Ionic liquid supported on magnetic nanoparticles as highly efficient and recyclable catalyst for the synthesis of β-keto enol ethers

Article abstract of DOI:10.1016/j.catcom.2013.11.025

A magnetically ionic liquid supported on γ-Fe₂O ₃ nanocatalyst (Al_xCl_y-IL-SiO ₂γ-Fe₂O₃) was synthesized and evaluated as a recoverable catalyst for the synthesis of β-keto enol ethers. The immobilized catalyst proved to be effective and provided the products in high to excellent yield at room temperature. Moreover, the catalyst could be easily recovered by magnetic separation and recycled for six times without significant loss of its catalytic activity.

Full text of DOI:10.1016/j.catcom.2013.11.025

Ionic liquid supported on magnetic nanoparticles as highly efficient and
recyclable catalyst for synthesis of β-keto enol ethers
Pei-He Li, Bao-Le Li, Hai-Chuan Hu, Xiao-Na Zhao, Zhan-Hui Zhang
PII:
DOI:
Reference:
S1566-7367(13)00454-8
doi: 10.1016/j.catcom.2013.11.025
CATCOM 3728
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Catalysis Communications
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Revised date:
Accepted date:
27 September 2013
22 November 2013
25 November 2013
Please cite this article as: Pei-He Li, Bao-Le Li, Hai-Chuan Hu, Xiao-Na Zhao, Zhan-
Hui Zhang, Ionic liquid supported on magnetic nanoparticles as highly efficient and
recyclable catalyst for synthesis of β-keto enol ethers, Catalysis Communications (2013),
doi: 10.1016/j.catcom.2013.11.025
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ACCEPTED MANUSCRIPT
Ionic liquid supported on magnetic nanoparticles as highly efficient and
recyclable catalyst for synthesis of β-keto enol ethers
Pei-He Li, Bao-Le Li, Hai-Chuan Hu, Xiao-Na Zhao, Zhan-Hui Zhang*
College of Chemistry & Material Science, Hebei Normal University, Shijiazhuang 050024, China
ABSTRACT
A magnetically ionic liquid supported on γ-Fe2O3 nano catalyst (AlxCly-IL-SiO2@γ-Fe2O3) was
synthesized and evaluated as a recoverable catalyst for synthesis of β-keto enol ethers. The
immobilized catalyst proved to be effective and provided the products in high to excellent yield at
room temperature. Moreover, the catalyst could be easily recovered by magnetic separation and
recycled for six times without significant loss of its catalytic activity.
Keywords:
Magnetic nanoparticle; Immobilization; Ionic liquid; aluminium chloride; Heterogeneous catalyst;
β-keto enol ethers
____________________________________________________________________________
*Corresponding author. Fax: +86 31180787431. E-mail address: zhanhui@126.com (Z.-H. Zhang)
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1. Introduction
Inspired by the current requirements for green chemistry and sustainable chemistry, the
development of heterogeneous catalysts in organic reactions has become a major areas of research
due to the potential advantages of these materials over homogeneous systems such as simplified
recover, reusability, easy product isolation and incorporation in continuous reactors and
microreactors. But the major limitation associated with heterogeneous catalysts is their lower
catalytic activity and selectivity. In address this issue, nanoparticles (NPs) have emerged as
alternative form of the support materials due to their high surface areas and catalyst loading capacity,
improved catalytic activity and dispersability in the reaction medium [1]. Among the various
nanoparticle supports, magnetic nanoparticles (MNPs) are arguably the most extensively studied
and emerged as promising supports [2]. The potential advantage of MNPs is that it can be separated
from the reaction medium by an external permanent magnet without using any filtration or
centrifugation after completion of the reaction [3-4]. Moreover, they are accessible from
inexpensive materials and can be easily tuned by structural appropriate surface modification [5-7].
Meanwhile, the concept of supported ionic-liquid catalysis (SILC) has also recently been introduced,
which combines the features of ionic liquids and heterogeneous systems [8]. These advantages
prompted us to explore ionic liquid supported on MNPs as an efficient and recoverable catalyst in
organic synthesis.
