Polyesters Containing Chiral Imidazolidinone Salts in Polymer Main Chain: Heterogeneous Organocatalysts for the Asymmetric Diels–Alder Reaction

Article abstract of DOI:10.1002/cctc.201700723

Novel main-chain polyesters functionalized with chiral imidazolidinone salts were successfully synthesized. Polycondensation of a chiral imidazolidinone dimer bearing two hydroxyphenyl groups with selected achiral dicarboxylic acid chlorides followed by t

Full text of DOI:10.1002/cctc.201700723

Accepted Article
Title: Polyesters Containing Chiral Imidazolidinone Salts in Polymer
Main Chain: Heterogeneous Organocatalysts for Asymmetric
Diels-Alder Reaction
Authors: Naoki Haraguchi, Thanh Liem Nguyen, and Shinichi Itsuno
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10.1002/cctc.201700723
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Polyesters Containing Chiral Imidazolidinone Salts in Polymer Main Chain:
Heterogeneous Organocatalysts for Asymmetric Diels–Alder Reaction
Naoki Haraguchi,*[a] Thanh Liem Nguyen,[a] and Shinichi Itsuno[a]
Abstract: Novel main-chain polyesters functionalized with chiral
imidazolidinone salts were successfully synthesized.
organometallic catalysts such as being metal free, exhibiting
higher stability against oxygen or moisture, and permitting ease
of operation. However, chiral imidazolidinone organocatalysis,
as well as most conventional organocatalysis, requires relatively
high amounts of catalyst loading (5 to 20 mol%) to facilitate an
asymmetric reaction at a reasonable reaction rate. In the
isolation of a product and separation of a chiral organocatalyst
from a reaction mixture, burdensome purification methods such
as silica gel column chromatography are required. Therefore,
chiral imidazolidinone is generally not reused.
Polycondensation of a chiral imidazolidinone dimer bearing two
hydroxyphenyl groups with selected achiral dicarboxylic acid
chlorides followed by the addition of an acid afforded polyesters with
chiral imidazolidinone salts incorporated into the polymer main chain.
Main-chain ionic polyesters functionalized with chiral imidazolidinone
salts in the polymer main chain were synthesized by the
neutralization polymerization of a chiral imidazolidinone dimer with
selected aromatic disulfonic acids. These polyesters were used as
heterogeneous organocatalysts in the asymmetric Diels–Alder
reaction of trans-cinnamaldehyde and 1,3-cyclopentadiene. By
optimizing the polymer main-chain structure, enantioselectivity of up
to 97% was achieved, which is higher than that attained using the
corresponding monomeric and dimeric chiral imidazolidinone salts.
These heterogeneous organocatalysts were easily recovered from
the reaction mixture and reused several times without significant loss
of the enantioselectivity.
To overcome these issues, immobilization of
a
chiral
organocatalyst onto a polymer is a potential solution.[3] Several
studies on the immobilization of chiral imidazolidinone salts onto
polymer side chains or chain ends have been reported.[4] We
have also developed some polymeric chiral imidazolidinone salts.
Ionically polymer-supported chiral imidazolidinone salts have
been successfully prepared by the ion-exchange reaction of a
chiral imidazolidinone salt with a sulfonated polymer, in which
the chiral imidazolidinone salts were immobilized at the polymer
pendants.[5] The non-covalent immobilization method has also
been widely used for the immobilization of chiral
organocatalysts.[6]
Keywords: asymmetric catalysis
・ Diels-Alder reaction ・
heterogeneous catalysis・organocatalysis・polymers
Recently, we developed polymers incorporating
a chiral
organocatalyst into the polymer main chain.[7-9] Polyether
incorporating chiral imidazolidinone into the polymer main chain
has been prepared by the Williamson reaction of a bisphenol-
type chiral imidazolidinone monomer with an achiral dihalide,
followed by the addition of an acid.[7] Furthermore, a unique ionic
polyether incorporating chiral imidazolidinone salts into the
polymer main chain has been successfully synthesized by the
neutralization reaction of a chiral imidazolidinone dimer with an
achiral disulfonic acid.[8] Various types of main-chain chiral
polyethers can be readily prepared because these building
blocks involve various simple amino acids, dihalides, and
disulfonic acids. Appropriate tuning of the polymer main-chain
structure resulted in high to excellent enantioselectivity in the
asymmetric Diels–Alder reaction of trans-cinnamaldehyde and
1,3-cyclopentadiene.
Introduction
Organocatalysis has been a rapidly growing area of research
over the last decade with the penetration of the concept of green
chemistry. Chiral organocatalysis is highly effective for the
synthesis of optically active compounds for medical and
pharmaceutical applications.[1] With the development of chiral
organocatalyst design and its application to asymmetric
reactions, high yields and enantioselectivities have been
reported for various asymmetric reactions using chiral
organocatalysts.
Chiral imidazolidinones and their salts developed by D.W.D.
