Synthesis of chiral polyesters of cinchona alkaloid catalysts for enantioselective Michael addition of anthrone to nitroalkenes

Article abstract of DOI:10.1016/j.jcat.2018.03.020

Chiral polyesters of cinchona alkaloid derivatives were synthesized via repetitive Mizoroki-Heck (MH) coupling reaction. Under MH reaction conditions, 4-iodobenzoylquinine and 4-iodobenzoylcupreine were easily polymerized via self polycondensation to affo

Full text of DOI:10.1016/j.jcat.2018.03.020

Journal of Catalysis 361 (2018) 398–406
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Journal of Catalysis
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Synthesis of chiral polyesters of cinchona alkaloid catalysts for
enantioselective Michael addition of anthrone to nitroalkenes
⇑
Bahati Thom Kumpuga, Shinichi Itsuno
Department of Environmental & Life Sciences, 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
Article history:
Chiral polyesters of cinchona alkaloid derivatives were synthesized via repetitive Mizoroki-Heck (MH)
coupling reaction. Under MH reaction conditions, 4-iodobenzoylquinine and 4-iodobenzoylcupreine
were easily polymerized via self polycondensation to afford the corresponding chiral polyesters. The
MH reaction conditions were also applied to the two-component polycondensation of cinchona ester
dimer and aromatic diiodide to obtain the chiral polyesters. These cinchona-based chiral polyesters were
successfully applied as polymeric organocatalysts in the asymmetric Michael addition reaction. The chiral
polyesters bearing free OH groups at the C60 position of the quinoline rings showed high catalytic activ-
ities (up to 97% isolated yield) and good enantioselectivities (up to 92% ee) in the Michael addition of
anthrone to nitroalkenes. Furthermore, these polymeric catalysts were stable and could be recycled
and reused several times.
Received 1 February 2018
Revised 17 March 2018
Accepted 19 March 2018
Keywords:
Cinchona alkaloids
Mizoroki-Heck polymerization
Polycondensation
Chiral polyester
Ó 2018 Elsevier Inc. All rights reserved.
Organocatalysts
Asymmetric catalysis
1. Introduction
alkaloids offer a unique platform for new reactions and methodolo-
gies in chiral catalyst design, including their application as
Cinchona alkaloids are some of the most important sources of
various kinds of efficient chiral organocatalysts [1,2]. Each of the
cinchona alkaloids, namely, quinine, cinchonidine, quinidine, and
cinchonine, contain several functionalities such as secondary
alcohol, quinuclidine, and quinoline rings in addition to the vinyl
group. These functionalities can be exploited for various chemical
modifications [1–3]. Several different kinds of chiral organocata-
lysts have been developed and they play a vital role in modern
asymmetric catalysis [3]. Cinchona alkaloids and their derivatives
are classified as privileged organic chirality inducers, efficiently
catalyzing many classes of organic reactions in a highly enantiose-
lective fashion [4,5].
Cinchona alkaloids modified at the C9-OH group [1–9], quinu-
clidine nitrogen [10–16], as well as C60 methoxy group [17–25]
are among the most prevalent effective enantioselective catalysts.
With the advancement in chemical synthesis methodologies that
are also in accordance with green chemistry practices, cinchona
polymer-immobilized catalysts [26–34]. Recently, chiral polymeric
catalysts have received significant attention owing to their easy
separation from the reaction mixture and their recyclability
[5,6,30]. Cinchona alkaloid derivatives have been attached to a
variety of synthetic polymers [27–30,33], which were used as chi-
ral modifiers and chiral ligands in metal complex catalysts [35–38],
and polymeric organocatalysts [6,34,38–42].
While many heterogeneous reactions using polymeric catalysts
exhibit poor reactivity by virtue of their heterogeneity, in some
cases, a well-designed polymeric chiral catalyst may lead to higher
selectivity with sufficient reactivity in asymmetric reactions [43].
Cinchona alkaloid C9 ester derivatives bearing free OH at the C60
position of the quinoline moiety have been widely explored and
successfully applied in many enantioselective reactions
[2,17–25,44].
Nevertheless, chiral polyesters having the cinchona-based
C60-OH derivative as a repeating unit in their main-chain have
not been reported yet. The most important aspect of obtaining an
efficient chiral polymer catalyst involves designing its synthesis.
Our design of chiral polyester synthesis involves the use of the
C3 olefinic double bond of the cinchona alkaloid. Mizoroki-Heck
(MH) coupling reaction of an aryl iodide represents one of the most
effective palladium-catalyzed carbon-carbon bond forming
processes [43]. We have previously utilized MH polymerizations
Abbreviations: BzCPN, benzoyl cupreine; CPD, cupreidine dimer; CPNa-CPNe,
cupreine dimers; IBzCPN, iodobenzoyl cupreine; IBzQN, iodobenzoyl quinine;
PBzCPN, polymeric benzoyl cupreine; PBzQN, polymeric benzoyl quinine; PCPD,
cupreidine polymer; PCPN, cupreine polymer; PQN, quinine polymer; QNa-QNe,
quinine dimers.
⇑
Corresponding author.
E-mail address: itsuno@ens.tut.ac.jp (S. Itsuno).
https://doi.org/10.1016/j.jcat.2018.03.020
0021-9517/Ó 2018 Elsevier Inc. All rights reserved.

