Asymmetric Transfer Hydrogenation in Water with Platinum Group Metal Catalysts

Asymmetric transfer hydrogenation (ATH) is the reduction of prochiral compounds with a hydrogen donor other than hydrogen gas in the presence of a chiral catalyst. The asymmetric reduction of a wide variety of ketone and aldehyde substrates has been carried out in water using catalysts based on complexes of ruthenium(II), rhodium(III) and iridium(III), affording fast reaction rates and good enantioselectivities without the use of organic solvents and with easy separation of catalyst and product. For ATH of ketones, the Rh(III) complexes appear to perform better than the Ru(II) and Ir(III) complexes in terms of activity, enantioselectivity and substrate scope. However, their performance varies with the choice of ligands, and simple Ir(III)-diamine complexes were found to be excellent catalysts for the reduction of aldehydes.