C2-symmetric ligands

Lewis acid-promoted asymmetric addition of dialkylzincs to aldehydes is also an acceptable procedure for the preparation of chiral secondary alcohol. Various chiral titanium complexes are highly enantioselective catalysts [4]. C2-Symmet-ric disulfonamide, chiral diol (TADDOL) derived from tartaric acid, and chiral thiophosphoramidate are efficient chiral ligands. C2-Symmetric chiral diol 10, readily prepared from 1-indene by Brown s asymmetric hydroboration, is also a good chiral source (Scheme 2) [17], Even a simple a-hydroxycarboxylic acid 11 can achieve a good enantioselectivity [18]. 

Besides the chiral C3- and Ci-symmetric ligands, C2-symmetric tacn analogues have been used (Figure 10.8) [75]. So far modest enantioselectivity has been obtained but the potential of these tmtacn analogues by further fine-tuning of the chiral ligand structure is evident... 

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