Optically active helical polymers with pendent thiourea groups: Chiral organocatalyst for asymmetric michael addition reaction

Article abstract of DOI:10.1002/pola.27630

This article reports a novel category of helical substituted polyacetylenes bearing pendant thiourea groups and showing remarkable asymmetric catalysis ability. Thiourea-based monomer and another chiral monomer underwent copolymerization, affording copoly

Full text of DOI:10.1002/pola.27630

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Optically Active Helical Polymers with Pendent Thiourea Groups: Chiral
Organocatalyst for Asymmetric Michael Addition Reaction
Haiyang Zhang,1,2 Wantai Yang,1,2 Jianping Deng1,2
1State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
2College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China
Correspondence to: J. Deng (E-mail: dengjp@mail.buct.edu.cn)
Received 31 January 2015; accepted 10 March 2015; published online 2 April 2015
DOI: 10.1002/pola.27630
ABSTRACT: This article reports a novel category of helical sub-
stituted polyacetylenes bearing pendant thiourea groups and
showing remarkable asymmetric catalysis ability. Thiourea-
based monomer and another chiral monomer underwent
copolymerization, affording copolymers with considerable opti-
cal activity. The copolymers were used as chiral organocatalyst
to homogeneously catalyze the asymmetric Michael addition of
diethyl malonate to trans-b-nitrostyrene. During catalysis, a
synergetic effect occurred between the pendant thiourea moi-
eties and the helical structures in the polymer backbones. The
enantioselectivity of the reaction was governed by the thiourea
moieties. Meanwhile, the concaves along the helices provided
specific domains where the substrates and catalytic groups
were packed together, leading to a remarkable enhancement of
product yield and enantioselectivity. Product with high yield
(85%) and satisfactory ee (up to 72%) can be obtained. The
present helical polymers open up new opportunities for devel-
oping macromolecules as mimetic enzymes catalyzing asym-
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2015 Wiley Periodicals, Inc. J. Polym. Sci.,
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KEYWORDS: asymmetric catalysis; catalysts; chiral; copolymer-
ization; helical polymer
INTRODUCTION Asymmetric catalysis has gathered ever-
increasing interest and become one of the most active
research areas in chemistry, since it provides an inexpensive
and readily accessible approach to obtain enantiomerically
pure compounds.1–4 Over the last decade, chiral organocata-
lysts have been established as an essential branch of asym-
metric catalysis,5,6 among which thiourea and the derivatives
play important roles for their high selectivity and efficiency.
Since the pioneering work by Vachal and Jacobsen7 and
Takemoto et al,8 a number of thiourea-based organocatalysts
have been designed and applied for a wide range of reac-
tions.9–11 However, some inherent shortcomings also exist in
these small molecule catalysts, such as large usage amount,
difficulty in recovery, and possible contamination. Mean-
while, artificial macromolecules as mimetic enzymes are
emerging for their outstanding catalytic efficiency and partic-
ularly the much ease in recycling. The investigations by
Reggelin et al.,12,13 Suginome and coworkers,14,15 and Roelfes
and coworkers16,17 demonstrated outstanding catalytic per-
formance of helical macromolecular scaffolds in asymmetric
catalysis. Nevertheless, taking into account their complicated
structures, designing and preparing such macromolecular
catalysts is still a challenging topic. The helical polymers
reported in this article provide a new strategy for combining
macromolecules with functional organic molecules to gener-
ate a novel category of green catalyst.
