A readily accessible porous organic polymer facilitates high-yielding Knoevenagel condensation at room temperature both in water and under solvent-free mechanochemical conditions

Article abstract of DOI:10.1016/j.catcom.2021.106304

A novel nitrogen-rich amorphous porous organic polymer has been synthesized using a microwave-assisted process. Its high chemical stability, reusability and poor solubility enable the use of the porous polymer as a metal-free heterogeneous catalyst for C–

Full text of DOI:10.1016/j.catcom.2021.106304

Catalysis Communications 154 (2021) 106304
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
A readily accessible porous organic polymer facilitates high-yielding
Knoevenagel condensation at room temperature both in water and under
solvent-free mechanochemical conditions
a
a
a
b
Parishmita Sarma , Kashyap Kumar Sarmah , Dharittri Kakoti , Sanjeev Pran Mahanta ,
c
d
,
g
a,*
Nadeesh Madusanka Adassooriya , Goutam Nandi , Pranab Jyoti Das
Dejan-Kre ˇs imir Bu ˇc ar , Ranjit Thakuria
,
e
a,*
a
Department of Chemistry, Gauhati University, Guwahati 781014, Assam, India
b
c
Department of Chemical Sciences, Tezpur University, Tezpur 784 028, India
Department of Food Science & Technology, Wayamba University of Sri Lanka, Makandura, Gonawila 60170, Sri Lanka
School of Chemistry, Sackler Faculty of Exact Sciences, Tel-Aviv University, Ramat-Aviv, 69978 Tel-Aviv, Israel
Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom
d
e
g
Department of Chemistry, Katwa College (Affiliated to the University of Burdwan) Katwa, Purba Bardhaman, West Bengal, PIN: 713130, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
A novel nitrogen-rich amorphous porous organic polymer has been synthesized using a microwave-assisted
process. Its high chemical stability, reusability and poor solubility enable the use of the porous polymer as a
Porous organic polymers
Catalysis
metal-free heterogeneous catalyst for C
–
C bond formation at ambient temperature under environmentally
Amorphous materials
Mechanochemistry
Knoevenagel condensation
benign conditions.
1
. Introduction
optoelectronics [22]. POPs are also extensively used as heterogeneous
catalysts [20,23,24] owing to their high thermal and chemical (i.e. hy-
drolytic) stability under extreme reaction conditions [25,26] and can, in
terms of functionality, easily compete with other classes of emerging and
established porous materials (e.g. metal-organic, covalent organic and
zeolitic imidazolate frameworks) [27]. To enhance their catalytic
properties, POPs are frequently doped with inorganic nanoparticles
[28]. A recent study, for example, has shown that such doped, organic-
inorganic hybrid catalysts enable Knoevenagel condensation reactions
in water with remarkable efficiency [29].
The Knoevenagel condensation reaction is a fundamentally impor-
tant reaction for the formation of C C bonds in organic synthetic
–
chemistry [1]. These condensation reactions are generally conducted in
organic solvents and are facilitated by a wide range of bases (e.g.
ammonia, amines, etc.) [2]. In recent years, the high utility of the
Knoevenagel condensation reaction was further promoted through the
development of numerous catalysts to enhance reaction yields and
shorten times [3–8]. Despite the appreciable variety of available cata-
lysts and their efficiencies [9], however, the need to develop catalysts
that enable fast access to condensation products in high yields under
environmentally friendly conditions still persists [10–15]. We report
herein the use of a readily accessible porous organic polymer (POP) as a
cost efficient, stable and reusable catalyst for fast and high-yielding
Knoevenagel condensation reactions under pH-neutral conditions in
water and at room temperature.
2. Experimental section
2.1. Synthesis of the AmPOP catalyst
Stoichiometric amounts of melamine (252 mg, 2 mmol) and ter-
ephthaloyl chloride (609 mg, 3 mmol) in DMSO (15 ml) with catalytic
amounts of triethyl amine (0.5 ml) were placed in a round bottom flask.
