Polyether-substituted thiazolium ionic liquid catalysts - A thermoregulated phase-separable catalysis system for the Stetter reaction

Article abstract of DOI:10.1039/b921796g

A series of polyether-substituted thiazolium ionic liquids have been synthesized and used as catalysts in the Stetter reaction. The ionic liquids are a thermoregulated phase-separable catalysis (TPSC) system, because they possess the properties of critical solution temperature (CST) and inverse temperature-dependent solubility in toluene-heptane. The results showed that the novel TPSC system exhibits high recycling efficiency, and provides a potential route for a environmentally benign Stetter reaction. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b921796g

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www.rsc.org/greenchem | Green Chemistry
Polyether-substituted thiazolium ionic liquid catalysts – a thermoregulated
phase-separable catalysis system for the Stetter reaction
Fengli Yu,a Ruili Zhang,a Congxia Xie*a and Shitao Yub
Received 19th October 2009, Accepted 17th February 2010
First published as an Advance Article on the web 18th May 2010
DOI: 10.1039/b921796g
A series of polyether-substituted thiazolium ionic liquids have been synthesized and used as
catalysts in the Stetter reaction. The ionic liquids are a thermoregulated phase-separable catalysis
(TPSC) system, because they possess the properties of critical solution temperature (CST) and
inverse temperature-dependent solubility in toluene–heptane. The results showed that the novel
TPSC system exhibits high recycling efficiency, and provides a potential route for a
environmentally benign Stetter reaction.
efficiently recycled twice. In addition, immobilized thiazolium
1. Introduction
and triazolium catalysts have been synthesized to improve the
The Stetter reaction is one of the most important reactions in
recycling of Stetter catalysts.23 However, they were only recycled
organic synthesis. It is widely used in stereoselective synthesis,1,2
twice, and no further studies on the recycling efficiency were
heterocyclic synthesis,3,4 total synthesis of natural products,5,6
carried out. To the best of our knowledge, no other articles
electrochemical materials synthesis,7 and so on. Thiazolium
about the recycling of Stetter catalysts have been published.
salts,8 cyanide ions,9 triazolium salts10-12 and tributylphosphine13
In this paper, a series of thermoregulated thiazolium ionic
can be used as catalysts for the Stetter reaction. Generally,
liquids catalysts for use in the Stetter reaction were designed
Stetter reactions are carried out under heterogeneous conditions
and synthesized by introducing a polyether chain to the thiazole
with low reaction rates. Homogeneous catalysts are of higher
compound. It was found that the catalysts possessed the
catalytic activity, but it is difficult to separate the catalyst from
properties of CST and inverse temperature-dependent solubility
the reaction mixture. So far, intensive work has been focused
in toluene–heptane. The catalysts were successfully applied in the
on developing efficient catalytic systems for heterogenization of
Stetter reactions of ethyl acrylate with furfural or butanal. Thus,
homogeneous catalysts.14 Recently, based on the critical solution
a novel TPSC system of polyether-substituted thiazolium ionic
temperature (CST) of nonionic tensioactive phosphine ligands, a
liquids catalyst has been developed for resolving the problem
novel catalytic system (termed thermoregulated phase-separable
associated with the separation and reuse of Stetter catalysts.
catalysis (TPSC)), in which the catalyst itself becomes one phase
at room temperature and entirely dissolves in the organic phase
2. Results and discussion
at higher reaction temperature, has been developed by Jin and
Wang.15 By simple decantation, products can be easily separated
from the catalyst upon cooling of the reaction solution. So TPSC
combines the advantages of one-phase homogeneous catalysis
with an easy means of catalyst separation.
