Mechanism of preferential enrichment, an unusual enantiomeric resolution phenomenon caused by polymorphic transition during crystallization of mixed crystals composed of two enantiomers

Article abstract of DOI:10.1021/ja020454r

The mechanism of Preferential Enrichment, an unusual enantiomeric resolution phenomenon observed upon recrystallization of a series of racemic crystals which are classified as a racemic mixed crystal with fairly ordered arrangement of the two enantiomers, has been studied. On the basis of the existence of polymorphs and the occurrence of the resulting polymorphic transition during crystallization from solution, the mechanism has been accounted for in terms of (1) a preferential homochiral molecular association to form one-dimensional chain structures in the supersaturated solution of the racemate or nonracemic sample with a low ee value, (2) a kinetic formation of a metastable crystalline phase retaining the homochiral chain structures in a process of nucleation, (3) a polymorphic transition from the metastable phase to a stable one followed by enantioselective liberation of the excess R (or S) enantiomers from the transformed crystal into solution at the beginning of crystal growth to result in a slight enrichment (up to 10% ee) of the opposite S (or R) enantiomer in the deposited crystals, together with an enantiomeric enrichment of the R (or S) enantiomer in the mother liquor, and (4) a chiral discrimination by the once formed S (or R)-rich stable crystalline phase in a process of the subsequent crystal growth, leading to a considerable enantiomeric enrichment of the R (or S) enantiomer up to 100% ee in the mother liquor. The processes (3) and (4) are considered to be directly responsible for an enrichment of one enantiomer in the mother liquor. The association mode of the two enantiomers in solution has been investigated by means of (i) the solubility measurement and (ii) the number-averaged molecular weight measurement in solution by vapor pressure osmometry, together with (iii) the molecular dynamics simulation of oligomer models. The polymorphic transition during crystallization has been observed visually and by means of the in situ FTIR technique and DSC measurement. Both metastable and stable crystals have been obtained, and their crystal structures have been elucidated by X-ray crystallographic analysis of their single crystals.

Full text of DOI:10.1021/ja020454r

Published on Web 10/12/2002
Mechanism of Preferential Enrichment, an Unusual
Enantiomeric Resolution Phenomenon Caused by
Polymorphic Transition during Crystallization of Mixed
Crystals Composed of Two Enantiomers
Rui Tamura,*,† Daisuke Fujimoto,† Zsolt Lepp,† Kentaro Misaki,† Hideyuki Miura,†
Hiroki Takahashi,† Takanori Ushio,§ Tadashi Nakai,‡ and Ken Hirotsu‡
Contribution from the Gradutate School of Human and EnVironmental Studies, Kyoto UniVersity,
Kyoto 606-8501, Japan, Taiho Pharmaceutical Co., Ltd., Kamikawa-cho,
Kodama-gun, Saitama 367-0241, Japan, and Graduate School of Science, Osaka City UniVersity,
Osaka 558-8585, Japan
Received March 28, 2002
Abstract: The mechanism of Preferential Enrichment, an unusual enantiomeric resolution phenomenon
observed upon recrystallization of a series of racemic crystals which are classified as a racemic mixed
crystal with fairly ordered arrangement of the two enantiomers, has been studied. On the basis of the
existence of polymorphs and the occurrence of the resulting polymorphic transition during crystallization
from solution, the mechanism has been accounted for in terms of (1) a preferential homochiral molecular
association to form one-dimensional chain structures in the supersaturated solution of the racemate or
nonracemic sample with a low ee value, (2) a kinetic formation of a metastable crystalline phase retaining
the homochiral chain structures in a process of nucleation, (3) a polymorphic transition from the metastable
phase to a stable one followed by enantioselective liberation of the excess R (or S) enantiomers from the
transformed crystal into solution at the beginning of crystal growth to result in a slight enrichment (up to
10% ee) of the opposite S (or R) enantiomer in the deposited crystals, together with an enantiomeric
enrichment of the R (or S) enantiomer in the mother liquor, and (4) a chiral discrimination by the once
formed S (or R)-rich stable crystalline phase in a process of the subsequent crystal growth, leading to a
considerable enantiomeric enrichment of the R (or S) enantiomer up to 100% ee in the mother liquor. The
processes (3) and (4) are considered to be directly responsible for an enrichment of one enantiomer in the
mother liquor. The association mode of the two enantiomers in solution has been investigated by means
of (i) the solubility measurement and (ii) the number-averaged molecular weight measurement in solution
by vapor pressure osmometry, together with (iii) the molecular dynamics simulation of oligomer models.
The polymorphic transition during crystallization has been observed visually and by means of the in situ
FTIR technique and DSC measurement. Both metastable and stable crystals have been obtained, and
their crystal structures have been elucidated by X-ray crystallographic analysis of their single crystals.
Introduction
racemates comprising nonracemizable enantiomers by crystal-
lization are classified into two categories; one is a method using
Nowadays, in connection with the industrial and pharmaceuti-
cal needs of chiral organic substances, exploitation of economi-
cally and environmentally acceptable enantiomeric resolution
methods along with catalytic asymmetric syntheses has been
the subject of considerable recent development.1-8 In this
context, the practical methods for enantiomeric resolution of
an external chiral element like a diastereomeric salt formation
followed by fractional crystallization4-8 or a diastereoselective
host-guest inclusion complexation,2 and the other is a straight-
forward method to separate enantiomers by crystallization in
the absence of an external chiral element. In the latter category,
there seem to be two contrastive cases. One is the well-known
“Preferential Crystallization” method using a racemic conglom-
erate composed of a mixture of homochiral R and S crystals, in
* To whom correspondence should be addressed. E-mail: tamura@
fischer.jinkan.kyoto-u.ac.jp.
† Kyoto University.
§ Taiho Pharmaceutical Co., Ltd.
(5) Chiral Separations, Applications and Technology; Ahuja, S., Ed.; American
Chemical Society: Washington, DC, 1997.
(6) Chiral Separation Techniques; Subramanian, G., Ed.; Wiley-VCH:
Weinheim, 2001.
(7) CRC Handbook of Optical Resolutions Via Diastereomeric Salt Fromation;
Kozma, D., Ed.; CRC Press: Boca Raton, FL, 2001.
(8) Vries, T.; Wynberg, H.; van Echten, E.; Koek, J.; ten Hoeve, W.; Kellog,
R. M.; Broxterman, Q. B.; Minnaard, A.; Kaptein, B.; van der Sluis, S.;
Hulshof, L.; Kooistra, J. Angew. Chem., Int. Ed. Engl. 1998, 37, 2349-
2354.
‡ Osaka City University.
(1) Newman, P. Optical Resolution Procedures for Chemical Compounds;
Optical Resolution Information Center: New York, 1978, 1981, and 1984;
Vol. 1-3.
(2) Toda, F. Top. Curr. Chem. 1987, 140, 43-69.
(3) Chromatographic Chiral Separations; Chromatographic Science Series;
Zief, M.; Crane, L. J., Eds.; Marcel Dekker: New York, 1988; Vol. 40.
(4) Chirality in Industry; Collins, A. N., Sheldrake, G. N., Crosby, J., Eds.;
Wiley: New York, 1992.
9
10.1021/ja020454r CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 13139-13153
13139

A R T I C L E S
Tamura et al.
the Preferential Crystallization technique by Gernez in the 19th
century,10 this latter ideal case had been believed to be totally
unfeasible for more than a century by taking into account the
crystalline nature of known homochiral and heterochiral crystals.
Contrary to this common recognition, however, we found the
first instance where this latter ideal enantiomeric resolution is
indeed the case (Figure 2), and this unusual enantiomeric
resolutionphenomenonwasreferredtoasPreferentialEnrichment.16-21
Preferential Enrichment has the following four features
(Figure 2): (1) Repeated crystallization of the racemate and
each crop of resulting deposited crystals from the 4-25-fold
supersaturated solution leads to a remarkable alternating enrich-
ment of the two enantiomers up to 100% ee in the mother liquors
(a considerable enantiomeric enrichment in the mother liquor).
