Synthesis Of Amino Acid-and Imidazolium-Tagged Chiral Pyrrolidinodiphosphine Ligands And Their Application For Catalytic Asymmetrc Hydrogenation

The transition-metal-catalyzed asymmetric hydrogenation reaction is one of themost efficient methods for preparing a wide range of enantiomerically purecompounds，homogeneous asymmetric hydrogenation is now a well-used methodologyin organic synthesis due to the high activities and stereoselectivities. However, Thesynthesis of chiral ligands is usually tedious and the corresponding chiral catalysts areoften expensive. It is highly demanded to develop efficient ways to separate thecatalyst and reactant after the catalytic reaction.Herein we report amino acid and imidazolium-tagged chiral pyrrolidinodiphosphineligands and catalytic efficiency and reusability of resulting Rh-catalysts in asymmetrichydrogenation of methyl (Z)-2-acetamido-cinnamate in different solvents, ionic liquid(IL)[bmim]BF4(1-n-butyl-3-methylimidazolium tetrafluoro-borate) and[bmim]BF4/cosolvents systems. The effects of different ionic tags structures, solventsor cosolvents, Rh precursor and halide impurities on the catalytic activity andenantioselectivity were studied in detail. The amino acid and imidazolium modifiedligands represent higher ee values than the classical pyrrolidinodiphosphine,(2S,4S)-N-(tert-butoxy-carbonyl)-4-(diphenylphosphino)-2-[(diphenylphosphino)methyl] pyrrolidine (BPPM), both in pure methanol and under IL/MeOH system. Thecatalyst recycling experiments demonstrate that the introduction of amino acid andimidazolium moieties at pyrrolidinodiphosphine backbone results in much betterimmobilization and reusability of Rh-catalysts in IL, thus no significant loss of activity,enantioselectivity or Rh was observed after several cycles. This work enriches thevariety of chiral phosphines ligands and provides a new methodology for separationand recycling of the expensive chiral catalysts.