The Modified Clay as a New and Eco-Friendly Catalyst for the Knoevenagel Reaction

Article abstract of DOI:10.1134/S1070427220090177

Abstract: This work aims the synthesis of substituted alkenes via Knoevenagel condensation using the clay collected from the Agadir region and modified by KF as a heterogeneous catalyst (KF-modified clay). In this context, the influence of various paramet

Full text of DOI:10.1134/S1070427220090177

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2020, Vol. 93, No. 9, pp. 1438–1444. © Pleiades Publishing, Ltd., 2020.
CATALYSIS
The Modified Clay as a New and Eco-Friendly Catalyst
for the Knoevenagel Reaction
S. Bentahara,*, M. Ait Talebb,c, A. Saboura, A. Dbika,
M. El Khomria, N. El Messaoudia, and A. Lacheraia
a Laboratory of Applied Chemistry and Environment, Department of Chemistry, Faculty of Science, Ibn Zohr University,
BP 8106, Agadir, 80000 Morocco
b Team of Materials, Catalysis and Valorization of Natural
Resources, Department of Chemistry, Faculty of Science, Ibn Zohr University, BP 8106, Agadir, 80000 Morocco
c Team of Bioorganic Applied Chemistry, Department of Chemistry, Faculty of Science, Ibn Zohr University,
Agadir, 80000 Morocco
*e-mail: safae.bentahar@edu.uiz.ac.ma
Received October 25, 2019; revised August 12, 2020; accepted August 12, 2020
Abstract—This work aims the synthesis of substituted alkenes via Knoevenagel condensation using the clay
collected from the Agadir region and modified by KF as a heterogeneous catalyst (KF-modified clay). In this
context, the influence of various parameters on the reaction yield was studied, and the reuse of catalyst was carried
out. The results obtained show that KF-modified clay is very effective for the preparation of substituted alkenes
giving excellent yields (87–99%) and in very short times (between 2–4 s and 8 min) under mild and heterogeneous
conditions. In addition, the recycling of our catalyst indicates that our material can be reused several times without
losing its catalytic activity.
Keywords: heterogeneous catalysts, Knoevenagel reaction, clay modified, substituted alkenes
DOI: 10.1134/S1070427220090177
poly(vinyl chloride) supported tetraethylenepentamine
[10], amino-functionalized mesoporous silica [11],
anion-exchange resins [12], guanidinium group [13],
task-specific ionic liquid [14], MgO/ZrO2 [15], modified
SBA-1 materials [16], Niobium (V) chloride [7], and
lipoprotein lipase [17]. Hence the need to research and
develop efficient, alternative, more environmentally
friendly methods.
INTRODUCTION
The Knoevenagel reaction remains the most
frequently used method for the synthesis of C=C double
bonds in organic chemistry [1, 2]. This condensation
occurs between the aldehydes and the ketones reacting
with the methylene compounds which become active in
the presence of a weak base and thus makes it possible
to obtain α, β-unsaturated dicarbonyl compounds
accompanied by the formation of water molecules.
Nowadays, one of the priority axes of research
in organic chemistry concerns the development of
alternative and more respectful methods for the
environment compared to conventional reactions.
In recent years, Knoevenagel products have
received significant interest because of their use in the
preparation of polymers [3], coumarin derivatives [4],
cosmetic products [5], perfumes [6], and pharmaceutical
chemicals [7].
In this study, we have established a simple, efficient
and economical method for the preparation of substituted
alkenes by the condensation of benzaldehyde with
malononitrile or ethyl cyanoacetate (Scheme 1) using
KF modified clay [18] as new catalyst under the mild
reaction conditions. Indeed, we optimized the reaction
Recently, several homogeneous and heterogeneous
catalysts have been developed for the synthesis of
substituted alkenes such as K3PO4 [8], zeolite [9],
1438

THE MODIFIED CLAY AS A NEW AND ECO-FRIENDLY CATALYST
1439
Scheme 1. General reaction scheme of Knoevenagel condensation.
The latter is taken up in dichloromethane (2 × 10 mL).