The obtained solution was refluxed under microwave condition at 420
Porous organic polymers (POPs) are widely used materials for ap-
plications in gas storage [16,17], separation sciences [18–21] and
*
Corresponding authors.
E-mail addresses: pranabjdas52@gmail.com (P.J. Das), ranjit.thakuria@gmail.com (R. Thakuria).
https://doi.org/10.1016/j.catcom.2021.106304
Received 8 November 2019; Received in revised form 26 February 2021; Accepted 12 March 2021
Available online 13 March 2021
1
566-7367/© 2021 The Authors.
Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(
http://creativecommons.org/licenses/by-nc-nd/4.0/).

P. Sarma et al.
Catalysis Communications 154 (2021) 106304
Fig. 1. Base-catalysed condensation of melamine (triaminotriazine) and terephthaloyl chloride to prepare the AmPOP catalyst under various reaction conditions
top); TEM images of AmPOP showing irregularly shaped particles under different magnifications (scale length 200 nm for bottom left; 50 nm for bottom right).
(
W for about 2 h. The solid obtained was filtered and slurried in 30 ml
MeOH, filtered and dried under vacuum.
3. Result and discussion
Our recent interest in porous organic materials led to the discovery of
an amorphous POP that is decorated with amide functional groups
2
.2. Mechanochemical Knoevenagel condensation reactions
(
hereafter abbreviated with AmPOP). The AmPOP can be efficiently and
A mixture of an aromatic (or a heteroaromatic) aldehyde (1 mmol)
rapidly synthesized in a condensation reaction involving melamine and
terephthaloyl chloride under reflux conditions or through a microwave-
assisted reflux process (see Supplementary Information, SI). Popular
mechanochemical methods [26], such as neat grinding or liquid-assisted
grinding, on the other hand, enabled the formation of the AmPOP, but
the reactions did not reach completion even after four hours of manual
milling. Powder X-ray diffraction and transmission small-angle X-ray
scattering (SAXS) revealed that the AmPOP material consists of amor-
phous particles that are 20 nm small, with radial size distributions (RSD)
of 38.5%. Results of transmission electron microscopy (TEM) studies of
the particle properties were also consistent with the results of the SAXS
studies (Fig. 1) and revealed that the particles exhibit irregular mor-
and an active methylene compound (1 mmol) were added to a mortar
along with the AmPOP catalyst (2.4 mol%). The mixture was ground at
room temperature for the lengths of time shown in Schemes 2 and 3.
Progress of the reaction was monitored by TLC, using an n-hexane:ethyl
acetate (5:1) solvent mixture as eluent. The reaction mixture was then
washed with ethanol and purified by recrystalisation from ethanol.
2
.3. Knoevenagel condensation reactions in an aqueous medium
A mixture of an aromatic (or a heteroaromatic) aldehyde (1 mmol),
an active methylene compound (1 mmol) and the AmPOP catalyst (2.4
mol%) was added to a mixture of 5 ml of distilled water. The suspension
was stirred at room temperature for the lengths of time shown in
Schemes 2 and 3. The reaction progress was monitored by thin-layer
chromatography using the above-mentioned eluent. After completion
of the reaction, the aqueous solution was filtered and the solid was
dissolved in ethanol. Again, the solution was filtered for removal of the
catalyst from the reaction mixture, and purified by recrystalisation from
ethanol to obtain the pure product. The characterization of products was
performed using 1H NMR, C NMR, MS, powder and single-crystal X-
ray diffraction.
phologies.
