An improved and practical approach to essentially enantiopure BINOLs: enantioselective inclusion complexation of (S)-proline

An improved and practical approach to (R)- and (S)-BINOLs has been developed. The rac-BINOL and (S)-proline were refluxed in acetonitrile for several hours, and the resulting white precipitate was recrystallized in ethanol to afford colorless crystals consisting of (S)-BINOL and (S)-proline, which were analyzed by single crystal X-ray structural analysis. Essentially enantiopure (S)- and (R)-BINOLs were obtained in high yields after decomposition of the colorless crystalline complex and evaporation of the acetonitrile mother liquor removed from the complex precipitate and successive crystallization. The (S)-proline can be recovered and reused without any decrease in efficiency.

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(S)-BINOLC20H14O2ee = 99.98% ee[α]D20=-35.4 (c 1, THF)Source of chirality: chiral separationAbsolute configuration: (S)