Solid acid catalyst TS-1 zeolite-assisted solvent-free one-pot synthesis of poly-substituted 2,4,6-triaryl-pyridines

Article abstract of DOI:10.1007/s11164-018-3305-4

Abstract: A new method is described for one-pot solvent-free synthesis of 2,4,6-triaryl pyridines in the presence of a solid acid catalyst, titanium silicate (TS-1) via cyclocondensation of acetophenone, aryl aldehyde and ammonium acetate. The present method illustrates several advantages, such as eco-friendly reaction conditions, simplicity, short reaction time (3?h), easy separation of catalyst and high yields of the products (85–93%). Furthermore, the TS-1 catalyst was reused for four catalytic cycles with consistent catalytic activity. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11164-018-3305-4

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3305-4
Solid acid catalyst TS‑1 zeolite‑assisted
solvent‑free one‑pot synthesis of poly‑substituted
2
,4,6‑triaryl‑pyridines
Sachin P. Gadekar · Machhindra K. Lande1
1
Received: 2 September 2017 / Accepted: 23 January 2018
©
Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract A new method is described for one-pot solvent-free synthesis of
2
1
,4,6-triaryl pyridines in the presence of a solid acid catalyst, titanium silicate (TS-
) via cyclocondensation of acetophenone, aryl aldehyde and ammonium acetate.
The present method illustrates several advantages, such as eco-friendly reaction con-
ditions, simplicity, short reaction time (3 h), easy separation of catalyst and high
yields of the products (85–93%). Furthermore, the TS-1 catalyst was reused for four
catalytic cycles with consistent catalytic activity.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/
s11164-018-3305-4) contains supplementary material, which is available to authorized users.
*
Machhindra K. Lande
mkl_chem@yahoo.com
1
Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad,
Maharashtra 431004, India
1
3

S. P. Gadekar, M. K. Lande
Graphical Abstract
NH OAc
4
+
COCH 3
CHO
TS-1, 0.04g
10 oC, 3h.
2
1
R1
Ar1
Ar
N
Ar
Keywords Titanium silicate (TS-1) · One-pot reaction · Solid acid catalyst ·
,4,6-Triaryl pyridine
2
Introduction
The one-pot synthesis method has played a signiꢀcant role in organic synthesis
because of its several advantages such as atom economy, high selectivity, mini-
mum reaction time and least by-products as compared to classical synthetic meth-
ods during last two decades, [1]. Nitrogen-containing heterocyclic compounds such
as 2,4,6-triaryl pyridine have proved to be important building blocks of alkaloids
and natural products [2]. They have also been found to possess biological and phar-
maceutical activity such as anti-oxidant, anticancer, antibacterial, antiparasitic and
antitubercular [3–5]. Some substituted pyridine derivatives show antifungal (a),
anti-HIV (b), anticancer (c) and DNA binding activities (d), while dihydroxylated
2
,6-diaryl-4-chlorophenyl pyridines are topoisomerase inhibitors and show anitcan-
cer activity [6–8].
1
3

Solid acid catalyst TS‑1 zeolite‑assisted solvent‑free one‑pot…
OH
H
N
CN
NH2
CN
NH2
N
N
N
OH
b)
HO
(a)
(
c)
Topo isomerase inhibitor
Anti-cancer)
(
Antifungal agent
Anti-HIV agent
(
S
O
N
O
N
H
N
H
N
N
(d)
G- quadruplex DNA binder
(
Anti-cancer)
They also show ꢁuorescence properties and are used in thin ꢀlm vortex ꢁu-
idic device development [9]. Additionally, 2,4,6-triaryl pyridine derivatives have
a unique position in medicinal chemistry, being building block synthons of supra-
molecular chemistry, because of their π staking and H-bonding ability [10–12].
Considering these wide and versatile applications, several workers have devel-
oped catalytic methods for the synthesis of 2,4,6-triaryl pyridine derivatives,
such as heteropolyacid H [NaP W O ], MIL 101-SO H, ionic liquid, BiCl ,
1
4
5
30 110
3
3
bismuthtriꢁate, ZnO, AlPO and MgAl O with acetic acid [13–20], titanium-
4
2
3
supported sulfated silica with toluene, and montmorilonite K10 clay [21–23].
Although the reported methods are quite eꢂcient, some of them suꢃer from one
or more limitations, such as longer reaction times, harsh reaction conditions, car-
cinogenic or volatile organic solvents, low yields and the use of expensive cata-
lysts. It has also been found that some of the reported catalysts are not reusable,
while a few methods require speciꢀc reaction conditions, such as microwave and
sonication in the homogenous phase, like KOH in DMSO [24–26]. Some chemi-
cal transformations have required a higher temperature to synthesize the desired
product from acetophenone with epoxy styrene [27]. Hence, to overcome these
limitations, it is imperative on the part of researchers to ꢀnd a sustainable,
1
3