MacMillan and coworkers are one of the most important classes
of chiral organocatalysts; this family of organocatalysts can be
applied to various asymmetric reactions.[2] Chiral imidazolidinone
In this article, we focused on polyester as a repeating unit of the
polymer main chain. Polyester is a common synthetic polymer
that is widely used in fibers, films, and beverage containers.[10] It
is an excellent plastic in terms of heat resistance, impact
resistance, gas barrier properties (resistance to gas permeation),
chemical resistance, and abrasion resistance, making polyester
organocatalysts
have
advantages
over
conventional
[a]
Prof. N. Haraguchi, T. L. Nguyen, Prof. S. Itsuno
Department of Environmental and Life Sciences, Graduate School
of Engineering
a
promising polymeric chiral catalyst for further industrial
application. Polyesters with chirality in the polymer main chain
have mainly been applied as functional materials with liquid
crystallinity or biodegradability.[11] However, little attention has
been paid to their application as support polymers for
heterogeneous asymmetric catalysis. Only a few examples have
been reported for these applications.[12]
Toyohashi University of Technology
Toyohashi, Aichi 441-8580 (Japan)
E-mail: haraguchi@ens.tut.ac.jp
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201700723
ChemCatChem
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Scheme 1. Synthesis of chiral polyesters 1.
Polyesters incorporating chiral imidazolidinone salts into the
polymer main chain 1 were synthesized by the polycondensation
of a chiral imidazolidinone bearing two hydroxyphenyl groups 4
with a few achiral dicarboxylic acid chlorides 5, followed by the
addition of an acid, as illustrated in Scheme 1. First, 4 was
synthesized from (S)-tyrosine 3 according to the process
described in our previous report.[7]
Figure 1. Polyesters containing chiral imidazolidinone salts in polymer main
Polyester is generally prepared using polymerization methods
such as alcoholic transesterification, azeotrope esterification,
ring-opening polymerization, and enzymatic synthesis. Among
these methods, we chose the polycondensation of a dicarboxylic
acid dihalide with a diol because this approach is general and
affords polyester in good yield and with high molecular weight. In
addition, the polymerization procedure is simple: the addition of
a diol to an equimolar amount of a dicarboxylic acid dihalide in
chain.
For the development of
a
polyester-supported chiral
imidazolidinone organocatalyst, we designed two types of novel
main-chain polyesters functionalized with chiral imidazolidinone
salts in the main chain 1 and 2 (Figure 1). These chiral
polyesters were used as heterogeneous polymeric chiral
organocatalysts in the asymmetric Diels–Alder reaction of trans-
cinnamaldehyde and 1,3-cyclopentadiene. The effects of
dicarboxylate, counteranions, and disulfonate of the chiral
polyester on the reactivity and stereoselectivity were
investigated in detail. The substrate generality and reusability of
these chiral polyesters were also tested in the same reaction. To
the best of our knowledge, this is the first report on the
asymmetric organocatalytic application of main-chain chiral
polyester.
the presence of
a
base. First, we performed the
polycondensation of 4 and 5 in the presence of a catalytic
amount of benzyltributylammonium chloride in 1.0 M NaOH and
CH2Cl2 biphasic solution.[13] We confirmed that only the ester
formation occurred and the secondary amines in 4 remained
intact under these polymerization conditions. Precipitates were
observed as soon as the CH2Cl2 solution of 5 was added to the
NaOH aqueous solution of 4. Various types of dicarboxylic acid
dihalides including aromatic, unsaturated aliphatic, and
saturated aliphatic dicarboxylic acid dichlorides were used to
prepare chiral polyester 6. In all the cases, the polycondensation
occurred smoothly to afford 1 in high yield under these
experimental conditions (Table 1). When the polycondensation
of 4 and dicarboxylic acid was used instead of 5 in the presence
of Sc(OTf)3,[14] no polymer was obtained.
Results and Discussion
Synthesis of main-chain chiral polyester functionalized with
chiral imidazolidinone salt (1)
Table 1. Yield of main-chain chiral polyester 6.
Entry
5
Chiral polyester 6
Yield [%]
1
2
3
4
5
6
5a
5b
5c
5d
5e
5f
6a
6b
6c
6d
6e
6f
99
86
98
80
90
84
Interestingly, the solubility of 6 was quite different from that of
main-chain chiral polymers we have previously synthesized.
Regardless of the difference in the spacer structure derived from
5, the resulting polyesters were completely insoluble in common
solvents such as hexanes, CH2Cl2, ethyl acetate, methanol, and
H2O and very slightly soluble only in N,N-dimethylformamide
(DMF), most likely because of the rigid ester repeating unit in the
polymer main chain.
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The slightly soluble part in DMF was
barely able to be analyzed by size-
exclusion chromatography (SEC). Table
1
summarizes these number- and
weight-averaged molecular weights and
molecular weight distributions. These
SEC traces were monomodal, and their
number-averaged molecular weights
were sufficiently high (Mn>7,000) (see
Table S1 in the supporting information).
Figure 2a presents a Fourier-transform
infrared (FT-IR) spectroscopy spectrum
of a chiral polyester functionalized with
chiral
imidazolidinone
6a.