(2) At the same time, whenever recrystallization is carried out,
the resulting deposited crystals always display the opposite
chirality (a slight enrichment of the opposite enantiomer in the
deposited crystals) with a full reproducibility. (3) Racemates
or nonracemates with low ee values (less than 10% ee) are the
more suitable starting material for Preferential Enrichment than
those with higher ee values, achieving a very efficient resolution;
the use of the nonracemic samples of more than 35% ee leads
to an enantiomeric enrichment in the mother liquor, but fails to
induce the characteristic enrichment of the opposite enantiomer
in the deposited crystals. Therefore, only the racemates or
nonracemates with low ee values (less than 10% ee) have to be
crystalline to carry out the Preferential Enrichment experiment.
In contrast to Preferential Crystallization, it is no matter whether
the enantiomerically enriched materials with high ee values exist
as the solids or the oils. (4) No addition of seed crystals is
necessary at all.
Figure 1. Principle of preferential crystallization of conglomerates.
These unique features are unusual and quite different from
those of Preferential Crystallization of a racemic conglomerate
in which a considerable enantiomeric enrichment occurs in the
deposited crystals. Therefore, characterization of the crystalline
nature of a series of compounds showing Preferential Enrich-
ment and elucidation of the mechanism of this unusual
phenomenon are crucial to the utilization of Preferential
Enrichment as a potent enantiomeric resolution method for
certain racemic substances.
Figure 2. Preferential Enrichment of 1a. Conditions: (a) EtOH (32 mL)
at 25 °C for 4 days. (b) EtOH (32 mL) at 25 °C for 2 days. (c) Removal
of the solvent by evaporation.
On the basis of the unique polymorphism observed for certain
sulfonium and ammonium sulfonates (1e and 2c) showing
Preferential Enrichment (Chart 1),16,28 it has been expected that
which by repeating crystallization from the supersaturated
solution with the aid of its enantiopure seed crystals the
enantiomerically enriched crystals are efficiently deposited in
the alternating chirality sense (Figure 1).9-15 The other is the
completely reverse case of Preferential Crystallization, that is,
it is in the mother liquor that considerable enantiomeric
enrichment occurs by recrystallization (e.g., Figure 2). In
principle, since the accomplishment of the manual resolution
of a racemic conglomerate by Pasteur9 and the discovery of
(16) (a) Ushio, T.; Tamura, R.; Takahashi, H.; Yamamoto, K. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 2372-2374. (b) Ushio, T.; Tamura, R.; Azuma,
N.; Nakamura, K.; Toda, F.; Kobayashi, K. Mol. Cryst. Liq. Cryst. 1996,
276, 245-252.
(17) Tamura, R.; Ushio, T.; Nakamura, K.; Takahashi, H.; Azuma, N.; Toda,
F. Enantiomer 1997, 2, 277-280.
(18) Tamura, R.; Ushio, T.; Takahashi, H.; Nakamura, K.; Azuma, N.; Toda,
F.; Endo, K. Chirality 1997, 9, 220-224.
(19) (a) Takahashi, H.; Tamura, R.; Ushio, T.; Nakajima, Y.; Hirotsu, K.
Chirality 1998, 10, 705-710. (b) Tamura, R.; Takahashi, H.; Hirotsu, K.;
Nakajima, Y.; Ushio, T. Mol. Cryst. Liq. Cryst. 2001, 356, 185-194.
(20) Tamura, R.; Takahashi, H.; Hirotsu, K.; Nakajima, Y.; Ushio, T.; Toda, F.
Angew. Chem., Int. Ed. 1998, 37, 2876-2878.
(9) Pasteur, L. Ann. Chim. Phys. 1848, 24, 442-459.
(21) Tamura, R.; Takahashi, H.; Ushio, T.; Nakajima, Y.; Hirotsu, K.; Toda, F.
Enantiomer 1998, 3, 149-157.
(10) Gernez, M. C. R. Hebd. Seances Acad. Sci. 1866, 63, 843.
(11) Eliel, E.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic
Compounds; Wiley: New York, 1994; pp 297-464.
(22) Coquerel, G. Enantiomer 2000, 5, 481-498.
(23) (a) Dobashi, A.; Saito, N.; Motoyama, Y.; Hara, S. J. Am. Chem. Soc.
1986, 108, 307-308. (b) Jursic B. S.; Goldberg, S. I. J. Org. Chem. 1992,
57, 7172-7174.
(12) Collet, A. Optical Resolution In ComprehensiVe Supramolecular Chemistry;
Reinhoudt, D. N., Ed.; Pergamon: Oxford, 1996; Vol. 10, pp 113-149.
(13) Kinbara, K.; Hashimoto, Y.; Sukegawa, M.; Nohira, H.; Saigo, K. J. Am.
Chem. Soc. 1996, 118, 3441-3449.
(24) Cung, M. T.; Marraud, M.; Neel, J. Biopolymers 1978, 17, 149-1173.
(25) Harger, M. J. P. J. Chem. Soc., Perkin Trans. 2 1977, 1882-1887.
(26) Harger, M. J. P. J. Chem. Soc., Perkin Trans. 2 1978, 326-331.
(27) Kitamura, M.; Suga, S.; Niwa, M.; Noyori, R.; Zhai, Z.-X.; Suga, H. J.
Phys. Chem. 1994, 98, 12776-12781.
(14) Jacques, J.; Collet, A.; Wilen, S. H. Enantiomers, Racemates and
Resolutions; Krieger Publishing Co.: Malabar, FL, 1994.
(15) Collet, A.; Brienne, M.-J.; Jacques, J. Chem. ReV. 1980, 80, 215-230.
9
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Mechanism of Preferential Enrichment
A R T I C L E S
Chart 1. Ammonium and Sulfonium Sulfonates: Compounds 1a,
1b, 1d, 1e,2a, 2b, 2c, 5a, and 5b Showed Preferential
Enrichment, while 1c, 3, 4, and 5c Failed to Do So
polymorphic transition from a metastable crystalline phase to a
thermodynamically stable one at the stage of nucleation or
crystal growth might be responsible for Preferential Enrich-
ment.14,22 To elucidate the mode of this polymorphic transition,
it is primarily necessary to clarify the stable crystal structures
of a series of compounds showing Preferential Enrichment and
compare them with the enantiomeric assembly mode in solution
or in the first-formed metastable crystal prior to polymorphic
transition.
Figure 3. Time profile of the changes in the ee values (a) in the deposited
crystals and (b) in the mother liquor during crystallization from the
supersaturated EtOH solution of S-rich 1a (0.9% ee, 1.0 g/16 mL) at 25
°C.
Evaluation of the enantiomeric association mode in solution
has not been an easy task. Enantiomeric discrimination in
solution was reported to be observed in the 1H NMR spectra of
the nonracemic samples of particular chiral organic compounds
such as carboxamides,23 dicarboxamides,24 phosphinamides,25
and phosphinothioic acids26 which can easily form intermo-
lecular hydrogen bonds. In these cases, it is essential that the
diastereomeric signal separations in their 1H NMR spectra
should be observed arising from the presence of both homochiral
and heterochiral molecular association species in solution.
Therefore, this method is not applicable to the case where a
solution of the racemic or nonracemic sample contains only
either the homochiral or heterochiral molecular association
species in equilibrium with the corresponding monomers. In
this case, as we indicate in this report, the combined use of the
solubility measurement under various conditions and the
number-averaged molecular weight measurement in solution by
vapor pressure osmometry27 may become a potent tool to predict
which molecular association mode is preponderant in solution,
the homochiral one or the heterochiral one.
have been determined. On the basis of these findings, we discuss
the mechanism of this polymorphic transition during crystal-
lization and correlate it with an unusual key phenomenon, a
slight enrichment of the opposite enantiomer in the deposited
crystals. Finally, we propose the overall mechanism of Prefer-
ential Enrichment.
