Enantioselective Extraction of Di-O-benzoyltartrate Anion by Ion-Pair Extractant Having Binaphthyl-Unit.

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A lipophilic diphosphonium salt 1 having binaphtyl-type axial chirality was developed as a novel ion-pair extractant. By use of (R)-1, (−)-di-O-benzoyltartrate ((−)-DBT) was extracted more effectively than (+)-DBT. The enantioselectivity (α=D(−)⁄D(+); D: distribution ratio) was 1.3–1.4. When monophosphonium-type extractants (2 and 3) having structures similar to that of 1 were used, the enantioselectivity was heavily lowered. Equilibrium studies revealed that the extraction occurs via formation of a 1 : 1 ion-pair complex between 1 and DBT at pH 7.5–8.0. The effects of pH, ionic strength, and temperature on the extractability and the enantioselectivity were investigated. The enantioselectivity decreased with lowering pH, and was higher at lower temperatures. The optical purity of DBT in the aqueous phase was gradually enhanced by multistage extraction. From an NMR analysis of the 1·DBT complex, the resonances of methylene-protons adjacent to phosphonium in the more extractable complexes ((R)-1·-(−)-DBT and (S)-1·(+)-DBT) shifted to a higher field compared to those of the less extractable complexes ((R)-1·(+)-DBT and (S)-1·(−)-DBT).
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