Application of Crystallization Inhibitors to Chiral Separations. 2. Enhancing the Chiral Purity of Mandelic Acid by Crystallization R. K. Mughal,† R. J. Davey,*,† and S. N. Black‡ The Molecular Materials Centre, School of Chemical Engineering and Analytical Sciences, The UniVersity of Manchester, SackVille Street, Manchester M60 1QD, and Process Engineering Group, PR&D, Silk Road Business Park, AstraZeneca, Charter Way, Macclesfield, Cheshire SK 10 2NA, UK
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 2 225-228
ReceiVed August 24, 2006; ReVised Manuscript ReceiVed October 25, 2006
ABSTRACT: In part 1 of this work, the successful design of additives for the selective inhibition of (R,S)-mandelic acid crystallization was described. Now, in part 2 of our study, we show how such additives may be used, together with the known ternary phase diagram, as the basis for a crystallization process that yields a chirally enriched product. Introduction It is well-known that the use of crystallization for separating chiral molecules from racemic solutions is limited due to the tendency toward the formation of racemic crystalline forms (racemic compounds).1 This problem is traditionally circumvented through the addition of chiral acids or bases and the resulting formation of diastereomeric salts that can be separated on the basis of their differing solubilities.2 However, as we have shown in part 1 of this work (the preceding paper in this issue), it is possible to design and select crystallization additives that are more effective inhibitors for the crystallization of a racemic compound than of its pure enantiomers. This suggests two possible alternative means of using direct crystallization to effect a chiral separation. On the one hand, during crystallization from a racemic solution if the crystallization of a compound can be sufficiently inhibited by an additive then this may allow the nucleation of a conglomerate (much in the same way as additives can be used to direct crystallization in a polymorphic system).3 In the mandelic acid (MA, Scheme 1)-water system described in part 1, we found no evidence for this outcome. This result is consistent with recent spectroscopic evidence relating to the solution chemistry of MA.4 Alternatively, if crystallization were initiated from a starting solution that already had an enantiomeric excess of one isomer, then this excess might be enhanced by inclusion of an additive that could selectively inhibit subsequent crystallization of a racemic compound. It is this latter possibility that we explore in this part 2 of our study. In particular, we show how, in the ternary system (R)-MA, (S)-MA, and water, it is possible to utilize this effect to produce a crystallized material with an enhanced enantiomeric excess. This work relies extensively on the availability of the ternary phase diagram for this system as reported previously5 and shown here both schematically and specifically in Figure 1. Figure 1a shows a schematic, isothermal, ternary phase diagram for a compound forming chiral system of two enantiomers plus a solvent. The vertices represent the pure components involved: thus, the (S)and (R)-apices represent systems comprising 100% pure enantiomers, while the top corner is 100% solvent. Points within the triangle then represent systems of specific compositions. If this diagram is used to represent the MA-water phase diagram, the solubility of the isomers in pure water at the chosen temperature is given by points on the left and right sides of the * To whom correspondence should be addressed. † The University of Manchester. ‡ Process Engineering Group, PR&D.
triangle. The area within the triangle can then be divided into a number of domains depending on whether, at equilibrium, there are solids present and depending on the composition of the solids. Thus, mixtures in the green region comprise single phase solutions. On reducing the water content, a number of regions emerge in which solids and solutions are in equilibrium. In the blue regions, crystals of pure enantiomers are in equilibrium with saturated solutions having compositions on the white line. In the red region, the racemic compound is in equilibrium with solutions of compositions on the yellow line. Since the physical properties of both enantiomers are identical, the diagram is symmetrical. There are two invariant (eutectic) points and corresponding yellow regions in which mixtures of pure enantiomer and racemic compound crystals are in equilibrium with solutions of fixed composition. It follows that crystallization in the blue and red regions would only yield one solid form, i.e., pure enantiomer or racemic crystals respectively, while crystallization in the yellow region will yield products that are mixtures of pure enantiomer and the racemic compound crystals. From a given starting composition, the final solution phase in equilibrium with such mixed solid phases has only one possible composition, and hence the equilibrium chiral purity of the solid product can be calculated. Choice of Crystallization Conditions. The measured ternary phase data for MA in water is shown in Figure 1b and is based on the data reported by Lorenz et al.5 The data for 25, 30, and 40 °C are superimposed onto a single triangle. It is clear that with decreasing temperatures solubilities fall, and the eutectic composition moves toward the water apex enlarging the threephase region. However, unusually the enantiomeric ratio in the eutectic composition is constant over this temperature range. Using this phase information, various crystallization pathways can be visualized. For example, starting at a given composition at 40 °C and cooling to 25 or 30 °C, one can choose the region in the phase diagram where crystallization will take place. The three starting compositions used in this work are marked 1, 2, and 3 in Figure 1b. Figure 1c is a version of this ternary diagram with temperature as the vertical axis and designed specifically to show these crystallization experiments. All three compositions (1, 2, and 3) start at 40 °C and are cooled to 30 or 25 °C resulting in six crystallization experiments. It can be seen that, although for a given crystallization temperature the three experiments start with different initial compositions, the liquid phases equilibrate to the same invariant (eutectic) point in the phase diagram.
