Chitosan as a biodegradable heterogeneous catalyst for Knoevenagel condensation between benzaldehydes and cyanoacetamide

Article abstract of DOI:10.1016/j.catcom.2020.105954

A natural biopolymer chitosan is being employed as heterogeneous solid base catalyst for the Knoevenagel condensation reaction between benzaldehydes and cyanoacetamide under mild reaction conditions. This reaction offers the direct synthesis of condensati

Full text of DOI:10.1016/j.catcom.2020.105954

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Chitosan as a biodegradable heterogeneous catalyst for
Knoevenagel
condensation
for
benzaldehydes
and
ethylcyanoacetamide
Nagaraj Anbu, Vimali Elamathi, Perumal Varalakshmi,
Amarajothi Dhakshinamoorthy
PII:
S1566-7367(20)30030-3
DOI:
https://doi.org/10.1016/j.catcom.2020.105954
CATCOM 105954
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
9 December 2019
5 February 2020
6 February 2020
Please cite this article as: N. Anbu, V. Elamathi, P. Varalakshmi, et al., Chitosan as a
biodegradable heterogeneous catalyst for Knoevenagel condensation for benzaldehydes
and ethylcyanoacetamide, Catalysis Communications (2020), https://doi.org/10.1016/
j.catcom.2020.105954
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Chitosan as a biodegradable heterogeneous catalyst for Knoevenagel
condensation for benzaldehydes and ethylcyanoacetamide
Nagaraj Anbu,1 Vimali Elamathi,2 Perumal Varalakshmi*2 Amarajothi
Dhakshinamoorthy*1
1School of Chemistry, Madurai Kamaraj University, Tamil Nadu, India.
2School of Biotechnology, Madurai Kamaraj University, Tamil Nadu, India.
E-mail: admguru@gmail.com
Abstract
A natural biopolymer chitosan is being employed as heterogeneous solid base catalyst
for the Knoevenagel condensation reaction between benzaldehyde and cyanoacetamide under
mild reaction conditions. This reaction offers the direct synthesis of condensation product
with two functional groups that can readily be functionalized. The biopolymer is used for five
cycles with no decrease in activity and hot filtration test proves heterogeneity of the reaction.
Some of the salient features of this strategy include biodegradability of catalyst, high catalyst
stability, mild reaction conditions and wide substrate scope. The structural integrity is
retained even after repeated cycles as evidenced by powder X-ray diffraction (XRD).
.
Keywords: Biopolymer; Chitosan; Heterogeneous Catalysis; Knoevenagel condensation;
Solid base
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1. Introduction
Among the wide ranges of naturally occurring biopolymers, chitosan is one of the
biopolymers that have attracted researchers for many applications due to their attractive
properties like non-toxicity, biocompatibility and biodegradability. Further, chitosan exist in
a three dimensional structure with functional groups such as amino and hydroxyl groups in its
skeleton. These functional groups in chitosan are often used as anchoring sites for further
functionalization or active sites in promoting organic reactions. Among the various
applications of chitosan, the use of chitosan as heterogeneous catalyst is highly promising due
to its insoluble nature in water and common organic solvents. Hence, chitosan or modified
chitosan has been employed as heterogeneous catalysts for various organic transformations.
