Immobilization of chiral catalysts on magnetite nanoparticles for highly enantioselective asymmetric hydrogenation of aromatic ketones

Abstract

Magnetite nanoparticle (MNP)-supported asymmetric catalysts were synthesized by direct immobilization of a 4,4′-bisphosphonic acid-substituted BINAP-Ru-DPEN complex onto MNPs. The resulting heterogeneous catalyst (7) was highly active for the asymmetric hydrogenation of a wide range of aromatic ketones with remarkably high activity (>99% conversion with 0.1 mol% catalyst) and enantioselectivity (up to 98.1% ee) as a result of the bulky 4,4′-substituents (i.e., phosphonic acid groups) on BINAP. Catalyst 7 could be readily recycled and reused through simple magnetic decantation 9 times without the loss of activity or enantioselectivity. An alternative strategy was also explored to immobilize the [4,4′-(TMS)₂-BINAP]-Ru-DPEN complex onto MNPs by first grafting a DPEN derivative onto the MNP surface followed by complexation of [4,4′-(TMS)₂-BINAP]-Ru(DMF)₂Cl₂ to the surface-bound DPEN, but this immobilization strategy did not provide a robust heterogeneous catalyst, presumably due to oxidation of the Ru(II) intermediate by iron oxide nanoparticles.