Polyelectrolyte-catalyzed Diels–Alder reactions

Article abstract of DOI:10.1016/j.reactfunctpolym.2016.07.021

Imidazolium-based poly(ionic liquids) bearing bromide and bis(trifluoromethane)sulfonimide as counteranions [poly(ViEIm)Br and poly(ViEIm)NTf₂] derived from reversible addition fragmentation chain transfer (RAFT) polymerization techniques are u

Full text of DOI:10.1016/j.reactfunctpolym.2016.07.021

Reactive and Functional Polymers 106 (2016) 132–136
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Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Polyelectrolyte-catalyzed Diels–Alder reactions
Nellepalli Pothanagandhi, Akella Sivaramakrishna, Kari Vijayakrishna ⁎
Department of Chemistry, School of Advanced Sciences, VIT University, Vellore 632014, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 May 2016
Received in revised form 28 July 2016
Accepted 29 July 2016
Available online 30 July 2016
Imidazolium-based poly(ionic liquids) bearing bromide and bis(trifluoromethane)sulfonimide as counteranions
2
[poly(ViEIm)Br and poly(ViEIm)NTf ] derived from reversible addition fragmentation chain transfer (RAFT) po-
lymerization techniques are used in polymer-assisted solution phase Diels–Alder reactions between isoprene
and a variety of dienophiles. With 5 mol% loading, these polyelectrolytes exhibit very good catalytic activity to-
ward Diels–Alder reaction with quantitative yields within 30 min at room temperature. These polyelectrolyte-
catalyzed reactions resulted in good para/meta-like product ratios. Here, the coordination nature of the positively
charged imidazolium groups in polyelectrolyte with dienophiles plays a key role in catalyst activity and selectiv-
ity. These polyelectrolytes were reused for 4 cycles without a significant loss in their catalytic activity and
selectivity.
Keywords:
Polyelectrolytes
Poly(ionic liquids)
Anion exchange
Organocatalysis
Diels–Alder reaction
Selectivity
© 2016 Elsevier B.V. All rights reserved
1
. Introduction
Poly(ionic liquids) (PILs), also known as polyelectrolytes, have
used in Diels-Alder (DA) reaction [25,26]. The first study of IL mediated
DA reaction of cyclopentadiene with methyl acrylate and methyl vinyl
ketone in [EtNH ][NO ] was reported by Jaeger [27]. There are several
3 3
attracted much attention because of their unique properties [1–3].
Their potential applications involve use as catalysts, electrolytes, coating
materials, stabilizers for metal nanoparticles, CO absorbing materials,
2
reports on DA reaction catalyzed by chloroaluminate- [28],
imidazolium- [29], pyridinium- [30], phosphonium- [31], and metal-
based [32] ILs. The combination of Lewis acids with imidazolium salts
catalyzing DA reactions is also reported [33]. In recent years, there
have been an increasing number of studies conducted on ionic liquid
(IL)-supported polymers for DA reactions. In 2007, Bae et al. used
pyridinium-based polymers supported by chloroaluminates for DA
reactions of cyclopentadiene with methyl methacrylate that yielded
DA products in good yields with low endo/exo (60.6/39.4) selectivities
[23]. In 2010, Haraguchi et al. reported the polymer-supported IL-
catalyzed DA reactions between cyclopentadiene and trans-
cinnamaldehyde with good conversions and less endo/exo (55/45) se-
lectivities [24]. Six-membered cyclohexyl derivatives as synthons in or-
ganic synthesis can be prepared through DA reactions between isoprene
and different dienophiles. Very few reports are available on polymer-
supported/catalyzed DA reactions of isoprene cyclized with different
dienophiles [34–36]. Organocatalyzed reactions are attracting much
attention over conventional metal-catalyzed reactions, as the final
products are free from metals [37–39]. However, these organocatalysis
reactions require high catalyst loading and are associated with difficul-
ties in product separation and catalyst reusability when compared
with conventional metal-mediated catalysis. Immobilization of
organocatalysis on inorganic or polymer materials is of significant inter-
est, as it can address most of the raised issues [40–43]. However, multi-
step synthesis and decay in the catalyst activity during recycling are the
two major concerns of these immobilized catalysts. In the present work,
for the first time, we have used simple Poly(ionic liquids) (PILs) as
use in vermicidal activity, water purification, DNA binding, etc. [1–13].
Polymerization of any type of monomers in a controlled manner is a
great challenge [14,15]. Reversible addition fragmentation chain trans-
fer (RAFT) polymerization technique has facilitated the polymerization
of a number of functionalized ionic liquid monomers in a much con-
trolled manner [16,17]. RAFT polymerization can be carried out in aque-
ous medium, and this technique has less impact on the environment.
