Synthesis of a 3D-network polymer supported Bronsted acid ionic liquid based on calix[4]resorcinarene via two post-functionalization steps: A highly efficient and recyclable acid catalyst for the preparation of symmetrical bisamides

Article abstract of DOI:10.1039/c4ra03463e

In this work, for the first time, a 3D-network polymer-supported Bronsted acid ionic liquid was synthesized via two post-functionalization steps. Initially, the active homogeneous catalyst was chemically immobilized onto a polymeric support based on calix[4]resorcinarene by silylation of the hydroxyl groups to form a cationic polymer that contains imidazolium moieties. The formation of this cationic polymer was confirmed by elemental analysis, scanning electron microscopy (SEM), thermal gravimetric analysis (TGA) and derivative thermogravimetric (DTG) analysis. Subsequently, HSO₄ anion was incorporated into the polymer along the imidazolium pendant groups via a well-known ion exchange reaction. Elemental analysis data revealed that the cationic polymer was conveniently loaded with the desired Bronsted acid anion; therefore, it provides a novel heterogeneous acid catalyst for achieving synthetic goals. The immobilized acidic ionic liquid effectively catalyzed the one-pot synthesis of symmetrical bisamides by the multicomponent condensation of two moles of amides with aldehydes. Interestingly, the catalyst exhibited a high turnover number (TON) and turnover frequency (TOF), which were even comparable with that of H₂SO₄. The unique features of this catalyst, such as superior thermal stability, recyclability, excellent catalytic activity in terms of yield and reaction time, high turnover number and turnover frequency, are potentially important for the applications of this catalyst in the industry. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra03463e

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PAPER
Synthesis of a 3D-network polymer supported
Bronsted acid ionic liquid based on calix[4]-
resorcinarene via two post-functionalization steps:
a highly efficient and recyclable acid catalyst for
the preparation of symmetrical bisamides†
Cite this: RSC Adv., 2014, 4, 31239
*
Arash Mouradzadegun, Somayeh Elahi and Fatemeh Abadast
In this work, for the first time, a 3D-network polymer-supported Bronsted acid ionic liquid was synthesized
via two post-functionalization steps. Initially, the active homogeneous catalyst was chemically immobilized
onto a polymeric support based on calix[4]resorcinarene by silylation of the hydroxyl groups to form a
cationic polymer that contains imidazolium moieties. The formation of this cationic polymer was
confirmed by elemental analysis, scanning electron microscopy (SEM), thermal gravimetric analysis (TGA)
and derivative thermogravimetric (DTG) analysis. Subsequently, HSO4 anion was incorporated into the
polymer along the imidazolium pendant groups via a well-known ion exchange reaction. Elemental
analysis data revealed that the cationic polymer was conveniently loaded with the desired Bronsted acid
anion; therefore, it provides a novel heterogeneous acid catalyst for achieving synthetic goals. The
immobilized acidic ionic liquid effectively catalyzed the one-pot synthesis of symmetrical bisamides by
the multicomponent condensation of two moles of amides with aldehydes. Interestingly, the catalyst
exhibited a high turnover number (TON) and turnover frequency (TOF), which were even comparable
with that of H2SO4. The unique features of this catalyst, such as superior thermal stability, recyclability,
excellent catalytic activity in terms of yield and reaction time, high turnover number and turnover
frequency, are potentially important for the applications of this catalyst in the industry.
Received 16th April 2014
Accepted 17th June 2014
DOI: 10.1039/c4ra03463e
www.rsc.org/advances
moreover, water easily co-adsorbs near the acidic sites, further
leading to the partial deactivation or leaching of the active
sites.5 Furthermore, the presence of organic halides containing
cations usually results in the destruction of the inorganic
supports, which largely constrains their wide application.6
Compared with inorganic supports, polymers have shown
adjustable hydrophobicity and good stability for various acids.
These features, coupled with their biodegradability and green
nature, make polymeric supports attractive materials to organic
and industrial chemists.