10.1021/cg060565s CCC: $37.00 © 2007 American Chemical Society Published on Web 01/05/2007
226 Crystal Growth & Design, Vol. 7, No. 2, 2007
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Figure 1. (a) The ternary phase diagram of a racemic compound forming system, (b) (R)-MA, (S)-MA, and water system (mass fractions) redrawn from Lorenz et al.5 (c) crystallization pathways. Initial compositions and the crystallization pathways of all three experiments (labeled 1, 2, and 3).
Scheme 1.
Molecular Structures of (RS)-MA and (RS)-2-PPA
Crystallization Control Part 1 of these two papers has discussed the crystallization of MA in some detail and in particular has shown how certain additives can be selected that inhibit the crystallization of the compound and yet have little impact on the pure enantiomer. In this current aspect of the work, one of the most active of these additives, 2-phenylpropionic acid (PPA, Scheme 1) was incorporated into the six experiments defined above. Since the final crystalline product should be a mixture of pure enantiomer and racemate, it was hoped that the presence of additives would enhance the enantiomeric excess of this product by preventing crystallization of the racemate. Experimental Procedures Crystallizations were carried out in which the impact of 3% additive on the time-resolved evolution of chiral purity of the product was compared to equivalent experiments with no additive. For each
composition, duplicate crystallization experiments were performed at both 25 and 30 °C. (R,S)-MA (99%), S-(+)-MA (99+%), and (R,S)2-phenylpropionic acid, (97%) were all obtained from Sigma Aldrich and used as supplied with crystallizations carried out in distilled water. Experiments were performed on a 10 g scale in a sealed, jacketed, glass crystallization vessel whose temperature was controlled within (0.1 °C using a Haake circulating water bath. All the solutions (even after crystallization) were agitated using a magnetic stirrer and follower. Table 1 provides the starting compositions of each experiment. Experiment 1 started as a slurry of pure S-enantiomer at 40 °C. After equilibrating for 1 h, this slurry was crash cooled to the chosen crystallization temperature. Experiments 2 and 3 started as solutions that were prepared at 70 °C and stirred for 1-2 h to fully dissolve the starting solids. The solutions were again crash cooled to the chosen crystallization temperature, and the induction time for the appearance of the first crystals was recorded. In all experiments, solid samples were taken at the onset of crystallization and daily over an equilibration period of up to 9 days. The chiral purity of the isolated solids was determined using a combination of NMR, gas chromotography (GC), and differential scanning calorimetry (DSC). From the DSC6 (TA DSC Q100 with auto sampler and refrigerated cooling system, controlled by a computer running TA Instruments Universal Analysis 2000 software) of products the melting of the pure enantiomer and the racemic compound could be discriminated. From the magnitude of these endotherms, and given the known two component phase diagram,1 the compositions of the mixtures were evaluated. The estimated maximum error (arising from calculating compositions and sample preparation) was 5%. For GC7 (Hewlett-Packard, models HP6850 & 6890, with a
Crystallization Inhibitors for Chiral Separations
Crystal Growth & Design, Vol. 7, No. 2, 2007 227
Table 1. Starting Compositions (as Mass Fractions), Supersaturations, Calculated Enantiomeric Excesses, and Maximum Yields (10 g Scale) for Experiments with Compositions 1, 2, and 3 composition 1
composition 2
composition 3
0.45 0.42 0.13 3.41 1.41 57.4
0.57 0.33 0.10 1.67 0.47 63.6
0.40 0.42 0.18 4.31 1.91 40.9
61.0
79.6
39.6
4.23
2.69
4.87
3.18
1.37
3.94
XWater X(S)-MA X(R)-MA σ25°C σ30°C calculated %ee of final solid at 25 °C calculated %ee of final solid at 30 °C max theoretical yield (25 °C) (g) max theoretical yield (30 °C) (g)
25 m × 0.25 mm Chirasil Dex CB capillary column, Varian, Inc.) only volatile samples can be analyzed. MA was therefore derivatized as a methyl ester triflate. DSC and GC results agreed within e1%. A selected number of product samples were further analyzed by NMR8 (400 MHz Bruker spectrometer). To determine the enantiomeric purity, the samples were examined in the presence of a chiral complexing reagent (-)-2,2,2-trifluoro (9-anthryl) ethanol, {(-)-TFAE}. For the three compositions chosen and for each crystallization temperature, Table 1 gives the maximum amount of solid that could be crystallized, the calculated equilibrium composition of this solid [i.e., %enantiomeric excess (ee), defined by eq 19] and the supersaturation (defined by eq 2).
ee )
|R - S| × 100 ) |%R - %S| R+S
(1)
The composition and %ee of the crystallized solid were calculated on the assumption that (R)-MA limits [as (S)-MA is in excess] the amount of (RS)-MA that can be crystallized and that the remaining (S)-MA would crystallize as the pure enantiomer. Supersaturation was calculated from the total concentration of MA in the starting solutions used (css) and the concentration of MA in a solution at the invariant point (ceq) at 25 and 30 °C, since this is the final solution composition in these experiments. The relative supersaturation, σ, is then calculated as
σ)
css - ceq ceq
(2)
with all concentrations in gsolute/100 gsolvent).
