A Simple and Highly Versatile Procedure for the Knoevenagel Condensation Promoted by an Efficient, Eco-Friendly, and Recyclable nano-ZnO Catalyst

Article abstract of DOI:10.1134/S1070428020020220

Abstract: A simple, highly versatile, and efficient protocol for the synthesis of substituted olefins from various aromatic aldehydes and active methylene compounds by Knoevenagel condensation using an efficient, eco-friendly, and recyclable nano-ZnO cata

Full text of DOI:10.1134/S1070428020020220

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2020, Vol. 56, No. 2, pp. 315–321. © Pleiades Publishing, Ltd., 2020.
A Simple and Highly Versatile Procedure
for the Knoevenagel Condensation Promoted
by an Efficient, Eco-Friendly, and Recyclable nano-ZnO Catalyst
a,
b
c
d
S. Tasqeeruddin *, Ya. I. Asiri , M. M. Alam , and S. Sh. Sulthana
a
d
Department of Pharmaceutical Chemistry, College of Pharmacy, King Khalid University, Abha, 61421 Saudi Arabia
b
Department of Pharmacology, College of Pharmacy, King Khalid University, Abha, 61421 Saudi Arabia
c
Department of Chemistry, College of Science, King Khalid University, Abha, 61421 Saudi Arabia
School of pharmacy and medical sciences, Singhania University, Pacheri Bari, Jhunjhunu, Rajasthan, 333515 India
*
e-mail: drsyedtasqeeruddin@gmail.com
Received August, 04, 2019; revised December 09, 2019; accepted December 20, 2019
Abstract—A simple, highly versatile, and efficient protocol for the synthesis of substituted olefins from vari-
ous aromatic aldehydes and active methylene compounds by Knoevenagel condensation using an efficient, eco-
friendly, and recyclable nano-ZnO catalyst in ethanol at a moderate temperature was developed. Due to its large
surface area, the nano-ZnO catalyst exhibited an excellent catalytic activity. The advantages of the method includ-
ed milder conditions, short reaction times, and easy work-up and purification of products by non-chromatographic
methods. The catalysts can be recovered for subsequent reactions and reused without any appreciable loss of
efficiency.
Keywords: Knoevenagel condensation, nano-ZnO catalyst, aromatic aldehydes, active methylene compounds,
substituted olefins
DOI: 10.1134/S1070428020020220
INTRODUCTION
ligands, as recyclable nanocatalysts [13–15]. The
synthesis of such systems have attracted intense interest
and has been reviewed extensively [16]. Many aldol [17–
Literature survey reveals that Knoevenagel reaction
is a facile and important method for the formation of
carbon–carbon bonds [1], with numerous applications
in the synthesis of intermediates of fine chemicals [2],
hetero-Diels–Alder reactions [3] and in the synthesis
of carbocyclic as well as heterocyclic compounds of
biological significance [4].
1
9], Knoevenagel [20–22], Henry [23, 24], and Michael
[25] reactions have been successfully carried out in
the presence of various effective heterogeneous base
catalysts and, in many cases, the solid base is recyclable
[26]. Acid-catalyzed methodologies include the use of
AlCl [27], dry HCl [28], Zn(bpy)(OAC) [29], TiCl
3
2
4
A number of carbon–carbon bond forming reactions
have been reported and their applications in different
fields of chemistry is well documented [5–9]. Base-
catalyzed chemical reactions, such as the Knoevenagel
[
30], CpZrH /NiCl [31], zeolites [32], RuCl [33], and
2 2 3
Selectfluor [34].
However, many of these procedures suffer from
one or more disadvantages such as long reaction time,
tedious workup procedure, and poor yields and required
large amounts of catalyst, which lead to environmental
pollution. Thus, a mild, efficient, and environmentally
friendly method for Knoevenagel condensation is
desirable.
[10], Henry [11] and aldol reactions [12], are frequently
used in both small- and large scale organic synthesis.
These reactions are commonly catalyzed by organic
amines and alkali metal alkoxides and hydroxides.
Although these catalysts are very effective, they are
quite difficult to separate and, in many cases, cannot
be recycled. In recent years, many researchers have
developed heterogeneous solid-supported catalytic
systems, based on metals, metal oxides, and organic
As part of our continuing effort on developing
environmentally benign synthetic methodologies, in
the present work we developed a mild and efficient
3
15

