Development of New Catalytic Asymmetric Reactions Based on the Synthesis of Optically Active Phosphine Ligands

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This study is directed toward the development of new catalytic asymmetric reactions based on the design and synthesis of novel chiral phosphine ligands. A new tetraphosphine ligand bearing their four chiral centers at phosphorus atoms has been synthesized via phosphine-boranes as the intermediates and the molecular structure of its dirhodium complex was determined by the single crystal X-ray analysis. The tetraphosphine lidand itself is C_2 symmetric, but each Rh-diphosphine sphere forms a C_1 symmetric three-hindered quadrant. This dirhodium complex exhibited excellent enantioinduction ability in Rh-catalyzed asymmetric hydrogenation of dehydroamino acids. Low temperature hydrogenation of the complex afforded a tetrahydrorhodium complex quantitatively, and the reactions of the tetrahydride complex with alkene substrates were studied in order to elucidate the detailed enantioselection mechanism. The results including the stereochemical outcome are reasonably interpreted by assuming tha … More t the enantioselection would be determined at the migratory insertion step.A new P-chiral phosphinophenol was prepared and employed in copper-catalyzed asymmetric conjugate addition to α,β-unsaturated carbonyl compounds. The reactions proceeded smoothly under mild conditions to afford the corresponding products with high enantiomeric excesses of up to 96%.Asymmetric hydrosilylation of simple ketones was tested by the use of several P-chiral phosphine ligands prepared in our laboratory. Very high enantioselectivity of up to 99% was observed when a bulky silylation reagent such as α-naphtyldiphenylsilane was used. The sense of the stereoselection has been rationalized by considering that the enantioselection occurs at the migratory insertion step.A new P-chiral diphosphine ligand, (R,R)-2,3-bis(tert-butylmethylphosphino)quinoxaline (QuinoxP^*)) was designed and synthesized by reacting enantiomerically pure (R)-tert-butyl(hydroxymethyl)methylphosphine-borane with 2,3-dichloroquinoxaline. This ligand, in contrast to most of the previously reported P-chiral ligands, is an air-stable solid and exhibits excellent enantioselectivities in both Rh-catalyzed asymmetric hydrogenations and Rh- or Pd-catalyzed carbon-carbon bond-forming reactions. Less