However, some of these polymeric supports suffer from one
or more defects in their synthetic or functionalization routes
and properties, including long reaction times, multiple steps
for synthesis, functionalization via copolymerization or further
post-polymerization under harsh reaction conditions, the use of
expensive catalysts, solubility in various solvents, and low
thermal and/or chemical stability. Hence, at present, devel-
oping and applying new supports for heterogeneous-type cata-
lytic systems, which may quickly move from the laboratory to
green industrial plants, have received increased attention.
It seems that the 3D-network polymer based on calix[4]-
resorcinarene may be a good candidate as a support material for
heterogeneous catalysis because of its ease of preparation and
1. Introduction
Ionic liquids including the Lewis and the Bronsted acid anion
have been proven to be efficient catalysts for various reactions.1
However, high viscosity, high cost, a tedious purication
procedure of the product, moisture sensitivity and decomposi-
tion of some ionic liquids under normal atmospheric condi-
tions, have restricted their widespread practical applications on
an industrial scale.2 In order to solve these problems, immo-
bilized ionic liquid catalysts, which combine the advantageous
characteristics of ionic liquids, inorganic acids and solid acids,
have been proposed.3
In this context, the graing of strong acidic ionic liquids
(ILs) onto inorganic materials, such as mesoporous silicas and
zeolites, has received considerable attention.4 However, the
hydrophilic inorganic networks of the supports largely affect
their catalytic activities due to the presence of water, which is
usually
a by-product in many acid catalyzed reactions;
Department of Chemistry, Faculty of Science, Shahid Chamran University, Ahvaz, Iran.
E-mail: arash_m@scu.ac.ir; Fax: +98(611)3337009; Tel: +98(611)3331042
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra03463e
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unique properties such as good dispersion of active sites,
With the above points in mind, as part of our continuing
versatile chemical modication (acylation, alkylation and sily- efforts to develop high performance and environmentally
lation of hydroxyl group and nucleophilic aromatic substitution friendly procedures for various important reactions and trans-
on the aromatic ring), its straightforward functionalization formations13 and due to our new interest in the application of
route via post-functionalization without undergoing copoly- the 3D-network porous polymer based on calix[4]resorcinarene
merization or further post-polymerization, porous surface (pore for the preparation of biologically important molecules,7 herein,
radius from approximately 100 to 500 nm), and high thermal we report for the rst time, the functionalization of polymeric
and chemical stability.7
calix[4]resorcinarene via a covalently anchored acidic ionic
To the best of our knowledge, despite its convenient prop- liquid and the investigation of its efficiency as a solid acid for
erties and signicant potential for functionalization under mild the synthesis of bisamide derivatives through the one-pot
reaction conditions without the use of acidic or basic catalysts, reaction of various aldehydes with aliphatic and aromatic
there have been no reports to date on the immobilization of amides.
acidic ionic liquid onto this polymeric structure.
Thus far, some immobilization processes for acidic ILs onto
solid support have been designed. Among them, the synthesis
of functionalized cationic polymers via covalently anchored ILs
2. Experimental
2.1. General
Chemicals were purchased from Fluka, Merck and Aldrich
onto a polymeric backbone, which could then be converted to
the acidic form by a facile ion exchange reaction, has generated
much interest.
Quaternary ammoniums (QAs) have been most widely
studied as cationic groups for the functionalization of various
polymers. However, environmental issues concerning the use of
highly volatile reagents, such as the trimethyl amine, which is
used for the introduction of these QAs, limits their large scale
synthesis and application.
To overcome this limitation, alkyl imidazolium has recently
been incorporated into polymers as a cationic moiety due to its
low volatility, and excellent thermal and chemical stabilities.8
Hence, it would be reasonable to consider the immobiliza-
tion of acidic ionic liquids containing imidazolium moieties
onto a 3D-network polymer based on calix[4]resorcinarene as a
promising solid support, which could then be used as an effi-
cient acidic catalyst in chemical reactions for the construction
of valuable target molecules.
chemical companies and used without further purication.