Chiral macromolecules extensively exist in nature, such as
DNA, enzymes, and peptides. Owing to their unique helical
structures, these biomacromolecules play extremely significant
roles in living organisms. Stimulated by natural helical macro-
molecules, so far various artificial polymers with chiral helical
structures have been successfully designed and prepared.18–27
These helical polymers demonstrated fascinating features and
found applications in enantioselective crystallization,28 chiral
recognition,29,30 and so forth. Particularly, helical polymers
with catalytic moieties can be elegantly utilized as mimetic
enzymes to catalyze asymmetric reactions efficiently. To date
some groups12–17,31–34 have made remarkable contribution to
this significant research field. Regrettably, as a typical class of
helical polymers, substituted polyacetylene has been rarely
involved in the literature concerning macromolecular catalysts
yet, except the studies from Yashima and coworkers,31,32
Masuda and coworkers,33,34 and from us.35,36 Therefore, the
investigations along this direction are of great significance in
both fundamental research and practical applications.
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 (A) Synthesis of monomer (M1R, M1S), (B) copolymerization of M1R or M1S with M2, and (C) asymmetric Michael
addition reaction catalyzed by chiral helical copolymers. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
In this article, we successfully manufactured a novel optically
active helical copolymer, which possessed catalytic ability
and further efficiently catalyzed asymmetric Michael reac-
tion. Thiourea-based monomer and another chiral monomer
(defined as M1 and M2, respectively) were copolymerized,
providing copolymers with stable helical conformations and
pendant thiourea groups, as shown in Scheme 1. Signifi-
cantly, a synergetic effect was observed between helical poly-
mer backbones and the side thiourea moieties in catalyzing
the asymmetric reaction. Additionally, the helical structures
existed in the polymer backbones can provide plenty of con-
caves, which, together with the pendant chiral thiourea units,
served as specific domains for catalyzing asymmetric reac-
tions. The present copolymers are of high importance for
further developing novel mimics of bioenzymes as efficient
green catalysts.
The molecular weight (Mn) and molecular weight polydisper-
sity (Mw/Mn) were determined by GPC (Waters 515–2410
system) calibrated by using polystyrenes as standards and
THF as eluent. FT-IR spectra were recorded with a Nicolet
NEXUS 670 spectrophotometer (in KBr tablet). 1H and 13C
NMR spectra were recorded on a Bruker AV 400 spectrome-
ter. Organic elemental analyses were carried out on varioEL-
cube elemental analyzer. HPLC analyses were performed on
FL2200-2 (FL2200-2 pump and Absorbance Detector). Chir-
alpak AD-H column was purchased from Daicel Chemical
Industries Ltd.
Polymerization
The typical procedure for polymerization is stated as below.
Taking homopolymerization as example, the polymerizations
were carried out in CHCl3 at 30 ꢀC for 24 h, with
(nbd)Rh1B2 (C6H5)4 (1 mol % of monomer) as catalyst. Sub-
sequently, the mixture was added into hexane to precipitate
the formed polymer. After filtered and dried, the homopoly-
mer was obtained. The copolymerizations were conducted in
a similar way, with varied feed ratio of monomers. FT-IR and
1H NMR spectra of the copolymers are shown in Supporting
Information Figures S3 and S4.
EXPERIMENTAL
Materials
(nbd)Rh1B2(C6H5)4 (nbd 5 2,5-norbornadiene) was pre-
pared according to the procedure reported in literature.37
Monomer M1R and M1S were synthesized according to the
method described in our previous work.38 Both the two
monomers were obtained in a high yield (ca. 80%), and
were structurally identified by FT-IR and 1H NMR spectra
(Figs. S1 and S2, Supporting Information). Monomer M2 was
prepared by the method reported earlier.39 Propargylamine,
trans-b-nitrostyrene, diethyl malonate, (R)-(1)21-phenyle-
thylamine, and (S)-(2)21-phenylethylamine were bought
from Aldrich and used without further purification. All sol-
vents were purified by distillation before use.