The
absorption peaks at 1739 and 1263
cm−1 are assigned to stretching
vibrations of C=O and C–O bonds for
the polyester, respectively. In addition,
the absorption peaks at 1640 and 1167
cm−1 are assigned to stretching
vibrations of C=O and C–N bonds for
Scheme 2. Synthesis of ionic chiral polyesters 2.
the imidazolidinone, respectively. In contrast, a good liquid 1H
NMR spectrum was not obtained because of the low solubility of
the chiral polyester 6 in common deuterated solvents (CDCl3,
dimethylsulfoxide (DMSO-d6), and D2O). Alternatively, solid state
13C NMR spectrum was measured. The signal ranging from 165
to 175 ppm assigned to ester carbon was obviously observed.
These results indicate that the polycondensation of 4 and 5
proceeded smoothly and that the imidazolidinone moiety was
incorporated into the polyester. The treatment of the main-chain
polyester of chiral imidazolidinone 6 with an acid yielded a main-
Synthesis of ionic main-chain polyester functionalized with
chiral imidazolidinone salt (2)
Ionic polyesters incorporating chiral imidazolidinone salts into
the polymer main chain 2 were prepared by the neutralization
polymerization of a chiral imidazolidinone dimer 8 with selected
aromatic disulfonic acids 9, as illustrated in Scheme 2. Chiral
imidazolidinone with a hydroxyphenyl group 7 was synthesized
from (S)-phenylalanine using a synthetic procedure similar to
that of 4. Dimerization of 7 was performed by esterification with
dicarboxylic acid dichloride 5. Chiral imidazolidinone dimers with
different dicarboxylate spacers 8 were obtained with moderate to
high yield.
chain polyester with chiral imidazolidinone salt moiety
1
(Scheme 1). For example, the reaction of 6a with p-
toluenesulfonic acid yielded 1a-OTs. The imidazolidinone salt
structure was confirmed by FT-IR spectroscopy, as shown in
Figure 2b. The broad absorption peaks in the range from 2800
to 2200 cm-1 are assigned to stretching vibrations of N-H for the
imidazolidinone salt. In addition, the absorption peaks at 2940,
1164, 810, 679, and 563 cm−1 are assigned to the p-
toluenesulfonate.
Table 2. Characterization of ionic main-chain chiral polyesters 2.
[a]
[a]
Entry
8
9
Ionic chiral
polyester 2
Yield
[%]
Mn
Mw
Mw/Mn
[a]
1
2
3
4
5
8a
8b
8c
8d
8e
8f
8e
8e
8e
8e
9A
9A
9A
9A
9A
9A
9B
9C
9D
9E
2aA
2bA
2cA
2dA
2eA
2fA
2eB
2eC
2eD
2eE
73
93
76
74
95
77
90
75
77
84
5,000
4,500
4,400
5,000
4,800
4,900
4,200
4,300
6,200
4,700
11,000
6,200
5,500
7,900
9,700
8,300
5,100
9,900
14,400
6,900
2.2
1.4
1.2
1.6
2.0
1.7
1.2
2.3
2.3
1.5
6
7
8
9
10
[a] Determined by SEC using DMF as an eluent at a flow rate of 1.0 mL
min−1 at 40 oC (polystyrene standards).
Figure 2. FT-IR spectra of chiral polyesters 6a (a) and 1a-OTs (b).
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Figure 3. 1H NMR spectrum of ionic chiral polyester 2eA.
Polymerization was conducted by adding an aqueous solution of
disulfonic acid 9 to a CH2Cl2 solution of chiral imidazolidinone
dimer 8 at room temperature. The successive intermolecular
neutralization reaction between 8 and 9 proceeded smoothly to
afford ionic chiral polyester 2. During the neutralization
polymerization, polymers were gradually precipitated in the
solution because these polymers were soluble neither in CH2Cl2
nor in H2O. Chiral imidazolidinone dimers 8 and disulfonic acids
9 were used to prepare various types of polyesters 2. We
prepared ionic chiral polyesters with selected aromatic
disulfonate spacers because a similar ionic polymeric chiral
imidazolidinone salt with naphthalenedisulfonate exhibited high
catalytic performance in the following Diels–Alder reaction.[8]
Table 2 summarizes the isolated yield, number- and weight-
averaged molecular weights, and molecular weight distribution
of chiral polyester 2. Unlike the chiral polyesters 1, the ionic
chiral polyesters 2 were highly soluble in DMF and DMSO;
therefore, their molecular weights and molecular weight
distribution were easily measured using SEC. The number-
averaged molecular weights determined by SEC ranged
between 4.2 and 6.2 kg/mol.
Asymmetric Diels–Alder reaction using model chiral
imidazolidinone salt
1
Figure 3 presents a H NMR spectrum of ionic chiral polyester
functionalized with chiral imidazolidinone salts 2eA in DMSO-d6.
Both signals assigned to imidazolidinone and disulfonate
moieties were observed with a 1:1 ratio. These results clearly
indicate that the neutralization polymerization of 8 and 9
proceeded smoothly to afford unique ionic main-chain chiral
polyesters incorporating chiral imidazolidinone salt into the
polymer main chain 2.
