An efficient, green solvent-free protocol for the synthesis of 2,4,6-triarylpyridines using reusable heterogeneous activated Fuller’s earth catalyst

Article abstract of DOI:10.1007/s13738-018-1434-8

Abstract: A simple, efficient, and green method of preparation for the synthesis of highly substituted pyridines by multicomponent reaction of acetophenones, aldehydes, and ammonium acetate using activated Fuller’s earth as an effective and reusable heterogeneous catalyst is described. The advantages of the present protocol include simple procedure with an easy workup procedure, mild reaction conditions, and high yields of the products. The performance of this reaction under solvent-free conditions using heterogeneous catalysts, such as activated Fuller’s earth, could enhance its efficiency from an economic as well as ecological point of view. Graphical abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s13738-018-1434-8

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-018-1434-8
ORIGINAL PAPER
An efficient, green solvent-free protocol for the synthesis
of 2,4,6-triarylpyridines using reusable heterogeneous activated
Fuller’s earth catalyst
Deelip S. Rekunge1 · Ishwari A. Kale1 · Ganesh U. Chaturbhuj1
Received: 4 April 2018 / Accepted: 7 June 2018
© Iranian Chemical Society 2018
Abstract
A simple, efficient, and green method of preparation for the synthesis of highly substituted pyridines by multicomponent
reaction of acetophenones, aldehydes, and ammonium acetate using activated Fuller’s earth as an effective and reusable
heterogeneous catalyst is described. The advantages of the present protocol include simple procedure with an easy workup
procedure, mild reaction conditions, and high yields of the products. The performance of this reaction under solvent-free
conditions using heterogeneous catalysts, such as activated Fuller’s earth, could enhance its efficiency from an economic as
well as ecological point of view.
Graphical abstract
O
solvent-free, 110 0
C
CH3
2 R2
R1
N
R2
R2
O
Activated Fuller's earth
c
A
O
H N
4
R1
H
19 examples
•
•
•
Recyclable and non-corrosive catalyst
Metal-free synthesis
No column chromatography
Keywords Activated Fuller’s earth · Triarylpyridines · Ammonium acetate · Solvent-free condition
Electronic supplementary material The online version of this
Introduction
article (https://doi.org/10.1007/s13738-018-1434-8) contains
supplementary material, which is available to authorized users.
The N-heterocyclic pyridine compounds, mainly 2,4,6-tri-
arylpyridine, are of enormous attention owing to their wide
range of biological and pharmaceutical properties such
as anti-convulsant, anesthetic, anti-malarial, vasodilator,
anti-epileptic character (Fig. 1), and they are also used in
* Ganesh U. Chaturbhuj
gu.chaturbhuj@gmail.com
1
Department of Pharmaceutical Sciences and Technology,
Institute of Chemical Technology, Mumbai,
Maharashtra 400019, India
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
agro-chemicals as pesticidal, fungicidal, and herbicidal
[1–3]. Nowadays, this molecule has made it a prime tar-
get for scientific research because of the presence of the
pyridine ring system in natural products, such as NAD
nucleotides, pyridoxal (vitamin B6), pyridine alkaloids,
and a number of pharmacologically significant molecules
[4]. They are important in supramolecular chemistry due
to their π-stacking ability and directional H-bonding capac-
ity [5–8]. In addition, due to excellent thermal stabilities of
these pyridines, they are used as monomeric building blocks
in organometallic polymers and thin films [9–12]. 2,4,6-Tri-
arylpyridines have been directed for photodynamic cell-
specific cancer therapy having structure similarities with
symmetrical triaryl-thiopyrylium, triarylselenopyrylium,
and triaryl-telluropyrylium photosensitizers [13], also these
molecules found to be useful for the synthesis of DNA bind-
ing ligands, as a possible target in cancer therapy targeting
G-quadruplex DNA [14–18].
conditions, and process simplicity. Therefore, to overcome
this problem associated with the synthesis of pyridines, it is
highly desirable to develop a powerful method of synthesis
to meet the requirement of green chemistry. In this regard,
Fuller’s earth is a commercially available, non-toxic, eco-
friendly, economic material. It played a pivotal role under
heterogeneous conditions by catalyzing various organic
transformations; this is due to tangible benefits such as non-
corrosiveness, ease of preparation, handling, regeneration,
low cost, and insolubility in most of the organic solvents.