Michael Reaction
Diethyl malonate (0.16 g, 1 mmol), triethylamine (0.0101 g,
0.1 mmol), helical copolymer (2 mol % of nitrostyrene,
based on thiourea unit), and 5 mL CH2Cl2 were added in a
reactor. The mixture was stirred for 20 min and then trans-
b-nitrostyrene (0.0745 g, 0.5 mmol) was added. 72 h later,
the reaction was completed based on TLC analysis. The
crude product was concentrated under reduced pressure
and purified by flash column chromatography (hexane/
EtOAc 5 4/1, v/v) to remove the copolymer and unreacted
substrates. The product (ethyl 2-carboethoxy-4-nitro-3-phe-
nylbutyrate) was subjected to FT-IR, 1H and 13C NMR spec-
troscopy measurements, as shown in Supporting Information
Figures S6–S8. Enantiomeric excess (ee%) was determined
Measurements
Circular dichroism (CD) and UV–vis absorption spectra were
conducted on a Jasco-810 spectropolarimeter. Specific rota-
tions were measured on a JASCO P-1020 digital polarimeter
with a sodium lamp as the light source at room temperature.
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TABLE 1 Data of Homopolymers and Copolymers From M1R,
the feed ratio increased to 2/8 (M1/M2), the copolymers’
molecular weights decreased. We further conducted copoly-
merization with the feed ratio of 3/7 (M1/M2), but the
molecular weights were rather low (<2000). The low poly-
merization activity of M1 should be due to the high tendency
of the polar thiourea groups to form intermolecular hydro-
gen bonding, which was ever observed before.38 Therefore,
the molecular weights of the resulting copolymers had a
downward trend as the proportion of M1 increased. To
ensure the helical structures in copolymers, we selected the
copolymers with the feed ratio of M1/M2 being 1/9 and 2/8
as macromolecular catalysts in the subsequent Michael
reactions.
M1S, and M2
Polymera
Mn
Mw/Mn
Yield (%)c [a]D (ꢀ)d
b
b
Poly-1R
3,100
2,900
8,600
5,400
4,800
3,900
3,500
1.31
1.21
2.24
2.40
2.55
2.12
1.97
73
70
96
82
78
75
72
1588
2572
2823
2603
2684
2476
2595
Poly-1S
Poly-2
Poly(M1R0.1-co-M20.9
Poly(M1S0.1-co-M20.9
Poly(M1R0.2-co-M20.8
Poly(M1S0.2-co-M20.8
)
)
)
)
a
With (nbd)Rh1B–(C6H5)4 as catalyst; [M]0 5 0.5 M; [M]0/[Rh] 5 100 (mol/
mol); T 5 30 ꢀC; in CHCl3 for 24 h.
To verify the copolymerization of M1 and M2, the two mono-
b
Determined by GPC (polystyrene as standards; THF as eluent).
mers and their copolymers (defined as poly(M10.1-co-M20.9
)
c
Determined gravimetrically, hexane-insoluble part.
Measured by polarimeter at room temperature, c 5 0.1 g dL21, CHCl3.
d
and poly(M10.2-co-M20.8)) were structurally identified by FT-
IR (Supporting Information Fig. S3). Both the copolymers
showed the characteristic absorption peak of benzene group
at 706 cm21, which only existed in monomer M1. Moreover,
the relative intensity of this peak in the latter copolymer
was stronger than in the former. This offers clear evidence
for the presence of M1 units in the copolymers. To further
determine their composition, copolymers with different
monomer feed ratios were subjected to 1H NMR spectros-
copy. We suppose that the content of M1 unit is x, and M2
unit is (1 2 x). The ratio of the peak areas of phenyl hydro-
gen and methyl hydrogen can be expressed as 5x/
[3x 1 9(1 2 x)]. By integration, we can easily determine the
real ratio of the two peak areas, and the value of x can be
consequently calculated. Taking poly(M10.1-co-M20.9; see Fig.
1) as example, the ratio of peak areas of (a 1 b 1 c) to
(d 1 e 1 f) is 16.3, and x is 0.103 accordingly. The composi-
tion of the copolymer was found to be equal to the
by HPLC analysis, Chiralpak AD-H, hexane/EtOH 5 80/20, v/
v, 1.0 mL min21, 254 nm.
