Synthesis of cinchona squaramide polymers by Yamamoto coupling polymerization and their application in asymmetric Michael reaction

Article abstract of DOI:10.1016/j.reactfunctpolym.2021.104913

Yamamoto coupling polymerization has been used for the synthesis of polymeric chiral organocatalysts. Cinchona squaramide derivatives with dibromophenyl moiety were polymerized under the Yamamoto coupling conditions to afford the corresponding chiral polymers in good yields. Using this technique, novel cinchona alkaloid polymers containing the squaramide moiety were designed and successfully synthesized. In addition to the homopolymerization of cinchona squaramide monomers with a dibromophenyl group, achiral comonomers such as dibromobenzene were copolymerized with the cinchona monomers to yield chiral copolymers. These chiral polymers were successfully utilized as polymeric catalysts in asymmetric Michael addition reactions. Good to excellent enantioselectivities were observed for different types of asymmetric Michael reactions. Using the chiral homopolymer catalyst P4, almost perfect diastereoselectivity (>100:1) with 99% ee was obtained for the reaction between methyl 2-oxocyclopentanecarboxylate 25 and trans-β-nitrostyrene 17. The polymer catalysts developed in this study have robust structures and can be reused several times without a loss in their catalytic activities.

Full text of DOI:10.1016/j.reactfunctpolym.2021.104913

Reactive and Functional Polymers 164 (2021) 104913
Contents lists available at ScienceDirect
Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Perspective Article
Synthesis of cinchona squaramide polymers by Yamamoto coupling
polymerization and their application in asymmetric Michael reaction
,
*
Sadia Afrin Chhandab, Shinichi Itsunoa
a National Institute of Technology, Gifu College, Motosu, Gifu 501-0495, Japan
b Department of Applied Chemistry & Life Science, Toyohashi University of Technology, Toyohashi 441-8580, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Yamamoto coupling polymerization has been used for the synthesis of polymeric chiral organocatalysts.
Cinchona squaramide derivatives with dibromophenyl moiety were polymerized under the Yamamoto coupling
conditions to afford the corresponding chiral polymers in good yields. Using this technique, novel cinchona
alkaloid polymers containing the squaramide moiety were designed and successfully synthesized. In addition to
the homopolymerization of cinchona squaramide monomers with a dibromophenyl group, achiral comonomers
such as dibromobenzene were copolymerized with the cinchona monomers to yield chiral copolymers. These
chiral polymers were successfully utilized as polymeric catalysts in asymmetric Michael addition reactions. Good
to excellent enantioselectivities were observed for different types of asymmetric Michael reactions. Using the
chiral homopolymer catalyst P4, almost perfect diastereoselectivity (>100:1) with 99% ee was obtained for the
reaction between methyl 2-oxocyclopentanecarboxylate 25 and trans-β-nitrostyrene 17. The polymer catalysts
developed in this study have robust structures and can be reused several times without a loss in their catalytic
activities.
Chiral organocatalyst
Cinchona squaramide
Yamamoto coupling
Copolymers
Enantioselectivities
1. Introduction
asymmetric catalysis [3].
Some cinchona alkaloid derivatives with catalytic activities were
Owing to the design and development of natural product-derived
chiral molecular frameworks as chiral organocatalysts, asymmetric
catalysis using these molecules is a popular strategy in asymmetric
synthetic methodologies [1]. For the synthesis of these catalysts, sig-
nificant efforts have been engrossed on the development of green and
sustainable procedures. Literary survey of Szollosi reported, heteroge-
neous chiral catalysts obtained by the immobilization of chiral metal
complexes, by anchoring chiral organocatalysts, or by modifying cata-
lytic surfaces with optically pure compounds has been applied in
asymmetric one-pot reactions, which may also integrate uncatalyzed
and homogeneously catalyzed steps [2]. Cinchona alkaloid-derived
organocatalysts are important examples of the catalysts used in asym-
metric reactions. Owing to their several functionalities such as second-
ary alcohols, quinoline rings, and quinuclidine and vinyl groups,
cinchona alkaloids are useful for suitable modifications to versatile
catalysts in asymmetric reactions. Several types of cinchona-based chiral
organocatalysts have been developed that play significant roles in
attached to various types of synthetic polymers [4–7] to act as polymeric
organocatalysts in asymmetric reactions [8–14]. Modified cinchona al-
kaloids of with different functionalities such as hydroxyl, amino, urea
and thiourea on position C9 and C6′ are used in asymmetric reactions
[15]. Supported bifunctional thioureas derived from cinchona alkaloids
were used as catalyst in enantioselective reaction by continuous flow
process reported by Rodriguez et al. [16]. Cinchona alkaloid–metal
complexes were also efficient catalysts in some asymmetric reactions.
Their polymeric counterparts were prepared and applied to asymmetric
catalysis [17,18].
–
The C C cross-coupling is one of the most powerful tools for the
construction of complex organic compounds [19]. Transition metal
complexes are used as catalysts for this purpose and have many ad-
vantages over the classical catalysts [20]. We have investigated several
types of polymeric cinchona-derived catalysts, where the organocatalyst
is incorporated into the polymer main chain as a repeating unit. Previ-
ously Mizoroki–Heck polymerization was developed for the synthesis of
Abbreviation: PPh3, triphenyl phosphine; SEC, size exclusion chromatography; HPLC, high performance liquid chromatography; TLC, thin layer chromatography.
* Corresponding author at: Molecular Functional Chemistry, Department of Applied Chemistry and Life Science, Toyohashi University of Technology, 441-8580
Toyohashi, Japan.
