Multiple Dendritic Catalysts for Asymmetric Transfer Hydrogenation

The first and second generation multiple dendritic ligands based on chiral diamine were synthesized in a convergent approach and were well-characterized by NMR and MS techniques. Their ruthenium complexes prepared in situ had good solubility in the reaction medium (azeotrope of formic acid and triethylamine) and demonstrated high catalytic activity and enantioselectivity comparable to monomeric catalysts in the asymmetric transfer hydrogenation of ketones and imines. Quantitative yields and for some cases a slightly higher enantioselectivity (up to 98.7% ee) were obtained in the dendritic catalysis. Considering the high local catalyst concentrations at the periphery, diones were tested for the possible synergic reactivity between catalytic units at the surface, while no apparent differences were noted.