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 (TOFmax) 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 TOFmax during the catalyst reuse could be recovered essentially to the pristine levels by replenishing certain amount of the fresh ligand into the used catalyst.