Study On The Synthesis And Applications Of Oligomer-type Catalysts Of Chiral Cobalt Complexes

In recent years, the syntheses and applications of Schiff base and its metal complexes have attracted more attention of chemists. One of those is the salen-metal complex. Compared to metalloporphyrin, the salen-M complexes have several advantages: First, it is easier to be synthesized. Second, an asymmetric surrounding can be introduced around the salen ligands to enhance the asymmetric selectivity. Finally, some appropriate substituted groups can be introduced into 3,5-position of the salen ligands to adjust their steric and electronic effect. This paper will do a brief review about the applications of chiral Schiff bases and their complexes as chiral cataltysts in the asymmetric reactions, such as epoxidation, hydrogenation of olefins;hydrogen transfer, cyanosilylation, hydrocyanation reactions of ketones, oxidation kinetic resolution of alcohol and asymmetric synthesis of cyclic carbonates.Although the asymmetric catalytic reactions can be efficiently carried out in homogeneous catalytic systems with high yields and ee value, it has also some disadvantages, such as complicated purification procedures and using the expensive catalysts that are quite difficult to recover and recycle. However, in heterogeneous systems, the reaction mixture can be easily separated from one another by simple filtration;the catalysts can be reused several times in most cases. Thus the heterogeneous systems are more convenient to be used in industry.In order to obtain some Schiff bases with high optical activity, high enantioselectivity and reused several times without loss of activity, we have modified salen-Co complexe with ferrocene and bisphenol A, respectively, and obtained two polymeric chiral Co complex that were charactized by IR and MALDI-TOF-MS. These catalysts were applied to the synthsis of chiral cyclic carbonate with satisfied results.A novel and efficient catalyst system, CAN/SiO₂, was developed to catalyze the bromination of aromatic aldehydes at room tempreture. Therefore, a series of mono-and di-bromo aromatic aldehydes were successfully obtained using this method.