A Facile and Green Synthesis of 2,4,6-Triarylpyridine Derivatives Using the Modified Mesoporous Organic Polymer Based on Calix [4]Resorcinarene: As an Efficient and Reusable Heterogeneous Acidic Catalyst

Article abstract of DOI:10.1134/S0023158419020071

Abstract: The work describes an efficient one-pot synthesis of 2,4,6-trisubstituted pyridine derivatives through a three-component catalytic reaction. The procedure involves mesoporous organic polymer based on calix[4]resorcinarene in and functionalized b

Full text of DOI:10.1134/S0023158419020071

ISSN 0023-1584, Kinetics and Catalysis, 2019, Vol. 60, No. 2, pp. 187–195. © Pleiades Publishing, Ltd., 2019.
Published in Russian in Kinetika i Kataliz, 2019, Vol. 60, No. 2, pp. 206–215.
A Facile and Green Synthesis of 2,4,6-Triarylpyridine Derivatives
Using the Modified Mesoporous Organic Polymer Based on Calix
[4]Resorcinarene: As an Efficient and Reusable Heterogeneous
Acidic Catalyst
A. Mouradzadeguna, b, *, M. A. Mostafavia, **, and M. R. Ganjalib, c, ***
aDepartment of Chemistry, Faculty of Science, Shahid Chamran University of Ahvaz, Ahvaz, Iran
bCenter of Excellence in Electrochemistry, School of Chemistry, College of Science, University of Tehran, Tehran, Iran
cBiosensor Research Center, Endocrinology and Metabolism Molecular-Cellular Sciences Institute,
Tehran University of Medical Sciences, Tehran, Iran
*e-mail: arash_m@scu.ac.ir
**e-mail: mostafavi.mahsa1366@yahoo.com
***e-mail: ganjali@khayam.ut.ac.ir
Received March 6, 2018; revised August 18, 2018; accepted October 19, 2018
Abstract—The work describes an efficient one-pot synthesis of 2,4,6-trisubstituted pyridine derivatives
through a three-component catalytic reaction. The procedure involves mesoporous organic polymer based
on calix[4]resorcinarene in and functionalized by N-propyl sulfamic acids a solid acid catalyst. Excellent
yields, short reaction times, catalyst reusability, and easy reaction handling under solvent-free and mild acidic
conditions are the most important advantages of the present protocol. The acidic catalyst was characterized
by a number of spectroscopic methods such as Fourier-transform infrared spectroscopy, field emission scan-
ning electron microscopy, transmission electron microscopy and elemental CHNS analysis.
Keywords: sulfamic acid functionalized mesoporous organic polymer, calix[4]resorcinarene, multi-compo-
nent reactions, 2,4,6-trisubstituted pyridines, solid acid catalysts
DOI: 10.1134/S0023158419020071
INTRODUCTION
mers of intrinsic microporosity, covalent organic
frameworks, and porous organic polymers with low or
high crystallinity were used [18–23]. Among different
types of mesoporous materials, considerable attention
has been given to design and synthesis of mesoporous
organic polymers that are composed of light and non-
metallic elements like C, H, O, and N. These materi-
als are of intense interest as catalysts or catalyst sup-
ports because they combine textural properties of mes-
oporous materials such as large surface areas, low den-
sities and tuneable structures with the advantages of
organic polymers that contain aromatic fragments and
show high thermal and chemical stability and high
degree of hydrophobicity. The porous materials can be
pre-synthetically or post-synthetically functionalized
with a wide range of functionalities to make them
promising candidates for heterogeneous catalysts in
various types of chemical reactions [24–26].
Multi-component reactions emerge as an attractive
strategy for organic synthesis. It is an advantage to
conduct several reactions in a single reactor without
the need to use isolation and purification steps. Oper-
ation simplicity that does not involve any separation
process and detection of intermediates reduces time,
labour and cost [1]. Accordingly, these reactions are
widely used for the synthesis of a wide variety of het-
erocyclic compounds, among which pyridine deriva-
tives [2–8] are of significant interest owing to their key
role in pharmaceuticals and agrochemicals production
[9–11].
