Optically active helical substituted polyacetylenes as chiral seeding for inducing enantioselective crystallization of racemic N -(tert -butoxycarbonyl)alanine

Article abstract of DOI:10.1021/ma2013628

Helical substituted polyacetylenes were investigated for inducing enantioselective crystallization of racemic N-(tert-butoxycarbonyl)alanine (BOC-alanine) enantiomers. For this purpose, helical substituted polyacetylenes [(R)-PSA and (S)-PSA)] were dissolved in supersaturated racemic BOC-alanine solutions. Upon cooling the solutions, (R)-PSA preferentially induced BOC-l-alanine to crystallize, while (S)-PSA facilitated the enantioselective crystallization of BOC-d-alanine, according to the characterizations by circular dichroism, XRD, SEM, and optical rotation analyses. As expected, no enantioselective crystallization was observed in such cases: racemic BOC-alanine enantiomers in the absence of optically active helical PSA and racemic BOC-alanine enantiomers in the presence of equal amount of (R)-PSA and (S)-PSA. The present study provides the first direct evidence for the role of artificially synthetic helical polymers in inducing efficiently enantioselective crystallization.

Full text of DOI:10.1021/ma2013628

ARTICLE
pubs.acs.org/Macromolecules
Optically Active Helical Substituted Polyacetylenes as Chiral Seeding
for Inducing Enantioselective Crystallization of Racemic
N-(tert-Butoxycarbonyl)alanine
Bo Chen, Jianping Deng,* Xin Cui, and Wantai Yang
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China
S Supporting Information
b
ABSTRACT: Helical substituted polyacetylenes were investi-
gated for inducing enantioselective crystallization of racemic
N-(tert-butoxycarbonyl)alanine (BOC-alanine) enantiomers.
For this purpose, helical substituted polyacetylenes [(R)-PSA
and (S)-PSA)] were dissolved in supersaturated racemic BOC-
alanine solutions. Upon cooling the solutions, (R)-PSA prefer-
entially induced BOC-L-alanine to crystallize, while (S)-PSA
facilitated the enantioselective crystallization of BOC-D-alanine,
according to the characterizations by circular dichroism, XRD,
SEM, and optical rotation analyses. As expected, no enantiose-
lective crystallization was observed in such cases: racemic BOC-alanine enantiomers in the absence of optically active helical PSA
and racemic BOC-alanine enantiomers in the presence of equal amount of (R)-PSA and (S)-PSA. The present study provides the
first direct evidence for the role of artificially synthetic helical polymers in inducing efficiently enantioselective crystallization.
’ INTRODUCTION
Among the relatively new chiral resolution techniques, chiral
HPLC (high-performance liquid chromatography) may be the
most powerful and the most widely studied one.10 However,
tedious process, high cost, and in particular the difficulty in
selecting a suitable chiral resolving medium largely limit further
extended practical applications of HPLC. Another technique,
crystallization11 of a long history, is an industrial process for
enantioseparation and thus attracts our special attention recently.12
In practice, enantioselective seeding of racemic solution is fre-
quently adopted to substantially shorten the separation process
and meanwhile to enhance the separation efficiency.13 So far,
chiral polymers,14 chiral polymer-based miniemulsions15 and
microspheres,16 self-assembled films,17 chiral solvents,18 chiral hostÀ
guest system,19 etc., have been investigated as chiral additives to
induce enantioselective crystallization. Nevertheless, no other
reports with the exception of our earlier one12 focus on enantio-
selective crystallization induced by synthetic helical polymers.