B.T. Kumpuga, S. Itsuno / Journal of Catalysis 361 (2018) 398–406
399
Scheme 1. Synthetic route for the preparation of main-chain chiral polyesters PBzQN and PBzCPN. (i) 4-iodobenzoylchloride, NaOH aq. solution, CH2Cl2, rt, 4 h. (ii) Ar, IBzQN,
1 M BBr3, CH2Cl2, –78 °C to rt, 26 h. (iii) IBzQN or IBzCPN, Pd(OAc)2, Et3N, DMF, 100 °C, 36 h.
Scheme 2. Preparation of quinine-derived ester dimers 2. (i) 1, Et3N, DMAP, CH2Cl2, rt. (ii) Ar, 2QNa – 2QNe, 1 M BBr3, Dry CH2Cl2, ꢀ78C to rt, 26 h.
to prepare several kinds of chiral polymers, with the exception of
chiral polyesters [32,34,38,39]. In this article, we have prepared
chiral polyesters from cinchona alkaloid by means of MH polymer-
ization. Repetitive MH reaction of the iodophenyl ester of cinchona
alkaloids may yield chiral polyesters. Another possible method to
obtain similar chiral polyesters from cinchona alkaloids involves
repetitive MH reaction between cinchona alkaloid ester dimers
and diiodides. Herein, through MH polymerization, quinine was
chosen as the starting cinchona alkaloid to synthesize main-
chain type chiral polyesters bearing free OH at the C60 position of
the quinoline structure. The chiral polyesters were obtained in
good yield and subsequently used as catalysts in the asymmetric
Michael addition of anthrone to nitroalkenes. The synthesis and
catalytic performance of the chiral polyesters have been discussed
in this article.
catalysts, we prepared polyesters with the C60-OH groups PCPN
and compared their catalytic performance with that of the
C60-OMe polyesters PQN. In case of polyesters prepared by
two-component polycondensation, various kinds of ester linkers
were introduced into the quinine ester dimer.
2.1. Preparation of quinine-based chiral polyesters by one-component
self-polycondensation
A monomer containing both groups, namely, the olefinic double
bond and aromatic iodide moieties, was polymerized via one-
component self-polycondensation. We prepared quinine-derived
ester monomers IBZQN and IBzCPN having iodophenyl groups by
esterification of quinine with 4-iodobenzoyl chloride (Scheme 1)
[45,46]. The monomer IBZQN was then treated with boron tribro-
mide in CH2Cl2 to give the IBzCPN monomer with C60-OH function-
ality. In the presence of Pd(OAc)2, repetitive MH reaction took
place smoothly with the IBZQN and IBzCPN monomers in DMF
to give the chiral polyesters PBzQN and PBzCPN respectively
[32,38,39]. As shown in Table 1 (entries 1, 2), the polyesters
obtained in good yields by the one-component self-
polycondensation method had over 7000 molecular weight.
2. Results and discussion
The quinine-based chiral polyesters bearing free OH at the C60
position of the quinoline ring were designed and synthesized. We
prepared two different types of chiral polyesters. The first type
was prepared by the one-component self-polycondensation
method shown in Scheme 1. The Mizoroki–Heck (MH) reaction
occurred between the olefinic double bond and the aromatic
iodide. This reaction took place repeatedly on the monomer con-
taining both olefinic double bond and aromatic iodide in its mole-
cule. The second type of polyesters were prepared by the two-
component polycondensation method. MH polycondensation
between the cinchona ester dimers 2 (Scheme 2) and aromatic
diiodide 3 yielded chiral polyesters as shown in Scheme 3. In order
to confirm the importance of the C60-OH group in the polymeric
2.2. Preparation of quinine-based chiral polyesters by two-component
polycondensation
MH polymerization between quinine-derived ester dimers
2QNa, 2CPNa–2CPNe, and aromatic diiodides is another method
to obtain cinchona-based chiral polyesters. We first prepared the
cinchona ester dimers 2QNa–2QNe as illustrated in Scheme 2.
These dimers were prepared by using two equivalents of quinine

400
B.T. Kumpuga, S. Itsuno / Journal of Catalysis 361 (2018) 398–406
Scheme 3. Preparation of main-chain cinchona-based chiral polyesters PQN and PCPN by the MH reaction from quinine dimers 2. (i) 3, Pd(OAc)2, Et3N, DMF, 100C, 2–3 d.
Table 1
Synthesis of chiral polyesters via Mizoroki-Heck coupling reaction.a
b
b
Entry
Dimeric compound
Diiodides (Ar)
Chiral Polyester
Time (h)
Yield (%)
Mn
Mw
Mw/Mn
1c
2
–
–
–
–
PBzQN
PBzCPN
PQNaa
36
36
48
72
72
72
72
72
72
72
72
48
72
93
97
98
81
74
77
98
82
72
98
85
86
70
7400
9300
6800
5000
5400
5000
7000
7000
11,400
12,300
8800
5100
6000
5500
8100
8900
1.5
1.3
1.2
1.1
1.1
1.1
1.2
1.3
3c
4
2QNa
2CPNa
2CPNd
2CPNb
2CPNe
2CPNc
2CPNd
2CPNe
2CPNe
2CPNa
4CPDe
3a
3a
3a
3a
3a
3a
3c
3b
3c
3c
3a
PCPNaa
PCPNda
PCPNba
PCPNea
PCPNca
PCPNdc
PCPNeb
PCPNec
PCPNac
PCPD
5
6
7
8
d
d
d
9
–
–
–
10
11
12
13e
9700
22,300
2.3
d
d
d
–
–
–
3000
7500
4400
14,600
1.2
2.0
a
b
c
Polymerized in dry DMF at 100 °C.
