Green condensation reaction of aromatic aldehydes with active methylene compounds catalyzed by anion-exchange resin under ultrasound irradiation

Article abstract of DOI:10.1016/j.ultsonch.2014.07.018

To realize a practical and green chemistry, two important challenges need to be addressed, namely the effective process for the activation of reaction and efficient, eco-friendly and robust chemical methods for the reaction conversion to target products v

Full text of DOI:10.1016/j.ultsonch.2014.07.018

Ultrasonics Sonochemistry 22 (2015) 559–564
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Green condensation reaction of aromatic aldehydes with active
methylene compounds catalyzed by anion-exchange resin under
ultrasound irradiation
⇑
Hafedh Belhadj Ammar , Manef Chtourou, Mohamed Hédi Frikha, Mahmoud Trabelsi
Laboratory of Applied Chemistry: Heterocyclics, Fats and Polymers, Faculty of Sciences, University of Sfax, Soukra Street Km 3.5, 3038 Sfax, Tunisia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 June 2014
Received in revised form 22 July 2014
Accepted 23 July 2014
Available online 30 July 2014
To realize a practical and green chemistry, two important challenges need to be addressed, namely the
effective process for the activation of reaction and efficient, eco-friendly and robust chemical methods
for the reaction conversion to target products via highly selective catalytic and reactions. Ultrasonic
energy promotes the conversion process through its special cavitational effects. Combined with
anion-exchange resin as a heterogeneous, reusable and efficient catalyst, Ultrasonic energy enhances
the Knoevenagel condensation and leads to reduced reaction time at lower reaction temperature with
less amounts of solvent and catalyst.
Keywords:
Green chemistry
Ultrasonic energy
Anion-exchange resin
Knoevenagel condensation
Ó 2014 Published by Elsevier B.V.
1
. Introduction
The application of ultrasound irradiation in organic synthesis
the energy intensity depends on the mean behavior of bubbles,
the population of active cavitation determines the cavitational effi-
ciency [14]. One of the most important aspects of applying Ultra-
sonic energy to systems is how its energy is transferred to
reactant solutions. This has three steps: 1) the transformation of
electrical input into mechanical energy through a piezoelectric or
piezomagnetic transducer; 2) the delivery of vibrational energy
(acoustic energy) from the emission tip of the transducer to the
liquid medium; 3) the conversion of the energy of ultrasonic
streaming to the energy that activates reactants by acoustic cavita-
tion [15]. Conventionally, convective heating devices such as heat
baths or electric heating mantles are used for these reactions,
which have major adverse effects on the environment as well as
consumption of energy for heating and cooling [16].
has been largely extended in recent years [1–9]. Compared with
traditional methods, ultrasound has been shown to have desirable
effects on both homogeneous and heterogeneous reactions, such as
good yields, short reaction times, easier work-up procedure, for-
mation of pure products in milder conditions and waste minimiza-
tion [6]. Ultrasound irradiation method is now recognized as a
viable and environmentally-benign alternative. Therefore, it is
more convenient for green chemistry [10]. Ultrasound enhance-
ments may be attributed either to its chemical or mechanical
effects or to both of them simultaneously. The chemical effect of
ultrasonic comes from local hotspots produced by cavitation.
Moreover, ultrasound is a mechanical acoustic wave with the fre-
quency [11]. It imparts high energy to reaction medium by cavita-
tion and secondary effects [12]. As regards cavitation, it is a
phenomenon characterized by the formation of vapor bubbles in
a liquid. This bubble formation occurs when a pressure drop pro-
duces a new thermodynamic state corresponding to a point on
the saturation curve. It is worthy to note that cavitation is the
major mechanism for ultrasound intensification [13]. The key to
the efficient application of ultrasound is the control and selection
of the energy intensity and population of active cavitation. While
Knoevenagel condensation reaction is one of the most primitive
route for the synthesis of a,b-unsaturated carbonyl compounds by
the condensation of aldehydes or ketones with active methylene
compounds [12]. Since it was introduced by Knoevenagel in 1896
[17], it has drawn so much attention, especially in the preparation
of a range of substituted alkenes and bioactive molecules. It is also
regarded as a key step in the synthesis of natural products, thera-
peutic drugs and pharmacological products [18]. Overall, the Knoe-
venagel condensation is carried out homogenously using
nitrogenous molecules such as aliphatic amines, urea and piperi-
dine or their corresponding ammonium salts and amino acids
[19,20]. Several Lewis bases and acids have also been reported as
catalysts in the Knoevenagel condensation, including phosphates
⇑
Corresponding author.