1
Products were characterized by physical data, IR, H NMR and
13C NMR spectra. IR spectra were obtained on a Bomen MB:102
1
FT-IR spectrophotometer. H NMR and 13C NMR spectra were
recorded on a Bruker spectrometer at 400 MHz and 100 MHz,
respectively, in CDCl3 or dimethyl sulfoxide (DMSO) with tet-
ramethylsilane as an internal standard. Elemental analyses
were performed on a Thermo Finnigan Flash EA 1112 CHNS-
Analyzer. The polymer morphology was examined by SEM (LEO-
1455VP). Thermal stability of the functionalized polymer was
investigated by NETZSCH STA 409 PC/PG under a nitrogen
atmosphere (rate of N2 z 1 L hꢀ1).
Monitoring of the reactions and the purity determination of
the products were accomplished by TLC on silica gel PolyGram
SILG/UV 254 plates.
2.2. Synthesis of calix[4]resorcinarene (1)
Because of the exceptional properties of the amide bond, it
plays an important role in amide derivatives, such as
biochemicals, structural subunit of polymers, and stable
synthetic intermediates in chemistry and materials science.9
Among a large variety of compounds containing amide bonds,
bisamides are one of the more important amide derivatives,
which have been used as versatile intermediates in the synthesis
of biologically active compounds such as amidoalkyl.10 They
have also been applied as ligands in Ullmann coupling reac-
tions;11 moreover, they are of considerable interest in the
synthesis of pharmacological materials such as peptidomimetic
compounds.12
Due to the aforementioned chemical and pharmacological
signicance, a great deal of effort has been focused on devel-
oping new and efficient synthetic routes to generate bisamide
derivatives in recent years. Although these methods are effec-
tive, they suffer from some drawbacks such as long reaction
time, multiple reaction steps, the use of expensive catalysts,
Calix[4]resorcinarene was prepared according to a previously
reported procedure.14
2.3. Synthesis of the 3D-network polymer based on calix[4]-
resorcinarene (2)
The desired polymer was synthesized by adding 42 mmol of
formaldehyde to 14 mmol of the prepared calix[4]resorcinarene
dissolved in 40 mL NaOH solution (10%)ꢁat room temperature.
The resultant mixture was heated to 90 C and maintained at
90 ꢁC for 20 h. Next, the excess alkali was washed out of the gel
formed with cold water. The gel was allowed to stand at 100 ꢁC
for 1 h. Then, the gel was transformed to the acidic form by
treatment with the 0.1 M HCl solution. The resulting solid was
dried at 100 ꢁC for 10 h.15
2.4. Synthesis of novel functionalized polymer (3) through
one-step chemical modication
special care in handling and storing the catalyst, and laborious The synthesized polymer (1.0 g), 3-chloropropyltrimethoxy-
work-up and purication steps. Consequently, the development silane (3.4 g, 16 mmol), and an excess of methyl imidazole
of more convenient, economical and eco-friendly protocols that (25 mmol) were added to a round-bottomed ask containing 10
use an efficient and reusable catalyst is highly desired.
mL H2O/EtOH at room temperature. The reaction mixture was
heated to 90 C and maintained at 90 C for 20 h. The desired
ꢁ
ꢁ
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cationic polymer was collected by ltration and washed with [pmim]HSO4 onto a 3D-network polymer based on calix[4]-
ꢁ
deionized water several times before drying at 80 C.
resorcinarene via two post-functionalization steps. The rst step
consists of the synthesis of novel cationic polymer via the
immobilization of IL ([pmim]Cl) onto polymeric backbone, and
the second step consists of [pmim]Cl transformation into
[pmim]HSO4 by an ion-exchange reaction.
