Simple Protocol for the Knoevenagel Condensation under Solvent Free Conditions using Tungstophosphoric Acid as Catalyst

Article abstract of DOI:10.14233/ajchem.2019.22072

The effect of calcination on the performance of tungstophosphoric acid for the product of Knoevenagel condensation was investigated. Substituted aldehydes and dimedone has been used in the presence of calcined tungstophosphoric acid as a heterogeneous catalyst using grinding method at room temperature. The results of reactions revealed that calcined tungstophosphoric acid has superior catalytic activity comparing to non-calcined catalyst in terms of yield and reaction time. Maximum yield of model compound was achieved by using 10 mol% of calcined catalyst in a reaction time that does not exceed 10 min, whereas the yield at same amount of non-calcined catalyst was 86 % in a reaction time of 35 min.

Full text of DOI:10.14233/ajchem.2019.22072

Asian Journal of Chemistry; Vol. 31, No. 10 (2019), 2181-2184
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2019.22072
Simple Protocol for the Knoevenagel Condensation Under Solvent
Free Conditions using Tungstophosphoric Acid as Catalyst
ABDULRAHMAN I. ALHARTHI
Department of Chemistry, College of Science and Humanities, Prince Sattam Bin Abdulaziz University, P.O. Box 83, Al-Kharj 11942, Saudi Arabia
Corresponding author: E-mail: a.alharthi@psau.edu.sa
Received: 13 March 2019;
Accepted: 22 April 2019;
Published online: 30 August 2019;
AJC-19519
The effect of calcination on the performance of tungstophosphoric acid for the product of Knoevenagel condensation was investigated.
Substituted aldehydes and dimedone has been used in the presence of calcined tungstophosphoric acid as a heterogeneous catalyst using
grinding method at room temperature. The results of reactions revealed that calcined tungstophosphoric acid has superior catalytic
activity comparing to non-calcined catalyst in terms of yield and reaction time. Maximum yield of model compound was achieved by
using 10 mol% of calcined catalyst in a reaction time that does not exceed 10 min, whereas the yield at same amount of non-calcined
catalyst was 86 % in a reaction time of 35 min.
Keywords: Tungstophosphoric acid, Knoevenagel condensation, Calcination, Catalyst.
INTRODUCTION
EXPERIMENTAL
Knoevenagel condensation is one of the most important
organic reactions [1]. The reaction is generally performed bet-
ween condensation of active methylene and carbonyl compounds
in presence of some basic [2,3] or Lewis acid catalyst [4-8]. The
Knoevenagel condensation products are further used in various
organic reactions such as Michael additions and Diels-Alder
reactions [9]. It has many applications including therapeutic
drugs, perfumery, cosmetics, polymers and insecticides [10].
Heteropoly acid (HPA) has special Keggin structure having
bronsted acid properties which has been used in many organic
reactions [11,12]. Tungstophosphoric acid is one of the most
important members of heteropoly acid family having high
thermal as well as high solubility in polar solvents [13]. Upon
calcinations, acidity of tungstophosphoric acid is increased
and acidic points on the surface developed [14,15].
Calcination of catalyst:An accurately weighed tungsto-
phosphoric acid was placed in a crucible and transferred in a
tube furnace for calcination at around 300 ºC upto 4 h.
General procedure for the synthesis of benzylidene
derivatives (3a-f) via one-pot two component reaction using
tungstophosphoric acid: A mixture of aldehydes (2 mmol)
and dimedone (2.5 mmol) were grounded in the presence of
varying mol % of calcined as well as non-calcined tungsto-
phosphoric acid at room temperature. The resulting product
was obtained by recrystallization of ethanol (Scheme-I).
Spectral data of few selected compounds
2-(4-Chlorobenzylidene)-5,5-dimethylcyclohexane-1,3-
dione (3c): White, m.p.: 240 ºC, m.f. C15H15O2Cl. FT-IR (ATR,
ν
max, cm-1); 2950 (C=H), 1678, 1658 (2C=O), 1623 (C=C); 1H
NMR (DMSO, δ ppm): 7.95 (s, 1H, CH=), 7.17-7.29 (4H, m,
In the present work, tungstophosphoric acid catalyst was
calcined and used for the synthesis of Knoevenagel conden-
sation product. Selected compounds were evaluated on the basis