β-keto enol ethers are useful intermediates for the formation of several organic and biologically
active natural products such as platencin [9], trisubstituted enones [10], acutumine [11-12], furan
derivatives [13], dihydronarciclasine [14], and sordarin [15]. Due to the importance of β-keto enol
ethers in organic synthesis and medicinal chemistry, several strategies have been developed for
synthesis of these compounds. Of these methods, the direct etherification of cyclic β-diketones with
alcohol is the simple and straightforward procedure for the synthesis of β-keto enol ethers. Some
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catalysts have been applied to promote this transformation such as iodine [16], B(C6F3) [17],
cerium(IV) ammonium nitrate [18], P2O5/SiO2 [19], ytterbium triflate [20], HClO4·SiO2 [21],
iron(III) tosylate [22], polyoxometalate-based acid [23], and H5CoW12O40 supported on nano silica
[24]. While the existing synthetic methods are effective for the preparation of target compounds,
some of them suffer from shortcomings such as high temperature, low yields, prolonged reaction
time, harsh reaction conditions, and the employment of unrecyclable catalysts as well as the
requirement of special apparatus. Consequently, there is need to develop a simple, efficient and
recyclable catalyst for the synthesis of β-keto enol ethers under mild conditions.
As part of our continuation efforts into the design of magnetic nanocatalysts and their
application in organic synthesis [25-29], we report herein on the preparation of a new type of ionic
liquid supported on magnetic γ-Fe2O3 catalyst (AlxCly-IL-SiO2@γ-Fe2O3, Scheme 1) and its
application for converting cyclic-β-diketones into their corresponding β-keto enol ethers at room
temperature (Scheme 2).
[Insert Scheme 1 here]
[Insert Scheme 2 here]
2. Experimental
2.1 General
X-ray diffraction analysis was carried out using a PANalytical X’Pert Pro X-ray
diffractometer. Surface morphology and particle size were studied using a Hitachi S-4800 SEM
instrument. Transmission electron microscope (TEM) observation was performed using Hitachi
H-7650 microscope at 80 KV. Elemental compositions were determined with a Hitachi S-4800
scanning electron microscope equipped with an INCA 350 energy dispersive spectrometer
(SEM-EDS) presenting a 133 eV resolution at 5.9 keV. The ICP-MS analyses were carried out with
an X series 2 spectrometer. Melting points were determined using an X-4 apparatus and are
uncorrected. IR spectra were obtained as potassium bromide pellets or as liquid films on potassium
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1
bromide pellets with a Bruker-TENSOR 27 instrument. H NMR (500 MHz) and 13C NMR (125
MHz) spectra were recorded on a Bruker DRX-500 spectrometer using CDCl3 as the solvent with
TMS as internal standard. Elemental analyses were performed by using a Vario EL III CHNOS
elemental analyzer.
2.1. Preparation of ionic liquid supported on magnetic γ-Fe2O3 catalyst (AlxCly-IL-SiO2@γ-Fe2O3)
1-Methyl-3-(trimethoxysilylpropyl)-imidazolium chloride (I) was synthesized according to the
procedure reported in the literature [28]. Compound I (5 mmol) was dispersed in absolute ethanol
(20 ml) and γ-Fe2O3@SiO2 (5.0 g) nanoparticles were added. Then the mixture was heated to 60 oC
for 12 h under nitrogen atmosphere. After cooling to room temperature, the material was collected
by a permanent magnet and washed with ethanol to remove the excess ionic liquid, and then was
dried to afford Cl-IL-SiO2@γ-Fe2O3.
The solution of anhydrous AlCl3 (1.0 mmol) in ethanol (20 ml) was added to a well stirred
ethanol solution suspension (50 ml) of Cl-IL-SiO2@γ-Fe2O3 (1.0 g). The mixture was refluxed for
24 h under nitrogen. After cooling to room temperature, the catalyst was collected by a permanent
o
magnet and rinsed with ethanol (3 × 20 ml), and then was dried at 110 C for 5 h to afford
AlxCly-IL-SiO2@γ-Fe2O3. The loading amount of Al metal was determined based on ICP-MS.
2.2 General procedure for synthesis of β-keto enol ethers:
A mixture of cyclic β-diketone (1, 1 mmol), alcohol (3 ml) and AlxCly-IL-SiO2@γ-Fe2O3 (20
mol%, 0.27 g) was stirred vigorously at room temperature. After completion of the reaction
(monitored by TLC), the catalyst was removed by using a magnet and washed with ethyl acetate.
The combined organic layers were dried over anhydrous MgSO4, and the solvent was evaporated
under reduced pressure. The crude product was purified by column chromatography on silica gel
(acetate: n-hexane, 1:4) to afford the pure β-keto enol ether.