In preceding studies, we have prepared a unique type of poly-
meric particles consisting of helical substituted polyacetylenes.20,21
Hollow microspheres bearing helical polymer segments demon-
strated remarkable capacity for chiral recognition.21 Chiral hy-
brid nanoparticles consisting of optically active helical polymer/
silica were capable of facilitating considerable chiral discrimi-
nation in aqueous systems.12,20f Although such chiral hybrid
Synthetic helical polymers have been actively studied over the
past two decades due to their unique structures and interesting
properties.1 More recently, helical polymers with much complex
structures have been elegantly created, including polymers with
double-helix,2 fun-structured pendent groups,3 etc. Helical poly-
mers are always considered being of some significantly potential
applications in particular for chiral catalysis,4 separation mate-
rials,5 optical materials,6 and advanced functional materials.7 For
chiral resolutions in the literature, a majority of the investigations
are conducted in the state of (hydro)gels made up of helical
polymers.8 We also prepared (hydro)gels consisting of helical
substituted polyacetylenes showing intriguing chiral recognition
performances.9 The present study attempted another unprece-
dented approach of general interest and high promise, i.e., selec-
tive chiral crystallization of racemic isomers induced by helical
polymers from initial homogeneous solution. Herein, the helical
polymers function as chiral seeding, as to be reported in detail
below.
We live in a chiral world containing chiral compounds at both
molecular and macromolecular levels. These compounds are
responsible for the maintenance of life. Even though the origin
for the nature to favor one of the enantiopure chiral structures
over the opposite one is still open to discussion, chiral resolution
of a chiral isomer from its mirror image form is thus an undoub-
tedly significant research theme. Despite the various techniques
developed for chiral resolution, considerable disadvantages of
them make more facile and effective approaches still required.
Received: June 15, 2011
Revised:
August 22, 2011
Published: September 01, 2011
r
2011 American Chemical Society
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Scheme 1. Schematic Illustration of the Strategy for Inducing Enantioselective Crystallization of Racemic BOC-alanine by Using
(R)-PSA or (S)-PSA
nanoparticles exhibited considerable chiral discrimination per-
formance, they are considered as being of some disadvantages
including the complex preparing processes and particularly the
utility limited in aqueous medium. More importantly, no direct
evidence has been achieved to identify the role of helical
structures in chiral resolution. With the aim to directly verify
the function of helical polymers in inducing enantioselective
crystallization, we in the present study directly used helical
polymers to induce enantioselective crystallization in solution,
following a novel, effective, and facile methodology schematically
outlined in Scheme 1.
In Scheme 1, helical polymer (PSA) was utilized to induce
enantioselective crystallization of racemic BOC-alanine (N-(tert-
butoxycarbonyl)alanine) in CHCl3. We found (R)-PSA enabled
BOC-L-alanine to crystallize preferentially, whereas (S)-PSA
favored enantioselective crystallization of BOC-D-alanine. The
chiral resolution results are satisfying, and the role of helical
polymers is clearly identified. Also noteworthy is that in the
methodology under discussion helical polymers are applicable in
organic solvents, while in the earlier paper12,20f the hollow parti-
cles are applicable in aqueous systems. Accordingly, the present
study together with the earlier ones12,20f is expected to provide a
novel efficient approach for chiral resolution from either organic
or aqueous solutions.
then heated to 35 °C. A predetermined amount of substituted poly-
acetylene (chiral PSA) was added in the above racemic BOC-alanine/
CHCl3 supersaturated solution and stirred for ca. 10 min, and then the
solution was left to cool spontaneously to room temperature (∼25 °C)
under ambient conditions. In the course of cooling, crystals first visually
formed and then grew larger. The crystals were filtrated, collected, and
subject to circular dichroism (CD), X-ray diffraction (XRD), and scan-
ning electron microscopy (SEM) analyses, and the residual filtrate was
subject to optical rotation measurements. The other processes are simi-
lar to those reported in our previous investigations.12,20f
Characterizations. CD and UVÀvis absorption experiments were
conducted on a Jasco-810 spectropolarimeter. Specific rotations were
measured on a JASCO P-1020 digital polarimeter with a sodium lamp as
the light source at room temperature by using the BOC-alanine CHCl3
solutions. The molecular weights and molecular weight polydispersities
were determined by GPC (Shodex KF-850 column) calibrated by using
polystyrenes, with THF as eluent. SEM was performed by a Hitachi
S-4700 electron microscope at an accelerating voltage of 20.0 kV. XRD
analyses of the crystals were conducted with a Shimadzu XRD-6000.
