Efficient heck reactions catalyzed by palladium(0) and -(II) complexes bearing n-heterocyclic carbene and amide functionalities

A palladium(0) NHC complex Pd⁰(LH¹)₂(MA) (MA = maleic anhydride) was prepared from the amide-imidazolium salt [LH ¹H²]Cl (H¹ = NH proton; H² NCHN proton). The X-ray diffraction studies confirmed that a η²-MA ligand and two monodentate NHC ligands with the H¹ protons remaining intact are coordinated. These NH protons are involved in intra- or intermolecular hydrogen bonds stabilizing the solid-state structure. Degradation of Pd(LH¹)₂(MA) in air leads to the formation of the chelate complex trans-Pd^IIL₂ and other unidentified products. Negative-ion electrospray mass spectrometry revealed some intriguing Pd(0) species, including a 14-electron [Pd⁰L]^- species that bears only a bidentate NHC/amido ligand. The anionic amido group imparts a high electron density on a palladium center, as shown by X-ray photoelectron study. The palladium(0) precatalyst is highly efficient in catalyzing Heck reactions with activated aryl chlorides in ionic liquid. For deactivating aryl chlorides and bulky aryl bromides, cis-Pd^IIL₂ is more effective. A range of Heck-coupled products can be prepared by Pd ⁰(LH¹)₂(MA) and cis-Pd^IIL ₂. The latter complex also successfully mediates one-pot sequential Heck/Heck and Suzuki/Heck coupling reactions with 4-bromochlorobenzene as substrate.