Highly Productive CNN Pincer Ruthenium Catalysts for the Asymmetric Reduction of Alkyl Aryl Ketones

Abstract

Chiral pincer ruthenium complexes of formula [RuCl(CNN)(Josiphos)] (2–7; Josiphos=1-[1-(dicyclohexylphosphano)ethyl]-2-(diarylphosphano)ferrocene) have been prepared by treating [RuCl₂(PPh₃)₃] with (S,R)-Josiphos diphosphanes and 1-substituted-1-(6-arylpyridin-2-yl)methanamines (HCNN; substituent=H (1 a), Me (1 b), and tBu (1 c)) with NEt₃. By using 1 b and 1 c as a racemic mixture, complexes 4–7 were obtained through a diastereoselective synthesis promoted by acetic acid. These pincer complexes, which display correctly matched chiral PP and CNN ligands, are remarkably active catalysts for the asymmetric reduction of alkyl aryl ketones in basic alcohol media by both transfer hydrogenation (TH) and hydrogenation (HY), achieving enantioselectivities of up to 99 %. In 2-propanol, the enantioselective TH of ketones was accomplished by using a catalyst loading as low as 0.002 mol % and afforded a turnover frequency (TOF) of 10⁵–10⁶ h⁻¹ (60 and 82 °C). In methanol/ethanol mixtures, the CNN pincer complexes catalyzed the asymmetric HY of ketones with H₂ (5 atm) at 0.01 mol % relative to the complex with a TOF of ≈10⁴ h⁻¹ at 40 °C.