New Pincer-heterocyclic Carbene Palladium Synthesis And Coupling Reactions In Aqueous Applications

A robust hydrophilic pincer-type N-heterocyclic carbene palladium complex 3 was designed and synthesized, which exhibited not only high catalytic activity but also robust against air, moisture and heat. In addition, the simple synthetic method, readily accessible from inexpensive and commercial precursors, and environment-friendly as replacement of air-sensitive phosphine ligands present a practical catalyst.In consideration of green chemistry, water was chosen as solvent, and catalytic activities of complex 3 were investigated in aqueous-phase C-C formation reactions, such as Suzuki-Miyaura couplings and Heck reaction. In Suzuki-Miyaura couplings, complex 3 tolerated a variety of functional groups of the (hetero)aryl bromide and aryl boronic acid varying from their electronic and steric properties in aqueous media with extremely low catalyst loading down to the 50 ppm. The products could be easily separated by filtration and the filtrate could be reused for several times. In Heck reaction, with Et3N as a base, the hydrolysis of the substrates and products could be efficiently inhibited, and resulted a good yield. The protocol tolerated various electronic properties of substituted functional groups of arenes with the catalyst loading down to the 50 ppm, excluded the bulky and heterocyclic iodide substrates as well as several vinyl compounds.Poisoning experiments indicated the pincer-type N-heterocyclic carbene palladium complex 3 acts as a molecular catalyst in couplings reaction, instead of forming Pd nanoparticles catalyst in situ as its imidazolium analogues. It may attribute to the steady Pd-C imposed by strongerσ-donating property of benzimidazolylidene and hardly broken to form Pd nanoparticles in the catalytic process.