Optically active microspheres from helical substituted polyacetylene with pendent ferrocenyl amino-acid derivative. Preparation and recycling use for direct asymmetric aldol reaction in water

Article abstract of DOI:10.1016/j.polymer.2017.08.022

The contribution reports a novel type of chiral microspheres in which ferrocenyl amino-acid derived catalytic moieties were integrated with optically active helical substituted polyacetylene. The microspheres were prepared by suspension copolymerization o

Full text of DOI:10.1016/j.polymer.2017.08.022

Polymer 125 (2017) 200e207
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Optically active microspheres from helical substituted polyacetylene
with pendent ferrocenyl amino-acid derivative. Preparation and
recycling use for direct asymmetric aldol reaction in water
Jinrui Deng, Jianping Deng*
State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing
100029, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The contribution reports a novel type of chiral microspheres in which ferrocenyl amino-acid derived
catalytic moieties were integrated with optically active helical substituted polyacetylene. The micro-
spheres were prepared by suspension copolymerization of achirally and chirally substituted acetylenic
monomers in a quantitative yield. SEM images showed the microspheres' regular spherical morphology.
Circular dichroism and UVꢀvis absorption spectra demonstrated that the copolymer chains constituting
the microspheres adopted predominantly one-handed helices, endowing the microspheres with optical
activity. The microspheres were further used to catalyze aldol reaction between cyclohexanone and p-
nitrobenzaldehyde in water media in the presence of Brøsted acid, providing the product with
remarkable yield (up to 91%) and enantiomeric excess (up to 93%). Synergistic effects occurred in the
helical polymer chains and the pendant catalytic moieties. The catalytic microspheres can be finely
recycled and reused three times without significant loss of catalytic ability and enantio-selectivity.
© 2017 Elsevier Ltd. All rights reserved.
Received 3 June 2017
Received in revised form
27 July 2017
Accepted 9 August 2017
Available online 10 August 2017
Keywords:
Optically active microspheres
Ferrocenyl-amino acid
Asymmetric reaction
1. Introduction
experienced. L-Proline, 4-hydroxy-L-proline, enzymes and other
organic molecules have attracted much attention and been widely
used as chiral organocatalysts in aldol reactions. Moreover, most
asymmetrically organocatalyzed direct aldol reactions used water
as media and achieved high diastereo- and enantioselectivity
[14e17].
Water becomes increasingly important to be used as the media
for chemical transformations because of the growing environ-
mental pollution due to numerous uses of harmful organic solvents
in chemical industries. Accordingly, using clean, eco-friendly and
the safest solvent water as environmentally benign synthetic media
has become a hot topic in chemistry. Compared to many organic
solvents, water possesses some unique features, for example, a very
high dielectric constant and cohesive energy density, which may
generate new chemical reactivity [1e10].
Optically active polymer microspheres have become a rapidly
expanding research topic since their combination of the properties
of chiral polymers and particulate materials. Therefore, optically
active polymer microspheres possess many intriguing advantages
such as chirality, large specific surface area and easy separability.
Taking these merits into consideration, chiral microspheres were
employed widely in significant applications like chiral recognition
[18e20], enantioselective crystallization [21,22], chiral assembly
[23], and asymmetric catalysis [24,25]. Particularly, optically active
microsphere-supported catalysts for asymmetric reactions can
efficaciously solve the problems associated with difficulties in
recovering and reusing homogeneous catalysts. Efficient methods
have been established in our group for preparing optically active
microspheres derived from helical polyacetylenes, e.g. emulsion
polymerization [26], precipitation polymerization [27], suspension
polymerization [28] and self-assembly [29].
Aldol reactions are recognized a powerful synthetic method for
preparing
of carbohydrates [12].
known to excellently catalyze asymmetric direct aldol reactions
since the early reports in the 1970s that the -proline catalyzed
b
-hydroxy carbonyl skeletons [11] and the biosynthesis
L-Proline and the derivatives have been
L
intramolecular aldol reactions [13]. From then on, an explosive
growth in developing organic catalysts towards aldol reactions for
enantiomerically enriched
b-hydroxy carbonyl motifs has been
* Corresponding author.