E-mail address: itsuno@chem.tut.ac.jp (S. Itsuno).
https://doi.org/10.1016/j.reactfunctpolym.2021.104913
Received 16 February 2021; Received in revised form 8 April 2021; Accepted 16 April 2021
Available online 20 April 2021
1381-5148/© 2021 Elsevier B.V. All rights reserved.

S.A. Chhanda and S. Itsuno
Reactive and Functional Polymers 164 (2021) 104913
cinchona-derived polymeric catalysts [10,11,21–25]. Mizoroki–Heck
reaction requires two components, an olefinic double bond and an ar-
omatic halide. In contrast, Yamamoto coupling reaction occurs between
aromatic halides. The homopolymerization of aromatic dihalides is
possible using this coupling reaction. Yamamoto coupling is the nickel-
catalyzed coupling reaction of organic halides in the presence of neutral
ligands (e.g., PPh3 and bipyridine) [26,27]. The most commonly used
nickel catalyst for the Yamamoto coupling reaction is bis(cyclo-
octadiene)nickel(0) (Ni(COD)2). This coupling reaction is particularly
interesting when it is applied to polymer synthesis. Yamamoto coupling
polymerization proceeds through reductive elimination from a dio-
rganonickel(II) intermediate [28–30]. Aromatic dihalides in the pres-
APCI) data were recorded on a Bruker micro OTOF II HRMS instrument.
High-performance liquid chromatography (HPLC) was carried out using
a Jasco HPLC system composed of a DG-980-50 three-line degasser, an
Intelligent HPLC pump (PU-2080), and a UV/Vis detector (UV-2075).
The instrument was equipped with a chiral column (Chiralpak AS-H,
Daicel) and hexane/2-propanol were used as the eluent at a flow rate
of 0.7 mL/min at room temperature. HPLC was also carried out on a
Jasco HPLC system composed of an HPLC pump (PU-980), a UV/Vis
detector (UV-975), and a column oven CO-2065 equipped with a chiral
column (Chiralcel OD-H, Chiralpak AD-H, Daicel) using hexane/2-
propanol as the eluent at a flow rate of 1.0 mL/min at room tempera-
ture. Size exclusion chromatography (SEC) was performed using a Tosoh
instrument with HLC 8020 UV (254 nm) or refractive index detector.
ence of a Ni catalyst simply react to afford
π-conjugated polymers [31].
Various types of -conjugated polymers have been synthesized via
π
Two polystyrene gel columns with a bead size of 10 μm were used, and
Yamamoto coupling polymerization [32]. However, to our knowledge,
only one example of chiral polymer synthesis using Yamamoto coupling
polymerization has been reported. Onimura et al. reported the Yama-
moto coupling polymerization of chiral oxazoline monomers containing
a diiodophenyl group [33]. They synthesized optically active poly(m-
phenylene)s bearing chiral oxazoline at the side chains. The structures
and chiroptical properties were characterized using spectroscopic and
thermal gravimetric analyses [33]. No application of the chiral polymers
to asymmetric catalysis has been reported till date.
DMF was used as the carrier solvent at a flow rate of 1.0 mL/min at
40 ◦C. A calibration curve was established to determine the number
average molecular weight (Mn) and molecular weight distribution (Mw/
Mn) values by comparison with the polystyrene standards. Optical ro-
tations were recorded on a JASCO DIP-149 digital polarimeter using a
10-cm thermostatted microcell.
2.2. Synthesis of squaramides 8 and 10
We designed new cinchona-based chiral polymers using Yamamoto
coupling polymerization. A novel type of polymeric chiral catalyst can
be synthesized via this polymerization. Cinchona squaramide derivative
was selected as an efficient catalyst for the Michael addition reactions.
For this purpose, a dibromophenyl group or two iodophenyl groups were
introduced into the cinchona alkaloids. For the polymerization reaction,
the original polymerization method reported by Yamamoto was fol-
lowed [34]. In addition to the chiral homopolymers, the copolymers of
these cinchona squaramide monomers with an achiral aromatic dihalide
were synthesized. For the linear chiral polymer synthesis, the olefinic
double bond (C3-vinyl group) in cinchona alkaloid was reduced to
prevent the Mizoroki–Heck-type coupling reaction. In the presence of
the C3-vinyl group in the cinchona squaramide monomer, both Yama-
moto coupling and Mizoroki–Heck coupling occurred simultaneously to
yield hyperbranched chiral polymers, which were also used as catalysts
in asymmetric reactions. In this study, the synthesis of novel chiral
polymers via the Yamamoto coupling polymerization of cinchona
squaramide derivatives is described. The design and synthesis of
monomers suitable for Yamamoto coupling polymerization and their
reaction conditions are discussed. The chiral polymers obtained by this
polymerization are applied to the asymmetric catalysis of Michael
addition reactions. The catalytic activities and stereoselectivities of the
chiral polymers are also described.
First, monosquaramide 3 (410 mg, 0.71 mmol) and 10 mL of ethanol
were added to a 30-mL volumetric flask. To the stirred solution, 0.86
mmol (253 mg) of 9-amino (9-deoxy)epi cinchonidine 7 in 10 mL
ethanol was slowly added. The mixture was stirred under reflux for
approximately 24 h in Ar atmosphere. A white precipitate was obtained,
which was filtered, washed with ethanol, and dried to yield 8 (460 mg,
78%) as a white solid. Rf: 0.48 (CH2Cl2/MeOH = 9.0/1.0; mp:
229–231 ◦C. 1H NMR (400 MHz, CDCl3):δ 0.90 (m, 1H), 1.29 (m, 2H),
1.61 (m, 4H), 2.27 (br,1H), 2.53–3.14 (m, 4H), 4.59 (br, 4H), 4.93 (m,
2H), 5.63 (m, 1H), 6.95(d, J = 8.0 Hz, 4H), 7.72 (m, 8H), 8.14 (d, J =
8.0 Hz, 1H), 8.86 (d, J = 4.4 Hz, 1H); 13C NMR (500 MHz, CDCl3): δ
25.9, 27.31, 27.69, 39.54, 40.77, 56.0, 94.20, 114.90, 123.57, 127.16,
129.56, 135.32, 138.41, 141.20, 149.99, 167.46, 168.0, 182.87, 183.50.
