Polyoxometalate-based acid salts with tunable separation properties as recyclable Br?nsted acid catalysts for the synthesis of β-keto enol ethers

Article abstract of DOI:10.1016/j.catcom.2012.04.009

Various polyoxometalate-based acid salts were synthesized by combining Keggin anions with sulfonated ammonium, imidazolium and pyridinium cations and used as catalyst for the synthesis of β-keto enol ethers. The sulfonated pyridinium cation with SiW₁

Full text of DOI:10.1016/j.catcom.2012.04.009

Catalysis Communications 25 (2012) 64–68
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Catalysis Communications
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Short Communication
Polyoxometalate-based acid salts with tunable separation properties as recyclable
Brönsted acid catalysts for the synthesis of β-keto enol ethers
⁎
Ezzat Rafiee , Sara Eavani
Faculty of Chemistry, Razi University, Kermanshah, 67149, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Various polyoxometalate-based acid salts were synthesized by combining Keggin anions with sulfonated ammoni-
Received 1 February 2012
Received in revised form 10 April 2012
Accepted 12 April 2012
um, imidazolium and pyridinium cations and used as catalyst for the synthesis of β-keto enol ethers. The sulfonated
4−
pyridinium cation with SiW12
O
anion ([PyPS]4SiW), showed the best activity and recyclability. Depending on the
40
polarity of reaction mixture, “self-separation” or heterogeneous catalysis was observed. The catalyst was conve-
niently separated from the products, easily recycled and reused several times without significant loss of its activity.
© 2012 Elsevier B.V. All rights reserved.
Available online 21 April 2012
Keywords:
Organic–inorganic hybrid material
Polyoxometalate
Self-separation catalyst
Sulfonated cations
β-Keto enol ethers
1. Introduction
terpenoids, 4-alkylated-2-cyclohexenones, 2-aryl and 2-alkenyl-3-
alkoxycyclohexenones and bicyclo [2,2,2] octenones [13–15]. Also,
In the last decade, the application of ionic liquids (ILs) in catalysis
has been the subject of intensive research in academia and industry be-
cause of their adjustable physical and chemical properties [1–3]. Recent
trends in the field of IL chemistry are the catalytic applications of the so-
called “task-specific ILs” which were prepared by incorporating a func-
tional group responsible for catalysis in their ionic partners [4,5].
Through the different combination of positive and negative ions, differ-
ent hybrid materials can be designed and synthesized [6,7]. An interest-
ing example of organic–inorganic hybrid materials was first reported by
Leng et al. [8]. The combination of propane sulfonate functionalized or-
ganic cations with Keggin type polyoxometalate led to the formation of
polyoxometalate-based acid salts that possesses unique structural and
physicochemical properties. They were used as “reaction-induced self-
separation catalysts” for various esterification reactions with one of
the reactants being polycarboxylic acid or polyol [8]. As far as we
know, the applications of these hybrid materials as “reaction-induced
self-separation catalysts” are limited to esterifications and transesterifi-
cation of trimethylolpropane [8–10]. In view of the emerging impor-
tance of polyoxometalate-based acid salts as novel, clean and
recyclable catalyst [11,12], extending their applications in various or-
ganic transformations can be of considerable interest.
they act as dienophiles in Diels–Alder reactions [16]. However, the syn-
thesis of β-keto enol ethers has received little attention despite their
wide range of applications [17–21] and developing a new, cost-effective
and green protocol for this transformation is still a challenging task.
In this study, we reported our efforts at extending the application
of polyoxometalate-based acid salts as “self-separation” catalysts in
the synthesis of β-keto enol ethers.
2. Experimental
2.1. Preparation of catalysts
The catalysts (Fig. 1) were synthesized according to previous litera-
ture [8]. For the synthesis of [PyPS]3PW catalyst, pyridine (0.11 mol)
and 1,3-propanesultone (0.10 mol) were dissolved in toluene (30 mL)
and stirred at 40 °C for 24 h under nitrogen atmosphere. A white precip-
itate (PyPS) was formed. It was filtered, washed with diethyl ether three
times and dried in vacuum. PyPS (0.06 mol) was added to an aqueous
solution of H3PW12O40 (PW) (0.02 mol), and then the mixture was stir-
red at room temperature for 24 h. Water was removed in vacuum to
give the final product as a solid. Other catalysts were prepared accord-
ingly. IR and 1H NMR data, elemental analyses and melting points of
the catalysts are given in the supplementary information.
Among the different protocols for the synthesis of useful building
blocks, we selected the synthesis of β-keto enol ethers because of their
very high impact as synthons for the preparation of bioactive compounds,
2.2. Catalytic reaction
The catalyst (0.2 g) was added to a solution of β-diketone
⁎
Corresponding author. Tel./fax: +98 831 427 4559.
E-mail address: ezzat_rafiee@yahoo.com (E. Rafiee).
(1 mmol) in alcohol (4 mL). The mixture was stirred at 50 °C. After
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.04.009

