Biphasic Asymmetric Hydrogenation In Ionic Liquid

The synthesis of chiral ligands is usually tedious and the corresponding chiral catalysts are often expensive. It is highly demanded to develop efficient ways to separate the catalyst and reactant after the catalytic reaction. Reportedly, the biphasic catalysis system composed of ionic liquid and organic liquid can not only maintain the property of homogeneous catalysis with high activity and selectivity, but also provide an easy way for the catalyst-reactant separation. Therefore, it is meaningful to develop novel biphasic systems of ionic liquid-organic liquid for the asymmetric catalysis.This dissertation has chosen the commercial available chiral ligands of (R)- and (S)-BINAP, (R,R)- and (S,S)-DPEN to prepare the water-soluble chiral ligands of (R)- and (S)-BINAP-nSO3Na (n=3～4) (labeled as (R)- and (S)-BINAPS), (R,R)- and (S,S)-DPEN-2SO3Na (labeled as (R,R)-and (S,S)-DPENS) by sulfonation and then neutralization. These ligands are complexed with Rh or Ru precursors to afford the water-soluble catalysts like [Rh(COD)((R)-BINAPS)]BF4, [(R)-BINAPS-Ru-(R, R)-DPENS]Cl₂ and [(S)-BINAPS-Ru-(S,S)-DPENS]Cl2. The asymmetric hydrogenations of dimethyl itaconate and aromatic ketones are investigated by using these water-soluble complexes in ionic liquid-organic liquid biphasic system like [bmim]BF₄/IPA (i-propanol) and [bmim]PF₆/IPA.When the water-soluble complex [Rh(COD)((R)-BINAPS)]BF₄ is used to catalyze the asymmetric hydrogenation of dimethyl itaconate, a moderate enantiomeric excess (ee) of 64.2% and initial turnover frequency (TOFmax) up to 1234 h-1 are obtained at 303 K and 2 MPa pressure in [bmim]BF₄/IPA. The catalyst can be reused several times without reducing the reactivity and enatioselectivity significantly, exhibiting rather good stability in catalytic performance. The ICP–OES element analysis shows that the total extraction of rhodium species by the reactants from the ionic liquid phase is about 3.24 wt% after the catalyst for four runs, which is about three times lower than the total loss of phosphorous species.The water-soluble complex [(R)-BINAPS-Ru-(R,R)-DPENS]Cl₂ in the biphasic system [bmim]BF4/IPA can catalyze the asymmetric hydrogenation of acetophenone with activity and selectivity comparable to these using the oleo-soluble complex [(R)-BINAP-Ru-(R,R)-DPEN]Cl₂ in IPA at room temperature and 2 MPa pressure. In this case an ee value of 83.5 % is achieved. When the water-soluble complex [(S)-BINAPS-Ru-(S,S)-DPENS]Cl₂ is used to catalyze the asymmetric hydrogenation of 1-acetonaphthone in the biphasic system, an ee value up to 92.9 % is obtained. The recycling uses of such water-soluble catalysts show that the activity and selectivity can be sustained for three times at least. But further recycling uses can cause declines in the catalytic performance, especially in conversion.