’ RESULTS AND DISCUSSION
We have previously examined the chiral recognition and chiral
resolution ability of helical substituted polyacetylenes. Such
polymers in the state of gels9 and spheres21 both showed the
expected ability. In particular, the hybrid hollow chiral particles
demonstrated remarkable ability for inducing enantioselective
crystallization.12 Nevertheless, the crucial role of the helical poly-
mers cannot be directly observed in both gels and spheres. There-
fore, taking helical substituted polyacetylene (PSA) and BOC-
alanine system as a typical model system in the present study,
we directly used the synthetic helical polymers for inducing
enantioselective crystallization from BOC-alanine enantiomers
solution. The major sequence and advantages of this practice are
illustratively presented in Figure 1.
Figure 1 depicts the major procedure for enantioselective
separation of BOC-alanine by optically active PSA as chiral
seeding in CHCl3 solution, taking (R)-PSA inducing BOC-
L-alanine as a representative. A typical enantioselective crystal-
lization experiment was performed as briefly stated below. A pre-
determined amount of chiral PSA (R- or S-PSA) was dissolved in
a racemic D,L-alanine supersaturated solution in CHCl3. The
related concept is schematically presented in Scheme 1 and
Figure 1. At beginning the racemic BOC-alanine solution is clear
and pale yellow (Figure 1A) (the yellowish color is derived from
the helical structures of PSA22). After staying for ca. 120 h at
room temperature (∼25 °C), precipitation began to occur in the
solution (Figure 1B). The precipitate grew larger gradually, and
’ EXPERIMENTAL SECTION
Materials. Solvents were purified by distillation under reduced
pressure under a N2 atmosphere in advance. Deionized water was used
for polymerizations. 1R-(À)- and 1S-(+)-10-camphorsulfonyl chloride
and propargylamine were purchased from Aldrich and used as received
without further purification. BOC-D- and L-alanine (D- and L-N-(tert-
butoxycarbonyl)alanine, respectively) were purchased from Aladdin
Reagent Co. (Shanghai, China) and were used directly. All the other
reagents were directly used. N-Propargyl-(1R and 1S)-camphor-10-
sulfamide, denoted as (R)-SA and (S)-SA, respectively, were synthesized
and polymerized (providing (R)-PSA and (S)-PSA, respectively) ac-
cording to the method described by us earlier.22 (nbd)Rh+BÀ(C6H5)4
was prepared through the synthetic procedure.23 The parameters of the
(R)-PSA and (S)-PSA used in this study are Mn (number-average mole-
cular weight) = 4000 and 3600 and Mw/Mn (polydispersity of the mole-
cular weight) = 1.86 and 1.98, respectively.
Enantioselective Crystallization Experiments. All enantiose-
lective crystallizations were accomplished in supersaturated BOC-ala-
nine solutions in CHCl3 under room conditions. A typical crystallization
experiment was performed as follows. BOC-D- and L-alanine (270 mg,
each) were simultaneously added in 3 mL of CHCl3. The solution was
stirred for ca. 5 min until BOC-alanine was completely dissolved and
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Figure 1. Schematic representation of the major procedure for enan-
tioselective crystallization of BOC-D- or L-alanine enantiomers induced
by chiral PSA, taking (R)-PSA inducing BOC-L-alanine as example:
(A) supersaturated racemic BOC-alanine/CHCl3 solution; (B) the
alanineÀPSA system after crystallization for 120 h and (C) after 160 h.