E-mail address: dengjp@mail.buct.edu.cn (J. Deng).
http://dx.doi.org/10.1016/j.polymer.2017.08.022
0032-3861/© 2017 Elsevier Ltd. All rights reserved.

J. Deng, J. Deng / Polymer 125 (2017) 200e207
201
In the context mentioned above and on the basis of our fore-
2.3. Preparation of chiral microspheres
going studies dealing with helical polyacetylenes [26e29], we
designed and prepared optically active helical substituted poly-
acetylene microspheres containing catalytic ferrocenyl amino-acid
derivative moieties. We highlight that ferrocene as unique molec-
ular scaffold [30] has been widely used towards catalysis, however
there has been no report yet on combining ferrocene with helical
macromolecules for asymmetric catalysis, except our earlier paper
[31]. The obtained microspheres possessed significant catalytic
ability towards asymmetric aldol reaction between cyclohexanone
and p-nitrobenzaldehyde in water media, which was taken as a
model reaction in the present study. Furthermore, synergistic ef-
fects occurred between the helical copolymer main-chains consti-
tuting the microspheres and the pendent ferrocenyl amino-acid
derivative moieties, which is favorable for the asymmetric aldol
reaction in yield and stereoselectivity. More importantly, the as-
prepared microspheres can be facilely recycled and performed
desirable reusability in the asymmetric aldol reaction, with much
improved catalytic performance relative to our previous studies
[24,25,27]. Accordingly, the present chiral microspheres are
remarkably advantageous over the corresponding analogous chiral
macromolecular catalyst limited in homogeneous catalysis [31]. We
further anticipate that the novel microspheres to demonstrate good
performance and large promise as chiral catalyst towards other
asymmetric reactions. The present study may also pave a new way
for immobilizing other expensive homogeneous catalysts to realize
easy recovery and reuse, thus avoiding product contamination.
The microspheres were synthesized by suspension polymeri-
zation, for which the major procedure is illustrated in Scheme 1 and
briefly stated as follows [34]. The polymerization was performed in
a 100 mL glass reaction vessel attached with a Teflon stirrer under a
nitrogen atmosphere. L-MTFc (18.4 mg, 0.05 mmol), M1 (90.4 mg,
0.45 mmol) and dibutynyl adipate (25.0 mg, 0.10 mmol) were dis-
solved in CHCl3 (0.5 mL). Then the solution was added in a PVP
aqueous solution (2 wt %, 50 mL) under vigorous stirring at a rate of
350 rpm in ice bath. After stirring for 30 min, a solution contained
Rh catalyst [Rh(nbd)Cl]2 (2.4 mg, 0.10 mmol) and Et3N (50 mL) was
added in the above reaction system. Subsequently, the reaction
system was heated at a rate of 10 ꢁC hꢀ1 till 30 ꢁC, and then kept for
2 h. The whole procedure was performed under nitrogen atmo-
sphere. After polymerization, the resulting yellow MPs suspended
in the aqueous media were collected by filtration, repeatedly
washed with ethyl alcohol and deionized water, and then dried.
2.4. Asymmetric aldol reactions
Aldehyde (0.075 g, 0.5 mmol), a predetermined amount of mi-
crospheres (10 mol % of aldehyde, based on catalytic unit) and 2 mL
of solvent water were added in sequence in a reactor. The reaction
mixture was stirred for 15 min and then cyclohexanone (0.5 mL)
was added. The reaction was monitored by TLC. After the asym-
metric aldol reaction, the mixture was quenched with saturated
NaHCO3 aqueous solution, extracted three times with ethyl acetate,
and the combined organic phases were dried, filtered, and
concentrated in vacuo to afford the crude product. The anti/syn
ratio of the product was determined by 1H NMR analysis. The mi-
crospheres were filtrated from the mixture, slightly rinsed with
deionized water, dried, and reused for next cycle. The enantiomeric
excess (ee) was determined by chiralphase HPLC analysis of the
pure anti-product, which was purified by flash column chroma-
tography (n-hexane/EtOAc ¼ 3/1, v/v) in advance. The product,
(2S,10R)-2-(hydroxyl- (p-nitrophenyl)methyl) cyclohexan-1-one,
was characterized with FT-IR, 1H and 13C NMR analyses.