E. Rafiee, S. Eavani / Catalysis Communications 25 (2012) 64–68
65
3-
3-
PW12O40
PMo12O40
(CH2)3SO3H
N
(CH2)3SO3H
N
3
3
[(3-sulfonic acid)propylpyridine]3PW12O40
[(3-sulfonic acid)propylpyridine]3PMo12O40
[PyPS]3PMo
[PyPS]3PW
4-
3-
SiW12O40
N
(CH2)3SO3H PW12O40
3
N
(CH2)3SO3H
4
[(3-sulfonic acid)propyltrimethylamine]3PW12O40
[(3-sulfonic acid)propylpyridine]4SiW12O40
[PyPS]4SiW
[TMAPS]3PW
3-
N (CH2)3SO3H
N
PW12O40
3
[3-(1-methylimidazolium-3-yl)propane-1-sulfonate]3PW12O40
[MIMPS]3PW
Fig. 1. Structures of various catalysts used in this study.
completion of the reaction, the catalyst was separated by decantation
(or filtration). The filtrate was concentrated to generate the crude
product. The crude products were purified by column chromatogra-
phy on silica gel using hexane/EtOAc (4:1) as eluent. All products
were characterized by 1H NMR and IR spectroscopy.
please see supplementary information). According to H0, all of the cata-
lysts have strong acidity (Table 1). Potentiometric titration curves
obtained for different catalysts are presented in Fig. 2. As a criterion to
interpret the obtained results, it was suggested that the initial electrode
potential (Ei) indicates the maximum acid strength of the sites. The
value of mmol amine/g catalyst, where the plateau is reached, indicates
the total number of acid sites. All catalysts present very strong acidity
and the same number of acidic sites.
3. Results and discussion
Acidity and solubility of polyoxometalate-based acid salts are major
factors that should be considered in their catalytic applications. These
factors have significantly affected by the choice of organic cations and/
or inorganic anions. The acidity of HPAs is often discussed in terms of
numbers and strength of acidic sites. These parameters can be estimat-
ed by potentiometric titration with n-butylamine [22]. However, the
Brönsted acidity of ILs was usually determined by the Hammett acidity
function (H0). Up to now, there was no report in the literature to deter-
mine the acidity of polyoxometalate-based acid salts by potentiometric
titration with n-butylamine. Therefore, the Brönsted acidity of the cata-
lysts was determined by the Hammett method (see supplementary in-
formation). For comparison purposes, the number and strength of acid
sites were estimated by potentiometric titration (for more details
The solubility of the catalysts was investigated in the model
reaction (Scheme 1) at different temperatures. All catalysts were in-
soluble in methanol at room temperature. When 1 mmol of 5,5-
dimethylcyclohexan-1,3-dione (dimedone) was added, [MIMPS]3PW
and [TMAPS]3PW catalysts started to be dissolve. The system became
250
[TMAPS]3PW
200
[MIMPS]3PW
150
[PyPS]3PW
100
[PyPS]3PMo
50
[PyPS]4SiW
Table 1
0
-50
Effect of organic cations in the acidity of hybrid catalysts.
a
Catalyst
Amax
[I] (%)
[IH+] (%)
H0
–
1.496
0.79
0.70
0.77
0.82
0.85
100
0
–
-100
-150
[TMAPS]3PW
[MIMPS]3PW
[PyPS]3PW
[PyPS]4SiW
[PyPS]3PMo
52.8
47.0
51.5
54.8
56.8
47.2
53.0
48.5
45.2
43.2
1.03
0.94
1.01
1.07
1.10
0
1
2
3
4
5
6
7
mmol amine / g catalyst
a
H0 =pKa +log[I]/[IH+]; indicator: 4-nitroaniline (pKa =0.99); concentration of
POM-IL: 0.1 mmol/L; concentration of 4-nitroaniline: 2 mmol/L.
Fig. 2. Potentiometric titration curves.