E-mail address: hbelhadjammar@yahoo.fr (H.B. Ammar).
http://dx.doi.org/10.1016/j.ultsonch.2014.07.018
350-4177/Ó 2014 Published by Elsevier B.V.
1

5
60
H.B. Ammar et al. / Ultrasonics Sonochemistry 22 (2015) 559–564
such as [(NP)/KF, NP/NaNO
and K NiP [23]. There are also reports on the use of ZrO
ZnCl [25], CdI [26], TiCl [27], Al [28], Ni–SiO
and ZnO [30], AlPO –Al [31], KF–Al [32], SiO
33], ionic liquid [34], fly ash supported CaO [35], poly(vinyl chlo-
3
] [21], Na
2
CaP
2
O7 [22], Ca
2
P
2
O
7
[22]
[24],
with hot ethanol (10 mL). Afterwards, the sample was taken and
analyzed by GC to determine the yield of the reaction.
2
2
O
7
2
2
2
4
2
O
3
2
[29], MgO
–NH OAc
4
2
O
3
2
O
3
2
4
2.3. Spectroscopic analysis
[
ride) supported tetraethylenepentamine [36], enzyme [37] and a
proline-functionalized polyacrylonitrile fiber [38] as catalysts in
the Knoevenagel condensation.
In general, no further purification method was required. All the
products were previously reported and characterized by the melt-
1
13
ing point, IR, H NMR, C NMR.
In most cases, organic synthesis catalyzed with homogeneous
basic or acid medium has various disadvantages, such as the cata-
lyst recovery and generation of secondary products. Besides, most
organic syntheses catalyzed under homogeneous conditions
involve not only the use of hazardous solvents, but also expensive
and toxic reagents, as well as special efforts needed to prepare cat-
alysts and starting materials.
Anion-exchange resin may be considered as an insoluble base,
and might consequently be expected to advance reactions cata-
lyzed by conventional bases. Only a few examples of reactions
catalyzed by anion-exchange resin are reported in the literature
The spectral data of some isolated compounds, taken as repre-
sentative examples, are listed below.
Ethyl (E) 2-cyano-3-(2-methoxyphenyl)-2-propenoate (c): IR [v,
ꢀ1
cm ] 1590 (C@C), 3073 (C@CAH), 2224 (CN); 1726 (OAC@O);
1
H NMR [d, ppm] 1.36 (3H, t, J = 7.2 Hz); 3.87 (3H, s), 4.34 (2H, q,
J = 7.2 Hz), 7.02 (1H, t), 7.47 (1H, m), 8.25 (1H, dd, J = 1,5 Hz and
13
J = 7.8 Hz), 8.72(1H, s); C NMR [d, ppm] 13.6, 55.2, 61.9, 101.9,
1
10.7, 115.4, 120.2, 120.4, 128.8, 134.5, 149.2, 158.7 162.3.
ꢀ1
2
-(2-methoxbenzylidene) malononitrile (i): IR [v, cm ] 1599
1
(
C@C), 3047 (C@CAH) 2221.44 (CN); H NMR [d, ppm] 3.90 (3H,
s), 7.03(2H, m), 7.56 (1H, m), 8.15 (1H, dd, J = 1.2 Hz; J = 7.1 Hz),
[
39–42]. The advantages of insoluble catalysts lies in the fact that
13
8
1
.27(1H, s); C NMR [d, ppm] 55.5, 80.9, 111.0, 112.5, 113.8,
the separation problems are manifestly simpler, these catalysts
are regenerated and can be recycled several times, and sensitive
molecules can, in some cases, react without polymerization or
other reactions. Moreover, ultrasound reactions using green cata-
lysts such as anion-exchange resin are attractive in the growing
field of green and more sustainable chemistry.