To achieve this objective, the 3D-network polymer 2 was
prepared by the reaction of resorcinol and acetaldehyde, fol-
lowed by the polycondensation of calix[4]resorcinarene 1 with
formaldehyde,14,15 and the synthesis steps are shown in
Scheme 1.
The structure of compound 1 was established unambigu-
ously from the spectroscopic (IR, 1H NMR, 13C NMR) data
(see ESI†). The formation of polymer 2 was also conrmed and
characterized by atomic force microscopy (AFM), X-ray diffrac-
tion (XRD) and scanning electron microscope (SEM)7 (available
in the ESI†).
Subsequently, in the rst step of post-functionalization, the
prepared polymer 2 was treated with 1-methyl imidazole and
(3-chloropropyl) trimethoxysilane in water/ethanol to undergo a
condensation reaction, yielding cationic polymer 3 in a one-step
modication reaction (Scheme 2). The loading amount of the
cationic moieties on the polymer was determined by elemental
analysis. The nitrogen analysis of supported IL 3 (N, 7%) indi-
cates that 2.5 mmol of the imidazolium moieties were graed
onto the surface per 1 g of the polymer. The formation of 3 was
also conrmed by SEM, TG and DTG analyses.
2.5. Determination of the amount of cationic moieties
graed onto the polymer
The amount of cationic moieties graed onto the polymer was
determined by elemental analysis. The amount of graed imi-
dazolium moieties was generally found to be 2.5 mmol per gram
of dry cationic polymer.
2.6. Preparation of polymer-supported acidic catalyst (4)
Dried cationic polymer (1.0 g) was suspended in 10 mL CH2Cl2.
During vigorous stirring, 3.5 mmol of concentrated H2SO4
(96%) was introduced drop by drop at 0 C. Then, the mixture
was warmed up to room temperature and reuxed for 48 h. The
desired catalyst was collected by ltration and washed with
deionized water several times.
ꢁ
2.7. Determination of the capacity of the polymer-supported
catalyst
The capacity of the polymer-supported catalyst was determined
by elemental analysis. The capacity was generally found to be
0.906 mmol H+ per gram of the dry polymer-supported
catalyst 4.
2.8. General procedure for the synthesis of bisamide
derivatives (5)
In order to gain an insight into the morphology of the
synthesized polymer 3, scanning electron microscopy was used
(Fig. 1).
Since the cationic polymer 3 shows smaller pores (about
97 nm in diameter) than polymer 2 (300 nm in diameter,
see ESI†), it can be concluded that the ILs are successfully
graed onto the polymeric backbones.
To a mixture of aldehyde (1 mmol) and amide (2 mmol) in
toluene (5 mL), 0.08 g of catalyst was added. The resultant
mixture was stirred under reux condition, and aer comple-
tion of the reaction (TLC monitoring), a white viscous solid was
obtained. The mixture was cooled and the solvent was removed
by ltration. Then, hot ethanol (10 mL) was added to the residue
and the catalyst was separated by ltration. Evaporation of the
ethanol under reduced pressure afforded the products, which
were then puried by recrystallization in a suitable solvent
(ethanol or acetone).
The stability of the functionalized polymer 3 was determined
by thermo gravimetric analysis at a heating rate of 10 ꢁC per
minute under a nitrogen atmosphere (Fig. 2). The TG curve
indicates an initial weight loss of 8.46% up to 100 ꢁC due to the
adsorbed water and organic solvent in the polymeric support,
which is common in the cases of immobilized catalysts onto
organic or inorganic supports.16 A complete loss of the IL chains
is seen in the range of 300–469 ꢁC. The weight loss in the
temperature range of 500–1100 ꢁC can be assigned to the
decomposition of the polymeric support. Moreover, the peak in
the DTG curve shows that decomposition of the IL chains
2.9. Recycling of catalyst 4
The multicomponent reaction of 1 mmol benzaldehyde and
2 mmol benzamide in toluene as a model reaction was studied.