Figure 4. Monomeric and dimeric chiral imidazolidinone salts.
To evaluate the catalytic activity of these chiral imidazolidinone
salt polyesters and their monomeric counterparts, the
asymmetric Diels–Alder reaction of trans-cinnamaldehyde 12
and 1,3-cyclopentadiene 13 was investigated. Initially, the
catalytic activity of the model chiral imidazolidinone salts in the
Diels–Alder reaction was examined (Table 3). The reaction was
conducted with 10 mol% chiral imidazolidinone salt in
a
CH3OH:H2O (95:5, v/v) mixed solvent at 25 °C for 24 h. The
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reaction catalyzed by the original chiral imidazolidinone
hydrochloride 10-Cl (Figure 4) afforded bicycle[2.2.1]heptene
derivatives 14 in quantitative yield with 93% ee for exo and endo
isomers (entry 1).[2g] The chiral imidazolidinone p-
toluenesulfonate 10-OTs and two hydroxyphenyl-functionalized
chiral imidazolidinone p-toluenesulfonate 4-OTs exhibited similar
enantioselectivity for the endo isomer to 10-Cl and somewhat
lower enantioselectivity for the exo isomer (entries 2–3). The
chiral imidazolidinone salt functionalized with two benzoyloxy
moieties 11-OTf (Figure 4) as a substructure for the repeating
unit of chiral polyester 1 exhibited comparable enantioselectivity
to 10-OTs (entry 4). These monomeric and dimeric chiral
imidazolidinone salts exhibited similar reactivity and
enantioselectivity for endo isomers.
has a significant effect on the catalytic activity.[5a,15] 1a-OTs
exhibited good affinity to CH2Cl2 and CH3CN but not to CH3OH
and H2O. When H2O or CH3OH was used for the reaction,
significant reductions of both the yield and enantiomeric excess
of these adducts were observed (entries 1–2). The catalytic
activity in CH3CN or CH2Cl2 was also comparable to those in
H2O or CH3OH (entries 3–4). The CH3OH–H2O (95:5, v/v) mixed
solvent used in the Diels–Alder reaction by the model chiral
imidazolidinone salt gave the desired adducts quantitatively with
high enantioselectivity (entry 5). The acceleration of the reaction
rate in the conventional Diels–Alder reaction with the addition of
H2O has been reported.[16] Similarly, the presence of H2O in the
Diels–Alder reaction catalyzed by the chiral imidazolidinone salt
enhanced the reactivity.[17] Based on these results, we chose the
CH3OH–H2O mixed solvent as a suitable reaction solvent for the
reaction catalyzed by the chiral imidazolidinone polyester.
Table 3. Asymmetric Diels–Alder reaction using chiral imidazolidinone salt[a]
.
Table 5. Asymmetric Diels–Alder reaction using chiral imidazolidinone salt
polyester 1.[a]
Entry
Chiral
imidazolidinone salt
polyester
14
Yield
[%][b]
ee (exo)
(%)[d]
ee (endo)
exo/endo[c]
(%)[d]
Entry
Chiral
imidazolidinone
salt
14
1
2
3
1a-OTf
1b-OTf
1c-OTf
1d-OTf
1e-OTf
1f-OTf
1a-BF4
1a-OMs
1a-OTs
1a-Cl
99
99
42
81
56
88
99
99
99
94
87
99
54/46
56/44
55/45
55/45
55/45
56/44
53/47
56/44
56/44
56/44
54/46
53/47
81
88
79
78
69
83
89
92
91
87
92
91
90
93
77
84
75
88
89
93
94
91
96
95
Yield
[%][b]
ee (exo)
ee (endo)
exo/endo[c]
(%)[d]
(%)[d]
4
5
6
7
8
9
10
11[e]
12[e]
1[e]
2[f]
3[g]
4
10-Cl
10-OTs
4-OTs
11-OTf
99
94
92
99
57/43
55/45
65/35
56/44
93
88
89
92
93
92
93
93
[a] Reaction conducted with 10 mol% of catalyst in CH3OH:H2O (95:5) at 25 °C
for 24 h. [b] Isolated yield. [c] Determined by 1H NMR. [d] Determined by GC
(CHIRALDEX -PH). [e] See ref 2g. [f] See ref 5a. [g] See ref 7.
1a-OTs
1b-OTs
[a] Reaction conducted with 10 mol% of catalyst in CH3OH:H2O (95:5) at 25 °C
for 24 h. [b] Isolated yield. [c] Determined by 1H NMR. [d] Determined by GC
(CHIRALDEX -PH). [e] Reaction conducted at 0 °C for 72 h.