The catalytic activity of activated Fuller’s earth is due to
their Bronsted as well as Lewis acidic characters in their
natural form. It has been used to analyze color additives in
food products and as an adsorbent in pharmaceutical and
cosmetics. These qualities make it safer and suitable for both
laboratories as well as industrial processes. In connection
with our continuing studies recently, our lab has prepared,
characterized stable, and easily handled clay-based catalyst,
such as activated Fuller’s earth and sulfated polyborate, as
well as aluminized polyborate, and demonstrated its effec-
tiveness as an acid catalyst for the development of novel
methodologies to synthesize important heterocycles [37–54,
67]. Herein, we wish to report an efficient, and green method
for the synthesis of 2,4,6-triarylpyridines by one-pot three-
component reaction of acetophenone, aryl aldehydes, and
ammonium acetate using activated Fuller’s earth catalyst
under solvent-free conditions. To the best of our knowledge,
there is no report in the literature on the use of activated
Fuller’s earth in the synthesis of 2,4,6-triarylpyridines.
Traditionally, 2,4,6-triarylpyridines (Kröhnke-pyridine)
synthesis occur through the reaction of N-phenacyl pyri-
dinium salts with α, β-unsaturated ketones in the presence
of ammonium acetate (NH4OAc) [19, 20], but this method
is relatively expensive, time-consuming, and harmful as eco-
logical point of view, also the pyridinium salts and unsatu-
rated ketones have to be synthesized first, in this method.
Recently, several improved methods have been developed
for the synthesis of 2,4,6-triaryl pyridines, viz., reaction
of α-ketoketene dithioacetals with methyl ketones in the
presence of NH4OAc [21], reaction of N-phosphinyletha-
nimines with aldehydes [22], solvent-free reaction of chal-
cones with ammonium acetate [23], solvent-free reaction
between acetophenones and benzaldehydes, and NH4OAc
in the presence of various catalysts such as PEG-400 [24],
HClO4·SiO2 [25], catalytic amount of acetic acid [26],
H14[NaP5W30O110] [27], molecular iodine [28], l-proline
[29], microwave irradiation without catalyst [30], [BmIm]
[BF4] [31], wet 2,4,6-trichloro-1,3,5-triazine [32], pentafluo-
rophenyl ammonium triflate [33], trichloroisocyanuric acid
[34], bismuth triflate [35], and MgAl2O4 nanocrystals [36]
have been employed to promote this transformation. Among
these, the multicomponent one-pot reaction of aromatic
ketones, aldehydes, and ammonium acetate is one of the
simplest methods.
Experimental
All the chemicals and solvents used were of LR grade and
purchased from SD fine, Avra Synthesis, and Spectrochem,
and used as received. Melting points of all the compounds
were recorded by Analab ThermoCal melting point appara-
tus in the open capillary tube and are uncorrected. The FTIR
spectra (KBr) were recorded on Shimadzu FTIRAffinity-1
1
Fourier Transform infrared spectrophotometer. H NMR
spectra were recorded on MR400 Agilent Technology NMR
spectrometer using tetramethylsilane (TMS) as an internal
standard and CDCl3 as a solvent. X-ray diffractograms
(XRD) were recorded on Shimadzu X-ray diffractometer.
The SEM-EDAX characterization was performed on a JEOL
JSM-638DLA scanning electron microscope equipped with
energy dispersive X-ray spectrometer. The purity determi-
nation of the starting materials and reaction monitoring
was accomplished by thin-layer chromatography (TLC) on
Merck silica gel G F254 plates. All the products are known
compounds and were identified by 1H NMR spectroscopy
for structural identification.
However, many of these protocols suffer from drawbacks
such as the use of costly and corrosive catalysts, acidic
media, and toxic organic solvents, undesired side products
of the reaction with harsh reagents, long reaction time, cum-
bersome product isolation procedures, and environmental
pollution. To avoid such drawbacks, the development of
greener, simple, economical, and efficient protocols are still
in demand.
A green chemistry protocol involves considerations, such
as atom economy, non-hazardous catalysts, solvent-free
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Journal of the Iranian Chemical Society
1H NMR spectra
Preparation of activated Fuller’s earth
of 4‑(4‑chlorophenyl)‑2,6‑diphenylpyridine (Table 3,
entry 2)
The activated Fuller’s earth catalyst was prepared as per
procedure reported in the literature [37].
1H NMR (400 MHz, CDCl3) δ 8.19 (d, J = 8.4 Hz, 4H),
7.84 (s, 2H), 7.68 (d, J = 7.6 Hz, 2H), 7.52–7.49 (m, 8H).