RESULTS AND DISCUSSION
Based on our earlier studies dealing with optically active hel-
ical polymers35,40–42 and the strategy for preparing chiral N-
propargylthiourea,38 we in the present study designed and
prepared a novel substituted polyacetylene bearing thiourea
pendant groups. The major processes for monomer synthesis
and copolymerization are schematically presented in Scheme
1(A and B). Thiourea-based monomers (M1R and M1S) were
used because thiourea organocatalysts showed high reactiv-
ity and selectivity in catalyzing asymmetric reactions like
Michael addition.8 We first polymerized M1R and M1S to
obtain the corresponding homopolymers. Unfortunately, the
homopolymers’ molecular weights were not satisfactory
(Table 1). To increase the polymerization degree of poly(N-
propargylthiourea)s, another monomer without the function
of catalysis, M2,35 was employed to carry out copolymeriza-
tions separately with M1R and M1S. Only one enantiomer of
M2 was used in the copolymerization, because the raw mate-
rial for preparing the other enantiomer is commercially
unavailable. As reported previously,39 the M2 units could
afford the copolymers with helical conformations and in turn
remarkable optical activity. In addition, it should be high-
lighted herein that the stable helical structures in the formed
copolymers can severe as catalytic domains to provide multi-
ple reactive sites for catalyzing asymmetric reactions.
We prepared copolymers with two varied monomer feed
ratios (M1/M2 5 1/9 and 2/8, mol/mol, M1 5 M1R and
M1S), yielding copolymers designated as poly(M1R0.1-co-
M20.9), poly(M1S0.1-co-M20.9), poly(M1R0.2-co-M20.8), and
poly(M1S0.2-co-M20.8), respectively. The relevant data are
presented in Table 1. It can be inferred that when the feed
ratio of M1/M2 was 1/9, the copolymers were obtained with
moderate molecular weights, higher than that of the corre-
sponding homopolymers (poly-1R and poly-1S, Table 1). As
FIGURE 1 1H NMR spectrum of poly(M10.1-co-M20.9), measured
in DMSO-d6 at r.t.
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FIGURE 2 (A) CD and (B) UV–vis spectra of (a) poly-2 (homopolymer of M2); (b) poly(M1R0.1-co-M20.9); (c) poly(M1R0.2-co-M20.8);
(d) poly-1R (homopolymer of M1R). Measured in CHCl3 (c 5 0.1 mM) at 25 ꢀC. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
corresponding monomer feed ratio. For poly(M10.2-co-M20.8
;
ical pitch along the copolymer backbones, rendering the
copolymers with lower absorption wavelength.
Supporting Information Fig. S4), the value of x is 0.218. The
copolymers’ compositions were also confirmed by elemental
analysis [M1 content: 9.75% in poly(M10.1-co-M20.9); 19.56%
Since M1S and M1R are a pair of enantiomers, poly-1S exhib-
ited negative CD signal at 320 nm, opposite to poly-1R (Fig.
3). Consequently, the copolymers composed of M1S and M2
have the same CD signal direction with poly-2. In addition,
the increasing proportion of M1S also caused a slight blue
shift, the reason for which is discussed previously. From the
CD and UV–vis spectra presented in Figures 2 and 3, we can
also see that with the content of poly-1 unit (poly-1R or
poly-1S) increased, the intensity of absorption peaks over
300 nm decreased gradually. On the contrary, the UV–vis
absorption below 300 nm was enhanced. Due to the inser-
tion of poly-1 units, the regularity of helical conformations
occurring in poly-2 backbones was somewhat affected, weak-
ening the chiral amplification effect.43,44 Therefore, the opti-
cal activity of the copolymers was reduced. The specific
rotations displayed in Table 1 also support the previous con-
sideration. Nevertheless, the copolymers still demonstrated
considerable optical activity, stronger than that of the
thiourea-based homopolymers.
in poly(M10.2-co-M20.8)]. Both poly(M10.1-co-M20.9
) and
poly(M10.2-co-M20.8) displayed high specific rotations (Table
1), reflecting that the copolymers were optically active, just
as expected. The intense optical activity derived from the
helical conformations of the copolymer backbones, as will be
discussed in detail later on.