Asymmetric
Diels–Alder
reaction
using
chiral
imidazolidinone salt polyester 1
Table 5 summarizes the reaction catalyzed by the polyester
functionalized with chiral imidazolidinone salt 1. The effect of the
dicarboxylate spacer on the catalytic activity was investigated in
detail. The chiral imidazolidinone trifluoromethanesulfonate
polyester with the saturated aliphatic (butylene) spacer 1a-OTf
produced Diels–Alder adducts 14 in quantitative yield but
somewhat lower ee for exo and endo isomers than those
obtained using the corresponding model chiral imidazolidinone
salts (entry 1). The chiral imidazolidinone salt polyester with the
longer aliphatic (octylene) spacer 1b-OTf exhibited comparable
enantioselectivity as these model catalysts (entry 2). Other
polymeric chiral imidazolidinone salts with an unsaturated
aliphatic spacer 1c-OTf and aromatic spacer 1d-OTf, 1e-OTf,
1f-OTf exhibited lower reactivity and moderate enantioselectivity
(entries 3–6). These results clearly indicate that the
dicarboxylate spacer structure introduced into the polymer main
chain affected the catalytic activity. The reason why 1b-OTf
Table 4. Solvent screening in asymmetric Diels–Alder reaction using chiral
imidazolidinone salt polyester 1a-OTs.[a]
Entry
Solvent
14
Yield
[%][b]
ee (exo)
(%)[d]
ee (endo)
exo/endo[c]
(%)[d]
1
2
3
4
5
H2O
CH3OH
CH3CN
CH2Cl2
58
49
41
43
99
52/48
56/44
58/42
58/42
56/44
68
75
59
55
91
73
77
56
58
94
CH3OH:H2O = 95:5
[a] Reaction conducted with 10 mol% of 1a-OTs at 25 °C for 24 h. [b] Isolated
yield. [c] Determined by 1H NMR. [d] Determined by GC (CHIRALDEX -PH).
Next, the solvent effect was briefly examined using a polyester
functionalized with chiral imidazolidinone p-toluenesulfonate 1a-
OTs in the same asymmetric Diels–Alder reaction (Table 4). The
affinity of a polymeric catalyst for a reaction solvent generally
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exhibited similar catalytic activity as the corresponding model
chiral imidazolidinone salt has not been clarified. We suppose
that the rigidity of the main-chain structure of the polymer due to
the ester bond is relaxed by introducing a flexible spacer such
as octylene, which leads to a conformation of the chiral
imidazolidinone moiety suitable for the asymmetric Diels–Alder
reaction.
Next, we investigated the effect of the counteranion of 1a in the
reaction (Table 5). Chiral imidazolidinone salts with
methanesulfonate 1a-OMs, p-toluenesulfonate 1a-OTs, and
hydrochloride 1a-Cl exhibited higher enantioselectivities than
that with trifluoromethanesulfonate 1a-OTf (entries 8–10).
Among them, the polyester with p-toluenesulfonate 1a-OTs
exhibited the highest enantioselectivity for the endo isomer
(entry 9). Reducing the reaction temperature to 0 °C increased
the enantioselectivity for both diastereomers of 14 (entries 11–
12). Based on these results, the counteranion and reaction
temperature were observed to affect the enantioselectivity to a
certain extent.
afforded a quantitative yield and high enantioselectivity (97% ee
for endo isomer) (entry 2). Encouraged by these results, we
investigated the catalytic activity of ionic polyester 2 in the same
reaction. Although 2 was completely insoluble in the CH3OH–
H2O (95:5, v/v) mixed solvent, it was well dispersed in the
reaction mixture. Higher catalytic activity of 2 is expected due to
the high surface area. Polymeric ionic chiral imidazolidinone
salts with aromatic dicarboxylate spacers 2aA and 2cA exhibited
low reactivity and enantioselectivity (Table 6, entries 3 and 5).
However, chiral imidazolidinone salt polyesters with a 1,3-
phenylene spacer 2bA and aliphatic linkers 2dA, 2eA, 2fA
exhibited quantitative yield and excellent enantioselectivity in
both exo (up to 94% ee) and endo (up to 97% ee) isomers
(entries 4 and 6–8). The effect of the disulfonate spacer of 2 was
also investigated in the reaction (Table 6).
A
chiral
imidazolidinone salt polyester with naphthalene 1,4-disulfonate
2eB exhibited comparable catalytic activity to that of the original
chiral imidazolidinone salt (entry 9). However, the catalytic
reactivity and enantioselectivity of the chiral imidazolidinone salt
polyester with hindered aromatic disulfonate 2eC, 2eD, and 2eE
were lower than those of 2eA (entries 10–12). The catalytic
activity of 2eA washed with the reaction solvent before the
reaction was comparable to that of 2eA without prewashed with
the solvent (entries 7 and 13), indicating that the catalytic site is
heterogeneous. The reaction using 2eA was completed within
12 h (entry 14). Reducing the reaction temperature to 0 °C did
not enhance the enantioselectivity, whereas the yield dropped to
78% despite the extended reaction time (48 h) (entry 15).
Appropriate tuning of the dicarboxylate spacer and disulfonate
structure of these ionic polyesters led to a suitable conformation
of the polymer main chain. The conformation affected the
microenvironment of the catalytic site, which enhanced the
catalytic performance in the asymmetric reaction.