The filtrate is dried over Na2SO4 and the solvent is
evaporated at reduced pressure. The product obtained is
purified by recrystallization from ethanol.
conditions as the effect of catalyst amount and solvent,
thus, we expanded the reaction with other aromatic
benzaldehyde substrates.
EXPERIMENTAL
2-(Phenylmethylene)malononitrile (3aa): mp 81–
83 °C, 99% yield, H NMR (300 MHz, CDCl3) δ 7.93
1
General. All commercial reagents were purchased
(d, 2H, HAr), 7.81 (s, 1H, HC=C), 7.59 (m, 3H, HAr);
FTIR (ATR, cm–1) 3060 (C–HAr), 2976 (C–Halkene),
2212 (C≡N), 1572 (C=Calkene), 1447 (C=CAr).
from Aldrich. All solvents were distilled before use.
1
Nuclear magnetic resonance spectra of H NMR were
recorded on a Bruker apparatus. IR spectra were per-
formed on a JASCO 4100 spectrometer, absorption
bands ν are expressed in cm–1. The thin-layer chroma-
tography was carried out on silica plates from ALBET.
The revelation was realized using the CAUTION Ultra-
Violet lamp (254 and 365 nm). Melting points were
measured with a Kofler bench, type Wagner & Munz
HEIZBANK.
2-(4-Methoxyphenylmethylene)-malononitrile
(3ab): mp 108–110°C, 94% yield, 1H NMR (300 MHz,
CDCl3) δ 7.89 (d, 2H, HAr), 7.6 (s, 1H, HC=C), 6.9 (d,
2H, HAr), 3.87 (s, 3H, CH3); FTIR (ATR, cm–1) 3088
(C–HAr), 2942 (C–Halkene), 2803 (C–HCH3), 2198 (C≡N),
1524 (C=Calkene), 1414 (C=CAr), 998 (C–Oether).
2-(4-Methylphenylmethylene)-malononitrile
(3ac): mp: 129–131°C, 95% yield, 1H NMR (300 MHz,
CDCl3) δ 7.8 (d, 2H, HAr), 7.65 (s, 1H, HC=C), 7.3
(d, 2H, HAr), 2.4 (s, 3H, CH3); FTIR (ATR, cm–1)
3182 (C–HAr), 3018 (C–Halkene), 2846 (C–HCH3), 2195
(C≡N), 1565 (C=Calkene), 1421 (C=CAr).
General procedure for the synthesis of substituted
alkenes. The synthesis of substituted alkenes was
realized by introducing into a flask an equimolar
mixture of aromatic aldehyde and malononitrile or
ethyl cyanoacetate in methanol by adding 20 mg of KF-
modified clay as a heterogeneous catalyst. The mixture is
stirred at room temperature. The condensation reaction
is monitored by thin-layer chromatography (eluent =
9 mL of hexane and 1 mL of ethyl acetate), the revelation
is carried out using (utilizing, employing, through) a UV
lamp. At the end of the reaction, the product precipitates.
2-(4-Chlorophenylmethylene)-malononitrile
(3ad): mp 157–159°C, 97% yield, 1H NMR (300 MHz,
CDCl3) δ 7.84 (d, 2H, HAr), 7.65 (s, 1H, HC=C),
7.49 (d, 2H, HAr); FTIR (ATR, cm–1) 3081 (C–HAr),
2949 (C–Halkene), 2204 (C≡N), 1704 (C=Calkene), 1475
(C=CAr).
Scheme 2. Optimization for the synthesis of 3aa and 3ba.
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BENTAHAR et al.
Table 1. Effect of KF-modified clay amount on the yield of
2-(phenylmethylene)-malononitrile and ethyl (E)-2-cyano-3-
phenyl-2-propenoatea
CDCl3) δ 8.23 (s, 1H, HC=C), 7.96 (d, 2H, HAr), 7.51
(d, 2H, HAr), 4.41 (q, 2H, CH2), 1.42 (t, 3H, CH3); FTIR
(ATR, cm–1) 3081–2851 (C–HAr, C–Halkene, C–HCH
,
3
Entry
no.
Quantity,
mg
C–HCH ), 2233 (C≡N), 1621 (C=O), 1558 (C=Calkene),
1440 (C=CAr), 1190 (C–Oester).