General procedure for the synthesis
of 2,4,6‑triphenylpyridine
1H NMR spectra
of 4‑(4‑nitrophenyl)‑2,6‑diphenylpyridine (Table 3,
entry 4)
A mixture of benzaldehyde (2 mmol), acetophenone
(4 mmol), ammonium acetate (2.4 mmol), and activated
Fuller’s earth (10 wt%) was heated at 110 °C in an oil bath.
The reaction was monitored by thin-layer chromatography.
After completion of the reaction, the insoluble product
was dissolved in hot ethanol to separate activated Fuller’s
earth by filtration, and the filtrate was evaporated to get
the product. The product was technically pure subject to
purification by recrystallization for spectral analysis. The
products obtained were known compounds and identified
by their melting point and 1H NMR spectroscopy, and the
analytical data were compared with the literature values.
1H NMR (400 MHz, CDCl3) δ 8.40 (d, J = 8.8 Hz, 2H),
8.21 (d, J = 7.2 Hz, 4H), 7.98–7.88 (m, 4H), 7.55–7.46
(m, 6H).
1H NMR spectra
of 4‑(4‑methoxyphenyl)‑2,6‑diphenylpyridine
(Table 3, entry 5)
1H NMR (400 MHz, CDCl3) δ 8.19 (d, J = 7.2 Hz, 4H),
7.88 (s, 2H), 7.71 (d, J = 8.8 Hz, 2H), 7.51 (t, J = 7.2 Hz,
4H), 7.45 (d, J = 7.2 Hz, 2H), 7.05 (d, J = 8.8 Hz, 2H),
3.88 (s, 3H).
Procedure for the recyclability study
Recyclability of the catalyst is an important attribute for
the industrial suitability. Therefore, reusability of the cata-
lyst in the model reaction of acetophenone, benzaldehyde,
and ammonium acetate under a solvent-free condition at
110 °C was evaluated. After completion of the reaction,
the insoluble crude product was dissolved in hot ethanol,
and activated Fuller’s earth was recovered by filtration.
After filtration catalyst was washed with ethanol to remove
the organic traces. The catalyst was dried at 60 °C for 1 h
and reused for successive four cycles with no remarkable
loss of yield.
1H NMR spectra
of 4‑(3‑bromophenyl)‑2,6‑diphenylpyridine
(Table 3, entry 7)
1H NMR (400 MHz, CDCl3) δ 8.20 (d, J = 5.6 Hz, 4H),
7.88 (s, 1H), 7.84 (s, 2H), 7.52–7.38 (m, 9H).
1H NMR spectra of 2,6‑diphenyl‑4‑styrylpyridine
(Table 3, entry 9)
1H NMR (400 MHz, CDCl3) δ 8.21 (d, J = 7.6 Hz, 4H),
7.89 (s, 2H), 7.75 (d, J=8.0 Hz, 2H), 7.53–7.44 (m, 11H).
Representative spectral data
1H NMR spectra of 2,4,6‑triphenylpyridine (Table 3,
entry 1)
1H NMR spectra of 2,6‑diphenyl‑4‑(thiophen‑2‑yl)
pyridine (Table 3, entry 11)
1H NMR (400 MHz, CDCl3) δ 8.32 (s, 2H), 8.11 (d,
J = 8.4 Hz, 2H), 7.87 (s, 2H), 7.74 (d, J = 6.8 Hz, 2H),
7.59–7.49 (m, 5H), 7.39 (t, J = 7.8 Hz, 2H).
1H NMR (400 MHz, CDCl3) δ 8.18 (d, J = 7.6 Hz, 4H),
7.87 (s, 2H), 7.63–7.62 (m, 1H), 7.54–7.45 (m, 7H),
7.19–7.17 (m, 1H).
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Journal of the Iranian Chemical Society
1H NMR spectra of 4‑phenyl‑2,6‑di‑p‑tolylpyridine
(Table 3, entry 13)
1H NMR spectra of 2,6‑bis(3‑bromophenyl)‑4‑(p‑to
lyl)pyridine (Table 3, entry 18)
1H NMR (400 MHz, CDCl3) δ 8.10 (d, J = 8.0 Hz, 4H),
7.83 (s, 2H), 7.74 (d, J = 7.6 Hz, 2H), 7.54–7.46 (m, 3H),
7.31 (d, J = 8.0 Hz, 4H), 2.43 (s, 6H).
1H NMR (400 MHz, CDCl3) δ 8.31 (s, 2H), 8.10 (d,
J=7.6 Hz, 2H), 7.85 (s, 2H), 7.64 (d, J=7.6 Hz, 2H), 7.57
(d, J=7.6 Hz, 2H), 7.46–7.33 (m, 4H), 2.45 (s, 3H).