Surface
area
measurements,
using
the
Bar-
rettꢀ Joynerꢀ Halenda method, showed that the material exhibits pore
2
ꢀ 1
sizes in the 15–70 Å diameter range and surface areas of 66–96 m g
2
ꢀ 1
(or 100–133 m
g
, as determined by the Brunauer–Emmett–Teller
method). The activated AmPOP was also shown to exhibit outstanding
chemical stability upon exposure to a range of commonly utilized sol-
vents (including water), as well as under acidic conditions (see SI,
Fig. S7). The synthesis and the characterization of the AmPOP are
detailed in the SI document.
13
Further studies showed that the AmPOP can be used as metal-free
heterogeneous catalyst with relatively low catalytic loadings (2.4 mol
%
) to facilitate Knoevenagel condensation reactions both in water and in
solvent-free mechanochemical reactions. To the best of our knowledge,
2

P. Sarma et al.
Catalysis Communications 154 (2021) 106304
Y
CHO
OR
Z
Y
X
Y
Z
2.4 mol% AmPOP
X
Z
CHO
OR
A. H O, rt
2
B. solvent free grinding
R
R1
1
2
3/6
1
4/7
5/8
O
O
/
O
O
Y
CN
R : EWG/EDG
=
/
1
O
O
X : N/O
CN
Z
3
6a
6b
Scheme 1. General scheme for the AmPOP-catalysed Knoevenagel condensation reaction.
Scheme 2. Synthetic targets (top) and the
CN
CHO
OR
obtained yields (an average of two runs) for
NC
NC
compounds 4 and 5 using two different
X
X
CN
CN
2.4 mol% AmPOP
OR
synthetic methods (bottom): stirring of the
reactants in a round bottom flask at room
temperature (A) and milling under solvent-
free conditions (B). Method A: Aldehyde (1
mmol), malononitrile (1 mmol), AmPOP
CN
CHO
A. H O, rt
2
NC
5a-b
B. Neat grinding
R1
X: N/O
2a-b
R1
1
a-e
3
4a-e
2
catalyst (2.4 mol%), H O (5 ml), room tem-
perature stirring. Method B: aldehyde (1
mmol), malononitrile (1 mmol), catalyst
AmPOP (2.4 mol%), in an agate mortar and
pestle for the above-mentioned times and in
the absence of liquid additives. The per-
centage yields are given for the isolated
products.
NC
ClNC
NC
CN
Cl
CN
CN
CN
O N
2
Cl
4
a
4b
4c
4d
A: 50min, 96%
A: 20min, 95%
A:50min, 97%
A: 50min, 95%
B: 60min, 91%
B: 90min, 95%
B: 30min, 94%
B:60min, 95%
CN
NC
H
N
O
CN
CN
NC
NC
5
a
5b
4
e
A: 90min, 94%
A: 55min, 90%
A: 60min, 98%
B: 105min, 92%
B: 90min, 86%
B: 90min, 95%
The general scheme for the studied Knoevenagel condensations in
Table 1
water, as well as under solvent-free manual grinding conditions, using
AmPOP as catalyst is shown in Scheme 1. To establish the optimal re-
action condition, an initial catalyst screen was conducted using an
equimolar mixture of 4-chlorobenzaldehyde 1c and malononitrile 3 in
water at neutral pH. Product 4c (Scheme 2) was obtained in 97% yield in
presence of 2.4 mol% AmPOP after 50 min (Table 1, entry 4). Notably,
the product was isolated in poor yields (10%) in the absence of catalyst
Screening of optimal reaction conditions (catalyst loading, solvent and reaction
time). Reaction conditions unless specified otherwise: 4-chlorobenzaldehyde (1
mmol), malononitrile (1 mmol), in each solvent (5 ml), room-temperature stir-
ring. Yields are given for isolated materials.
