Experimental Data and Modelling of the Interactions in Solid State and in Solution between (R) and (S) N-Acetyl-α- Methylbenzylamine. Influence on Resolution by Preferential Crystallization

Abstract

The binary phase diagram between (R) and (S) N-acetyl-α-methylbenzylaminehas been established by using differential scanning calorimetry and X-ray diffraction. Up to 337 K, the two enantiomers form an eutectic mixture without partial solid solution (i.e. conglomerate). At 337 K, the eutectoid equilibrium: conglomerate⇔racemate (Δ H ≥ 1.25 kJ.mol⁻¹) is observed. Several hypotheses are made about the upper part (T > 340K) of the binary diagram to explain the complex succession of stable and metastable thermal phenomena.

The structures of enantiomer and metastable racemate have been solved by X-ray diffraction on single crystals at 293 K. Infinite Chains of H-bonded Molecules (CHBMs) exist in the two structures: isotactic in enantiomer and syndiotactic in racemate. M.O. calculations (PM3) show that conformation of the molecule in enantiomer structure is more stable by 9.4 kJ.mol⁻¹ than the conformation of the molecule in racemate. The other energy differences(H-bonds and van der Waals contacts) between the two structures do not reverse the order of relative stabilities.

Two isothermal solubility studies at 273 K and 287 K have been carried out in the system: (R)-(S)-toluene and in the system: (R)-(S)-acetonitrile. In toluene, a significant departure from the ideal behaviour appears when temperature increases from 273 K upwards. A hypothesis involving a dynamic equilibrium between solvated CHBMs containing n monomers is postulated, the n value being dependent on the temperature and nature of the solvent. The more the optical purity departs from 100%, the more atactic the CHBMs and the weaker the interactions between these chains. The strength of the interactions between CHBMs versus the optical purity and the temperature seems to be responsible for the departure from the ideal behaviour.

The homogeneous nucleation of the metastable racemate at 293 K, in highly supersaturated racemic solution, is discussed in terms of diffusion of molecules in solution.

Preferential crystallization has been achieved in toluene and acetonitrile, but the difficulties which arose in its implementation appear to be roughly independent of the solvent and are connected to R ⇔ S interactions at the crystal-solution interface as soon as the composition of the solution departs from racemic.

Key Words:

• Enantiomers and racemates
• binary and ternary phase diagrams
• crystal structures
• preferential crystallization
• MO calculations
• interactions in solid state and in solutions