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

Abstract

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 Fréchet-type polyether dendritic benzyl bromide, followed by deprotection of methoxymethyl groups by i PrOH/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).