Over the recent years, synthetic porous organic
frameworks (POFs) with micro (<2 nm) and meso
(2–50 nm) pore size range have received an increasing
attention in a wide range of applications such as het-
erogeneous catalysis, organic synthesis, chemical
sensing, light harvesting, gas storage and separation
[12–17]. In this area, several different classes of porous
Solid acids are recommended as suitable catalytic
systems in organic synthesis because they have a num-
ber of advantages over their homogeneous counter-
materials such as hyper-cross-linked polymers, poly- parts. Non-toxicity, long catalytic life, environmental
187

188
MOURADZADEGUN et al.
compatibility, low cost, ease of isolation and reusabil- PC/PG (NETZSCH, Germany) under a nitrogen
ity are the most important properties of these hetero- atmosphere (rate of N2 ~1 L/h). Elemental analysis
geneous acidic systems [27–29]. In this way, poly-
meric supports can be equipped with organic spacers
and the addition of the acidic agent to these active sites
will result in the solid acid catalyst formation [30, 31].
Accordingly, the addition of acidic functional groups
like sulfamic acid moieties into solid supports is one of
the ways to produce efficient heterogeneous catalysts
[32–35].
was recorded on a Flash EA 1112 CHNS analyzer
(Thermo Finnigan, Italy).
Synthesis of Calix [4]Resorcinarene 1
Calix[4]resorcinarene 1 was prepared according to
the previously reported procedure [43].
Polycalix [4]resorcinarene, which is composed of
monomeric units of cupped hydrophobic cavities of
calixarene molecules surrounded by hydrophilic
groups, is among the novel mesoporous organic poly-
mers amenable to post-synthetical functionalization
for catalytic purposes [36].
This work is a continuation of our studies aimed at
developing safer methods in organic synthesis [37–
42]. It was of interest to explore the possibility of
anchoring SO3H-entities into our novel NH2-func-
tionalized mesoporous polymer based on calix
[4]resorcinarene to produce a promising solid acid
catalyst. We hoped that this catalyst would be suitable
for the preparation of 2,4,6-trisubstituted pyridine
derivatives through a one-pot reaction between ace-
tophenone, aryl aldehydes and NH4OAc under mild
and solvent-free conditions.
Synthesis of the 3D-Network Polymer Based on Calix
[4]Resorcinarene 2
The desired polymer was synthesized by adding
42 mmol of formaldehyde to 14 mmol of the prepared
calix [4]resorcinarene 1 dissolved in 40 cm3 aqueous
NaOH solution (10%) at room temperature. The
resultant mixture was heated to 90°C and was main-
tained at this temperature for 20 h. Next, the excess
alkali was washed out of the gel formed with cold
water. The gel was allowed to stand at 100°C for 1 h.
Then, the gel was transformed into the acidic form by
being treated with the 0.1 M HCl solution. The
resulted solid was dried at 100°C for 10 h [48].
Synthesis of the NH2-Functionalized Polymer
Based on Calix [4]Resorcinarene 3
The synthesized polymer 2 obtained from the above
procedure (1.00 g) and 3-(triethoxysilyl)propylamine
(4.00 g, 18 mmol) were introduced into a 50 cm3 round-
bottomed flask containing 10 cm3 H2O/EtOn at room
temperature. The reaction mixture was heated to 90°C
and stirred at this temperature for 20 h. The desired
amine-functionalized polymer 3 was collected by fil-
tration and washed several times with hot toluene
before being dried at 100°C [45].