S. P. Gadekar, M. K. Lande
eꢂcient, eco-friendly and cost-eꢃective protocol for the synthesis of 2,4,6-triaryl
pyridine derivatives.
Titanium silicate (TS-1) zeolite has been found to be an alternative eꢂcient het-
erogeneous solid acid catalyst with unique properties, such as high surface area,
thermal stability, low toxicity, reusability, ion exchange ability and ease of handling,
and have hence been used in various industrial processes such as oxidation, epoxida-
tion, alkylation, acylation and cyclic condensation reactions [28–33]. In view of the
ever-increasing importance of green/sustainable synthetic protocols, it is planned to
develop eco-friendly, eꢂcient catalytic methods for organic synthesis [34, 35].
Silicate-1 zeolite is crystalline with a micro-porous MFI framework, and weak
Lewis acidity. The acidity of silicate-1 can be enhanced by doping with transition
metal cations, such as Ti (IV), which generates strong Lewis acidic sites in the
framework, including Bronsted acidic sites. It has been reported that the acidity of
TS-1 increases with increasing titanium content in a zeolite framework [36–39].
Considering this view and in continuation of our research work on the develop-
ment of heterogeneous solid acid catalysts, we report here the synthesis of a TS-1
zeolite catalyst by incorporating Ti (IV) cations in the framework of silicate-1 [40,
4
1].
Experimental
General Procedure for the Synthesis of the Catalyst
Amounts of 24.37 mL of tetraethyl orthosilicate and 25 mL, 20% tetrapropyl ammo-
nium hydroxide were added to a ꢁask with vigorous stirring. The resulting reac-
tion mixture was stirred for 10 min at room temperature to form a silica sol. Next,
1
.16 mL of Titanium tetrabutoxide was mixed in 30 mL of dry isopropyl alcohol,
stirred vigorously for 10 min, and the resulting mixture was added dropwise into
the silica sol with constant stirring at 70–75 °C with the pH maintained above 12.
The resulting transparent viscous gel was transferred to a Teꢁon-lined stainless steel
autoclave, and treated hydrothermally under static condition and autogenous pres-
sure at 175 °C for 24 h. The solid product was ꢀltered, dried in an oven at 100 °C for
1
h, and ꢀnally calcined at 500 °C for 4 h in a muꢄe furnace, under air atmosphere.
The resulting material was cooled naturally, characterized and used as the catalyst.
General reaction procedure synthesis of modal reaction 3a
A mixture of acetophenone (240.30 mg, 2 mmol), 4-chloro-benzaldehyde
(
140.56 mg, 1 mmol), NH OAc (100.20 mg, 1.3 mmol), and 40.00 mg of the
4
TS-1 catalyst was added and reꢁuxed for 3–4 h, as shown in Scheme 1. The pro-
gress of the reaction was monitored by thin-layer chromatography using petroleum
ether:ethyl acetate (8:2) as a solvent system. After completion of the reaction, the
catalyst and product were separated. Then, the catalyst was activated and used for
the next batch of reactions. The ꢀltered organic layer containing the product was
1
3

