An effective approach for the immobilization of chiral Mn(III) salen complexes through a supported ionic liquid phase

Article abstract of DOI:10.1016/j.tetlet.2006.07.048

An effective method based on supported ionic liquid system was employed to immobilize chiral Mn(III) salen complexes. The prepared heterogeneous catalysts exhibited excellent activity and enantioselectivity in the asymmetric epoxidation of unfunctionalize

Full text of DOI:10.1016/j.tetlet.2006.07.048

Tetrahedron Letters 47 (2006) 6513–6516
An effective approach for the immobilization of chiral Mn(III)
salen complexes through a supported ionic liquid phase
Lan-Lan Lou, Kai Yu, Fei Ding, Wei Zhou, Xiaojie Peng and Shuangxi Liu*
Institute of New Catalytic Materials Science, College of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
Received 30 March 2006; revised 12 July 2006; accepted 13 July 2006
Available online 1 August 2006
Abstract—An effective method based on supported ionic liquid system was employed to immobilize chiral Mn(III) salen complexes.
The prepared heterogeneous catalysts exhibited excellent activity and enantioselectivity in the asymmetric epoxidation of unfunc-
tionalized olefins. Especially, in the epoxidation of a-methylstyrene, both the conversion and ee value could exceed 99%. Further-
more, the immobilized catalysts were stable and could be recycled three times without loss of activities.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
as well as increased activity in the asymmetric epoxida-
tion of olefins than that obtained without the employ-
ment of ionic liquid. Here, we employed the supported
ionic liquid system as carrier media to immobilize chiral
Mn(III) salen complexes. Mehnert14,15 has described the
concept of supported ionic liquid catalysis, which
involves the treatment of a monolayer of covalently
attached ionic liquid on the surface of a support mate-
rial with additional adsorbed ionic liquid. These layers
serve as the reaction phase in which the homogeneous
catalyst is dissolved. The supported ionic liquid catalyst
combines the advantages of ionic liquids and heteroge-
neous supports. Active species in the ionic liquid phase
acts like a homogeneous catalyst. Several studies14–20
have successfully confined various ionic liquid phases
to the surface of different supports and applied them
in some catalytic processes. In the present work, we have
prepared chiral Mn(III) salen complexes immobilized on
the imidazolium-based ionic liquid modified mesopor-
ous silicate MCM-48, and [bmim][PF6] was used as the
additional adsorbed ionic liquid. The obtained catalyst
was employed as an efficient catalyst for the asymmetric
epoxidation of unfunctionalized olefins. It is expected
that high activity and enantioselectivity can be obtained
from these immobilized catalysts due to the solution
property of ionic liquid phase and the topological struc-
ture of MCM-48.
The asymmetric epoxidation of unfunctionalized olefins
is an important reaction to synthesize a variety of valu-
able chiral compounds, which are widely applied in the
synthesis of pharmaceuticals and agrochemicals. To
date, the chiral Mn(III) salen complexes have been dem-
onstrated to be very effective catalysts for the enantio-
selective epoxidation of unfunctionalized olefins. In
consideration of the advantages of the heterogeneous
catalysis system, for example, easy catalyst/product sep-
aration and simple catalyst recycling, many researchers
have been devoting to the studies of heterogenization
of chiral Mn(III) salen complexes.1–4 In most cases,
the heterogenized chiral Mn(III) salen complexes would
lead to lower enantioselectivity than their homogeneous
counterparts; only a few documented work have
reported higher ee value, however, lower activity is
always obtained in this situation.5–10
Recently, room temperature ionic liquids (RTIL) have
attracted noticeable attention as important solvent in
the synthesis and catalysis owing to their strong solvent
power for many organic and inorganic substances.11,12
It was reported by Song and Roh13 that the chiral
Mn(III) salen epoxidation catalyst immobilized in ionic
liquid [bmim][PF6] ([bmim]+ = 1-butyl-3-methylimid-
azolium cation) offered comparable enantioselectivity
2. Results and discussion
Keywords: Supported ionic liquid phase; Mn(III) salen; Asymmetric
epoxidation; Unfunctionalized olefin.
*
1-Methyl-3-(3-trimethoxysilylpropyl)-imidazolium chlo-
ride was prepared by the reaction of 1-methylimidazole
Corresponding author. Tel./fax: +86 22 23509005; e-mail: sxliu@
nankai.edu.cn
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.07.048

