FULL PAPER Pre-Nucleation Self-Assembly and Chiral Discrimination Mechanisms during Solution Crystallization of Racemic Diprophylline

The crystallization behavior of (RS)-Diprophylline (DPL) in two different solvents is investigated in order to assess the incidence of solvated pre-associations on nucleation, crystal growth and chiral discrimination. In the solvated state, Raman spectroscopy shows that dimeric associations similar to those depicted in the crystalline solid solution (ssRII) predominate in isopropyl alcohol (IPA), which may account for the systematic spontaneous nucleation of this crystal form from this solvent. By contrast, spontaneous nucleation in dimethylformamide yields the stable racemic compound RI, consistently with the distinct features of the Raman spectrum collected in this solvent. A crystal growth study of ssRII in IPA reveals that the crystal habitus is impacted by the solution enantiomeric excess which is explained by an increased competition between homo and heterochiral pre-associations. This is supported by a molecular modelling study on the enantiomeric selectivity of the DPL crystal lattices. The combination of assessment methods on solution chemistry, nucleation and chiral discrimination provides methodological tools from which the occurrence of solid solutions can be rationalized.