Dendritic BINOL ligands for asymmetric catalysis: effect of the linking positions and generations of the dendritic wedges on catalyst properties

Three types of new chiral BINOL ligands (2, 3 and 4) bearing dendritic wedges have been synthesized through coupling reaction between 3-hydroxymethyl-2,2'-bis(methoxymethyl)-1,1'-binaphthol (7), 6,6'-dihydroxymethyl-2,2'-bis(methoxymethyl)-1,1'-binaphthol (12), 6-hydroxymethyl-2,2'-bis(methoxymethyl)-1,1'-binaphthol (15) and Frechet-type polyether dendritic benzyl bromide, followed by deprotection of methoxymethyl groups by (PrOH)-Pr-i/HCl, respectively. These new ligands obtained were assessed in enantioselective Lewis acid-catalyzed addition of diethylzinc to benzaldehyde. Compared to the enantioselectivity observed with dendrimer 1 bearing the dendritic wedges at 3,3'-positions of the binaphthyl backbone, higher enantioselectivity for all these ligands was observed. Difference in the effect of linking positions and generations on enantioselectivity and/or activity for all three kinds of dendritic ligand-derived catalysts was observed. Among these dendritic ligands, (R)-3/Ti(IV) catalyst with the dendritic wedges at 6,6-positions of BINOL gave the highest enantioselectivity (up to 87% ee). (C) 2003 Elsevier Ltd. All rights reserved.