Solid acid catalyst TS‑1 zeolite‑assisted solvent‑free one‑pot…
R1
O
O
TS-1, 0.04 g ,Neat.
H
2
+
o
R2
R1
110 C, 3h.
N
NH OAc
4
R2
R2
3a-k
Scheme 1 Synthesis of 2,4,6-Triaryl-pyridine derivatives model reaction, 3a
dried by using sodium sulfate, and then the ꢀltrate was concentrated to obtain the
crude product which was recrystallized from the ethanol solvent to aꢃord the pure
products (3a–l).
Spectral data
1
2
,4,6‑Triarylpyridine (3a) Yiled = 93%, M. P. 132–134 °C, H NMR (CDCl ):
3
1
3
δ 7.25–8.2 (m, 17H, Ar–H); C NMR (CDCl ): δ 157.37, 150.07, 139.45, 138.93,
3
1
28.99, 128.92, 128.85, 128.58, 128.33, 127.06, 127.00, and 117.01; FT‑ IR (KBr,
+1
−1
cm ): 3058, 1576, 1549, 1449, 1399, 1074, 870, 735, and 692; LC–MS: M ,
3
08.14 found 308.13.
4
‑(4‑Methylphenyl)‑2,6‑(biphenyl)pyridine (3b) Yiled = 89%, M. P. 124–
1
1
25 °C, H NMR (CDCl ): δ 8.20 (dd, 4H, Ar–H), 7.22–7.65 (m, 10H, Ar–H), 7.86
3
1
3
(
s, 2H, Pyridyl-H), and 2.42 (s, 3H, CH ); C NMR (CDCl ): δ 157.49, 150.07,
3
3
1
39.71, 139.12, 136.11, 129.87, 129.03, 128.73, 127.17, 127.03, 116.94, and 21.31;
FT‑ IR (KBr): 3031, 2919, 1595, 1545, 1449, 1418, 1391, 1252, 1209, 1178, 1032,
+1
8
74, 769, and 619; LC–MS: M , 322.15 found 322.13.
‑(2,6‑Diphenylpyridin‑4‑yl)phenol (3c) Yiled = 90%; M. P. 197–199 °C; H
1
4
NMR (CDCl ): δ 8.21 (dd, 4H, Ar–H), 7.75 (s, 2H, Pyridyl-H), 7.69 (dd, 2H, Ar
3
1
3
H), 7.4–7.5(m, 6H, Ar H), 6.99 (d, 2H, Ar H), and 5.14 (s, 1H, OH); C NMR
(
CDCl ): δ 157.58, 156.61, 149.67, 139.75, 131.61, 129.08, 128.78,128.66, 127.22,
3
−
1
1
16.73, and 116.10; FT‑IR (KBr, cm ): 3035, 2954,2850, 1603, 1545, 1518, 1399,
+1
1
348, 1206, 733, and 688; LC–MS: M , 324.13 found 324.12.
1
4
‑chlorophenyl (3d): HNMR (CDCl ) Yiled = 93%; M. P. 126–127 °C;
3
1
−1
HNMR (CDCl ): δ 7.26–8.21 (m, 17 H, Ar–H); FT‑IR (KBr, cm ): 3031, 1696,
3
+1
1
649, 1348, and 1233; LC–MS: M , 342.10 found 342.09.
5
‑methoxy‑2‑(2,6‑diphenylpyridin‑4‑yl)phenol (3k)) Yiled = 88%; M. P. 167–
1
1
68 °C; HNMR (CDCl ): δ 3.05–3.11 (dd, 1H, CH ), 3.78 (dd, 1H CH ), 3.81 (s,
3
2
2
1
3

S. P. Gadekar, M. K. Lande
3
1
1
H OCH ), 3.80–3.81 (s, 3H OCH ), 5.15–5.20 (dd, 1H, CH), and 6.77–7.29 (m,
3
3
1
3
3 Ar–H); C NMR (CDCl ): δ 139.88, 129.45, 129.26, 129.12, 128.81, 128.62,
3
28.54, 127.29, 127.17, 127.02, 119.26, 116.07, 56.20, and 56.07; FT‑IR (KBr,
−1
+1
cm ): 3495, 2924, 1666, 1476, 1256, 1064, and 694; LC–MS: M , 354.14 found
3
54.107.
Results and discussion
The TS-1 catalyst was synthesized and characterized by X-ray diꢃraction, Fourrier-
transform infrared spectroscopy, scanning electron microscopy and BET surface
area analysis. The XRD of TS-1, depicted in Fig. S1 (Supplementary information),
shows intense peaks at 2θ° = 14.63, 21.81, 22.76, 22.59, 24.04, and 26.62, with
the corresponding planes (112), (023), (150), (511), (250), and (004) respectively;
the planes (150) and (511) indicate the presence of an ordered orthorhombic TS-1
framework [42, 43].
The FT-IR spectrum of TS-1 is shown in Fig. S2 (Supplementary information).
−1
The absorption band at 530–551 cm corresponds to the presence of S5R, which
conꢀrms the MFI topology of TS-1 [44, 45]. The band appearing between 550 and
−
1
6
23 cm is due to Ti–O bond vibration [46]. The sharp absorption band appearing
−1
at 800 cm is ascribed to Si–O–Si symmetric stretching vibration [47]. The absorp-
−1
tion band observed in the range 952–967 cm is due to Si–O–Ti bond vibration,
which conꢀrms the presence of Ti (IV) content in the TS-1 framework. The charac-
−
1
teristic band observed in the range 1228–1265 cm is for the MFI topology of the
−
1
TS-1 zeolite [43]. However, the broad band in the range 3404–3799 cm is due to
Si–OH–Si or Si–OH–Ti vibrations [48].
The surface morphology and textural properties of the TS-1 catalyst have been
studied by scanning electron microscopy, which shows uniform sphere-shaped crys-
tals, which is a characteristic morphology of TS-1-type zeolites. The SEM image
suggests that the synthesized catalyst has a uniform particle size and ordered mor-
phology, as shown in Fig. 1. The BET surface area, average pore diameter and pore
2
2
volume of calcined TS-1 were found to be 408.3 m /g, 20.87 Å and 0.2130 cm /gm
respectively calculated by the N adsorption–desorption BJH isotherm.
2
The synthesized catalyst was used for the one-pot solvent-free synthesis of poly-
substituted 2,4,6-triaryl-pyridines via cyclocondensation of acetophenone, aryl
aldehyde and NH OAc (Scheme 1). The structure of representative derivatives have
4
1
13
been conꢀrmed by H and C NMR, Mass, and FTIR techniques.
In order to explore the scope and importance of the investigated method, we have
synthesized several pyridine derivatives using electronically diverse aldehydes (R )
1
and acetophenones (R ), and the results are summarized in Table 1. It was found that
2
acetophenones (R ) and aldehydes (R ) with electron-donating groups like H, CH ,
2
1
3
4
-OH, 4-OCH gave maximum yields as compared to electron-withdrawing groups
3
like 4-Cl and 4-NO . However, 4-Cl gave more yield as compared to NO because of
2
2
a mesomeric eꢃect.
1
3