316
TASQEERUDDIN et al.
Fig 1. FTIR spectrum of the synthesized ZnO nanorods.
Fig 2. XRD pattern of the ZnO nanorods.
protocol for the Knoevenagel condensation of aromatic
aldehydes with active methylene compounds.
Experimental) and characterized by FTIR spectroscopy
(Fig. 1), XRD analysis (Fig. 2), and SEM (Fig. 3).
RESULTS AND DISCUSSION
The FTIR spectrum (Fig. 1) of the ZnO nanorods
shows an absorption band at about 515 cm–1 corres-
ponding to Zn–O stretching vibration modes, which is
a typical characteristic band of pure ZnO. The weak
Catalytic applications of metal nanoparticles are
always advantageous in various organic transformations.
The reason for this high catalytic efficiency is mainly
due to a high surface area–to–volume ratio of metal
nanoparticles. nano-ZnO catalyst is of special interest
ununderstable when compare to their macro counter part
due to their small size. They possess unique properties,
regardless of their chemical properties they generally
have high surface area to volume ratio.
–
1
band at 1394 cm may be due to symmetric stretching
modes of carboxylate groups, probably from a small
amount of residual zinc acetate used in the synthesis.
–1
The broad absorption bands at 3437 and 1456 cm are
assignable to the OH groups of water molecules on the
–1
ZnO surface. The bands at 2925 and 2856 cm are due
to acetate C–H stretching vibrations [36].
Synthesis and characterization of nano–ZnO
catalyst. Zinc oxide nanoparticles were synthesized as
by the procedure in [35] with a few modifications (see
The XRD pattern of the ZnO nanorods (Fig. 2)
showed strong diffraction peaks at 2θ 31, 34, 37, 47, 51,
5, 63, 67 and 69°, corresponding to (100), (002), (101),
102), (110), (103), (200), (112) and (201) lattice planes.
5
(
No other peaks were detected, providing evidence for a
high purity of the synthesized ZnO nanorods.
Fig. 3 shows the SEM image of the synthesized ZnO
nanorods prepared using PEG 2000 as a capping agent.
The average diameter of these nanorods is 35–70 nm. It
should be noted that the average diameter of most of the
synthesized nanorods, as estimated from SEM images,
is approximately 50 nm [37].
Optimization of conditions of the synthesis of
substituted olefins. We started with the condensation
reaction of benzaldehyde with ethyl cyanoacetate as a
model reaction in the presence of the nano-ZnO catalyst
(
3
5mol%)inaqueousethanolandobtainedethyl2-cyano-
-phenylacrylate in an excellent yield (Scheme 1).
Fig. 3. SEM image of the synthesized ZnO-PEG nanorods.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 2 2020

