Chiral BINOL-centered dendrimers for the enantioselective Lewis acid catalyzed diethylzinc addition to aldehydes

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.