Crystal Structures of the Salts of Chiral Primary Amines with Achiral Carboxylic Acids:  Recognition of the Commonly-Occurring Supramolecular Assemblies of Hydrogen-Bond Networks and Their Role in the Formation of Conglomerates

Abstract

X-ray crystallographic studies were carried out for seven conglomerates, seventeen racemic compounds, and four enantiomerically pure salts of chiral primary amines with achiral monocarboxylic acids. The crystal structures of the conglomerate salts revealed that these crystals can be regarded as being an assembly of a characteristic columnar hydrogen-bond network in which the ammonium cations and the carboxylate anions are aligned around a 2-fold screw axis (21-column). On the other hand, the crystal structures of the racemic-compound salts could be broadly classified into two types. One type is a crystal consisting of 21-columns; the other type is a crystal consisting of a different type of columnar hydrogen-bond network in which the ammonium cations and the carboxylate anions are related by inversion centers (i-column). Our results suggest that both the formation and the assembly of 21-columns are essential in the formation of conglomerates from these salts, and that the difference in the packing of 21-columns between conglomerate salts and racemic-compound salts is governed by van der Waals interaction between the 21-columns.

Chiral discrimination of 2‐arylalkanoic acids by (1S,2S)‐1‐aminoindan‐2‐ol and (1S,2S)‐2‐aminoindan‐1‐ol: Correlation of the relative configuration of the amino and hydroxy groups with the pattern of a supramolecular hydrogen‐bond network in the less‐soluble diastereomeric salt

“…12,13 Contrary to our expectation, the hydroxy group of (1S,2S)-2, however, forms another kind of hydrogen bond with the carboxylate oxygen within a column to reinforce the columnar structure. Thus, in this case, not a hydrogen-bond sheet but a reinforced hydrogen-bond column is formed in the less-soluble salt.…”

“…The infrared spectra were recorded on a Jasco IR-810 spectrophotometer, and the 1 H and 13 C NMR spectra were measured on a Varian Mercury-300 instrument using tetramethylsilane as an internal standard. Analytical HPLC was performed using Daicel Chiralcel OJ, OB, OD (eluent: hexane/2-propanol), or OJ-R (eluent: 0.5 M H 3 PO 4 -KHP 2 O 4 buffer (pH 2)/CH 3 CN) columns with the detector wavelength at 254 nm.…”

Chiral Symmetry Breaking Phenomenon Caused by a Phase Transition

“…As the typical example of this latter category, well known is the "preferential crystallization" method to resolve a racemic conglomerate composed of a mixture of homochiral R and S crystals, in which by repeating crystallization from the supersaturated solution with the aid of its enantiopure seed crystals the enantiomerically enriched crystals are efficiently deposited in the alternating chirality sense [15][16][17][18]. However, the racemates existing as a racemic conglomerate occupy only less than 10% of the characterized crystalline racemates, and more than 90% of them are supposed to belong to racemic crystals [19], which are further classified into either a racemic compound consisting of a regular packing of a pair of R and S enantiomers or a racemic mixed crystal (in other words, a pseudoracemate or a solid solution) composed of a random alignment of the two enantiomers in the defined positions ( Figure 2) [9].…”

Polarization-Modulated Infrared Reflection Absorption Spectroscopic Studies of a Hydrogen-Bonding Network at the Air−Water Interface

“…Numerous artificial molecular recognition systems can be designed by incorporating complementary hydrogen bond donors and acceptors into host and guest molecules. [1][2][3][4][5][6][7][8] These artificial systems function as convenient and direct models to mimic complicated biological activities. Recently, these studies have been further expanded to the airwater interface.…”