Results and Discussion Figure 2 a-c shows representative data for the three starting compositions crystallized at 30 °C. Overall, it can be seen clearly that the addition of 3% PPA results in solid products of higher enantiomeric excess both at the onset of crystallization and well into the equilibration period. The use of additives increased the observed crystallization induction times typically from a few tens of minutes to a few hours and also led to lower yields (as
Figure 2. The evolution of chiral purity over the crystallization and equilibrating periods at 30 °C for (a) composition 1, (b) composition 2, and (c) composition 3.
low as 50% compared to 85-90% in pure solutions) than in crystallizations with no additives. Looking at when the various experiments reached the equilibrium composition, it can be seen (Figure 2) that starting compositions 1 and 3, both with and without additive had reached the equilibrium composition by day 9. However, for the crystallization of composition 2 in the presence of 3% PPA the equilibrium composition was not reached over the entire equilibration period. This is presumably the result of the lower supersaturation that prevails at this composition. This pattern was repeated for crystallizations performed at 25 °C, and Table 2 summarizes the outcome of the experiments
Table 2. Summary of the Maximum Increases in Enantiomeric Excess for All Experiments
experimental details
supersaturation (σ)
calc equilibrium % ee
measured initial % ee
maximum % ee increase (measured - calculated)
experiment 1 - 3% PPA at 30 °C experiment 1 - 3% PPA at 25 °C experiment 2 - 3% PPA at 30 °C experiment 2 - 3% PPA at 25 °C experiment 3 - 3% PPA at 30 °C experiment 3 - 3% PPA at 25 °C
3.41 1.41 1.67 0.47 4.31 1.91
61 57.4 79.6 63.6 39.6 40.9
80.2 65.5 97.3 93.8 81.3 76.9
19.2 8.1 17.7 30.2 41.7 36.0
228 Crystal Growth & Design, Vol. 7, No. 2, 2007
in terms of the maximum enhancements in enantiomeric excess. It is clear that starting composition 3, crystallized at either temperature, gives the best outcome of the three compositions chosen. This is an indication that increasing supersaturation favors the initial nucleation of the pure enantiomer. Taken overall, these data show that the additive is, as planned in part 1 of this work, inhibiting the crystallization of the racemate while allowing the pure enantiomer to crystallize. As expected, this inhibition of the racemate by the additive results in lower yields. It can also be seen (Figure 2b,c) that even in the absence of additives, the initial products from all starting compositions showed an enantiomeric enrichment (compared to the calculated ee) which decayed with time as the system reached equilibrium. This indicates that from these starting compositions the pure enantiomer nucleates preferentially, this enhancement increasing with increasing initial supersaturation. Conclusions Crystal chemistry, phase equilibria data, and crystallization in the presence of additives have all been used in combination to enhance the enantiomeric excess of a chiral product. It has been successfully shown that by rational additive design the crystallization of the racemate, compared to the pure enantiomer, has been strongly inhibited and improvements of up to approximately 40% in enantiomeric excess were achieved. This
Mughal et al.
approach clearly offers potential process options for separation of chiral molecules. Interestingly, it appears that at compositions near the eutectic, where the supersaturation with respect to racemic compound and single enantiomer are very similar, it is the pure enantiomer that nucleates preferentially even in the absence of additives. References (1) Jacques, J.; Collett, A.; Wilsen, S. H. Enantiomers, Racemates, and Resolutions; John Wiley and Sons Inc.: New York, 1994. (2) Wood, W. M. L. In Chirality in Industry II. DeVelopments in the Commercial Manufacture and Application of Optically ActiVe Compounds; Collins, A. N., Sheldrake, G. N., Crosby, J., Eds.; John Wiley & Sons, Chichester, UK, 1997. (3) Davey, R. J.; Blagden, N.; Potts, G. D.; Docherty, R. J. Am. Chem. Soc. 1997, 119, 1767-1772. (4) Davey, R. J.; Dent, G.; Mughal, R. K.; Parveen, S. Cryst. Growth Des. 2006, 6, 1788-1796. (5) Lorenz, H.; Seidel-Morgenstern, A. Thermochim. Acta 2002, 382, 129. (6) Brittain, H. G. Physical Characterisation of Pharmaceutical Solids; Marcel Dekker, Inc.: New York, 1995; Chapter 8, pp 223-251. (7) Subramanian, G. Chiral Separation Techniques: A Practical Approach, 2nd ed.; Wiley-VCH: Weinheim, 2001. (8) Hanna, G. M. Enantiomer 2000, 5, 303-312. (9) Carey, F. A. Organic Chemistry, 2nd ed.; McGraw-Hill: New York, 1992.
CG060565S
