Microwave-assisted Knoevenagel condensation in aqueous over triazine-based microporous network

Article abstract of DOI:10.1007/s11164-013-1456-x

A high nitrogen-containing triazine-based microporous polymeric (TMP) network was used as an efficient metal-free catalyst for Knoevenagel condensation of ethylcyanoacetate with aromatic aldehydes. The reactions were performed in water as an environmental

Full text of DOI:10.1007/s11164-013-1456-x

Res Chem Intermed (2014) 40:67–75
DOI 10.1007/s11164-013-1456-x
Microwave-assisted Knoevenagel condensation
in aqueous over triazine-based microporous network
•
Mohd Bismillah Ansari Mst. Nargis Parvin
Sang-Eon Park
•
Received: 14 November 2012 / Accepted: 9 April 2013 / Published online: 26 October 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract A high nitrogen-containing triazine-based microporous polymeric
(TMP) network was used as an efficient metal-free catalyst for Knoevenagel con-
densation of ethylcyanoacetate with aromatic aldehydes. The reactions were per-
formed in water as an environmentally benign medium, under microwave
irradiation within a short reaction time of 10 min. The conversions of substituted
aromatic aldehydes and selectivities for Knoevenagel products were found to be in
the ranges of 44–99 and 65–99 %, respectively. The electron-withdrawing sub-
stituent showed higher conversion and selectivity as compared to electron-donating
substituents. The TMP network can be readily recovered and reused up to three runs
without loss in catalytic activity and selectivity.
Keywords Triazine ꢀ Microporous polymer network ꢀ Aqueous media ꢀ
Knoevenagel condensation ꢀ Microwave
Introduction
Green catalysis has attained a major status in scientific disciplines for the
development of clean and benign chemical processes [1]. Current emphasis on green
processes involves utilization of safer solvents and the design for energy-efficient
and time-saving methodologies. The choice of green solvent is not only based upon
its abundance but also the following factors should be taken into consideration, such
as energy to manufacture, cumulative energy demand, and impact on health and the
environment. In this regard, there is widespread current debate over relative
‘‘greenness’’, and undoubtedly water can be regarded as the cleanest solvent
M. B. Ansari ꢀ Mst. N. Parvin ꢀ S.-E. Park (&)
Laboratory of Nano-Green Catalysis and Nano Center for Fine Chemicals Fusion Technology,
Department of Chemistry, Inha University, Inchon 402-751, Korea
e-mail: separk@inha.ac.kr
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M. B. Ansari et al.
available [2]. On the other hand, for many chemical processes, a major adverse
effect on the environment is the consumption of energy for heating and cooling. To
overcome these problems, it is highly desirable to develop efficient methods that use
alternative energy sources such as ultrasound or microwave irradiation to facilitate
chemical reactions. In this context, microwaves hold superiority as most of the
reactions can be performed in a very short time and, moreover, they provide
significant energy savings in most of the chemical transformations compared to
heating by conduction and convection currents [3, 4]. An important perspective for
greenness is the design and development of methodologies using metal-free
heterogeneous catalysts for clean organic transformations under microwave
irradiation in water [5].
Knoevenagel condensation is a base catalyzed reaction for the synthesis of a,b-
unsaturated carbonyl compounds by condensation of aldehydes or ketones with C–H
acidic methylene group-containing compounds [6]. Metal-free Knoevenagel reac-
tions have been well investigated in homogenous reactions using nitrogenous
molecules such as aliphatic amines, urea, and piperidine or their corresponding
ammonium salts and amino acids as catalyst [7, 8]. These homogenous reactions are
prone to problems of catalyst recovery and separation. In order to overcome these
limitations, enormous efforts have been dedicated to the immobilization of amines
and ionic liquids over silica support [9–11]. However, these materials are associated
with the drawback of fewer active sites and leaching of anchored catalysts. In this
context, it is desirable to design a catalyst which possesses inbuilt catalytic
functionalities as a part of a nanoporous framework, and one such promising design
of catalysts is porous organic polymers (POPs) [12–14].
In regard to these perspectives, here we report the catalytic performance of a
nitrogen-containing triazine-based microporous polymer network as an efficient
base catalyst for Knoevenagel condensation in aqueous under microwave
irradiation.
Experimental
Chemicals
All chemicals were of analytical grade and were used without further purification.
Aldehydes, terephthalaldehyde, and ethylcyanoacetate were purchased from Sigma
Aldrich. Melamine was purchased from TCI. The water was deionized by aqua
MaxTM basic water purification system (Young Lin, Korea).
