Asymmetric Reduction of Ketones Catalyzed by a Polymeric Chiral Ru Complex

Abstract

Two polystyrene-supported chiral ligands 1 and 2, based on (1S,2S)-N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine (TsDPEN), were prepared from sodium 4-hydroxybenzenesulfonate dihydrate in seven and eight steps, respectively. The catalytic properties of their Ru complexes prepared in situ were examined for the asymmetric transfer hydrogenation of aromatic ketones with formic acid-triethylamine azeotrope as the hydrogen source to exhibit the high catalytic activities and enantioselectivities. This reduction protocol also displayed excellent chemoselectivities: the keto-carbonyl group was reduced selectively under these reaction conditions whereas the adjacent ester, amide, or cyano groups remained intact. The immobilized catalyst was easily recovered by simple filtration and subsequent washing with dichloromethane, and was reused without a significant drop in enantioselectivity. The resulting optically active alcohols are known to be appropriate intermediates for the synthesis of (S)-fluoxetine, which is an important antidepressant.