L.-L. Lou et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 20–27
21
clable heterogeneous catalyst for asymmetric hydrogenation of
prochiral ketones. A few researches have successfully applied
the concept of supported ionic liquid catalysis in asymmetric
catalytic systems [38]. The organocatalyst l-proline was immo-
bilized in a SiO2-supported ionic liquid phase, and comparable
yields and enantiomeric excess (ee) values with respect to the
homogeneous catalysis were obtained in aldol reactions between
acetone and aromatic aldehydes [39,40]. Dioos and Jacobs [41]
immobilized a dimeric Cr(salen) catalyst in ionic liquids onto the
surface of SiO2, and the resulting material showed better reac-
tivity and selectivity for asymmetric ring-opening reactions of
epoxides than the Cr(salen) complex immobilized by impregna-
tion. An MCM-48 supported ionic liquid phase catalyst containing
chiral Mn(salen) complex was previously reported by our group
[22], and the catalytic system exhibited comparable activity and
higher enantioselectivity for the asymmetric epoxidation of olefins
than the homogeneous counterpart. In the present work, the chi-
ral ruthenium complex containing a cheap achiral monophosphine
[RuCl2(PPh3)2(S,S-DPEN)] was prepared and immobilized in the
ionic liquid [bmim][BF4] (bmim = 1-butyl-3-methylimidazolium),
which was confined on the surface of imidazolium-based ionic
liquid modified mesoporous silicates, including MCM-48, MCM-
41, SBA-15, and amorphous SiO2. The obtained materials were
employed as efficient catalysts for the asymmetric hydrogenation
of aromatic ketones, and the effects of various reaction condi-
tions on the catalytic performance were studied in detail. It was
found that all of these immobilized catalysts exhibited comparable
catalytic activity and enantioselectivity to those of homogeneous
counterpart, and they could be recycled at least four times with no
obvious decrease in catalytic performance.
ratus. The specific surface areas were determined by the BET
(Brunauer–Emmett–Teller) equation using adsorption data in the
relative pressure range from 0.05 to 0.2, and the pore size dis-
tributions were estimated using the BJH (Barret–Joyner–Halenda)
model. The content of Ru leaching in the reaction solution was
determined by inductively coupled plasma-atomic emission spec-
trometry (ICP-AES) on an ICP-9000(N+M) spectrometer (TJA Co.).
Diffuse-reflectance UV–vis (DR UV–vis) spectra were obtained on a
Jasco V-570 UV–vis spectrophotometer in the range of 220–800 nm.
The products of hydrogenation reaction were determined by
gas chromatography (GC) on a Rock GC7800 gas chromatograph
equipped with a flame ionization detector and a chiral capillary col-
umn (BETA-DEX 325, 30 m × 0.25 mm × 0.25 m), using ultrapure
nitrogen as a carrier gas. The injection and detector temperatures
were set at 523 K and 513 K, respectively, and the column tem-
perature was programmed in the range of 373–393 K (10 min hold
at 373 K, 1 K/min from 373 K to 393 K). The injection volume was
0.2 L using a split ratio of 1:50.
2.2. Synthesis of ionic liquids
2.2.1. Synthesis of ionic liquid [bmim][BF4]
1-Methylimidazole (4.0 mL, 50 mmol) and 1-chlorobutane
(5.2 mL, 50 mmol) were stirred in air at 343 K for 24 h [44]. The reac-
tion mixture was cooled to room temperature and then repeatedly
washed with ethyl acetate (3× 20 mL). The residual ethyl acetate
was evaporated at 313 K under reduced pressure, and the residue
was dried under vacuum at 323 K to give [bmim]Cl as a white crys-
talline solid. 1H NMR (CDCl3, 300 MHz, Me4Si): ı (ppm) 0.93–0.98
(t, 3H, CH2CH3), 1.34–1.42 (m, 2H, CH2CH3), 1.84–1.94 (m, 2H,
CH2CH2CH2), 4.12 (s, 3H, NCH3), 4.31–4.36 (t, 2H, NCH2), 7.41 (s,
1H, NCH), 7.54 (s, 1H, NCH), 10.49 (s, 1H, NCHN).
2. Experimental
To a solution of [bmim]Cl (7.0 g, 40 mmol) in acetone (50 mL)
was added NaBF4 (5.1 g, 46 mmol) [45]. The resulting mixture
was vigorously stirred at room temperature for 72 h and then
filtrated. The filtrate was evaporated at 303 K under reduced pres-
sure to remove acetone, and dried at 313 K under vacuum to give
[bmim][BF4] as a clear, pale yellow liquid, which was stored in a
vacuum desiccator before use. 1H NMR (CDCl3, 300 MHz, Me4Si):
ı (ppm) 0.93–0.98 (t, 3H, CH2CH3), 1.25–1.41 (m, 2H, CH2CH3),
1.72–1.91 (m, 2H, CH2CH2CH2), 3.96 (s, 3H, NCH3), 4.16–4.21 (t,
2H, NCH2), 7.28 (s, 1H, NCH), 7.32 (s, 1H, NCH), 8.83 (s, 1H, NCHN).