Asymmetric
Diels–Alder
reaction
using
chiral
imidazolidinone salt polyester 2
Table 6. Asymmetric Diels–Alder reaction using chiral imidazolidinone salt
polyester 2.[a]
Entry
Chiral
imidazolidinone salt
polyester
14
Yield
[%][b]
ee (exo)
(%)[d]
ee (endo)
exo/endo[c]
(%)[d]
1
2
3
7-OTs
8e-OTs
2aA
88
99
37
99
49
99
99
99
96
85
47
59
98
99
78
56/44
60/40
52/44
56/44
54/46
58/42
56/44
55/45
56/44
55/45
53/47
54/46
56/44
56/44
55/45
84
92
75
92
73
91
94
94
91
88
81
84
94
94
94
86
97
72
97
71
97
97
97
94
90
76
80
97
97
97
4
5
6
7
8
9
10
11
12
13
14[f]
15[g]
2bA
2cA
2dA
2eA
2fA
2eB
2eC
2eD
2eE
2eA[e]
2eA
Substrate scope and reusability of chiral imidazolidinone
salt polyester in asymmetric Diels–Alder reaction
An ionic polyester functionalized with chiral imidazolidinone salt
2eA was used for asymmetric Diels–Alder reactions of some
aldehydes and ketones to verify the substrate generality (Table
7). The reaction between 4-fluorocinnamaldehyde 15 and 1,3-
cyclopentadiene 13 afforded these adducts 16 in 93% yield and
with 90% ee (exo) and 90% ee (endo) (entry 1). The reaction
between 2-methoxycinnmahyde 17 and 13 produced 18 in
quantitative yield and with excellent ee (99% ee for exo
isomer)(entry 2). The reaction of acrolein 19 and 13 proceeded
with moderate enantioselectivity (entry 3). The same polymeric
imidazolidinone salt 2eA was used for the asymmetric Diels–
Alder reaction of 19 and 2,3-dimethylbutadiene 21 and exhibited
high enantioselectivity (entry 4). Therefore, the novel ionic
polyester functionalized with chiral imidazolidinone salt 2eA has
the substrate generality in the reaction.
2eA
[a] Reaction conducted with 10 mol% of catalyst in CH3OH:H2O (95:5) at 25 °C
for 24 h. The catalyst amount is based on the repeating unit of 2. [b] Isolated
yield. [c] Determined by 1H NMR. [d] Determined by GC (CHIRALDEX -PH).
[e] Prewashed with CH3OH:H2O (95:5). [f] Reaction conducted for 12 h. [g]
Reaction conducted at 0 °C for 48 h.
The catalytic activity of ionic polyester 2 and the corresponding
monomeric and dimeric chiral imidazolidinone salts were also
examined in the Diels–Alder reaction. The reaction using model
chiral imidazolidinone p-toluenesulfonate with a hydroxyphenyl
group 7-OTs afforded 14 in 88% yield with 84% ee (exo) and
86% ee (endo) (Table 6, entry 1). The dimer of chiral
imidazolidinone p-toluenesulfonate with butylene spacer 8e-OTs
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10.1002/cctc.201700723
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not changed at all in the reuse (see Table S2 in the supporting
information).
Table 7. Scope of substrates in asymmetric Diels–Alder reaction using chiral
imidazolidinone salt polyester 2eA.[a]
Entry Aldehyde
(R=)
Diene
Product
Yield
(%)[b]
exo
ee
ee
/endo[c] (exo) (endo)
(%)[d] (%)[d]
4-F-Ph
15
1
13
93
57/43
90
90
16
2-
2
3
MeOPh
17
13
13
99
99
57/43
15/85
99
68
94
81
18
20
H
19
2,3-
dimethyl-
butadiene
21
Figure 5. Reusability of chiral imidazolidinone salt polyester 2eA in
asymmetric Diels–Alder reaction of 12 and 13.
H
19
94
4
99
-
-
[e]
22
[a] Reaction conducted with 10 mol% of 2eA in CH3OH:H2O (95:5) at 25 °C for
24 h. The catalyst amount is based on the repeating unit of 2eA. [b] Isolated
yield. [c] Determined by 1H NMR. [d] Determined by HPLC after reduction of
aldehydes (CHIRALCEL OJ-H). [e] ee (%) determined by GC (CHIRALDEX -
PH).
Conclusions
We have successfully synthesized covalent and ionic main-chain
chiral polyesters functionalized with chiral imidazolidinone salts.
These chiral polyesters were used as polymeric chiral
organocatalysts in the asymmetric Diels–Alder reaction of trans-
cinnamaldehyde
and
1,3-cyclopentadiene.
The
chiral
Chiral polyester 1a-OTs was insoluble in general organic
solvents such as hexanes, acetone, CH2Cl2, ethyl acetate, THF,
methanol, DMF, DMSO, and H2O. Regardless of the
heterogeneous system, the Diels–Alder reaction proceeded
smoothly, as demonstrated in Table 4 and Table 5. After
completion of the Diels–Alder reaction, the polyester was easily
separated from the reaction mixture by simple filtration or
decantation. The recovered chiral imidazolidinone salt polyester
was dried with a vacuum pump at room temperature and used
for the next reaction (procedure A, see experimental section).