b
Entry
Catalyst (mol%)
Solvent
Stirring time
Yield of 4c (%)
1
2
3
4
5
6
7
8
9
0
H
H
H
2
O
2
O
2
O
5 h
10
30
80
97
97
82
87
97
32
10
23
22
65
93
0.1
2.0
2.4
3.8
2.4
2.4
2.4
2.4
2.4
2.4
2.4
2.4
2.4
5 h
(
Table 1, entry 1) (see SI, Fig. S19). Further investigations showed that
2 h
the use of smaller amounts of the catalyst (0.1–2.0 mol%) yields less
product, while increasing the amount of the catalyst (to 3.8 mol%) did
not further improve the yields. Using the optimal catalytic loadings of
H
2
O
O
O
O
O
50 min
50 min
30 min
40 min
1.5 h
2.5 h
2.5 h
2.5 h
2.5 h
2 h
H
H
H
H
2
2
2
2
2
.4 mol%, a subsequent solvent study showed that water is the most
effective solvent for the Knoevenagel condensation at room temperature
using the same model reaction (Table 1, entry 4, 9–14). The model re-
action was then further studied to compare the effect of reaction time on
the product yield. It was found that stirring at room temperature for 50
min in water affords the best yields (Table 1, entry 4, 6–8).
THF
10
11
12
13
14
CH
CH
3
CN
Cl
2
2
CHCl
3
MeOH
EtOH
1 h
Once the most effective reaction conditions have been determined
for the formation of compound 4c (Scheme 2), the scope and generality
of the developed protocol was further tested by subjecting various re-
actants in form of substituted aromatic and heteroaromatic aldehydes
and active methylene compounds (viz. malononitrile, 5,5-
this study describes the first example of a metal-free amorphous organic
polymer that acts as a highly efficient heterogeneous catalyst for C ꢀ C
bond formation under solvent-free conditions.
3

P. Sarma et al.
Catalysis Communications 154 (2021) 106304
Scheme 3. Synthetic targets (above) and the
O
Y
obtained yields (an average of two runs) for
compounds
7 and 8 using two different
CHO
OR
O
Y
Y: C/O
6a-b
2.4 mol% AmPOP
Y
O
synthetic methods: stirring of the reactants
in a round bottom flask at room temperature
(A) and milling under solvent-free conditions
(B). Method A: Aldehyde (1 mmol), cyclic
active methylene compound (1 mmol for 7e-
f, 8a-b; 2 mmol for 7b-d), catalyst AmPOP
X
X
Y
O
CHO
A. H O, rt
OR
O
2
O
O
B. Neat grinding
R1
O
R1
b-f
X: N/O
2a-b
7
8a-b
1
b-f
(
2
2.4 mol%), H O (5 ml), room temperature
NO2
Cl
stirring; method B: Aldehyde (1 mmol), cy-
clic active methylene compound (1 mmol for
MeO
O
OMe
O
7
e-f, 8a-b; 2 mmol for 7b-d), catalyst
AmPOP (2.4 mol%), in agate mortar & pestle
for above mentioned time period in absence
of added liquid. The percentage yields are
given for isolated products.
O
O
O
O
OH
OH
O
O
OH
OH
7
b
7c
7d
A: 45min, 85%
B: 90min, 82%
A: 90min, 90%
B: 120min, 87%
A: 50min, 96%
B: 120min, 95%
O
O
HO
O
Cl
O
O
O
H
N
O
O
OH
O
O
O
O
O
Cl
O
O
7
e
7f
8a
8b
A: 60min, 98%
A: 90min, 94%
A: 50min, 96%
A: 45min, 90%
B: 90min, 96%
B: 105min, 91%
B: 90min, 94%
B: 60min, 87%
dimethylcyclohexane-1,3-dione and 2,2-dimethyl-1,3-dioxane-4,6-
dione) to the same reaction conditions (yields: 82–97%). Knoevenagel
condensation reactions involving heterocyclic aldehydes (2a-2b) also
gave excellent yields (yields for 5a and 5b: 92–98%) whereby no
occurrence of a polymerization reaction was observed (Scheme 2)
discussed above a few more Knoevenagel condensation products have
been synthesized using AmPOP and discussed elsewhere [33]. PXRD was
used as a characterizing tool to compare the products obtained using
solvent-free milling with the respective products obtained from room
temperature stirring (SI document, Figs. S21-S29).