Before investigating the helical structures of the copolymers,
the corresponding homopolymers were first subjected to CD
and UV–vis spectroscopy measurements, which have been
widely used for exploring the secondary structures of macro-
molecules.35,38,40 In the CD and UV–vis spectra in Figure 2,
poly-2 (homopolymer of M2, the homopolymers below fol-
low the same denotation) showed negative CD signal and
strong UV–vis absorption at ꢁ350 nm, while poly-1R demon-
strated positive CD signal and UV–vis absorption around
320 nm. According to our earlier studies concerning optically
active helical substituted polyacetylenes,35,40 we can con-
clude that the two homopolymers formed helices of prefer-
ential helicity. However, their helical senses were just
opposite, which is in accordance with our earlier reports.38
When M1R underwent copolymerization with M2 with the
feed ratio of 1/9 and 2/8 (mol/mol), the resulting copoly-
mers exhibited considerable CD signal and UV absorption at
about 350 nm, indicating that the copolymer backbones
adopted helical conformations of predominantly one-handed
screw sense. Particularly, both the two copolymers showed
negative CD effects, similar to that of poly-2. Furthermore, a
slight blue shift was found to occur in the CD and UV–vis
spectra of the copolymers compared with poly-2. As the ratio
of M1R/M2 rose, the degree of blue shift increased. This is
because the screw pitch of the helical conformations formed
by poly-1R was shorter than that formed by poly-2. As a
result, the addition of poly-1R units reduced the average hel-
Next, we explored the helical structures of the copolymers in
varied solvents. Taking M1R0.1-co-M20.9 as example, the rele-
vant CD and UV–vis spectra are illustrated in Figure 4. It is
obvious that the copolymer showed stronger CD signal and
UV–vis absorption in CH2Cl2 and CHCl3 than in other sol-
vents. The reason may lie in the different helical content of
the copolymer in varied solvents. It has been proved that
helical structures became more stable in CHCl3 and CH2Cl2
than in DMF and MeOH because polar solvent can destroy
the intramolecular hydrogen bonding in the copolymer,
which is essential for the formation of helical conforma-
tions.45 Another observation is that the CD signal and UV–vis
absorption of the copolymer became stronger when the tem-
ꢀ
perature decreased from 25 to 0 C. According to our previ-
ous work,46 lower temperature is more desirable for
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FIGURE 3 (A) CD and (B) UV–vis spectra of (a) poly-2; (b) poly(M1S0.1-co-M20.9); (c) poly(M1S0.2-co-M20.8); (d) poly-1S (homopoly-
mer of M1S). Measured in CHCl3 (c 5 0.1 mM) at 25 ꢀC. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
polymers to form stable helical structures. CD and UV–vis
spectra of M1S0.1-co-M20.9 in different solvents and at varied
temperatures showed similar results (Supporting Informa-
tion Fig. S5).
[7,47. Before discussing the factors affecting the reaction, it
should be specially noted that the absolute configuration of
the reaction products was related to the composition units
of the copolymers. It seemed that the M1R-derived (co)poly-
mers tended to yield R-configuration product. Meanwhile,
when catalyzed by the polymers from M1S (homopolymer or
copolymer), the absolute configuration of the product was
determined to be S. Taking the products from Entries 8 and
16 (Table 2) as representative, the specific rotations were,
respectively, 15.75ꢀ and 24.30ꢀ. The CD and UV–vis spectra
in Supporting Information Figure S11 also indicate the two
types of products were enantiomers. These facts clearly sug-
gest that the thiourea moieties totally control the enantiose-
lectivity during catalysis. Furthermore, the ee value of the
two types of products varied significantly (the M1R-derived
organocatalyst gave a higher ee). This phenomenon was also
found in literature when a pair of enantiomeric catalysts
were used for asymmetric catalysis.48,49 The inversion of
configuration and the differentiation in ee is attributed to
With the helical copolymers in hand, we further conducted
asymmetric reactions using the copolymers as chiral catalyst.