IR (KBr):
ν = 3324, 3065, 2933, 2862, 1788, 1667, 1561, 1484, 1343,
1285, 1182, 1088, 981, 839, 771, 649, 564 cmꢀ 1; HRMS (APCI) m/z for
23.7
C
37H35I2N4O2 [M + H+] calcd. 821.0849, found 821.0844; [
α]
=
D
ꢀ 166 (c 0.07, DMF).
Further, monosquaramide 3 (410 mg, 0.71 mmol) was added with
10 mL of ethanol to a 30-mL volumetric flask. To the stirred solution, 9-
amino (9-deoxy) 3-ethyl epi cinchonidine 9 (255 mg, 0.86 mmol), 10 mL
of ethanol was slowly added. The mixture was stirred under reflux for
approximately 24 h in Ar atmosphere. A white precipitate was obtained,
which was filtered, washed with ethanol, and dried to afford 10 (480
mg, 81%) as a white solid. Rf: 0.48 (CH2Cl2/MeOH = 9.0/1.0; mp:
224–226 ◦C. 1H NMR (400 MHz, CDCl3): δ 0.78 (t, J = 7.6, 3H), 0.88 (m,
1H), 1.16 (m, 4H), 1.42–1.62 (m, 4H), 2.23 (br,1H), 2.52–3.14 (m, 4H),
4.59 (br, 4H), 6.95(d, J = 8.0 Hz, 4H), 7.72 (m, 8H), 8.15 (d, J = 7.6 Hz,
1H), 8.86 (d, J = 4.4 Hz, 1H); 13C NMR (500 MHz, CDCl3): δ 12.08,
24.97, 25.52, 27.60, 28.36, 37.31, 40.82, 57.69, 94.17, 123.63, 127.04,
129.46, 135.35, 138.41, 149.99, 167.82, 168.33, 182.89, 184.00. IR
2. Experimental section
2.1. Materials and methods
All reagents and solvents used during the investigation were pur-
chased from Sigma–Aldrich, Wako Pure Chemical Industries, Ltd., or
Tokyo Chemical Industry (TCI) Co., Ltd. To monitor the progress of the
reactions, thin layer chromatography (TLC) was performed using pre-
coated silica gel plates (Merck TLC silica gel, 60F254). To purify the
synthesized compounds, column chromatography was performed using
a silica gel column (Wakogel C-200, 100–200 mesh). The 1H NMR
spectra were recorded using JEOL JNM-ECS400 and JEOL JNM-ECX500
spectrometers operated at 400/500 MHz. The 13C NMR spectra were
recorded at 100/125 MHz in CDCl3 or DMSO‑d6 at room temperature.
The chemical shifts are reported in parts per million (ppm) using tet-
ramethyl silane (TMS) as the reference, and the J values are reported in
Hertz (Hz). Infrared (IR) spectroscopy was performed using “KBr” pellets
on a JEOL JIR-7000 FTIR spectrometer, and the wavenumbers are re-
ported in cmꢀ 1. High-resolution mass spectrometry (HRMS; ESI and
(KBr):
ν = 3246, 2915, 2857, 1789, 1669, 1559, 1435, 1319, 1249,
1005, 839, 788, 673 cmꢀ 1; HRMS (ESI) m/z for C37H37I2N4O2 [M + H+]
16.7
calcd. 823.1006, found 823.1000; [
α]
= ꢀ 190 (c 0.09, DMF).
D
2.3. Synthesis of squaramides 11 and 12
Monosquaramide 6 (198 mg, 0.53 mmol) and 9-amino (9-deoxy)epi
cinchonidine 7 (171 mg, 0.58 mmol) were added to a 30-mL flask with
10 mL of MeOH and stirred at reflux temperature under Ar atmosphere
for 48 h. White precipitate was obtained, which was filtered and washed
with cold MeOH and finally dried in a vacuum oven to yield 11 (187 mg,
56%) as a white solid. Rf: 0.57 (CH2Cl2/MeOH = 1.0/1.0); mp:
1
◦
296–298 C. H NMR (500 MHz, DMSO‑d6): δ 0.64 (m, 1H), 1.34 (m,
2

S.A. Chhanda and S. Itsuno
Reactive and Functional Polymers 164 (2021) 104913
I
I
EtO
O
OEt
O
I
EtOH, Et3N
Reflux, 24 h
+
HN
EtO
O
N
O
I
2
1
3
Br
2, 35 o
Br
+
H
N
MeO
OMe
O
CH2Cl
C, 24 h
H2N
MeO
O
Br
O
Br
O
6
5
4
Scheme 1. Synthetic route to monosquaramides 3 and 6.
2H), 1.55 (m, 4H), 2.25 (br,1H), 2.66 (m, 2H), 3.12–3.32 (m, 2H), 4.64
(br, 2H), 4.96 (m, 2H), 5.90 (m, 1H), 7.53 (s, 1H), 7.63–7.80 (m, 4H),
8.07 (d, J = 8.5 Hz, 2H), 8.48 (d, J = 8.5 Hz, 1H), 8.95 (d, J = 4.0 Hz,
1H); 13C NMR (500 MHz, DMSO‑d6): δ 26.40, 27.75, 39.9, 46.15, 56.09,
59.80, 114.85, 123.18, 124.01, 126.80, 127.64, 129.97, 130.28, 130.35,
2.5. Enantioselective Michael addition reaction of anthrone to trans-
β-nitrostyrene
Trans-β-nitrostyrene 17 (29.8 mg, 0.20 mmol) and the polymeric
catalyst (5.0 mol%; calculated based on the unit molecular weight of the
polymer catalyst) were added to a reaction vessel with 2.0 mL of CH2Cl2.