3. Results and Discussion
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3.1. Synthesis and characterization of Al-IL-SiO2@γ-Fe2O3
Imidazolium-based ionic liquid supported on γ-Fe2O3 magnetic nanoparticle catalyst
(AlxCly-IL-SiO2@γ-Fe2O3) was obtained by the following steps (Scheme 1). Firstly,
1-methyl-3-(trimethoxysilylpropyl)-imidazolium chloride (I) was prepared by reaction of
N-methylimidazole and 3-chloropropyltrimethoxysilane [28]. Then, grafting of I onto NMPs coated
by silica was carried out through the formation of Si-O bonds via the reaction of alkoxy with the
abundant surface hydroxyl groups on SiO2@γ-Fe2O3. Finally, reaction between the grafted NMPs
-
(Cl-IL-SiO2@γ-Fe2O3) and AlCl3 was performed in ethanol under reflux for 48 h to form the AlCl4
anion as counter ion of the imidazolium cation of the IL (AlxCly-IL-SiO2@γ-Fe2O3) [30]. The Al
amount of the immobilized catalyst was found to be 0.73 mmol·g-1 based on inductively coupled
plasma mass spectrometry (ICP-MS) analysis. The energy dispersive spectrum (EDS) showed that
the mass percent of C, Fe, Si, O, Cl and Al are 18.61%, 15.28%, 8.26%, 54.30%, 1.60% and 1.95%
respectively (Fig. 1). Scanning electron microscopy (SEM) revealed that the catalyst presented the
uniform particles with spherical particle morphology. Using the Debye-Scherrer equation, the
average nanopartical diameter was estimated to be 28 nm, which was in a good agreement with the
results from SEM. Transmission electron microscopy (TEM) further confirmed this catalyst was
encapsulated by a thin silica layer. FT-IR spectra of AlxCly-IL-SiO2@γ-Fe2O3 displayed peaks at
3134 (unsaturated C-H stretching), 3085, 2973, 2943 (aliphatic C-H stretching), 1626 (C=N
stretching of imidazolium ring), 1576 (C=C stretching of imidazolium ring) cm-1, which clearly
differs from that of the unfunctionalized silica-coated γ-Fe2O3 (Figure 3). These results provided the
evidence that IL was successfully attached to the surface of γ-Fe2O3 nanoparticle. XRD
measurements of SiO2@γ-Fe2O3 and Al-IL-SiO2@γ-Fe2O3 exhibit the typical diffraction peaks at
around 18.28o，30.07o，35.42o，43.05o，53.41o，56.93o and 62.52o, which corresponded to the (111),
(220), (311), (400), (422), (440) and (511) diffraction planes. The observed diffraction peaks agree
well with the tetragonal structure of γ-Fe2O3 (1999 JCPDS file no. 13-0458) (Figure 4) [31],
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suggesting that the surface modification of MNPs did not significantly affect the phase composition
of SiO2@γ-Fe2O3.
[Insert Figures 1-4 here]
3.2. Catalytic activity of AlxCly-IL-SiO2@γ-Fe2O3
The activity of the prepared catalyst was next evaluated in the model reaction of
5,5-dimethylcyclohexane-1,3-dione and ethanol at room temperature. As shown in Table 1, the
reaction occurred smoothly in the presence of AlxCly-IL-SiO2@γ-Fe2O3, affording a single desired
product 3b in 93% yield without any side-products (Table 1, entry 7). The optimum amount of
catalyst was 20 mol%. Increasing the amount of catalyst did not improve the yield and the reaction
time. A control reaction conducted in the absence of the catalyst under identical conditions, only a
trace amount of the target product was observed (Table 1, entry 1). Non-functionallized magnetic
nanoparticles Fe3O4, γ-Fe2O3, γ-Fe2O3@SiO2 showed almost no catalytic effect (Table 1, entries
2-4). We also examined the feasibility of the present method for large-scale synthesis with
5,5-dimethylcyclohexane-1,3-dione (1.40 g, 10 mmol) and ethanol (30 ml) at room temperature.
The reaction was found to proceed smoothly affording the desired product in 94% yield within 50
min, almost similarly in all respects with 1 mmol scale (Table 1, entry 9). This experiment
demonstrated the efficiency of the catalyst for large production as well.
To realize the efficiency and the scope of AlxCly-IL-SiO2@γ-Fe2O3 catalyzed synthesis of β-keto
enol ethers, 5,5-dimethylcyclohexane-1,3-dione were treated with various alcohols under optimized
conditions. As shown in Table 2, the reaction could be successfully applied to various alcohols
including primary alcohols such as methanol, ethanol, butan-1-ol and heptan-1-ol (Table 2, entries
1-4), secondary alcohols such as propan-2-ol and cyclohexanol (entries 5 and 6), benzyl alcohol
(entry 7) as well as allyl alcohols (entry 8) to furnish the desired β-keto enol ethers in high to
excellent yields. When ethylene glycol reacted with 5,5-dimethylcyclohexane-1,3-dione the
corresponding mono-β-diketone 3i was obtained. Furthermore, cyclohexana-1,3-dione and
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cyclopentane-1,3-dione participated well in this transformation.