Yield, %b
2
Products
t, min
3aa
3aa
3aa
3aa
3ba
3ba
3ba
3ba
1
2
3
4
5
6
7
8
10
20
30
40
10
20
30
40
3
3
3
3
8
8
8
8
90
99
99
97
84
91
90
90
Ethyl (E)-3-(4-Nitrophenyl)-2-cyano-2-propeno-
ate (3be): mp 129–131°C, 87% yield, 1H NMR
(300 MHz, CDCl3) δ 8.71 (s, 1H, HAr), 8.44 (2d in coin-
cidence, 2H, HAr), 8.33 (s, 1H, HC=C), 7.7 (t, 1H, HAr),
4.44 (q, 2H, CH2), 1.44 (t, 3H, CH3); FTIR (ATR, cm–1)
3074 (C–HAr), 2991–2928 (C–Halkene, C–HCH3), 2858
(C–HCH ), 2204 (C≡N), 1711 (C=O), 1517 (C=Calkene),
1475 (C2=CAr), 1078 (C–Oester).
a
Reaction condition: benzaldehyde (1 mmol), malononitrile or ethyl
cyanoacetate (1 mmol), room temperature, solvent: CH3OH.
RESULTS AND DISCUSSION
b Isolated yields.
The effect of the catalyst amount is a major factor
affecting the yield of the Knoevenagel condensation.
For this reason, we have studied the effect of the
amount of catalyst on the yield of the products obtained
during the condensation of benzaldehyde (1 mmol)
with malononitrile (1 mmol) or ethyl cyanoacetate
(1 mmol) in methanol at room temperature and varying
the amount of the catalyst from 10 to 40 mg (Table 1,
Scheme 2). The results gathered in Table 1 show that
increasing the catalyst amount from 10 to 20 mg results
in an increase in the yield of the desired product from
90 to 99% after 3 min for the malononitrile substrate
(entries 1 and 2, Table 1) and from 84 to 91% after 8
min for the cyanoacetate ethyl substrate (entries 5 and
6, Table 1). An amount of KF-modified clay greater than
20 mg does not influence the yield of the condensation
reaction of benzaldehyde with the two substrates (entries
3, 4, 7б and 8, Table 1). From these results, it can be
concluded that 20 mg is the optimal catalytic amount for
this condensation.
2-(3-Nitrophenylmethylene)-malononitrile (3ae):
mp 112–114°C, 93% yield, 1H NMR (300 MHz, CDCl3)
δ 8.59 (s, 1H, HC=C), 8.41 (d, 1H, HAr), 8.26 (d, 1H,
HAr), 7.8 (s, 1H, HAr), 7.72 (t, 1H, HAr); FTIR (ATR,
cm–1) 3123 (C–HAr), 2935 (C–Halkene), 2170 (C≡N),
1565–1467 (C=Calkene, C=CAr), 1329 (C–NO2 Ar).
Ethyl (E)-2-cyano-3-phenyl-2-propenoate (3ba).
1
mp 47–49°C, 91% yield; H NMR (300 MHz, CDCl3)
δ 8.28 (s, 1H, HC=C), 8.01 (d, 2H, HAr), 7.55 (m,
3H, HAr), 4.41 (q, 2H, CH2), 1.41 (t, 3H, CH3); FTIR
(ATR, cm–1) 3046–2845 (C–HAr, C–Halkene, C–HCH3
,
C–HCH2), 2198 (C≡N), 1684 (C=O), 1572 (C=Calkene),
1450 (C=CAr), 1182 (C–Oester).