A SIMPLE AND HIGHLY VERSATILE PROCEDURE FOR THE KNOEVENAGEL CONDENSATION
317
Scheme 1.
H
H
CN
CO C H
2 2 5
CN
O
ZnO-Nano catalyst
+
o
C H OH/Reflux/80 C
2
5
CO C H
2 5
2
1
2
3
Table 1. Optimization of the amount of nano-ZnO catalyst
Run no.
Catalyst, mol %
Time, min
Yield, %a
1
2
3
4
2.5
5
60
30
30
30
60
98
70
65
10
15
a
Isolated yields.
After successful model reaction, we focused on the
optimization of the reaction conditions with respect
to the reaction temperature and time, solvent, and
amount of the catalyst. It was found the 5 mol % of the
catalyst was enough to complete the reaction and obtain
the highest yield of the final product. The results are
summarized in Table 1.
The model reaction between benzaldehyde and ethyl
cyanoacetate under the optimized conditions was also
used to assess the reusability of the nano-ZnO catalyst.
To this end, the catalyst was recovered by filtration from
the reaction mixture after dilution with ethyl acetate,
washed with methanol, and recycled in subsequent
experiments under similar reaction conditions. The yield
of ethyl 2-cyano-3-phenylacrylate in four consecutive
cycles remained comparable, which established the
recyclability and reusability of the catalyst without any
significant loss of activity (Table 3).
With 5 mol % of nano-ZnO catalyst, we performed
screening of various solvents and a solvent-free system
for the model reaction to find that the highest yield in
a shorter reaction time was obtained in ethanol under
reflux (Table 2).
Further on we studied the Knoevenagel condensation
of different aromatic aldehydes with different active
Table 2. Optimization of the reaction conditions at 5 mol % of nano-ZnO catalyst
Run no.
Solvent
No solvent
Ethanol
Time, min
160
120
30
Temperature
Reflux
rt
Yield, %a
20
1
2
3
4
5
6
7
8
9
60
Ethanol
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
98
Water
120
120
60
30
Ethylene glycol
DMF
40
60
Acetic acid
Acetonitrile
THF
60
75
60
70
60
60
1
0
DMS
60
60
a
Isolated yields.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 2 2020

318
TASQEERUDDIN et al.
Table 3. Recyclability of the nano-ZnO catalysta
Cycle no.
Catalyst recovery, %
Yield, %b
1
2
3
95
90
85
80
98
95
88
80
4
a
Reaction conditions: benzaldehyde (10 mmol), ethyl cyanoacetate (10 mmol), catalyst (5 mol %), aqueous ethanol (10 mL).
Isolated yields.
b
methylene compounds (Scheme 2). In all cases, the
was refluxed at 80°C for 5 h and then concentrated
to 10–15 mL and heated for an additional 7 h. The
fraction of the solution is removed after 30 min. The
resulting reaction mixture was stirred for 10 min with
50 mL of water. The upper layer was discarded again
after 30 min. This procedure was repeated twice. The
resulting ZnO nanorods were dried at 100°C in an
