Sulfated tungstate catalyzed synthesis of C3-symmetric 1,3,5-triaryl benzenes under solvent-free condition

Article abstract of DOI:10.1016/j.tetlet.2017.06.055

Sulfated tungstate catalyzed environmentally benign, simple, one pot, and solvent-free method has been developed for the synthesis of 1,3,5-triarylbenzenes via cyclic self-condensation of three molecules of aryl ketones. High yields, short reaction time, easy work-up procedure and recycling of the catalyst endorse advantage.

Full text of DOI:10.1016/j.tetlet.2017.06.055

Accepted Manuscript
Sulfated Tungstate Catalyzed Synthesis of C3-Symmetric 1,3,5- Triaryl ben-
zenes Under Solvent-Free Conditions
Ganesh D. Wagh, Krishnacharya G. Akamanchi
PII:
S0040-4039(17)30785-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.06.055
TETL 49047
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
27 May 2017
16 June 2017
18 June 2017
Please cite this article as: Wagh, G.D., Akamanchi, K.G., Sulfated Tungstate Catalyzed Synthesis of C3-Symmetric
1,3,5- Triaryl benzenes Under Solvent-Free Conditions, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/
j.tetlet.2017.06.055
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Tetrahedron Letters
Sulfated Tungstate Catalyzed Synthesis of C3-Symmetric 1,3,5- Triaryl benzenes
Under Solvent-Free Conditions
Ganesh D. Wagh, Krishnacharya G. Akamanchi*
Department of Pharmaceutical Sciences and Technology, Institute of Chemical Technology, Matunga, Mumbai, 400019, India
ARTICLE INFO
ABSTRACT
Sulfated tungstate catalyzed environmentally benign, simple, one pot, and solvent-free method
has been developed for the synthesis of 1,3,5-triarylbenzenes via cyclic self-condensation of
three molecules of aryl ketones. High yields, short reaction time, easy work-up procedure and
recycling of the catalyst endorse advantage.
Article history:
Received
Received in revised form
Accepted
2009 Elsevier Ltd. All rights reserved.
Available online
Keywords:
Aryl alkyl ketones
Sulfated tungstate
Self-condensation
1,3,5-Triarylbenznes
Introduction
1,3,5-Triarylbenzene and its derivatives have long been
recognized as a pivotal class of aromatic compounds which are
Lewis acids such as SiCl415, 37, TiCl4 38-41, Bi(OTf)341, B(HSO4)3
42
,
Amberlyst-1543,
HOTf44,
amine-TFA13,
PTSA45,46
,
47
widely used in the electrode devices1-3, resistance materials4,
PTSA/SnCl4
,
DBSA48, Zirconocene bis (perfluorooctane-
6
organic light emitting diodes5, and conducting polymers7-10
.
sulfonate)49, hetropolyacids50, 51, Bronsted acid ionic liquids52,
NCP/HDTMA53, and SOCl2/EtOH54. However, these numerous
methods suffer from one or more drawbacks such as the use of a
precious metal catalyst, poor reaction yields, tedious workup
procedure, formation of side products, non-recyclability of
catalyst and use of hazardous organic solvents. This clearly
signifies the need and scope for the better method that can meet
the requirements of green chemistry.
Moreover, 1,3,5-triarylbenzenes are important moieties in
14
dendrimers11, 12, polycyclic aromatic hydrocarbons (PAHs)13,
,
fullerene fragments6, 15, and bulky ligands16-19. Accordingly,
many efforts have been devoted to the development of effective
methods for synthesis of 1,3,5-triarylbenznes. Cyclotrimerization
of alkynes catalyzed by complexes of transition metals such as
Pd, Rh, Co, Ni, Ir, and Nb are few such methods20-29. However,
alkynes are not readily available, expensive and generate 1,2,5-
trisubstituted arenes as side products. Recently, a combination of
Amberlyst-15 and Ionic liquids(ILs) as a solvent was found to be
effective for improved synthesis of 1,3,5-triarylbenzenes but its
application is restricted due to high cost30. The metal-catalyzed
coupling of 1,3,5-trihaloarenes with various arylating agents have
been another dominant way to construct 1,3,5-triarylbenzenes31-
36, but these methods suffer from drawbacks such as the
requirement for functionalized starting materials, stoichiometric
amount of salt formation, multiple steps, and some methods even
produce toxic waste materials. Consequently, synthesis of 1,3,5-
triarylbenzenes starting from aryl methyl ketones has been
regarded as comparatively straight forward method and has
received more attention. Starting from aryl methyl ketones the
1,3,5-trialkylbenzenes have been synthesized using Bronsted or
Scheme 1. Synthesis of 1,3,5-triarylbenzene
In recent years, our research group has introduced sulfated
tungstate a mildly acidic, non-corrosive, reusable, easy to
prepare, heterogeneous green catalyst and has established its
effectiveness in amidation55, Ritter reaction56, Biginelli
reaction57, Kindler reaction58, Strecker synthesis59, Willgerodt-
Kindler reaction60, N-formylation61, transamidation62, N-