It is well known that room-temperature ionic liquids (RTILs)
with many unique properties have provided a class of superior
reaction media in synthesis.16-20 Gre´e and his group were the
first to use imidazolium-type RTILs as Stetter reaction media.21
Thiazolium salts and Et3N are efficient catalysts for Stetter
reactions performed in RTILs, but it is impossible for the
catalyst to be recycled. Yang has reported the first example of a
microwave-assisted intramolecular Stetter reaction in RTILs.22
Although the thiazolium catalyst in RTILs is recyclable and
reusable, this paper reported that the catalyst could only be
2.1 Synthesis of thermoregulated thiazolium ionic liquid
catalysts
The process for synthesis of the thermoregulated thiazolium
ionic liquid catalysts 4 is shown in Scheme 1. Firstly,
poly(ethylene glycol) monomethyl ethers 1 with different num-
bers of polymerized ethylene oxide units (n = 11, 16, 42) was
reacted with PBr3 to obtain bromine-substituted poly(ethylene
aKey Laboratory of Eco-chemical Engineering, Ministry of Education,
College of Chemistry and Molecular Engineering, Qingdao University of
Science and Technology, Qingdao, 266042, China. E-mail: xiecongxia@
126.com; Fax: +86 532 8402 3927; Tel: +86 532 8402 3927
bCollege of Chemical Engineering, Qingdao University of Science and
Technology, Qingdao, 266042, China. E-mail: yushitaoqust@126.com
Scheme 1 The synthesis of the thermoregulated thiazolium ionic liquid
catalysts.
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Table 1 The solubility of catalysts 4 in toluene–heptane
Toluene–heptane (v/v)
n
Toluene
Soluble
Soluble
Soluble
1 : 1
1 : 1.2
1 : 1.5
1 : 2
11
16
42
Slightly soluble
Insoluble; insoluble on heating
Insoluble; insoluble on heating
Insoluble; insoluble on heating
Insoluble; insoluble on heating
Insoluble; insoluble on heating
Insoluble; soluble on heating;
inverse
Insoluble; insoluble on heating
Insoluble; soluble
on heating; inverse
Slightly soluble
Insoluble; soluble on heating;
inverse
Insoluble; insoluble on heating
glycol) monomethyl ethers 2. Then, compounds 2 were re-
acted with 4-methyl-5-(hydroxyethyl)thiazole in the presence of
Et3N to produce polyether-substituted compounds 3. Lastly,
the thermoregulated thiazolium ionic liquid catalysts 4 with
different polyether chain lengths were obtained by the reaction
of compounds 3 with bromoethane. Catalysts 4 are pale yellow,
viscous liquids at room temperature. The viscosity increases with
the length of the polyether chain. The structures of compounds
1
3 and 4 were determined by NMR. In the H NMR spectrum
of catalysts 4, there is clearly the resonance of the CH-acidic
thiazolium proton at about d = 10, which indicates the existence
of the heterazolium precursor of the active carbene species.
2.2 Critical solution temperature (CST) of thermoregulated
thiazolium ionic liquid catalysts
The critical solution temperature (CST) is the temperature
at which a mixture of two liquids, immiscible at ordinary
temperature, ceases to separate into two phases. The upper
critical solution temperature (UCST) is the critical temperature
above which a mixture is miscible in all proportions. The CST
properties of polyether-substituted nonionic phosphine ligands
in nonpolar aprotic solvents has been reported.24 We have
discovered that the polyether-substituted thiazolium ionic liquid
catalysts 4 (n = 11, 16, 42) also possess the property of CST
(UCST) in mixture of toluene and heptane. The results of the
solubility of catalysts 4 are shown in Table 1.
According to Table 1, the catalysts 4 (n = 11, 16, 42) exhibit
good thermoregulated solubility in toluene–heptane (v/v)=1 : 2,
1 : 1 and 1 : 1.2, respectively. Moreover, inverse temperature-
dependent solubility and the property of UCST of catalysts
4 have been found. The relationship between the solubility and
temperature is shown in Fig. 1. It can be seen that catalyst 4 (n =
16) exhibits the best property of CST. At room temperature,
catalyst 4 (n = 16) is hardly soluble, but when the temperature
raises to about 60 ◦C, the solubility increases markedly. Above
60 ◦C, the catalyst can completely dissolve in the solvent mixture
and the whole system becomes one phase. Therefore, catalyst 4
(n = 16) has a CST of 60 ◦C in toluene–heptane (v/v=1 : 1).