Anthrone 16 (46.6 mg, 0.24 mmol) was added to the resulting solution.
The reaction was stirred at room temperature and monitored by TLC.
The reaction mixture was then filtered, and the filtrate was concentrated
in vacuo. The crude product was purified by silica gel (100–200 mesh)
column chromatography using hexane/EtOAc (5:1) as the eluent to
afford 18 as a white solid. The enantiomeric excess was determined
using HPLC by employing a Chiralpak AS-H column and 5:1 hexane:2-
propanol solution. Experiments to determine the effects of the solvent,
substrate scope, and recyclability were performed according to this
procedure. The results are summarized in Tables 2, 3, 4, and 5 (Scheme
4).
132.94, 142.70, 143.98, 148.62, 150.95, 167.5, 182.63. IR (KBr):
ν =
3196, 3067, 2952, 2864, 1799, 1638, 1512, 1474, 1342, 1207, 1132,
980, 814, 766, 682 cmꢀ 1; HRMS (APCI) m/z for C30H29Br2N4O2 [M +
17.4
H+] calcd. 637.0657, found 635.0652; [
α
]
D = ꢀ 43 (c 0.11, DMF).
Monosquaramides 6 (198 mg, 0.53 mmol) and 9-amino (9-deoxy) 3-
ethyl epi cinchonidine 9 (173 mg, 0.58 mmol) were added to a 30-mL
flask with 10 mL of MeOH and stirred under reflux conditions with Ar
gas for 48 h. White precipitate was obtained, which was filtered and
washed with cold MeOH and finally dried in a vacuum oven to afford 12
(185 mg, 55%) as a white solid. Rf: 0.57 (CH2Cl2/MeOH = 1.0/1.0); mp:
294–296 ◦C. 1H NMR (400 MHz, DMSO‑d6): δ 0.61 (m, 1H), 0.78 (t, J =
7.6 Hz, 3H), 1.24–1.57 (m, 8H), 2.32–2.43 (m, 1H), 2.55–2.67 (m, 1H),
3.10–3.29 (m, 3H), 4.65 (br, 2H), 7.53 (s, 1H), 7.61–7.81 (m, 4H), 8.05
(d, J = 8.8 Hz, 2H), 8.48 (d, J = 8.4 Hz, 1H), 8.94 (d, J = 4.4 Hz, 1H); 13
C
2.6. Enantioselective Michael addition reaction of β-ketoesters to trans-
β-nitrostyrene
NMR (500 MHz, DMSO‑d6): δ 12.51, 25.49, 26.20, 27.49, 28.55, 37.32,
46.13, 57.68, 59.9, 123.18, 124.03, 127.61, 129.95, 130.27, 130.45,
132.94, 144.00, 148.85, 150.96, 167.50, 182.50. IR (KBr):
ν = 3196,
Trans-β-nitrostyrene 17 (82.1 mg, 0.55 mmol) and the polymeric
3066, 2955, 2861, 1799, 1638, 1563, 1420, 1342, 1276, 1101, 958, 852,
catalyst (5 mol%) were added to a reaction vessel with 2.0 mL of solvent.
766, 681 cmꢀ 1; HRMS (APCI) m/z for C30H31Br2N4O2 [M + H+] calcd.
Methyl 2-oxocyclopentanecarboxylate 25 (63 μL, 0.50 mmol) was added
17.1
637.0814, found 637.0808; [
α]
D = ꢀ 34 (c 0.06, DMF).
via a syringe into the resulting solution (Scheme 5). The reaction was
stirred at room temperature, and its progress was monitored by TLC. The
reaction mixture was then filtered, and the filtrate was concentrated in
vacuo. The crude compound was purified by silica gel (100–200 mesh)
column chromatography using hexane/EtOAc (6:1) as the eluent to
afford the addition product 26 as a colorless oil. The enantioselectivity
(ee) and diastereomeric ratio (dr) were determined using HPLC by
employing a Chiralcel OD-H column and 4:1 hexane:2-propanol solu-
tion. The results are summarized in Table 6.
2.4. Synthesis of chiral cinchona-based squaramide copolymer using
Yamamoto coupling polymerization reaction
Squaramide 12 (160 mg, 0.19 mmol), 1,3-dibromo benzene 14 (22.7
μ
L, 0.19 mmol), 1,5-cyclooctadiene (0.21 mmol), 2,2′-bipyridyl (0.21
mmol), and Ni(COD)2 (0.28 mmol) were added to a test tube with DMF
(4.0 mL), and the reaction was carried out at 85 ◦C under N2 flow. After
48 h, the polymer was precipitated in diethyl ether and washed twice
with this solvent. Then, this polymer was washed with THF, HCl, and
EDTA solution. After filtration, the yellowish-brown product P2a was
dried in a vacuum oven. Yield: 158 mg (99%). 1H NMR (400 MHz,
2.7. Enantioselective Michael addition reaction of acetylacetone to trans-
β-nitrostyrene
DMSO‑d6): δ 0.50–2.00, 4.69, 7.1–8.94; IR (KBr): ν = 3398, 3065, 2955,
Trans-β-nitrostyrene 17 (37.3 mg, 0.25 mmol) and the polymeric
2871, 1771, 1646, 1599, 1443, 1331, 1156, 1024, 969, 844, 768, 652
catalyst (5 mol%) were added to a reaction vessel with 1.0 mL of CH2Cl2.
