Highly Enantioselective And Efficient Asymmetric Epoxidation Catalysts Based On Inorganic Nanosheet Modified Alpha-Amino Acids As Ligands

Highly enantioselective and efficient synthesis of enantiopurity is the aim of asymmetric catalysis. The elaborately design of chiral ligands is one of the most effective ways to obtain sigle isomer. Steric structure or solid surface of inorganic materials has been used to modified chiral ligands to enhance the catalytic asymmetric selective, which is a hotspot in recent years. Mesoporous materials, montmorillonite and carbon nanotubes, etc, immobilized with metal containing chiral ligands have been reported with success to present excellent enantioselectivity. The focus of our work is to utilize the rigid and ordered structure of layered double hydroxides (LDHs) and the property that the LDHs can be delaminated into nano lamella to improve asymmetric selectivity and catalytic activity. The main results in our work are listed as follows:LDHs modified L-alanine (Ala), L-serine (Ser) and L-glutamate (Glu) ligands are prepared based on the assumption that the anionic layered materials, layers of LDHs, can serve as substituents of chiral alpha-amino acids ligands. In the vanadium catalyzed asymmetric epoxidation of 2-methyl-3-phenyl-allyl alcohol, a disubstituted allylic alcohol, ee value to 97% for the layer modified amino acid as ligand from 50% for homogeneous counterpart. Meanwhile, significant enhancement of enantioselectivity is also obtained in the asymmetric epoxidation of monosubstituted allylic alcohol and homoallylic alcohol. The'huge'and'rigid'brucite-like layer is supposed to provide the necessary rigidity and more effective steric effects to direct the approach trajectory and restrict the coordination of allylic alcohol molecules to the activated vanadium centres, which enhances the chiral induction of ligand and promote asymmetric selectivity.To make efficient use of the catalytic center in the layer, the layered compounds containing alpha-amino acid can be further delaminated due to the interlayer space can be expanded. Higher yield can be achieved by the pseudo-homogeneous catalyst in a shorter time than intercalated system, while the yield almost reaches the homogeneity level during the same time. Using the delaminated alpha-amino acid-LDHs, It makes the catalytic reactions to be carried out under a pseudo-homogeneous condition. The pseudo-homogeneous catalysis using delaminated L-alanine-LDHs increased the yield to 89% in only half the time 520 min from 71% in 1050 min, while the yield almost reaches the homogeneity level (93%) during the same time (520 min).The solid catalysts (the intercalated LDH materials which contain chiral alpha-amino acids are used in solid together with vanadium center) could be readily recovered from the reaction medium by simple filtration. Using environmentally friendly water as solvent, the colloidal catalyst could be directly separated by simple liquid/liquid separation. The catalysts could be recycled almost without loss of catalytic activity and enantioselectivity in three runs in the asymmetric epoxidation of 2-methyl-3-phenyl-allyl alcohol. Futher, the relationship between the recycle performance and the host-guest interaction or the coordination structure of vanadium with ligand is studied.