Fig. 1 The relationship between the solubility of catalysts 4 in toluene–
heptane and temperature (n = 11, toluene–heptane (v/v)=1 : 2; n = 16,
toluene–heptane (v/v)=1 : 1; n = 42, toluene–heptane (v/v)=1 : 1.2).
Fig. 2 The general principle of TPSC of polyether-substituted thia-
zolium ionic liquids catalyst. (S: substrate; Os: organic solvent; C: ionic
liquids catalyst; P: product).
solvent, and the system is biphasic. When heated to temperatures
above the CST, the ionic liquid catalyst is completely soluble in
organic solvent, and the system becomes homogeneous. After
reaction, upon cooling to a temperature lower than the CST, the
system becomes biphasic again, accompanied by separation of
the products in the organic phase from the ionic liquid catalyst.
After simple decantation, the lower ionic liquid catalyst can be
recycled.
2.4 Optimization of reaction conditions in the novel TPSC
system
2.3 General principle of TPSC of polyether-substituted
thiazolium ionic liquid catalysts
The novel TPSC system of the polyether-substituted thiazolium
ionic liquid catalyst has been successfully used in Stetter
reactions of ethyl acrylate with furfural or butanal (Scheme 2).
The results of the Stetter reaction of ethyl acrylate with
furfural in the novel TPSC system are shown in Table 2. The
% conversion and yield were determined by GC. According
to entries 1–3, the length of the polyether chain of catalyst 4
The CST and inverse temperature-dependent solubility in
toluene–heptane enable the polyether-substituted thiazolium
ionic liquid catalysts 4 to meet the requirements of a TPSC. The
general principle of TPSC is illustrated in Fig. 2. At room
temperature, the ionic liquid catalyst is insoluble in organic
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Table 2 Stetter reaction of ethyl acrylate with furfural in the novel
TPSC system
Table 3 Stetter reaction of ethyl acrylate with furfural catalyzed by
different catalysts
Toluene:
Entry
Catalyst
Anion
Conversion (%)
Yield (%)
Catalyst heptane Tempera- Time Conversion Yield
ture (◦C) (h)
(%)
(%)
1
2
3
4
5
6
7
Cl-
Br-
I-
97.4
99.0
99.2
99.1
98.7
96.5
38.7
42.8
35.6
41.7
40.8
37.5
Entry
n
(mol%) (v/v)
4 (n = 16)
1
2
3
4
5
6
7
8
11 15
16 15
42 15
16 15
16 15
16 15
1 : 2
1 : 1
1 : 1.2
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
80
80
85
40
60
90
80
80
80
80
80
80
12
12
12
12
12
12
12
12
12
5
99.2
99.0
95.4
88.7
92.2
99.5
87.2
90.5
99.3
85.8
95.7
99.0
43.0
42.8
35.1
28.4
28.5
36.7
28.2
32.8
39.8
27.4
37.4
41.1
8
9
-
BF4
-
10
11
PF6
Br-
16 5
To increase the selectivity of the Stetter product, the thiazolium
ionic liquid catalysts 7–10 were synthesized with different anions
and used in the Stetter reaction of ethyl acrylate with furfural
under the same reaction conditions as catalyst 4 (n = 16).