Results and Discussion
Preferential Enrichment, Binary Melting Point Phase
Diagram and Crystal Structure of 1a. To find out an
appropriate compound that (1) shows Preferential Enrichment,
(2) can explicitly give the unique melting point phase diagram,
and (3) exhibits a behavior of polymorphic transition in response
to the occurrence of Preferential Enrichment, we have carried
out screening of various ammonium sulfonate analogues which
would be more easily crystallized than the corresponding
sulfonium sulfonate. Finally we have found a new compound,
(()-[2-[4-(3-ethoxy-2-hydroxypropoxy)phenylcarbamoyl]ethyl]-
trimethylammonium p-nitrobenzenesulfonate [(()-1a], which
satisfies the above requirements.
In this work, we report that (1) a newly found compound
(1a) showing Preferential Enrichment has a unique melting point
phase diagram implying the existence of two polymorphs, and
this compound shows a visually observable phase transition at
the early stage of crystallization, (2) the compounds (1a and
2c) showing Preferential Enrichment favor a homochiral mo-
lecular association over a heterochiral one in solution, (3)
another new compound (1d) showing Preferential Enrichment,
which indeed exhibits the gradual polymorphic transition of the
first-formed metastable crystalline form to give the more stable
forms during crystallization, has been found, and (4) the crystal
structures of the key metastable form as well as the stable one
Very efficient Preferential Enrichment has been achieved
when 1a was recrystallized from ethanol (EtOH) at 25 °C
(Figure 2). Although 1a was synthesized from (()-epichloro-
hydrin, the solid powder sample of 1a obtained after final
methylation of the precursor amine with methyl p-nitroben-
zenesulfonate in acetone was no longer racemic but contained
either enantiomer in small excess (less than 5% ee) (eq 1).
Repetition of recrystallization of this nonracemic sample of 1a
and each crop of resulting deposited crystals successively from
its 20-fold or less supersaturated EtOH solution led to a slight
enrichment of the opposite enantiomer in the deposited crystals
regularly, along with a considerable enantiomeric enrichment
in the mother liquors in the alternating chirality sense (Figure
2). Thus, by collecting the enantiomerically enriched mother
(28) Takahashi, H.; Tamura, R.; Fujimoto, D.; Lepp, Z.; Kobayashi, K.; Ushio,
T. Chirality 2002, 14, 541-547.
9
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A R T I C L E S
Tamura et al.
Figure 5. (a) Time development of the in situ FTIR spectra monitered by
means of React IR spectroscopy during crystallization from the supersatu-
rated EtOH solution of (()-1a (0.1 mol/L). (b) Comparison of spectra at
the beginning (solid line, -) and the end (dotted line, ‚‚‚) of crystallization,
taken from the above spectra, with the spectra in the solid state of (()-la
(dashed line, - - -, δ-form crystal structure) and (()-3 (dashed and dotted
line, -‚-‚-‚, γ-form crystal structure). Black arrows indicate the absorptions
corresponding to the δ- or γ-form supramolecular structure. A white arrow
points the absorption at 90 min after crystallization began.
Figure 4. Visual observation of polymorphic transition and redissolution
of one enantiomer from the once formed crystals during crystallization from
the supersaturated EtOH solution of (()-1a (0.1 mol/L); (a) 15 min and
(b) 60 min after crystallization began.
liquors with the same handedness, very efficient separation of
the two enantiomers (>96% ee) has been achieved, leading to
the 35-40% recovery of both enantiomers after four consecutive
recrystallizations.
described in the following section, this redissolution behavior
of one enantiomer seems to correspond to the polymorphic
transition of the metastable crystalline phase (γ-form) to the
stable one (δ-form). Similarly, by means of the in situ FTIR
measurement using ReactIR spectroscopy which can detect
absorptions by small particles less than 1 µm as well as both
solute and solvent molecules in suspensions, it has been
observed that three new absorption bands at 1246, 1225, and
1198 cm-1 corresponding to S-O stretching vibrations of the
powder sample of the stable δ-form of 1a appear after
crystallization begins and their intensity gradually increases for
the first 90 min (Figure 5). Since the crystal structure of the
metastable crystalline phase (γ-form) is very similar to the
supramolecular structure in solution as described in the following
section, it is reasonable that only single-phase change can
seemingly be observed by the in situ FTIR measurement.
To characterize the crystalline nature of 1a, the binary melting
point phase diagram was constructed from the temperatures of
crystal fusion which were measured by differential scanning
calorimetry (DSC). Microcrystals for DSC measurement were
produced in a kinetically controlled manner by rapid concentra-
tion of each of the EtOH solutions of 1a (ca. 0.1 mol/L) with
various ee values which were prepared by mixing two solutions
of 1a with high and low ee values and then heating this mixture
at 70 °C followed by cooling, because slow crystallization
always led to the convergent deposition of crystals of less than
The time profiles of the changes in the ee values in the
deposited crystals as well as in the mother liquor during
crystallization are shown in Figure 3. A slight enrichment of
the opposite enantiomer in the deposited crystals occurred at
the incipient stage of crystallization and the ee value in the
deposited crystals became constant at around 5% after 40 min,
while the ee value in the mother liquor gradually increased.
The photographs shown in Figure 4 indicate that during the first
90 min of crystallization, one of the two enantiomers continues
to redissolve predominantly from the just-made crystals into
solution, as can be seen as a lot of convective streams evolved
from the crystal surface. In fact, the ee values in solution in the
proximity of the crystals were always higher by about 4-6%
than those in the area distant from the crystals. After 90 min,
the average ee value in solution reached about 60%. As
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Mechanism of Preferential Enrichment
A R T I C L E S
that the two phase curves intersect at two points around 35%
ee (Figure 6). From the relatively high temperatures of fusion
in the range of 0-15% ee in this phase diagram, it is conceivable
that the crystals in this range are liable to be deposited during
crystallization from the supersaturated solution.
X-ray crystallographic analysis of the single crystal of almost
racemic 1a of less than 0.4% ee which was prepared by very
slow crystallization from the 2-fold supersaturated EtOH
solution at 25 °C revealed that the crystal structure is character-
ized by two kinds of centrosymmetric cyclic dimers; one is
formed by the hydrogen bonds between the hydroxy groups and
the ethoxy oxygen atoms (O‚‚‚O distance: 2.820(4) Å) in a
pair of R and S molecules resulting in the formation of a head-
to-head cyclic dimer (type A), and the other is formed by the
hydrogen bond between an oxygen atom of a sulfonate ion and
the amide NH (N‚‚‚O distance: 2.942(4) Å) and the electrostatic
interactions between another oxygen atom of the same sulfonate
ion and the ammonium nitrogen atom in the neighboring long-
chain cation (N+‚‚‚O- distance: 4.098(4) Å), giving a head-
to-head cyclic dimer (type B) (Figure 7 and Table 1). By virtue
of these intermolecular interactions, a heterochiral one-
dimensional (1D)-chain is formed. Furthermore, each 1D-chain
interacts with two adjacent chains by another electrostatic
Figure 6. Binary melting-point phase diagram of 1a. White and black
circles represent the temperatures of the beginning and the end of fusion,
respectively.
10% ee, a phenomenon typical of Preferential Enrichment,
irrespective of the original ee values of 1a in solutions. The
average ee value of microcrystals (ca. 1.0 mg) in each small
DSC pan was determined by HPLC analysis immediately after
each DSC measurement. The deviation of the ee values of the
microcrystals in each flask was (1, (2, and (5% for the
microcrystals of 0-8, 10-35, and more than 50% ee, respec-
tively. The rough phase diagram thus obtained clearly suggests
Figure 7. Crystal structure of the δ-form of racemic 1a. (a) A view down the a axis. The carbon, oxygen, nitrogen, and sulfur atoms are representated by
black, red, blue, and yellow ellipsoids, respectively. (b) Schematic representation of the intermolecular interactions in the crystal. The ellipsoid and circle
in the inset indicated the long-chain cation and sulfonate ion, respectively. The dashed lines show the intermolecular hydrogen bonds and electrostatic
interactions.
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A R T I C L E S
Tamura et al.