Determined by SEC (polystyrene standard) using DMF as an eluent at a flow rate of 1.0 mL min–1 and 40 °C.
C60-OMe chiral polyesters.
d
e
SEC measurements failed because of poor solubility in DMF.
Chiral polyester derived from quinidine linked by decanedioyl chloride 1e.
with dicarboxylic acid dichlorides 1 by following the previously
reported procedures [47,48]. We utilized different kinds of
dicarboxylic acid dichlorides 1a–1e in order to evaluate the effect
of the linker structure on the catalytic performances of the chiral
polyester organocatalysts PQN and PCPN. These cinchona ester
dimers were then treated with boron tribromide in CH2Cl2 to
obtain the C60-OH derivatives 2CPNa–2CPNe in good yields. We
also prepared another ester dimer (4CPD) with the C60-OH group
from quinidine, which is a pseudoenantiomer of quinine. The
chemical structure of the dimer is presented in Fig. 1.
the two-component polycondensation system are summarized in
Table 1 (entries 3–13). In all cases, the chiral polyesters PCPN were
successfully obtained in high yields (from 70% to 98%). These poly-
mers were all insoluble in commonly used organic solvents such as
ether, ethyl acetate, acetonitrile, toluene, and tetrahydrofuran.
Most of them were also soluble in polar aprotic solvents such as
DMF and DMSO. Owing to the poor solubility of the chiral polye-
sters PCPNdc and PCPNec, attempts of SEC measurements of these
polymers were unsuccessful (Table 1, entries 9 and 11).
These cinchona ester dimers possessed two olefinic double
bonds in their molecule, which made their polymerization with
aromatic diiodides feasible by the two-component polycondensa-
tion process under MH reaction conditions. The cinchona ester
dimer 2QNa was reacted with 1,4-diiodobenzene 3a in the pres-
ence of Pd catalyst in DMF. The MH polymerization occurred
smoothly to give the polyester PQN in 98% yield (Table 1, entry
3). The number average molecular weight of PQN was 6800. The
polymerization of the C60-OH dimers 2CPNa–2CPNe also
proceeded under the MH polymerization conditions to yield the
corresponding polymers PCPNa–PCPNe in good yields (Scheme 3).
MH polymerization is relatively tolerant to functional groups,
including the phenolic hydroxyl group at the C60 position. Various
combinations of the cinchona ester dimers (2CPNa–2CPNe) and
aromatic diiodides 3 were chosen to prepare the corresponding
chiral polyesters (Scheme 3). The MH polymerization results of
2.3. Asymmetric Michael addition of anthrone and b-nitrostyrene with
monomeric and dimeric cinchona derived ester catalysts
Shi et al. have reported that the asymmetric Michael addition of
anthrone and b-nitrostyrene in the presence of O-benzoyl cupreine
BzCPN as a catalyst gave the Michael adduct in 94% yield with 96%
ee at ꢀ40 °C (Table 2, entry 1) [46]. We examined the catalytic
potential of the ester monomers IBzQN and IBzCPN, and the
dimers 2QNa, 2CPNa–2CPNe, and 4CPD for the same asymmetric
transformation of anthrone and b-nitrostyrene (Scheme 4). The
results of these asymmetric catalytic reactions are summarized
in Table 2. While IBzQN with its OMe group at the C60 position gave
very low enantioselectivity in the Michael reaction (entry 2), the
C60-OH derivative IBzCPN showed almost the same activity as
BzCPN.

B.T. Kumpuga, S. Itsuno / Journal of Catalysis 361 (2018) 398–406
401
Fig. 1. Chemical structure of cupreidine dimer 4CPD and its corresponding chiral polyester PCPD.
Table 2
Effect of monomeric and dimeric cinchona-derived ester catalysts in the Michael addition of anthrone to b-nitrostyrene.a
Entry
Chiral catalyst
Temperature (°C)
Time (h)
Yieldb (%)
eec (%)
1d
2e
3
BzCPN
IBzQN
IBzCPN
2QNa
2CPNa
2CPNb
2CPNc
2CPNd
2CPNe
4CPD
–40
rt
–40
rt
–40
–40
–40
–40
–40
rt
12
24
16
24
24
24
24
24
24
24
3
94
58
89
80
75
69
81
79
69
97
92
95
96
9
94
11
94
98
94
98
90
90
90
86
4e
5
6
7
8
9
10f
11
12
2CPNd
IBzCPN
50
50
3
a
Reactions (entries 2–9) were conducted by using 8 (0.48 mml) and 9 (0.40 mmol) as the substrates with 10 mol% of the quinine-derived catalysts in CH2Cl2 (3.0 mL) for a
specified time to obtain 10 with R configuration [45].
b
Isolated yield of the product.
c
Enantiomeric excess (ee) values determined by using the HPLC -Chiralcel AS-H column with hexane/isopropyl alcohol = 5/1 as eluent at a flow rate of 0.7 mL/min.
d
Reference data with 5 mol% catalyst loading of BzCPN at –40 °C as reported in literature [45].
e
C60-OMe group quinine-derived ester dimer.
f
Reaction involving 8 (0.24 mmol), 9 (0.20 mmol), and 15 mol% of C60-OH cupreidine ester dimer 4CPD as a catalyst in CH2Cl2 (2.0 mL) for the formation of 10 with S
configuration.