Synthesis of TMP-network
The 1,3,5-triazine-based microporous network was synthesized by our earlier
reported procedure [12]. The TMP network was synthesized by condensation
reaction between terephthalaldehyde and melamine at 180 °C up to 48 h under an
inert atmosphere (Scheme 1). The isolated TMP network gave 75 % yield. The
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Microwave-assisted Knoevenagel condensation
69
detailed synthetic procedure and characterization of TMP network have been
reported by us elsewhere [12].
Catalytic activity
Knoevenagel condensation
The Knoevenagel condensation reactions were carried out under green solvent
(water) at 120 °C using single mode microwave irradiation (power 300 W). A
flame-dried two-necked round-bottom flask fitted with a condenser and a magnetic
stirring bar was charged with 1 mmol of aldehyde and 1 mmol of active methylene
compound ethylcyanoacetate. To this mixture, 20 mg of catalyst was added and
stirred for 2 min, followed by addition of 2 ml solvent. The reaction mixture was
maintained in inert atmosphere and air was removed by flushing nitrogen in order to
prevent oxidation of the aldehyde to acid.
The temperature of the reaction mixture was raised to 120 °C within 1.5 min and
maintained for an appropriate time (max. 12 min). After completion of the reaction,
the mixture was cooled to room temperature, diluted with dichloromethane, and
poured into a separating funnel. The organic layer was collected and dried over
anhydrous sodium sulfate, filtered, concentrated by rota-evaporator, and subjected
to GC and GC–MS analysis (Agilent Technologies 5975). From the aqueous layer,
the catalyst was recovered by filtration and washed with water, followed by
subsequent washing with acetone, and dried at 120 °C in a vacuum oven for reuse.
A similar procedure was followed for subsequent cycles.
The conversions were calculated on the basis of mole percent of aldehyde; the
initial mole percent of aldehyde was divided by initial area percent (CYA peak area
from GC) to get the response factor. The unreacted moles of aldehyde remaining in
the reaction mixture were calculated by multiplying the response factor by the area
percentage of the GC peak for the CYA obtained after the reaction. The conversion
and selectivity were calculated as follows:
Aldehyde Con:ðmol%Þ ¼ ½ðInitial mol% ꢁ Final mol%Þ=initial mol%ꢂ ꢃ 100;
Knoevenagel product Sel:%
¼ ðGC peak area of aldehyde=R GC peak area of all productsÞ ꢃ 100:
Results and discussion
The TMP network was synthesized by catalyst-free polymerization by our earlier
reported procedure with 75 % yield. The detailed synthesis and characterization
procedure is presented elsewhere [12]. The TMP network is nitrogen-enriched
(elemental analysis C, 41.03; H, 4.55; N, 40.30; S, 1.3 %) and can be used as a base
catalyst in Knoevenagel condensation reaction. As an initial step, we investigated
the effect of various solvents in microwave-assisted Knoevenagel condensation.
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M. B. Ansari et al.
Scheme 1 Synthesis of TMP network
Effect of solvent
The investigations on the effect of the solvent over the reaction revealed that the
highest conversion of benzaldehyde (99 and 85 %; Table 1, entries 1 and 4) was
found in the case of ethanol and water, respectively, whereas the lowest conversion
was observed in toluene (34 %) and chloroform (54 %). The selectivity for the
Knoevenagel product in all the cases was approximately 99 %.
These observations revealed that the solvent plays a significant role in the
reaction. Polar protic solvents such as ethanol and water give better conversions due
to dH hydrogen bonding interaction and dipole interaction with the intermediate
formed during the course of the reaction. The acetonitrile and DMF are considered
to be polar aprotic solvents lacking hydrogen bonding interaction and depicting a
little lower conversion. The lowest conversion in the case of toluene and chloroform
is attributed to their relative non-polar nature. The effect of solvent property on the
conversion of benzaldehyde can be summarized in the following order: polar protic
solvent [ polar aporotic solvent [ non-polar solvent [15].
In the absence of solvent, the conversion of benzaldehyde was 55 % (Table 1,
entry 7), and without catalyst, 10 % conversion (Table 1, entry 8) was observed.
These results suggested that microwave-assisted Knoevenagel condensation reac-
tions described here are purely catalytic in nature.
Effect of reaction time
The difference in the conversion of ethanol and water has only a small deviation,
owing to the eco-friendly and green nature of water as solvent, and further
investigation on the effect of time over the reaction was investigated in
condensation reactions between benzaldehyde and ethylcyanoacetae. The increase
in reaction time showed an increase in conversion. Maximum conversion of
benzaldehyde (99 %) and selectivity of Knoevenagel product (99 %) was observed
at 12 min of microwave irradiation time (Table 2, entry 5).