EXPERIMENTAL
Materials and Methods
Commercially available starting materials were
purchased from Merck, Fluka and Aldrich chemical
companies and used without further purification. TLC
accomplished monitoring of the reaction progress. IR
spectra were recorded using a BOMEM MB-102 FT-
IR spectrophotometer (Hartmann & Braun, Canada,
1
13
KBr). H- and C-NMR spectra were obtained on a
Bruker NMR spectrometer (Bruker, Germany) at 400
and 100 MHz, respectively, in CDCI3 with tetrameth-
ylsilane as an internal standard. Mass spectra were
acquired on an Agilent 5975 mass spectrophotometer
(Agilent, USA). Melting points were checked with a
Thermo Scientific 9200 melting point apparatus
(Thermo Scientific, USA), whereas pH measure-
ments were recorded on a Metrohm 827 pH lab pH-
meter apparatus (Metrohm AG, Zwisserland). Poly-
meric matrix morphology was examined by atomic
force microscopy (AFM) with Nano Wizard II AFM
microscope (JPK instruments, Germany). Field emis-
sion scanning electron microscopy (FE-SEM) images
were collected on a MIRA 3 TESCAN apparatus
(TESCAN, Czechia). The particle morphology and
porosity were also evaluated using transmission elec-
tron microscopy (TEM) technique on a Zeiss-EM
10C (Germany) in 80 kV. In order to evaluate the ther-
mal stability of the functionalized polymer, its TGA
Determination of the Amount of Amine Functionalities
Grafted onto the Polymer 2
The amount of amine functionalities grafted onto
the polymer 2 was determined by elemental analysis to
be 3.5 mmol per gram of dry polymer [45].
Preparation of Polymer-Bonded N-Propyl Sulfamic Acid
Based on Calix [4]Resorcinarene 4
To a magnetically stirred mixture of the NH2-func-
tionalized polymer described in the previous section
(5 g) in CHCl3 (20 cm3) at 0°C, chlorosulfonic acid
(5 cm3, 75.2 mmol) was added dropwise over 2 h. After
the addition was completed, the mixture was stirred for
3 h until all HCl was removed from the reaction
medium. The mixture was filtered, washed with etha-
curves were recorded on a NETZSCH STA 409 nol (30 cm3) and finally dried at room temperature.
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A FACILE AND GREEN SYNTHESIS
189
1
[M+]; HNMR (400 MHz, CDCl3), δ, ppm: 7.47–
7.77 (m, 11H, Ar–H), 8.22–8.24 (m, 4H, Ar–H), 7.92
(s, 2H, H1); 13CNMR (100 MHz, CDCl3), δ, ppm:
117.2 (C3, C5), 127.2, 128.7, 129.0, 129.1 (ArC), 139.1,
139.5 (ArCq), 150.1 (C4), 157.4 (C2, C6).
Determination of the Capacity of Sulfamic Acid
Functionalities Grafted onto the Catalyst 4
The capacity of sulfamic acid functionalities in the
polymer-supported catalyst was determined by ele-
mental analysis. The capacity was generally found to
be 1.55 mmol H+ per gram of the dry polymer-sup-
ported catalyst 4.
In addition, the capacity of the acidic functionalities
was determined through pH analysis. To this end, to an
aqueous solution of NaCl (1 M, 25 cm3) with a primary
pH equal to 6.01, catalyst 4 (50 mg) was added and the
resulting mixture was stirred for 2 h after which the pH of
the solution decreased to 2.49. This is equal to a loading
of 1.62 mmol SO3H per gram of dry polymer [46].
Recyclability of Catalyst 4
At the end of the reaction, the catalyst was filtered,
the residue was consecutively washed with CH2Cl2
(5 cm3), Et2O (5 cm3), EtOH (5 cm3), and n-hexane
(5 cm3). After the catalyst had been dried in an oven at
60°C for 4 h, it could be reused directly without fur-
ther purification.
