Silicotungstic acid catalysed Claisen Schmidt condensation reaction: An efficient protocol for synthesis of 1,3-diaryl-2-propenones

Article abstract of DOI:10.1016/j.tetlet.2011.11.109

An efficient and clean synthesis of 1,3-diaryl-2-propenones has been carried out by Claisen Schmidt condensation reaction of aryl methyl ketones with a series of aromatic aldehydes at room temperature in the presence of the catalyst silicotungstic acid (STA). This method provides an ecofriendly, chemoselective, efficient and green synthesis of 1,3-diaryl-2-propenones in excellent yields.

Full text of DOI:10.1016/j.tetlet.2011.11.109

Tetrahedron Letters 53 (2012) 646–649
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Tetrahedron Letters
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Silicotungstic acid catalysed Claisen Schmidt condensation reaction: an
efficient protocol for synthesis of 1,3-diaryl-2-propenones
⇑
Jaspreet Kaur Rajput , Gagandeep Kaur
Department of Chemistry, Dr. B. R. Ambedkar National Institute of Technology, Jalandhar 144011, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient and clean synthesis of 1,3-diaryl-2-propenones has been carried out by Claisen Schmidt con-
densation reaction of aryl methyl ketones with a series of aromatic aldehydes at room temperature in the
presence of the catalyst silicotungstic acid (STA). This method provides an ecofriendly, chemoselective,
efficient and green synthesis of 1,3-diaryl-2-propenones in excellent yields.
Received 30 September 2011
Revised 17 November 2011
Accepted 23 November 2011
Available online 2 December 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Claisen Schmidt condensation
Silicotungstic acid (STA)
Solvent-free conditions
1
,3-Diaryl-2-propenones
The 1,3-diaryl-2-propenones represent an important class of
acid having these properties. Heteropoly acids (HPA) belonging
to a large family of oxygen cluster compounds known as polyoxo-
metalates have attracted much attention as catalyst having high
1
compounds, as they possess a broad spectrum of biological activity
2
3
4
such as anti-inflammatory, anti-cancer, anti-malarial, anti-
miotic, cardiovascular, anti-tuberculosis.7 In addition they are
5
6
Bronsted acidity and redox properties. Among the HPAs, HPA
31
nÀ
5+
useful synthonsfor the preparation ofbiologicallyimportanthetero-
with inorganic—cage type anions of the form [XM12
O
40
]
adopt
8
9
4+
cyclic compounds, and also have application in natural products.
Some of its derivatives are also used as sweeteners, drugs and sun-
a keggin structure, where X is the heteroatom (e.g., P or Si )
6
+
6+
and M is the addendum atom (W or Mo ) providing a strong op-
tion for efficient and cleaner technology compared to polluting and
1
0
screen agents. Thus, the synthesis of 1,3-diaryl-2-propenones
has attracted the organic/pharmaceutical chemists worldwide.
The synthesis of 1,3-diaryl-2-propenones involves Claisen
Schmidt condensation in basic or acidic conditions. The various
types of catalysts are used for the synthesis of 1,3-diaryl-2-prope-
3
2
corrosive liquid acid catalysts. These keggin type heteropoly
acids have received much attention due to their dual character as
they are strong acids and can also be used as good oxidants.33
In the beginning of the study, 4-chlorobenzaldehyde and aceto-
phenone were used as model reactants in order to find the opti-
mum reaction conditions by Claisen Schmidt condensation using
STA as catalyst (Scheme 1).
2
b,4,5,8c,11,12
8a,13
nones including bases such as NaOH,
KOH,
Ba(O-
2
a,14
15
16
,17 TiCl
18
H)
BF
2
,
LiHMDS, LiOHÁH
2
O
and acids such as RuCl
3
4
,
1
9
20
21
22
23
3
ÁOEt, dry HCl, SOCl
2
–EtOH, Zn(bpy)(OAc)2, ZrCl4, bam-
24
,25 clay,
26,27
28
boo char sulfonic acid, BiCl
3
ionic liquids and
Initially the reaction was carried out between the 4-chlorobenz-
aldehyde and acetophenone using STA catalyst under solvent-free
conditions. In the first stage, optimisation of catalyst amount was
carried out. At first, no product was observed in the absence of cat-
alyst. The reaction started with 1 mol % of catalyst. Further we
observed that the yield of the product gradually increased on
increasing the amount of catalyst. The maximum yield was
obtained using 10 mol % of catalyst. No improvement in yield
was observed with further increase in the amount of catalyst
(Table 1, Fig. 1).
29
sulfonic acid-functional ionic liquids. 1,3-Diaryl-2-propenones
are also formed by Suzuki reaction.30 However some of these
procedures suffer from many shortcomings such as long reaction
2
a,7,13,14,22,24,29
22,24,25,28,29
23
time,
reagents,
high temperature,
use of expensive
1
5,30
formation of side products and use of hazardous
2
2
solvents. Keeping in mind these drawbacks and as an extension
to our work on eco-friendly acid catalysed reactions, here we have
studied the solvent-free Claisen Schmidt condensation reaction
using silicotungstic acid (STA) as acid catalyst.
In recent years the focus is on finding ecofriendly, green solid
acid catalysts. Herein we report such a catalyst that is heteropoly
The effect of solvent was studied using various solvents as well
as under solvent-free conditions. We observed that in the presence
of solvents the reaction was sluggish and no improvement was
seen in yield of the product. The best results were obtained under
solvent-free conditions (Table 1, Fig. 2).
⇑
Corresponding author.
E-mail address: rajputj@nitj.ac.in (J.K. Rajput).
0
040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.11.109