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Microwave-assisted Knoevenagel condensation
71
Table 1 TMP-catalyzed Knoevenagel condensation of benzaldehyde with ethylcyanoacetate in different
solvents
N
N
O
O
H
O
O
MW irradiation10 min
TMP
O
+
S. no.
Solvent
Benzaldehyde
conversion (%)a
Knoevenagel product
selectivity (%)a
1
2
3
4
5
6
7
8
Water
85
68
71
99
34
54
54
10
99
99
99
99
99
99
99
96
Acetonitrile
DMF
Ethanol
Toulene
Chloroform
No
Ethanolb
Reaction conditions: catalyst 20 mg, benzaldehyde 1 mmol, ethylcyanoacetate 1 mmol, solvent 2 ml,
microwave irradiation 300 W, temperature 120 °C and time 10 min
a
Analyzed by GC and GC–MS
b
Without catalyst
Table 2 Effect of microwave irradiation time in TMP catalyzed Knoevenagel condensation reactions
S. no.
Time (min)
Benzaldehyde
conversion (%)a
Knoevenagel product
selectivity (%)a
1
2
3
4
5
2.5
5
50
66
77
85
99
92
90
84
99
99
7.5
10
12
Reaction conditions: catalyst 20 mg, benzaldehyde 1 mmol, ethylcyanoacetate 1 mmol, solvent 2 ml,
microwave irradiation 300 W, temperature 120 °C and time 2.5–12 min
a
Analyzed by GC and GC–MS
Effect of substituents
The effect of substituents over the aromatic ring of benzaldehyde was investigated
with various electron-withdrawing and electron-donating groups at different
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M. B. Ansari et al.
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Microwave-assisted Knoevenagel condensation
73
Fig. 1 Recyclability of the TMP network in Knoevenagel condensation of benzaldehyde and
ethylcyanoacetate
Fig. 2 Plausible mechanism for TMP-catalyzed Knoevenagel condensation of benzaldehyde and
ethylcyanoacetate
positions (Table 3). The conversions of aromatic aldehydes were obtained in the
range of 44–99 %, whereas the selectivities for Knoevenagel products were in the
range of 65–99 % within a short reaction time of 10 min. These results indicated
that the TMP network can be used as an efficient catalyst with substrate versatility.
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Electron-withdrawing groups such as –Cl and –NO2 on the the aromatic ring
(Table 3, entries 2–4) increases the conversion due to the -I effect (electron-
withdrawing inductive effect). A slight decrease in conversion was observed for the
4-hydroxybenzaldehyde, 4-methoxybenzaldehyde, and 3-phenoxybenzaldehyde
(Table 3, entries 6, 8 and 10) due to the cumulative role of ?M (electron-donating
mesomeric effect) and the -I effect. In the case of salicylaldehyde, the
intramolecular hydrogen bonding between carbonyl ‘‘O’’ and hydroxyl ‘‘H’’
restricts the reaction. The electron-donating group such as 4-isopropyl-(Table 3,
entry 9) on the aromatic ring decreases the yields due to the existence of ?I
(electron-donating inductive effect) effect. These reaction results are in agreement
with those reported earlier [16–19]. The catalyst recyclability was tested for three
cycles (Fig. 1) in Knoevenagel condensation between benzaldehyde and ethyl-
cyanoacetate, and no significant loss in catalytic activity was observed.
Plausible mechanism
As mentioned earlier, the TMP is nitrogen-enriched which plays a role as a Lewis
base site for Knoevenagel condensation. Figure 2 depicts a plausible mechanism for
the TMP-catalyzed Knoevenagel condensation of benzaldehyde with ethylcyanoac-
etate. The amine nitrogen from the TMP network abstracts the acidic proton from
the active methylene group of ethylcyanoacetate. The generated anion makes a
nuleophilic attack on the carbonyl-carbon atom of the benzaldehyde to form
oxyanion. The oxyanion abstracts H? from the protonated amine group of TMP
network and the catalyst is regenerated. The formed hydroxyl molecule eliminates
the water molecule, leading to the formation of Knoevenagel product [20].
Conclusion
In summary, we have developed a new metal-free heterogeneous catalyst for
microwave-mediated Knoevenagel condensation operative under water as a green
solvent media. The TMP catalyst exhibited a wide substrate scope for Knoevenagel
condensation reaction with high to optimum conversions and selectivities. The
catalyst can be easily recovered and reused without loss of its catalytic activity and
selectivity.
Acknowledgments This work was supported by the National Research Foundation of Korea[NRF]
grant funded by the Korea government [MSIP] [No.20090083525] and INHA University, Korea.
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