One-pot, solvent-free and efficient synthesis of 2,4,6-triarylpyridines catalyzed by nano-titania-supported sulfonic acid as a novel heterogeneous nanocatalyst

Article abstract of DOI:10.1016/j.cclet.2015.06.013

Nano titania-supported sulfonic acid (n-TSA) has found to be a highly efficient, eco-friendly and recyclable heterogeneous nanocatalyst for the solvent-free synthesis of 2,4,6-triarylpyridines through one-pot three-component reaction of acetophenones, aryl aldehydes and ammonium acetate. This reported method illustrates several advantages such as environmental friendliness reaction conditions, simplicity, short reaction time, easy work up, reusability of catalyst and high yields of the products. One new compound is reported too. Furthermore, the catalyst could be recycled after a simple work-up, and reused at least six times without substantial reduction in its catalytic activity.

Full text of DOI:10.1016/j.cclet.2015.06.013

Accepted Manuscript
Title: One-pot, solvent-free and efficient synthesis of
2,4,6-triarylpyridines catalyzed by nano-titania-supported
sulfonic acid as a novel heterogeneous nanocatalyst
Author: Elham Tabrizian Ali Amoozadeh Salman Rahmani
Elham Imanifar Saeede Azhari Masoumeh Malmir
PII:
S1001-8417(15)00269-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.cclet.2015.06.013
CCLET 3370
To appear in:
Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
26-2-2015
10-4-2015
8-6-2015
Please cite this article as: E. Tabrizian, A. Amoozadeh, S. Rahmani, E.
Imanifar, S. Azhari, M. Malmir, One-pot, solvent-free and efficient synthesis
of 2,4,6-triarylpyridines catalyzed by nano-titania-supported sulfonic acid
as a novel heterogeneous nanocatalyst, Chinese Chemical Letters (2015),
http://dx.doi.org/10.1016/j.cclet.2015.06.013
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Graphical Abstract
One-pot, solvent-free and efficient synthesis of 2,4,6-triarylpyridines catalyzed by nano-titania-supported sulfonic acid as a
novel heterogeneous nanocatalyst
Elham Tabrizian, Ali Amoozadeh*, Salman Rahmani, Elham Imanifar, Saeede Azhari, Masoumeh Malmir
Department of Organic Chemistry, Faculty of Chemistry, Semnan University, Semnan 35131-19111, Iran
H
3
R1
O
R2
H
3
S
H
O
O 3
S
S
H3C
O
O
3
H O S
CHO
H3C
Nano-TiO2
OAc
NH4
SO3H
R1
R2
N
S
O
S
3
O
H
3
H
R1
R1
Nano-titania-supported sulfonic acid as an efficient and recyclable heterogeneous nanocatalyst is reported for the synthesis of 2,4,6-
triarylpyridines under solvent-free conditions. One new compound is reported too.
2
Page 1 of 8