When the reaction was completed, the mixture was cooled and
the solvent was removed by ltration. Then, hot ethanol was
added to dissolve the product. The solid catalyst was ltered off,
washed with hot ethanol, dried under vacuum aer each cycle,
and then reused for the next reaction.
ꢁ
mainly occurred in two steps at 395 and 430 C, which can be
attributed to the decomposition of the imidazolium moieties
and propylsilane linkers, respectively. Thus, the TGA and DTG
curves also convey the clear information that IL chains are
successfully graed onto the polymeric backbones.
3. Results and discussion
Aer the characterization of 3, in the second step of post-
As stated earlier, the search for more efficient catalytic systems functionalization, the resultant cationic polymer was further
that might combine the advantages of both homogeneous and reacted with H2SO4 in dichloromethane to form the corre-
heterogeneous catalysis is one of the most exciting challenges in sponding 3D-network polymer-supported acidic catalyst 4
modern chemistry. Considering this fact, we have chosen (Scheme 3).
the immobilization of Bronsted acidic ionic liquid (1-methyl-3-
The sulfur content of the catalyst, measured by CHNS anal-
(trimethoxysilylpropyl)
imidazolium
hydrogensulfate) ysis, showed the value of about 2.9%. Based on this value, the
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Scheme 1 Synthesis of calix[4]resorcinarene (1) and the 3D-network polymer (2).
Scheme 2 Synthesis of cationic polymer (3) containing imidazolium moieties through the one-step chemical modification of (2).
amount of HSO4 anions, which were supported on the polymer, applicability of 4 as an acid catalyst for the rapid and efficient
is about 0.906 mmol per gram polymer. This result gave us construction of bisamide derivatives through a multicompo-
condence that the imidazolium side-chains and polymeric nent reaction.
backbones are completely stable under acidic conditions.
To develop suitable reaction conditions, the reaction of
It is noteworthy that this polymeric support showed more 2 mmol benzamide with 1 mmol benzaldehyde was chosen as a
potential for covalently anchored acidic ionic liquids in model reaction (Scheme 4) and parameters such as solvent and
comparison with some previous solid supports reported in amount of catalyst (4) were examined in detail.
literature.17 Moreover, the synthesized polymer supported acid
The solvent optimization revealed that the solubility of both
catalyst (4) is effectively insoluble in various solvents, including the substrate and product play an important role in the process.
H2O, CH3OH, EtOH, DMSO, DMF, CH3CN, CH2Cl2, and CHCl3. It was found that when the substrate was highly soluble and the
These features render it suitable for conducting chemical product precipitated the process came together with the
transformations in various reaction media as an efficient increased ease of work-up using ltration, the high yield and
heterogeneous acid catalyst.
the purity of product.18 Thus, tetrahydrofuran, acetone, aceto-
In this regard, owing to the widespread applications of nitrile and ethyl acetate failed to afford a homogeneous solution
bisamide derivatives, we decided to investigate the synthetic at the rst stage, which diminished the yield and purity of the
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benzaldehyde in the presence of 0.08 g catalyst in reuxing
toluene was chosen as the optimized reaction condition.
To conrm the catalyst efficiency, further experiments were
carried out. Initially, in a control experiment, we carried out the
model reaction in the absence of any catalyst. In this case, only a
trace amount of product was detected aer 6 h (entry 9, Table 1).
Next, the model reaction was examined in the presence of
H2SO4. As shown in Table 1, the yield of N,N0-phenylidene
bisamide (5) was much higher with solid protic acid (4)
compared with sulfuric acid (entries 7 and 10, respectively,
Table 1).
The superiority of this solid analog of sulfuric acid to
commercial sulfuric acid is attributed to its porosity. The
adsorption of substrate onto the surface of the functionalized
polymer based on hydrophobic interaction between the
substrates and polymeric backbone could increase the local
concentration of substrates around the active sites of the
polymer and effectively promote the reaction. These obser-
vations clearly highlight the existence of a synergistic effect
between the cavities of the polymeric support and the
immobilized acidic ILs, which leads to high catalytic
activities.
