Ion-Tagged Chiral Ligands for Asymmetric Transfer Hydrogenations in Aqueous Medium

We report the design and synthesis of novel ion-tagged chiral ligands for asymmetric transfer hydrogenation (ATH) in aqueous medium. Based on (R,R)-1,2-diphenylethylene diamine (DPEN) as structural motif, a straightforward three-step protocol was developed that gave access to novel chiral ligands with carbamate-substructure and pyridinium headgroup. The careful optimization of steric and electronic properties in combination with the adaption of solubility via choice of the anion gave a set of chiral and water-soluble ligands for use in ruthenium-catalyzed asymmetric transfer hydrogenations in aqueous medium. Eventually, a pool of aliphatic and aromatic ketones as well as two imine substrates were reduced with excellent isolated yields up to 95% and enantioselectivities >90% ee under environmentally benign conditions in the absence of additional surfactants.