Scheme 4. Asymmetric Michael addition of anthrone 8 to b-nitrostyrene 9 in the presence of organocatalysts.

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B.T. Kumpuga, S. Itsuno / Journal of Catalysis 361 (2018) 398–406
In case of the dimeric catalysts, similar catalytic performances
organocatalysts, C60-OH chiral polyesters PCPNba, PCPNea,
PCPNdc, and PCPNeb showed good enantioselectivities (88–92%
ee, entries 6, 8, 9, 11, and 13) good yields of 10. As a result of the
heterogeneity of the polymeric catalysts, longer reaction time was
required for some cases (entries 4–6, 11). Among the evaluated cat-
alysts shown in Table 3, the linearly spaced chiral polyesters
PCPNea, PCPNdc, PCPNeb, and PCPNec showed good performance
in terms of their catalytic activity and enantioselectivity in the for-
mation of Michael product 10 when the reaction was carried out at
room temperature (entries 9, 12–14). In addition, PCPD prepared
from the quinidine dimer 4CPD (Fig. 1) showed good enantioselec-
tivity with the chiral product having opposite enantiomer (Table 3,
entry 16).
We then surveyed the effect of solvent on the catalytic perfor-
mance of the chiral polyesters by using PCPNea. The results of
asymmetric Michael reaction with PCPNea and various solvents
have been summarized in Table 4. It was found that the chiral
polyesters were tolerant to the different types of solvents. For all
selected solvents except for toluene, the reaction was highly enan-
tioselective and 10 was obtained with ee above 80%. In toluene as
the reaction medium, only poor enantioselectivity (16% ee) was
attained (Table 4, entry 6).
were observed. The C60-OMe dimer 2QNa gave the product with
low enantioselectivity (entry 4), while the reaction with the C60-
OH dimer 2CPNa–2CPNe was highly enantioselective in all cases
(entries 5–9). In the cases of both 2CPNb and 2CPNd, 98% ee was
attained for the same reaction. When dimer 2CPNd and monomer
IBzCPN were used as catalysts at 50 °C, the achieved enantioselec-
tivity of 10 was lower in comparison to the reaction performed at
ꢀ40 °C (entries, 11 and 12 vs entries 3 and 8, respectively). Dimer
4CPD was prepared from quinidine, which is a pseudo-enantiomer
of quinine. When this dimer was used as a catalyst in the same
reaction, the chiral product with the opposite stereochemistry
was predominantly formed, as compared to the product obtained
by using quinine-derived catalysts (entry 10).
2.4. Asymmetric Michael addition of anthrone and b-nitrostyrene with
chiral polyester catalysts
In order to evaluate the catalytic performance of the chiral
polyesters PBzQN, PBzCPN, PQN, and PCPN derived from quinine,
we used these polymers as organocatalysts in the asymmetric
Michael addition reaction (Scheme 4). We first used the chiral
polyester PBzQN, which was prepared by one-component self-
polycondensation, as the polymeric organocatalyst. Although the
insoluble polymeric catalyst resulted in a heterogeneous mixture,
We next examined the effect of temperature on the activity of
the polymeric catalyst PCPNea in the asymmetric Michael
reaction. Interestingly, the enantioselectivities obtained showed
no significant changes between ꢀ40 °C and 50 °C (Table 5). A
higher temperature ensured that a relatively shorter time was
required to complete the reaction (entry, 8). The increase in
catalyst loading from 5 mol% to 20 mol% had no significant effect
on the enantioselectivity (entries, 4–7).
the asymmetric Michael addition of anthrone
8 and b-
nitrostyrene 9 proceeded smoothly to give the corresponding
Michael adduct 10 in 61% yield with 33% ee (Table 3, entry 1). In
the case of PBzCPN with C60-OH as a catalyst, higher enantioselec-
tivity (88% ee) was obtained. This was in accordance with the
results obtained with low molecular weight catalysts. Next, chiral
polyesters prepared by the two-component MH-polycondensation
reaction (PQN and PCPN) were used as catalysts for the same reac-
tion; the results of these tests are summarized in Table 3. The polye-
ster PQNaa containing C60-OMe catalyzed the asymmetric reaction
to give 10 with 11% ee (entry 3). When the chiral polyesters PCPN
containing C60-OH were used as catalysts, the asymmetric Michael
reaction occurred under heterogeneous condition to give 10 with
much higher enantioselectivities (70–92% ee, entries 4–16). Com-
pared with the results obtained by using the corresponding dimeric
catalysts 2CPN, somewhat lower enantioselectivities were obtained
with the polymeric catalysts. Among the screened polymeric
2.5. Substrate scope
Under the optimized reaction conditions, the performance of
the chiral polyester PCPNea was evaluated by changing the
Michael acceptor substituents 11, as presented in Scheme 5. The
aryl nitroalkene derivatives bearing electron withdrawing or
electron donating substituents on the benzene ring showed high
enantioselectivities and good catalytic activity in the formation
of the corresponding Michael adducts 12a and 12b. The thiophene
derived nitroalkene also showed good enantioselectivity and
catalytic activity when it was used as an acceptor in the Michael
Table 3
Cinchona based chiral polyester catalysts screening for the formation of 10 in the Michael addition of anthrone to b-nitrostyrene.a
Entry
Chiral polyester catalyst
Temperature (°C)
Time (h)
Yieldb (%)
eec (%)
1d
2
PBzQN
PBzCPN
PQNaa
rt
–40
rt
ꢀ40
ꢀ40
ꢀ40
rt
36
48
40
84
72
72
24
48
40
48
72
20
40
40
48
48
61
75
62
50
61
52
73
73
84
67
51
85
85
85
63
78
33
88
11
79
80
90
79
88
90
71
92
86
88
84
70
82
3d
4
PCPNaa
PCPNda
PCPNba
PCPNba
PCPNea
PCPNea
PCPNca
PCPNdc
PCPNdc
PCPNeb
PCPNec
PCPNac
PCPD
5
6
7
8
ꢀ40
9
rt
10
11
12
13
14
15
16e
ꢀ40
ꢀ40
rt
rt
rt
rt
rt
a
Reactions were performed with 8 (0.48 mmol or 0.24 mmol), 9 (0.40 mmol or 0.20 mmol), and with 10 mol% or 15 mol% of the polymeric catalyst in CH2Cl2 (3 mL or 2.0
mL) under the specified reaction temperature and time for the formation of 10 with R configuration.