Figure 2. SEM images of crystals formed via enantioselective crystal-
lization in racemic BOC-alanine supersaturated solutions induced by
(A) (R)-PSA and (B) (S)-PSA under otherwise identical experimental
conditions (time = 180 h, c = 2.25 M; the solutions were left to cool
spontaneously from 35 °C to ∼25 °C). (C) Racemic BOC-alanine
crystals obtained by using the equal amount of (R)- and (S)-PSA (time =
200 h). (D) Typical morphology of racemic BOC-alanine crystals
obtained in the absence of PSA under otherwise identical conditions
(time = 240 h).
then crystals can be observed visually (Figure 1C). About 160 h
later, the crystallization was ended and the precipitate was
filtrated, providing pale yellow solid crystals. (Both BOC-alanine
and PSA are simutaneously present. See below.) According to
their different solubility, the crystallized BOC-alanine and PSA
can be readily separated. For the recovered PSA, it can be reused
for another cycle of enantioselective crystallization. We find that
(R)-PSA prompted the enantioselective crystallization of BOC-
L-alanine while (S)-PSA enabled BOC-D-alanine to preferentially
crystallize. This consideration was evidently confirmed by CD,
XRD, and optical rotation measurements, as to be introduced in
detail below. Herein it should be noted that, after optimization of
the structure of the helical polymers and the experimental
conditions for enantioselective crystallization, the crystallization
time shall be shortened largely.
After crystallization and separation, the as-obtained enantio-
pure BOC-alanine was subject to measurements. The morpho-
logy of the formed crystals is observed by SEM, as presented in
Figure 2. Both BOC-L- and BOC-D-alanine formed needlelike
crystals (Figure 2A,B). The two crystals demonstrate little
difference in visual appearance and in SEM images. We further
observed the orginal pure BOC-D- and L-alanine enantiomers,
and the SEM images are presented in Figure S1 of the Supporting
Inormation. Both the two enantiopure isomers also form needle-
like crystals. This is rather different from our earlier investigations.12,20f
Therein racemic alanines were used as the chiral compound for
chiral separation via enantioselective crystallization by using
PSA/silica core/shell particles. The obtained two enantiopure
alanines formed different crystal morphologies.12,20f In more de-
tail, L-alanine formed octahedral crystals while D-alanine formed
needlelike crystals. Therein we also revealed that (S)-PSA pre-
ferentially induced L-alanine to crystallize, while (R)-PSA is
favorable for D-alanine. On the basis of these two cases (the
present study and the preceding ones12,20f), it is thus clearly
indicated that the enantioselective crystallization is dependent on
the racemics, the chiral additives, the chiral environment, etc. It
further indicates that the crystal morphologies are closely related
with the chiral additives, which largely influence the nucleation
and orientation of the crystals.24 Nonetheless, the enantioselec-
tive crystallization is an elegant process, and more investigations
are required for us to obtain deep understanding of it. The SEM
images in Figure 2 cannot offer much information for us to analyze
the enantioselective crystallization. Fortunately, additional experi-
ments provide remarkable evidence, as to be reported next.
In order to verify and understand enantioselective crystal-
lization of soluble chiral PSA toward BOC-alanine enantiomers,
PSA consisting of equal amounts of (R)-PSA and (S)-PSA were
used for crystallization of racemic BOC-alanine. In this case no
pure and regular BOC-D- or L-alanine enantiomer crystals gene-
rated under the otherwise identical experimental conditions. The
relevant SEM images are exhibited in Figure 2C. Figure 2D
presents the SEM images of the obtained crystals for racemic
BOC-alanine via self-crystallization in the absence of PSA. In
both Figures 2C and 2D, no regular crystals are observed. The
e.e. of the residual racemic BOC-alanine solutions in these two
cases was determined to be ∼8%. The two experiments in
Figures 2C and 2D plus Figures 2A and 2B clearly demonstrate
the significant role of helical PSA as chiral selector for enantio-
selective crystallization. Namely, the chirality of PSA was the
decisive origin for the regular crystals formed in enantioselective
crystallization process. Additionally, we also used DMF as
solvent instead of CHCl3 for performing crystallization. Just
as expected, DMF provided not-so-satisfactory effects as in
Figures 2A,B (see Figure S2), since DMF is not favorable for
PSA to maintain helical structures.22 Figure S2 shows that the
crystals, even though they can be formed, are not so regular in
morphology when compared with the corresponding crystals in
Figures 2A,B. Furthermore, when DMF is used as the solvent,
crystallization took a much longer time (∼360 h) than in CHCl3.
Accordingly, it is further identified the dominant role of helical
structures of PSA in inducing enantioselective crystallization.
