Syntheses Of Surfactant Palladium Complexes And Their Applications To Cross-Coupling Reactions In Water

As a non-toxic, cheap, readily available and environmentally benign solvent, water has been widely used in organic synthesis. However, the reasons of water not used as solvent in some organic synthesis reactions include mainly:(1) Most of organic substrates are insoluble in water; (2) Many catalysts in water are easily decomposed or inactivated. In addition, the coordination catalytic reaction is in line with the development requirements of green chemistry because of mild conditions, rapid reaction and good selectivity. Therefore, the development of a suitable catalyst that achieves formation of carbon-carbon bond smoothly in water possesses good application prospect and theoretical significance.Air-and water-stable transition metal complexes with catalytic activity and surfactant performance are a key to achieve catalytic reactions in water. In this paper, on the basis of our group's preliminary work the polydentate nitrogen Pd(II) complexes were designed and synthesized as follows:New bis(imino)pyridine palladium complexes 58-68 were synthesized, and their structures were determined by spectrum analysis. The X-crystallographic analysis of complexes 63,64, and 68 revealed the square planar geometry of three nitrogen ligands chelated Pd(II).Salen-Pd complexes 74-77 and half-Salen-Pd complex 78 were synthesized, and their structures were determined by spectrum analysis. The X-crystallographic analysis of complexes 75,77, and 78 revealed the square planar geometry of two nitrogen ligands chelated Pd(II).Polydentate nitrogen ligands 80-88 with long alkyl chain and new surfactant Pd(II) complexes 89-93 were synthesized, and their structures were determined by spectrum analysis.The above air-and water-stable polydentate nitrogen Pd(II) complexes had some solubility and surface activity in hot water. And the catalytic activity of these complexes was investigated in water by using the Suzuki cross-coupling as model reaction.Complexes 62,64, and 68 could catalyze effectively the Suzuki cross-coupling reactions of aryl bromides with small molecular weight in water at 80℃, and the yield is up to 99%. But the employment of aryl bromides with larger molecular weight, higher melting point, and low water solubility failed to give the desired coupling products. The reaction could accomplish smoothly by addition of the surfactant or using ethanol as solvent, and the yield was up to 96%. This showed the complexes 62,64, and 68 were also effective catalysts for Suzuki cross-coupling reactions of aryl bromides with larger molecular weight.Half-Salen-Pd complex 78 could effectively catalyze the Suzuki cross-coupling reaction of various aryl bromides in ethanol at 60℃. The yield was up to 99% and TON was up to 7.6×105. However, Salen-Pd complexes 75-78 showed no catalytic activity.Complexes 90-94 could catalyze effectively not only Suzuki cross-coupling reactions of aryl bromides with low water-solublity and small molecular weight in water at 80℃, but also the reactions of aryl bromides with large molecular weight and poor water-solublity. And the yield is up to 90%. This showed surfactant polydentate nitrogen Pd(Ⅱ) complex was an excellent catalyst for Suzuki cross-coupling reaction in water, and this catalytic system also offered an effective method to eliminate solvent contamination in organic reactions.