New soluble bifunctional polymeric chiral ligands for enantioselectively catalytic reactions

Two new soluble bifunctional polymeric ligands (R,R)-4 and (R,R)-5 have been prepared via the direct condensation reaction of (R)-3,3′-diformyl-1,1′-bi-2-naphthol (R)-1 with (R)-5,5′-diamino BINAP (R)-2 and with (R)-5,5′-diamino BINAPO (R)-3, respectively. The different types of catalytic centers, BINOL and BINAP or BINAPO, were alternatively organized in a regular chiral polymer chain. Both polymeric ligands were found to be effective in the addition of diethylzinc to benzaldehyde either in the presence or in the absence of Ti(OPri)4 with different enantioselectivities. (R,R)-4/Ti(IV) catalyst, which showed similar efficiency to the parent catalyst BINOL/Ti(IV), was more enantioselective than (R,R)-5/Ti(IV). (R,R)-4 was also found to be highly effective in the Ru(II)-catalyzed asymmetric hydrogenation of 2-arylacrylic acids. The results demonstrated that the use of the co-polymer catalyst rather than a mixture of monomer catalysts not only simplified the recycling of the catalyst, but also improved the enantioselectivity and/or the activity in some cases.