RESULTS AND DISCUSSION
General Procedure for the Synthesis of 2,4,6-
Trisubstituted Pyridine Derivatives 8a–g
Immobilization of SO3H—acidic group on highly
efficient mesoporous materials provides the system
that combines the advantages of homogeneous acidic
catalysts with those of heterogeneous systems. In this
project, attention was focused on the catalytic activity
of the novel porous organic polymer based on calix
[4]resorcinarene functionalized by sulfamic acid. The
preparation of 2,4,6-trisubstituted pyridine derivatives
served as a model reaction. The multi-step functional-
ization process involved in the catalyst preparation is
A mixture of aromatic aldehyde (1 mmol), ace-
tophenone (2 mmol), NH4OAc (1.3 mmol, 0.103 g)
and 0.05 mmol catalyst (0.025 g) was stirred in a sealed
tube at 60°C. Upon the reaction completion moni-
tored by TLC, hot ethanol was added to the mixture and
the insoluble catalyst was filtered off. Pure products were
obtained by recrystallization from hot ethanol.
2,4,6-Triphenyipyridine (8a): white crystals, yield
90%, m.p. 134–136°C (from EtOH), MS m/z: 307 designated in Scheme 1.
H2C
HO
OH
HO
OH
R
R
R
OH
HO
HO
OH
H2
HO
HO
C
R
CH3 H3C
CH3 H3C
CH3 H3C
CH3 H3C
HO
4
OH
+
H2
H+
n
NaOH
4CH COH
+
CH2O
3
C
OH
OH
OH
OH
OH
OH
OH
OH
HO
OH
HO
n
H2C
1
2
NH2
NHSO3H
NH2
NHSO3H
OH
OH
Si
Si
HO
HO
NH2
Si
O
Si
O
OH
OH
OH
OH
R
HO
HO
OH
OH
OH
OH
OH
OH
OH
OH
O
O
O
O
OH
HO
HO
H
N
O
Si
O
ClSO3H
CHCl3, 3 h, 25°C
O
O
S
H2
C
+
R
H2
C
H2
C
CH3
O
R
R
R
R
n
CH3
O
H
R
R
R
R
CH3
R
3
n
R
4
n
2
Scheme 1. Synthesis of calix [4]resorcinarene 1, 3D-network polymer 2, amine-functionalized polymer 3 and the N-pro-
pyl sulfamic acid functionalized polymeric catalyst 4.
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MOURADZADEGUN et al.
NH2-functionalized polymer 3 was characterized
through a series of analytical methods including IR
spectroscopy, atomic force microscopy (AFM), field
emission scanning electron microscopy (FE-SEM),
thermal gravimetric analysis (TGA) and energy-dis-
persive X-ray spectroscopy (EDS) (see Electronic
Supporting materials). In the following step, –SO3H
groups were anchored to the polymeric matrix 3 via
the addition of chlorosulfonic acid to the reaction
medium.
a
b
c
This new reported heterogeneous sulfamic acid
functionalized catalyst 4 was prepared by the reaction
of previously synthesized NH2-functionalized poly-
mer 3 and chlorosulfonic acid in chloroform. It can be
postulated that a strong basicity of amine functional
groups towards acidic reagents like chlorosulfonic acid
is a driving force of intense acidic-basic interaction
that plays a decisive role in the preparation of the cat-
alyst 4.
4000
3000
2000
1000
Wavenumber, сm–1
According to the IR spectral analysis of the acidic
catalyst 4, the N–S the band due to stretching vibra-
tions was observed at 685 cm–1. The bands assignable
to Si–O–Si symmetric and asymmetric stretching
vibrations appeared at 1035 and 747 cm–1. The bands
at 1217 and 1124 cm–1 can be assigned to sulfonyl moi-
eties in catalyst 4. The presence of bending vibrations
related to amine functionalities suggests the occur-
rence of amine groups, which avoided transformations
into sulfamic acids (Fig. 1).
Fig. 1. The comparative FT-IR spectra for polymeric network
2 (a), amine functionalized polymer 3 (b), catalyst 4 (c).