2.1. General
3-Chloropropyltriethoxysilane (95%), Pluronic P123 (EO20
-
PO70-EO20, 30 wt% PEG) and 1-methylimidazole (≥99%) were
purchased from Aldrich. (1S,2S)-(+)-1,2-Diphenylethylenediamine
(S,S-DPEN, ≥97%, ≥99% ee) was obtained by Likai Chiral Tech. Co.
Ltd. in Chengdu. Acetophenone (99%) was provided by Kermel
Chemical Reagent Co. Ltd. in Tianjin. o-Chloroacetophenone (>99%)
and o-fluoroacetophenone (>98.5%) were purchased from Jintan
Kingun Chemical Factory. Tris(triphenylphosphine)ruthenium(II)
chloride [RuCl2(PPh3)3] (≥99.9%) was supplied by Shaanxi Kaida
Chemical Engineering Co. Ltd. Cetyltrimethylammonium bromide
(CTAB, 99%), tetraethyl orthosilicate (TEOS, AR), 1-chlorobutane
Technology Co. Ltd. in Tianjin. The hydrogen gas used in the hydro-
genation experiments was of 99.99% purity and was supplied by
Tianjin Skarn Gas Co., Ltd. The highly ordered siliceous mesoporous
materials MCM-41 and MCM-48 were synthesized according to the
literature method [42], using CTAB as template and TEOS as silica
source. The purely siliceous SBA-15 was synthesized using a tri-
block organic copolymer Pluronic P123 as template and TEOS as
silica source [43]. Amorphous SiO2 was purchased from YuMinYuan
silica-gel reagent factory. All of the solvents used in the present
work were dried over 4A molecular sieves before use.
2.2.2. Synthesis of ionic liquid
1-methyl-3-(3-triethoxysilylpropyl)-imidazolium
tetrafluoroborate ([Simim][BF4])
The ionic liquid [Simim][BF4] was obtained through the synthe-
sis sequence given in Scheme 1 [37]. 1-Methylimidazole (3.0 mL,
38 mmol) was reacted with 3-chloropropyltriethoxysilane (9.1 mL,
38 mmol) at 368 K under nitrogen atmosphere for 24 h. The reac-
tion mixture was cooled to room temperature and then washed
completely with diethyl ether. After dried under vacuum at 303 K,
a slight yellow viscous liquid 1-methyl-3-(3-triethoxysilylpropyl)-
imidazolium chloride ([Simim]Cl) was obtained. 1H NMR (CDCl3,
300 MHz, Me4Si): ı (ppm) 0.59–0.65 (t, 2H, SiCH2), 1.20–1.25 (t,
9H, CH2CH3), 1.97–2.07 (m, 2H, CH2CH2CH2), 3.79–3.86 (q, 6H,
CH2CH3), 4.13 (s, 3H, NCH3), 4.32–4.37 (t, 2H, NCH2), 7.26 (s, 2H,
NCH), 11.01 (s, 1H, NCHN).
To a solution of [Simim]Cl (9.7 g, 30 mmol) in acetone (40 mL),
NaBF4 (3.8 g, 35 mmol) was added slowly. The resulting mixture
was vigorously stirred at room temperature for 72 h and then fil-
trated. The liquid phase was evaporated at 303 K under reduced
pressure to remove acetone, and dried under vacuum at 313 K to
give [Simim][BF4] as a yellow viscous liquid, which was stored in a
vacuum desiccator before use. 1H NMR (CDCl3, 300 MHz, Me4Si): ı
(ppm) 0.56–0.62 (t, 2H, SiCH2), 1.20–1.25 (t, 9H, CH2CH3), 1.95–2.05
1
1H and 13C{ H} NMR spectra were obtained on a Varian Mer-
cury Vx-300 spectrometer, 300 MHz for 1H and 75 MHz for 13C,
respectively. The elemental analysis of C, H and N was performed
on a PerkinElmer 240C analyzer. XRD patterns were recorded on
an Rigaku D/max-2500 diffractometer with Cu K␣ radiation at
40 kV and 100 mA. FT-IR spectra were carried out on a Bruker Vec-
tor 22 spectrometer using KBr pellets. N2 adsorption–desorption
analysis was done at 77 K on a Micromeritics TriStar 3000 appa-