[
30,31]. Reactions involving malononitrile and aryl aldehydes bearing
After completion of the reaction, the AmPOP catalyst was recovered
quantitatively by dissolving the reaction mixture in ethanol and through
subsequent separation of the virtually insoluble catalyst by filtration.
The re-activation of the catalyst is achieved by soaking the recovered
material in methanol, followed by drying under reduced pressure for one
hour. The catalyst was shown to exhibit a slight decrease in surface area
once recycled although no signs of chemical decomposition were
detected (see SI, Fig. S14 and Table S1), thus indicating that the recy-
cling procedure needs to be further optimised. The recyclability of
AmPOP was evaluated using the standard reaction condition in presence
of 4-chlorobenzaldehyde and malononitrile. The recycled AmPOP
catalyst yielded compound 4c in no less than 85% within 60 mins once it
was recycled up to five times (see SI, Fig. S18).
electron-withdrawing or donating groups completed within 105 min to
give the alkene product in high yields.
The versatility of the investigated AmPOP-assisted Knoevenagel
condensation reactions was further examined by expanding the range of
suitable reactants to cyclic methylene derivatives (Scheme 3). The
Knoevenagel condensation reaction was pursued under mild experi-
mental conditions, namely through stirring of the reactants in water at
room temperature and through solvent-free milling. All products were
obtained under both reaction conditions in high yields (82–98%) and in
short reaction times (45–120 min). In addition, the reaction of one mole
equivalent of aldehydes (4-nitrobenzaldehyde 1b, 4-chlorobenzalde-
hyde 1c and 3,5-dimethoxy benzaldehyde 1d) with a two mole equiv-
alent of dimedone 6a resulted in the formation of Knoevenagel-Michael
addition products (Scheme 3, 7b: 85%, 7c: 90% and 7d: 96%), similar to
those reported by Rostamizadeh et. al [32], while reactions involving
substituted aldehydes and Meldrum’s acid resulted in the formation of
arylidene products with good to excellent yields (7e-7f & 8a-8b;
4. Conclusions
In conclusion, we have described the synthesis of a chemically stable
porous organic polymer that effectively catalyses Knoevenagel
condensation reactions of various substituted aromatic/ heteroaromatic
aldehydes in very mild and environmentally benign experimental con-
ditions in water and at room temperature. To the best of our knowledge,
only a few reports [34,35] are available for porous organic polymer that
catalyses Knoevenagel condensation reaction under metal-free and pH-
neutral conditions in water. We also demonstrated that the condensa-
tion reaction can be effectively catalysed with the polymer under
solvent-free mechanochemical conditions. The high relevance and broad
applicability of the Knoevenagel condensation reactions and the high
8
7–98%). It is noteworthy that reactions pursued in stirred aqueous
suspensions reached completion faster than those conducted under
solvent-free mechanochemical conditions. This may be attributed to the
faster diffusion rates of the reactants into the AmPOP in water, as
compared to the diffusion of molecules through the AmPOP under
1
solvent-free conditions. All products were characterized using H and
1
3
C NMR spectroscopy, MS (SI document, Fig. S19) and crystallographic
methods (either single crystal or powder X-ray diffraction, PXRD) (SI
document, Figs. S20-S29 and Table S2). In addition to the examples
4

P. Sarma et al.
Catalysis Communications 154 (2021) 106304
catalytic efficiency and recyclability of the polymeric catalyst offers the
development of environmentally-friendly industrial processes relying in
such type of chemical transformation.
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The authors declare no competing financial interest.
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Catalysis Study of AmPOP and characterization of the Knoevenagel
products (NMR spectroscopic analysis, GC–MS, HR-MS analysis, SC-XRD
analysis, PXRD analysis of representative products prepared using NG).
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