A PEG bridged tertiary amine functionalized ionic liquid exhibiting thermoregulated reversible biphasic behavior with cyclohexane/isopropanol: Synthesis and application in Knoevenagel condensation

Article abstract of DOI:10.1039/c2nj40890b

A novel poly(ethylene glycol) bridged tertiary amine functionalized ionic liquid PEG₈₀₀-DPIL(Cl) was synthesized. It can form a temperature driven reversible biphasic system with cyclohexane/isopropanol mixed solvent. This biphasic system was applied in Knoevenagel condensation up to 99%. PEG ₈₀₀-DPIL(Cl) could be simply recovered and recycled for several runs.

Full text of DOI:10.1039/c2nj40890b

New Journal of Chemistry
A journal for new directions in chemistry
www.rsc.org/njc
Volume 37 | Number 2 | February 2013 | Pages 253–544
ISSN 1144-0546
LETTER
Jun Luo et al.
A PEG bridged tertiary amine functionalized ionic liquid exhibiting
thermoregulated reversible biphasic behavior with cyclohexane/isopropanol:
synthesis and application in Knoevenagel condensation

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LETTER
A PEG bridged tertiary amine functionalized ionic
liquid exhibiting thermoregulated reversible biphasic
behavior with cyclohexane/isopropanol: synthesis and
application in Knoevenagel condensation†
Cite this: NewJ.Chem., 2013,
37, 269
Received (in Montpellier, France)
4th October 2012,
Accepted 21st November 2012
Jun Luo,* Tantan Xin and Yinglei Wang
DOI: 10.1039/c2nj40890b
www.rsc.org/njc
A novel poly(ethylene glycol) bridged tertiary amine functiona- By heating up to the CST, the mixture homogenises and the
lized ionic liquid PEG800-DPIL(Cl) was synthesized. It can form a reaction can be carried out under monophasic conditions. After
temperature driven reversible biphasic system with cyclohexane/ completion of the reaction, catalyst recovery can be obtained by
isopropanol mixed solvent. This biphasic system was applied in cooling to ambient temperature and phase separation. A fluorous
Knoevenagel condensation up to 99%. PEG800-DPIL(Cl) could be biphasic system (FBS) is also widely used. This system is based on
simply recovered and recycled for several runs.
the development of fluorous-tagged catalysts with a high affinity
for perfluorinated solvents and a poor solubility in conventional
It is well known that homogeneous catalysis generally has the organic solvents. The reaction procedure is similar to TPSC.6
drawback of difficult catalyst recovery. Immobilization of
Ionic liquids (ILs) have attracted increasing attention owing
catalyst on solid ‘‘stationary’’ supports is a conventional way to their excellent properties such as negligible volatility,
to make catalyst recovery easy and simple.1 However, most excellent thermal stability and high designability.7 Unfortu-
catalysts tend to show reduced catalytic activity after being nately, traditional ILs show remarkable solubility in commonly
immobilized on inert supports. To complement each other’s used solvents, which causes inconvenience of product separa-
advantages, chemists have developed a variety of thermo- tion and catalyst recovery, especially in homogeneous systems.
regulated, also named ‘‘temperature-dependent’’ or ‘‘temperature Recently, chemists have developed some temperature driven
driven’’, reversible biphasic systems. For water-involved reac- reversible IL–liquid biphasic systems. For example, based on
tions, thermoregulated phase-transfer catalysis has been under the conception of a fluorous biphasic system, a highly fluorous-
development for more than ten years.2 In this reaction, cata- tagged ionic liquid was reported to have the appropriate
lysts are located in the water phase at low temperature, then temperature-dependent phase behavior with toluene.8 Some
they are transferred to the organic phase after being heated to other functionalized ionic liquids such as polyether-based