19.7, 120.7, 128.4, 136.0, 154.0, 158.5.
3. Results and discussion
3.1. Ultrasonic induced reaction
This research work is interested in developing cleaner reaction
profiles and operational simplicity for the Knoevenagel condensa-
tion of aromatic aldehydes with active methylene groups catalyzed
by anion-exchange resins under ultrasound irradiation. This
method allows for a high yield in a short time under mild reaction
conditions. A reusable, easily separable, eco-friendly and highly
effective resin-catalyst is also reported.
A
series of preliminary tests between the stoichiometric
amount of benzaldehyde and ethyl cyanoacetete in small volumes
of ethanol (2 mL) over anion-exchange resin through conventional
9
8
7
6
5
4
0
0
0
0
0
0
2
. Experimental
2.1. Chemicals and apparatus
All reagents were purchased and used without further purifica-
IRA-410 St
IRA-96 St
tion. Two commercial anion-exchange resins (IRA-410 and IRA-96)
were used as catalysts for the preliminary reactions between
benzaldehyde and ethyl cyanoacetete. The two basic resins (IRA-96
30
IRA-410 US
IRA-96 US
2
1
0
0
ꢀ
and IRA-410) are marketed as Cl , so they are activated by a
sodium hydroxide solution to be used as catalysts for the prelimin-
ary reactions between benzaldehyde and ethyl cyanoacetete.
The ultrasonication was performed in a Bioblock ꢀ750 W ultra-
sound cleaner with a low frequency of 20 kHz (amplitude of 30%).
The melting points were determined by using Perkin Elmer
Spectrum apparatus version 10. The yields of the reactions were
determined using an analysis by gas chromatography (GC). The
device type is ‘‘Shimadzu GC-2014’’, equipped with an FID detector
and a capillary column DB-5.
0
10 20 30 40 50 60 70 80 90 100
Time (min)
Fig. 1. Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate
through the conventional stirring and ultrasound irradiation.
90
8
7
6
5
0
0
0
0
The NMR of the isolated products was recorded in solution in
1
13
CDCl
3
on a spectrometer type AC Bruker ( H at 350 MHz and
C
at 75 MHz). The internal reference was CDCl
3
.
2.2. General procedure for Knoevenagel condensation
IRA-410
IRA-96
Aldehyde (10 mmol), the active methylene compound
10 mmol), 0.20 g/0.01 mol is the ratio of resin and 2 mL of ethanol
40
(
3
0
were charged in a 10 mL glass reactor. The glass was located at the
maximum energy area in the ultrasonic cleaner and the addition or
removal of water was used to control the temperature of the water
bath at room temperature (25–30 °C). After each test, the reaction
mixture was filtered to recover the catalyst. It was then washed
5
10
15
20
25
30
Frequency (KHz)
Fig. 2. Effect of ultrasonic frequency on Knoevenagel condensation of benzaldehyde
with ethyl cyanoacetate.

H.B. Ammar et al. / Ultrasonics Sonochemistry 22 (2015) 559–564
561
1
00
The comparison of the results shown in Fig. 1 reveals the
remarkable influence of ultrasonic activation on the kinetics of
the reaction. Compared to the magnetic stirring, ultrasonic activa-
tion has achieved the reactions in a very short time. For example,
using IRA-410, the yield passes from 42% through the conventional
stirring to 58% after 60 min under ultrasound irradiation, and using
IRA-96, it passes from 49 to 68% after 60 min.
9
0
0
0
0
0
0
8
7
6
5
4
IRA-410
IRA-96
3.2. Ultrasonic frequency
As shown in Fig. 2, the experimental data indicate that the acti-
vation energy increases with the increase in ultrasound frequency
at 10% of amplitude. The optimal yields of 78% for IRA-410 and 86%
or IRA-96 were obtained at the frequency of 20 kHz after 60 min,
from which the reaction performance did not have any significant
change.
0
5
10 15 20 25 30 35 40 45
Amplitude (%)
Fig. 3. Effect of ultrasonic amplitude on Knoevenagel condensation of benzalde-
hyde with ethyl cyanoacetate.