The 3D-network polymer 2 was produced by the
reaction between resorcinol and acetaldehyde, fol-
lowed by the polycondensation of calix [4]resorcin-
arene 1 with formaldehyde as designed in Scheme 1.
The structure of compound 1 was determined unam-
1
biguously by the spectroscopic (IR, H NMR, 13C
The sulphur content obtained from the elemental
analysis showed a loading of 1.55 mmol H+/g on the
polymeric matrix (Table 1).
NMR) data (see Electronic Supporting materials).
The formation of polymer 2 was also confirmed by
atomic force microscopy (AFM), X-ray diffraction
(XRD) and scanning electron microscopy (SEM) (see
Electronic Supporting materials). Subsequently,
according to the schematic procedure describing the
first step of post-functionalization, the prepared
porous polymer 2 was functionalized through reacting
with 3-(triethoxysilyl)propylamine as an organic
spacer. In this way proper positions accessible to acidic
In order to determine the percentage of amine
groups converted to the corresponding sulfonic acid
groups, pH analysis was carried out through a pH-
meter apparatus and a loading equal to 1.62 mmol
SO3H per 1 g of the solid acid catalyst at 25°C was
found. This value represents the catalyst proton
exchange capacity, which is in good accordance with
agents can be created. The structure of the synthesized CHNS analysis results.
Table 1. Elemental analysis data for polymer 2, NH2-fimctionalized polymer 3 and the sulfamic acid functionalized cata-
lyst 4
Sample
C, wt %;
H, wt %
N, wt %
S, wt %
Polymer 2
45.720
45.500
44.429
4.960
5.020
6.071
0.030
4.950
5.001
–
–
NH2-functionalized polymer 3
Sulfamic acid functionalized catalyst 4
4.782
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A FACILE AND GREEN SYNTHESIS
191
architecture, is stable up to 800°C. These results
achieved via TGA technique shows high stability of the
synthesized structure (Fig. 2).
Weight, %
Loss: 3.38%
171.1°C
100
23.4°C
Field emission scanning electron microscopy
(FE-SEM) and transmission electron microscopy
(TEM) techniques were employed to examine the
morphology of this new developed polymeric system.
As illustrated in Fig. 3, the obtained structure is con-
sisted of meso-scale pores and spherical particles that
are reminiscent of the morphology of the initial poly-
meric network (Figs. 3a and 3b).
As shown in Fig. 4, nitrogen adsorption–desorp-
tion isotherm of the new sulfamic acid functionalized
catalyst is classified as nearly type IV isotherm with
surface area, pore volume and mean pore diameter
equal to 199 m2/g, 0.023 cm3/g and 4.67 nm, respec-
tively.
90
80
70
60
50
40
30
Loss:
62.23%
Residue:
33.96%
796.5°C
0
100 200 300 400 500 600 700 800
Temperature, °С
Fig. 2. TGA profile of the synthesized catalyst 4.
In order to investigate the catalytic efficiency of
novel catalyst, a three-component reaction between
benzaldehydes, acetophenone and NH4OAc was
elected as the model reaction for the initial screening
experiments (Scheme 2). To illustrate the usefulness of
the catalyst for this reaction, firstly the reaction was
carried out without the catalyst. In this case, as it was
anticipated the reaction did not proceed properly
(Table 2, entry 6). In the continuation of our research
efforts, we tried to optimize the reaction conditions in
the presence of the novel catalyst. A brief report of the
results of the optimized parameters is depicted in
The thermal stability of the prepared catalyst was
accessed by TGA measurements under N2 atmo-
sphere. A mass loss of about 3% up to 172°C is due to
the removal of water and residual amounts of solvents
trapped inside the pores during the synthesis proce-
dure. The second mass loss of about 62%, which
occurs in the temperature range of nearly 172 to 796°C
is mainly pertain to the cleavage of the grafted linkers
and organic moieties. The retained mass of about
34%, which is attributed to the remained polymeric Table 2.