We took the direct Michael addition of diethyl malonate to
trans-b-nitrostyrene as a model, as shown in Scheme 1(C).
To promote the catalytic rate, we added triethylamine as a
promotor, which had no catalytic effect when individually
used. The product was characterized by FT-IR and NMR
spectroscopy, as illustrated in Supporting Information Fig-
ures S6–S8.
Detailed data of the asymmetric catalysis are listed in Table
2. We present typical HPLC spectra for the products of the
Michael reaction in Supporting Information (Figs. S9 and
S10). The HPLC results were analyzed by referring to Refs.
FIGURE 4 (A) CD and (B) UV–vis spectra of poly(M1R0.1-co-M20.9) in CH2Cl2 (0 ꢀC and 25 ꢀC), CHCl3, DMF and MeOH (25 ꢀC). [Color
figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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TABLE 2 Asymmetric Michael Addition Reactions Catalyzed by Homopolymers and Copolymersa
Entry
Catalyst
Solvent
Temp. (ꢀC)
Yield (%)b
ee%c (config.)d
e
1
Poly-2
M1R
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
DMF
25
25
25
25
25
25
25
0
–
–
2
43
45
15 (R)
38 (R)
62 (R)
60 (R)
–
3
Poly-1R
4
Poly(M1R0.1-co-M20.9
Poly(M1R0.1-co-M20.9
Poly(M1R0.1-co-M20.9
Poly(M1R0.1-co-M20.9
Poly(M1R0.1-co-M20.9
Poly(M1R0.2-co-M20.8
M1S
)
)
)
)
)
)
58
5
55
Tracef
6
7
MeOH
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
DMF
–
–
8
85
72 (R)
68 (R)
6 (S)
22 (S)
42 (S)
38 (S)
2
9
25
25
25
25
25
25
25
0
70
10
11
12
13
14
15
16
17
25
Poly-1S
41
Poly(M1S0.1-co-M20.9
Poly(M1S0.1-co-M20.9
Poly(M1S0.1-co-M20.9
Poly(M1S0.1-co-M20.9
Poly(M1S0.1-co-M20.9
Poly(M1S0.2-co-M20.8
)
)
)
)
)
)
62
60
Trace
2
MeOH
CH2Cl2
CH2Cl2
2
78
51 (S)
45 (S)
25
65
a
c
d
e
f
The reaction was conducted with trans-b-nitrostyrene (1 equiv) and
diethyl malonate (2 equiv) in the presence of various additives (0.05
equiv).
Determined by HPLC.
Determined by specific rotation.
“–” indicates no reaction occurred.
b
Isolated yield.
According to TLC, the amount was too little to be determined.
the reaction mechanism, which will be specifically discussed
later.
at 0 ꢀC (Table 2, Entries 8 and 16), at which temperature
the helical conformations of the copolymers are more stable.
Taking the effect of solvents and temperature into considera-
tion, the helical structures in the copolymer played an
important role in the asymmetric reaction.