Solid acid catalyst TS‑1 zeolite‑assisted solvent‑free one‑pot…
Fig. 1 SEM images of TS-1 zeolite calcined at 500 °C
Table 1 TS-1 catalyzed
synthesis of 2,4,6-triaryl-
pyridine derivatives
Yield (%)a
M. P. (°C)b
Entry
R2
R1
3
3
3
3
3
3
3
3
3
3
3
3
ac
b
c
d
e
f
g
h
i
H
H
H
H
H
CH3
93
89
90
93
88
89
89
90
90
85
88
85
132–134
124–125
197–199
126–127
134–136
99–100
4-OH
4-Cl
4-OCH3
4-OCH3
H
4-CH3
4-Cl
4-NO2
2-OH
4-OCH3
4-OCH3
H
4-OCH3
4-OCH3
4-OCH3
4-OCH3
H
98–100
157–158
113–114
143–145
167–168
142–144
j
k
l
NO2
a
Isolated yields
b
Melting points are in good agreement with those reported in the lit-
erature [19–23]
c
The TS-1 was run for four consecutive cycles
1
3

S. P. Gadekar, M. K. Lande
Proposed mechanism for the formation pyridine derivatives
The aldehyde and acetophenone initially formed a chalcone intermediate in the pres-
ence of the TS-1 catalyst. It has been assumed that the reaction was initiated by the
polarization of the > C=O group of the aldehyde, which perhaps simultaneously
interacts with the > C=O group of the acetophenone to enhance the electrophilic
interactions. In second step, a Michael addition reaction was facilitated by the two
components, chalcone and enamine, which are in close proximity to each other. Sub-
sequently, ring cyclization–dehydration–air oxidation–aromatization gave rise to the
desired 2,4,6-triaryl-pyridine derivatives.
The posssible reaction mechanism for the formation of triaryl pyridines is
depicted in Scheme 2. The enol form of acetophenone (I) gives a nucleophilic addi-
tion on the aldehyde (II) in the presence of TS-1 to aꢃord the condensation product
of chalcone intermediate (III). Then, the second molecule of acetophenone reacts
with ammonia to form an amine, which undergoes a Michal addition reaction with
(
III) to form the (IV) intermediate. Simultaneously, cyclization and dehydration
result in the formation of compound (V) in the presence of TS-1. Finally, the air oxi-
dation process gave the target molecule, triaryl pyridines (3a–l).
To specify the advantages of the proposed method, the results of diꢃerent
reported methods are compared with our results and are summarized in Table 2,
which conꢀrms that the TS-1 catalyst promotes the reaction more eꢂciently than the
other reported methods.
H O
O
O
2
O
O
O
Si
Ar
O
O
O
Ar
Ti
O
NH OAc
4
NH2
Ar
O
O
Ti
O
O
Si
O
OH
O
Ar1
Ar
Ar1
(
H
O
O
O
Ar
(III)
II)
(
I)
Ar1 NH
Air [O]
Ar1
O
O
Si
O
Ar1
OH
Ti
O
O
O
Ar
Ar
Ar1
Ar N Ar
O
(IV)
O
Ar
N
H
Ar
Ar
(
V)
H N
2
(VI)
Scheme 2 Proposed reaction mechanism for the synthesis of 2,4,6-triaryl-pyridine derivatives
1
3