1H NMR spectra
of 2,6‑bis(3‑bromophenyl)‑4‑phenylpyridine
(Table 3, entry 14)
1H NMR (400 MHz, CDCl3) δ 8.32 (s, 2H), 8.11 (d,
J = 7.6 Hz, 2H) 7.87 (s, 2H), 7.73 (d, J = 6.8 Hz, 2H),
7.59–7.49 (m, 5H), 7.39 (t, J = 7.8 Hz, 2H).
1H NMR spectra of 2,4,6‑tri‑p‑tolylpyridine (Table 3,
entry 17)
1H NMR (400 MHz, CDCl3) δ 8.08 (d, J = 8.0 Hz, 4H),
7.81 (s, 2H), 7.63 (d, J = 8.0 Hz, 2H), 7.30 (t, J = 7.2 Hz,
6H), 2.41 (s, 9H).
Fig. 1 Pharmacologically active 2,4,6-trisubstitutedpyridines
Table 1 Effect of catalyst loading for the synthesis of 2,4,6-triph-
enylpyridine
Entry
Catalyst (wt%)
Tempera-
ture (°C)
Time (min)
Yielda (%)
1
2
3
4
5
6
7
8
–
5
110
110
110
110
110
60
360
180
120
90
Traces
60
79
91
91
7.5
10
15
10
10
10
90
120
120
360
45
90
rt
75
NRb
Fig. 2 Comparison of the efficiency of activated Fuller’s earth with
the literature reported catalysts for the synthesis of 2,4,6-triphe-
nylpyridine
aIsolated yield
bNo reaction
Table 2 Effect of the
Entry
Catalyst (wt%)
Solvent
Temperature (°C)
Time (min)
Yielda (%)
solvents for the synthesis of
2,4,6-triphenylpyridine
1
2
3
4
5
10
10
10
10
10
Solvent free
H2O
EtOH
MeCN
DMF
110
90
240
240
240
240
91
22
54
30
68
Reflux
Reflux
Reflux
110
aIsolated yield
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Journal of the Iranian Chemical Society
Table 3 Synthesis of
2,4,6-triarylpyridine using
a variety of aldehydes and
acetophenones
Entry
Aldehydes
Acetophenones Products
Time
(h)
Yielda Melting point (°C)
(%)
Obs.
Lit.
1.
2.
1.5
91
135–
136
136–137 [60]
1.5
90
128–
130
127–128 [60]
136–138 [61]
199–201 [62]
3.
4.
1.5
2.5
89
87
135–
137
198–
200
5.
2.5
87
101–
102
100–101 [62]
116–119 [61]
6.
7.
2.5
2
86
86
117–
118
105–
107
106.3–107.5
[63]
8.
9.
2.5
3
84
84
110–
112
109–112 [61]
123 [64]
122–
124
10.
3
83
76–78
76 [65]
11.
12.
13.
14.
2.5
2
86
88
85
86
160–
162
162–164 [66]
177–179 [60]
156–158 [60]
172–174 [60]
178–
179
2.5
2
155–
157
173–
175
15.
2
89
266–
267
268–270 [62]
16.
17.
2
2
87
84
130–
132
–
178–
180
179–181 [62]
18.
19.
2
84
85
120–
122
–
–
2.5
124–
126
aIsolated yield
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Journal of the Iranian Chemical Society
entry 5). Therefore, this was the optimum temperature for
performing the reaction (Scheme 1).
Results and discussion
The effect of various solvents in model reaction on time
and yield of the reaction was ascertained (Table 2, entries
2–5). None of the solvents presented the advantage of time
and yield over solvent-free condition. Hence, the solvent-
free condition was regarded as the best for the cost and envi-
ronmental acceptability.
We structured our study to investigate the suitability of acti-
vated Fuller’s earth as a catalyst for synthesis of 2,4,6-triar-
ylpyridines, for this benzaldehyde (2 mmol), a representative
substrate, acetophenone (4 mmol), and ammonium acetate
(2.4 mmol) were used to afford 2,4,6-triphenylpyridine
(Tables 1, 2).
In comparison with the literature reported other catalysts
such as PPA–SiO2 [55], silica sulphuric acid [56], silica
vanadic acid [57], mesoporous nanocrystalline MgAl2O4
[36], ultrasound-mediated nanocrystalline MgAl2O4 [58],
and montmorillonite K10 Clay [59] used for the synthesis
of 2,4,6-triphenylpyridine, activated Fuller’s earth catalyst
showed an advantage with respect to reaction condition,
workup procedure, time, and yields (Fig. 2).