Original article
One-pot, solvent-free and efficient synthesis of 2,4,6-triarylpyridines catalyzed by
nano-titania-supported sulfonic acid as a novel heterogeneous nanocatalyst
Elham Tabrizian, Ali Amoozadeh , Salman Rahmani, Elham Imanifar, Saeede Azhari, Masoumeh Malmir
Department of Organic Chemistry, Faculty of Chemistry, Semnan University, Semnan 35131-19111, Iran
ARTICLE INFO
ABSTRACT
Nano titania-supported sulfonic acid (n-TSA) has found to be a highly efficient, eco-friendly
and recyclable heterogeneous nanocatalyst for the solvent-free synthesis of 2,4,6-
triarylpyridines through one-pot three-component reaction of acetophenones, aryl aldehydes
and ammonium acetate. This reported method illustrates several advantages such as
environmental friendliness reaction conditions, simplicity, short reaction time, easy work up,
reusability of catalyst and high yields of the products. One new compound is reported too.
Furthermore, the catalyst could be recycled after a simple work-up, and reused at least six
times without substantial reduction in its catalytic activity.
Article history:
Received 26 February 2015
Received in revised form 10 April 2015
Accepted 8 June 2015
Available online
Keywords:
Nano-titania-supported sulfonic acid
Multicomponent reaction
2,4,6-Triarylpyridines
Solvent free
Heterogeneous nanocatalyst
1. Introduction
In the last two decades, multicomponent reactions (MCRs) have been widely used in organic synthesis due to their efficacy for the
generation of heterocyclic compounds in a single synthetic step [1]. These reactions represent very powerful chemical technology
procedures from both economical and synthetic points of view and they are one of the most influential techniques in green chemistry,
industrial chemistry and the modern drug discovery process [2]. Therefore, the discovery of novel MCRs is an interesting topic for
synthetic chemistry researchers.
Recently, solid-supported heterogeneous catalysts have gained considerable interest in organic synthesis because of their unique
properties such as high efficiency due to more surface area, more stability, reusability, low toxicity and ease of handling [3-8]. We
have recently reported nano titania-supported sulfonic acid (n-TSA) [9], as an effective heterogeneous acidic nanocatalyst for the
promotion of a wide range of organic reactions as for its Lewis and Bronsted acidity features. Ease of preparation, simple work-up
procedures and improved product yields, facile purification, shorter reaction times, mild reaction conditions and recyclability of the
catalyst are the main superiorities of n-TSA.
Pyridine ring systems, particularly 2,4,6-triarylpyridines represent an important class of heterocyclic compounds because of their
unique position in medicinal chemistry [10, 11]. Moreover, they are prominent synthons in supramolecular chemistry, with their π-
stacking ability along with directional H-bonding capacity [12, 13]. In addition, the excellent thermal stabilities of these pyridines have
instigated a growing interest for their use as monomeric building blocks in thin films and organometallic polymers [14].
Since, Krohnke’s original report on the synthesis of 2,4,6-triarylpyridines [15], there has been a plethora of research targeting their
syntheses [16, 17]. Recently, the synthetic approach to 2,4,6-triarylpyridines involves cyclo-condensation reaction of acetophenones,
benzaldehydes and ammonium acetate in the presence of different catalysts under heating condition or sonication and microwave
irradiation [18-22].
Wide variety of 2,4,6-triarylpyridine chemical and pharmacological applications, encouraged us to develop and validate an efficient
protocol for their synthesis. Herein, we have reported nano titania-supported sulfonic acid catalyzed synthesis of 2,4,6-triarylpyridines
under solvent-free conditions.
2. Experimental
Corresponding author.
E-mail address: aamozadeh@semnan.ac.ir (A. Amoozadeh).
Page 2 of 8
3