The catalyst 7 was synthesized according to the same synthesis
route as catalyst 4 (n = 16) by using chloroethane instead
of bromoethane. The catalysts 8–10 were obtained by anion
exchange reaction of catalyst 4 (n = 16) using a corresponding
sodium salt. A thiazolium salt catalyst 11 without a polyether
chain was also used to catalyze the Stetter reaction of ethyl
acrylate with furfural in DMF. The obtained results are shown
in Table 3. The results showed that catalyst 4 (n = 16) gave
the highest yield, and catalyst 8 with I- gave the lowest yield
16 10
16 20
16 15
16 15
16 15
9
10
11
12
10
15
-
-
(entry 3). BF4 or PF6 did not improve the selectivity of the
Stetter products (entries 4 and 5) compared with Br-. According
to entries 2 and 6, the thiazolium ionic liquid catalyst 4 (n =
16) gave a higher conversion and yield than the thiazolium salt
catalyst 11. This is due to the advantage of the novel TPSC
system over a conventional reaction system. In all, catalyst 4
(n = 16) is the best catalyst for Stetter reactions of ethyl acrylate
with furfural.
Scheme 2 Stetter reactions of ethyl acrylate with furfural and butanal.
affects the catalytic property. When n = 11–16, the conversion
of furfural and yield of 5 vary little, and the conversion is above
99.0%. When n is increased to 42, the conversion decreases. This
is possibly due to the diffusion control from the presence of the
bulkier catalyst. Larger mass transfer resistance results in a lower
reaction rate. Moreover, the larger spatial effect of the catalyst
means it can not contact well with the reaction substrates. In
addition, when n = 11, the color of the upper organic layer
after reaction and cooling to room temperature indicates a small
loss of ionic liquid catalyst, which will influence the recycling
efficiency of the catalyst. Therefore, n = 16 seems to give the
best result. The conversion and yield of the Stetter reaction
increased with increasing temperature (entries 4, 5 and 2). When
the temperature is higher than the CST (60 ◦C), the conversion
of furfural and yield of 5 reach 99.0% and 42.8%, respectively.
The obvious increase of conversion indicates that the reaction
occurs in the homogeneous phase. The result provides further
proof for TPSC. However, further increase of temperature results
in lower selectivity for product 5 because of side reactions such
as polymerization of ethyl acrylate and condensation of furfural
(entry 6). The conversion and yield increase with increasing the
amount of thiazolium ionic liquid catalyst 4 from 5 to 15 mol%
(entry 7, 8 and 2). The use of 20 mol% of catalyst cannot
give much improved conversion and yield (entry 9). When the
reaction time is over 5 h, the conversion of furfural and yield of 5
increase significantly (entry 10, 11 and 2). But upon prolonging
the reaction time, no significant increase of conversion and
yield is obtained (entry 12). Compared with the corresponding
reaction catalyzed by thiazolium salt,8,9 the reaction time could
be significantly shortened, because the reaction is homogeneous.
From Table 2, it can be seen that the yield of product 5 is
only 42.8% under the optimum conditions (Table 2, entry 2).
Why is the selectivity of the Stetter product relatively low? In
general, intramolecular Stetter reactions can give relatively high
selectivity, but it is difficult for intermolecular Stetter reactions to
achieve high selectivity. To the best of our knowledge, the highest
yield is 67% in the Stetter reaction of p-fluorobenzaldehyde
with methyl acrylate.21 The lower selectivity or yield is mainly
due to the reaction of the catalytic carbanion with a second
aldehyde molecule, which affords a condensation byproduct.25
In addition, the polymerization of ethyl acrylate could occur
at higher temperature, although hydroquinone is added as an
inhibitor. The main byproducts of compounds 12–14 were
determined by GC-MS in a Stetter reaction of ethyl acrylate
with furfural. Compound 12 is a self-condensation product of
furfural catalyzed by a thiazolium carbanion. Compound 13
may be afforded by the reaction of furfural with ethanol derived
from the hydrolysis of ethyl acrylate. Acrylic acid derived from
ethyl acrylate hydrolysis could react with another molecule of
ethyl acrylate by a Diels–Alder reaction to afford compound 14.
The yield of compounds 12–14 were 31.2%, 11.6% and 8.3%,
respectively.
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the presence of the active proton of the thiazole ring. Therefore,
the catalytic activity of catalyst 4 can be retained to some extent.