Table 1. Summary of X-ray Crystallographic Data
(±)-1a
(±)-3
(±)-1d
(±)-4
empirical formula
formula weight
crystal system
space group
a, Å
C
23H33O9N3S
C24H35N3O9S
541.62
C23H34N2O7S
482.59
C24H35N3O8S
525.62
527.59
triclinic
P1h (No. 2)
9.852(2)
15.148(4)
9.040(9)
96.52(5)
92.57(4)
107.19(2)
1276(1)
2
triclinic
P1h (No. 2)
9.337(2)
16.2933(7)
9.338(3)
100.57(2)
103.63(1)
80.88(1)
1347.2(4)
2
triclinic
P1h (No. 2)
9.4967(13)
14.947(3)
9.0688(13)
99.301(7)
102.637(10)
89.263(9)
1239.3(3)
2
triclinic
P1h (No. 2)
13.120(1)
16.299(2)
6.5805(4)
96.957(5)
99.590(5)
76.310(4)
1343.2(2)
2
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å
Z
F(000)
560.00
576.00
516.00
560.00
D
calc, g/cm3
1.373
1.335
1.293
1.300
T, °C
23
23
23
23
crystal size, nm
µ (Cu KR), cm-1
µ (Mo KR), cm-1
2θmax, deg
0.35 × 0.25 × 0.05
0.32 × 0.07 × 0.07
0.25 × 0.20 × 0.15
0.30 × 0.30 × 0.30
16.18
15.46
-
-
-
-
1.75
1.71
120.0
135.9
57.5
56.6
no. of reflns measd
no. of reflns obsd
no. of variables
4039
4132
5718
14071
3242 (I > 2.00σ(I))
1476 (I > 3.00σ(I))
1834 (I > 3.00σ(I))
2853 (I > 3.00σ(I))
325
366
331
359
R a, Rw
0.062; 0.120
1.88
0.065; 0.089
1.14
0.091; 0.157
1.46
0.049; 0.077
0.93
b
GOF
2
2
2
2
2
2
a R ) ∑(Fo - Fc )/∑Fo . b Rw ) (∑ω(Fo - Fc )2/∑ω(Fo ))1/2; ω ) 1/σ2(Fo).
Figure 8. X-ray diffraction patterns of 1a, simulated from the X-ray crystallographic data of almost racemic single crystal (a) and measured for polycrystalline
powder samples; (b) 5% ee, (c) 41% ee, and (d) 94% ee.
interaction between the third oxygen atom of the same sulfonate
ion and the ammonium nitrogen atom in the adjacent chains,
eventually forming a rigid two-dimensional sheet structure. We
call this type of crystal structure a δ-form, which is commonly
found in most of the compounds showing Preferential Enrich-
ment.17,19,20,28-30 Thus, the phase curve in the range of 0-15%
ee in Figure 6 has proved to correspond to either a racemic
compound or a highly ordered racemic mixed crystal.14 The
powder X-ray diffraction pattern simulated from the above X-ray
crystallographic data was almost identical to the experimental
diffraction pattern of the polycrystalline powder of less than
10% ee obtained by the Preferential Enrichment experiment
(Figure 8, a and b).
and gave no single crystal of adequate quality for the X-ray
analysis at all, from the shape of the overall flat phase curve in
this range and the similarity in the powder X-ray diffraction
patterns of the nonracemates with various ee values more than
40% ee (Figure 8, c and d), we concluded that the flat phase
curve must correspond to the mixed crystals composed of
different amounts of the R and S enantiomers in a crystal lattice
similar to that of the pure enantiomer.
Thus, it can be deduced that during crystallization from the
supersaturated solution of 1a with a low ee value, the metastable
mixed crystals are initially formed in a kinetically controlled
manner, followed by its polymorphic transition into the stable,
highly ordered crystals via either a solid-to-solid type31,32 or a
solvent-mediated type of polymorphic transition according to
Although both the pure enantiomers and nonracemates (>40%
ee) of 1a showed a tendency to form polycrystalline powder
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Mechanism of Preferential Enrichment
A R T I C L E S
Figure 9. Binary melting-point phase diagram of 1c. White and black
circles represent the temperatures of the beginning and end of fusion,
respectively.
the “Ostwald’s law of stages”.33-35 The intersection of the two
phase curves implies that the energy difference between the two
polymorphs is small enough to allow such a polymorphic
transition to proceed at a moderate rate during crystallization,
in good agreement with the visual observation and the conse-
quence of the in situ FTIR measurement shown in Figures 4
and 5, respectively.
Besides the melting point phase diagram of 1a, another
compound 1e showing Preferential Enrichment has been re-
ported to have a different type of binary melting point phase
diagram which consists of two somewhat convex curves.28 In
this case, the two curves do not intersect but are located closely
to each other; one is a stable polymorph belonging to a fairly
ordered racemic mixed crystal, and the other is a metastable
one which cannot be characterized. This result also implies the
importance of polymorphic transition for the occurrence of
Preferential Enrichment.
In contrast, compound 1c, the p-toluenesulfonate derivative
of 1a, has failed to show Preferential Enrichment and has a
single convex curve (Figure 9), indicating that this compound
exists only as a single type of a mixed crystal composed of the
two enantiomers over a wide range of ee values like 5c.19 This
classification of the crystalline nature has been confirmed by
the fact that the powder X-ray diffraction pattern of (()-1c is
almost identical to that of the corresponding almost pure
enantiomer (Figure 10).
Figure 10. X-ray diffraction patterns of powder samples of 1c; (a) 0% ee
and (b) 99% ee.
Preferential Enrichment did not occur by recrystallization of
the racemate. This is because only molecular conformation is
different from each other with respect to the two polymorphs
of oxime of carvone, and there is no possibility of rearrangement
of hydrogen bonds in the crystal lattice.36
Assembly Mode of Enantiomers in Solution. Since com-
pounds showing Preferential Enrichment like 1a and 2c have
1
failed to exhibit the signal separations in the H NMR spectra
of the nonracemic samples with various ee values taken in
CDCl3, C2D5OD, or (CD3)2CDOD at -30-25 °C, the 1H NMR
technique has proved not to be applicable for deciding which
molecular association mode is more stable in solution, the
homochiral one or the heterochiral one. Instead, we have
measured the solubility of the sulfonium sulfonate 2c as well
as that of the ammonium sulfonate 1a. Compound 2c shows
much higher solubility in organic solvents due to its sulfonium
salt structure than 1a. The saturated solution of 2c was prepared
by stirring the crystals in 2-PrOH for 48 h at 25 °C. The
solubility of the pure R enantiomer (11.4 mg/mL) was higher
than that of the racemic sample (6.6 mg/mL). Furthermore,
under the conditions for supersolubility measurement in which
the crystals are dissolved in 2-PrOH by heating at 70 °C
followed by cooling and standing for 45 h at 25 °C, the pure R
enantiomer has exhibited a drastic increase of the supersolubility
(more than 150 mg/mL) compared to the racemate (10 mg/mL).
In this case, to avoid the complexity arising from the occurrence
In addition to the presence of polymorphs, structural require-
ments must be satisfied for the occurrence of Preferential
Enrichment. For example, it was reported that there exist two
polymorphs for oxime of carvone, both of which are classified
as a mixed crystal over a wide range of ee values, and the two
melting point phase curves are located closely to each other,
but not intersected,36 similar to the case of 1e.28 However,
(29) Tamura, R.; Takahashi, H.; Miura, H.; Lepp, Z.; Nakajima Y.; Hirotsu,
K.; Ushio, T. Supramol. Chem. 2001, 13, 71-78.
(30) Takahashi, H.; Tamura, R.; Lepp, Z.; Kobayashi, K.; Ushio, T. Enantiomer
2001, 6, 57-66.
(31) For polymorphic transformation, see: Dunitz, J. D.; Bernstein, J. Acc. Chem.
Res. 1995, 28, 193-200.
(32) Parkinson, G. M.; Thomas, J. M.; Williams, J. O.; Goringe, M. J.; Hobbs,
L. W. J. Chem. Soc., Perkin 2 1976, 836-838.
(36) (a) Oonk, H. A. J.; Kroon, J. Acta Crystallogr. 1976, B32, 500-504. (b)
Kroon, J.; van Grup, P. R. E.; Oonk, H. A. J.; Baert, F.; Fouret, R. Acta
Crystallogr. 1976, B32, 2561-2564. (c) Baert, F.; Fouret, R.; Oonk, H. A.