J. K. Rajput, G. Kaur / Tetrahedron Letters 53 (2012) 646–649
647
O
O
(
a) STA (10mol%) with stirring at r.t.
CHO
(
b)
R1
R
2
1
3
R1
R2
2
R
1
= H, 2-OH, 4-Cl and R
2
=H, 4-Cl, 2-Cl, 3-NO
2
, 4-OCH
3
, 3, 4-di (OCH )
3
Scheme 1. Claisen Schmidt condensation reaction using STA.
Table 1
Optimisation of Claisen Schmidt condensation of acetophenone and 4-chlorobenzaldehyde under various conditions
Entry
Catalyst (mol %)
Different optimisation conditions
Solvents
Yield (%)
1
2
3
4
5
6
7
8
9
STA (1)
STA (3)
STA (5)
STA (7)
At room temperature with magnetic stirring
At room temperature with magnetic stirring
At room temperature with magnetic stirring
At room temperature with magnetic stirring
At room temperature with magnetic stirring
At room temperature with magnetic stirring
Water bath (55 °C)
Water bath (65 °C)
Water bath (85 °C)
Refluxing
At room temperature with magnetic stirring
At room temperature with magnetic stirring
At room temperature with magnetic stirring
At room temperature with magnetic stirring
At room temperature with magnetic stirring
At room temperature with magnetic stirring
At room temperature with magnetic stirring
At room temperature with magnetic stirring
—
—
—
—
—
—
—
—
—
—
EtOH
CH
DMF
H O
2
DCM
Neat
—
40
60
79
86
94
83
92
83
85
87
80
84
65
70
73
94
60
55
STA (10)
STA (12)
STA (10)
STA (10)
STA (10)
STA (10)
STA (10)
STA (10)
STA (10)
STA (10)
STA (10)
STA (10)
PTA (10)
PMA (10)
10
11
12
13
14
15
16
17
18
3
CN
—
Figure 2. Effect of various solvents on synthesis of 1,3-diaryl-2-propenones in case
of acetophenone and 4-chlorobenzaldehyde. (1 = EtOH, 2 = CH CN, 3 = DMF,
4 = H O, 5 = DCM, 6 = neat).
Figure 1. Effect of mol % of catalyst STA on yield of 1,3-diaryl-2-propenones
3
(
1 = 1 mol %, 2 = 3 mol %, 3 = 5 mol %, 4 = 7 mol %, 5 = 10 mol %, 6 = 12 mol %).
2
Various conditions were examined for carrying out the Claisen
carried out in the presence of STA (Table 2) to give the desired
product in excellent yields. When the reaction was carried out tak-
ing aldehyde first with the catalyst and then acetophenone was
added, the reaction did not yield any product. But on taking aceto-
phenone and the catalyst first and then adding aldehyde resulted
in the desired product. The results showed that the reactions were
clean and products were isolated by filtration and obtained in pure
form after recrystallization. As shown in Table 2 the aromatic alde-
Schmidt condensation between 4-chlorobenzaldehyde and aceto-
phenone. Carrying out the reaction at different temperatures
resulted in 92%, 83% and 85% yields of the product. The reaction
under refluxing conditions did not improve the yield (Fig. 3).
The optimum reaction condition was found to be 10 mol % of
catalyst using magnetic stirring at room temperature under sol-
vent-free conditions. The results are summarised in Table 1.
To establish the scope of Claisen Schmidt condensation of vari-
ous aryl methyl ketones with aromatic aldehydes the reaction was
hydes bearing electron–donating groups such as 3,4-di-(OCH
3
) and
OCH took longer reaction time as compared to benzaldehyde
3