Ethyl (E)-2-cyano-3-(4-methoxyphenyl)-2-prop-
1
enoate (3bb): mp 79–81°C, 87% yield, H NMR (300
MHz, CDCl3) δ 8.2 (s, 1H, HC=C), 8.03 (d, 2H, HAr),
7.01(d, 2H, HAr), 4.38 (q, 2H, CH2), 3.91 (s, 3H, CH3),
1.42 (t, 3H, CH3); FTIR (ATR, cm–1) 3080-2886 (C–
HAr, C–Halkene, C–HCH , C–HCH2), 2198 (C≡N), 2212
3
To concretize the solvent effect on the Knoevenagel
condensation in the presence of our catalyst, the reaction
was carried out by reacting 1 mmol of benzaldehyde with
1 mmol of malononitrile or ethyl cyanoacetate as shown
in Scheme 2 and keeping the same catalyst amount
(20 mg). The reaction is carried out at room temperature
and in various solvents.All the results found are grouped
in Table 2. Analysis of these results indicates that
the reaction appears to proceed more rapidly in polar
solvents such as methanol (dipole moment μ = 1.71),
ethanol (μ = 1.74), and acetonitrile (μ = 3.45) with 99, 90,
and 88%, respectively, after 3 min for the condensation
(C≡N), 1700 (C=O), 1579–1440 (C=Calkene, C=CAr),
1162 (C–Oester), 1000 (C–Oether).
Ethyl(E)-2-cyano-3-(4-methylphenyl)-2-propeno-
ate (3bc): mp 92–94°C, 89% yield, 1H NMR (300 MHz,
CDCl3) δ 8.22 (s, 1H, HC=C), 7.91 (d, 2H, HAr), 7.34
(d, 2H, HAr), 4.4 (q, 2H, CH2), 2.45 (s, 3H, CH3), 1.42
(t, 3H, CH3); FTIR (ATR, cm–1) 3060–3845 (C–HAr, C–
Halkene, C–HCH , C–HCH2), 2191 (C≡N), 1684 (C=O),
3
1544 (C=Calkene), 1467 (C=CAr), 1204 (C–Oester).
Ethyl (E)-3-(4-chlorophenyl)-2-cyano-2-propeno-
ate (3bd): mp 87–89°C, 90% yield, 1H NMR (300 MHz,
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THE MODIFIED CLAY AS A NEW AND ECO-FRIENDLY CATALYST
1441
Table 2. Influence of solvent on the yield of
2-(phenylmethylene)malononitrile and ethyl (E)-2-cyano-3-
phenyl-2-propenoatea
of benzaldehyde with malononitrile (entries 1, 2 and 3,
Table 2), and 91, 84, and 79% after 8 min in the case of
using the cyanoacetate ethyl substrate (entries 8, 9 and
10, Table 2), with a preference for protic solvents. For
moderately polar solvents as chloroform (μ = 1.15) and
dichloromethane (μ = 1.55), the yields obtained are low
for the same duration of the reaction (entries 5, 6, 12,
and 13, Table 2). We note that the reaction gave lower
yields for the two condensations in water (entries 4 and
11, Table 2) which is very polar (μ = 1.80) and this may
be due to the non-solubility of reagents in this solvent
as well as a possible interaction of the latter with our
catalyst. From these results, it can be concluded that only
protic solvents characterized by high polarity solubilize
the reactants and promote this condensation [19].
Entry Products
Solvents
CH3OH
t, min
Yield, b
3aa
3aa
3aa
3aa
3aa
3aa
3aa
3ba
3ba
3ba
3ba
3ba
3ba
3ba
1
3
3
3
3
3
3
_
8
8
8
8
8
8
–
99
90
88
7
2
EtOH
3
CH3CN
H2O
4
5
CHCl3
6
6
CH2Cl2
Petroleum ether
CH3OH
EtOH
4
7
_
8
91
84
79
8
9
10
11
12
13
14
CH3CN
H2O
In order to expand the reaction to other reagents,
we have carried out the condensation of 1 mmol of
substituted benzaldehyde with 1 mmol of malononitrile
or ethyl cyanoacetate under conditions previously
optimized (20 mg of KF-modified clay in methanol).
The results obtained are summarized in Table 3. We
note that excellent yields were obtained for both
condensations. The kinetics of the reaction involved
CHCl3
7
CH2Cl2
Petroleum ether
4
–
a
Reaction condition: benzaldehyde (1 mmol), malononitrile or ethyl
cyanoacetate (1 mmol), room temperature, catalyst amount 20 mg.
b Isolated yields.
