Effects of solvent on inclusion complexation of a chiral dipeptide toward racemic BINOL

The effects of reaction solvent on inclusion complexation of a chiral dipeptide (3S,6S)-1 derived from (S)-proline toward racemic BINOL was investigated, discovering that the reaction solvent played a crucial role in determining the inclusion complexation behavior of dipeptide (3S,6S)-1 toward rac-BINOL. (3S,6S)-1 did not show any chiroselective or achiroselective complexation toward rac-BINOL in polar protic solvents such as methanol and ethanol, polar aprotic solvents including trichloromethane and THF, while in polar aprotic solvent ethyl acetate and apolar aprotic solvents benzene, (3S,6S)-1 displayed achiroselective complexation toward rac-BINOL. However, the resulting heterocomplex HC-2 from benzene and HC-3 from ethyl acetate have a different composition. Single crystal X-ray diffraction analysis demonstrates that the two heterocomplexes are formed via different H-bond interaction patterns, in which the reaction solvent has a dramatic effect. Furthermore, this work provides a relatively green method for quantitative enantiomeric enrichment of nonracemic BINOL, in which unacceptable and toxic benzene was replaced by ethyl acetate.