Moreover, this technique facilitates the preparation of polymers with
different architecture because this technique belongs to the group of re-
versible deactivation radical polymerization techniques [14–17].
The Diels–Alder (DA) reaction is a [4 + 2] cycloaddition-type pericy-
clic reaction, and is one of the important synthetic tools for C–C bond
forming reactions in the construction of new adduct molecules having
a significant impact on the organic synthesis [18–22]. The DA reactions
proceed extremely slowly unless a catalyst is present; to accelerate the
3 3 3 3
reactions, different Lewis acid catalysts such as AlCl , Sc(CF SO ) and
BF have been frequently used [23,24]. The use of Lewis acid catalysts
3
has some drawbacks such as the requirement of laborious work-up pro-
cedures, formation of environmentally hazardous wastes, and difficulty
in catalyst recovery and reuse [23]. To overcome these problems, in re-
cent years, ionic liquids and solid supported ionic liquids are widely
⁎
Corresponding author.
E-mail addresses: kari@vit.ac.in, vijayakrishnakari@gmail.com (K. Vijayakrishna).
http://dx.doi.org/10.1016/j.reactfunctpolym.2016.07.021
381-5148/© 2016 Elsevier B.V. All rights reserved
1

N. Pothanagandhi et al. / Reactive and Functional Polymers 106 (2016) 132–136
133
reusable catalyst for DA reactions between isoprene and different
2.3. Synthesis of chain transfer agent
dienophiles at room temperature with low catalyst loading (5 mol%)
that resulted in quantitative yields with good para/meta selectivity.
Potassium ethyl xanthogenate (5 g, 31.7 mmol) was dissolved in
ethanol. To this mixture, ethyl-2-bromopropionate (5 g, 27.6 mmol)
was added and stirred at room temperature overnight to produce 2-
ethoxythiocarbonylsulfanyl-propionic acid ethyl ester (Scheme 1) in
good yield [36].
2
. Experimental section
2
.1. Materials
2.4. Synthesis of poly-1-vinyl-3-ethylimidazolium bromide [poly(ViEIm)Br]
1
-Vinylimidazole, n-bromoethane, acrylic acid, acrylonitrile, iso-
prene, methyl acrylate, ethyl acrylate, potassium ethyl xanthogenate,
bis(trifluoromethane)sulfonimide lithium salt, and ethyl-2-
bromopropionate were purchased from Sigma-Aldrich Chemicals,
India, and used as received. 2,2′-Azo-bis isobutyronitrile (AIBN) was
purchased from AVRA Chemicals, India, and recrystallized from metha-
nol before further use. All solvents used in the reactions were purchased
from Rankem Chemicals, India, and distilled before use.
1
-Vinylimidazole (5 g, 53.1 mmol) was taken in a 100-mL round-bot-
tomed flask. To this, n-bromoethane (6.947 g, 63.7 mmol) was added and
heated at 60 °C for 3 h to produce 1-vinyl 3-ethylimidazolium bromide
ViEIm]Br in quantitative yields.
[
[ViEIm]Br was polymerized through the RAFT technique using AIBN
as an initiator (Scheme 2). A general procedure for RAFT polymerization
of [ViEIm]Br is as follows. A flame-dried Schlenk flask was charged with
2
g (9.90 mmol) of [ViEIm]Br and 5 mL of dry dimethylformamide
(DMF). To this mixture, 44 mg of chain transfer agent (CTA,
.19 mmol) and 6.5 mg of AIBN (0.039 mmol) were added and stirred
2
.2. Measurements
0
1H and 13C nuclear magnetic resonance (NMR) spectra were recorded
to obtain a homogeneous mixture. Three freeze–thaw cycles were con-
ducted in this reaction mixture to eliminate any dissolved gas. Subse-
quently, the reaction mixture under nitrogen protection was heated to
70 °C for 24 h. The aliquots were precipitated in cold chloroform
(3 × 100 mL) and dried under vacuum to produce poly(ViEIm)Br in
82% yield [6].