For example, Cu-metal-organic framework supported on chitosan was used for condensation
reaction,[1] Knoevenagel condensation reactions were performed with chitosan,[2-4]
modified chitosan-based hydrogels were also employed to perform Knoevenagel
condensation,[5] chitosan hydrogel was used for aldol reactions,[6] chitosan-TiO2 hybrid
catalyst was reported for nitroaldol condensation,[7] chitosan was employed as solid catalyst
for the synthesis of jasminaldehyde,[8] chitosan biohydrogel beads were reported for C-C
bond reaction,[9] carboxymethyl chitosan Schiff-base palladium(II) catalyst was used for
Suzuki-Miyaura cross-coupling,[10] melamine-functionalized chitosan was used to obtain
cyanocinnamonitrile,[11] multicomponent synthesis realized with chitosan as solid catalyst
under microwave conditions for the synthesis of bis(2-amino-tetrahydro-4H-chromene-3-
carbonitrile,[12] chitosan-based Schiff-base Ni complex was shown to promote the synthesis
of pyrazolophthalazinediones [13] and chitosan-supported Cu(II) Schiff-base for the
synthesis of 5-substituted 1H-tetrazoles and homocoupling of terminal alkynes.[14]
Knoevenagel condensation is one of the well-known C-C bond forming reactions in
organic chemistry which is performed between carbonyl compounds and active methylene
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compounds and the resulting condensation product finds application in polymers and
pharmaceuticals. Furthermore, the most frequently used active methylene compounds for
Knoevenagel condensation reaction were malononitrile and ethylcyanoacetate catalyzed by
acids or bases either in homogeneous or in heterogeneous fashion. Hence, this reaction is
considered as one of the benchmark reactions to rank different catalysts. This reaction has
been reported with a wide range of heterogeneous solid catalysts like mesoporous silica,[15]
microporous titanosilicate,[16] methylammonium-FAU zeolite,[17] rare-earth exchanged
NaY
zeolite,[18]
Cs-NaY,[19]
nitrided
ITQ-2,[20]
mesoporous
sodalite,[21]
aluminophosphates oxynitrides,[22] alkali metal-modified oxide,[23] mesoporous K-
ZrO2,[24] amine modified graphene oxide,[25] hexagonal boron nitride,[26] nickel iron
hydrotalcite,[27] IRMOF-3,[28] MIL-53(Al)-NH2,[29] and MIL-100(Fe).[30] Although
considerable numbers of reports have been reported for Knoevenagel condensation reaction,
some of the main limitations are tedious synthesis procedure of catalyst, easy deactivation,
high expensive nature, requires activation of catalyst prior catalytic test and limited substrate
scope. Considering these limitations in the reported methods, there is still a scope for the
development of a simple and facile heterogeneous catalyst from naturally occurring
biopolymer to promote this reaction.
In the present work, chitosan is employed as an eco-friendly heterogeneous solid base
catalyst for the Knoevenagel condensation reaction between benzaldehyde and
cyanoacetamide under mild reaction conditions. Although this reaction has been effectively
promoted between benzaldehyde and malononitrile, no reports are available in the literature
for the condensation between benzaldehyde and cyanoacetamide. This reaction is an
important reaction since the resulting product has two different functional groups for further
synthetic modifications. Besides, this condensation reaction has many benefits including the
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employment of a naturally available biopolymer as heterogeneous catalyst, environmentally
benign, absence of functionalization and it is commercially available.
2. Experimental Section
2.1 Material
Chitosan, cyanoacetamide and aldehydes were purchased from Sigma Aldrich, Alfa
Aesar and Merck, used as received without any purification. Ethanol, water and other
solvents were procured from Chemical Drug House and Sisco Research Laboratory and used
without any additional purification.
2.2 Instruments
The powder XRD patterns were measured in the refraction mode in a Bruker D2
Phase X-ray diffractometer (30 kV, 10 mA) using the Cu Kα (λ = 1.5406 Å) radiation. FT-IR
analysis was carried out with Bruker tensor 27 series FT-IR spectrometer in the region of
-1
-1
400-4000 cm with 2 cm resolution and 16 scans. SEM images were collected using a
Hitachi S-3000H scanning electron microscope. Conversion and selectivity were determined
by Agilent 7820A gas chromatography (GC) using HP-5 column of 30 m length and high
pure nitrogen as carrier gas with flame ionization detector. Finally, the obtained products
were confirmed by 5890A model of Agilent GC-MS instrument using HP-5 column of 30 m
1
length and high pure helium gas as carrier gas. H and 13C NMR Avance III HD Nanobay 400
MHz FT-NMR spectrometer using DMSO-d6 and CDCl3 appropriately based on the solubility
of the compounds.
2.3 Catalytic procedure Knoevenagel condensation between 1 and 2
A 10 mL schlenk tube was charged with chitosan (20 mg), 1 (0.5 mmol) and 2 (0.6
mmol). To this mixture 1:1 ratio of water and ethanol (0.4 mL) was added and these mixtures
o
were homogeneously mixed. This slurry was then placed in an oil bath maintained at 70 C
for 4 h. The kinetics of the catalytic reaction was monitored by collecting samples at different
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time intervals from the reaction mixture and analyzed by GC using 7820A model instrument.
Also, the yields of the resulting products were determined by GC using internal standard
method. Finally, the observed products were identified using Agilent 5779B GC-MS
technique. The catalyst was reused in subsequent cycles after recovering the solid from the
reaction mixture, washed three times with absolute ethanol (5 mL x 3) followed by drying at
o
80 C for 2 h. Later, this dried solid catalyst was subjected for reusability tests with the fresh
1
reactants 1 and 2 in the next cycle. The as-synthesized products were characterized by H-
NMR, 13C-NMR and GC-MS techniques and copies of these spectra are provided in Figures
S1-S33.