6
48
J. K. Rajput, G. Kaur / Tetrahedron Letters 53 (2012) 646–649
good yield of product. The proposed mechanism is shown as
Scheme 2. STA acts as bifunctional catalyst as this catalyst activates
both the carbonyl groups. The 1,3-diaryl-2-propenones are formed
with excellent chemoselectivity. No side products are formed
such as product of decomposition, self condensation or Michael
addition.
To check the efficiency of the present method it was compared
with the reported methods. The Claisen Schmidt condensation of
benzaldehyde and 4-chlorobenzaldehyde with acetophenone
afforded 81% and 72% yields, respectively, after 18 h at 80 °C in
2
2
DMF in the presence of Zn(bpy)(OAc)
2
and 75% and 69.7% yields,
respectively, after 24 h at 120 °C using bamboo char sulfonic
2
4
17
acid. RuCl
1
3
catalysed condensation (90% and 94% yields at
2
7
20 °C), montmorillonite as catalyst (91% yield in 60 min), and
2
9
sulfonic acid-functional ionic liquid (85% yield in 5 h at 120 °C).
But by using STA, Claisen Schmidt condensation afforded 83%
and 94% yields after 20 min and 5 min, respectively at room
temperature.
Figure 3. Synthesis of 1,3-diaryl-2-propenones with STA using different methods.
1 = heating at 65 °C, 2 = heating at 85 °C, 3 = refluxing, 4 = heating at 55 °C,
= room temperature with acetophenone and 4-chlorobenzaldehyde).
(
5
To check the catalytic activity of STA it was compared with
other heteropoly acids that is phosphotungstic acid (PTA) and
phosphomolybdic acid (PMA) (Table 1), STA was found to be very
effective among those in terms of reaction time as well as yield.
In conclusion, we have demonstrated an efficient, green, simple
catalyst and method for Claisen Schmidt condensation of aryl
(
entries 4 and 6). In the case of substrates containing electron
withdrawing groups the reaction occurred in shorter time (entries
and 3). Further the presence of electron donating group on aryl
1
methyl ketone increased the reaction time. The reaction of various
substituted acetophenone with unsaturated aldehydes also gave
Table 2
STA catalysed Claisen Schmidt condensation of various substituted acetophenone and various substituted benzaldehyde
Entry
R
1
R
2
Product
Time (min)
Yield
Mp (observed)
Mp (reported)
2
8
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
2-OH
2-OH
2-OH
2-OH
2-OH
2-OH
4-Cl
4-Cl
4-Cl
4-Cl
4-Cl
H
2-Cl
4-OCH
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
5
94
85
87
80
87
85
89
80
79
75
77
72
80
89
81
87
85
112–115
54–56
48–50
113–114
56–572
47–492
8
20
10
65
20
55
35
15
40
45
35
70
45
7
7
28
3
70–72
74–75
3-NO
2
140–142
138–140
94–98
148–150
138–140
88–90
130–134
93–95
152–155
152–154
80–81
143–14437
3,4-di(OCH
Ph-CH@CH
4-Cl
2-Cl
H
3-NO
4-OCH
Ph-CH@CH
4-Cl
2-Cl
H
3
)
—
—
15236
—
3
6
10
11
12
13
14
15
16
17
89
—
92
—
2
36
3
27
155–156
15
18
40
81–8227
3
3
96–98
120–122
98–100
120–12127
4-OCH
3
STA
O
STA
STA
O
O
O
O
O
CH3
STA
CH2
R1
R1
CHO
R2
R1
I
R1
II
R2
III
R2
O
CH
3
R1
O
O
OH
R1
R2
R
1
R2
IV
Scheme 2. Mechanism of synthesis of 1,3-diaryl-2-propenones using STA.

J. K. Rajput, G. Kaur / Tetrahedron Letters 53 (2012) 646–649
649
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