18.8
cmꢀ 1; [
1.03.
α
]
= ꢀ 9.0 (c 0.06, DMF); Mn (SEC) = 12,600; Mw/Mn =
D
Next, acetylacetone 27 (30.6 μL, 0.275 mmol) was added to the resulting
solution using a syringe (Scheme 5). The reaction was stirred at room
temperature and monitored by TLC. The reaction mixture was then
filtered, and the filtrate was concentrated in vacuo. The crude com-
pound was purified by silica gel (100–200 mesh) column chromatog-
raphy using hexane/EtOAc/CH2Cl2 (6:1:1) as the eluent to afford
addition product 28 as a white solid. The enantiomeric excess values
Other chiral polymers used in this study were synthesized using this
procedure.
3

S.A. Chhanda and S. Itsuno
Reactive and Functional Polymers 164 (2021) 104913
3.1. Preparation of chiral cinchona-based squaramide polymers using
Yamamoto coupling polymerization
Since Onimura et al. reported the synthesis of chiral polymers using
Yamamoto coupling polymerization [33], we used this procedure for the
synthesis of cinchona squaramide polymers. First, this polymerization
was applied to bis(4-iodobenzyl) derivative 10 of cinchona squaramide.
Under standard conditions of Yamamoto coupling polymerization for
aromatic dihalides [30], cinchona squaramide monomer 10 was poly-
merized to afford polymer P1. The results are summarized in Table 1. In
the presence of Ni(COD)2, monomer 10 underwent
a
self-
polycondensation reaction at 85 ◦C in dimethylformamide (DMF) to
provide linear polymer P1 with a molecular weight of 7400 in quanti-
tative yield (Table 1, entry 1). Copolymer P1a was also synthesized
using cinchona squaramide 10 and 1,4-diiodobenzene 13 under the
same Yamamoto coupling conditions (Scheme 3). Using squaramide 12
with a dibromobenzyl group, another linear polymer P2 was synthe-
sized. The polymerization conditions were investigated using 12 as a
monomer. The polymerization of 12 in DMF at 85 ◦C for 24 h provided
chiral polymer P2 with a molecular weight of Mn = 7800 in 95% yield
Scheme 2. Synthetic route to cinchona squaramide monomers 8, 10, 11
and 12.
, COD
Ni(COD)2
were determined using HPLC by employing a Chiralpak AD-H column
and hexane:2-propanol (9:1) as the solvent. The results are summarized
in Table 6.
P1a
+
10
I
I
Bipyridyl, DMF, 85 °C
48 h
13
3. Results and discussions
Cinchona squaramide derivatives 8, 10, 11, and 12 were synthesized
as monomers for the Yamamoto coupling polymerization. Scheme 1
describes the preparation of monosquaramides 3 and 6, which contain
two haloaryl moieties. These monosquaramides easily reacted with 9-
amino (9-deoxy)epi cinchonidine 7 or 9-amino (9-deoxy) 3-ethyl epi
cinchonidine 9 [25] to afford the corresponding cinchona squaramides
8, 10, 11, and 12 (Scheme 2). These are chiral monomers for the
Yamamoto coupling polymerization. Using these cinchona squaramide
monomers (8, 10, 11, and 12), the polymerization conditions of
Yamamoto coupling polymerization were studied.
Br
Br
, COD
Ni(COD)2
+
12
P2a
Bipyridyl, DMF, 85 °C
48 h
14
, COD
Ni(COD)2
+
12
Br
Br
P2b
Bipyridyl, DMF, 85 °C
48 h
15
Scheme 3. Synthesis of polymers, P1a, P2a and P2b.
Table 1
Synthesis of chiral polymers P1–P4 from cinchona squaramide 8, 10, 11, and 12 and their SEC data.a
b
Entry
Monomer
Solvent
Chiral Polymers
Yield [%]
Mn
Mw
Mw/Mn
1
10
DMF
P1
99
99
99
92
55
97
98
95
99
98
98
91
99
90
60
98
98
7400
3900
8000
4800
6000
4000
4300
7800
12,600
5000
4700
3600
7500
4900
4400
8000
4400
14,000
4100
1.87
1.05
1.74
1.23
1.02
1.15
1.14
1.20
1.03
2.46
1.48
1.03
1.77
1.32
1.53
1.02
1.13
2
10 + 13 (1:1)
DMF
P1a (1:1)
3
12
DMF
P2
14,000
6000
4
12
DMSO
Toluene
DMF
P2
5
12
P2
6100
6c
7d
8e
9
12
P2
4600
12
DMF
P2
5000
12
DMF
P2
9300
12 + 14 (1:1)
DMF
P2a (1:1)
P2a (1:1.5)
P2a (1:0.7)
P2b (1:1)
P3
13,000
12,000
7000
10
11
12
13
14
15
16d
17
12 + 14 (1:1.5)
DMF
12 + 14 (1:0.7)
DMF
12 + 15 (1:1)
DMF
3700
8
DMF
13,300
6500
8
DMSO
Toluene
DMF
P3
8
P3
5000
8
P3
8200
11
DMF
P4
5000
a
Reaction was carried out with 0.19 mmol monomers, 0.21 mmol 2, 2′ bipyridyl, 0.21 mmol 1,5-cyclooctadiene and 4.0 mL solvent for 48 h at 85 ◦C.
b
c
Determined by size exclusion chromatography (SEC) using DMF as the solvent at a flow rate of 1.0 mL/min at 40 ◦C (polystyrene standard).
Reaction was carried out at 60 ◦C.
Reaction was carried out at 110 ◦C.