66
E. Rafiee, S. Eavani / Catalysis Communications 25 (2012) 64–68
100
80
60
40
Scheme 1. Model reaction.
0.05 g
0.1 g
0.15 g
0.2 g
clear gradually with heating continually, and these catalysts were dis-
solved completely at 50 °C. However, [PyPS]3PW catalyst remained
insoluble at room temperature and its solubility was partially in-
creased at 50 °C.
20
0
0.25 g
Considering that the catalysts were insoluble in the reaction medium
at room temperature, the model reaction was carried out at 50 °C
(Table 2). Control experiment showed that the substrates hardly reacted
in the absence of catalyst (entry 1). For the hybrid catalysts with the
same anion groups, the change of cations has little influence on the cat-
alytic activity (entries 2–4). But solubility of the catalysts has significant-
ly changed. At the beginning of the reaction, [MIMPS]3PW and
[TMAPS]3PW catalysts, were dissolved in the reaction medium to form
a homogeneous mixture. With the consumption of dimedone during
the reaction, the system became turbid and the catalyst was precipitated
at the end of the reaction (entries 2, 3). In the case of [PyPS]3PW catalyst,
the cloudy mixture at the beginning of the reaction became heteroge-
neous with the consumption of dimedone (entry 4). These catalysts
were separated from the reaction mixture, dried in vacuum and
weighed. The wt.% of precipitated catalysts was increased in the follow-
ing order: [PyPS]3PW>[TMAPS]3PW>[MIMPS]3PW (entries 2–4).
Therefore, PyPS was selected as the best cation for the preparation of
polyoxometalate-based acid salts. The influence of various HPAs in the
activity and solubility of catalysts with the same PyPS cation is shown
in entries 4–6. [PyPS]4SiW showed the best recyclability and 96 wt.% of
the catalyst was precipitated at the end of the reaction (entry 6). The cat-
alytic activity of various hybrid catalysts was compared with counterpart
cations and HPAs (Table 2). The results showed that the counterpart cat-
ions TMAPS and PyPS were insoluble in the reactants and/or products,
and the yields of products were lower than those obtained using hybrid
catalysts (entries 7, 8). However, when MIMPS was used as catalyst, a
homogeneous system resulted which gave a moderate yield of the prod-
uct (entry 9). The product was obtained in good yields by using the pure
HPAs. But, the reaction proceeded in a homogeneous system with diffi-
cult catalyst recovery (entries10–12).
0
10
20
30
40
50
Time (min)
Fig. 3. Effect of catalyst loading in the model reaction.
the product yield (it should be noted that catalyst solubility was not
changed with increasing the amount of the catalyst even to 0.25 g).
Catalytic reusability of [PyPS]4SiW without any regeneration steps
was investigated in the model reaction (Fig. 4). At the fourth reaction
run, 87.5 wt.% of the catalyst (compared with the amount of fresh one
used in the first run) was recovered (see supplementary information)
and yield of the product decreased from 95% to 83%. This decrease in
the product yield may be due to the weight loss in the operation for re-
covering and/or deactivation of the catalyst. In order to find which one
is the main reason for decreasing the product yield, a control test was per-
formed using the same amount of fresh catalyst (0.2×87.5%=0.175 g)
and 88% yield of the product was obtained. Therefore, taking into account
the weight loss in the operation for the recovering of the catalyst, only 5%
decrease in the product yield is due to very slow catalyst deactivation. In
light of this, [PyPS]4SiW catalyst showed excellent reusability and self-
separation performance during the four cycles.
To evaluate the scope of the [PyPS]4SiW as “self-separation catalyst”
for the synthesis of β-keto enol ethers, different alcohols were used as
reactants (Table 3). Methyl, primary, and secondary alcohols, reacted
with dimedone without any significant difference to give the corre-
sponding products in good to excellent yields. tert-Butanol was also
reacted with dimedone to provide the corresponding β-keto enol
ether in 30% yield (entry 8), while some previous approaches did not
apply this alcohol or gave negative results for similar reactions [17–21].
The model reaction was carried out in the presence of different
amounts of [PyPS]4SiW as the best catalyst (Fig. 3). Excellent yield of
the product was obtained when only 0.2 g of [PyPS]4SiW was used.
The use of higher amounts of the catalyst had no significant effect in
96.1
100
90.7
87.5
90
80
70
60
50
40
30
20
10
0
Table 2
Efficacies of various catalysts in the model reaction.a
Entry
Catalyst (g)
Reaction
phenomenon
Recyclability
(wt.%)
Time
(min)
Yield
(%)b
1
2
3
4
5
6
7
8
–
–
–
180
45
45
45
45
45
90
90
90
30
30
30
10
98
98
90
87
82
31
15
66
91
84
89
[TMAPS]3PW
[MIMPS]3PW
[PyPS]3PW
[PyPS]4SiW
[PyPS]3PMo
TMAPS
PyPS
MIMPS
PW
PMo
Phase separation
Phase separation
Phase separation
Phase separation
Homogeneous
Heterogeneous
Heterogeneous
Homogeneous
Homogeneous
Homogeneous
Homogeneous
42
36
78
96
–
–
1
2
3
4
–
9
–
Run
Yield of product
wt.% of recovered catalyst
10
11
12
–
–
SiW
–
a
Reaction condition: dimedone (1 mmol), methanol (4 mL), catalyst (0.1 g), 50 °C.
Isolated yield.
b
Fig. 4. Catalytic reusability in the model reaction (after 30 min).