More specifically, the chirality of the induced chirally pure BOC-
alanine is dominated by the added chiral PSA. It should be
highlighted that this is the first report on using synthetic helical
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macromolecules to induce enantioselective crystallization from a
homogeneous solution system.
The relevant results are illustrated in Figure 4. For [(R)-PSA +
BOC-L-alanine] enantioselective crystallization system, the in-
duced BOC-L-alanine crystals containing chiral (R)-PSA simul-
To acquire more insights into the induced crystals via pre-
ferential crystallization, we characterized the crystals by XRD.
Also, taking BOC-L-alanine crystals as the representative, the
obtained XRD profile are illustrated in Figure 3. The two profiles
for original pure BOC-L-alanine and BOC-L-alanine obtained via
enantioselective crystallization are almost completely identical,
since PSA cannot form crystal structures under the investigated
conditions. BOC-D-alanine crystals and the corresponding ori-
ginally enantiopure BOC-D-alanine showed the same phenom-
enon, as shown in Figure S3. Similar observations were also
attained in preferential crystallization induced by hollow two-
layered chiral PSA-silica NPs and PSA-silica core/shell NPs in
our earlier study.12,20f
taneously exhibited the CD signals of both (R)-PSA (λmax
=
415 nm) and BOC-L-alanine (λmax = 215 nm) (see Figure 4A).
UVÀvis spectra in Figure 4B demonstrate similar results.
In addition to the CD and UVÀvis spectra, optical rotation of
the residual racemic alanine/CHCl3 solution after crystallization
was also measured. Figure 5 indicates the change of e.e. as a func-
tion of crystallization time. After ca. 180 h, the maximum chiral
resolution was achieved. The preferential crystallization process
seem to be somewhat slow; however, e.e. could be as high as 78%.
The results obtained in the case of [(S)-PSA + BOC-D-alanine]
system gave similar data, for which BOC-D-alanine was prefer-
entially crystallized and a maximum e.e. up to 77% was also
achieved in the residual BOC-L-alanine. The related CD and
UVÀvis spectra and the e.e. plot against crystallization time in
the [(S)-PSA + BOC-D-alanine] system are presented in Figures
S4 and S5.
The obtained crystals were further investigated by CD and UVÀvis
spectra measurements, referring to our previous investigations.12,20f
After dissolving the induced BOC-D- or L-alanine crystals
containing the chiral PSA in acetone and then centrifuging the
solution mixture, the chiral additive PSA can be easily isolated, as
Figure 5. Plot of e.e. (%) as a function of crystallization time (the
remaining BOC-D-alanine in excess in solution). 15 mL racemic alanine
supersaturated solutions (BOC-D-alanine/BOC-L-alanine = 1735/1735,
in mg), 2.5 mg of chiral (R)-PSA. For crystallization conditions, refer to
Figure 2.
Figure 3. Typical X-ray diffraction profiles of BOC-alanine crystals:
(A) original enantiopure BOC-L-alanine and (B) BOC-L-alanine crystals
induced by chiral (R)-PSA. For the crystallization conditions, refer to
Figure 2.
Figure 4. (A) CD and (B) UVÀvis spectra of the crystals [BOC-L-alanine + (R)-PSA] obtained via enantioselective crystallization, pure (R)-PSA, and
pure BOC-L-alanine (for crystallization conditions, refer to Figure 2; crystallization for 180 h). All the spectra were measured in CHCl3 solution;
concentration of the samples = 0.1 mM; at room temperature.
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Scheme 2. Schematic Showing the Possible Mechanism for Chiral Helical Polymers To Induce Enantioselective Crystallization
ARTICLE
conceptually illustrated in Figure 1. The white crystals of
BOC-alanine enantiomer were obtained via recrystalliza-
tion from the clear acetone solution, and the residual yellow-
green solid was identified to be chiral PSA and thus can
be reused. The present study undoubtedly provided new in-
sights and better understanding of optically active polymers
with helical structures regarding practical applications and
remarkably broadened new applications of helical substituted
polyacetylenes.
