Development of Highly Stereoselective Asymmetric Reactions Using P-Chiral Phosphine Ligands

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This study is directed toward the development of new asymmetric catalysts that exhibit exceedingly high enantioselectivity and catalytic activity. In order to achieve this purpose, we designed and prepared new P-chiral phosphine ligands based on the phosphine-borane methodology. The newly synthesized phosphine ligands (QuinoxP^* and AlkynylP^*) have been proved very high enantioselectivities up to 99.9% ee in not only rhodium-catalyzed asymmetric hydrogenation but also rhodium- or palladium-catalyzed carbon-carbon bond forming reactions.A new synthetic method for the synthesis of the counter enantiomers of P-chiral phosphine ligands has been studied. A key intermediate (S)-t-butylmethylphosphine-borane, which is prepared by the use of (-)-sparteine, was converted to (R)-enantiomer by the bromination and reduction with lithium aluminum hydride. The synthesized (R)-enantiomer was used for the preparation of (R,R) t-Bu- QuinoxP^*.Mechanistic study was carried out in order to investigate the enantioselection mechanism in the rhodium-catalyzed asymmetric hydrogenation of dehydroaminoacids using (Z)-α-acetamidocinnamate as a model substrate and R-(tert-butylmethylphosphino)(di-tert-butylphosphino)methane as the chiral ligand. Based on the experimental facts and DFT caluculation, it is concluded that the enantioselection takes place during the association step when the chelating dihydride is formed from the nonchelating dihydride.