pubs.acs.org/crystal Published on Web 07/27/2010 r 2010 American Chemical Society DOI: 10.1021/cg100605t 2010, Vol. 10 4005–4013 Phase Diagram of a Chiral Substance Exhibiting Oiling Out in Cyclohexane Lorenzo Codan, Matth€aus U. B€abler, and Marco Mazzotti* Institute of Process Engineering, ETH Zurich, Sonneggstrasse 3, CH-8092 Zurich, Switzerland Received May 6, 2010; Revised Manuscript Received June 12, 2010 ABSTRACT: This work investigates the ternary phase diagram of the enantiomers of ethyl-2-ethoxy-3-(4-hydroxyphenyl)- propanoate (EEHP) in cyclohexane. The enantiomers of EEHP form a conglomerate, and both the pure enantiomer and the racemic mixture exhibit stable oiling out in cyclohexane. Our analysis shows that the ternary phase diagram of such a system assumes a unique structural evolution around the onset temperature of oiling out; that is, we found that the onset of oiling out strictly occurs through the emergence of a second liquid phase of racemic composition. Furthermore, we found that the further evolution of the ternary phase diagram, i.e., above the onset temperature of oiling out, is dictated by the properties of the phase diagrams pure enantiomer/solvent and racemic mixture/solvent. Our theoretical considerations are in excellent agreement with experimental measurements of EEHP in cyclohexane. 1. Introduction Oiling out refers to the appearance of a second liquid phase during an attempt to crystallize a compound from solution. 1,2 The underlying phenomenon is a liquid-liquid phase separa- tion (LLPS), which under this terminology is a well-known phenomenon in the field of protein crystallization that is exhi- bited by several experimental systems. 3-11 With respect to these works, relatively few studies have focused on oiling out occur- ring in systems of small organic molecules. Oiling out often occurs for substances having a low-melting point when in a sol- vent exhibiting low solubility. 2 Cases of binary systems exhibit- ing LLPS have been studied by Sv€ard et al. 1 and Groen and Roberts, 12 whereas Bonnett et al., 13 Veesler et al., 14 Deneau and Steele, 15 Lai et al., 16 and Kiesow et al. 17 report LLPS occurring in systems involving solvent mixtures. One of the key purposes of crystallization of small organic molecules is their purification. Thereby, LLPS prior to crystal- lization is unwanted, as the solute rich phase is typically a good solvent for impurities. The crystallization of the solute rich phase yields, in fact, impure crystals. 15 Also, design and scale up of the process is difficult, since the criterion of constant power input gives emulsions consisting of smaller droplets. 14,16 Furthermore, oiling out leads to the formation of droplets around the crystal surface, which slows down crystal growth and decreases crystal quality. 16 Bonnett et al. 13 and Lafferrere et al. 18 report the formation of highly agglomerated quasi-spherical particles as crystallization occurred in the dense liquid droplets. In tune with the Ostwald rule of stages, oiling out was found to slow down subsequent crystallization, as the first formed metastable liquid phase hinders primary and secondary nucleation. 18,19 Thus, for purification purposes, the aim is to avoid oiling out. Strategies to avoid oiling out include seeding according to some detailed protocols or choosing operation conditions out- side the region where oiling out occurs. In particular, seeding was shown to be a good method to avoid oiling out for systems admitting metastable LLPS. 13,15,20 For both strategies, however, knowledge of the phase diagram is indispensable. Regarding small chiral organic molecules, so far, no systema- tic studies have dealt with systems exhibiting oiling out, although a high number of API and their intermediates are chiral 21 and cases where oiling out occurs are known. 22 Driven by the strict purity requirements of the regulatory agencies, that include also enantiomeric purity, crystallization has become established as a valid pathway to obtain enantiomeric resolution. Besides classi- cal preferential crystallization and diastereomeric resolution, 23 also the hybrid process combining simulated moving bed chro- matography (SMB) with crystallization 24-26 has gained in inte- rest. The applicability of such resolution techniques is restricted if the system undergoes a LLPS. A better understanding of oiling out is also in this case necessary. In this work, the generic phase diagram of a ternary system consisting of two conglomerate forming enantiomers and a pure solvent is derived for the case where both the pure enantiomer and the racemic mixture exhibit stable oiling out. We show that in the low temperature range the shape of the phase diagram is unique whereas in the high temperature range it is determined by the sequence on the temperature axis of a few key events. Our findings are applied to construct the com- plete phase diagram of ethyl-2-ethoxy-3-(4-hydroxyphenyl)- propanoate (EEHP) (Figure 1) in cyclohexane. EEHP is a chiral compound that forms conglomerates and that undergoes stable oiling out in cyclohexane. Its S-enantiomer is an inter- mediate of Tesaglitazar, an active compound for the treatment of type II-diabetes. 27 2. Materials and Methods 2.1. Materials. Purified enantiomers of EEHP were supplied by AstraZeneca (S€ odert€alje, Sweden). Cyclohexane of purity g99.9%, supplied by Sigma-Aldrich (Buchs, Switzerland), was used for all phase equilibrium measurements. The eluent for HPLC-analysis was a mixture of hexane with a purity of g98% supplied by Merck (Darmstadt, Germany) and ethanol with a purity of g99.9% supplied by Scharlau (Sentmenat, Spain). 2.2. Determination of the Binary Phase Diagram. The liquidus and the solidus lines were determined using differential scanning calori- metry (DSC) (Mettler DSC822 e , Mettler Toledo GmbH, Schwer- zenbach, Switzerland). At first, powder mixtures of different com- positions of the two enantiomers were ground in a mortar and *To whom correspondence should be addressed. E-mail: marco.mazzotti@ ipe.mavt.ethz.ch. Phone: þ41 44 632 24 56. Fax: þ41 44 632 11 41.