2. Experimental section
2.1. Materials
Achiral monomer (M1) was prepared according to a method
reported earlier [32], and chiral monomer (L-MTFc) was synthe-
sized as reported [31]. A bifunctional butynyl ester (dibutynyl
adipate) was prepared referring to the method in literature [33] and
was used as cross-linker to prepare cross-linked polymer micro-
spheres. Triethylamine (Et3N) and polyvinylpyrrolidone (PVP K90)
were purchased from Beijing Chemical Reagent Co. and used
2.5. Chiral HPLC analysis of aldol reaction product
without further purification. [Rh(nbd)Cl]2 (nbd
¼
2,5-
norbornadiene) (Alfa Aesar), cyclohexanone and p-nitro-
benzaldehyde (Aldrich) were used as received. Solvents were pu-
rified by distillation. Deionized water was used for the experiments.
For the product (2S,10R)-2-(hydroxyl-(p-nitrophenyl) methyl)
cyclohexan-1-one [15,35,36], the percent ee was determined by
HPLC analysis, Chiralpak AD-H, n-hexane/i-PrOH ¼ 90/10, v/v,
1.0 mL minꢀ1
,
254 nm; tr (minor)
¼
13.01 min, tr
(major) ¼ 16.75 min). The obtained four stereoisomers of 2-[hy-
droxyl-(4-nitrophenyl)-methyl] cyclohexanone were defined as
MSR, MRS, MSS, and MRR, respectively. Among the isomers, MSR and
MRS are a pair of enantiomers with anti-configuration, while MSS
and MRR are the syn-configuration enantiomers. The diastereomer
ratio of aldol transformation is defined as dr ¼ anti/syn, and the
enantiomer excess of the anti-configuration products is defined as
ee% (anti) ¼ (MSRꢀMRS)/(MSR þ MRS) ꢂ 100 (MSR as the major
product).
2.2. Measurements
The morphology of the microspheres was observed with a
Hitachi Sꢀ4800 scanning electron microscope (SEM). Circular di-
chroism (CD) and UVꢀvis absorption spectra were recorded on a
Jasco-810 spectropolarimeter. FT-IR spectra were measured on a
Nicolet NEXUS 870 infrared spectrometer. Raman spectra were
recorded on a Renishaw inVia-Refl exconfocal Raman microscope
with an excitation wavelength of 785 nm. Elemental analysis (C, H,
N, O) was recorded by vairo EL cube Elementar Analysensysteme
GmbH. Thermogravimetric analysis (TGA) was carried out with a
Q50 TGA at a scanning rate of 10 ꢁC/min under N2. The products
from the asymmetric catalysis were purified by preparative thin
layer chromatography (TLC) on silica gel by using the mixture n-
hexane/EtOAc as eluent. 1H and 13C NMR spectra were recorded on
a Bruker AV 400 spectrometer. High performance liquid chroma-
tography (HPLC) analysis was performed on FL2200-2 (FL2200-2
pump and absorbance detector). Chiralpak AD-H column was
purchased from Daicel Chemical Industries Ltd.
3. Results and discussion
3.1. Preparation and characterization of the microspheres
Based on our earlier studies about optically active particles
[18,26e29,34,37], we successfully prepared optically active micro-
spheres with ferrocenyl amino-acid derivative pendants in the
present work, aiming to solve the problem of unrecyclable ability of
catalysts. The optically active microspheres consisting of helical
substituted polyacetylenes were manufactured through catalytic

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J. Deng, J. Deng / Polymer 125 (2017) 200e207
Scheme 1. Illustrative strategy for preparing the microspheres (MPs) (A) and asymmetric Aldol reaction catalyzed by MPs (B).