Effect of the catalyst loading on time and yields of the
reaction was assessed (Table 1, entries 2–5). The reaction
does proceed at 110 °C with low product yield in the absence
of a catalyst (Table 1, entry 1). An increase in the cata-
lyst loading increased the product yield with a reduction in
reaction time (Table 1, entries 2–5). The catalyst loading
beyond 10 wt% was not advantageous (Table 1, entries 4 and
5), hence a 10 wt% catalyst loading was chosen for further
study. Temperature played an important role in the synthesis
of 2,4,6-triphenylpyridine (Table 1, entries 6 and 7). The
temperature effect was examined at ambient, 60 and 90 °C
under the solvent-free condition with activated Fuller’s earth
as a catalyst. The reaction does not proceed at room tem-
perature. Further increasing temperature to 110 °C resulted
in increased product yield in shorter reaction time (Table 1,
To investigate the substrate scope, optimized reaction
conditions were applied to substituted aromatic/heterocyclic/
α,β-unsaturated aldehydes, and acetophenones. All the sub-
strate variants reacted well and afforded high yields of the
corresponding 2,4,6-triarylpyridines within short reaction
time (Table 3). Several electron-releasing or electron-with-
drawing substituents at ortho and para positions of aro-
matic aldehydes have been examined. However, for 4-nitro,
4-methoxy, and 4-methyl substrates, the reaction time was
longer with comparable product yield (Table 3, entries 4–6).
This protocol was also applicable to cinnamaldehyde, phe-
nylacetaldehyde, and thiophene 2-carboxaldehyde (Table 3,
entries 9–11). On the other hand, the applicability of this
protocol on substituted acetophenones was also examined
(Table 3, entries 12–14). All the acetophenone variants
reacted well and afforded good yields in shorter reaction
time.
Scheme 1 Schematic representation of activated Fuller’s earth cata-
lyzed the synthesis of 2,4,6-triphenylpyridine
O
O
AFE
R2
R2
Al
R2
O
R1
NH3
NH2
H
R1
O
-H
2O
Al
OH
R2
AFE
R2
R1
AF
E
R1
R2
Al
R2
air oxidation
O
NH
OH
R1
N
N
H
H2N
R2
R2
R2
R2
R2
R1
R2
Scheme 2 Proposed mechanism for the synthesis of 2,4,6-triphenylpyridine
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Journal of the Iranian Chemical Society
5. G.W. Cave, M.J. Hardie, B.A. Roberts, C.L. Raston, Eur. J. Org.
Chem. 2001, 3227 (2001)
The scope of the present protocol has been explored
for the synthesis of 2,4,6-triarylpyridine using substituted
aldehyde and substituted acetophenones, wherein, electron-
withdrawing substituents variants showed comparable yield
and time to the substituted aldehyde or substituted acetophe-
nones 2,4,6-triarylpyridine. (Table 3, entries 15–19).
The postulated mechanism for the synthesis of 2,4,6-tri-
phenylpyridine is shown in Scheme 2; the reaction involves
four steps aldol condensation, Michael addition, cyclisa-
tion, and finally air oxidation. Condensation of an aldehyde
and acetophenone forms aldol product; on the other hand,
a molecule of acetophenone with ammonia forms an enam-
ine adduct. The addition of enamine to the aldol product
followed by cyclisation gives dihydropyridine. Finally, air
oxidation afforded the final product. Activated Fuller’s earth
aids aldol condensation, Michael addition, and precipitated
oxidation of dihydropyridine to the final product. The reac-
tion with electron-withdrawing groups of aromatic alde-
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Conclusion
In conclusion, A mild, efficient, and ecological approach
for the synthesis of 2,4,6-trisubstituted pyridines via the
condensation of aromatic ketones with aromatic aldehydes
and ammonium acetate in the presence of activated Fuller’s
earth clay as a recyclable heterogeneous solid acid catalyst
has been developed. Due to the mild reaction conditions,
good to excellent yields and easy workup procedure, the
present protocol has edge over the other methods. Moreo-
ver, present method tolerates a wide variety of substituents.
The trisubstituted pyridines were produced without forma-
tion of any other side product. Activated Fuller’s earth is an
inexpensive, eco-friendly, efficient heterogeneous catalyst.
The catalyst can be easily prepared, used, recovered, and
recycled with no loss of significant catalytic activity.
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