Chemicals were purchased from the Merck chemical companies. Thin-Layer Chromatography (TLC) on commercial plates of silica
gel 60 F254 was used to monitor the progress of reactions. The products were characterized by FT-IR spectra, 1H NMR and 13C NMR.
FT-IR spectra were recorded on Shimadzu FT-IR 8400 instrument. 1H NMR and 13C NMR spectra were recorded on Bruker Advance
Spectrometer 300 MHz and 75 MHz respectively, using CDCl3 as solvent. The chemical shifts are expressed in parts per million (ppm)
and tetramethylsilane (TMS) was used as an internal reference. Melting points were recorded on a THERMO SCIENTIFIC 9100
apparatus.
2.1. General procedure for the synthesis of catalyst (n-TSA)
Chlorosulfonic acid (1 mL, 15 mmol) was added dropwise to a suspension of powdered nano TiO2 (4 g) in dry CH2Cl2 (20 mL) over
a period of 30 min while the mixture was stirred slowly in an ice bath. The mixture was stirred for 30 min at room temperature until
HCl evolution was seized. Then, the CH2Cl2 was removed under reduced pressure and the solid powder was washed with ethanol (10
mL) and dried at 70 °C. n-TSA was obtained as a white solid powder [9].
The mmol of H+ per gram of catalyst was also evaluated. For this purpose, the surface acidic protons of nano-TiO2-SO3H (100 mg)
were ion-exchanged with a saturated solution of NaCl (10 mL) by sonicating. This process was repeated twice more, yielding 30 mL of
proton-exchanged brine solution. Therefore, to determine the loading of acid sites on the synthesized catalyst the obtained solution
titrated with NaOH (0.1 mol/L) solution in the presence of phenol red indicator solution or pH meter. The amounts of acidic protons
found to be 4.5 mmol.g-1 of n-TSA. This finding means the effective density of the acid sites.
The catalyst was characterized in our previously published paper by the X-ray diffraction (XRD), scanning electron microscopy
(SEM), FT-IR spectroscopy, thermal gravimetric analysis (TGA) and the acid strength of n-TSA was determined by the Hammett
acidic function too [9].
2.2. General procedure for the synthesis of 2,4,6-triarylpyridine derivatives
A mixture of acetophenones (2 mmol), aromatic aldehydes (1 mmol), ammonium acetate (1.5 mmol) and 0.009 g of n-TSA was
stirred at 110 °C under solvent-free condition for an appropriate time. After completion of the reaction (monitored by TLC), the
reaction mixture was cooled, eluted with hot ethanol (2 mL) and was centrifuged to separate the catalyst. The title compounds were
obtained in their crystalline forms by recrystallization of ethanol solution.
2,6-Bis(4-bromophenyl)-4-(p-tolyl)pyridine (5c): Light green crystal; mp: 236-240 °C; IR (KBr, cm-1) νmax: 3028, 1601, 1577, 1516,
1489; 1H NMR (300 MHz, CDC13):
δ
2.46 (s, 3H), 7.33 (d, 2H, J = 8.12 Hz), 7.60 (m, 6H), 7.80 (m, 2H), 8.05 (dd, 4H, J = 7.66, 4.02
Hz); 13C NMR (75 MHz, CDCl3): δ 21.3, 116.8, 123.6, 126.9, 128.6, 129.9, 131.8, 135.6, 138.2, 139.3, 150.3, 156.3.
3. Results and discussion
Considering our researches toward evaluating the catalytic activity of n-TSA for the synthesis of organic compounds [9, 23], we
have investigated the synthesis of 2,4,6-triarylpyridines (4-7) via one-pot, three-component cyclo-condensation reaction of
acetophenones (1), aldehydes (2) and ammonium acetate (3) under solvent-free conditions (Scheme 1).
R2
O
CHO
n-TSA
(0.009 g)
CH3
NH4OAc
+
+
R1
Solvent free
R2
1
110 o
C
N
1a:
1b:
1c:
1d:
R = H
1
3
2
R1
R1
R = Br
1
R = Cl
4, 5, 6, 7
1
R = NO2
Scheme 1. Synthesis of 2,4,6-triarylpyridine derivatives by n-TSA.
To choose the most appropriate condition in this reaction and to understand the influence of different variables, several experiments
were studied. Initially, we optimized the amount of catalyst in order to find the required catalyst load. The obtained results are listed in
Table 1.
Table 1
Optimum amount of catalyst on the synthesis of 2,4,6-triphenylpyridine.a
O
CHO
CH3
n-TSA
NH4OAc
+
+
Solvent free
110 o
C
N
Page 3 of 8
4