suspension polymerization, as schematically illustrated in Scheme
1. The chiral monomer with catalytic group named as L-MTFc and
achiral monomer M1 underwent suspension co-polymerization
with a feed ratio of 1:9 (mol/mol) according to our previous work
[31]. The obtained copolymers through solution polymerization
showed obviously positive CD signals around 390 nm, demon-
strating the copolymers adopting predominantly one-handed he-
lical conformations. Especially, among the copolymers, the CD
signal reached a maximum intensity with the feed ratio of L-MTFc
and M1 being 1:9 (mol/mol), due to the synergistic effects occur-
ring between the co-monomer units. The synergistic effects
maximized the excess single-handed helicity in the co-polymer
main chains. Moreover, we introduced the dibutynyl adipate as
cross-linker for preparing cross-linked MPs. It should be mentioned
herein that we used [Rh(nbd)Cl]2/Et3N system other than (nbd)
RhþBꢀ(C6H5)4 in order to enhance catalytic activity. However, we
failed to obtain ideal microspheres due to the excess of amphipathy
of Et3N at the beginning of this work. Later, the anticipated mi-
crospheres were facilely prepared by reducing the dosage of Et3N
without impacting the catalytic activity.
agent was not used in the process during preparing micro-
spheres. Additionally, element distribution of the microspheres was
characterized by EDS mapping, as depicted in Fig. 2(BeE), showing
the organic components, especially iron from catalytic group of
ferrocenyl, were randomly distributed, which provides largely
dispersive catalytic sites for the subsequent aldol reactions. The
composition of the resulting microspheres was then investigated
by elemental analyses and the practical element contents are: C
73.08%, H 7.22%, N 5.72%, O 11.91%; the content of Fe was further
determined to be 2.05%. The data are in well agreement with the
theoretical values, i.e. C 73.04%, H 7.20%, N 5.75%, O 11.94%, and Fe
2.08%. The result also verified the conclusion that the suspension
polymerizations can provide microspheres in a quantitative yield.
To further confirm the successful fabrication of the micro-
spheres, monomers L-MTFc, M1, crosslinker and the corresponding
crosslinked microspheres were structurally identified by FT-IR
spectroscopy, as presented in Fig. 3. For the microspheres, the
peaks at 1655, 1562, and 3300e3400 cmꢀ1 indicate amide I and II
bands and the NeH stretching band, respectively. The peak at
1720 cmꢀ1 indicates the C]O structures in the cross-linker. The
peaks around 1100e1400 cmꢀ1 and 490 cmꢀ1 demonstrate the CeC
bond and CeFe bond of cyclopentene, respectively. Summarily, FT-
IR spectra, together with the images in Fig. 1, ascertain the forma-
tion of microspheres as expected. Furthermore, the stereo-
regularity of the polymer [(poly(L-MTFc0.1-co-M10.9), obtained by
solution polymerization) and the microspheres was measured by
Raman spectroscopy (Fig. 4). Raman spectra show that the
substituted polyacetylene chains forming the microspheres
possessed high cis content and high stereo-regularity, which is
helpful for the polymer chains to form helical structures [41e44].
Circular dichroism (CD) and UVꢀvis absorption spectroscopies
have been proved as effective and straightforward methods to
identify helical conformations in synthetic polymers. With the
The photograph of the obtained microspheres is presented in
Fig. 1(A). They were further observed by SEM, as presented in
Fig. 1(B,C,D). The typical photograph (Fig. 1(A)) clearly shows that
the resulting microspheres possessed regular spherical
morphology and uniform size, and exhibited a yellow color, which
originated from helical substituted polyacetylenes [38e40]. SEM
images (Fig. 1(BeD)) show that a mean diameter of approximately
450 mm with a satisfactory particle size distribution (the coefficient
of variation (CV) of the microspheres was 9.82%) and surface pore
structures in the resulting microspheres, which was caused by
phase separation inside the monomer drops during suspension
polymerization [25]. Nonetheless, the pore volume and surface area
were not large enough for measurement, because pore-forming

J. Deng, J. Deng / Polymer 125 (2017) 200e207
203
Fig. 1. Typical photograph of the microspheres: (A) scale bar, 1.00 cm; typical SEM images: (B) scale bar, 1.00 mm; (C) scale bar, 400 mm; (D) scale bar, 200 mm.
