Asymmetric Hydrogenation of Acetophenone and Its Derivatives Catalyzed by L-Proline Stabilized Iridium

Abstract

A supported iridium catalyst stabilized by L-proline was prepared under mild conditions and was applied to the heterogeneous asymmetric hydrogenation of acetophenone and its derivatives.The effect of the support and L-proline on the supported iridium catalyst was studied.The catalyst was characterized by transmission electron microscopy(TEM) and X-ray photoelectron spectroscopy(XPS).The results showed that L-proline had a marked ability to stabilize and disperse the iridium particles.Reaction conditions for the asymmetric hydrogenation of acetophenone were examined in detail.LiOH enhanced the activity and enantioselectivity of the reaction.The alkali metal cation plays an important role in influencing the enantioselectivity.In addition,a synergistic effect between(1S,2S)-1,2-diphenyl-1,2-ethylenediamine((1S,2S)-DPEN) and L-proline was observed,which significantly accelerated the reaction rate and enhanced the enantioselectivity.In the presence of(1S,2S)-DPEN as chiral modifier the catalyst 5%(w,mass fraction)Ir/15(the molar ratio of L-proline to Ir)(L-Proline)-γ-Al2O3 exhibited good catalytic performance in the asymmetric hydrogenation of acetophenone and its derivatives.Under optimum conditions the enantiomeric excess(ee) values of(R)-phenylethanol and(R)-2?-(trifluoromethyl)phenylethanol were 71.3% and 79.8%,respectively.Without using phosphine as a stabilizer the preparation of the catalyst was simple and the catalyst was reused several times without a significant loss in activity and enantioselectivity.