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.

X-ray results for the highly organised superstructure (space group P6(5)22) of the 1:2 salt of (1S,2S)-diaminocyclohexane with (R,R)-tartaric acid (1) are compared with the previously reported [1] crystal structures of two di astereomeric 1:1 salts of (R,R)-diaminocyclohexane with (R,R)- and (S,S)-ta rtaric acid. The asymmetric unit of the unit cell of 1 consists of one mole cule of (S,S)-diaminocyclohexane, one molecule of (R,R)-hydrogentartrate, h alf 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)-tar trate in one crystal is most unusual, as is the anti conformation of the ca rboxyl group in (R,R)-tartaric acid. Association of (S,S)-diaminocyclohexan e and (R,R) hydrogentartrate involves three-centre hydrogen bonds between a mmonium groups as donors and alpha-hydroxycarboxylate and alpha-hydroxycarb oxyl moieties as accepters. The remaining hydrogen bonds stabilise this mot if and expand it to three dimensions. The results provide some insights int o the enantioselective association of trans-1,2-diaminocyclohexane with opt ically active tartaric acids, the main driving force being the hydrogen bon ding. While in a homochiral 1:1 salt the two ions combine by a scissors-sha ped hydrogen bonding motif formed by two ammonium functions as donors and v icinal hydroxyl groups as accepters, in heterochiral salts the cation and a nion species join together via three-centre hydrogen bonds between ammonium functions and planar or nearly planar alpha-hydroxycarboxylate moieties. ( C) 1998 Elsevier Science B.V. All rights reserved.