Fig. 2. EDS mapping patterns of (A) carbon, (B) oxygen, (C) nitrogen and (D) iron in a specific microsphere.
microspheres in hand, we next characterized them for optically
activity by CD and UVꢀvis absorption spectroscopies. To acquire
more insights into optical activity of the MPs (Fig. 5(A), spectrum b),
about 380 ꢁC under nitrogen atmosphere. TGA analysis shows that
the microspheres possessed good thermostability, which is favor-
able for asymmetric catalysis when used as catalyst even at a
relatively high temperature.
we made a comparison with copolymer poly(L-MTFc0.1-co-M10.9
(Fig. 5(A), spectrum a), as illustrated in Fig. 4. In our previous study
[31], the copolymer poly(L-MTFc0.1-co-M10.9 displayed an
)
)
3.2. Catalysis of aldol reactions with microspheres
apparent positive CD pattern around 390 nm, suggesting that the
chiral ferrocenyl amino-acid derivative pendants were arranged
into a one-handed helical array along the helical copolymer poly(L-
MTFc0.1-co-M10.9) backbones with an excess handedness. Similarly,
the CD signal and UVevis absorption around 390 nm of the mi-
crospheres powerfully demonstrate that the polymer chains
forming the crosslinked microspheres possessed helical structures
of predominant onehanded screw sense, enabling the microspheres
to possess noticeable optical activity. It should be pointed out
herein that the shifts in both CD and UV-vis spectra are due to the
specific measurements. For poly(L-MTFc0.1-co-M10.9), it was
measured in chloroform solution, while for the microspheres, they
were welled by chloroform and then compressed between two
pieces of quartz glass. However, the intense CD signals and UV-vis
absorptions can be clearly observed in both cases.
Water as the nature's most friendly media is gaining more and
more attention in asymmetric reactions [45e47]. With crosslinked
microspheres in hand, we selected water and brine as the friendly
media for asymmetric aldol reaction between cyclohexanone and
p-nitrobenzaldehyde to investigate the catalytic ability of the chiral
microspheres. The catalytic results are summarized in Table 1. For a
comparison, homopolymer polyM1 was also used as catalyst to
catalyze the asymmetric aldol reaction both in water and brine, but
failing to show catalytic activity owing to lack of catalytic group
(Table 1, run 1 and run 2). Subsequently, we employed chirally
substituted acetylenic monomer L-MTFc and the copolymer poly(L-
MTFc0.1-co-M10.9) as catalysts for the asymmetric aldol condensa-
tion between cyclohexanone and p-nitrobenzaldehyde in the
presence of 10 mol% of the primary amine in L-MTFc units (Table 1,
run 3e6). The catalytic results show that both the chiral monomer
L-MTFc and copolymer poly(L-MTFc0.1-co-M10.9) displayed a better
catalytic performance in brine than in H2O, giving a conversion of
Thermal property of the microspheres was measured by TGA
technique, as shown in Fig. 6. The microspheres began to decom-
pose at about 200 ꢁC and thermal decomposition temperature was

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J. Deng, J. Deng / Polymer 125 (2017) 200e207
remarkable improvement in both yield and ee data revealed the
fact that the chirally helical polymer chains and chiral ferrocenyl
amino-acid derivative moieties underwent synergistic effects in
catalysis process. To further explore the role of helical structures in
catalysis, we examined microspheres constructed by helical
copolymer chains, as discussed below.
To demonstrate the catalytic activity of the microsphere-
supported organocatalyst, the microspheres were investigated in
asymmetric aldol reactions under the same conditions aforemen-
tioned. The asymmetric aldol condensation in the presence of
10 mol% of the primary amine unit in microspheres was chosen as
the model for optimizing the reaction conditions. What should be
mentioned herein is that the suspension polymerizations can
provide microspheres in a quantitative yield. Accordingly, the
composition of the microspheres can be approximately referred to
the corresponding feed ratio. At the onset of this work, we used
water and brine as solvent respectively for performing aldol reac-
tion. The reaction mixture was emulsion-like in the condition of
powerful stirring and the microspheres were suspended in the
reaction system, in which the condensation effect was not found. As
expected, the reactions proceeded smoothly and afforded the anti-
aldol products (2S,10R)-2-(hydroxyl-(p-nitrophenyl) methyl) cyclo-
hexan-1-one formed mainly in high yields with up to excellent
diastereo- and enantioselectivities, which were determined by FTIR
(Fig. S1), NMR (Fig. S2), and HPLC techniques (Fig. S3) (Detailed
information is presented in Supporting Information). The MPs were
found to be more active in brine than in water and afforded the
aldol product with higher yield and enantiomeric excess (Table 1,
run 7 vs run 8) in the former case. Furthermore, the microspheres
as catalyst gave asymmetric aldol product with higher yield and
stereoselectivity as well, compared to monomer L-MTFc (Table 1,
run 3 vs run 7, and run 4 vs run 8) under the same reaction con-
ditions. This also indicates that synergistic effect occurred to the
microspheres in catalyzing the aldol reactions, as reported earlier
[31].