These results encouraged us to demonstrate the efficiency
and the applicability of the present acid-catalyzed multicom-
ponent reaction. For this purpose, the optimized reaction
conditions were applied to a variety of aldehydes containing
electron-withdrawing or electron-donating groups. Represen-
tative results of this synthetic modication are listed in Table 2.
Both electron-rich and electron-decient aldehydes gave the
N,N0-arylidene bisamide in good to excellent yields (entries A–F,
Table 2).
Fig. 1 SEM image of synthesized polymer 3.
nal product (entries 2–5, Table 1). To our surprise, among the
solvents investigated, boiling toluene boosted the yield to 80%
(entry 6, Table 1), due to its perfect solubility for the substrate
and insolubility for the product.
The reaction was also evaluated in the presence of various
amounts of catalyst. The results showed that 0.08 g of catalyst
efficiently catalyzed the reaction to give the desired product (5)
(entry 7, Table 1). It was also found that increasing or
decreasing the amount of catalyst not only improves the reac-
tion times but also reduces the yield of the desired product
signicantly (entries 6 and 8, respectively, Table 1). This may be
rationalized by considering the fact that small amounts of
acidic catalyst could not activate the aldehyde effectively, but
large amounts of it inactivate the amide via its protonation.
Thus, the reaction of 2 mmol benzamide with 1 mmol
In order to further expand the scope of this catalytic system,
acetamide was treated with a variety of aldehydes and success-
fully converted to their corresponding bisamide (entries G–L,
Table 2).
Fig. 2 TGA and DTG curves of cationic polymer 3.
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Scheme 3 Synthesis of polymer supported Bronsted acid ionic liquid (4).
Scheme 4 The reaction of model compounds with polymer supported catalyst.
All products were identied by comparing their physical and synthesis of these privileged compounds in the presence of
spectral data with those of authentic samples.19
polymer supported Bronsted acid ionic liquid as an efficient
It is noteworthy that the formation of 5A–F by this method solid acid catalyst.
are of considerable interest, since this is the rst attempt at the
On the basis of the results obtained above, a possible
mechanism for the sequence of events is shown in Scheme 5.
It seems that initially, the solid acid catalyst 4 activates
the carbonyl group of the aldehyde for reaction with the
amide to form a hemiamidal intermediate (6). Subsequently,
elimination of water from 6, forms N-acyliminium interme-
diate (7), which is then converted to the desired
product 5 by a nucleophilic reaction with another molecule of
amide.
It is well known that the reusability, turnover numbers (TON)
and turnover frequencies (TOF) of a catalytic system are the key
factors that identify whether it has potential applications in
industry.
Table 1 Optimization of the reaction conditions
Amount of catalyst
Entry Solvent
Catalyst (g)/mmol H+
Time
Yield (%)
1
2
3
4
5
6
7
8
9
None
THF
4
4
4
4
4
4
4
4
0.1/0.092
0.1/0.092
0.1/0.092
0.1/0.092
0.1/0.092
0.1/0.092
0.08/0.074
0.07/0.065
—
6 h
6 h
6 h
6 h
6 h
30 min 80
30 min 85
30 min 78
0
5
15
13
20
Acetone
Acetonitrile
Ethyl acetate
Toluene
Toluene
Toluene
Toluene
Toluene
Considering this fact, in order to test the catalyst reusability,
the reaction of 2 mmol benzamide with 1 mmol benzaldehyde
None
H2SO4
6 h
60 min 50
10
10
0.004/0.074
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Table 2 Synthesis of bisamides catalyzed by polymer supported catalyst 4
Entry
Amide
Aldehyde
Product 5
Time (min)
Yield (%)
mp (ꢁC)
A
25
83
258–261
B
30
84
260–262
C
D
E
20
30
30
91
85
87
265–267
237–238
241–244
F
50
80
225–227
G
35
83
254–257
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Table 2 (Contd.)