of catalyst amount, yield and reaction time by running two
parallel reactions for calcined as well as non-calcined tungsto-
phosphoric acid.
13
Ar-H); C NMR (DMSO, δ ppm): 196.59, 163.56, 143.72,
130.39, 128.31, 114.42, 50.42, 32.34, 31.41, 29.07, 26.95.
2-(4-Methoxybenzylidene)-5,5-dimethylcyclohexane-
1,3-dione (3d):Yellowish white, m.p.: 250 ºC, m.f. C16H18O3.
FT-IR (ATR, νmax, cm-1): 2957 (C=H), 1677, 1607 (2C=O),
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2182 Alharthi
Asian J. Chem.
O
A comparative study of FT-IR for calcined (HPW-Cal)
and non-calcined (HPW) sample is depicted in Fig. 2. From
the IR spectra, it is evident that Kegging structure unaltered
upon heat treatment [16]. A clear defined peak at 1075.48 and
963.93 cm-1 was observed for P-O and W=O stretching vibra-
tions. On the other hand, a peak at 883.07 cm-1 signified for
W-O-W modes of vibration.
Ar
O
O
HPW
ArCHO
room temp.
O
1
3a-f
2
Scheme-I: Synthesis of benzylidene derivatives (3a-f) from a aromatic
aldehydes (1) and dimedone (2)
1584 (C=C); 1H NMR (DMSO, δ ppm): 7.89, (s, 1H, CH=),
6.76-7.07 (4H, m, Ar-H); 13C NMR (DMSO, δ ppm): 196.58,
163.14, 158.00, 136.88, 129.45, 115.06, 113.68, 55.37, 50.51,
32.32, 30.74, 29.14, 26.93.
HPW
HPW-Cal
200
5,5-Dimethyl-2-(2,4,6-trimethoxybenzylidene)cyclo-
hexane-1,3-dione (3e): White, m.p.: 207 ºC, m.f. C18H22O5.
FT-IR (ATR, νmax, cm-1): 2955 (C=H), 1665, 1625 (2C=O),
100
1
1591 (C=C); H NMR (DMSO, δ ppm): 7.26 (s, 1H, CH=),
6.41-6.61 (2H, m, Ar-H); 13C NMR (DMSO, δ ppm): 196.69,
163.53, 152.80, 140.36, 136.38, 114.71, 105.84, 60.37, 56.23,
50.50, 32.33, 31.63, 29.16, 26.81.
1075.48
1614.33
0
2-(Indoline-3ylmethylene)-5,5-dimethylcyclohexane-
1,3-dione (3f): Light pink, m.p.: 112 ºC, m.f. C17H19NO2. FT-
IR (ATR, νmax, cm-1): 2930 (C=H), 1631, 1611 (2C=O), 1575
746.81
963.93
3000
2500
2000
1500
1000
500
1
Wavenumber (cm–1)
(C=C); H NMR (DMSO, δ ppm): 8.29 (s, 1H, CH=), 7.20-
8.10 (4H, m,Ar-H); 13C NMR (DMSO, δppm): 185.45, 138.94,
137.50, 123.93, 122.59, 121.28, 112.88, 102.88, 32.59, 28.41.
Fig. 2. FT-IR spectra analysis for non-calcined (HPW) and calcined
tungstophosphoric acid (HPW-Cal)
RESULTS AND DISCUSSION
The TGA profile for tungstophosphoric acid catalyst
(HPW) is shown in Fig. 3. It can be seen that there are three
weight loss regions at about 50, 175 and 450 ºC, respectively.
The first weight loss, around 5 % can be attributed to the removal
of physisorbed water, whereas the second weight loss, roughly
3 %, can be ascribed to the bonded water to acidic protons in
H3PW12O40. The weight loss beyond 450 ºC was due to the
loss of the remaining water molecules that corresponds to the
loss of all acidic protons and the beginning of decomposition
of the Keggin structure. This result is consistent with previous
studies [17]. This indicates that calcination of HPW at 300 ºC
does not effect the structure of HPW, in good agreement with
FT-IR spectrum, and increases the acidity of HPW by removal
of water molecules.
Characterization of the catalyst was performed using
different techniques such as XRD, SEM and FT-IR analysis
before and after calcination. The powder XRD patterns of non-
calcined and calcined tungstophosphoric acid samples are
illustrated in Fig. 1. From the pattern of non-calcined tungsto-
phosphoric acid (HPW), it is obvious that there is a sharp
crystalline peak at 2θ = 5º which matches with the position of
tungsten metal. On the other hand, it was absent in calcined
tungstophosphoric acid (HPW-Cal), which can be attributed
to the evaporation of water molecules adhered before calci-
nation. Generally, tungstophosphoric acid appeared to be more
crystalline after calcination and eventually due course bronsted
acid capacity increased [14].
SEM images for non-calcined (HPW) and calcined tungsto-
phosphoric acid (HPW-Cal) are shown in Fig. 4. From the
HPW
HPW-Cal
102
100
98
0.4
0.2
200
100
0
0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
96
94
92
90
10
20
30
40
50
60
70
80
150
300
450
Temperature (°C)
Fig. 3. TGA analysis of tungstophosphoric acid
600
750
900
2θ (°)
Fig. 1. Powder X-ray diffraction patterns for non-calcined (HPW) and
calcined tungstophosphoric acid (HPW-Cal)