Fig. 3. Typical FT-IR spectra of (a) M1, (b) L-MTFc, (c) cross-linker, and (d) micro-
spheres (MPs) (in KBr tablet).
Further aldol reactions were then carried out in brine and with
acid additives, an effective approach that can enhance the catalytic
performance of amide-based organocatalysts. As reported,
improvement in the diastereo- and enantioselectivity was observed
with all the examined acids [48,49]. The catalytic activity of
copolymer poly(L-MTFc0.1-co-M10.9) with Brøsted acid additives
has been studied in our preceding work [31] and the results showed
that Brøsted acid additives could enhance the catalytic activity of
copolymer poly(L-MTFc0.1-co-M10.9) indeed. In this study, benzoic
acid, trifluoroacetic acid, acetic acid, and p-toluenesulfonic acid
were investigated as typical Brøsted acids (0.1 equiv) to explore
their effects towards microspheres in asymmetric catalysis. The
best result was obtained in the presence of TsOH (Table 1, run 12), in
which a conversion of 91% and an ee of 93% were achieved. In the
case using PhCOOH (run 9), the yield was found to be 88%, and ee
was 90%. For TFA (run 10), the product was obtained with a yield of
82% and ee of 87%. Using AcOH afforded a yield of 85% and ee of 89%
(run 11).
After reaction, the crosslinked microspheres can be easily and
finely separated and recycled from reaction mixture, just by the
adding chloroform and then extraction and filtration. The reus-
ability of the microspheres was examined for three cycles in the
direct aldol reaction between cyclohexanone and p-nitro-
benzaldehyde under the conditions optimized above, i.e. in the
presence of TsOH with brine as media. As shown in Table 2, the
reaction yields, diastereo- and enantioselectivities changed little
during the recycling process, indicating the high catalytic activities
of our crosslinked microspheres. The reusability further indicates
that the helical polymer chains forming the microspheres are
highly stable.
Fig. 4. Raman spectra of copolymer poly(L-MTFc0.1-co-M10.9) (a) and microspheres
(MPs, b).
80%, ee of 75% (Table 1, run 4) and conversion of 87%, ee of 84%
(Table 1, run 6). We also found that polymeric organocatalyst
copolymer poly(L-MTFc0.1-co-M10.9) is more effective than the
corresponding chiral monomer L-MTFc, affording asymmetric aldol
product with higher yield and stereoselectivity. These results can
be explained by the helical polymer structures, which seem to be
favorable for the asymmetric reaction. More importantly, the

J. Deng, J. Deng / Polymer 125 (2017) 200e207
205
Fig. 5. CD (A) and UVevis (B) spectra of copolymer poly(L-MTFc0.1-co-M10.9) in CHCl3 solution (c ¼ 0.1 mM) at 25 ꢁC (a) and microspheres (MPs) measured by direct compression
method (b).
Table 2
Reusability of the microspheres for catalyzing asymmetric aldol reactions.a
Cycles
Time (h)
Yield (%)
dr (anti/syn)b
ee (%)(anti)c
1
2
3
48
48
48
91
90
90
87/13
89/11
86/14
93%
93%
92%
a
The reactions were conducted with MPs, under the same condition in Table 1,
Run 12.
b
c
Determined by 1H NMR.
Determined by HPLC using a Daicel CHIRALPAK AD-H.