R1
CHO
+ 2
O
CH3
0.05 mmol cat.
60−70°C
Oil bath,
+ NH4OAc
R2
R1
N
stirring
7
5
6
8а−g
R2
R2
Scheme 2. Synthesis of 2,4,6-trisubstituted pyridines
At the next step, different parameters of the reac-
tion such as the catalyst amount, time and tempera-
ture were optimized. We were able to found that an
increase in the catalyst quantity above 0.025 g did not
appreciably affect the yield of products. Consequently,
0.025 g was selected as the optimum amount of cata-
lyst (Table 2, entry 2). In the following, the optimum
temperature was obtained in the range of 60–70°C.
Unexpectedly, the reaction was completed in a
short period of time with high yields. This result can be
attributed to the specific porous structure and high
surface area of the catalyst which can facilitate the
transportation of the reactants to the exposed active
sites and consequently increase the reaction yields. In
addition, the aromatic hydrophobic cavities of polyca-
lixarene monomeric units provide proper pockets to
accumulate the reactants and promote the reaction
efficiency.
It is found that the best yield was obtained using
5 mol % of catalyst 4 within 10 min in 60–70°C under
solvent-free conditions (Table 2, entry 5).
At the next step, as shown in Table 3, an attempt
was made to evaluate the scope and limitations of the
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MOURADZADEGUN et al.
(а)
(b)
L4 = 13.00 nm
L3 = 11.54 nm
L1 = 10.19 nm
L5 = 14.86 nm
L2 = 9.01 nm
L6 = 16.61
36 nm
Fig. 3. FE-SEM (a) and TEM (b) images of the synthesized catalyst 4.
methodology. The reaction was carried out under as NO2 and haloides, Table 3, entries 5, 6, and 7) at
para-position of the aromatic ring afforded higher
yields than those with electron-donating groups (such
as CH3, OCH3, and N(CH3)2) (Table 3, entries 2, 3,
and 4) in this position. At the same time, there is no
adverse effect of substituent either electron-withdraw-
ing or electron-donating groups on the product yield
(Table 3). All the products are known compounds and
they were characterized by comparing their IR and
1H-NMR spectral data as well as melting points with
those reported in literature [2, 47].
optimized conditions for a variety of substituted aro-
matic aldehydes and in each case, the corresponding
2,4,6-trisubstituted pyridines were obtained in good to
excellent yields. The remarkable point is that aromatic
aldehydes bearing electron-withdrawing groups (such
Va/cm3 (STP)g−1
1.8
It is worth noting that the final reaction products
were easily purified through recrystallization without
any need to use costly and perilous chromatographic
methods.
1.2
0.6
The plausible mechanism for the formation of
2,4,6-triarylpyridine derivatives is designated in
Scheme 3. A series of processes including Aldol con-
densation, Michael addition, cyclization and oxida-
tion eventually result in the desired product 8. Ini-
tially, the activated aryl aldehyde 5, in the presence of
the catalyst 4 undergoes nucleophilic addition by the
enol form of acetophenone 6 to give rise to the aldol
condensation product A. Then the second mole of
acetophenone, in the presence of ammonium acetate
is converted to the enamine molecule B, followed by
attacking A in a Michael addition affording intermedi-
ate C. In the next step, dihydropyridine D is formed by
cyclization and finally air oxidation of D gives the final
product 8 (Scheme 3).
0
0.5
1.0
P/P0
Fig. 4. Nitrogen adsorption–desorption isotherm of N-pro-
pyl sulfamic acid functionalized polymeric catalyst 4.
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A FACILE AND GREEN SYNTHESIS
Table 2. Optimization of the reaction conditions
193
Entry
Catalyst amount, mmol
Temp, °C
Time, min
Yield, %*
1
2
3
4
5
6
0.025
0.050
0.075
0.050
0.050
–
100
100
100
25
60–70
120
15
10
10
120
10
120
85
92
90
–
90
Negligible
* Reported yields.