However, ultrasonic frequency influences the behavior of bub-
ble cavitation through the change in the duration time of acoustic
cycle. On the other hand, high frequency does not favor the occur-
rence of active cavitations as the time for the growth, radial motion
and collapse of bubbles may be insufficient [43].
1
00
9
8
7
6
5
4
3
2
0
0
0
0
0
0
0
0
3.3. Ultrasonic amplitude
As can be seen from the Amplitude effect study in Fig. 3, the
reaction conversation reaches its maximum at 30% of ultrasonic
amplitude at the frequency of 20 kHz where the yields are using
IRA-410 and IRA-96 after 60 min.
IRA-410
IRA-96
Generally, the increase of ultrasonic power means that the
higher intensity of ultrasound was introduced into the reaction
vessel, which would accelerate the reactions. It can be seen that
such increase led to relatively higher yield and shorter reaction
time before the ultrasound power intensity reached 30%, and then
the yield decreased slightly with the increase in ultrasound power
intensity. Ultrasound which is used in heterogeneous chemistry
has been shown to increase the reaction rate of processes by emul-
sifying liquid–liquid systems or by producing the erosion, disrup-
tion and dispersion of solid particles. The parameters affecting
the solid catalyst such as the weight percentage of the catalyst
and reusability in the reaction were studied.
0
.5
1.0
1.5
2.0
2.5
3.0
3.5
Catalyst ratio (g/0.01mol)
Fig. 4. Effect of catalyst ratio on Knoevenagel condensation of benzaldehyde with
ethyl cyanoacetate under ultrasound irradiation.
H
O
CN
Z
Z
resin
Ar
+
CH2
Ar
H
3.4. Catalyst ratio
CN
Scheme 1. Schematic representation of Knoevenagel condensation.
Solid catalysts provide a green and promising way for chemical
synthesis with less corrosion than homogeneous acid/base cata-
lysts and simplified recovery of the catalysts. The effect of catalyst
ratio was then studied. The same reaction model (0.01 mol each
reagent) was carried out using different amounts of resins at
stirring (St) or under ultrasound irradiation (US) at the frequency
of 10 kHz and 10% Amplitude were performed in order to optimize
the main experimental parameters involved in the conduct of the
Knoevenagel condensation reaction.
2
5–30 °C for 60 min under ultrasound irradiation. As shown in
Fig. 4, the maximum yield was obtained when the catalyst/reagent
Table 1
Knoevenagel condensation catalyzed by anion-exchange resins under ultrasound irradiation.
Entry
Ar
Z
Time (min)
Yield (%)
IRA-96
mp (°C)
IRA-410
Found
Reported
a
b
c
d
e
f
g
h
i
C
6
H
5
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CN
CN
CN
CN
CN
CN
Et
Et
Et
Et
Et
Et
60
15
60
60
60
60
15
15
15
15
15
15
96
98
93
85
90
84
79
92
93
89
90
87
85
87
88
49–50
87–88
72–74
85–86
53–54
91–92
81–82
67–68
85–86
114–115
94–95
159–160
50–51 [46]
86–87 [46]
_
86–87 [46]
52–53 [47]
89–90 [46]
80–82 [46]
65–66 [46]
_
2-Furyl
2-CH
4-CH
2-ClC
4-ClC
6 5
C H
2-Furyl
2-CH
4-CH
2-ClC
4-ClC
3
OC
OC
6
H
H
4
3
6
4
89
6
H
H
4
100
100
98
98
97
95
93
95
6
4
3
OC
OC
6
H
H
4
j
k
l
3
6
4
114–116 [46]
94–96 [47]
161–162 [46]
6
H
H
4
6
4

5
62
H.B. Ammar et al. / Ultrasonics Sonochemistry 22 (2015) 559–564
Table 2
the reaction and that a resin ratio equal to 0.25 g/0.01 mol is opti-
mal in these reaction conditions.
Comparison of our results with those of some reports.
