Asymmetric Diels-Alder Reactions On Silica-Supported Bis(Oxazoline) Metal Complexes

Chiral bis(oxazoline) metal complexes have been widely applied to homogeneous Diels-Alder (DA) reactions. However, these homogeneous catalysts are usually used in large amounts and difficult to be recycled. It is highly desirable to immobilize homogeneous complexes on solid supports in order to increase the efficiency of the utilization. Heterogeneous catalysis has the potential advantages such as easy seperation, efficient recycling and thus lower costs comparing with the homogeneous catalysis. In this thesis, a detailed research has been carried out on the preparation of supported bis(oxazoline) metal catalysts and the application to DA reactions. Furthermore, several bis(oxazolinyl)pyridine scandium complexes have been synthesized and electronic and steric effects of the bis(oxazolinyl)pyridine ligands on the homogeneous DA reactions were investigated.Bis(oxazoline) metal complexes were immobilized on the supports activated silica, SBA-15, cesium salts of 12-tungstophosphoric acid and periodic mesoporous ethane-silicas functionalized with sulfonic acid via non-covalent interactions. Asymmetric DA reaction between 3-((E)-2-butenoyl)-1,3-oxazolin-2- one and cyclopentadiene shows that the catalyst with activated silica calcinated at 300 oC gave the best enantioselectivity. Hydrogen bonding between the anion of the homogeneous complex and the silanol groups was confirmed by FT-IR characterization, which is considered to be the main interactions for the catalystimmobilization on the surface of activated silica.A series of bis(oxazoline) metal complexes of copper(II), zinc(II) and magnesium(II) were immobilized on silica for DA reactions. Up to 93% ee was obtained and the heterogeneous catalyst could be recycled at least three times without losing enantioselectivity. It is found that the absolute configuration of the product is reversed from S to R upon immobilization of the (S)-PhBOX-Cu(II) complex. Experimental and theoretical studies show that the reversal of the absolute product configuration is triggered by the anion dissociation from Cu(II) onto the surface of the support.In addition, several bis(oxazolinyl)pyridine ligands with different electronic and steric properties were synthesized in order to develop new catalysts for DA reactions. The results show that electron-withdrawing groups significantly increase both the activity and enantioselectivity of the catalyst. A study of steric effect of the ligands reveals that the bulky tBu-substituted ligand gave the highest enantioselectivity of 96% ee under mild reaction conditions.