Palladium(II) Complexes with Small N‐Heterocyclic Carbene Ligands as Highly Active Catalysts for the Suzuki–Miyaura Cross‐Coupling Reaction

• DOI: 10.1002/cctc.201200428
• Publication Date: 2012-10-26T20:55:16Z
• Authors: Michał S. Szulman...; Andrzej Gniewek; Wojciech Gil; Anna M. Trzeciak
• Keywords: Suzuki reaction; Coupling reaction

Abstract Four new palladium(II) complexes of the type [Pd(NHC) 2 X ] with N‐heterocyclic carbene (NHC) ligands relatively small steric hindrance were prepared and characterized by using spectroscopic X‐ray methods. For [Pd(bmim‐y) Br (bmim‐y=1‐butyl‐3‐methylimidazol‐2‐ylidene), crystals both cis trans isomers obtained. All studied demonstrated very high activity in Suzuki–Miyaura cross‐coupling ethylene glycol, which yielded turnover numbers up to 760 000. High was also observed if NaBPh 4 used instead PhB(OH) , best results (turnover number=580 000) obtained [Pd(emim‐y) Cl (emim‐y=1‐ethyl‐3‐methylimidazol‐2‐ylidene). In reaction mixture, different forms containing [Pd x y + fragments ( =1–4, =2–5) identified ESI‐MS. presence substrates, catalytic palladium intermediates aryl groups—[Pd(NHC) Ph] 3 —were detected. Additional mechanistic investigations, such as TEM observations mercury poisoning experiments, substantiated formation nanoparticles a catalyst resting state. These heterogeneous particles serve reservoir for soluble species—atoms or clusters that function homogeneous catalysts reaction.