Entry
a
Report
Yield (%)
85
Our work
Yield (%)
96
However, other variables can have ultrasound effects, such as
the concentration of solids (in our case the clay-catalyst) and
mainly the viscosity. Raso et al. [44] have confirmed that at higher
viscosities, cavitation is more difficult to induce and the number of
cavitating bubbles per unit volume is reduced. Besides, Duran-
Valle et al. [45] have reported that when one of the phases is a solid
the ultrasonic irradiation has several additional enhancement
effects, which are particularly convenient when the solid acts as
catalyst. The cavitation effects form microjects of liquid, which
bombard the solid surface, which causes the exposition of unre-
acted surfaces of solid, increasing the interphase surface able to
react. In general, the sonication presents beneficial effects on the
chemical reactivity. In fact, it can accelerate the reaction, reduce
the induction period and enhance the catalyst efficiency.
After optimizing the reaction conditions and in order to gener-
alize this model reaction, the reactivity of different aromatic alde-
hydes and various active methylene compounds such as ethyl
cyanoacetate and malononitrile (Scheme 1) was tested in the same
conditions: T = 25–30 °C, catalyst ratio = 0.25 g/0.01 mol under
ultrasound irradiation (frequency of 20 KHz, 30% amplitude). The
results of the obtained yields and selectivities are listed in Table 1.
The obtained results have revealed that, in general, the reac-
tions were clean and the isolated compounds were obtained after
crystallization in pure form (IR, NMR and GC) without further puri-
fication. The yields are less than 100% due to the remaining starting
reagents. Compared to the Knoevenagel reactions of malononitrile
with aromatic aldehydes, the reactions of ethyl cyanoacetate with
the same aromatic aldehydes need longer time. Because the elec-
tron withdrawing ability of the CN group is stronger than that of
carbonyl or carboxylic group, the methylene group of malononitri-
le is more activated than ethyl cyanoacetate.
Time (min)
Time (min)
60
480
180
240
900
150
240
150
570
180
420
24O
30
[33]
[35]
[38]
[29]
[36]
[38]
[36]
[33]
[24]
[33]
[38]
[24]
[36]
[29]
[24]
[46]
[36]
[24]
[46]
[33]
8
9
7
9
c
d
e
f
75
93
90
92
93
89
100
100
60
60
60
60
85
g
89
98
15
8
9
7
8
h
i
87
91
98
97
15
15
60
70
900
150
20
60
120
20
j
80
97
15
68
k
l
90
88
93
95
15
15
6
9
5
0
390
ratio is equal to 0.25 g/0.01 mol of each reagent. From this ratio the
yield remains constant. The results have demonstrated that the
maximum yield is obtained when the catalyst/reagents ratio is
equal to 0.25 g/0.01 mol of each reagent. A higher ratio (0.3 g/
0
.01 mol) seems to decrease the yield, because the reaction mix-
ture became very viscous, under a small volume of solvent, thus
reducing the diffusion of reagents molecules. This might be
explained by a higher adsorption of reagents on active sites of res-
ins. A lower catalyst ratio (0.2 g/0.01 mol) was shown to afford
reduced yield. It seems that this ratio is insufficient to catalyze
CH CH2 CH
CO Et
2
Resin
CO2Et
CH3
H
CN
.
.
CH2
N
CH CH2 CH
OH
CH3
−
H
H
H
CN
CH3
H
O
CH3
OH
CH2
N
CH3 CH3
HO
CO2Et
O
CO2Et
CN
H
CN
H
H
H
H
OH
−
H2O
H
Et = CH3−CH2−
CO2Et
CN
Scheme 2. The proposed mechanism of synthesis of Ethyl
a-cyanocinnamate.

H.B. Ammar et al. / Ultrasonics Sonochemistry 22 (2015) 559–564
563
IRA-410
IRA-96
can be easily recovered and reused several times without the sig-
nificant loss of its catalytic activity.
9
6
1
00
8
8
8
5
7
8
7
5
8
6
4
2
0
0
0
0
0
6
7
68
6
2
Acknowledgment
This work was financially supported by the Tunisian Higher
Education and Scientific Research Ministry.
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2
3
4
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.6. Catalyst reusability
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