J.; Kroon, J. Acta Crystallogr. 1978, B34, 222-226. (d) Oonk, H. A. J.;
Tjoa, K. H.; Brants, F. E.; Kroon, J. Thermochim. Acta 1997, 19, 161-
171. (e) Meijer, E. L.; Blok, J. G.; Kroon, J.; Oonk, H. A. J. Thermochim.
Acta 1977, 20, 325-334. (f) Calvet, T.; Oonk, H. A. J. Calphad 1995, 19,
49-56.
(33) Bernstein, J.; Davey, R. J.; Henck, J.-O. Angew. Chem., Int. Ed. 1999, 38,
3440-3461.
(34) Ostwald, W. Grundriss der Allgemeinen Chemie; W. Engelmann: Leipzig,
1899.
(35) McCrone, W. C. Polymorphism. In Physics and Chemistry of the Organic
Solid State; Fox, D., Labes, M. M., Weissberger, A., Eds.; Interscience:
New York, 1965; Vol. II, pp 726-767.
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A R T I C L E S
Tamura et al.
Figure 11. Number-averaged molecular weight measurement of the
racemate and nonracemic sample of 2c in CHCl3 at 30 °C by vapor pressure
osmometry.
of Preferential Enrichment, this supersolubility measurement for
racemic 2c was carried out carefully by using a much lower
concentration of 2c in 2-PrOH than the standard experimental
conditions (250 mg/mL) used for the Preferential Enrichment
experiment. These results indicate that the homochiral supramo-
lecular structure in 2-PrOH is very different from that in the
enantiopure crystalline state and is much more stable than any
heterochiral molecular association structure in 2-PrOH solution.
As expected, the equimolar mixing of the respective enantiopure
solutions of the R and S enantiomers in 2-PrOH (125 mg/mL
each) at 25 °C has resulted in gradual deposition of nonracemic
crystals with low ee values to induce Preferential Enrichment.
These results and the fact that it takes long (sometimes more
than 24 h) before crystallization begins from the supersaturated
solution of the racemic or nonracemic 2c suggest that even in
a high concentration (0.50 mol/L) of racemic 2c in 2-PrOH the
stable, homochiral R and S supramolecular structures are
preferentially formed, gradually interact with each other prob-
ably due to dipole-dipole interactions, and finally undergo
phase transition to form new heterochiral supramolecular
structures, which show low solubility in 2-PrOH and are
deposited as the crystals.
Figure 12. Crytal structure of the γ-form of racemic 3. (a) A steroview
down the c axis. The carbon, oxygen, nitrogen, and sulfur atoms are
representated by black, red blue, and yellow ellipsoids, respectively. (b)
Schematic representation of the intermolecular interactions in the crystal.
The ellipsoid and circle in the inset indicate the long-chain cation and
sulfonate ion, respectively. The dashed lines show the intermolecular
hydrogen bonds.
To confirm the preferential homochiral molecular association
in a solution of racemic 2c, we have measured the number-
averaged molecular weights of the racemic and nonracemic
(87% ee) samples of 2c in CHCl3 over the range of concentra-
tions of 1.0 × 10-3 to 1.4 × 10-2 mol/L at 30 °C by using a
vapor pressure osmometer (Figure 11), because the measurement
in CHCl3 is considered to reflect strong solute-solute interac-
tions even in the diluted solutions and the measurements in
EtOH or 2-PrOH have indicated that the anion and cation
moieties of 2c are almost separated by solvation in the same
measurable concentration range. In CHCl3 the molecular weight
increased in proportion to the concentration of 2c to give the
tetramer at the concentration of 1.3 × 10-2 mol/L with respect
to both samples, and there was no appreciable difference
between the racemate and the nonracemate of 87% ee (with an
experimental error of (10%) (Figure 11). This result suggests
that (1) the molecular assembly structure is a 1D-chain and (2)
in a diluted solution of racemic 2c, the preferential molecular
association mode is homochiral or random. In combination with
the results of solubility measurements, the preferential molecular
association mode in a solution of racemic 2c is probably
homochiral.
Likewise, an S-rich sample of 1a of 93% ee showed higher
solubility (71.1 mg/mL) than an almost racemic one of less than
0.4% ee (3.1 mg/mL) in EtOH at 25 °C. Furthermore, the
number-averaged molecular weight of the S-rich sample in
CHCl3 was also in proportion to the concentration of 1a to give
a tetramer at the concentration of 1.5 × 10-2 mol/L, although
measurement of the racemic sample was unfeasible due to its
very low solubility in CHCl3 even at elevated temperatures. In
fact, the equimolar mixing of the respective solutions of the R
and S enantiomers in CHCl3 (1.5 × 10-2 mol/L each) resulted
in prompt deposition of almost racemic solid. Probably, this
insolubility of the racemic sample in CHCl3 stems from stronger
dipole-dipole interactions between the homochiral R and S
chains followed by prompt phase transiton to form new
heterochiral supramolecular structures. In EtOH, the equimolar
mixing of the respective solutions of the R and S enantiomers
of 1a led to gradual deposition of nonracemic crystals with low
ee values to induce Preferential Enrichment; this behavior is
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Mechanism of Preferential Enrichment
A R T I C L E S
Figure 13. DSC aspects of gradual polymorphic transition of the deposited crystals observed during crystallization of (()-1d from EtOH: (a) γ-form, (b)
a mixture of γ, ú, and η-forms after 8 days, (c) a mixture of ú, η and ꢀ-forms after 13 days, and (d) a mixture of η and ꢀ-forms after 19 days. The
endothermal peak at ca. 85 °C corresponds to the desorption of EtOH remaining around the crystals.
the same as that in the usual Preferential Enrichment experiment.
These results strongly sugggest that a homochiral molecular
assembly is essentially in preference to a heterochiral one in
the supersaturated 2-PrOH or EtOH solutions of the racemic
samples of 1a and 2c as well as in the diluted CHCl3 solutions
of racemic 2c and that the homochiral supramolecular structure
must be a 1D-chain.
morph. Compound 1d, the phenylsulfonate derivative of 1a, has
turned out to fit this requirement.
When racemic 1d was initially recrystallized from 2-PrOH,
the desired γ-form crystal exhibiting an endothermic peak at
116.2 °C by DSC measurement was obtained as a stable form
(Figure 13a). The crystal structure of the γ-form polymorph of
racemic 1d is very similar to that of racemic 3; each homochiral
chain comprises the long-chain cation and the sulfonate anion
in alternating alignment by two hydrogen bonds between one
oxygen atom of the sulfonate ion and the hydroxy group (O‚‚
‚O distance: 2.797(12) Å) and between the same oxygen atom
and the amide NH (O‚‚‚N distance: 2.869(12) Å) (Figure 14
and Table 1). There is no appreaciable interchain interaction.
Assembly Mode of Enantiomers in the Metastable Crystal
and Polymorphic Transition. Next, to predict the homochiral
supramolecular structure in solution, we have searched a
compound which possesses a homochiral 1D-chain structure in
the crystal, because it is very possible that a molecular
conformation or a molecular assembly structure in solution
would be retained in the first-formed crystal by crystallization
from the same solvent.37,38 Consequently, we have found that
racemic 3, the terminal propoxy derivative of 1a, possesses a
crystal structure (γ-form) which is composed of alternating
alignment of homochiral R and S 1D-chains in an antiparallel
direction with a space group P1h (No. 2) (Z ) 2) (Figure 12 and
Table 1). Furthermore, the FTIR absorption spectra of an EtOH
solution (0.20 mol/L) of racemic 1a is very similar to those of
the solid racemic 3 rather than the solid racemic 1a (Figure 5),
implying the similarity between the supramolecular structure
in the supersaturated EtOH solution of racemic 1a and that in
the solid state of racemic 3. However, since racemic 3 failed to
show Preferential Enrichment, we have sought another com-
pound which can show Preferential Enrichment and possess an
analogous homochiral chain structure in the metastable poly-
Although in 2-PrOH the racemate of 1d failed to show
Preferential Enrichment, the phenomenon began to occur slowly
when recrystallization was carried out in EtOH (Figure 15). DSC
measurements of the deposited crystals successfully showed the
occurrence of the successive polymorphic transitions as the
enantiomeric enrichment in the mother liquor proceeded; the
first-formed γ-form polymorph was gradually transformed into
the most stable one (ꢀ-form), corresponding to the endothermic
peak at 127 °C via two different metastable polymorphs (ú-
and η-forms) in turn during crystallization from EtOH (Figure
13 b, c, and d). Once Preferential Enrichment occurred in EtOH,
the pure γ-form crystal was no longer obtained as a stable crystal
even by crystallization from 2-PrOH; Preferential Enrichment
began to occur very slowly in 2-PrOH, also. These results
indicate that the polymorphic transition of the metastable γ-form
into the more stable forms should be a key process for the
occurrence of Preferential Enrichment.