In order to check the role of helical macromolecular scaffold,
we first examined catalytic ability of the thiourea-based
monomers, as shown in Table 2 (Entries 2 and 10). It is
obvious that whether R- or S-isomer was used, the results
were very poor. Then (co)polymers were used in asymmetric
catalysis. As mentioned previously, the prepared copolymers
possessed higher content of helical conformations compared
to the corresponding thiourea-based homopolymers. So we
examined their catalytic efficiency under the same reaction
conditions. Excitingly, the copolymers displayed more satis-
factory results, and they were both better than the mono-
mers (Table 2, Entries 2–4 and 10–12). On the basis of our
earlier studies,35,36 we conclude that stable helical structures
are favorable for the asymmetric reaction. To verify our
assumption, we conducted several groups of catalytic reac-
tions in different solvents and at varied temperatures. Taking
poly(M1R0.1-co-M20.9) as representative, solvents with weak
polarity were suitable for its catalysis (Table 2, Entries 4 and
5). However, in polar solvents like DMF and MeOH, the
copolymer hardly performed catalytic function (Table 2,
Entries 6 and 7). Poly(M1S0.1-co-M20.9) gave similar results
(Table 2, Entries 12–15). This variation tendency accorded
with the effect of solvents on the helices formed in the
copolymers (shown in Fig. 4 and Fig. S5). Furthermore, we
investigated the effects of reaction temperature. Both the
two copolymers generated products with higher yield and ee
Additionally, we also explored the influence of copolymer
composition on the Michael reaction. As the content of M1
increased from 10% to 20%, products with enhanced yield
and ee were obtained, as shown in Table 2 (Entries 9 and
17). It means increasing proportion of catalytic thiourea
groups will lead to more satisfactory reaction results, on
condition that enough degree of helical structures was
formed in the copolymer. Referring to our earlier study and
according to the investigations previously, we can conclude
that the presence of catalytic groups and stable helical con-
formations are two essential factors toward asymmetric
catalysis. Taking into account the unique helical structures of
the copolymer and meanwhile according to the reported
mechanism of asymmetric Michael reaction catalyzed by thi-
ourea,50 we herein proposed a possible reaction mechanism,
as depicted in Scheme 2.
In the process of reaction, the promotor triethylamine initially
deprotonated an acidic proton of diethyl malonate, generating
a
cyclic complex A (Scheme 2). Meanwhile, trans-b-
nitrostyrene was activated by the thiourea groups on the heli-
cal copolymers through hydrogen bonding, forming the com-
plex B or C. The steric hindrance of helical macromolecular
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SCHEME 2 Proposed catalytic mechanism of Michael reaction catalyzed by chiral helical copolymers with pendent thiourea
groups. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
scaffold and the benzene ring connected with thiourea group
influenced the attack direction of complex A, consequently
leading to adduct bearing R or S configuration with high enan-
tioselectivity. Particularly, the concaves along the helices
formed by the copolymer can serve as domains to provide a
three-dimensional environment for the asymmetric reaction.
Within the scope of the concaves, the concentration of reac-
tion substrates and catalytic groups were relatively increased,
resulting in a higher probability of molecular interaction. In
conclusion, the thiourea pendants and the helical polymer
backbones worked well together in a favorable synergy effect
in the asymmetric reaction. To acquire deeper insights into
the detailed catalyzing mechanism and to elucidate the advan-
tages of the novel chiral macromolecules catalysts, much more
investigations will be performed. Nonetheless, the present
study provides a novel class of chiral macromolecular cata-
lysts. We are continuing this important work in our group.
cal structures and highly functional thiourea groups into
one entity. The copolymers with pendant thiourea groups
demonstrated remarkable catalyzing ability toward asym-
metric Michael addition reaction. In the process of reaction,
the thiourea moieties totally control the enantioselectivity.
Furthermore, the macromolecular helical structures and the
thiourea moieties undertook synergic effects in the asym-
metric catalysis. This is the first report on successfully
using thiourea-based helical substituted polyacetylene as
chiral catalyst. We are highly convinced that by optimiza-
tion of the copolymer composition and reaction conditions,
product with higher yield and ee value can be further
obtained.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foun-
dation of China (21474007, 21274008, and 21174010), the
Funds for Creative Research Groups of China (51221002), and
the “Specialized Research Fund for the Doctoral Program of
Higher Education” (SRFDP 20120010130002).
CONCLUSIONS
We successfully prepared a novel category of optically
active helical copolymers, which combined the unique heli-
1822
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28 D. D. Medina, J. Goldshtein, S. Margel, Y. Mastai, Adv.
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