Chiral Ru/Ir bimetallic dendronized polymer catalysts constructed through sequential metal coordination and applied in asymmetric hydrogenation of quinaldine

Abstract

A new kind of chiral conjugated copolymer P-2 with (S)-BINAP [BINAP=2,2′-bis(diarylphosphino)-1,1′-binaphthyl] and fluorene as the building blocks was synthesized involving Pd[P(p-tolyl)3]3-catalyzed Suzuki-coupling reaction together with reduction of phosphine oxide with HSiCl3/NEt3. The chiral polymer was used to construct chiral Ru/Ir bimetallic dendronized polymer catalysts P-5-m with different generations of Fréchet-type dendritic DPEN [DPEN=1,2-diphenylethylenediamine] ligands through a programmed metal–ligand coordination strategy. These dendronized polymer catalysts were applied to the Ir-catalyzed asymmetric hydrogenation of quinaldine and exhibited excellent activity with moderate enantioselectivity. The reaction rate was increased with increasing generation of the dendronized polymer catalysts, which are better than that obtained from the monomeric catalyst. This interesting positive dendrimer effect might be due to effective shielding of the catalytically active iridium center by the bulky dendritic pendants, which reduced the formation of the inactive iridium dimer.