2
monoalkylation of primary amines63, etherification64, and
amidine synthesis65. To further explore its catalytic potential we
have successfully employed it for the synthesis of 1,3,5-
triarylbenzenes from aryl alkyl ketones under solvent free
conditions and the results are presented in this letter.
whereas increasing the catalyst loading to 30 wt% there was no
improvement in the yield (Table 1, entry 8).
Table 2: Synthesis of various triaryl benzenes a
Entry
1
Aryl ketones
Product
Yield(%) b
94
Result and discussion
For preliminary investigations acetophenone was used as a
substrate. The investigations included catalyst loading, reaction
time, and reaction temperature. The results are presented in Table
1. The reaction was performed in presence of different solvents
such as toluene, xylene and dimethylformamide (DMF) using
25 wt% of the catalyst at reflux temperature for 12 h and gave
1,3,5-triphenylbenzene 2a in 5, 12 and 15 % yield, respectively
(Table 1, entries 1-3).
1a
2a
2
91
1b
Table 1 Optimization of reaction conditions for synthesis of 2aa
2b
3
78
Temperature Time(h) Yieldb
(°C)
1c
Entry Solvent
Sulfated
tungstate
(wt%)
1
2
Toluene
Xylene
DMF
Neat
25
25
25
25
25
25
20
30
15
20
20
20
Reflux
Reflux
Reflux
150
12
12
12
12
12
12
12
12
12
10
16
8
5
2c
12
15
94
94
60
94
94
72
94
94
80
4
80
3
4
5
Neat
130
1d
6
Neat
120
7
Neat
130
2d
8
Neat
130
5
78
9
Neat
130
10
11
12
Neat
Neat
130
1e
130
2e
Neat
130
6
82
a Reaction conditions: 1a (3mmol), sulfated tungstate, different solvents, and
temperature. b Isolated yield.
A reaction was performed under solvent-free conditions and at
150 °C, the temperature just below the boiling point of DMF, and
it was satisfying to find that yield of 2a increased dramatically to
94% (Table 1, entry 4). To find the optimum reaction
temperature the reactions were carried out at lower temperatures
of 130 °C and 120 °C and the temperature of 130 °C was found
to be optimum giving 94% yield (Table 1, entries 5 and 6). The
catalyst loading study results showed 20 wt % as optimum (Table
1, entry 7). Because decrease in the catalyst loading to 15 wt% ,
lead to decrease in the yield of 2a to 74% (Table 1, entry 9)
1f
2f