Reaction was carried out for 24 h.
d
e
4

S.A. Chhanda and S. Itsuno
Reactive and Functional Polymers 164 (2021) 104913
m
N
N
O
O
HN
HN
N
O
N
O
N
N
n
n
P1
P1a
m
HN
HN
O
O
N
N
HN
HN
O
O
N
N
n
n
P2
P2a
(n:m)
m
HN
O
N
HN
O
N
n
P2b
Fig. 1. Cinchona squaramide linear homopolymers, P1 and P2, and copolymers, P1a, P2a and P2b.
(Table 1, entry 8). The completion of polymerization required 48 h, and
P2 with a molecular weight of Mn = 8000 was obtained (entry 3). Other
solvents including DMSO and toluene were used for the polymerization
of 12 at 85 ◦C for 48 h (entries 4 and 5). Low molecular weight and low
yield in DMSO and toluene, respectively, were observed. At a low tem-
perature (60 ◦C), polymerization occurred smoothly to afford polymer
P2 with a low molecular weight (Mn = 4000, entry 6). At a high tem-
polymerization may be 85 ◦C.
Under optimized conditions, copolymer P2a was synthesized from
squaramide 12 and 1,3-dibromo benzene 14 (Fig. 1) using different
ratios of 12 and 14. An equimolar ratio of 12 to 14 afforded the
copolymer P2a (1:1). The 1H nuclear magnetic resonance (NMR) spectra
of 12 and polymer P2a (1:1) are shown in Fig. 3. By comparison the
spectra of monomer and polymer, we can conclude that Yamamoto
polymerization successfully occurred. Polymers P2a (m:n) with
different ratios of 12 and 14 were prepared using the same method. For
copolymerization of 12 and 14, depending on the comonomer ratio, the
◦
perature (110 C), no increase in the molecular weight was observed
(entry 7). We tested polymerization temperature from 60 to 110 ◦C
(entries 6, 3, and 7). From these results, the suitable temperature for the
5

S.A. Chhanda and S. Itsuno
Reactive and Functional Polymers 164 (2021) 104913
EtOAc, and THF. These were insoluble in diethyl ether and hexane, and
readily precipitated in diethyl ether.
–
The transition-metal-catalyzed C C coupling reaction between aryl
N
halide and olefin is known as the Mizoroki–Heck coupling. Ni-catalyzed
Mizoroki–Heck coupling is also possible [35]. In the presence of the Ni
catalyst, both Yamamoto and Mizoroki–Heck coupling reactions occur
simultaneously. For example, cinchona squaramide monomers 8 and 11
with a C3-vinyl group can be polymerized by either Yamamoto or
Mizoroki–Heck coupling to yield hyperbranched polymers P3 and P4, as
shown in Fig. 2. For confirmation of that fact, under the reaction con-
dition (in DMF, 85 ◦C, 48 h) established for the polymerization, we did
the polymerization of dimer (only C3-vinyl group, no halide group) 29
and diiodobenzene 13. The corresponding polymer P5 was obtained in
O
HN
HN
N
O
O
N
HN
O
N
N
P3
P4
1
96% yield which was confirmed by H NMR (Supporting Information
Fig. 2. Cinchona squaramide branched polymers, P3 and P4.
S1.11).
molecular weights varied from 4700 to 12,600 (entries 3, 11, 9, and 10).
The molecular weights were reproducible. However, no specific ten-
dency was observed between the comonomer ratio and the molecular
weight. An additional copolymer P2b was synthesized using 12 and 4,4′-
dibromo biphenyl 15 (Fig. 1). These cinchona squaramide polymers are
synthesized via the Yamamoto coupling reaction for the first time. These
chiral polymers were soluble in DMF, DMSO, CH2Cl2, and CHCl3 and
partly soluble in other commonly used organic solvents such as MeOH,
3.2. Asymmetric Michael addition reaction of anthrone to trans-
β-nitrostyrene
Polymeric catalysts P1 ꢀ P4 were studied as organocatalysts in the
asymmetric Michael addition reaction of anthrone 16 and trans-
β-nitrostyrene 17 (Scheme 4). The absolute configuration of the Michael
adducts was assigned as (S) by comparing the reported value in the
literature [36]. First, the catalytic activities of low-molecular-weight
Fig. 3. 1H NMR spectra of monomer 12 and polymer P2a.
O
O
NO2
x mol % catalyst
CH2Cl
+
NO2
18 (S)
2, 25 °C
17
16
Scheme 4. Asymmetric Michael addition reaction of anthrone to trans-β-nitrostyrene using chiral polymeric catalysts.
6

S.A. Chhanda and S. Itsuno
Reactive and Functional Polymers 164 (2021) 104913
hours, which was much faster than that with the low-molecular-weight
catalyst 12, which required 24 h. The faster reaction was observed
because of its high substrate accumulation efficiency in the polymer
microenvironment. The chiral polymer chain of P2a effectively solubi-
lized the substrate molecule for the reaction. Low-molecular-weight
catalyst 12 was not soluble in the solvent and a heterogeneous reac-
tion occurred. P2a could be prepared using different molar ratios of 12
and 14. When the molar ratio was changed from 1:1 to 1:1.5 or 1:0.7, the
polymeric catalysts P2a (1:1.5) and P2a (1:0.7) afforded lower enan-
tioselectivities (entries 7 and 8). The structure of the comonomer also
affected the enantioselectivity, mainly due to the change in the polymer
conformation. With the use of biphenyl comonomer 15 instead of 14, a
much lower enantioselectivity (25% ee) was obtained (entry 9). Similar
to the linear polymers P1 and P1a, the hyperbranched polymer P3
provided an almost racemic product, and the reaction proceeded
appropriately (Table 2, entry 10). An additional hyperbranched polymer
P4 showed similar catalytic activity as P2 (entries 5, 11).