b
Isolated yield of the product.
c
Enantiomeric excess (ee) values were determined using a HPLC -Chiralcel AS-H column with hexane/isopropyl alcohol = 5/1 as the eluent at a flow rate of 0.7 mL/min.
d
C60-OMe group chiral polymeric catalyst was used for the formation of 10 with R configuration.
e
C60-OH quinidine-derived polymeric catalyst was used for the formation of 10 with S configuration.

B.T. Kumpuga, S. Itsuno / Journal of Catalysis 361 (2018) 398–406
403
Table 4
2.6. Recyclability of chiral polyesters
Effect of solvent in the Michael addition of anthrone 8 to b-nitrostyrene 9 in the
formation of 10 with PCPNea as a catalyst.a
As all synthesized chiral polyesters were insoluble in different
solvents, recovery of the polymeric catalyst after the completion
of the reaction was possible. To confirm the recyclability of the
polyester catalyst, PCPNdb was used as a catalyst (Scheme 4). In
order to compare the stability of the catalyst, the reaction was
performed at room temperature as well as under reflux conditions
in dichloromethane. Under reflux, PCPNdb showed good perfor-
mance and the asymmetric Michael addition reaction was com-
plete within 5 h. After completion of the reaction in each cycle,
the catalyst was separated by centrifugation and reused in the next
cycle. The results of the enantioselectivity and catalytic activity of
PCPNdb are summarized in Table 6. There was no deterioration of
enantioselectivity and catalytic activity under both reaction condi-
tions and the catalyst could be reused up to three cycles.
Entry
Solvent
Time (h)
Yieldc (%)
eed (%)
1
2
3
4
5
6
7
8
9
EtOH
96
70
96
96
48
96
70
72
96
48
44
29
51
73
34
25
51
67
82
80
84
90
88
16
86
88
90
Acetone
EtOAc
Et2O
CH2Cl2
Toluene
Hexane
CH3CN
THF
a
All reactions were carried out with 8 (0.48 mmol), 9 (0.40 mmol) and 10 mol%
catalyst loading at ꢀ40 °C for the specified time in the formation of 10 with R
configuration [45].
c
Isolated yield of the product.
The enantiomeric excess (ee) values determined by HPLC-chiralcel AS-H col-
d
umn with Hexane/IPA = 5/1 as eluent at a flow rate of 0.7 mL/min).
3. Conclusion
Table 5
Main-chain cinchona based chiral polyesters bearing free OH at
the C60 position of the quinoline moiety were successfully synthe-
sized by MH polymerization and isolated in good yields. The chiral
polyesters were used as catalysts in the enantioselective Michael
addition of anthrone to nitroalkenes. These chiral polyester
organocatalysts showed good activities and enantioselectivities
(up to 92% ee) in the Michael addition reaction. The activities of
the polymeric catalysts were also affected by the type of linker used
in the preparation of the chiral polyesters. It was found that the lin-
early spaced cinchona polymers were more effective catalysts in the
formation of Michael product 10 as compared to the chiral polye-
sters with aromatic linkers. The linearly spaced chiral polyesters
PCPNea and PCPNeb showed good stability under reflux conditions
and even at relatively low reaction temperatures; the reaction enan-
tioselectivity and catalytic activity were unaffected. Furthermore,
the catalyst with the lower molecular weight led to somewhat lower
enantioselectivity when the catalytic reaction was performed under
reflux conditions as compared to the reaction carried out at a
lower reaction temperature. The chiral polyesters could be easily
Optimization of reaction conditions for the Michael addition of anthrone to b-
nitrostyrene to form 10 with PCPNda as a catalyst.a
Entry
Catalyst (mol %)
T (OC)
Time (h)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
10
20
20
5
10
15
20
15
ꢀ40
ꢀ40
ꢀ20
RT
RT
RT
48
48
48
24
24
24
24
12
68
79
69
55
68
88
97
87
88
88
86
84
84
90
86
88
RT
50
a
All reactions were carried out with 8 (0.24 mmol), 9 (0.20 mmol) in CH2Cl2 (2.0
mL) under specified reaction conditions for the formation of 10 with R configuration
[45].
b
Isolated yield of the product.
Enantiomeric excess was determined by HPLC (Chiralcel AS-H column with
c
hexane/isopropyl alcohol = 5/1 as eluent at a flow rate of 0.7 mL/min).
reaction for the formation of 12c. The structural configuration of
these Michael products was not determined.