Table 3. One-pot synthesis of 2,4,6-trisubstituted pyridines*
R1
R2
Tm, °C (Tm, °C [Ref])
Entry
Product
Yield, %** Time, min
1
2
3
4
5
6
7
H
H
H
H
H
H
H
H
8a
8b
8c
8d
8e
8f
90
85
82
88
98
96
95
10
15
20
5
5
10
5
134–136 (134–135 [47])
122–124 (123–124 [47])
99–101 (100–103 [47])
141–143 (138–140 [47])
126–128 (125–127 [47])
166–168 (164–166 [2])
198–200 (195–197 [47])
4-Methyl
4-Methoxy
4-N,N-dimethyl amine
4-Chloro
4-Bromo
4-Nitro
8g
* Reaction conditions: aromatic aldehyde (1 mmol), acetophenone (2 mmol), NH OAC (1.3 mmol), catalyst (0.05 mmol), 60–70°C,
4
solvent-free.
** Isolated yields.
H
N
O
H
N
S
O
H
N
O
NH
S
O
O
H
O
S
O
H
O
O O
S
O
H
O
O
O
H
NH2
O
H
H2O
O
OH
CH2
NH4OAc
H2C
H3C
CH3
7
R2
R2
R1
B
R2
A
6
R2
R2
R1
5
R1
NH
O
S
OH
O
O
H
NH
4
R2
H
N
O
R2
C
S
R1
R1
R1
O O
H
O
[O]
N
N
H
NH2
R2
R2
R2
R2
R2
8
D
R2
Scheme 3. Plausible mechanism for the synthesis of 2,4,6-trisubstituted pyridines catalyzed by catalyst 4.
As seen in Table 4, catalyst 4 emerges as a more known catalytic systems in terms of the product yields
efficient catalyst in comparison with the previously and also the reaction time and conditions.
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MOURADZADEGUN et al.
Table 4. The comparison of this work catalyst with reported catalysts in the synthesis of triarylpyridines
Catalyst
Time, h
Temperature, °C
Yield, %*
Ref.
Montmorillonite K10 clay catalyst
4
120
120
100
120
120
60
97
85
97
87
87
90
[48]
[1]
Mesoporous nanocrystalline MgAl2O4
3
AcOH
4
[47]
H2SO4–SiO2
AlPO4
3
[49]
4
[50]
Catalyst 4
0.17
This work
* 2,4,6-Triphenylpyridine as the base product in all reported yields.
Finally, the recycling potential of the catalyst was
examined. For this purpose, upon the completion of
the reaction, the catalyst was recovered by simple fil-
tration through a sintered glass-bed, washed with
dichloromethane, diethyl ether, ethanol and n-hexane
and after being dried it was reused in 5 consecutive
runs without considerable decrease in catalytic activity
(Fig. 5).
CONCLUSIONS
In summary, applying this new porous organic
polymer as a heterogeneous catalytic system in the
three-component synthesis of multi-substituted pyri-
dines resulted in the reaction that can be accomplished
in a short span of time through solvent-free conditions
with excellent yields of products. Additionally, simple
work-up in products isolation, mild reaction condi-
tions, a green methodology and also the catalyst recy-
clability are the most noteworthy features of this novel
procedure.
A noteworthy synthetic advantage of this facile
cost-effective one-pot reaction conducted under sol-
vent-free conditions is that it can be accomplished
with readily available starting materials and with a
simplified work up and purification. The synthesis
affords excellent product yields and does not involve
any need to separate and employ hazardous materials
or column chromatography. These are the main
advantages of this ecofriendly multi-component pro-
tocol in the synthesis of triarylpyridine derivatives. In
addition, the catalyst can be recycled for at least five
consecutive runs without appreciable loss in the prod-
uct yields.
ACKNOWLEDGMENTS
This work was supported by the Research Council
at the University of Shahid Chamran.
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