on a Bruker AC-400 spectrometer in appropriate deuterated solvents. Gas
chromatography–mass spectrometry (GC–MS) images were recorded on
PerkinElmer Clarus 680 (GC) and Clarus 600 (EI Mass). Molecular weights
of poly-1-vinyl-3-ethylimidazolium bis(trifluoromethane)sulfonimide
[
poly(ViEIm)NTf
tography instrument with Styragel columns equipped with refractive index
RI) detector using tetrahydrofuran (THF) as solvent at an elution rate of
mL/min. Differential scanning calorimetry (DSC) of poly(ViEIm)Br was
performed using a DSC Q2000 V24.10 Build 122 instrument, between
2
] were determined by a Waters gel permeation chroma-
(
1
2.5. Synthesis of poly-1-vinyl-3-ethylimidazolium bis(trifluoromethane)
sulfonimide [poly(ViEIm)NTf
2
]
−
1
−
70 and 150 °C at a heating rate of 10 °C min . DSC analysis of
poly(ViEIm)NTf was performed using a DSC Q2000 V24.11 Build 124
instrument, between −80 and 150 °C at a heating rate of 10 °C min
Poly-1-vinyl-3-ethyl-imidazolium
bis(trifluoromethane)
2
sulfonimide was synthesized by simple anionic metathesis reaction
(Scheme 3) between poly-1-vinyl-3-alkyl-imidazolium bromide and
lithium bis(trifluoromethane)sulfonimide according to our previous re-
ports [6]. Briefly, 1 g (4.95 mmol) of poly(ViEIm)Br was dissolved in
10 mL of distilled water; to this mixture, 1.84 g (6.43 mmol) of lithium
bis(trifluoromethane)sulfonimide dissolved in distilled water (10 mL)
was added dropwise and stirred at room temperature until the reaction
completes. Then, the product was washed with water several times and
dried in vacuum.
−
1
.
Thermogravimetric analysis (TGA) of poly(ViEIm)Br was conducted
using a TA Thermogravimetric analyzer model TGA Q500 V20.13 Build 39,
−
1
from 10 to 800 °C at a heating rate of 10 °C min . TGA analysis of
poly(ViEIm)NTf was conducted using a TA Thermogravimetric analyzer
model TGA Q50 V6.7 Build 203, from 10 to 600 °C at a heating rate of
2
−
1
1
0 °C min
Single-crystal X-ray diffraction measurements for 4-methyl-
cyclohex-3-enecarboxylic acid were acquired at room temperature
293 K) on a Bruker axs SMART APEXII single-crystal X-ray diffractom-
.
(
2.6. General procedure for DA reaction
eter equipped with graphite monochromatic Mo Kα (λ = 0.71073 Å)
radiation and CCD detector. The structure was solved by direct methods
and refined by full-matrix least-squares method with the program
SHELXL-97 refining on F2 [44,45]. Packing diagrams were produced
using the program PovRay and graphic interface X-seed [46]. The crys-
The PILs-catalyzed DA reaction of isoprene with different
dienophiles was carried out as follows. In a typical reaction,
poly(ViEIm)Br (0.012 g, 0.059 mmol) was dissolved in an appropriate
solvent. To this mixture, isoprene (0.24 g, 3.60 mmol) and acrylic acid
(0.086 g, 1.20 mmol) were added and stirred at room temperature
(30 °C) for 30 min. After completion of the reaction, all the volatiles
were removed under reduced pressure. As the catalyst was insoluble
in ethyl acetate, the products were extracted into it, and crude products
were analyzed by NMR and GC–MS.
tals of C
8 10 2
H O are triclinic, space group P1; (a) 6.870(5) Å, (b)
7
.695(5) Å, (c) 8.948(5) Å. α 66.949(5)°, β 68.280(5)°, γ 82.533(5)°.
2
V = 404.3(5), Goodness-of-fit on F = 1.793. A total of 5877 reflections
were collected, and 1681 reflections were used for further refinement.
The structure of 4-methyl-cyclohex-3-enecarboxylic acid was solved
2
by direct methods and refined by a full-matrix technique against F in
the anisotropic approximation. Crystallographic data for 4-methyl-
cyclohex-3-enecarboxylic acid have been deposited with the Cambridge
Crystallographic Data Centre and assigned the number CCDC 1018086.
The data can be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre at www.ccdc.cam.ac.uk/structures.
3. Results and discussion
In this study, imidazolium-based PILs, namely poly-1-vinyl-3-ethyl-
imidazolium bromide poly(ViEIm)Br, was synthesized through RAFT po-
lymerization of 1-vinyl-3-ethylimidazolium bromide using xanthate-
Scheme 1. Synthesis of CTA.

