Asymmetric Addition Reactions to Aliphatic Aldehydes

After successful application in the diethylzinc addition to aromatic aldehydes, we applied our ligands to aliphatic substrates. These substrates still pose a challenge for most types of ligands. 

While cyclohexanecarbaldehyde and pivaldehyde, being a-branched aldehydes, are excellent substrates for most ligands leading to nearly perfect enantioselec-tion, unbranched aldehydes still remain cumbersome substrates. 

The application of diastereomeric ligands in the diethylzinc addition to aliphatic substrates demonstrated that two ligands, (Sp,S)-5b and (Rp,S)-5b, always yield appropriate esters 13 with opposite configurations. 

A comparison of these results with those obtained from ligand (Sp,S)-5a demonstrates that the ligands (Rp,S)-5b and (Sp,S)-5b can raise the enantiomeric excesses by about 10% ee [18]. However, use of pivaldehyde as the substrate already showed excellent ee values (98%) when the first-generation ligand systems (Sp,S)-5a and (Sp,S)-6a, respectively, were employed [17]. 

In conclusion, one can say that although the effectiveness of our ligands is only moderately good for diethylzinc additions to aromatic aldehydes, it is, however, excellent in applications involving aliphatic aldehydes. The ee values for 

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