(37) Maruyama, S.; Ooshima, H. J. Cryst. Growth 2000, 212, 239-245.
(38) (a) Kitamura, M.; Ueno, S.; Sato, K. Molecular Aspects of the Polymorphic
Crystallization of Amino Acids and Lipids. In Crystallization Processes;
Ohtaki, H., Ed.; Wiley: Chichester, 1998; pp 99-129. (b) Kitamura, M.
J. Cryst. Growth 1989, 96, 541-546.
Molecular Dynamics Simulation of Supramolecular Struc-
tures in Solution. As a model for the homochiral supramo-
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A R T I C L E S
Tamura et al.
Figure 15. Preferential Enrichment of 1d. Conditions: (a) EtOH (6.0 mL)
at 25 °C for 30 days. (b) EtOH (6.0 mL) at 25 °C for 17 days. (c) Removal
of the solvent by evaporation. (d) EtOH (4.0 mL) at 25 °C for 11 days. (e)
EtOH (3.0 mL) at 25 °C for 9 days.
there was only a small energy difference between the two
structures in the case of 2c (-132.1 ( 9.9 and -129.8 ( 10.0
kcal/mol for SSS and SRS, respectively), while in the case of
1a the homochiral trimer structure was appreciably more stable
than the heterochiral one (102.0 ( 9.5 and 117.2 ( 9.5 kcal/
mol for SSS and SRS, respectively). Noteworthy is the fact that
the stable homochiral SSS trimer structures for both 1a and 2c
adopt a similar compact conformation so as to allow further
elongation of the chain, whereas the stable heterochiral trimer
structures of both 1a and 2c adopt an analogous U-shaped
structure which is not advantageous for elongation of the chain
(Figure 16, a and b). In fact, the stable homochiral tetramer
(SSSS consisting of four cations and three anions) of 1a and 2c
exhibited the expected homologous chain structure after the
same molecular dynamics simulation (Figure 16, c and d). Thus,
the calculated homochiral oligomer structure seems to be a good
candidate in solution.
Figure 14. Crystal structure of the γ-form of racemic 1d. (a) A stereoview
down the a axis. The carbon, oxygen, nitrogen, and sulfur atoms are
representated by black, red, blue, and yellow ellipsoids, respectively. (b)
Schematic representation of the intermolecular interactions in the crystal.
The ellipsoid and circle in the inset indicate the long-chain cation and
sulfonate ion, respectively. The dashed lines show the intermolecular
hydrogen bond.
Assembly Mode of Enantiomers in the Stable Crystal. So
far we have finished X-ray crystallographic analyses of seven
racemates (1a, 1b,29 1e,28 2b,17,30 2c,16,30 5a,19 and 5b20), two
pure enantiomers (1e28 and 2c16), and three nonracemates (1e,28
2c,16 5b20) among a series of compounds showing Preferential
Enrichment. Of these racemates and nonracemates, there are
two important stable crystal structures, R- and δ-forms, which
are closely associated with the mechanism of Preferential
Enrichment. It has been indicated that (1) the formation of the
R-form crystal (observed only in the case of 2c) which is a
racemic compound type is not essential to the occurrence of
Preferential Enrichment and is considered to be caused by further
polymorphic transition of the once-formed δ-form polymorph,
because other compounds which have the stable R-form crystal
structure due to no structural possibility of giving the δ-form
polymorph have failed to show Preferential Enrichment,16,21,30,39
and (2) the δ-form crystal (Figures 7 and 17) which is commonly
lecular structure in solution, the 1D-chain structure observed
in the γ-form crystal of racemic 1d is supposed to be
appropriate, because (1) the homochiral 1D-chain is formed only
by intermolecular hydrogen bonds and no appreciable electro-
static interaction is observed, (2) there is no interchain interaction
in the crystal, and (3) the γ-form crystal has a space group of
P1h (No. 2) with the second-lowest symmetry, implying the
flexible conformation of this chain in solution. To verify the
possibility of the presence of this model structure in solution,
the molecular dynamics simulation (see the Experimental
Section for the detail) of the homochiral and heterochiral trimers
(SSS vs SRS) has been carried out at 300 K for 1a and 2c. The
used trimer model, consisting of three long-chain cations and
two sulfonate ions, was taken from the γ-form crystal structure
of racemic 1d. The reason only two sulfonate ions were used
inside the trimer model instead of three ions was that we could
save the calculation time without affecting both the energy
difference between two trimers and the conformation of each
trimer. The hydrogen bonds in each trimer are fixed to prevent
the component ionic species from dispersing. Consequently,
(39) Compound (()-4, in which the ethoxy oxygen atom in (()-1a is replaced
by CH2 group, has been found to fail to show Preferential Enrichment.
The crystal structure of (()-4 has been determined to be an R-form (Table
1). See also ref 21.
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Mechanism of Preferential Enrichment
A R T I C L E S
Figure 16. Oligomeric structure of γ-form for 1a optimized by molecular dynamics simulations; (a) SSS trimer, (b) SRS trimer, (c) SSSS tetramer, and (d)
SRSR tetramer. The dashed lines indicate the fixed hydrogen bonds. See the Experimental Section for the detail of calculations.
found as the stable form in the other six racemates seems to be
crucial to the occurrence of Prefrerential Erichment.
ondary alcohols containing a glycerol moiety, an amide group,
and a sulfonium sulfonate or ammonium sulfonate structure
(Chart 1). Their molecular weights are about 500. Scheme 1
summarizes the structural requirements for the occurrence of
Preferential Enrichment: (1) The amide, hydroxy, and terminal
alkoxy groups are all indispensable for the formation of
hydrogen bonds.21,39 (2) The terminal alkoxy group must be
ethoxy or methoxy; larger alkoxy groups fail to show Prefer-
ential Enrichment. (3) The onium sulfonate salt structure is
advantageous for the polymorphic transition, owing to the
mobility of the anion in the crystal lattice. (4) Hence, the
selection of para-substituent X on the benzenesulfonate ion is
very important; the sulfonate ions with low basicity having an
electron-withdrawing group X form weak hydrogen bonds to
allow the rearrangement of hydrogen bonds in the crystal lattice
and hence polymorphic transition, while highly basic sulfonate
ions which make strong hydrogen bonds fail to induce poly-
morphic transition.18,19
Mechanism of Polymorphic Transition. By comparing the
crystal structures of γ- and δ-forms, it can be easily deduced
that a solvent-assisted solid-to-solid type of polymorphic
transition of the metastable γ-form crystal,31,32 which is
considered to retain the supramolecular structure in solution,
into the stable polymorph like the δ-form during crystallization
would be responsible for Preferential Enrichment. Indeed, we
have indicated that in case of racemic 1d polymorphic transition
With regard to δ-form crystals, strictly speaking, there are
two types: One is a completely ordered mixed crystal very
similar to a racemic compound as observed in the racemic
crystals of 1a, 2b, and 5a (e.g., Figure 7), and the other is a
fairly ordered mixed crystal found in both racemic or nonra-
cemic 1b, 1e, and 5b; the crystal structure of the nonracemate
is virtually isomorphous with that of the racemate and similar
to that of the pure enantiomer.20,28 For example, the intermo-
lecular interactions observed in the nonracemic crystal of 1e of
20% ee are shown in Figure 17 in more detail than in ref 28.