3. Conclusion
A new type of thermoregulated thiazolium ionic liquid catalyst
for the Stetter reaction was synthesized by introducing a
polyether chain to the thiazole compound. This novel ther-
moregulated phase-separable catalysis (TPSC) system has been
developed for resolving the problems associated with separation
and reuse of Stetter catalysts. We also successfully applied it for
the Stetter reaction of ethyl acrylate with furfural or butanal.
Two Stetter reactions were fully investigated, and the reaction
conditions were optimized. Though the yield is low under the
optimum reaction conditions, the novel TPSC system exhibits
good recycling efficiency. There is no obvious decrease in the
conversion of furfural and yield after the catalyst is reused
six times. In the Stetter reaction of butanal with ethyl acrylate,
the catalyst can be reused five times without obvious loss of
catalytic activity. Therefore, this novel TPSC system provides a
potential route for an environmentally benign Stetter reaction.
Our novel TPSC system was also used in the Stetter reaction of
aliphatic aldehydes. The reaction of butanal with ethyl acrylate
was investigated. The optimum reaction conditions were found
to be using 15 mol% ionic liquid catalyst 4 (n = 16), toluene–
heptane (v/v=1 : 1) as the solvent, with a reaction temperature of
80 ◦C and a reaction time of 12 h. Under the above conditions,
the conversion of butanal and the yield of 6 were 99.5% and
37.3% respectively. The selectivity of the Stetter product is low,
but higher than that of the corresponding reaction catalyzed
by the thiazolium salt.9 The main byproducts determined by
GC-MS are compounds 14–16. Compounds 14 and 15 are the
polymerization products of ethyl acrylate. Compound 15 is the
product of the Diels–Alder reaction between compound 14 and
ethyl acrylate. Compound 16 is the self-condensation product
of butanal catalyzed by the thiazolium carbanion. Under the
above conditions, the yields of compounds 14–16 were 15.4%,
8.7% and 32.6% respectively.
4. Experimental
4.1 Materials and product analysis
All chemicals were used as purchased and were of reagent
grade. The solvents were dried by standard procedures and
used freshly distilled. The reaction products were analyzed by a
gas chromatograph (SP6800A) equipped with a flame ionization
detector, a capillary column (OV-17, 30 m ¥ 0.25 mm ¥ 0.25 mm).
1H and 13C NMR spectra (500 and 125 MHz, respectively) were
recorded with a Bruker AV 500 MHz spectrometer.
2.5 Recycling of the thermoregulated thiazolium ionic liquid
catalyst
4.2 Bromine-substituted poly(ethylene glycol) monomethyl
ether (2)
After the reaction, the organic phase was separated from the
ionic liquid catalyst by simple decantation. The ionic liquid
catalyst was directly recycled by addition of fresh solvents and
substrates. Table 4 shows the recycling efficiency of the catalyst
4 (n = 16) in the Stetter reaction of furfural with ethyl acrylate.
According to Table 4, there is no obvious decrease in the
conversion of furfural and yield of 5 after the catalyst is reused
six times. However, a slight decrease in catalytic activity is
observed after the catalyst is reused seven times. In the Stetter
reaction of butanal with ethyl acrylate, the ionic liquid catalyst
4 (n = 16) can be reused five times without obvious loss of
catalytic activity. The catalyst, after being reused five times, was
characterised using 1H NMR. The resonance at d = 9.5 indicates
All the glass vessels were dried before use. Poly(ethylene glycol)
monomethyl ether 1 (7.5 g, 10 mmol) and 20 mL CCl4 were
added to a 100 mL four-neck flask equipped with a thermometer,
a reflux condenser and a dropping funnel. With stirring,
fresh distilled PBr3 (4.05 g, 15 mmol) in 30 mL CCl4 was
added dropwise. During the dropwise addition, the reaction
temperature was kept at 30–40 ◦C. After finishing the addition,
the temperature was elevated to 65–70 ◦C and the reaction
solution was stirred for 4–5 h. Then, the product was separated
from the reaction mixture with a separatory funnel, followed by
washing with CCl4 to obtain a pale yellow solution of 2. Yield:
3.12 g (38.4%).