oven and characterized by FTIR spectroscopy (Fig. 1),
XRD analysis (Fig. 2), and SEM (Fig. 3).
reaction proceeded smoothly with 5 mol % of nano-
ZnO catalyst in ethanol under reflux. Aldehydes with
both electron-donor and electron-acceptor substituents
reacted equally easily and gave the corresponding olefin
derivatives in very good yields, but with electron-donor
aldehydes reacted the reaction times were slightly
shorter and the yields of the condensation products were
slightly lower (Table 4).
EXPERIMENTAL
Synthesis of Knoevenagel condensation products
All chemicals were purchased from Sigma–Aldrich
and used as received. All chemicals and reagents
used in the present study were of analytical grade.
The melting points were determined in open capillary
tubes and are uncorrected. Thin-layer chromatography
was run on Merck silica gel GF-254 plates with spot
visualization by exposure to iodine vapor or UV light.
The FTIR spectra were obtained on a Shimadzu 8000
spectrophotometer. The XRD patterns were recorded
on a Philips X-ray diffractometer (Cu Ka l radiation,
λ 1.7889°A, cobalt anode, 40 kV, 40 mA). The SEM
images were obtained on a KYKY-EM3200 digital
3
a–3j (general procedure). A mixture of aromatic
aldehyde 1 (10 mmol), active methylene compound
(10 mmol), and nano-ZnO catalyst (5 mol %) in
2
aqueous ethanol (10 mL) was stirred under reflux. The
formation of the products was monitored by TLC. After
completion of the reaction (by TLC), the mixture was
cooled to room temperature and filtered. The filtrate
was concentrated to obtain a crude product, which was
purified by recrystallization from aqueous ethanol.
Ethyl 2-cyano-2-p-tolylacrylate (3a). Colorless
–
1
solid, mp 91–92°C. IR spectrum, ν, cm : 3100, 2850,
2210, 1710, 1600, 1460, 1370, 1270, 1200, 1100. H
NMR spectrum (300 MHz, CDCl ), δ, ppm: 1.40 t (3H,
1
scanning electron microscope. The 1H and 13C NMR
spectra were recorded on Varian spectrometer.
3
J 7.1 Hz), 2.43 s (3H), 4.38 q (2H, J 7.1 Hz), 7.26 d (2H,
J 7.7 Hz), 7.86 d (2H, J 8.1 Hz), 8.21 s (1H).
Synthesis of the nano-ZnO catalyst. A mixture of
1
g of zinc acetate dihydrate and 0.3 g PEG 2000 (0.3 g)
in 126 mL of double distilled water was heated to
0°C until a to obtain a transparent solution formed.
Ethyl 3-(4-chlorophenyl)-2-cyanoacrylate (3b).
–1
6
White solid, mp 88–89°C. IR spectrum, ν, cm : 2985,
A solution of 1.2 g of KOH in 70 mL of water was
added to this solution, and the resulting mixture
2225, 1739, 1600, 1452, 1373, 1242, 1047, 933, 842,
1
788, 634. H NMR spectrum (300 MHz, CDCl ), δ, ppm:
3
Scheme 2.
R1
H
CN
R2
H
E
ZnO-Nano catalyst
O
+
o
C
H
OH/Reflux/80 C
2
5
R1
CN
1
2
3a–3j
1
2
R = Ph, 4-CH -Ph, 4-Cl-Ph, 4-OH-Ph, 4-OCH -Ph; R = CN, CO Et, 2-NO -Ph.
3
3
2
2
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 2 2020