Indeed, the size of the particles does have an effect on the cat-
alytic activity for the reaction, referring to the relevant impressive
studies [50,51]. Based on our earlier work dealing with optically
Fig. 6. TGA curve of the microspheres (MPs). Heating at 10 ꢁC/min, nitrogen
atmosphere.
Table 1
Data of asymmetric aldol reactions catalyzed by microspheres.a
Run
Cat. (10 mol%)
Solvent
Additive
Time (h)
Yield (%)
dr (anti/syn)b
ee% (anti)c
d
1
2
3
4
5
6
7
8
polyM1
polyM1
L-MTFc
L-MTFc
poly(L-MTFc0.1-co-M10.9
poly(L-MTFc0.1-co-M10.9
MPs
MPs
MPs
MPs
MPs
MPs
H2O
Brine
H2O
Brine
H2O
Brine
H2O
Brine
Brine
Brine
Brine
Brine
-
72
72
48
48
48
48
48
48
48
48
48
48
e
e
e
e
e
e
e
e
78
80
85
87
86
90
88
82
85
91
75/25
79/21
80/20
83/27
78/22
80/20
88/12
83/17
82/18
89/11
71
75
82
84
80
85
90
87
89
93
e
)
)
e
e
e
e
9
PhCOOH
TFA
AcOH
TsOH
10
11
12
a
Aldol reactions between cyclohexanone and p-nitrobenzaldehyde were carried out at 25 ꢁC under the conditions as listed in the table; the time for full conversion in all the
cases was monitored by TLC.
b
Determined by 1H NMR.
Determined by HPLC using a Daicel CHIRALPAK AD-H.
“-” indicate no additive added or no reaction occurred.
c
d

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J. Deng, J. Deng / Polymer 125 (2017) 200e207
(2002) 2751e2772.
active particles, we found that the size of particles was mainly
depended on the preparation methods. For example, emulsion
polymerization [26], precipitation polymerization [27] and sus-
pension polymerization [28] can provide particles of nano-, sub-
micron- and micron-sized scale, respectively. As for the chiral
acetylenic monomer containing ferrocenyl amino-acid derivative
substituent, L-MTFc, so far we have used it for preparing optically
active particles only through suspension polymerization, yielding
particles of micro-scale. In the future studies, we will take other
methods to polymerize the chiral monomer L-MTFc for preparing
particles with different sizes, thereby to further test the effects of
particles size on asymmetric catalysis. In addition, we also will
examine the catalytic ability of the optically active particles to-
wards other aldol reactions, to determine the potential applicability
of the novel chiral catalyst; these catalytic reactions will be per-
formed under varied conditions, e.g. in different media, for
different reaction time, etc. All these specific directions are worthy
to be investigated in detail, from which new findings are expected
to be obtained.
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We successfully prepared a novel category of optically active
microspheres through suspension copolymerization of chirally and
achirally substituted acetylenic monomers in quantitative yield.
The resulting microspheres were in regularly spherical morphology
and made up of copolymer chains adopted predominantly one-
handed helices, endowing the microspheres with optical activity.
The helical polymer chains with ferrocenyl amino-acid derivative
pendants afforded the spheres catalytic ability towards asymmetric
aldol reactions. Importantly, the synergistic effect occurring be-
tween the helical main-chain chirality and the ferrocenyl amino-
acid derivative pendants was found to favour the asymmetric re-
action in brine media in the presence of TsOH, yielding product
with both satisfactory yield and stereoselectivity. The microspheres
can be finely recycled and reused in catalyzing asymmetric aldol
reaction with high catalytic activity. We will continue our efforts to
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€
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Notes
The authors declare no competing financial interest.
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substituted acetylenes in chiral micelles, Angew. Chem. Int. Ed. 50 (2011)
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Acknowledgments
[27] D.Y. Zhang, C. Song, J.P. Deng, W.T. Yang, Chiral microspheres consisting
purely of optically active helical substituted polyacetylene: the first prepa-
ration via precipitation polymerization and application in enantioselective
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This work was supported by the National Natural Science
Foundation of China (21474007, 21274008).
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2017.08.022.
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