Sdudies And Applycation Research Of Pd Catalysts With Chiral Imidazole Thioether Ligand Supported On SiO₂

Imidazole and its derivatives play an important role in the fields of biology and pharmacology as well as chemistry. Recently, an interest to these heteroaromatic systems has been further expanded as the precursors of imidazolium-based ionic liquids and N-heterocyclic carbenes (NHCs). Among them, imidazole derivatives having a N-substituent with stereogenic center(s) have attracted special attention, because of their potential utility in a wide range of fields related to chiral recognition and asymmetric catalysis. When thioethers coordinated to metals, they could form strong bonds with soft metals like Pd. Meanwhile, there were fewer reports on the bidentate Pd complexes coordinated by sulphur and carbene. This thesis used natural amino acids as chiral reagents to synthesis a series of chiral thioethers containing imidazole ring. The thioethers were supported on silica and coordinated to PdCl2 combined with the carbene on the imidazole ring. At last, the catalytic capability of such catalysts supported on silica was studied in the Suzuki coupling reaction. The detailed research content was as follows:Using natural amino acids as starting materials, the imidazole ring was condensed by ring fragments. Hydroxyl group was induced into the N-substituent by the reduction of carboxyl of the amino acids, and then through chlorination and subsequent substitution by thiols, about 25 intermediates and the final products were synthesized, in which 9 thioethers were all novel compounds that haven't been reported. All the structure of these compounds were characterized by 1H NMR, 13C NMR, ESI-MS and IR. Their physical parameter were determined. While the chiral imidazole alcohols and thioether were synthesized, both were explored 2 routes and the synthetic conditions were optimized.Using silica as supports, 4 thioethers containing imidazole rings were loaded. They were separately coordinated to PdCl2 and the catalysts were characterized by IR, DTA, XPS and Elemental Analysis. The results showed that covalent bond was formed between thioether and silica, at the same time the coordination bond was formed between Pd and sulphur atom and the carbene C atom might participate in the coordination to Pd. The capability of the catalysts was studied in the Suzuki coupling reaction, which included the effect of reaction time, species of bases, amounts of bases, amounts of catalysts, reaction temperature, structure of substrate and ligands on the Suzuki reaction. The coupling reaction conditions were optimized. The study results showed that: using the catalysts supported on silica synthesized in this thesis to catalyze the Suzuki coupling reaction, best yields could be obtained under the conditions of 0.75 mol% Pd catalysts, 4 times of amounts potassium carbonate of aryl bromides, 80°C for 5 h. The capability of such catalysts was high, for the yields could reach to 90% when the Pd catalyst was only 0.25 mol% of aryl bromides.