Catalytic performance of chiral rhodium complex with water-soluble sulfonated (R)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl for enantioseletive hydrogenation in ionic liquid biphasic system

Asymmetric hydrogenation of dimethyl itaconate by the chiral rhodium complex ligated with water-soluble sulfonated (R)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl (labeled as (R)-BINAPS) was performed in ionic liquid-isopropanol biphasic catalytic system. A moderate enantiomeric excess (ee) of 64% and initial turnover frequency (TOF_max) as high as 1234 h⁻¹ could be obtained at 303 K and 2.0 MPa when the molar ration of substrate to rhodium was set at 1000. Application of imidazolium ionic liquid [bmim]BF₄ (bmim = 1-butyl-3-methyl imidazolium cation) made it possible to reuse the catalyst several times, showing rather good stability in the catalyst performance. According to the ICP-OES analyses, the total extraction of rhodium species by the reactants from the ionic liquid phase was about 3.24 wt% after the catalyst for four runs, which was about three times lower than that of total loss of phosphorous species. The slight decreases in the ee value and TOF_max during the catalyst reuse could be recovered essentially to the pristine levels by replenishing certain amount of the fresh ligand into the used catalyst.