Of Co <sub> 2 </ Sub> And The Asymmetry Of The Propylene Oxide Ring Addition Reaction Catalyst System Design And Research

A series of chiral Salen ligands derived from (7)-l,2-diaminocyclohexane, were employed in asymmetric cycloaddition of propylene oxide with CO2.A series of alkyl-, halo- and nitro-substituted chiral Salen ligands were prepared. SalenCo( II) complexes derived from the reaction of chiral Salen ligands and Co(OAc)2 were oxidized by different acid groups to form SalenCo (III) complexes, which were employed in asymmetric cycloaddition of propylene oxide with CO2. The e.e. value of propylene carbonate was determined by GC with a chiral chromato-graphic column; and the e.e. value of unreacted propylene oxide was determined after it was transformed into propylene carbonate by reaction with CO2 using SalenAlEt/ n-Bu4NI as catalyst.It was found that the chiral SalenCo(III)X complexes could effectively catalyze the cycloaddation reaction of CO2 with epoxides only in the presence of quaternary ammonium halides(-Bu4NY) at room temperature. The anion Y of n-BuNY in the binary catalyst systems has a great effect on enantiomeric purity and reaction rate. It seems that the use of -Bu4NCl was more beneficial for improving enantiomeric purity of the products, but had a pronounced negative effect on the rate. The axial X-group of chiral SalenCo(III)X complexes also affect enantiomeric purity of the resulted propylene carbonate and propylene oxide. A sterically bulky axial X-group is more beneficial for high enantioselectivity in this reaction. The performance of catalyst is also affected by the substituent on the aromatic rings of chiral Salen ligands. The decrease of reaction temperature is effective for improving enantiomeric purity of the products, but resulting in significant decrease in catalytic activity. Among the synthesized chiral metal-Salen complexes(such as chromium, manganese and cobalt), SalenCo(III)X complexes are more effective in catalytic activity as well as enantioselectivity.