Ionic liquid functionalized silica gel: novel catalyst and fixed solvent

Article abstract of DOI:10.1016/j.tetlet.2006.07.122

Imidazolium cation-based ionic liquid functionalized silica gel was prepared, and Knoevenagel condensation as well as cycloaddition of propylene oxide and carbon dioxide without any additional organic solvents has been investigated. It was found that imid

Full text of DOI:10.1016/j.tetlet.2006.07.122

Tetrahedron Letters 47 (2006) 6951–6953
Ionic liquid functionalized silica gel: novel catalyst and fixed solvent
Guoqiao Lai,a,b Jiajian Peng,b Jiayun Li,b Huayu Qiu,b Jianxiong Jiang,b
Kezhi Jiangb and Yongjia Shena,*
aLaboratory for Advanced Materials, Institute of Fine Chemicals, East China University of Science and Technology,
Shanghai 200237, PR China
bKey Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Teachers College,
Hangzhou 310012, PR China
Received 16 May 2006; revised 25 July 2006; accepted 26 July 2006
Available online 14 August 2006
Abstract—Imidazolium cation-based ionic liquid functionalized silica gel was prepared, and Knoevenagel condensation as well as
cycloaddition of propylene oxide and carbon dioxide without any additional organic solvents has been investigated. It was found
that imidazolium cation-based ionic liquid functionalized silica gel was an effective recyclable catalyst.
Ó 2006 Elsevier Ltd. All rights reserved.
Room temperature ionic liquids, entirely constituted of
ions with negligible vapour pressure, excellent chemical
and thermal stability, have attracted considerable atten-
tion as novel and environmentally friendly benign
media. Many organic reactions have been reported
to proceed in room temperature ionic liquids with excel-
lent yields and chemo- and/or enantio-selectivities.1–3
Although the application of ionic liquids in diverse areas
has been well developed over the last two decades, prob-
lems such as cost and better recycling methods need to
be addressed. In view of the increasing necessity for
clean manufacturing processes and environmentally
benign technologies, solvent-free and highly efficient
catalytic technologies for chemical production are
highly desirable. In fact, one of the continuing efforts
of several groups is to develop new catalytic processes
using supported ionic liquid catalysis.4–6
dependency, and therefore environmentally undesirable
organic solvents have been inevitably employed.7 Knoe-
venagel condensation can be catalyzed by ionic liquids
under organic solvent-free conditions.8 The cycloaddi-
tion of carbon dioxide to propylene oxide with ionic
liquids as catalyst without any additional organic
solvents had also been investigated.9–11 Though the
ionic liquid used as catalyst was recyclable by distillation
of the product from the resulting mixture, simpler cata-
lyst separation processes still remains a challenge. Very
recently, phosphonium salts immobilized onto silica
for synthesizing propylene carbonate with excellent
catalytic activities have been reported.12
Herein, we report the Knoevenagel condensation as well
as cycloaddition of propylene oxide and carbon dioxide
with imidazolium cation-based ionic liquid functional-
ized silica gel 213,14 as recyclable catalyst and fixed
solvent.
Knoevenagel condensation of aldehydes and/or ketones
is a fundamental transformation in organic synthesis
and one of the most powerful means of effecting car-
bon–carbon bond formation. The classical Knoevenagel
condensation was catalyzed by ammonia and ammo-
nium salts, primary and secondary amines and their
salts in a liquid phase system with pronounced solvent
Knoevenagel condensation of ethylcyanoacetate and
malononitrile with a variety of aromatic aldehydes with
ionic liquid functionalized silica gel 2 as catalyst were
investigated (Scheme 1). The results are summarized in
Table 1.
Results listed in Table 1 show efficient Knoevenagel con-
densation of aromatic aldehydes with active methylene
compounds in the presence of catalyst 2. Aromatic alde-
hydes with electron-drawing or electron-donating
Keywords: Ionic liquid functionalized silica gel; Knoevenagel conden-
sation; Cycloaddition; Recyclable.
*
Corresponding author. Tel.: +86 21 64252967; fax: +86 21
64100558; e-mail: yjshen@ecust.edu.cn
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.07.122