Table 4 Recycling efficiency of the catalyst 4 (n = 16) in the Stetter
reaction of furfural with ethyl acrylate
4.3 Polyether-substituted thiazole (3)
Recycle number
Conversion (%)
Yield of 5 (%)
Bromine-substituted poly(ethylene glycol) monomethyl ether 2
(8.13 g, 10 mmol), 4-methyl-5-(hydroxyethyl)thiazole (1.43 g,
10 mmol), Et3N (1.01 g, 10 mmol) and toluene (40 mL)
were added to a 100 mL round-bottomed flask with a reflux
condenser. The resulting mixture was stirred and heated under
reflux for 6 h. Then, the mixture was cooled to room temperature.
Et3N salt was separated from the solution by filtration, and the
solvent was removed under atmospheric distillation to obtain
1
2
3
4
5
6
7
8
99.0
99.2
99.0
98.7
98.5
98.8
97.5
95.0
42.3
42.2
42.2
41.2
40.9
41.2
38.9
35.5
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◦
compound 3. Yield: 5.91 g (67.6%); 1H NMR (500 MHz, D2O):
d = 2.29 (s, 3H), 2.95 (t, J = 7.0 Hz, 2H), 3.52 (s, 3H), 3.55–
3.64 (m, 64H), 3.66 (t, J = 7.1 Hz, 2H), 8.71 (s, 1H); 13C NMR
(125 MHz, D2O): d = 16.86, 30.22, 60.35, 68.77–71.72, 125.00,
137.14, 138.35.
was distilled under vacuum to obtain product 6 (88–90 C/13
mmHg). 1H NMR (500 MHz, CDCl3): d = 0.83 (t, J = 7.5 Hz,
3H), 1.13 (t, J = 7.2 Hz, 3H), 1.52 (m, 2H), 2.33 (t, J = 7.0 Hz,
2H), 2.62 (t, J = 6.9 Hz, 2H), 2.47 (t, J = 7.0 Hz, 2H), 4.03
(q, J = 7.2 Hz, 2H); 13C NMR (125 MHz, CDCl3): d = 13.63,
14.08, 17.20, 24.95, 36.97, 44.60, 60.53, 172.82, 209.09.
4.4 Thermoregulated thiazolium ionic liquid catalyst (4)
Polyether-substituted thiazole 3 (5.91 g, 6.7 mol), bromoethane
(0.74 g, 6.7 mol), and dried acetonitrile (20 mL) were mixed
in a 100 mL round-bottomed flask with a reflux condenser
(potassium hydroxide drying tube) and heated under reflux for
24 h. After cooling, the acetonitrile was removed to give the
product 4. Yield: 4.75 g (72%); 1H NMR (500 MHz, D2O): d =
1.60 (t, J = 7.3 Hz, 3H), 2.36 (s, 3H), 2.98 (t, J = 7.0 Hz, 2H),
3.45 (s, 3H), 3.47–3.69 (m, 64H), 3.70 (t, J = 7.0 Hz, 2H), 4.52 (q,
J = 7.2 Hz, 2H), 9.50 (s, 1H); 13C NMR (125 MHz, CDCl3): d =
14.76, 18.29, 30.39, 53.06, 61.34, 70.01–72.53, 128.23, 148.79,
148.94.
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acrylate (0.05 mol), Et3N (15 mol%), hydroquinone (3.6 mol%),
thermoregulated thiazolium ionic liquid catalyst 4 (15 mol%),
and 40 mL toluene–heptane mixture (v/v=1 : 1) were added to
a 100 mL round-bottomed flask with a reflux condenser. The
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for a given time. After completion of the reaction, the mixture
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upper organic phase (containing products) was separated from
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7.2 Hz, 1H), 7.24 (d, J = 6.9 Hz, 1H), 7.59 (d, J = 7.0 Hz, 1H);
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1200 | Green Chem., 2010, 12, 1196–1200
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