Racemic compound formation–conglomerate formation. Part 3. Investigation of the acidic salts of α-phenylethylamine by achiral dicarboxylic acids. Optical resolution by preferential crystallization and a structural study of (R)-α-phenylethylammonium hydrogen itaconate

Abstract

Acidic salts of eight achiral dicarboxylic acids with α-phenylethylamine are prepared and investigated by IR and density measurements, and X-ray crystallography to distinguish between racemic compound and conglomerate formation. It was found that conglomerate formation takes place when the protonated and deprotonated carboxylic groups form hydrogen bonded chains rather than forming cyclic intramolecular hydrogen bonds. Racemic compound formation, and indeed intramolecular H-bond formation, seems to be preferred when the two carboxylates are constrained in the same plane close to each other by a double bond, or they are ortho-substituents of a planar ring. The rules found for conglomerate formation of acidic salts of α-phenylethylamine are compared with Saigo's rules for conglomerate formation. The crystal structure of the conglomerate forming (R)-α-phenylethylammonium hydrogen itaconate is (OARPHI) also presented and the optical resolution by preferential crystallization is described. The hydrogen bonding network of OARPHI [N₁=C(7)C₂²(6), N₂=R₃³(8)R₅⁵(22)] is very similar to the hydrogen bonding network of the other conglomerate forming salt of this series: α-phenylethylammonium hydrogen succinate [N₁=C(7)C₂²(9), N₂=R₃³(8)R₃³(13)].