Table 2
Reaction optimization: asymmetric Michael addition reaction of anthrone 16 to
trans-β-nitrostyrene 14 using polymeric catalysts.a
Entry
Catalyst
Catalyst loading
(mol %)
Reaction time
[h]
Yieldbc
[%]
eec
[%]
1
2
3
4
5
6
7
10
5
5
5
5
5
5
5
7
93
89
90
88
90
92
91
58
84
20
5
12
24
16
24
12
3
P1
P1a
P2
66
78
61
P2a (1:1)
P2a
6
(1:1.5)
P2a
8
5
8
90
54
(1:0.7)
P2b
9
5
8
91
82
90
92
90
92
25
2
10
11
12
13
14
P3
5
24
12
3
P4
5
62
75
73
72
P2a (1:1)
P2a (1:1)
P2a (1:1)
2.5
10
15
3
The catalyst loading sometimes affects the catalytic performance in
asymmetric reactions. Using P2a, different molar percentages of the
catalyst were tested in the asymmetric reaction of anthrone 16 to trans-
β-nitrostyrene 17. Interestingly, even when the catalyst loading was
decreased to 2.5 mol%, the reaction was completed in three hours (entry
12).
3
a
Asymmetric reaction was carried out using 16 (0.24 mmol), 17 (0.2 mmol),
and 5 mol% cat. in CH2Cl2 (2.0 mL) at 25 ◦C.
b
Isolated yield of the product after column chromatography.
c
Enantioselectivity (ee) was determined using HPLC (CHIRALPAK AS-H
column).
Solvent screening was performed using P2a. The results are sum-
marized in Table 3. Various types of solvents were tested, and
dichloromethane afforded the best result in the asymmetric reaction
with P2a. The polymeric catalyst P2a was insoluble and shrank in
hexane, requiring a long time to complete the asymmetric reaction.
Relatively polar solvents such as acetonitrile, THF, acetone, methanol,
and ethyl acetate resulted in low enantioselectivities (entries 5–9). The
temperature effect was also examined (Table 3). At high temperature
(50 ◦C), the reaction time decreased, and it was completed in 1.5 h,
affording a slightly low enantioselectivity of 72% (Table 3, entry 11).
Decreasing the temperature to ꢀ 20 ◦C resulted in high enantioselectivity
(82% ee, entry 12). Increased enantiomeric excess of 84% was obtained
when the reaction was performed at ꢀ 40 ◦C, but a long time of 48 h was
needed (Table 3, entry 13).
cinchona squaramides 10 and 12 were examined as model catalysts.
Cinchona squaramides 10 and 12 showed excellent catalytic activities in
the asymmetric Michael addition reaction of anthrone and β-nitro-
styrene to afford Michael adduct 18 in high yield with 58% ee and 84%
ee (Table 2, entries 1 and 2). For monomeric catalyst 12, insolubility in
solvent may be responsible for longer reaction time. For linear polymer
P1, 20% enantioselectivity was observed with good yield (Table 2, entry
3). The corresponding copolymer P1a afforded an almost racemic
product in only 5% ee with a prolonged reaction time, but the yield was
good (Table 2, entry 4). Linear polymer P2 was then used as a catalyst
for the same reaction. In the presence of P2, the asymmetric reaction
was completed within 12 h at 25 ◦C. Michael adduct 18 was obtained in
90% yield with 66% ee (entry 5). The enantioselectivity increased to
78% ee using copolymer P2a (1:1) in the same asymmetric reaction
(entry 6). By employing P2a (1:1), the reaction was completed in three
Table 3
Asymmetric reaction of anthrone 16 and trans-β-nitrostyrene 17 using copolymer P2a (1:1).a
O
O
NO2
P2a
5 mol %
+
NO2
18 (S)
Solvent, temp
17
16
Entry
Solvent
Temperature [◦C]
Reaction time [h]
Yieldb [%]
eec [%]
1
CH2Cl2
25
3
92
91
89
83
90
90
92
87
89
88
92
90
89
78
70
63
42
6
2
CHCl3
25
9
3
Toluene
Hexane
Acetonitrile
THF
25
10
48
9
4
25
5
25
6
25
6
17
9
7
Acetone
MeOH
25
4
8
25
8
17
17
57
72
82
84
9
Ethyl acetate
Diethyl ether
CH2Cl2
25
5
10
11
12
13
25
27
1.5
22
48
50
CH2Cl2
ꢀ 20
ꢀ 40
CH2Cl2
a
Asymmetric reaction was carried out using 16 (0.24 mmol), 17 (0.2 mmol), and 5 mol% P2a (1:1) in 2.0 mL solvent at different temperatures.
Isolated yield of the product after column chromatography.
b
c
Enantioselectivity (ee) was determined using HPLC (CHIRALPAK AS-H column).
7

S.A. Chhanda and S. Itsuno
Reactive and Functional Polymers 164 (2021) 104913
Table 4
Substrate scope using copolymer catalyst P2a (1:1) in the reaction of anthrone to nitroolefins.a
ʦ
ʦ
Entry
Michael acceptor
Product
Reaction time [h]
Yieldb [%]
eec [%]
1
2
3
4
17
19
21
23
18
20
22
24
22
36
30
24
90
90
91
89
82
93
84
55
a
b
c
Asymmetric reaction was carried out using 16 (0.24 mmol), nitroolefin (0.2 mmol), and 5 mol% P2a(1:1) in CH2Cl2 (2.0 mL) at ꢀ 20 ◦C.
Isolated yield of the product after column chromatography.
Enantioselectivity (ee) was determined using HPLC (CHIRALPAK AS-H column).
methyl-β-nitrostyrene 21 (Table 4, entry 3). Decreased enantiose-
lectivity of 55% ee was observed for trans-2-thiophenyl-β-nitrostyrene
23 (Table 4, entry 4).