3. Results and discussion
To verify the structural features of chitosan, series of characterization methods like
elemental analysis, powder X-ray diffraction (XRD), Fourier Transform infrared
spectroscopy (FT-IR) and scanning electron microscope (SEM) were conducted and the
observed results are discussed below. Elemental composition of chitosan was measured and
the atomic weight percentages of C, H and N remained identical with earlier data as well as
matched with the data provided by commercial supplier. The structural integrity and the
physical nature of chitosan were ascertained by performing powder XRD analysis and the
observed results are shown in Figure 1. As it can be seen in Figure 1, two broad diffraction
patterns are seen at around 10 and 20 degrees, suggesting amorphous nature of chitosan. But,
the positions of 2θ values of these two broad peaks have slightly altered after the reaction, but
it is retained during the reusability tests. In any way, these changes in the structural skeleton
did not affect its catalytic performance. Further, chitosan was also characterized by FT-IR to
understand the existence of various functional groups and the observed results are shown in
Figure 2. The characteristic stretching frequencies like O-H, N-H and C-H stretching
-1
frequencies were observed at around 3300-3500, 3200-3300 and 2900 cm , respectively. In
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addition to these bands, C-H bending vibration is also seen at 1450 cm-1. These characteristic
stretching frequencies indicate the presence of O-H in the form of primary alcohols, free
amino groups and C-H in glucose units. Thus, the recovered chitosan after the catalytic
reaction and reused chitosan did not show many changes in its skeleton thus indicating the
absence of organic products on chitosan.
Figure 1. Powder XRD patterns of (a) fresh, (b) recovered after the reaction and (c) five
times used chitosan. Reaction conditions: 1 (0.5 mmol), 2 (0.6 mmol), chitosan (20 mg),
ethanol:water (0.4 mL), 70 oC, 4 h.
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Figure 2. FT-IR spectra of (a) fresh, (b) recovered after the reaction and (c) five times used
chitosan. Reaction conditions: 1 (0.5 mmol), 2 (0.6 mmol), chitosan (20 mg), ethanol:water
(0.4 mL), 70 oC, 4 h.
Benzaldehyde (1) and cyanoacetamide (2) were selected as model substrates to
evaluate the catalytic performance of chitosan in the Knoevenagel condensation reaction.
This reaction was optimized with respect to various solvents, effect of temperature and
catalyst loading. A blank experiment in the absence of chitosan exhibited around 20 % of the
o
expected condensation product at 70 C after 4 h. Later, the catalytic performance of chitosan
was tested for the reaction between 1 and 2 in methanol and ethanol was 54 and 79% yield,
respectively under identical conditions. The product yield was enhanced to 86% in water
under similar conditions. Then, the reaction between 1 and 2 using chitosan as catalyst in
water/ethanol (1:1) mixture afforded 97% yield under identical conditions while the
corresponding blank control in the absence of chitosan provided 26% yield. These catalytic
results certainly indicate that this condensation reaction is catalyzed by the basic sites present
in chitosan. The expected condensation product reached to 65% yield in the presence of
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chitosan for this reaction in toluene. Further, the reaction of 1 and 2 showed comparatively
lower yields in acetonitrile, THF, 1,2-dichloroethane of 33, 7 and 4%, respectively. Finally,
the activity of chitosan in the condensation of 1 and 2 in DMF and DMSO was 52 and 61%
yields, respectively under similar conditions. Hence, the optimum conditions to achieve high
o
activity with chitosan were the use of water/ethanol mixture at 70 C in 4 h. These conditions
were further used to optimize catalyst loading and effect of temperature.
Table 1. Optimization of the reaction conditions for Knoevenagel condensation between 1
and 2 using chitosan as solid heterogeneous base catalyst.a
Entry
1
Solvent
Methanol
Ethanol
Yieldb (%)
54
79
86
97
26
65
33
7
2
3
Water
4c
5c,d
6
Ethanol : Water
Ethanol : Water
Toluene
7
ACN
8
THF
9
DCE
4
10
11
DMF
52
61
DMSO
aReaction conditions: 1 (0.5 mmol), 2 (0.6 mmol), chitosan (20 mg), solvent (0.4 mL), 70
b
c
oC, 4 h; Yield was determined by GC using internal standard method; 1 : 1 solvent ratio was
d
used; Blank control experiment.
