Enantiopure <i>O</i>-Substituted Phenylphosphonothioic Acids: Chiral Recognition Ability during Salt Crystallization and Chiral Recognition Mechanism

The chiral recognition ability of enantiopure O-methyl, O-ethyl, O-propyl, and O-phenyl phenylphosphonothioic acids (1a−d) for various kinds of racemic amines during salt crystallization and the chiral recognition mechanism were thoroughly investigated. The chiral recognition abilities of enantiopure 1a−d for a wide variety of racemic amines varied in a range of 6 to >99% enantiomeric selectivity. Deposited less-soluble diastereomeric salts were classified into two categories, prism- and needle-type crystals; the prism-type crystals were composed of a globular molecular cluster, while there existed a 21 column in the needle-type crystals. In contrast to a general observation of a similar 21 column in the less-soluble diastereomeric salt crystals of chiral primary amines with chiral carboxylic acids, the globular molecular cluster is a very unique hydrogen-bonding motif that has never been constructed in diastereomeric salt crystals. Excellent chiral recognition was always achieved when the less-soluble diastereomeric salts were prism-type crystals. Significant correlations were found between the degree of the chiral recognition with 1a−d, the crystal shape of the less-soluble diastereomeric salts, and the hydrogen-bonding motif (molecular cluster/21 column). The chiral recognition mechanisms via the molecular cluster and the 21 column formations are discussed in detail on the basis of X-ray crystallographic analyses.