Entry
Catalyst (g)
0
Yield (%)b
TONc
-
58.4
34
16.53
12.27
9.1
TOFd (h-1)
-
29.2
17
8.26
6.13
4.55
3.64
1
2
3
4
5
6
7
65
73
85
93
92
91
91
0.002
0.004
0.009
0.012
0.016
0.020
7.28
a Reaction conditions: acetophenone (2 mmol), benzaldehyde (1 mmol), NH4OAc (1.5 mmol) at 110 °C under solvent-free conditions, reaction
time: 2 h.
b Isolated yields.
c TON= mmol of product per mmol of catalyst.
d TOF= TON/time.
As indicated in Table 1, the best yield was obtained in the presence of 0.009 g catalyst, which is enough to carry out the reaction
efficiently (Table 1, entry 4).
In the next step, the effect of temperature, solvent and ammonia source was examined over the reaction and the obtained results are
shown in Table 2.
Table 2
Optimum condition on the synthesis of 2,4,6-triphenylpyridine.a
O
CHO
n-TSA
Ammonia 0.009 g
+ source
CH3
+
N
Entry
1
2
3
4
5
6
7
8
Temp (oC)
60
Solvent
Ammonia source
Yield (%)b
-
-
-
-
-
-
-
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4Cl
0
70
80
90
20
51
75
82
93
90
10
22
31
15
70
81
10
20
15
0
100
110
120
110
110
110
110
110
110
110
110
110
110
110
CH2Cl2
CH3CN
CHCl3
H2O
EtOH
PEG-400
9
10
11
12
13
14
15
16
17
18
-
-
-
-
-
NH4Br
NH4I
(NH4)2SO4
(NH4)2CO3
50
a Reaction conditions: acetophenone (2 mmol), benzaldehyde (1 mmol), ammonia source (1.5 mmol), n-TSA (0.009 g), reaction time: 2 h.
b Isolated yields.
As can be seen in Table 2, the product yield increased as the reaction temperature was raised at 110 °C and the product was obtained
in high yield which was considered as the optimum temperature (Table 2, entry 6), but at higher temperature the yield was decreased
(Table 2, entry 7) due to side reactions monitored by TLC. On the other hand, neither polar nor non-polar solvents are suitable for this
reaction (Table 2, entries 8-13) and the best yield was obtained under solvent-free condition (Table 2, entry 6), which is due to the
appropriate interaction between the catalyst and solvent. Eventually, the effect of various ammonia sources on yield of the product was
shown that the maximum yield was obtained for NH4OAc (Table 2, entry 6), while other ammonia sources required longer reaction
times and produced lower yields (Table 2, entries 14-18).
To develop the scope of the reaction, a wide range of aldehydes and acetophenones was subjected to n-TSA-catalyzed reaction under
the optimized conditions (Table 3).
Table 3
Synthesis of 2,4,6-triarylpyridine derivatives in the presence of nano-TiO2-SO3H.a
Page 4 of 8
5

R2
O
CHO
n-TSA
CH3
(0.009 g)
NH4OAc
+
+
R1
Solvent free
R2
110 o
C
N
R1
R1
Product
Entry
1
2
3
4
5
6
7
8
R1
H
H
H
H
H
H
H
H
R2
4-H
4-Cl
Time (min)
120
100
90
Yield (%)b
93
Melting point (°C)
135-138
123-124
165-166
196-198
194-195
127-129
100-101
115-117
135-137
104-106
>200
4a
4b
4c
4d
4e
4f
4g
4h
4i
95
94
84
87
88
82
94
87
95
96
92
93
88
89
87
83
4-Br
4-OH
4- NO2
4-CH3
4-OCH3
2-Cl
4-N(CH3)2
4-H
4-Cl
140
80
135
150
105
130
100
110
120
80
120
110
125
130
120
100
110
115
9
H
10
11
12
13
14
15
16
17
18
19
20
21
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
5a
5b
5cc
5d
6a
6b
6c
6d
6e
7a
7b
7c
4-CH3
3-NO2
4-H
236-240
>200
123-125
169-171
202-204
187-188
140-142
316-318
312-313
143-145
2-Cl
4-CH3
4-OCH3
4-N(CH3)2
4-H
4-CH3
4-OCH3
84
90
88
87
NO2
NO2
NO2
a Reaction conditions: acetophenone (2 mmol), aldehyde (1 mmol), ammonium acetate (1.5 mmol), n-TSA (0.009 g) at 110 °C under solvent-
free conditions.
b Isolated yields.
c New compound.
In all cases, aromatic aldehydes with substituents carrying either electron-donating or electron-withdrawing groups reacted
successfully and gave the products in good yields (Table 3). It was found that aromatic aldehydes with electron-withdrawing groups
(Table 3, entries 2, 3, 5, 8, 11, 13, 15) reacted faster than those with electron-donating groups (Table 3, entries 4, 6, 7, 9, 12, 16-18, 20,
21), as would be expected [24, 25]. These results justified one more time the efficiency of n-TSA.
The proposed mechanism for the formation of 2,4,6-triarylpyridines is depicted in Scheme 2. The reaction involves four major
stages: Aldol condensation, Michael addition, cyclization, and oxidation. Initially, acetophenone (I) in the presence of nano-TiO2-
SO3H is converted into its enol form which gives nucleophilic addition to the arylaldehyde (II), which is activated by the catalyst, to
afford aldol condensation product (III). Then, a molecule of acetophenone is condensed with an ammonia source (IV) and enamine (V)
is formed. SO3H group of n-TSA enhances the electron deficiency on carbonyl group of (III) which easily could be attacked by (V) in
a Michael addition reaction to afford intermediate (VI). The following cyclisation provides dihydropyridine (VIII). Finally, air
oxidation gives the final product (IX). As indicated in Scheme 2, the SO3H groups of n-TSA plays principal role in the catalytic
activity of the catalyst and promotion of the reaction rate.
Page 5 of 8
6