Scheme 5. Substrate scope of asymmetric Michael addition of anthrone 8 to nitro alkene derivatives 11 with PCPNea as a catalyst.

404
B.T. Kumpuga, S. Itsuno / Journal of Catalysis 361 (2018) 398–406
Table 6
(0.93 g, 3.500 mmol) and a 30% w/w NaOH aqueous solution (4.3
mL) were added sequentially at room temperature. The mixture
was stirred vigorously for 4 h, as reaction completion was con-
firmed by TLC (EtOAc:Et3N = 50:1). Subsequently, H2O (15 mL)
and CH2Cl2 (10 mL) were added simultaneously to the reaction
mixture. The two layers were separated and the aqueous layer
was extracted by CH2Cl2. The combined organic extracts were
dried over MgSO4, filtered, and evaporated under reduced pressure.
The residue was purified by flash chromatography with EtOAc:
Et3N = 50:1 as an eluent mixture to afford IBzQN as white solid
Evaluation of recyclability of PCPNeb as a catalyst in the formation of 10.a
Entry
Cycle no.
Reaction temperature
rt
50 °C
Yieldb (%)
eec (%)
Yieldb (%)
eec (%)
1
2
3
4
Initial
First
Second
Third
85
80
79
80
88
86
84
84
80
82
85
75
88
90
86
85
a
All reactions were carried out with 8 (0.72 mmol), 2 (0.60 mmol), and 15 mol%
(1.44 g, 84% yield); mp: 198.0–199.7 °C; ½a D30
ꢁ
= +166.17 (c 1.0,
of catalyst loading for 24 h at room temperature and for 5 h at 50 °C in CH2Cl2 for
the formation of 10 with R configuration [45].
DMF); 1H NMR (CDCl3, TMS, 400 MHz,) dH = 1.50–1.60 (m, 2H),
1.70–1.80 (m, 2H), 2.10–2.18 (m, 1H), 2.31 (br, 1H), 2.59 (m, 4H),
2.50–2.54 (m, 1H), 2.91–2.97 (m, 1H), 3.25 (br, 1H), 3.59–3.63
(m, 1H), 4.05 (2, 3H), 5.08–5.15 (t, 2H), 6.04–6.13 (m, 1H), 6.6 (d,
J = 8.86 Hz, 1H), 7.51–7.55 (dd, J = 2.75, 1H), 7.65–7.70 (m, 2H),
7.8 (m, 2H), 7.90 (d, J = 1.83 Hz, 1H), 8.00–8.10 (m, 3H), 8.6–8.7
(d, J = 4.58 Hz, 1H). 13C NMR (CDCl3, TMS = 77.16, 400 MHz) dc =
8.1, 24.3, 27.7, 28.0, 39.7, 42.6, 42.8, 55.7, 56.8, 59.5, 74.8, 101.5,
114.7, 118.7, 122.0, 127.0, 129.3, 131.1, 132.0, 138.1, 141.7,
143.5, 144.9, 147.5, 158.1, 165.2. IR (KBr) vmax [cm–1]: 3057 (C–
b
Isolated yield of 10 with R configuration.
Determined by HPLC-chiralcel AS-H column with hexane/isopropyl alcohol = 5/
c
1 as eluent at a flow rate of 0.7 mL/min).
recovered from the reaction mixture and used up to three times
without loss in the catalytic activity and enantioselectivity. The
notable features of the main-chain cinchona-based chiral polyesters
as catalysts for the Michael addition of anthrone to nitroalkenes
include high catalytic activity and enantioselectivity (even at high
temperatures), recyclability, and stability.
H
arom), 1721 (C@O), 1459 (C@Carom), 1261 (CAO). HRMS (ESI) m/z
for C27H27IN2O3 [M+H+] calcd. 555.1139, found 555.1066.
4. Experimental part
4.3. Synthesis of dimer 2QNb
4.1. General methods and materials
A slightly modified synthetic procedure for the dimeric com-
pounds with aromatic linkers was employed [47]. To a solution
of quinine (3.00 g, 9.259 mmol) in 25 mL of anhydrous CH2Cl2 at
0 °C, 4.3 mL of Et3N was added and the mixture was stirred for
10 min. Next, powdered 4,40-biphenyl dicarbonyl chloride 1b was
added portionwise and the solution was stirred for 1 h at 0 °C.
The mixture was cooled to room temperature and stirred for 5 h.
Upon reaction completion, water (25 mL) was added and the
organic layer was separated from the aqueous layer, washed with
NH4Cl aqueous solution, extracted by CH2Cl2, dried over MgSO4,
and concentrated in vacuo. The residue was purified by flash chro-
matography on silica gel with EtOAc:EtOH:Et3N = 7:3:0.5 as the
eluent to afford 2QNb as a white solid (4.00 g, 51% yield); mp:
All solvents and reagents were purchased from Sigma-Aldrich,
Wako Pure Chemical Industries, Ltd., and Tokyo Chemical Industry
Co., Ltd. at the highest available purity and were used as received,
unless otherwise stated. Analytical thin-layer chromatography
(TLC) was conducted using pre-coated silica gel plates (Merck TLC
silica gel, 60F254) for monitoring the reaction progress. Column
chromatography was performed using silica gel columns (Wakogel
C-200, 100–200 mesh). Melting points (mp) were recorded with a
Yanaco micro melting apparatus and the average values of the ana-
lyzedsampleswere recorded. Opticalrotationwasdeterminedusing
a JASCO DIP-149 digital polarimeter and 10 cm-long thermostated
microcell. NMR spectra were recorded on JEOL JNM-ECS400 spec-
trometers in CDCl3 or DMSO-d6 at room temperature operating at
400 MHz (1H) and 100 MHz (13C{1H}). Tetramethylsilane (TMS)
was used as an internal standard for 1H NMR and 13C NMR in CDCl3.