7
83
14
72
1n
1g
2n
2g
15
64d
8
83
1o
1h
2o
aReaction conditions: aryl methyl ketones (3mmol), sulfated tungstate (20
wt%), 130 °C, 10 h. b Isolated yields. c time 8 h. d time 17 h
2h
9
84
Successively, the effect of reaction time was also investigated
and reaction time of 10 h was found to be adequate (Table 1,
entries 10-12). From the foregoing investigations a 20 wt%
loading of sulfated tungstate, the temperature of 130 °C and
solvent-free conditions were found to be optimum for performing
the reaction and were used for further studies.
1i
The substrate scope was studied using a variety of aryl alkyl
ketones and the results are presented in Table 2. All the
substrates reacted smoothly giving yields in the range of 64-94
%. In case of aryl methyl ketones having electron donating
groups like -CH3 ,-OMe , -OEt at the para position yields were 91
%, 78 % and 80%, respectively (Table 2, entries 2-4).
The reaction also works well with substrates with electron
donating group at ortho as well as meta positions (Table 2,
entries 5, 6) giving good yields. Further, it was found that
substrates with halogen at para and meta positions (F, Cl, Br and
I) (Table 2, entries 7-11) substituent were compatible and
generated the desired products in good yields. These halogenated
1,3,5-trialryproducts could be very valuable building blocks for
further elaboration into dendrimers and other structural motifs. It
was gratifying to note that disubstituted aryl methyl ketone,
heterocyclic aryl methyl ketone and naphthyl methyl ketone
(Table 2, entries 12-14) underwent the reaction successfully
giving good yields. The reaction also works with phenyl ethyl
ketone with moderate yield (Table 2, entry 15) and offers
important per substituted benzene.
2i
10
11
12
89
83
78
1j
2j
1k
2k
The recyclability of the catalyst was tested on the model reaction
by recovering the catalyst by adding ethyl acetate to the reaction
mixture followed by filtration and heating in an oven at 120 °C
for 1 h to remove solvent and moisture. It can be seen from the
results given in Figure 1 that the catalyst retained activity and
was reusable five times without any significant loss in yield. The
yields dropped marginally from 94% to 88% with fresh and fifth
recycle, respectively.
1l
2l
13
84c
To understand the reaction mechanism following studies were
carried out
i) To rule out the possibility of sulfated tungstate just acting as a
solid support without showing any catalytic role, a reaction was
performed using only neutral silica (20 wt %). The reaction did
not occur even after 24 h. Tungsten trioxide (WO3), a material
close to sulfated tungstate but lacking the acidity, was also
1m
2m

4
Figure 1: Catalyst Recyclability study
100
90
80
70
60
50
40
30
20
10
0
fresh
cycle 1
cycle 2
cycle 3
cycle 4
cycle 5
catalystcycle
Catalyst recyclability study. Reaction condition: acetophenone 1a (3 mmol), sulfated tungstate (20 wt%)b . Loss of the catalyst (<5%) during handling
Scheme 2: Plausible reaction mechanism for synthesis of 1,3,5-triarylbenzenes 2
screened however the product 2a was not formed even after
heating for 24 h. These results indicate that sulfated tungstate
indeed plays a catalytic role.
between 3a and 1a is faster and the formation of 3a is the rate
controlling step.
ii) In another study sulfated tungstate was stirred in 1a for 2 h at
130 °C filtered off, washed with chloroform, air dried for 2 h and
FT-IR was recorded. The IR spectra showed peaks at 1674 cm-1
and 1638 cm-1 corresponding to dypnone (1,3-diphenylbut-2-en-
1-one) 3a indicating that the reaction is occurring stepwise via
formation of 3a 46 as intermediate.
Scheme 3: Reaction of dypnone 3a and acetophenone 1a
iii) To confirm this observation a control experiment was carried
out using dypnone 3a and acetophenone 1a in 1:1 mole ratios in
the presence of the catalyst at 130 °C. The desired product 2a
was obtained in 91 % yield (Scheme 3), proving that 3a is indeed
an intermediate. The high yield obtained indicate that the reaction
iv) In another study 3a (1 eq) and 1a (1 eq) were mixed and
stirred in presence of the catalyst (20 wt %) at 100 °C for 1 h.
The catalyst was recovered by filtration and quickly washing
with chloroform to remove mother liquor. The recovered catalyst

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successive steps of condensation, electrocylization and
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The present work describes a simple and highly efficient
protocol, for the synthesis of 1,3,5-triarylbenzenes starting from
the corresponding aryl alkyl ketones under solvent-free
conditions resulting in very good to excellent yields using
sulfated tungstate as the catalyst. Operational simplicity, the
catalyst stability, reusability and water as a green by-product are
remarkable features. Synthetic applications of the reaction are
still under investigation.
32
33
Acknowledgments
34
35
36
37
The authors thank the CSIR-UGC, New Delhi, India, for
financial support.
Supplementary data
Experimental procedures, compound characterization data, 1H
and 13C NMR spectra and GC chromatographs are provided in
supporting information.
38
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8
Research Highlights





New method for synthesis of C3-symmetric triarylbenzenes.
Heterogeneous catalysis using Sulfated tungstate.
Starting from readily available aryl alkyl ketones.
High yields, easy workup, no use of solvent and the catalyst recyclable.
Environmentally benign method.