Switching of the diastereomer deposited during the crystallization of N-[(S)-1-phenylethyl]-2'-carbamoyl-1,1'-binaphthalene-2-carboxylic acid: investigation of the mechanism of dielectrically controlled resolution.

Dielectrically controlled resolution (DCR) has been achieved during the crystallization of (S)-1-phenylethylamides of racemic 1,1'-binaphthalene-2,2'-dicarboxylic acid (RS(a),S)-1. For example, a water well-shaped plot is obtained for the diastereomeric excess (de) of the deposited amide versus the solvent permittivity (ε) for the crystallization of (RS(a),S)-1 from three-component mixed solvents, consisting of 25 vol % of dichloromethane and 75 vol % of varying ratios of two solvents (i.e., an alcohol and either hexane or water). The de value drastically changes within two narrow ε ranges and diastereomerically pure crystals of either (R(a),S)-1 (13.9 ≤ ε ≤ 17.9) or (S(a),S)-1·CH(2)Cl(2) (ε ≤ 11.9 and ε ≥ 21.8) deposit, depending on the solvent permittivity. X-ray crystallographic analyses reveal that the major difference between the crystal structures of (S(a),S)-1 and (R(a),S)-1 is the presence of solvent molecules that fill the spatial voids in the (S(a),S)-1 crystals. The ε-dependence of the chemical shifts of (S(a),S)-1 and (R(a),S)-1 suggests that their aggregation states are similar in the same solvents and change discontinuously at two ε values. The ε-dependence of the C═O stretching vibrations suggests that the lower ε is a transition point where the amide molecules, which aggregate through intermolecular hydrogen bonds in low-permittivity solvents, begin to dissociate. An absorption experiment suggests that dichloromethane is easily incorporated into solvent-free (S(a),S)-1 crystals in high-permittivity solvents. On the basis of these observations, a feasible molecular mechanism is proposed for the present DCR phenomenon.