One of the important parameters to be optimized in identifying suitable reaction
conditions in heterogeneous catalysis is catalyst loading. In this aspect, four different
o
loadings of chitosan were tested for the condensation between 1 and 2 at 70 C in
water/ethanol mixture. Figure 3 shows the time yield profile for all these catalyst loading
under similar conditions. These catalytic results confirm that the evolution of product yield
gradually increases upon increasing the catalyst loading and the maximum yield was
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observed with 20 mg of chitosan. These data suggest that the number of active sites is higher
with 20 mg catalyst loading compared to other loadings under identical conditions, thus
showing the highest catalytic activity. Later, the performance of chitosan in promoting the
Knoevenagel condensation between 1 and 2 was also optimized at three different
o
temperatures like room temperature, 50 and 70 C. The observed catalytic profile is shown in
Figure 4. The condensation product yield gradually enhanced to achieve quantitative yield
o
upon increasing the reaction temperature from room temperature to 70 C in water/ethanol
mixture with 20 mg catalyst loading.
Figure 3. Effect of catalyst loading for the Knoevenagel condensation between 1 and 2 (a) 5
mg (b) 10 mg (c) 15 mg and (d) 20 mg. Reaction conditions: 1 (0.5 mmol), 2 (0.6 mmol),
ethanol:water (0.4 mL), 70 oC, 4 h.
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Figure 4. Effect of temperature for the Knoevenagel condensation between 1 and 2 (a) room
o
o
temperature (b) 50 C and (c) 70 C. Reaction conditions: 1 (0.5 mmol), 2 (0.6 mmol),
chitosan (20 mg), ethanol:water (0.4 mL), 4 h.
Hot filtration test (leaching test) is one of the mandatory tests to be performed in
heterogeneous catalysis mainly to ascertain that the catalytic reaction is driven by the active
sites located in the solid catalyst and not due to the leached active sites in the reaction
mixture. Hence, this test was also performed under the optimized reaction conditions. The
condensation reaction was initiated between 1 and 2 in water/ethanol mixture using chitosan
o
as catalyst at 70 C and chitosan was removed at reaction temperature from the reaction
mixture after 30 min. Then, the reaction mixture without chitosan was continued up to 4 h
and aliquots were periodically withdrawn from the reaction mixture. These aliquots were
analyzed by GC. Comparison of the activity with and without catalyst proved that the
evolution of the condensation yield is totally inhibited upon removal of chitosan from the
reaction medium (Figure 5). These catalytic data indicate that no active sites are leached to
the solution and hence, no activity is observed after removal of the catalyst. Hence, it can be
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claimed that the reaction is promoted by the active sites located on chitosan and the catalytic
reaction is heterogeneous in nature under these conditions.
Reusability tests are performed very often in heterogeneous catalysis to prove
catalyst stability. In this aspect, chitosan stability in this reaction was checked by performing
reusability test under the optimized reaction conditions. The catalytic reaction was started
under optimized conditions and the catalyst was recovered by filtration, washed and dried.
This recovered solid was reused in the subsequent cycle with fresh 1 and 2. In this way,
chitosan was used up to five cycles with no apparent decay in its catalytic activity (Figure
S34). These results indirectly indicate that the structural integrity and active sites are mostly
retained during the course of the reaction as well as in the reusability tests. This hypothesis
was further verified by characterizing the reused samples by powder XRD, FT-IR, SEM and
comparing with the fresh chitosan. Although minor changes have been observed in the XRD
patterns of the used sample than the fresh solid, SEM image of the used chitosan exhibits a
similar morphology to the fresh solid (Figure 6), thus confirming the stability of these solids
under the optimized reaction conditions. Furthermore, the atomic weight percentages of the
elements (C, H and N) remain almost similar with that of the fresh solid by EDX analysis.
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Figure 5. Time yield plots for the Knoevenagel condensation between 1 and 2 (a) in the
presence of chitosan as a solid base catalyst (b) hot filtration test and (c) blank control
experiment in the absence of chitosan. Reaction conditions: 1 (0.5 mmol), 2 (0.6 mmol),
chitosan (20 mg), ethanol:water (0.4 mL), 70 oC, 4 h.