Chemical shifts and J values are reported in parts per million (ppm)
and hertz (Hz), respectively. The IR spectra were recorded on a JEOL
JIR-7000 FTIR spectrometer by using KBr pellets and the wavenum-
bers are reported in cm–1. HRMS (ESI) spectra were recorded using a
Bruker micro OTOF II HRMS instrument. High-performance liquid
chromatography (HPLC) was performed with a JASCO HPLC system
composed of a DG-980-50 three-line degasser, the 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. Size exclusion chromatography
(SEC) was performed using a Tosoh instrument with either HLC
8020 UV (254 nm) or refractive index detection. Two polystyrene
gel columns of bead size 10 mm were used and dimethylformamide
(DMF) was used as the carrier solvent with a flow rate of 1.0 mL/min
at 40 °C. A calibration curve was obtained to determine the number-
average molecular weight (Mn) and molecular weight distribution
(Mw/Mn) in comparison with polystyrene standards.
218.6–220.2 °C; ½a D31
ꢁ
= +222.63 (c 1.0, DMF); 1H NMR (DMSO-d6,
400 MHz) dH = 1.02–1.07 (m), 1.14–1.19 (m), 1.68–1.83 (m),
1.97–1.98 (br, s), 2.47–2.51 (m), 2.82–2.89 (m), 3.88–3.89 (br, s),
3.92–3.96 (m), 4.91–4.97 (m), 4.98–4.99 (m), 5.04–5.07 (m), 5.74
(br, s), 5.95 (br, s), 5.97–5.99 (d, J = 2.44 Hz), 6.00 (br, s), 6.01–
6.03 (d, J = 2.44 Hz), 6.54–6.58 (d, J = 8.54 Hz), 7.36–7.39, (dd, J =
2.75 Hz), 7.41–7.45 (dd, J = 2.75 Hz), 7.49–7.50 (d, J = 3.97 Hz),
7.58–7.62 (dd, J = 4.58), 8.14–8.17 (d, J = 8.24), 8.66–8.67 (d, J =
4.58 Hz), 8.66–8.70 (d, J = 4.58). 13C NMR (DMSO-d6, 400 MHz) dC
= 11.7, 11.8, 22.1, 24.5, 27.9, 28.0, 28.1, 28.2, 40.0, 40.2, 42.9,
43.4, 46.5, 55.9, 56.0, 57.0, 57.2, 59.7, 60.3, 72.2, 74.9, 77.2, 77.3,
77.6, 77.8, 101.5, 101.7, 114.7, 114.8, 118.9, 121.8, 122.2, 126.98,
127.2, 127.8, 129.7, 130.6, 132.1, 142.0, 143.9, 145.0, 145.0,
147.7, 147.8, 147.9, 158.3, 165.5. IR (KBr) vmax [cm–1]: 3073 (C–
H
arom), 1720 (C@O), 1508 (C@Carom), 1264 (CAO). HRMS (ESI) m/z
for C54H54N4O6 [M+H+] calcd. 855.4116, found 855.4106.
4.4. Synthesis of dimer 2QNe
The typical literature procedure for the synthesis of dimers with
linear spacers was slightly modified [48]. To a solution of quinine
(2.00 g, 6.173 mmol) in 20 mL of anhydrous CH2Cl2, 0.5 equivalents
4.2. Synthesis of monomer IBzQN
of decanedioyl chloride 1e and
a catalytic amount of 4-
A typical synthetic procedure as reported in literature was
employed [45]. To a stirred solution of quinine (1.00 g, 3.086
mmol) in anhydrous CH2Cl2 (20 mL), 4-iodobenzoyl chloride
dimethylaminopyridine (DMAP) were added sequentially. After 2
d of vigorous stirring, 2 mL of triethylamine was added to the mix-
ture. After further 2 h of stirring, the solvent was removed in vacuo

B.T. Kumpuga, S. Itsuno / Journal of Catalysis 361 (2018) 398–406
405
and the residue was further dried in a vacuum oven at 40 °C for 1 d
without further purification. 2QNe was obtained as a solid orange
stirred together in 2 mL of dry DMF at 100 °C for 72 h in the
presence of 6 mol% of Pd(OAc)2 (15 mg) and Et3N (0.07 mL). Subse-
quently, the mixture was allowed to cool to room temperature,
then precipitated into distilled water (200 mL) with stirring. The
mixture was stirred for 24 h at room temperature. The precipitate
was filtered and sequentially washed by water, ethyl acetate, hex-
ane, and diethyl ether, and then dried in a vacuum oven at 40 °C to
afford a brown powder of PCPNea (0.43 g, 98% yield). IR (KBr) vmax
[cmꢀ1]: 3447 (OAH), 2928 (C@Harom), 1747 (C@O), 1465 (C@Carom),
1231 (CAO). Mn (SEC) = 7000; Mw/Mn = 1.2.
