Construction of P-Chiral Reaction Field by Y-P-Y type Chiral Ligands and Application to Multi-site Controlled Asymmetric Reactions

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Symmetrical Y-P-Y ligands having two coordinating units would be expected to form a P-chiral center if selective ligation of one of Y-units to metals occurs and to construct an effective chiral filed near the reaction site for asymmetric reactions. Residual Y unit would serve to interact with substrate or reaction reagent to realize the multi-site recognition to cause higher chiral induction. As N-P-N type ligand, various chiral bisoxazolylphosphine ligands having two chiral oxazoline units ((S,S)-R-P(Ox-R')_2) (R=Ph, ^iPr ; R'=Me, ^iPr, ^tBu) were prepared and their coordination mode to Pd, Pt, Rh, Ni, or Ru were examined by NMR technique. In the square planar complexes, large deviation in equilibrium to one of diastereomeric complexes was observed by the selective ligation of one of two oxazoline units to metal. This selective ligation to afford P-chiral center was also clarified by the X-ray analysis of a single crystal of [Pd(C_3H_5)-((S,S)-PhP(Ox-i^Pr)_2]PF_6.Bisoxazolylphosphine Pd complexes were applied to the asymmetric allylic alkylation and animation; In the enantiotopos differentiating allylic alkylation, high enantioselectivity more than 90 %ee were obtained, especially in the nucleophilic reaction of 1,3-dimethyl-π-allyl intermediate with a small steric factor 94% ee was obtained, which is the highest value reported so far. These results indicate the effectiveness of the P-chiral center formed by the selective ligation. In the reaction of alkyl zinc compound with π-allyl intermediate by Pd catalyst, the contribution of the free oxazolyl unit was observed to cause multi-site controlled reaction.