Scheme 2. Plausible mechanism for the synthesis of 2,4,6-triarylpyridines by n-TSA.
In order to exhibit the merit of the present work, our results are compared with some other previously reported studies in the Table 4.
Table 4
Comparison n-TSA with other reported catalysts in the synthesis of 2,4,6-triphenylpyridine.
Entry Catalyst (amount mol%)
Condition
Time (h)
Yield (%)
TONb
12.14
8.70
2.33
4.40
TOFd (h-1)
4.05
2.17
0.47
1.47
Reference
[26]
[27]
[28]
[24]
1
2
3
4
5
6
MgAl2O4 (7)
120 °C, solvent-free
120 °C, solvent-free
120 °C, solvent-free
120 °C, solvent-free
100 °C, solvent-free
110 °C, solvent-free
3
4
5
3
4
2
85
87
70
88
91
93
AlPO4 (10)
TBAHSa (30)
[HO3S(CH2)4MIM][HSO4] (20)
ZrOCl2 (15)
6.07
16.53
1.52
8.26
[19]
This work
n-TSA (5.62)
a Trichloroisocyanuric acid.
bTON= mmol of product per mmol of catalyst.
c TOF= TON/time.
As shown in Table 4, n-TSA can act as an effective catalyst with respect to the reaction time, yield of the product, TON (turnover
number) and TOF (turnover frequency). Moreover, it can be recovered and reused for several times.
The principle advantage of the use of heterogeneous catalysts in organic transformations is their reusability. For this purpose, after
completion of the reaction, the reaction mixture was cooled, eluted with hot ethanol (2 mL) and was centrifuged to filter the catalyst.
The catalyst was then washed with ethanol, dried at 50 °C and used directly for the synthesis of 2,4,6-triphenylpyridine. The recovered
n-TSA was reused for 6 consecutive reactions and obtained the yield 90%-93%. This indicates that n-TSA does not lose its activity and
can be recyclable (Fig. 1).
Fig. 1. Reusability of n-TSA for the synthesis of 2,4,6-triphenylpyridine.
The particle size of the recovered catalyst was confirmed by the scanning electron microscopy of the six-time reused catalyst, which
clearly demonstrated the same particle size (spherical powder with average size of 35-60 nm) and high stability of the nano TiO2-based
solid acid catalyst (Fig. 2).
Page 6 of 8
7

Fig. 2. SEM images of n-TSA before use (a) and after reuse six times (b).
4. Conclusion
In summary, an efficient and rapid route for the synthesis of 2,4,6-triarylpyridines under solvent-less conditions is reported.
Operational simplicity, short reaction time, simple work-up without column chromatographic purification, reusability of the catalyst,
tolerance of various functional groups, excellent yields of products and solvent free conditions are the advantages of this protocol.
Moreover, one new compound is reported.
Acknowledgment
We gratefully acknowledge the Faculty of Chemistry of Semnan University for supporting this work.
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