Chiral recognition in salts of trans-1,2-diaminocyclohexane and optically active tartaric acids: crystal structure of 1:2 salt of ( S, S)-diaminocyclohexane with ( R, R)-tartaric acid

• Published in: Journal of molecular structure Vol. 474; no. 1; pp. 235 - 243
• Main Author: Rychlewska, Urszula
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 1999
• Subjects: Chiral recognition; Mode of acid/amine self assembly; Natural tartaric acid in its three ionisation states; Tartaric acid/diaminocyclohexane superstructure; Three-dimensional hydrogen bonding; Tartaric acid/diaminocyclohexane superstructure; Mode of acid/amine self assembly; Natural tartaric acid in its three ionisation states; Chiral recognition; Three-dimensional hydrogen bonding
• ISSN: 0022-2860; 1872-8014
• DOI: 10.1016/S0022-2860(98)00577-8

X-ray results for the highly organised superstructure (space group P6 522) of the 1:2 salt of (1 S,2 S)-diaminocyclohexane with ( R, R)-tartaric acid ( 1) are compared with the previously reported [1] crystal structures of two diastereomeric 1:1 salts of ( R, R)-diaminocyclohexane with ( R, R)- and ( S, S)-tartaric acid. The asymmetric unit of the unit cell of 1 consists of one molecule of ( S, S)-diaminocyclohexane, one molecule of ( R, R)-hydrogentartrate, half a molecule of ( R, R)-tartaric acid, half of an ( R, R)-tartrate anion, and a water molecule. The presence of all three ionisation states of ( R, R)-tartrate in one crystal is most unusual, as is the anti conformation of the carboxyl group in ( R, R)-tartaric acid. Association of ( S, S)-diaminocyclohexane and ( R, R)-hydrogentartrate involves three-centre hydrogen bonds between ammonium groups as donors and α-hydroxycarboxylate and α-hydroxycarboxyl moieties as acceptors. The remaining hydrogen bonds stabilise this motif and expand it to three dimensions. The results provide some insights into the enantioselective association of trans-1,2-diaminocyclohexane with optically active tartaric acids, the main driving force being the hydrogen bonding. While in a homochiral 1:1 salt the two ions combine by a scissors-shaped hydrogen bonding motif formed by two ammonium functions as donors and vicinal hydroxyl groups as acceptors, in heterochiral salts the cation and anion species join together via three-centre hydrogen bonds between ammonium functions and planar or nearly planar α-hydroxycarboxylate moieties.