THE ROLE OF SOLVATES IN OPTICAL RESOLUTION - A STUDY OF THE DIASTEREOISOMERIC SALTS FORMED FROM ENANTIOMERIC 2-AMINO-2-PHENYLETHANOL AND (R)-MANDELIC ACID, THEIR CRYSTAL-STRUCTURES AND PHYSICOCHEMICAL PROPERTIES

The reactions between (R)-mandelic acid and the pure enantiomers of 2- amino-2-phenylethanol in water saturated ethyl acetate resulted in the diastereoisomeric salts, (R)-2-hydroxy-1-phenylethylammonium (R)-mand elate hydrate, (R APE)(R-MA).H2O and (S)-2-hydroxy-1-phenylethylammoni um (R)-mandelate, (S-APE)(R-MA). Similar reactions from pure ethyl ace tate gave the latter salt and unhydrated (R-APE)(R-MA). The crystal st ructures at 122 K have been determined for the salts obtained from wat er-saturated ethyl acetate. The less soluble salt, (R-APE)(R-MA) H,O. crystallizes in C2, a = 18.407(2), b = 5.5940(6), c = 15.1863(10) Angs trom, beta = 105.735(6)degrees, Z = 4, R = 0.029 based on 2950 observe d reflections. (S-APE)(R-MA) crystallizes in P1, a = 10.0046(12), b = 10.2466(11), c = 15.869(2) Angstrom, alpha = 100.347(10), beta = 98.77 0(10), gamma = 108.437(9)degrees, Z = 4, R = 0.033 based on 6085 contr ibuting reflections. Five independent cations and anions are found in different conformations. Hydrogen bonding is important for the crystal packing in both structures. The water molecule in (R-APE)(R-MA).H2O p lays an important role linking cations and anions. The solubilities in pure and water saturated ethyl acetate and the thermal behaviour have been investigated for the three compounds (R-APE)(R-MA).H2O, (R-APE)( R-MA) and (S-APE)(R-MA). The difference in thermochemical properties c an be related to the differences in the crystal packing and explain wh y the resolution of 2-amino-2-phenylethanol with (R)-mandelic acid can only be achieved in water-saturated ethylacetate. From the measured s olubilities Delta G(theta) has been calculated for the reaction: (R-AP E)(R-MA) + H2O --> (R-APE) (R-MA).H2O; Delta G(theta) = -3.4 kJ mol(-1 ).