Abstract
Two kinds of dendritic chiral BINOL ligands have been synthesized through the condensation reaction between 2,2'-dihydroxy-1,1'-binaphthyl-3,3'-dicarboxylic acid and Frechet-type polyether dendrons with primary and secondary amine at the focal point, respectively. These dendritic chiral BINOL ligands were found to be effective in the enantioselective addition of diethylzinc to benzaldehyde both in the presence and absence of Ti[OCH(CH3)2]4. Much different enantioselectivity was observed in both cases. In the former case, (R)-2 showed moderate enantioselectivity, which was much higher than that of (R)-3. The size/generation of the dendrimer 2 did not significantly influence the enantioselectivity. In the latter case, (R)-3 gave the highest enantioselectivity as compare to (R)-1, (R)-2 and BINOL, which decreased on going from 3-G0 to 3-G1.
Highlights
During the last decade, enormous progress has been obtained in the field of dendrimers research owing to their unique physical and chemical properties.[1]
We have developed two types of chiral dendritic ligands for asymmetric catalysis through the incorporation of BINAP6 and BINOL7 into the core of the Fréchet-type dendrimers, respectively
The commercially available (R)-BINOL was chosen as model ligand for this study due to its very extensive applications in asymmetric catalysis.[13]
Summary
Enormous progress has been obtained in the field of dendrimers research owing to their unique physical and chemical properties.[1]. The dendritic BINOL ligands 1 were found to be highly effective in the enantioselective addition of diethylzinc to benzaldehyde both in the presence and in the absence of Ti[OCH(CH3)2]4, albeit gave lower enantioselectivity with the increasing generation of the dendrimer.[7] This indicated that the microenvironment of the catalytic sites of the dendritic catalysts is very important for their effectiveness in steric control.
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