134
N. Pothanagandhi et al. / Reactive and Functional Polymers 106 (2016) 132–136
Scheme 2. Synthesis of poly(ViEIm)Br through RAFT polymerization.
reactions are very important. In these reactions, obtaining selective
products is of prime importance. For the first time in the literature, the
synthesized PILs were tested for their catalytic efficiency in the DA reac-
tion of isoprene with different dienophiles in this study (Scheme 4). The
blank reaction (without any catalyst) between isoprene and acrylic acid
yielded only 15% cyclic adduct within 2 h. When the same reaction was
carried out under identical conditions in the presence of a small amount
of poly(ViEIm)Br, it produced only 4-methyl-cyclohex-3-enecarboxylic
acid (para-like product) in quantitative conversion. The structure of the
obtained product as 4-methyl-cyclohex-3-enecarboxylic acid was con-
firmed by its single-crystal XRD (Fig. 2).
2 2
Scheme 3. Anion exchange of poly(ViEIm)Br to poly(ViEIm)NTf using LiNTf .
based CTA 2-ethoxythiocarbonylsulfanyl-propionic acid ethyl ester with
AIBN as a radical initiator (Scheme 2). The obtained poly(ViEIm)Br was
reacted with excess of LiNTf to produce poly(ViEIm)NTf by anionic me-
2 2
A series of reactions were carried out to optimize the reactions con-
ditions. It was found out that 5 mol% poly(ViEIm)Br as catalyst loading
and 30 min of reaction time are the optimum conditions for these DA re-
actions to achieve quantitative conversion. Under identical reaction
conditions, no significant change was noted with respect to product
yield and p/m ratio when catalyst loading was increased from 5 to
10 mol%.
tathesis (Scheme 3). These two polyelectrolytes were well characterized
by NMR, DSC, TGA and gel permeation chromatography (GPC) (for spec-
−
tral data, see supporting information). After anion exchange from Br to
−
NTf
2
, a distinct downfield shift for imidazolium protons was observed in
1
their H NMR spectra (see Figs. S4 and S5). From GPC analysis, the molec-
ular weight of the synthesized poly(ViEIm)NTf is found to be M
990, with narrow molecular weight distribution M /M = 1.27
Fig. 1). When compared with its bromide counterpart, poly(ViEIm)
NTf showed distinct T value at around 47 °C (see Figs. S6 and S7). Ther-
We compared the catalytic activities of these polyelectrolytes with
=
2
their monomeric analogies [EMIM]Br and [EMIM]NTf and a standard
2
n
7
(
w
n
Lewis acid AlCl .with one representative DA reaction between isoprene
3
and acrylonitrile.
2
g
mal stability of these polymers was studied using TGA analysis. From TGA
analysis of poly(ViEIm)Br, major weight loss was observed between
2
250 °C and 323 °C. However, its corresponding poly(ViEIm)NTf showed
relatively higher thermal stability, and its major decomposition was ob-
served between 325 °C and 445 °C (Figs. S8 & S9).
Recently, PILs have been extensively used in many catalytic reac-
tions. In the organic synthesis, “C–C” formation reactions such as DA
3
2
1
00
00
00
0
-
100
0
5
10
15
20
Fig. 2. ORTEP diagram of 4-methyl-cyclohex-3-enecarboxylic acid. The single-crystal XRD
data of 4-methyl-cyclohex-3-enecarboxylic acid showed a distinct double bond between
C2 and C7 with a bond length of 1.350(7) Å, and other single bonds between carbons
present in the molecule were found in the range of 1.420(7)–1.487(6) Å. The bond
lengths of C=O and C–O were 1.217(5) Å and 1.245(6) Å, respectively.
Retention time (min)
2
Fig. 1. GPC spectrum of poly(ViEIm)NTf .
Scheme 4. PIL catalyzed Diels-Alder reaction.

N. Pothanagandhi et al. / Reactive and Functional Polymers 106 (2016) 132–136
135
Table 1
Diels-Alder reactions using poly ionic liquids.
a
Run
Catalyst
Solvent
Dienophile
Time (min)
Selectivity (para:meta)b
% conversionc
1
2
3
4
5
6
7
8
9
Without catalyst
poly(ViEIm)Br
Methanol
Methanol
Toluene
Acrylic acid
Acrylic acid
120
30
30
30
30
30
30
30
30
30
30
30
30
120
–
100
15
98
72
86
95
96
85
AlCl
3
Acrylonitrile
Acrylonitrile
Acrylonitrile
Methyl acrylate
Ethyl acrylate
1-vinylimidazole
Acrylonitrile
Acrylic acid
Acrylonitrile
Methyl acrylate
Ethyl acrylate
1-vinylimidazole
52.5:47.5
59.5:40.5
76.5:23.5
71.5:28.5
55:45
[EMIM] Br
poly(ViEIm)Br
″
″
″
Methanol
Methanol
Methanol
Methanol
Methanol
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
–
No reaction
76
97
95
97
86
[EMIM] NTf
2
58.8:41.2
100
77.5:22.5
71.6:28.4
57.6:42.4
–
10
11
13
14
15
poly(ViEIm)NTf
2
″
″
″
″
No reaction
a
b
c
All reactions were carried out with 5 mol% of catalyst. In all the above reactions isoprene is used as diene.