Table 3. Effect of other benzaldehyde substrates on the yield of reactionsa
Entry
R
Products
3aa
Time
3 min
Yield, %b
mp, °C
81–83
mp, °C, references
83–83.5 [10]
113–115 [10]
134.5–135 [22]
162–163 [10]
100–101 [23]
50–51 [10]
1
2
3
4
5
6
7
8
9
10
H
99
94
95
97
93
91
87
89
90
87
3ab
4-OCH3
4-CH3
4-Cl
2–4 s
2–4 s
2–4 s
1 min
8 min
5 min
5 min
5 min
8 min
108–110
129–131
157–159
112–114
47–49
3ac
3ad
3ae
3-NO2
H
3ba
3bb
3bc
4-OCH3
4-CH3
4-Cl
79–81
80–81 [10]
92–94
90–92 [8]
3bd
3be
87–89
89.5–90.5 [22]
129–131 [24]
3-NO2
129–131
a
Reaction condition: Various aromatic aldehydes (1 mmol), malononitrile or ethyl cyanoacetate (1 mmol), catalyst amount= 20 mg, solvent:
CH3OH, room temperature.
b Isolated yields.
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BENTAHAR et al.
Table 4. Comparison with other heterogeneous catalysts
Products synthesized
Catalyst
PVC-TEPA
C/Z-30/70
Solvent
EtOH
T, °C
Reflux
Reflux
r. t
Time
1 h
R, %
75
References
[10]
[25]
EtOH
MeOH
EtOH
EtOH
MeOH
EtOH
EtOH
MeOH
50 min
3 min
2.5 h
75 min
8 min
1 h
82
99
80
78
91
91
89
94
Modified clay
PVC-TEPA
C/Z-30/70
In this study
[10]
Reflux
Reflux
r. t
[25]
Modified clay
PVC-TEPA
Chitosan
In this study
[10]
Reflux
40
6 h
[26]
Modified clay
r. t
2-4 s
In this study
C/Z-30/70
EtOH
Reflux
r. t
60 min
2-4 s
78
[25]
Modified clay
MeOH
95.2
In this study
C/Z-30/70
EtOH
Reflux
r. t
90 min
5 min
72
[25]
Modified clay
MeOH
89.05
In this study
PVC-TEPA
C/Z-30/70
Chitosan
EtOH
EtOH
EtOH
MeOH
EtOH
Reflux
Reflux
40
1h
90
81
85
97
78
[10]
60 min
6h
[25]
[26]
Modified clay
C/Z-30/70
r. t
2-4 s
60 min
In this study
[25]
Reflux
Modified clay
MeOH
r. t
5 min
90
In this study
C/Z-30/70
EtOH
Reflux
r. t
45 min
8 min
83
87
[25]
Modified clay
MeOH
In this study
are very rapid for the para-substituted benzaldehyde,
which varies from the instantaneous for the first series
(entries 2 to 4, Table 3) to a few minutes for the second
(entries 7 to 9, Table 3). The difference noted for the
yields of all the reactions is rather due to the recovery
and purification of the product and not to the state at the
end of the reaction since they are all complete. We also
note that the electron-donating groups (by the inductive
or mesomeric effect) have a very positive effect on
the reaction rate. These effects increase the electronic
charge around the oxygen atom of the carbonyl group.
We deduce that the first intervention of the aldehyde
in the reaction is done through this oxygen atom. On
the other hand, it can be noticed that the reactivity of
malononitrile is faster than that of ethyl cyanoacetate.
This can be attributed to the fact that the CN group is
more attractive than the CO2Et group, which makes the
proton of the methylene group more acidic in the case
of malononitrile than in the case of ethyl cyanoacetate
[20, 21].
The reuse of modified clay was carried out after
condensation of 1 mmol of benzaldehyde with 1 mmol of
malononitrile in the presence of 20 mg of KF-modified
clay at room temperature. After each reaction, the
catalyst was recovered by simple filtration and washed
successively with acetone and ether for 30 min and
dried at 80°C for 8 h [18]. The catalyst is then reused in
successive cycles under the same operating conditions.
The results obtained are grouped in Fig. 1. We note from
Fig. 1 that the yield of compound 3aa remains constant
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THE MODIFIED CLAY AS A NEW AND ECO-FRIENDLY CATALYST
ACKNOWLEDGMENTS
1443
100
80
60
40
20
0
We sincerely thank the Professors A. Khallaayoun for him
valuable assistance.
CONFLICT OF INTEREST
The authors declared no conflict of interest
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