High asymmetric induction using a diastereomeric mixture of axially dissymmetric ligands

We have reported that our new axially dissymmetric ligand with two chiral centers, (Ra)-2,2′-bis[(R)-1H-1-hydroxyperfluorooctyl]biphenyl ((Ra)-(R)2-1c, or tentatively called as (Ra)-(R)2-PFCAB-7), worked as a good asymmetric inducer for the reaction of benzaldehyde with diethylzinc. Now, a mixture of (Ra)-(R)2- and (Sa)-(R)2-PFCAB-7 even in 1:4 ratio (−60% de) was found to give nearly the same asymmetric induction as pure (Ra)-(R)2-PFCAB-7 of the corresponding molar percents. This result suggests that both isomers do not form complex and that (Ra)-(R)2-PFCAB-7 accelerates the reaction and induces high asymmetry, while (Sa)-(R)2-1c does not accelerate the reaction significantly and does not induce asymmetry at all. This ligand of low ee, (Ra)-(R)2-PFCAB-7 of 20% ee, did not show appreciable asymmetric amplification, suggesting no formation of heterochiral complex.