Para/meta ratios were calculated from GC–MS.
%
conversions were calculated from NMR.
As shown in Table 1, the polyelectrolytes showed better catalytic ac-
tivity and selectivity over their monomeric analogies and AlCl . These
results encourage us to explore the catalytic activity of poly(ViEIm)Br
and poly(ViEIm)NTf toward DA reaction between isoprene with five
(Table 1, runs 6 and 11), which support the above mechanical hypothesis.
We believe that this type of coordination of the imidazolium group with
dienophile triggers the selectivity toward para-like products as explained
by Xiao and Malhotra [30].
3
2
different dienophiles, namely acrylic acid, acrylonitrile, vinylimidazole,
ethyl acrylate, and methyl acrylate, as shown in Scheme 4, and the re-
sults are provided in Table 1.
The catalysts were easily recovered from the reaction mixtures and
reused for up to four cycles. When the reactions were carried out with
poly(ViEIm)Br and poly(ViEIm)NTf , the reaction mixtures were
2
As shown in Table 1, all these DA reactions produced two different
products – para- and meta-like products – with different ratios. The
para/meta ratios were calculated from crude GC–MS spectra. In all GC–
MS spectra, two peaks with different run times were observed with
the same m/z values, which correspond to para and meta cyclic adducts.
Among all these reactions reported here, better para/meta selectivity
was observed in the case of acrylic acid cyclized with isoprene, whereas
the cyclization of ethyl acrylate with isoprene encountered with low
selectivity.
washed with ethyl acetate and chloroform, respectively, to precipitate
the corresponding polymers. The extracted catalysts were dried under
vacuum and reused in the reactions for four cycles. As shown in Fig.
S23, no much change in catalytic activity and selectivity was noted for
the first four recycle runs.
After comparing the catalytic activity and selectivity (toward para/
2
meta ratio) of poly(ViEIm)Br and poly(ViEIm)NTf with their mono-
meric analogies ([EMIM]Br and [EMIM]NTf ), Lewis acid AlCl3, and
2
existing literature reports (Tables 1 and S1), it can be concluded that
these polyelectrolytes are promising candidates as a reusable catalysts
for DA reactions between isoprene and different dienophiles with
good catalytic activity and selectivity.
The hydrophilic poly(ViEIm)Br was replaced with hydrophobic
polyelectrolyte poly(ViEIm)NTf to carry out the same set of DA reac-
2
tions. To make the reaction homogenous, acetone was used as solvent.
Irrespective of the counteranions, both poly(ViEIm)Br and
poly(ViEIm)NTf
2
showed almost identical catalytic activity and selec-
4. Conclusion
tivity toward DA reaction. Although these counteranions do not have a
significant effect on these reactions, they enable us to carry out the reac-
tions in either the protic or aprotic solvent, as desired.
The catalytic activity and selectivity of these polyelectrolytes could be
attributed to the coordination of positively charged imidazolium groups
of polyelectrolytes with dienophiles, which eventually increased its activity
toward diene, as shown in Fig. 3. Because such coordination does not exist
between polyelectrolytes and 1-vinylimidazole, these polyelectrolytes
could not catalyze the DA reaction between 1-vinylimidazole and isoprene
RAFT derived polyelectrolytes poly(ViEIm)Br and poly(ViEIm)NTf
were successfully used as highly active, reusable homogenous catalyst
for DA reaction of isoprene with a variety of dienophiles. Both
2
2
poly(ViEIm)Br and poly(ViEIm)NTf exhibited remarkable catalytic
activity with quantitative conversions including good para/meta selec-
tivities. 4-Methyl-cyclohex-3-enecarboxylic acid, one of the DA adduct
derived from the DA reaction of isoprene cyclized with acrylic acid,
was structurally characterized by single-crystal XRD data. These ion-
Fig. 3. A possible mechanism of polyelectrolyte-catalyzed Diels–Alder reaction.

136
N. Pothanagandhi et al. / Reactive and Functional Polymers 106 (2016) 132–136
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role in catalyst activity and selectivity. These polyelectrolytes can be
reused several times without a significant loss of catalytic activity.
6
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Kari thanks DST-SERB, India (Project NO: SR/S1/OC-22/2012), for the
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also thank VIT for NMR, GC–MS, and other instrumentation facilities.
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