reaction temperature, and at last come back to the water phase ILs,9 heteropolyanion-based acidic ILs,10 quaternary ammonium-
when the mixture is cooled to room temperature. As for water- based thermoregulated ILs,11 dicationic acidic ILs12 have been
sensitive reactions, organic liquid–liquid biphasic catalysis3 reported to possess temperature driven biphasic properties. We
and poly(ethylene glycol) (PEG) biphasic catalysis4 have been recently have also developed a PEG-1000 linked dicationic
suggested, but these systems usually suffer from remarkable acidic ionic liquid (PEG1000-DAIL), which has the temperature-
catalyst leaching. Recently, based on the critical solution dependent reversible miscibility with toluene.13 The results of a
temperature (CST) of polyether-functionalized ligands, a novel quantitative solubility test indicated that the leaching of
biphasic catalysis system named ‘‘thermoregulated phase- PEG1000-DAIL in toluene was as low as 0.6% as the average of
separable catalysis (TPSC)’’ has been proposed.5 The character ten tests. There is still a very low average leaching of 0.75%
of this catalytic process can be described as follows: At ambient in a three-component condensation for the preparation of
temperature the catalyst is immiscible in the solvent. benzopyrans.13a
Knoevenagel condensation is a widely used reaction for the
formation of carbon–carbon double bonds. It can be catalyzed
School of Chemical Engineering, Nanjing University of Science and Technology,
by numerous catalysts such as acids,14 bases,15 acid–base
200 Xiaolinwei, Nanjing, 210094, China. E-mail: luojun@njust.edu.cn
bifunctional catalysts,16 iodine,17 supported catalysts,18 metal
† Electronic supplementary information (ESI) available: Detailed experimental
catalysts19 and enzymes.20 Some physical techniques including
procedures, analytical data and spectra, See DOI: 10.1039/c2nj40890b
c
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Scheme 1 Synthesis of PEG800-DPIL(Cl).
microwave,21 supersonic wave22 and grinding23 have also been isopropanol (volume ratio of 3 : 1) was revealed to fulfil this
utilized to promote this reaction. target. 3.6 mL of mixed solvent and 0.6 g of PEG800-DPIL(Cl) can
We report here a novel thermoregulated tertiary amine func- form a thermoregulated biphasic system, and the critical
tionalized dicationic ionic liquid PEG800-DPIL(Cl) (3, Scheme 1). temperature was found to be 80 1C. The phase diagram and
It presents temperature-dependent biphase properties with a thermoregulated process of the PEG800-DPIL(Cl)/CYH/IPA system
cyclohexane/isopropanol mixed solvent and was used as an are shown in Fig. 2 and 3 respectively.
efficient and recyclable catalyst and co-solvent in Knoevenagel
condensation to afford benzylidenes in excellent yields.
The application of this thermoregulated system in the
Knoevenagel reaction was then investigated. The model reac-
PEG800-DPIL(Cl) was prepared from commercially cheap tion was conducted with benzaldehyde and ethyl cyanoacetate.
starting materials via two routes (Scheme 1). Route 1 was The results indicated that PEG800-DPIL(Cl) displayed high
studied first but was found to be inefficient, especially for the catalytic activity in this reaction. Compared to traditional
quaternization reaction of poly(ethylene glycol) bridged imid- catalysis, the PEG800-DPIL(Cl)/CYH/IPA system has the combined
azole (2) with N-(2-chloroethyl)piperidine. This route suffered advantages of homogeneous and heterogeneous catalysis.
from tedious workup and it was almost impossibile to purify The catalysis process can be described as follows: the
the ILs. Alternatively, route 2 was found to be much better. The PEG800-DPIL(Cl)/CYH/IPA biphasic system is charged with
target ionic liquid 3 could be obtained in light color and high
purity.
According to the thermogravimetric analysis (TGA),
PEG800-DPIL(Cl) is quite stable since it does not decompose