Since two δ-form crystal structures of 1a and 1e are virtually
identical except that the orientational disorder was observed at
the position of the hydroxy group on an asymmetric carbon atom
in 1e, it is conceivable that the nonracemic crystal of 1a with
a low ee value which is always produced by the Preferential
Enrichment experiment would also adopt such a fairly ordered
δ-form structure to accommodate excess enantiomers flexibly
in the crystal lattice. Therefore, it is concluded that the crystalline
nature of the racemates of the compounds showing Preferential
Enrichment should be classified as a racemic mixed crystal
composed of the two enantiomers.
Molecular Structure Required for Preferential Enrich-
ment. Thus far compounds which have been found to show
Preferential Enrichment are analogous linear asymmetric sec-
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A R T I C L E S
Tamura et al.
Figure 17. Schematic representation of the intermolecular hydrogen bonds in the δ-form crystal of S-rich 1e (20% ee). The ellipsoid and circle in the inset
indicate the long-chain cation and sulfonate ion, respectively. The dotted lines represent the intermolecular hydrogen bonds and electrostatic interactions.
The hydroxy group on the asymmetric carbon atom is disordered over two positions. The R and S enantiomers in the sites with higher ocuppancy factor
(0.75) are designated R and S, and those in sites with lower occupancy factors (0.25) r and s. The contents of three dimer structures in the inset were
estimated from the occupancy factors of the orientationally disordered hydroxy groups and the ee value (20%) of the crystal. See also refs 20 and 28.
Scheme 1. Structural Requirements for the Occurrence of
Preferential Enrichment
is closely associated with Preferential Enrichment (Figure 13).
Figure 18. Polymorphic transition in the case where equal numbers of
In the γ-form crystal of racemic 1d (Figure 14), the shortest
interatomic distances between the hydroxy oxygen atoms in the
R chain and the ethoxy oxygen atoms in the adjacent S chain
are only 5.063(12) Å. Therefore, it is very possible for the
nearest R and S enantiomers between the two adjacent chains
to form new hydrogen bonds between the hydroxy groups and
the ethoxy oxygen atoms by slight movement of the two
molecules along the [100] plane in the crystal. This rearrange-
ment of hydrogen bonds accompanied by slight movement of
the sulfonate ions so as to form the cyclic dimers of types A
and B shown in Figures 7 and 17 occurs one after another in
the crystal lattice to lead to the heterochiral 1D-chain structure
(Figure 18). Probably the reason racemic 3 takes a stable γ-form
crystal structure but fails to show Preferential Enrichment is
due to the steric hindrance of the terminal propoxy group, which
would prevent the molecular rearrangement in the crystal lattice.
The fashion of alignment of the two homochiral chains in
the γ-form metastable crystal must largely define the type of
the resulting δ-form crystal structure after phase transition; (1)
when equal numbers of two homochiral chains are alternatingly
aligned in an antiparallel direction along one axis, a completely
homochiral R and S chains are alternatingly aligned in the γ-form crystal,
leading to the completely ordered δ-form crystal without crystal disintegra-
tion. White arrows show the direction of alignment of homochiral chains.
Black arrows indicate the formation of new hydrogen bonds and electrostatic
interactions. See Figures 7 and 14 for the details of each schematic
representation.
ordered δ-form crystal structure will be formed after polymor-
phic transition (Figure 18), (2) when an odd number of
homochiral R (or S) chains are aligned between two S (R) chains
locally, after polymorphic transition, the fairly ordered δ-form
crystal structure will locally be formed without disintegration
of the crystal (Figure 19), and (3) when an even number of
homochiral R (S) chains are aligned between two S (R) chains
locally, after polymorphic transition, partial disintegration of
the crystal occurs to release the R (S)-rich area into solution
(Figure 20), because the R (S)-rich area was surrounded by the
sites, where hydrogen bonds of type A cannot be formed, as
well as the sites with weak electrostatic interactions (Figure 21).
This third case can well explain the origin of a slight enrichment
of the opposite enantiomer in the deposited crystals. If the
metastable γ-form crystals contain the R enantiomer in a small
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Mechanism of Preferential Enrichment
A R T I C L E S
(ca. 5% ee) supersaturated solution is illustrated in Figure 22.
In solution, homochiral oligomers are formed preferentially and
aggregate to form a supramolecular cluster, which undergoes
phase transition to give a metastable crystal of γ-form in a
process of nucleation. Since the R enantiomer is in solution
slightly in excess, the resulting metastable crystal becomes
slightly R-rich, too, and this crystal contains large R-chain-rich
areas (represented by four consecutive R chains in Figure 22)
and small S-chain-rich areas (represented by three consecutive
S chains) except alternating R- and S-chain alignment. The next
step is polymorphic transition in a process of crystal growth
arising from rearrangement of hydrogen bonds in the crystal
lattice as described in the previous section. After polymorphic
transtion, the large R-rich area can no longer stay inside the
transformed crystal due to partial disintegration of the crystal,
dissolving into solution (see the photographs shown in Figure
4). However, the small S-rich area can be maintained in the
crystal lattice by newly formed hydrogen bonds to form an
S-rich crystal with a fairly ordered δ-form. Eventually, together
with a slight enrichment of the opposite S enantiomer in the
deposited crystals, an enrichment of the R enantiomer in the
mother liquor occurs to some extent at this stage. Subsequently,
the resulting slightly S-rich δ-form crystals induce chiral
discrimination to promote the growth of the crystals with the
same handedness and ee values. Thus, the enantiomeric purity
of the R enantiomer in the mother liquor is gradually raised
until the crystal growth ceases. In addition, higher solubility of
the materials with high ee values than those with low ee ones
accelerates the R enantiomer to be enriched in the mother liquor.
Such a chiral discrimination process induced by the trans-
formed crystals in a process of crystal growth is proved by the
following experiment with five independent runs (Figure 23).
By recrystallization of R-rich 2c of 8.2% ee from 2-PrOH at 25
°C, the initially deposited S-rich microcrystals of 5.4% ee were
separated by decantation. To the S-rich microcrystals (ca. 0.5
g) was added immediately an independently prepared super-
saturated solution of S-rich 2c (1.0 g) of 7.3% ee in 2-PrOH (4
mL). On standing at 25 °C, S-rich crystals of 5-6% ee continue
to be deposited during crystallization, and thereby the original
S-rich solution gradually becomes R-rich through the racemic
solution during crystallization. After 4 days at 25 °C, the
enantiomeric purity of the R enantiomer in the mother liquor
reaches 64-69% ee. This result was fully reproducible in all
five runs. In contrast, without addition of the S-enriched
Figure 19. Polymorphic transition in the case where an odd number of
homochiral R chains are aligned between two S chains in the γ-form crystal,
leading to the fairly ordered δ-form crystal without crystal disintegration.
White arrows show the direction of alignment of homochiral chains. Black
arrows indicate the formation of new hydrogen bonds and electrostatic
interactions. See Figures 14 and 17 for the details of each schematic
representation.
Figure 20. Polymorphic transition in case that an even number of
homochiral R chains are aligned between two S chains in the γ-form crystal,
leading to the local crystal disintegration in the δ-form crystal, which occurs
between the sites a and b (see Figure 21, also). White arrows show the
direction of alignment of homochiral chains. Black arrows indicate the
formation of new hydrogen bonds and electrostatic interactions.
excess (e.g., ca. 5% ee), probably the sum of the R chains is
larger than that of the S ones in the crystal, and the probability
for an even number of R chains to be aligned between two S
chains should be higher than the opposite case. If so, after
polymorphic transition, a substantial amount of the R enanti-
omers must be liberated into solution eventually to give slightly
S-rich crystals.
Mechanism of Preferential Enrichment. The proposed
mechanism of Preferential Enrichment in the case of enrichment
of the R enantiomer in the mother liquor from the slightly R-rich
Figure 21. Site of crystal disintegration. There is one hydrogen bond between the R and r enantiomers (or the s and S enantiomers), but on hydrogen bond
at the site a between the r and s enantiomers. Crystal disintegration occurs at this site a and the site b with weak electrostatic interactions (see Figure 20,
also).