A SIMPLE AND HIGHLY VERSATILE PROCEDURE FOR THE KNOEVENAGEL CONDENSATION
319
Table 4. Synthesis of various olefins catalyzed by nano-ZnO in aqueous ethanol
Product no.
Reactant 1
Reactant 2
Product 3
Time, min
35
Yield, %a
3
a
90
98
95
90
88
3
b
30
35
37
40
3
c
3
d
3
e
3
f
30
30
30
95
95
90
3
g
3
h
3
i
35
30
88
90
3
j
a
Isolated yields.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 2 2020

320
TASQEERUDDIN et al.
1
.38 t (3H, J 7.2 Hz), 4.38 q (2H, J 7.2 Hz), 7.48 d
2-(2-Nitrophenyl)-3-phenylacrylonitrile(3i).White
solid, mp 106–115°C. IR spectrum, ν, cm : 3098, 2936,
1
3
–1
(2H, J 8.5 Hz), 7.93 d (2H, J 8.5 Hz), 8.18 s (1H). C
NMR (300 MHz, CDCl ), δ, ppm: 14.26, 62.03, 103.37,
2845, 2506, 2401, 2204, 1930, 1900, 1802, 1502, 1511,
1332, 1204, 1087, 1028, 901, 840, 810, 742, 630. H
3
1
115.94, 129.50, 132.02, 139.49, 153.37, 161.87.
NMR spectrum (300 MHz, CDCl ), δ, ppm: 7.51 t (2H),
7.69 s (1H), 7.88 d (2H, J 7.9 Hz), 7.95 s (2H, J 7.9 Hz),
Ethyl 2-cyano-3-phenylacrylate (3c). White crys-
talline solid, mp 49–50°C. IR spectrum, ν, cm : 2924,
3
–
1
8
.31 t (2H, J 7.9 Hz).
2
1
224, 1726, 1604, 1529, 1452, 1367, 1265, 1199, 1093,
1
014, 954, 767, 684. H NMR spectrum (300 MHz,
3
-(4-Chlorophenyl)-2-(2-nitrophenyl)acrylo-
CDCl ), δ, ppm: 1.70 t (2H, J 7.1 Hz), 4.67 q (2H, J
nitrile (3j). White solid, mp 140–142°C. IR spectrum,
ν, cm–1: 3099, 2926, 2835, 2596, 2441, 2214, 1940, 1911,
3
7
(
.1 Hz), 7.84 t (2H, J 7.3 Hz), 8.30 t (2H, J 7.3 Hz), 8.55 s
1H).
Ethyl 2-cyano-3(4-hydroxyphenyl)acrylate (3d).
1817, 1519, 1510, 1342, 1203, 1097, 1008, 910, 858,
1
8
28, 750, 669. H NMR spectrum (300 MHz, CDCl ),
3
–
1
δ, ppm: 7.51 d (2H, J 6.8 Hz), 7.63 s (1H), 7.87 m (4H),
.31 d (2H, J 6.8 Hz). 13C NMR spectrum (300 MHz,
CDCl ), δ, ppm: 110.08, 116.94, 124.44, 126.80, 129.57,
Yellow solid, mp 169–171°C. IR spectrum, ν, cm :
8
3
1
414, 2235, 1716, 1579, 1512, 1402, 1271, 1207, 1172,
055, 910, 819, 734, 648. H NMR spectrum (300 MHz,
1
3
1
30.93, 131.30, 137.80, 140.24, 143.95, 148.03.
CDCl ), δ, ppm: 1.40 t (3H, J 7.1 Hz), 2.93 s (1H), 4.31 q
3
(
8
2H, J 7.1 Hz), 6.93 d (2H, J 8.8 Hz), 7.96 d (2H, J
.8 Hz), 8.30 s (1H).
CONCLUSIONS
We have developed an environmentally friendly,
Ethyl 2-cyano-3-(4-methoxyphenyl)acrylate (3e).
efficient, clean, and simple method for the synthesis of
substituted alkenes using nano-ZnO as a heterogeneous,
readily available, reusable, and eco-solid catalyst at
ambient temperature in an aqueous ethanol solvent.
Significant advantages of this method include a
reasonably simple experimental workup procedure and
catalyst preparation, ease of product isolation, high to
excellent yields, and short reaction time.
–
1
Yellow crystals, mp 79–80°C. IR spectrum, ν, cm :
2
1
990, 2916, 2215, 1710, 1584, 1561, 1513, 1431, 1262,
1
211, 1184, 1127, 1089, 1017, 837. H NMR spectrum
(
(
(
300 MHz, CDCl ), δ, ppm: 1.44 t (3H, J 7.1 Hz), 3.92 s
3
3H), 4.38 q (2H, J 7.1 Hz), 6.97 d (2H, J 9.0 Hz), 8.02 d
1
3
2H,J9.0Hz),8.20s(1H). CNMRspectrum(300MHz,
CDCl ), δ, ppm: 14.2, 55.6, 62, 99.36, 114.7, 116.2,
1
3
24.37, 133, 154.4, 163.13, 163.79.
ACKNOWLEDGMENTS
2
-(4-Chlorobenzylidene)malononitrile (3f). White
–
1
The authors would like to express their gratitude to King
Khalid University, Saudi Arabia, for providing administrative
and technical support. The research has no funding support
and was carried out as part of employment (employee King
Khalid University).
solid, mp 90–91°C. IR spectrum, ν, cm : 3807, 3724,
3
1
6
7
317, 3032, 2935, 2584, 2303, 2225, 2075, 1928, 1788,
585, 1419, 1410, 1290, 1219, 1095, 937, 779, 704,
1
17. H NMR spectrum (300 MHz, CDCl ), δ, ppm:
3
.53 d (2H, J 8.0 Hz), 7.74 s (1H), 7.84 d (2H, J 8.0 Hz).
1
3
C NMR spectrum (300 MHz, CDCl ), δ, ppm: 83.62,
CONFLICT OF INTEREST
The authors declare no conflict of interest.
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1
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