below 250 1C (Fig. 1). It is soluble in water and some polar
organic solvents including methanol, ethanol, isopropanol,
isobutanol and chloroalkanes even at room temperature, while
it is insoluble in less polar solvents such as toluene, diethyl
ether, hexane, cyclohexane, heptane and petroleum. No solvent
alone was found to have the prospect of forming a temperature
driven reversible biphasic mixture with PEG800-DPIL(Cl).
Fortunately, the mixed solvent containing cyclohexane and
Fig. 2 Phase diagram of PEG800-DPIL(Cl)/CYH/IPA thermoregulated system
(0.6 g PEG800-DPIL(Cl), VCYH : VIPA = 3 : 1).
Fig. 1 The TG spectrum of PEG800-DPIL(Cl).
270 New J. Chem., 2013, 37, 269--273
Fig. 3 Thermoregulated process of PEG800-DPIL(Cl)/CYH/IPA system.
c
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Table 1 (continued )
Yieldb
t(min) (%)
Entry
8
R
R0
Product
4-MeOC6H4
CO2Et
50
60
93
81
9
4-OHC6H4
CO2Et
CO2Et
CO2Et
3-MeO-4-
OHC6H3
10
11
30
89
Fig. 4 Recycling of PEG800-DPIL(Cl) in a model Knoevenagel condensation.
pyridin-2-yl
30
94
96
substrates then heated to 80 1C, the reaction takes place under
homogeneous conditions. After completion of the reaction,
PEG800-DPIL(Cl) would precipitate from the reaction mixture by
cooling to room temperature. The recycling experiments resulted
in the catalytic activity of PEG800-DPIL(Cl) remaining almost
unchanged after three cycles (Fig. 4). However, obvious deactiva-
tion was observed at the sixth run, which might be assigned to the
N-oxidation of PEG800-DPIL(Cl) along with the workup process.
In order to explore the application scope of this thermo-
regulated biphasic catalysis, a variety of aldehydes were then
applied to react with ethyl cyanoacetate and malononitrile
(Table 1). In general, all the benzaldehydes reacted smoothly
12
13
14
furan-2-yl
n-Pr
CO2Et
CO2Et
CN
30
120
5
Trace
C6H5
98
92
15
2,3-Cl2C6H3
CN
5
16
17
18
19
20
21
3,4-Cl2C6H3
4-BrC6H4
CN
CN
CN
CN
CN
5
10
5
91
Table 1 Knoevenagel condensation in PEG800-DPIL(Cl)/CYH/IPA biphasic systema
90
Yieldb
t(min) (%)
4-ClC6H4
90
94
Entry
R
R0
Product
3-BrC6H4
5
1
2
C6H5
CO2Et
CO2Et
30
6
99 (6)c
4-MeOC6H4
4-OHC6H4
10
97
4-NO2C6H4
99
CN
CN
10
5
84
3
2,3-Cl2C6H3
CO2Et
20
99
3-MeO-4-
OHC6H3
22
91
a
4
5
3,4-Cl2C6H3
4-BrC6H4
CO2Et
CO2Et
30
50
90
Reaction conditions: aldehyde (2 mmol), active methylene compound
(2.1 mmol), PEG800-DPIL(Cl) (0.6 g), CYH (2.7 mL), IPA (0.9 mL); 80 1C.
b
c
Isolated yield. Control experiment without PEG800-DPIL(Cl), 80 1C,
2 h, yield was calculated from 1H NMR.
95
with ethyl cyanoacetate and malononitrile to afford ethyl
6
7
3-BrC6H4
3-ClC6H4
CO2Et
CO2Et
30
30
99
98
2-cyano-3-phenylacrylates
or
2-benzylidenemalononitriles
respectively in good to excellent yields. Nevertheless, the control
experiment without PEG800-DPIL(Cl) gave very low conversion
even after being conducted for two hours (Table 1, entry 1).
c
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As expected, electron-withdrawing substituents were a little Notes and references
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PEG800-DPIL(Cl), which could form a temperature driven rever-
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has properties that are characteristic of
a temperature-
dependent reversible biphasic system: it has simple and facile
recyclability, high catalytic activity and extensive substrate
adaptability.
Experimental
General procedure for the Knoevenagel condensation and
recycling of catalyst
PEG800-DPIL(Cl) (0.6 g), cyclohexane (2.7 mL) and isopropanol
(0.9 mL) were charged into a condenser-equipped flask;
aldehyde (2 mmol) and active methylene compound (2.1 mmol)
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This project was financially supported by the National Natural
Science Foundation of China (No. 21002050), the ‘‘Zijin Star
Project’’ and the ‘‘Excellence Initiative Project’’ of NUST.
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