E. Rafiee, S. Eavani / Catalysis Communications 25 (2012) 64–68
67
Table 3
Synthesis of β-keto enol ethers using [PyPS]4SiW catalyst.a
Entry
1
Alcohol
MeOH
Product
Reaction phenomenon
Phase separation
Time (min)
30
Yield (%)b
95
2
EtOH
Phase separation
40
84
3
4
Phase separation
Phase separation
47
95
91
86
5c
6c
7c
Heterogeneous
Heterogeneous
Heterogeneous
90 (110)
45 (60)
70 (80)
80 (81)
96 (94)
91 (88)
8c
Heterogeneous
100 (120)
30 (27)
a
b
c
Reaction condition: diketone (1 mmol), alcohol (4 mL), catalyst (0.2 g), 50 °C.
Isolated yields.
Reaction condition: diketone (3 mmol), alcohol (4 mL), catalyst (0.2 g), 50 °C, results in parenthesis.
Reaction phenomena varied with the catalyst's solubility in reactants
4. The catalyst was conveniently separated from the products, easily
recycled and reused several times without significant loss of its
activity.
containing different polar groups. The phase-separation behavior was
observed in the cases of methanol, ethanol, propanol and cyclohexanol
(entries 1–4) and the reaction medium switched from a homogeneous
system to heterogeneous one at the end of the reactions. In these cases,
the phase transfer catalysis combined the advantages of both homoge-
neous and heterogeneous systems. The other entries were heteroge-
neous systems (entries 5–8). In order to increase the catalyst's
solubility, the excess amount of dimedone was used in these reactions.
In all cases, the catalyst remained insoluble and reaction times were sig-
nificantly increased (entries 5–8, results in parenthesis). However, high
yields were also obtained in these heterogeneous systems possibly be-
cause of the pseudo-liquid phase behavior of polyoxometalate-based
acid salts [11].
Acknowledgments
We thank the Razi University Research Council and Iran National
Science Foundation (INSF) for support of this work.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2012.04.009.
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