14/4/2010
Abstract
A series of well-defined chain-end-functionalized polystyrenes having a definite number of chiral ephedrine moieties dendritically distributed at the periphery of their hyperbranched chain-ends were evaluated as chiral catalysts for the enantioselective addition of dialkylzinc reagents to aldehydes. These dendritic macromolecules worked well as homogeneous chiral catalysts and exhibited high catalytic activity and enantioselectivity very similar to those observed for the corresponding monomeric chiral catalysts. The optimum amount of chiral catalyst was found to be 5 mol %. A profound number effect of the chiral ephedrine moieties was observed, and PS(Ephed)8 having eight chiral ephedrine moieties at the periphery was found to be superior to other dendritic chiral catalysts. The enantioselectivity reached a value of 95% in the addition of diisopropylzinc to 3-phenylpropanal. The dendritic chiral catalysts could be easily recovered from the reaction solution by using a solvent precipitation method, and the recovered catalyst showed no significant loss of its catalytic activity or enantioselectivity. _ 2008 Elsevier Ltd. All rights reserved.
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