Figure 6. SEM images of (a) fresh and (b) five times used chitosan. Reaction conditions: 1
(0.5 mmol), 2 (0.6 mmol), chitosan (20 mg), ethanol:water (0.4 mL), 70 oC, 4 h.
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These interesting results encouraged us to expand the scope of chitosan with other
substrates bearing electron withdrawing and donating substituents under optimized reaction
conditions. The observed catalytic data are given in Table 2. The reaction of 1 and 2 afforded
97% yield in the presence of chitosan as heterogeneous solid catalyst in water/ethanol
mixture at 70 oC after 4 h. Under similar conditions, 4-methyl, 4-methoxy and 4-t-
butylbenzaldehydes reacted with 2 efficiently to provide respective condensation products in
95, 96 and 89% yields, respectively. The reaction of 2-hydroxybenzaldehyde and 2 afforded
the corresponding 53% yield of the condensation product. This moderate activity of this
substrate is believed to be due to the intramolecular hydrogen bonding between carbonyl and
hydroxyl groups, thus lowering the reactivity of carbonyl group with 2. On other hand, 4-
nitro and 4-bromobenzaldehydes provided their respective condensation products in 98%
yields. Similarly, chitosan also promoted the condensation of furfural with 2 effectively to
afford 99% yield. Then, phenylbenzaldehyde was reacted with 2 and observed 24% yield of
the corresponding product. On other hand, cinnamaldehyde and 2 provided their
corresponding condensation product in 64% yield under similar conditions. Interestingly, a
bulkier substrate with larger dimension than 1 also effectively reacted with 2 using chitosan
as catalyst under identical conditions. A similar behavior was also noticed with
1-naphthaldehyde to provide 93% yield of respective condensation product. Finally,
9-anthracenealdehyde was reacted with 2 to give respective condensation product in 63%
yield. This lower activity may be due to the steric nature of the substrate compared to 1-
naphthaldehyde and 1.
Table 2. Knoevenagel condensation between different aldehydes and 2 in the presence of
chitosan as catalyst.a
Entry
1
Substrate
Product
Yieldb (%)
97
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2
3
95
96
4
89
5
6
7
53
98
98
8
99
9
24
64
10
11
12
97
93
13
63
aReaction conditions: aldehyde (0.5 mmol), 2 (0.6 mmol), chitosan (20 mg), ethanol:water
(0.4 mL), 70 oC, 4 h;
bYields were determined by GC.
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4. Conclusions
In summary, chitosan was shown to be an efficient heterogeneous solid base catalyst
for the promotion of Knoevenagel condensation reaction between 1 and 2 to afford high yield
of the desired product under mild reaction conditions. As commented earlier, this reaction has
not been reported either with homogeneous or heterogeneous catalysts, thus it provides a very
simple strategy to obtain condensation product bearing two different functional groups which
further can be modified to reach target products. Chitosan enjoys stability under the present
experimental conditions and the catalytic activity remains identical up to five cycles with no
decay in its performance. Further, powder XRD, FT-IR, SEM and elemental analyses
indicated that the structural integrity and morphology remains almost similar, but in any case,
the minor changes in the structure did not affect the catalytic performance. Chitosan has also
shown wide substrate scope leading to the formation of large number of condensation
products.
Acknowledgements
AD thanks the University Grants Commission, New Delhi, for the award of an Assistant
Professorship under its Faculty Recharge Programme. AD also thanks the Department of
Science and Technology, India, for the financial support through Extra Mural Research
Funding (EMR/2016/006500).
Conflicts of Interest
There are no conflicts to declare
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Graphical Abstract
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CRediT author statement
Conceptualization: Perumal Varalakshmi , Amarajothi Dhakshinamoorthy
Methodology: Amarajothi Dhakshinamoorthy
Formal analysis: Nagaraj Anbu, Vimali Elamathi
Investigation: Nagaraj Anbu, Vimali Elamathi,
Writing: Nagaraj Anbu, Amarajothi Dhakshinamoorthy
Writing - Review & Editing: Perumal Varalakshmi , Amarajothi Dhakshinamoorthy
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Declaration of interests
x The authors declare thatthey have no known competing financial interests orpersonal
relationships thatcould have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
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Highlights
 Chitosan is used as solid base for Knoevenagel condensation reaction
 Catalyst enjoys high stability under the reaction conditions
 Direct synthesis of condensation products is reported
 Catalyst is used for five cycles with no loss in its activity
20