Entry
Amide
Aldehyde
Product 5
Time (min)
Yield (%)
mp (ꢁC)
H
30
90
269–271
I
25
92
271–273
J
50
35
80
92
233–236
270–273
K
L
50
79
216–219
was carried out under optimal reaction conditions. Aer optimized conditions (entries 1 and 2, Table 3). The results
completion of each reaction, hot ethanol was added, and the show a greater superiority of catalyst 4 to H2SO4.
catalyst was then ltered. The recovered catalyst was washed
In order to show the merit of this novel catalyst, Table 3
with ethanol, dried and reused. The results illustrated in Fig. 3 compares our results obtained from the model reaction in the
show that the catalyst can be reused for up to ve cycles without presence of catalyst 4 with the results reported in literature. It
signicant loss in its activity. This result indicates that the can be seen that the heterogeneous catalyst 4 exhibited a
catalyst was very stable and could endure this reaction's signicantly higher turnover number and turnover frequency
conditions without decomposition or leaching of the active than those reported for other catalytic systems (entries 3–5,
species in the reaction media.
Table 3).
Based on these results, we expect that this catalyst 4 will be a
Next, the catalytic performances as TON and TOF of this
reusable analog of sulfuric acid were compared with commer- suitable alternative to the existing acid catalyst in laboratory
cially available sulfuric acid for the model reaction under and industrial applications.
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Scheme 5 The proposed mechanism.
(H2SO4) under the same reaction conditions. The porosity of
the polymeric support and covalently anchored acidic ionic
liquid might indeed provide a synergistic means for the effi-
cient approach of the starting materials to acidic sites, enough
space for the preparation of product and a suitable hydro-
phobicity to drive out the water formed during the reaction
from the support.
Catalyst 4 possesses a combination of a stable and
hydrophobic polymeric support, strong acid strength, excel-
lent catalytic activity and good recyclability in acid
catalyzed reactions, and may open up a new route for the
synthesis of novel and efficient heterogeneous acid
catalysts via modication of the 3D-network polymer
based on calix[4]resorcinarene. Unique properties, such as
uniform reaction site and signicant potential for function-
alization, will allow researchers to optimize the
properties of this polymer for immobilization of the desired
catalyst via post-functionalizations without undergoing
Fig. 3 Recyclability of catalyst.
4. Conclusion
copolymerizations
or
further
post-polymerization
modications.
Bronsted acid ionic liquid has been immobilized onto a 3D-
network polymer based on calix[4]resorcinarene to form a
novel heterogeneous solid acid catalyst. The synthesized cata-
lyst shows remarkably and somewhat extraordinary catalytic
activity for the synthesis of bisamide derivatives. This catalyst
is even more reactive than the homogeneous strong acid
Thus, this study suggests that the 3D-network polymer
based on calix[4]resorcinarene can be used as an important
and versatile green solid support for a variety of acid
catalysts.
Table 3 Comparison of the catalytic performance of catalyst 4 with H2SO4, MSAa, SSAb and SBSSANPsc
Entry
Catalyst
Acid capacity
Yield (%)
Time (min)
TON
TOF
References
1
2
3
4
5
4
0.074 mmol H+ per 0.08 g
0.074 mmol H+ per 0.004 g
0.2 mmol H+ per 0.029 g
0.125 mmol H+ per 0.05 g
0.096 mmol H+ per 0.08 g
85
50
91
90
93
30
60
30
30
240
1148
676
455
720
969
38.26
11.26
15.2
24
Present work
Present work
19d
19d
19a
H2SO4
MSA
SSA
SBSSANPs
4.04
a
Molybdate sulfuric acid. b Silica sulfuric acid. c Silica-bonded S-sulfonic acid nanoparticles.
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B. S. Shwetha, D. Hegde and U. Santhosh, Tetrahedron
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Acknowledgements
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This work was supported by the Research Council at the Shahid
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