’ CONCLUSIONS
Helical substituted polymers (PSAs) were added in a super-
saturation solution of BOC-alanine racemate in CHCl3, forming
a homogeneous system. Cooling the solution resulted in enantio-
selective crystallization of BOC-alanine, in which PSA served as
chiral seeds. SEM, CD, XRD, and optical rotation measurements
demonstrated that (R)-PSA induced preferential crystallization
toward BOC-L-alanine, while (S)-PSA toward BOC-D-alanine.
The enantioselective crystallization is decided by the screw sense
of the helical structures of PSA. This is the first direct evidence to
demonstrate the role of helical structures in inducing enantiose-
lective crystallization. We believe that after optimizing the struc-
tures of PSA and crystallization conditions a higher chiral separa-
tion efficiency will be realized. Such investigations are currently
in progress in our lab.
The investigations presented above convincingly demon-
strate the vital role of helical chiral PSAs as chiral nucleating
agent for inducing enantioselective crystallization on BOC-
alanine isomers. With the crystalys in hand, now we consider
another question, i.e., what induced the enantioselective crys-
tallization? We proposed our opinion below for this question.
For the enantioselective crystallization of racemic BOC-ala-
nine induced by chiral helical PSA, again taking (R)-PSA
toward BOC-L-alanine as an example, the possible underlying
mechanism is presented in Scheme 2. In the solution of
racemic BOC-alanine in CHCl3 in the presence of (R)-PSA,
BOC-L-alanine molecules are preferentially adsorbed on (R)-
PSA macromolecules at the beginning of cooling. This process
results from the enantioselective affinity of (R)-PSA toward
BOC-L-alanine molecules due to the different stereospecific
configurations of racemic BOC-alanines. As temperature con-
tinues to decrease, more BOC-L-alanine molecules are ad-
sorbed and agglomerated surrounding (R)-PSA macromole-
cules, which then act as chiral nucleation sites for the formation
of crystals. Along with this reasoning, further cooling leads to
tiny crystals appeared; they gradually grow larger forming a
suspension from the initial clear solution. In this process, an
increase in the crystals size and concentration gives rise to a
slight decrease in the transparency of the sample (Figure 1A,
B). Finally, the induced crystals griw so large that precipitation
(finally formed crystals) occurs from the CHCl3 solution, as
observed in Figure 1C.
The present methodology is expected to find remarkable
applications in practice. Moreover, the flexibility of designing
helical polymers with favorable functional groups in side chains
and the easy recovery of the chiral additives make the present
concept considerably advantageous over common chiral poly-
mers in inducing enantioselective crystallization. This study
opens up new possibility toward effective and facile enantiose-
parations. However, more studies are still required for us to
understand well the enantioselective crystallization induced by
chiral helical polymers. Some fundamental questions need to be
solved including the correlation between the specific conforma-
tion of helical polymers with the efficiency and chiral configura-
tion of the enantiomer preferentially crystallized, the detailed
enantioselective crystallization mechanism, etc. For answering
these questions, the novel concept is currently tried on other
chiral helical polymers and chiral isomers.
’ ASSOCIATED CONTENT
S
Supporting Information. Crystal morphology of pure
b
BOC-D- and L-alanine; typical X-ray diffraction profile of BOC-
D-alanine crystals; CD and UVÀvis spectra of the crystals ob-
tained via enantioselective crystallization toward [(S)-PSA +
BOC-D-alanine]; and e.e. (%) as a function of crystallization
time (BOC-L-alanine in excess). This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Tel +86-10-6443-5128; Fax +86-10-6443-5128; e-mail dengjp@
mail.buct.edu.cn.
’ ACKNOWLEDGMENT
This work is supported by the “National Natural Science
Foundation of China” (20974007, 20574004), the “Fundamen-
tal Research Funds for the Central Universities” (ZZ1117), the
“Program for New Century Excellent Talents in University”
(NCET-06-0096), and the “Project of Polymer Chemistry and
Physics, Beijing Municipal Commission of Education”.
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