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A R T I C L E S
Tamura et al.
Figure 22. Mechanism of Preferential Enrichment characterized by homochiral molecular association, polymorphic transition, followed by crystal disintegration,
and chiral discrimination.
microcrystals, that is, under the standard conditions for the
Preferential Enrichment experiment, the S enantiomer is enriched
in the mother liquor up to 75% ee after 4 days at 25 °C, and
enrichment of the opposite enantiomer occurs in the deposited
crystals as expected.
enriched in solution later. In fact, when exactly racemic 2c was
used for recrystallization, the probability of obtaining one
particular enantiomer in excess in the mother liquor after a
number of independent recrystallizations was 0.5.16
Conclusions
In addition, it should be noted that when the original
supersaturated solution for crystallization is strictly racemic (0%
ee), the probability for either the R or the S enantiomer to be
enriched in solution after crystallization is 0.5; in other words,
initial capricious formation of the very first nonracemic
metastable crystal nucleus seems to doom which enantiomer is
We have proposed the mechanism of Preferential Enrichment
based on (1) the unique polymorphism of the compounds
showing Preferential Enrichment, (2) their metastable and stable
crystal structures, (3) the association mode of the two enanti-
omers in solution, and (4) the aspects of polymorphic transition
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Mechanism of Preferential Enrichment
A R T I C L E S
using the KNAUER vapor pressure osmometer K-7000 with dry CHCl3
(H2O content: 5-10 ppm) and benzil as the standard sample for
calibration, and the results of the triplicate experiments were reproduc-
ible. The initial structure for molecular dynamics simulations was
energetically minimized by molecular mechanics calculations, in which
the Quasi-Newton (BFGS) method with convergence criterion of 0.001
kcal/mol/Å was used for minimization along with the COMPASS force
field.41,42 Then molecular dynamics simulations were conducted by using
Discover41 as the software with COMPASS force field under the
following calculation conditions: cutoff distance, 15.5 Å; spline width,
5.00 Å; buffer width, 2.00 Å; temperature control, 300 K by the Nose
method; time step, 1.0 fs; calculation time, 6000 ps (3000 ps each for
equilibration and production runs). The average energies were calculated
from the last 200 ps of production run.
Typical Crystallization Procedure for Preferential Enrichment.
In the case of 1a (Figure 2): S-Rich 1a (2.000 g) of 2.5% ee was
dissolved in EtOH (32 mL) on heating. The resulting ca. 20-fold
supersaturated solution was stirred at 25 °C until crystallization began
(usually within 2 h) and then allowed to stand for 4 days at the same
temperature. The deposited crystals were separated from the mother
liquor by filtration (or centrifugation if the amount is small). From the
S-rich solution, 0.192 g (96.4% ee) of 1a was obtained as a viscous oil
after evaporation of the solvent. The deposited R-rich crystals (1.802
g, 7.5% ee) were subsequently recrystallized from EtOH (32 mL) in a
similar way, leading to the deposition of antipodal S-rich crystals (1.585
g, 4.6% ee) and an enrichment of the R enantiomer in the mother liquor,
from which R-rich 1a (0.212 g, 98.0% ee) was obtained as a viscous
oil. Similar crystallizations were repeated five times in all.
X-ray Crystallographic Analysis. For the X-ray crystallographic
analysis, the single crystal was mounted in a sealed capillary. The data
collections were performed at 293 K on a Rigaku AFC7R diffractometer
with graphite-monochromated Cu KR radiation to 2θmax of 120.0° for
(()-1a, an Enraf-Nonius CAD4 diffractometer with graphite-mono-
chromated Cu KR radiation to 2θmax of 135.9° for (()-3, and an Enraf-
Nonius Kapp CCD diffractometer with graphite-monochromated MoKR
radiation to 2θmax of 57.5° and 56.6° for (()-1d and (()-4, respectively.
All of the crystallographic calculations were performed by using teXan
software package of the Molecular Structure Corp. The crystal structure
was solved by the direct methods and refined by full-matrix least
squares. All non-hydrogen atoms were refined anisotropically. The
summary of the fundamental crystal data and experimental parameters
for structure determinations is given in Table 1. The experimental details
including data collection, data reduction, and structure solution and
refinement as well as the atomic coordinates and Biso/Beq, anisotropic
displacement parameters, have been deposited in the Supporting
Information.
Figure 23. Chiral discrimination by initially formed crystals with a low
ee value.
observed during crystallization. A slight enrichment of the
opposite enantiomer in the deposited crystals observed regularly
after each recrystallization could be rationalized in terms of the
solvent-assisted solid-to-solid type of polymorphic transition
from the kinetically formed metastable crystal to the thermo-
dynamically stable one. A considerable enantiomeric enrichment
in the mother liquor has been correlated with this polymorphic
transition and the subsequent chiral discrimination by the once
formed stable crystals with low ee values. We anticipate that
such a polymorphism will be ubiquitous in chiral organic
compounds, in which the homochiral chain structure is stable
in solution while the heterochiral chain structure is stable in
the crystal. Probably this type of chiral compound comprises
large molecules, which can form homochiral chain structures
in solution and in the metastable crystalline phase, and must
have at least three hydrogen bond-forming functional groups
at the appropriate positions so as to allow the subsequent
polymorphic transition. Furthermore, the onium salt structure
seems to be advantageous for rearrangement of hydrogen bonds
to undergo polymorphic transition. We believe that Preferential
Enrichment could be extended to a powerful general method
for the enantiomeric resolution of organic racemates which are
classified as a fairly ordered mixed crystal composed of the
two enantiomers and show a similar polymorphism and thereby
polymorphic transition during crystallization.
Experimental Section
Acknowledgment. The present work was supported by the
Grant-in-Aid for Scientific Research from the Ministry of
Education, Science, Culture, Sports and Technology of Japan.
R.T. is grateful for supports from the Asahi Glass Foundation,
the Mitsubishi Foundation, and the Takeda Science Foundation.
We thank Daiso Co., Ltd. for the generous gifts of R- and
S-epichlorohydrins.
Generals. Melting points are measured by DSC. Differential
scanning calorimetry (DSC) was performed at the scanning rate of 5
°C/min. FT Infrared spectra were recorded as KBr pellets. The in situ
FTIR spectra were recored in solution or suspension by using the
attenuated total reflection (ATR) method on ReactIR 4000 (ST Japan).
1H NMR spectrum was recorded at 400 MHz, and 13C NMR was
recorded at 100 MHz; CDCl3 or CD3OD was used as the solvent. HPLC
analysis was carried out by using a chiral stationary phase column
(Daicel Chiralcel OD-H, 0.46 × 25 cm), a mixture of hexane, ethanol,
trifloroacetic acid, and diethylamine (800:200:5:1) as the mobile phase
at the flow rate of 0.5 mL/min, and a UV-vis spectrometer (254 nm)
as the detector.40 Powder X-ray diffraction patterns were recorded at a
continuous scanning rate of 2° 2θ/min using Cu KR radiation (40 kV,
20 mA) with the intensity of diffracted X-rays being collected at
intervals of 0.02° 2θ. A Ni filter was used to remove Cu Kâ radiation.
The number-averaged molecular weights of the solute in CHCl3 were
measured on the basis of the differential vapor pressure technique by
Supporting Information Available: X-ray crystallographic
file (CIF) for (()-1a (alternative name, NNMe3), (()-1d
(NPMe3), (()-3 (NNMe3-OPr), and (()-4 (NNMe3-C); the
synthetic scheme of (()-1a, and experimental procedures for
the synthesis of compounds (()-1a, (()-1c, (()-1d, (()-3, and
(()-4 (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA020454R
(41) Discover and COMPASS are distributed along with a software package
Materials Studio (Version 2.0); Accelrys Inc.: San Diego, 2001.
(42) Sun, H. J. Phys. Chem. B 1998, 102, 7338-7364.
(40) Ushio, T.; Yamamoto, K. J. Chromatogr. A 1994, 684, 235-242.
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