3.3. Recycling of the catalyst
An important aspect for heterogeneous catalyst is simple recovery and easily reusability. For this
purpose, the reusability of the catalyst was tested for the model reaction. After completion of the
reaction, the catalyst was separated from the reaction mixture simple by applying a strong external
permanent magnet, washed with ethyl acetate to remove residual product, dried under vacuum, and
used directly for the next cycle without further purification. The catalyst could be reused and
recycled for six times with only a slight loss of catalytic activity (Fig. 5). The slight decrease of
catalytic activity should be due to the normal loss of catalyst during the work-up stage.
4. Conclusion
In summary, a magnetically recoverable ionic liquid supported on γ-Fe2O3 nano catalyst was
successfully prepared and applied for synthesis of β-keto enol ethers. The significant features of this
newly developed proceduce are easy separation of the catalyst from reaction mixture by the
application of an external magnet, and its reusability, operational simplicity, room temperature and
high yields.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China
(21272053 and 21072042) and Nature Science Foundation of Hebei Province (B2011205031).
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Figures
Scheme 1 Synthesis of AlxCly-IL-SiO2@γ-Fe2O3
Scheme 2 Synthesis of β-keto enol ethers
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Fig. 1 EDS spectrum of AlxCly-IL-SiO2@γ-Fe2O3
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Fig. 2 SEM (A) and TEM images of AlxCly-IL-SiO2@γ-Fe2O3
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a
1576
1626
b
2973
3085
2943
3134
3500
3000
2500
2000
1500
1000
500
Wavenumber cm-1
Fig. 3 IR spectra of SiO2@γ-Fe2O3 (a) and AlxCly-IL-SiO2@γ-Fe2O3 (b)
Fig. 4 XRD pattern of AlxCly-IL-SiO2@γ-Fe2O3
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100
90
80
70
60
50
40
0
30
20
10
0
1
2
3
4
5
6
Cycles
Fig. 5 Reusability of the catalyst
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Tables
Table 1
Screening of reaction conditionsa
Entry
Catalyst
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
no
180
120
120
120
180
120
50
trace
trace
trace
trace
85
nano Fe3O4 (10 mol%)
nano γ-Fe2O3 (10 mol%)
nano γ-Fe2O3@SiO2 (10 mol%)
nano AlxCly-IL-SiO2@γ-Fe2O3 (10 mol%)
nano AlxCly-IL-SiO2@γ-Fe2O3 (15 mol%)
nano AlxCly-IL-SiO2@γ-Fe2O3 (20 mol%)
nano AlxCly-IL-SiO2@γ-Fe2O3 (25 mol%)
nano AlxCly-IL-SiO2@γ-Fe2O3 (20 mol%)
87
93
50
93
94b
50
a Experimental conditions: 5,5-dimethylcyclohexane-1,3-dione (1 mmol), ethanol (3 ml), room temperature.
b 10 mmol scale.
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Table 2
Synthesis of β-keto enol ethers in the presence of AlxCly-IL-SiO2@γ-Fe2O3
a
Entry β-Diketone
Alcohol
Product
Time (min) Yield (%)b
1
3a
MeOH
50
50
60
90
50
95
90
120
60
94
93
89
87
88
86
87
90
86c
2
3
4
5
6
7
8
9
3b
3c
3d
3e
3f
EtOH
3g
3h
3i
PhCH2OH
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10
11
3j
MeOH
EtOH
30
30
92
95
3k
12
13
3l
MeOH
EtOH
35
90
3m
35
93
a Reaction conditions: 1,3-dione (1 mmol), alcohol (3 ml), AlxCly-IL-SiO2@γ-Fe2O3 (0.27 g), room temperature.
b Isolated yield.
c The reaction was performed at 50 oC.
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Graphical Abstract:
Ionic liquid supported on magnetic nanoparticles as highly efficient and recyclable catalyst for
synthesis of β-keto enol ethers
Pei-He Li, Bao-Le Li, Hai-Chuan Hu, Xiao-Na Zhao, Zhan-Hui Zhang*
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Highlights
 The use of functionalized ionic liquid to synthesize grafted magnetic nanoparticle catalyst
 The synthesized catalyst shows excellent catalytic activity for synthesis of β-keto enol ethers.
 The catalyst can be easily recovered by magnetic separation.
 The supported catalyst exhibited high stability and could ba reused several times without
significant loss of its catalytic activity.
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