Anchored [RuCl2(p-cymene)]2 in hybrid zirconium phosphate–phosphonate coated and pillared with double-stranded hydrophobic linear polystyrene as heterogeneous catalyst suitable for aqueous asymmetric transfer hydrogenation

Abstract

A novel type of phosphonate-containing polystyrene copolymers 1a–e bearing an N′-alkylated TsDPEN chiral ligand and double-stranded polystyrene chains were prepared for the first time using simple radical copolymerization of 1-phosphonate styrene with (R,R)-N′-4′-vinylbenzyl-N-4-vinylbenzenesulfonyl-1,2-diphenylethylene-1,2-diamine. Through the coprecipitation of their supported Ru polystyrene copolymers 2a–e and NaH₂PO₄ with ZrOCl₂, pillared hybrid zirconium phosphate–phosphonate-anchored Ru catalysts 3a–e and 4d₁1–d₅5 were obtained as heterogeneous catalysts suitable for aqueous asymmetric transfer hydrogenation. In the aqueous asymmetric transfer hydrogenation of aromatic ketones, the anchored Ru catalysts showed good catalytic activities, chemoselectivities (∼100%), and enantioselectivities (73.6% ee to 95.6% ee). The Ru catalysts retained their catalytic properties even at the fifth recycle time (92.2% conv., 92.1% ee). However, corresponding supported Ru catalyst 3d′ resulted in disappointing reusability because of the loss of ruthenium in every recycle process. The conversions of aromatic ketones were closely related to the o-, m- or p-positions of the substituents on the aromatic ring caused by shape-selective matching.