6952
G. Lai et al. / Tetrahedron Letters 47 (2006) 6951–6953
Table 2. Cycloaddition of propylene oxide and carbon dioxide17
R
H
CN
CN
R1
catalyst
100 ºC
RCHO
+
Run
1
2
3
4
R1
Amount of CO2 added (g)
Conversion (%)
8.5
86.3
9.1
84.5
8.4
88.6
8.3
74.7
R = C6H5, 4-(CH3O)-C6H4, 4-(Cl)-C6H4, 2-(Cl)-C6H4, 4-(NO2)-C6H4,
4-(OH)-C6H4, 4-(CH3)-C6H4,
R1 = CO2Et, CN
In summary, Knoevenagel condensation of a series of
aromatic aldehydes with active methylene compounds
proceeds smoothly in the presence of ionic liquid func-
tionalized SiO2 without any additional volatile organic
solvents. The results have shown that this ionic liquid
functionalized SiO2 co-presented with zinc chloride
was an effective catalyst for the cycloaddition of carbon
dioxide with propylene to form propylene carbonate.
And the catalyst can be recyclable. Our experiments
show that imidazolium cation-based ionic liquid func-
tionalized silica gel can be used as recyclable catalyst
and/or fixed solvent in some organic reactions. Novel
applications of this type of catalyst and/or fixed solvent
are ongoing.
Scheme 1.
Table 1. Knoevenagel condensation catalyzed with ionic liquid func-
tionalized silica gel 215
Entry
R
R1
Reaction
time (h)
Yield (%)
1
C6H5
C6H5
C6H5
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CN
CN
CN
CN
CN
CN
CN
CN
5
5
5
5
5
5
5
5
5
4
4
4
4
4
4
4
4
95.2
93.3
91.8
60.4
93.7
91.4
96.9
95.1
87.8
93.1
94.9
93.8
96.8
97.1
96.4
97.6
95.6
2a
3b
4
4-CH3O–C6H4
4-Cl–C6H4
2-Cl–C6H4
4-OH–C6H4
4-(NO2)–C6H4
4-CH3–C6H4
C6H5
4-CH3O–C6H4
4-CH3O–C6H4
4-Cl–C6H4
2-Cl–C6H4
4-OH–C6H4
4-(NO2)–C6H4
4-CH3–C6H4
5
6
7
8
9
10
11
12a
13
14
15
16
17
Acknowledgements
We are grateful to Zhejiang Natural Science Foundation
(R203154) for financial support.
References and notes
Reaction conditions: aldehyde 25 mmol, active methylene compound
25 mmol, 100 °C, catalyst 80 mg.
a The second run.
b The third run.
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substituents could react very well with malononitrile and
excellent yields were achieved. In addition, the catalyst
was recyclable.
Our attention then turned to study the utility of this type
of catalyst and/or fixed solvent in cycloaddition of pro-
pylene oxide and carbon dioxide (Scheme 2). As men-
tioned above, imidazolium-based ionic liquids have
been introduced as effective catalysts for the fixation of
carbon dioxide to cyclic carbonate, and the catalytic
activity can be significantly enhanced by the co-catalyst
zinc chloride.16 The results of recycle experiments are
summarized in Table 2.
6. Mehnert, C. P.; Mozeleski, E. J.; Cook, R. A. Chem.
Commun. 2002, 3010.
7. Jones, G.. In Organic Reactions; Wiley: New York, 1967;
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Sakakura, T. Chem. Commun. 2006, 1664.
13. Preparation of 1-methyl-3-(3-triethoxysilylpropylimidazo-
lium chloride 1: (3-Chloropropyl)triethoxysilane (48.2 g,
0.2 mol) and N-methylimidazole (16.4 g, 0.2 mol) were
placed in a 100 mL stainless steel autoclave, which was
evacuated and sealed, then heated to 120 °C for 24 h. The
sticky residue was washed with ethyl acetate/diethyl ether
and dried at 60 °C in vacuum to afford 1 (58.1 g, 90%
yield), a white solid under room temperature.
14. Preparation of imidazolium cation-based ionic liquid
functionalized silica gels 2 and 3: Tetraethoxysilane
(41.6 g, 0.2 mol) and 1 (32.2 g, 0.1 mol) were dissolved in
80 ml of ethanol, then 9.2 mL of 1% HCl aqueous was
added. The resulting solution was stirred at 55 °C for 6 h
After the catalyst 313,14 had been used 4 times, the cata-
lytic activity showed a small decrease, indicating that the
ionic liquid functionalized silica gel catalyst 3 is recycla-
ble for the cycloaddition.
O
O
O
O
+ CO2
Scheme 2.

G. Lai et al. / Tetrahedron Letters 47 (2006) 6951–6953
6953
and at room temperature for 24 h, and then the mixture
was evaporated to dryness under reduced pressure. The
residue was heated to 200 °C for 8 h to form 2 quantita-
tively (Scheme 1). Catalyst 2 (7.82 g) and zinc chloride
(1.414 g) were suspended in methylene dichloride (50 mL),
after stirring overnight at room temperature. Evaporation
of the suspension afforded zinc chloride co-presented
catalyst 3, which was used as catalyst for synthesis
propylene carbonate from carbon dioxide and propylene
oxide.
filtered and dried in vacuum at 40 °C for 24 h to afford
products with high purity (>99.5% purity as judged with
GC–MS). The product was further purified by flash
chromatography. All compounds were characterized by
1
MS, H and 13C NMR.
16. Kim, H. S.; Kim, J. J.; Kim, H.; Jang, H. G. J. Catal.
2003, 220, 44.
17. Cycloaddition of propylene oxide with CO2: Cycloaddi-
tion reactions were performed in a 100 mL stainless steel
autoclave equipped with a magnetic stirrer. Propylene
oxide (7.0 mL, 0.10 mol) and 1.025 g of 3 were successively
charged into the reactor, then CO2 was introduced, and
the amount of CO2 added was measured by weight. The
autoclave was heated up to 120 °C with stirring for 8 h.
After cooling the remaining CO2 was released, the
resulting mixture was filtered, and the filtrate was analyzed
by GC–MS, while the catalyst was charged with propylene
and CO2 into the autoclave for the next run.
15. Typical Knoevenagel condensation experimental proce-
dure: A mixture of 25 mmol of aldehyde and 25 mmol of
ethyl cyanoacetate or malononitrile and 80 mg of 2 were
charged into a 50 mL round-bottomed flask equipped with
a magnetic stirrer. The reaction proceeded at 100 °C with
constant stirring for 4–5 h. After cooling to room
temperature, the resulted solid mass was ground and
washed with a cool solution of 1% aqueous alcohol,