Solid acid catalyst TS‑1 zeolite‑assisted solvent‑free one‑pot…
Table 2 Catalytic performance comparison of TS-1 and other reported catalysts
S. no.
Catalysts
Conditions
Time (h)
Yield (%)
1
2
AlPO /NH OAc
Solvent-free, 120 °C
Solvent-free, 120 °C
4–5
3.5
68–88 [18]
98 [13]
4
4
Polyamide-supported tung-
stophosphoric acid.
3
4
5
40% KOH/DMSO
Bi (OTf)3
TS-1
Solvent-free, 80 °C
Solvent-free, 120 °C
Solvent-free, 110 °C
2–3
2
3
80 [24–26]
89 [16]
93 (this work)
Optimization of reaction conditions
In order to ꢀnd the optimum loading of the catalyst, acetophenone (2 mmol),
4
-chloro-benzaldehyde (1 mmol), and ammonium acetate (1.3 mmol) were reꢁuxed
with the TS-1 catalyst. Ammonium acetate was used as a source of ammonia which
gave better results [13]. Initially, several protic and aprotic solvents with appropri-
ate times and temperatures were screened and the obtained results for model reac-
tion 3a, are summarized in Table 3. Then, the eꢃect of varying the amount of the
catalyst was tested, as summarized in Table 4. It has been found that, in the absence
of the catalyst, the reaction did not give satisfactory results of the desired product in
the stipulated period; however, the reaction yield was found to be a maximum in the
solvent-free condition, and therefore further optimization of the catalyst amount was
performed in a solvent-free reaction condition. It was found that 0.04 g of the TS-1
catalyst is suꢂcient for the model reaction (Table 4). Finally, the eꢃect of varying
the temperature and time was optimized and the results are presented in Table 5.
After optimizing the reaction conditions, eꢃorts were made towards the recovery
and reusability of the catalyst. After completion of the reaction, the TS-1 catalyst
was recovered from the reaction mixture by ꢀltration and washed with acetone or
another polar solvent and activated at 105 °C under vacuum for 10 min. The reus-
ability of the catalyst was investigated for four runs on themodel reaction and it was
found that the catalyst retained almost consistent catalytic activity (Fig. 2).
Table 3 Eꢃect of various
Yield (%)a
Entry
Solvent
T (°C)
Time (h:min)
solvents on the model reaction
3
a in the presence of 0.04 g of
1
2
3
4
5
6
7
8
Water
Water
Ethanol
Dichloromethane
Chloroform
Acetonitrile
Solvent free
Solvent free
r.t.
100
78
40
61
81
110
120
4:00
4:00
5:00
3:30
4:00
4:00
3:00
3:30
–
TS-1 catalyst
Trace
Trace
55
36
60
93
93
aIsolated yield
1
3

S. P. Gadekar, M. K. Lande
Table 4 Eꢃect of the amount of
TS-1 on the model reaction 3a
Yield (%)a
Catalyst (g)
–
Time (h:min)
3:00
3:00
3:00
3:00
3:00
3:00
Trace
25
40
60
93
0.01
0.02
0.03
0.04
0.05
93
aIsolated yield
Table 5 Eꢃect of temperature
on the yield under solvent-free
condition, 3a
Yield (%)a
T (°C)
Time (h:min)
5
8
1
1
1
0
0
00
10
20
3:00
3:00
3:00
3:00
3:00
Trace
70
85
93
93
aIsolated yield
Fig. 2 Reusability test of TS-1 for the synthesis of 3a
1
3

Solid acid catalyst TS‑1 zeolite‑assisted solvent‑free one‑pot…
Conclusions
In summary, we have developed a conventional approach for the synthesis of
2
,4,6-triaryl pyridines, assisted by the solid acid catalyst, TS-1 zeolite, from ace-
tophenone, aryl aldehyde and ammonium acetate. It shows excellent catalytic
potency for the synthesis of poly-substituted pyridines via a cyclic condensation
reaction under solvent-free conditions. This method oꢃers remarkable advan-
tages over reported methods because of the non-toxic, non-corrosive and reusable
nature of the TS-1 catalyst with excellent yields.
Acknowledgements We gratefully acknowledge the Head, Department of Chemistry, Dr. Babasa-
heb Ambedkar Marathwada University, Aurangabad–431004 (MS), India, for his support and constant
encouragement during the progress of this work and for providing necessarily laboratory facilities. The
corresponding author, MKL, is grateful to UGC New Delhi for providing ꢀnancial support under major
research project F. No. 43-184/2014 (SR). The authors are grateful to STIC Cochin and SAIF Chandigarh
for characterization facilities.
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