The Synthesis Of Chiral Pybox-metal Complexes And The Catalytic Performance Of The Mukaiyama Aldol Reaction In Aqueous Media

Pyridine-2, 6-bis(oxazolino) (pybox) is a C₂-symmetric chiral ligand with excellent asymmetric inductivity, which has been developed greatly in recent years. With people's increasing attention to the live environment and the rising of green chemistry, stereoselective organic reactions in aqueous media are attracting more and more attention. The aldol reaction is one of the most important reactions for the stereoselective construction of carbon-carbon bonds. Among other methodologies, the chiral Lewis-acid-catalyzed condensation of silylenol ethers with aldehydes (the Mukaiyama reaction) constitutes one of the most convenient and relevant variants of the classical aldol reaction. The main work of this article is synthesis metal complexes containing pybox and catalyzes asymmetric Mukaiyama aldol reaction in aqueous media to find the stable and effective catalyst in aqueous media.In the first chapter, the asymmetric synthesis and the synthesis of pybox ligand and the metal complexes are summarized. Research development of aldol reactions and present research status of Mukaiyama aldol reactions in water are also introduced.In the second chapter, a water soluble ligand: 2, 6-bis(4R-hydroxy methyloxazlyl)pyridine (pybox-hm) was synthesized. On the basis of literatures, we designed two routes to synthesis pybox-hm from pyridine-2, 6-dicarboxylic acid and L-serine, one is the amincation reaction with the esterification amino acid; the other is the amincation reaction with amino acid directly. The experimental results indicate that: in the first route, the ligand can obtained smoothly from L-serine, after the esterification of L-serine, then the amincation reaction, the chlorination reaction, the reduction and the ring closure. We explored reductive methods of ester group by NaBH₄, but no good results were obtained. LiBH₄ was used finally, which has strong reducing power but expensive. In the second route, we use acylchloride which is easy to be reducted, but the yield of the amincation using L-serine directly is slow, and the follow chlorination was not success. The structures of the intermediate products and the target compound were characterized by IR and ¹H NMR.In the third chapter, the coordination with metals was studied. Cobalt( II), Nickel( II), Zinc( II) and Copper( II) complexes containing Pybox-hm ligand were synthesized. Their structures were characterized by IR, UV, ESI-MS, elementary analysis (EA), molar conductivity measurements and electrochemical studies.In the fourth chapter, catalytic performance of the four metal complexes in the Mukaiyama aldol reaction in aqueous medial was studied. The effects of solvent, temperature and the amount of catalyst were studied, the results of experiments showed the optimum reaction conditions are: the solvent is THF:H₂O=9:1, 0℃is the best temperature, and the catalyst concentration is 20mol%. The catalytic performance of trifluoromethanesulfonate is better than chloride-salt, the Zinc( II ) complex shows the best catalytic performance, the yield can up to 93%.