(3.95 g, 76% yield); mp: 67.2–68.5 °C. ½a D31
ꢁ
= –10.75 (c 1.0, DMF);
1H NMR (CDCl3, TMS, 400 MHz) dH = 0.81–0.84 (m), 1.1–1.29 (m),
1.49–1.56 (m), 1.49–1.56 (m), 1.68–1.74 (m), 1.81–1.84 (m),
2.17–2.36 (m), 2.87–2.91 (m), 3.00–3.10 (m), 3.30–3.40 (m), 3.94
(s), 4.96–5.01 (m), 5.73–5.85 (m), 6.48–6.50 (d, J = 6.71 Hz), 7.22–
7.43 (m), 7.98–8.00 (d, J = 9.16 Hz), 8.66–8.71 (d, J = 4.58 Hz). 13C
NMR (CDCl3, TMS = 77.16, 400 MHz) dc = 0.2, 9.4, 12.2, 24.3, 24.9,
27.7, 27.8, 29.1, 29.3, 29.4, 34.5, 35.7, 37.3, 39.3, 39.7, 42.5, 42.6,
45.6, 55.6, 55.9, 56.0, 56.6, 58.2, 58.9, 59.1, 73.6, 73.7, 77.5, 77.6,
101.6, 106.7, 114.8, 118.8, 119.0, 122.1, 127.1, 131.8, 141.7,
143.9, 144.8, 147.5, 158.2, 172.8, 172.9, 178.4. IR (KBr) vmax
[cmꢀ1]: 3074 (C–Harom), 1740 (C@O), 1508 (C@Carom), 1228
(CAO). HRMS (ESI) m/z for C50H62N4O6 [M+H+] calcd. 815.4742,
found 815.4717.
4.8. Asymmetric catalysis with cinchona ester derivatives
A representative procedure for the Michael addition of anthrone
to nitroalkenes (Table 3 entry 9) using chiral polymeric catalyst
PCPNea is described [46]. Trans-b-nitrostyrene 9 (0.03 g, 0.200
mmol) and 15 mol% (0.033 g) of PCPNea were dissolved in CH2Cl2
(2.0 mL) and stirred for 10 min at room temperature. Next,
anthrone 8 (0.047 g, 0.24 mmol) was added and the mixture was
stirred for 40 h. Subsequently, the reaction mixture was filtered
and washed thoroughly with CH2Cl2. The filtered solution was then
concentrated in vacuo. The residue was purified via column
chromatography using hexane/EtOAc = 5/1 as the eluent and 10
was obtained as a white solid (0.07 g, 84% yield). HPLC analysis
(Chiralcel AS-H column with hexane/isopropanol = 5/1, v/v, at a
flow rate of 0.7 mL/min) of 10 was conducted at room temperature
and ee was determined to be 90%, tmajor = 18.78 min, tminor = 21.28
min. Thus, in the same way, except for recyclability evaluation, all
synthesized cinchona-derived chiral organocatalysts were used as
catalysts for enantioselective synthesis in the Michael addition of
anthrone to nitroalkenes.
4.5. Synthesis of dimer 2CPNe
The literature reported procedure for the preparation of C60 OH-
free compounds was slightly modified [45,46]. Under argon atmo-
sphere, 2QNe (1.80 g, 2.209 mmol) was dissolved in dry CH2Cl2 (25
mL) and stirred at ꢀ78 °C for 15 min. Next, 35 mL of 1 M boron tri-
bromide solution in CH2Cl2 was added slowly over 10 min and the
mixture was stirred vigorously for 2 h. It was then cooled to room
temperature and allowed to stir for 24 h. Subsequently, water (20
mL) and 10% w/w NaOH aq. solution (40 mL) were added carefully
and simultaneously to the reaction mixture. The solution pH was
adjusted to ꢂ12. The organic phase was separated and the aqueous
phase was neutralized by 2 N HCl aq. solution to pH ꢂ8. The aque-
ous phase was extracted several times using CH2Cl2. The combined
organic extracts were dried over anhydrous MgSO4 and concen-
trated in vacuo to obtain 2CPNe as an orange solid (1.40 g, 86%
Acknowledgements
yield); mp 183.2–184.3 °C. = ½a D31
ꢁ
–17.18 (c 1.0, DMF); 1H NMR
The authors are grateful to Dr. Naoki Haraguchi at the
Toyohashi University of Technology for useful discussions. This
work was financially supported by JSPS KAKENHI Grants
JP15H00732 and JP15K05517.
(DMSO-d6, 400 MHz) dH = 0.76–0.79 (m), 1.15–1.90 (m), 1.40–
1.50 (m), 2.32–2.44 (m), 4.97–5.20 (m), 5.83–5.92 (m), 6.23–6.35
(m), 7.30–7.50 (m), 7.87–7.97 (m), 8.60–8.62 (d, J = 4.27 Hz),
10.10 (br, s). 13C NMR (DMSO-d6, 100 MHz) dC = 7.8, 9.0, 24.3,
24.6, 24.8, 26.3, 27.1, 27.2, 27.3, 28.6, 28.8, 33.9, 34.0, 38.9, 42.6,
46.1, 49.5, 55.3, 55.3, 58.8, 105.1, 119.0, 122.1, 131.7, 143.6,
146.9, 156.2, 172.5. IR (KBr) vmax [cm–1]; 3356 (O–H), 3072 (C–
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.jcat.2018.03.020.
H
arom), 1726 (C@O), 1618 (C@Carom), 1241 (C–O). HRMS (ESI) m/z
for C48H58IN4O6 [M+H+] calcd. 788.4465, found 788.4456.
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