Vol. 31, No. 10 (2019)
Knoevenagel Condensation Under Solvent Free Conditions using Tungstophosphoric Acid as Catalyst 2183
Fig. 4. SEM images of tungstophosphoric acid before (HPW) and after calcination (HPW-Cal)
images, it is possible to understand the morphology of catalyst
and the changes that have been occurred on it. At the same
magnification, shape of both physical forms was quite different.
For non-calcined tungstophosphoric acid, it seems to be aggre-
gated as flakes and bit amorphous. However, it was observed
that calcined sample showed cubical shaped catalyst particles
which support the XRD result (Fig. 1), where the particles
seemed to be more crystalline.
Reaction optimization: Benzylidene derivatives (3a-f)
were synthesized from aldehydes and dimedone in the presence
of tungstophosphoric acid catalyst. An optimized condition
was achieved using calcined and non-calcined tungstophosphoric
acid. The synthesis of compound 3e from 2,4,6-trimethoxy
benzaldehyde and dimedone was considered as a model comp-
ound to evaluate the outcome of calcined (P) and non-calcined
(Q) tungstophosphoric acid in a solvent free condition as shown
in Table-1.
(Table-1, entries 4 and 9). Again, the amount of catalyst was
increased to 15 % using calcined (P) and non-calcined (Q)
tungstophosphoric acid and no further increase in yield was
noted (entries 5 and 10). Thus, 10 mol % of catalyst is the optimum
amount for such reaction. Moreover, amount and physical nature
of catalyst played a crucial role to get the results.
Above results also highlighted the importance of calcined
(P) version of catalyst as compared to non-calcined (Q). Further-
more, catalytic activity of calcined tungstophosphoric acid can
be attributed to the increase in Brønsted acid property [14,15].
To established the scope of Knoevenagel condensation reaction,
aldehydes were reacted with dimedone in the presence of calcined
(P) as well as non-calcined tungstophosphoric acid (Q) (Table-2).
TABLE-2
CALCINED (P) AND NON-CALCINED (Q)
TUNGSTOPHOSPHORIC ACID (10 % mol) CATALYZED
KNOEVENAGEL CONDENSATION REACTION
Time (min)
Yield (%)
TABLE-1
Entry
1
R
Product
P
Q
P
Q
EFFECT OF CALCINED (P), NON-CALCINED (Q)
TUNGSTOPHOSPHORIC ACID SYNTHESIS OF
MODEL COMPOUND 3e
3a
20
45
92
80
Yield2
(%)
Time
(min)
Temp.
(°C)
OH
Catalyst
(mol %)
Entry1
OCH3
2
3b
12
38
96
84
1
2
3
4
5
6
7
8
9
P (4)
P (6)
P (8)
P (10)
P (15)
Q (4)
Q (6)
Q (8)
Q (10)
Q (15)
70
77
85
100
91
55
65
74
86
78
45
38
25
10
20
60
50
42
35
40
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
Cl
OCH3
O
3
4
3c
14
15
42
40
95
94
85
85
3d
O
5
6
3e
3f
10
15
35
50
100
95
86
81
O
10
1Reaction conditions were optimized to 100 % conversion. 2Isolated
yield.
N
H
The yield of product 3e of model reaction was increased
with the amount of catalyst increased. The product yield (%)
was evaluated with the catalyst mol % and time (min). Maximum
yield (100 %) of model compound was obtained using 10 mol %
of calcined catalyst with the less expense of time (10 min), but
at the same amount of a non-calcined catalyst only 86 % yield
was achieved with the reaction completion time of 35 min
Catalyst reusability: Calcined tungstophosphoric acid
was further evaluated to assess the reusability ability after four
runs considering model reaction under mild conditions. After
reaction completion, water was added to separate the catalyst
as their high solubility in aqueous system. The product was
then washed again to remove any trace amount of catalyst and
purified by ethanol. The catalyst recovered by first reaction

2184 Alharthi
Asian J. Chem.
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A simple protocol was used for Knoevenagel condensation
reactions using aldehydes and dimedone in the presence of
catalytic amount of tungstophosphoric acid at room temper-
ature under solvent-free conditions. Work-up procedure has
been very easy and reaction was completed in few minutes which
offered an excellent yield. In conclusion, calcined tungstophos-
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CONFLICT OF INTEREST
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