Crystallization of chiral compounds: thermodynamical, structural and practical aspects

Chirality determination in crystals

This tutorial review article discusses chirality determination in the solid state, both in single crystals and in crystal assemblies, with an emphasis on X-ray diffraction. The main principles of using X-ray diffraction to reliably determine absolute structure are summarized, and the complexity which can be encountered in chiral structures-kryptoracemates, scalemates, and inversion twinning-is illustrated with examples from our laboratories and the literature. We then address the problem of the bulk crystallization and discuss different techniques to determine chirality in a large assembly of crystal structures, with a special prominence given to an X-ray natural circular dichroism mapping technique that we recently reported.

Stereoselective Crystallization of Chiral 3,4-Dimethylphenyl Glycerol Ether Complicated by Plurality of Crystalline Modifications

Spontaneous resolution of Pasteur’s salt was historically the first way to obtain pure enantiomers from the racemate. The current increase in interest in the direct racemates resolution during crystallization is largely due to the opened prospects for the industrial application of this approach. The chiral 3-(3,4-dimethylphenoxy) propane-1,2-diol 1 is a synthetic precursor of practically useful amino alcohols, the enantiomers of which exhibit different biological effects. In this work, it was first discovered that racemic diol 1 is prone to spontaneous resolution. However, the crystallization process is complicated by the existence, along with the conglomerate, of two other crystalline forms. Using the differential scanning calorimetry (DSC) approach, methods have been developed to obtain individual metastable phases, and all identified modifications ((R)-1, (R+S)-1, α-rac-1, β-rac-1) were ranked by energy. The IR spectroscopy and powder X-ray diffraction (PXRD) methods demonstrated the identity of the first two forms and their proximity to the third, while β-rac-1 is significantly different from the rest. The crystal structure of the forms (R)-1 and α-rac-1 was established by the single crystal X-ray diffraction (SC-XRD) method. Preliminary information on the structure of β-rac-1 phase was obtained by the PXRD approach. Based on the information received, the experimental conditions for a successful direct resolution of racemic 1 into individual enantiomers by a preferential crystallization procedure were selected.

Thermal properties, crystal structures, and phase diagrams of ionic plastic crystals and ionic liquids containing a chiral cationic sandwich complex

Salts of a chiral ruthenium sandwich complex with various anions were synthesized and their phase diagrams were investigated.

Intermolecular Interactions in Functional Crystalline Materials: From Data to Knowledge

Intermolecular interactions of organic, inorganic, and organometallic compounds are the key to many composition–structure and structure–property networks. In this review, some of these relations and the tools developed by the Cambridge Crystallographic Data Center (CCDC) to analyze them and design solid forms with desired properties are described. The potential of studies supported by the Cambridge Structural Database (CSD)-Materials tools for investigation of dynamic processes in crystals, for analysis of biologically active, high energy, optical, (electro)conductive, and other functional crystalline materials, and for the prediction of novel solid forms (polymorphs, co-crystals, solvates) are discussed. Besides, some unusual applications, the potential for further development and limitations of the CCDC software are reported.

“Doubly enantiophobic” behavior during crystallization of racemic 1,5-dihydro-2H-pyrrol-2-one thioether

One racemate forms two conglomerates and no racemic compound.

Structural aspects of partial solid solution formation: two crystalline modifications of a chiral derivative of 1,5-dihydro-2H-pyrrol-2-one under consideration

Two opposite structural factors control the formation of a partial solid solution of the enantiomers of the titled compound.

Non-equimolar discrete compounds in binary chiral systems of organic substances

1 : 3 and 1 : 2 discrete compounds are verified in two chiral systems and discussed with respect to known cases in the literature.

Prenucleation Self-Assembly and Chiral Discrimination Mechanisms during Solution Crystallisation of Racemic Diprophylline

The crystallisation behaviour of (RS)-diprophylline (DPL) in two different solvents is investigated 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 isopropanol (IPA), which may account for the systematic spontaneous nucleation of this crystal form from this solvent. By contrast, spontaneous nucleation in DMF 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; this is explained by 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 rationalised.