} and precatalyst 6 , bearing 4,5‐dicyano‐1‐benzyl‐3‐picolylimidazol‐2‐ylidene {(CN) 2 IBzPic} (Scheme ) 11…”

“…Unexpectedly, the most efficient precatalyst 5 , outperformed its literature known analogue [PdCl(dmba)(IMes)]. Poisoning experiments have shown that reactions using these complexes are catalyzed by palladium nanoparticles and not by a molecular [Pd 0 (NHC)] species 11. Thus, we endeavored to overcome the effects of nanoparticle formation.…”

“…We performed the Suzuki–Miyaura reaction, which proceeds under milder conditions than those required of the Mizoroki–Heck reaction. Reaction yields using 12 – 15 were compared to those employing established precatalysts 1 ,8a 4 11 and 5 11. The reaction rates associated with the use of 1 , 4 , 5 and 12 – 15 were compared.…”

Cross coupling reactions catalyzed by (NHC)Pd(II) complexes

“…The new precatalysts showed high activity in the Heck reactions, giving good-to-excellent product yields with 0.1 mol% precatalyst. 47 The nanoparticle nature of the catalytically active species of C9, C10, and C11 was confirmed by poisoning experiments with mercury and transmission electron microscopy. Precatalyst C10 showed the best overall catalytic performance ( The complexes C13a-c, connected with different kinds of coordination anions, were applied in Heck reactions.…”