Both the reactivity and enantioselectivity were decreased with
the reuse (5th cycle: 93% yield for 72 h, 59% ee for endo isomer).
The recovered polymeric catalyst was treated with p-
toluenesulfonic acid monohydrate before the next reaction
(procedure B, see experimental section), which was effective for
the reuse of chiral imidazolidinone salt polyether probably due to
the recovery of the counteranion leaching from the polymeric
chiral imidazolidinone salt,[7] slightly inhibited the reduction of the
enantioselectivity (5th cycle: 69% ee for endo isomer). The
reusability of the ionic polyester functionalized with chiral
imidazolidinone salt 2eA was also examined using procedure A
(Figure 5). The polymeric catalyst could be reused several times
with high enantioselectivity comparable to that of the
corresponding model catalyst. To evaluate the catalytic activity
in the reuse experiment, the yield at 3 h after starting the each
reaction was investigated. The yield tended to decrease slightly
with the repeated reaction, but the high reactivity was
maintained. The exo/endo selectivity and enantioselectivity were
imidazolidinone salt polyester with 1,4-phenylene spacer
exhibited high reactivity and enantioselectivity (94% ee for endo
isomer), comparable to those of the corresponding model
catalyst. In addition, the chiral imidazolidinone salt ionic
polyester with 2,6-naphthalene disulfonate afforded these Diels–
Alder adducts with quantitative yield and excellent
enantioselectivity of up to 97% ee for the endo isomer, higher
than that obtained by the corresponding monomeric chiral
imidazolidinone salt. We observed that the dicarboxylate spacer,
counteranion, and disulfonate structure affected the catalytic
activity in the reaction. The ionic chiral polyester was easily
recovered and could be reused several times without significant
loss of the enantioselectivity. A study on the influence of the
molecular weight of main-chain ionic polymeric chiral catalyst on
the catalytic performance is underway.
Experimental Section
General procedure for synthesis of covalent main-chain
chiral polyesters 1
A 50-mL round-bottomed flask equipped with a magnetic
stirring bar was charged with 4[7] (0.340 g, 1.00 mmol),
benzyltributylammonium chloride (3.1 mg, 0.01 mmol), and 1
M NaOH aqueous solution (5.0 mL, 5.00 mmol). Then, 5
(1.00 mmol) in 12 mL of CH2Cl2 was added dropwise to the
mixture at room temperature and the mixture was vigorously
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10.1002/cctc.201700723
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stirred for 3 h. After the solvent was removed by a vacuum
pump, the residue was washed with hot water and ethyl
acetate. The solid was dried under vacuum to produce 6 as a
vacuum pump, and the residual liquid was purified using silica
gel column chromatography (with 1:19 ethyl acetate/hexanes as
an eluent). The Diels–Alder adducts 14 were obtained as
colorless liquids. The yield and exo/endo ratio were
determined using 1H NMR through comparison of the proton
signals of the aldehyde. The enantiomeric excess of exo and
endo isomers was determined by GC analysis (Astec
CHIRALDEX -PH; injection temperature of 180 °C, detection
temperature of 180 °C, the column temperature was increased
from 120 °C to 150 °C with 5 °C/min and then to 180 °C with
1 °C/min; retention time: 26.2 min (exo(2R)), 26.8 min (exo(2S)),
27.3 min (endo(2R)), and 27.7 min (endo(2S)).
pale yellow powder.
6a: 99% yield; IR (KBr):  = 1739 (C=O), 1640 (C=O), 1263
(C–O), 1167 (C–N) cm−1; solid state 13C NMR: (100 MHz):  =
36.7, 42.8, 50.5, 70.2, 85.8, 89.0, 109.0, 123.7, 130.2, 138.9,
141.5, 145.9, 158.3, 173.2, 187.3; Mn(SEC) = 7.2 kg/mol ,
Mw(SEC) = 18.8 kg/mol, Mw/Mn = 2.61.
An acid (0.60 mmol) was added to a suspension of 6 (0.20
mmol) in methanol (2.0 mL). Then, the reaction mixture was
stirred at room temperature for 2 h. After the suspension was
filtered using a glass filter, the residue was washed with
methanol (2 x 5 mL). The solid was dried under vacuum to
produce 1 as a white solid, which was used for the following
asymmetric reaction without further purification.
General procedure for reuse of main-chain chiral polyester
in the asymmetric Diels–Alder reaction
Procedure A:
The main-chain chiral polyester was isolated from the reaction
mixture by simple filtration or decantation. The recovered main-
chain chiral polyester was dried with a vacuum pump at room
General procedure for synthesis of ionic main-chain chiral
polyesters 2
A 50-mL round-bottomed flask equipped with a magnetic
stirring bar was charged with 9 (0.500 mmol) and 3 mL of
H2O. Then, 8 (0.500 mmol) in 9 mL of CH2Cl2 was added to
the mixture at room temperature, followed by vigorous stirring
for 6 h. After the solvent was removed using a vacuum pump,
the residue was washed with CH2Cl2 and H2O. The solid was
dried under vacuum to produce 2 as an off-white powder.