Table 5
Recyclability test of copolymer catalyst P2a (1:1) in the enantioselective reac-
tion of anthrone 16 to trans-β-nitrostyrene 17 in CH2Cl2 at ꢀ 20 ◦C.a
Entry
Reaction time [h]
Yieldb[%]
eec [%]
3.4. Recyclability of chiral copolymer P2a (1:1)
Fresh polymer
Cycle 1
22
24
24
24
24
90
82
84
85
81
82
84
83
84
82
Chiral copolymer P2a (1:1) was completely soluble in CH2Cl2, and
the asymmetric reaction occurred in a homogeneous system in this
solvent. After completion of the reaction, the polymeric catalyst was
precipitated in diethyl ether and then easily separated and recovered via
simple filtration. The recovered polymeric catalyst P2a (1:1) was reused
in the same reaction. The recyclability of P2a (1:1) was also examined,
and in the recycling experiments, it could be used four times (Table 5);
no significant differences in enantioselectivity were observed. The re-
action was completed within 24 h using the reused polymeric catalyst
P2a (1:1).
Cycle 2
Cycle 3
Cycle 4
a
Asymmetric reaction was carried out using 16 (0.24 mmol), 17 (0.2 mmol),
and 5 mol% P2a (1:1) in CH2Cl2 (2.0 mL) at ꢀ 20 ◦C.
b
c
Isolated yield of the product after column chromatography.
Enantioselectivity (ee) was determined using HPLC (CHIRALPAK AS-H
column).
3.3. Substrate scope of chiral copolymer P2a (1:1)
Other phenyl-substituted trans-β-nitrostyrene derivatives were tested
in the asymmetric reaction of anthrone 16 using P2a (1:1) in CH2Cl2 at
ꢀ 20 ◦C (Table 4). Good (89–91%) yields were obtained in all cases.
Excellent enantioselectivity of 93% was obtained with the Michael
acceptor trans-4-fluoro-β-nitrostyrene 19 (Table 4, entry 2). The
electron-withdrawing group at the para position of the benzene ring
favored the enantioselectivity; 84% ee was obtained using trans-4-
3.5. Michael addition reaction of other substrate combinations
An additional Michael donor, methyl 2-oxocyclopentanecarboxylate
25, instead of anthrone was investigated. The chiral polymers P1–P4
were used as organocatalysts in the reaction of 25 with 17 (Scheme 5).
In all cases, the asymmetric reaction proceeded smoothly to provide
chiral product 26 in good yield and excellent enantioselectivity with P1,
O
O
COOCH3
NO2
5 mol %catalyst
+
NO2
COOCH3
CH2Cl
2, 25 °C
H
25
17
26 (2S,3R)
O
O
O
O
H
3C
CH3
NO2
5 mol % catalyst
CH2Cl
+
H
3C
CH3
17
2, 25 °C
27
28 (S)
Scheme 5. Enantioselective Michael addition reaction using chiral polymers.
8

S.A. Chhanda and S. Itsuno
Reactive and Functional Polymers 164 (2021) 104913
Table 6
Asymmetric Michael addition reaction using chiral polymeric catalysts.a
Entry
Catalyst
Michael donor
Product
Reaction time [h]
Yieldc [%]
drd
eed [%]
1
P1
25
25
25
25
25
25
25
27
27
26
26
26
26
26
26
26
28
28
24
48
24
19
24
48
24
18
23
96
80
95
94
92
85
93
82
80
58:1
9:1
99
72
98
98
98
48
99
78
65
2
P1a
3
P2
92:1
>100:1
>100:1
6:1
4
P2a (1:1)
P2b
5
6
P3
7
P4
>100:1
–
–
8b
P2a (1:1)
P4
9b
a
Asymmetric reaction was carried out using 25 (0.5 mmol), 17 (0.55 mmol), and 5 mol% catalyst in CH2Cl2 (2.0 mL) at 25 ◦C.
Asymmetric reaction was carried out using 27 (0.275 mmol), 17 (0.25 mmol), and 5 mol% catalyst in CH2Cl2 (2.0 mL) at 25 ◦C.
Isolated yield of the product after column chromatography.
b
c
d
Enantioselectivity (ee) and diastereomeric ratio (dr) were determined using HPLC (CHIRALCEL OD-H for entries 1–7 and CHIRALPAK AD-H for entries 8 and 9).
P2, and P4 (Table 6). The absolute configuration of the Michael adducts
26 was assigned as (2S, 3R) by comparing the reported value in the
literature [37,38]. Polymers P1a (1:1) and P3 afforded low enantiose-
lectivities (72% ee and 48% ee) in the prolonged reaction time of 48 h
(entries 2 and 6). High diastereoselectivity (>100:1) was obtained with
P2a (1:1), P2b, and P4 catalysts (entries 4, 5, and 7). Chiral polymers
P2a (1:1) and P4 were also used in the asymmetric reaction of acety-
lacetone 27 with trans-β-nitrostyrene 17. The reaction occurred
smoothly with polymer P2a (1:1) to provide chiral Michael adduct 28 in
78% enantioselectivity with 82% yield (entry 8). With P4 catalyst, the
product was obtained in 65% ee with 80% yield (entry 9). The absolute
configuration of the Michael adducts 28 was assigned as (S) by
comparing the reported value in the literature [39]. Based on these re-
sults, a suitable microenvironment for the asymmetric reaction was
created in the chiral polymer network of the polymeric catalyst P2a
(1:1).
Declaration of Competing Interest
The authors declare no conflicts of interest associated with this
manuscript.
Acknowledgement
We thank Dr. Naoki Haraguchi at the Toyohashi University of
Technology for useful discussions.
A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.reactfunctpolym.2021.104913.
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