2eA: An off-white powder. 95% yield; 1H NMR (400 MHz,
DMSO-d6, = 2.50 (DMSO)):  = 1.47 (s, 6H, CH3), 1.58 (s,
6H, CH3), 1.73 (br, 4H, CH2CH2), 2,64 (br, 4H, CH2CO), 2.84
(br, 4H, CH2CH2), 2.94–3.00 (m, 2H, CH2Ph), 3.46 (br, 2H,
CH2N), 4.67 (br, 2H, CH), 7.07 (d, J = 7.9 Hz, 4H, Ar), 7.30–
7.38 (m, 14H, Ar, Ph), 7.71 (d, J = 8.2 Hz, 2H, Ar), 7.91 (d, J
= 8.5 Hz, 2H, Ar), 8.12 (s, 2H, Ar); 13C NMR: (100 MHz,
DMSO-d6,  = 39.5 (DMSO)): = 23.7, 24.7, 26.2, 33.2, 33.8,
35.9, 41.3, 57.9, 76.5, 121.7, 123.8, 124.3, 126.6, 128.1,
128.4, 129.3, 129.9, 132.0, 137.5, 145.9, 149.0, 171.7;
Mn(SEC) = 4.8 kg/mol, Mw(SEC) = 9.7 kg/mol, Mw/Mn = 2.00;
temperature.
A 5-mL round-bottom flask equipped with a
magnetic stirring bar was charged with 12 (0.264 g, 2.00 mmol),
recovered main-chain chiral polyester, and 1.0 mL of
CH3OH/H2O (95/5, v/v) mixed solvent. After three freeze–thaw
cycles under liquid nitrogen, the mixture was stirred at room
temperature for 10 min. Then, 13 (0.397 g, 6.00 mmol) was
added to the mixture, which was stirred at 25 °C. The reaction
was monitored by TLC or 1H NMR. The procedures of the
following work and hydrolysis were the same as those of the
first reaction.
Procedure B:
The main-chain chiral polyester was isolated from the reaction
mixture by simple filtration or decantation. A methanol solution of
p-toluenesulfonic acid monohydrate (3 equivalents relative to
imidazolidinone) was added to the recovered main-chain chiral
polyester at room temperature, and the mixture was stirred for 2
h. The polymer was washed with methanol and dried with a
vacuum pump at room temperature. The following procedure
was as same as that used for procedure A.
20
[]D = −47.5 (c = 0.990 g/dL in DMF).
General procedure for asymmetric Diels–Alder reaction of
trans-cinnamaldehyde and 1,3-cyclopentadiene catalyzed
by main-chain chiral polyester
Acknowledgements
A 5-mL round-bottom flask equipped with a magnetic stirring bar
was charged with trans-cinnamaldehyde 12 (0.264 g, 2.00
mmol), main-chain chiral polyester 1 or 2 (0.20 mmol of catalyst),
and 1.0 mL of CH3OH/H2O (95/5, v/v) mixed solvent. After three
freeze–thaw cycles under liquid nitrogen, the mixture was stirred
at room temperature for 10 min. Then, 1,3-cyclopentadiene 13
(0.397 g, 6.00 mmol) was added to the mixture, which was
stirred at 25 °C for 24 h. After removing the solvents using a
vacuum pump, the residue was extracted with CH2Cl2. The
solvent of the organic layer was removed with a vacuum pump,
and the residue was added to CH2Cl2:H2O:trifluoroacetic acid
(2:2:1 mL). After stirring the mixture for 2 h, the mixture was
neutralized by saturated NaHCO3 aqueous solution. The mixture
was extracted with 5 mL of Et2O three times, and the organic
layer was dried with anhydrous MgSO4. After removing MgSO4
by filtration, the solvent of the filtrate was removed with a
This work was partially supported by JSPS KAKENHI Grant Number
2475103 and 26105729 (Grant-in-Aid for Scientific Research on
Innovative
Areas
“Advanced
Molecular
Transformations
by
Organocatalysts”).
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Entry for the Table of Contents
Layout 2:
FULL PAPER
Naoki Haraguchi*, Thanh Liem Nguyen,
Shinichi Itsuno
Page No. – Page No.
Polyesters Containing Chiral
Imidazolidinone Salts in Polymer
Main chain: Heterogeneous
Organocatalysts for Asymmetric
Diels-Alder Reaction
Novel polyesters containing chiral imidazolidinone salts in the main chain were
successfully synthesized by polycondensation or neutralization polymerization and
applied them as heterogeneous polymeric chiral organocatalysts to the asymmetric
Diels-Alder reaction of trans-cinnamaldehyde and 1,3-cyclopentadiene. These
polymeric catalysts exhibited high enantioselectivity of up to 97% ee for the endo
isomer.
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