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L.-L. Lou et al. / Tetrahedron Letters 47 (2006) 6513–6516
with (3-chloropropyl)trimethoxysilane. The resulting
chloride was treated with potassium hexafluorophos-
phate in acetone for 3 days to give the corresponding
ionic liquid 1. Parent silica MCM-48 materials were
synthesized in autoclaves at 383 K for 72 h according to
a procedure outlined earlier.21,22 A gel (molar) composi-
tion of 1TEOS/0.46CTAB/0.41NaOH/52.95H2O was
used. The template was removed by calcination at
823 K in air for 5 h. Pretreated mesoporous silicate
MCM-48 (dried under vacuum and heated to 393 K over-
night, 3.00 g) was stirred with a chloroform solution
(50 mL) of ionic liquid 1 (2.16 g, 5 mmol) under refluxing
for 26 h to prepare ionic liquid-anchored MCM-48
material (1-MCM-48). FT-IR spectrum of 1-MCM-48
revealed specific IR adsorption characteristics of ionic
liquid 1, which was reflected in: m(C–H) at 3171 and
3122 cmÀ1, m(C@C) at 1578 and 1461 cmÀ1, d(C–H) at
1168 cmÀ1, qs(–CH2–CH2–CH2–) at 1467 cmÀ1, and
m(P–F) at 840 cmÀ1. Elemental analysis of 1-MCM-48
indicated that 0.68 mmol of ionic liquid fragments per
gram of 1-MCM-48 existed. Next, 1-MCM-48 (1.00 g)
was treated with ionic liquid [bmim][PF6] (300 mg) and
an acetone solution of chiral Mn(III) salen pre-catalyst
a or b (Fig. 1, the two (S,S)-Mn(III) salen chlorides were
synthesized according to the literature method,23,24
0.30 mmol). After the volatile components were removed
under reduced pressure, brown powder was obtained.
The prepared catalysts were designated as [bmim][PF6]/
1-MCM-48 a and [bmim][PF6]/1-MCM-48 b. The struc-
ture of the catalysts is showen in Figure 1, the ionic liquid
phase [bmim][PF6] containing the chiral Mn(III) salen
complexes was immobilized on the surface of the modi-
fied support material 1-MCM-48. The XRD patterns of
the two catalysts confirmed that the mesoporous struc-
ture of the support still retained good periodicity after
the immobilization. The amount of Mn(III) salen in the
prepared catalysts was 0.2 mmol gÀ1, based on Mn as
determined by the ICP-AES.
m-CPBA/NMO (m-chloroperoxybenzoic acid/N-meth-
ylmorpholine N-oxide) as oxidant in CH2Cl2 at 273 K
for 2 h. The following substrates were employed: sty-
rene, a-methylstyrene, 1-phenylcyclohexene, and indene.
Conversions and enantiomeric excess values were deter-
mined by gas chromatography with a chiral b-cyclo-
dextrin capillary column. For comparison, the chiral
Mn(III) salen pre-catalysts a and b were also investi-
gated under the reaction conditions mentioned above.
The results are listed in Table 1.
As shown in Table 1, the homogeneous catalyst a gave
high conversion in the epoxidation of a-methylstyrene;
however, the ee value was not very high (entry 5). Com-
pared with the homogeneous catalyst, the supported
ionic liquid catalyst [bmim][PF6]/1-MCM-48 a exhibited
notably higher enantioselectivity and comparable cata-
lytic activity in the a-methylstyrene epoxidation: the
enantiomeric excess was increased from 50% to >99%
(entries 5 and 6), and the conversion of a-methylstyrene
reached 99% (entry 6). Both the obtained conversion and
ee value were significantly higher than those reported in
the literatures related to inorganic supports.5,7,8,10
Besides the smaller olefin substrates, the heterogenized
Mn(III) salen catalysts also showed satisfactory catalytic
activity and enantioselectivity for relatively bulkier
olefins such as 1-phenylcyclohexene and indene. The
epoxidation of 1-phenylcyclohexene by [bmim][PF6]/1-
MCM-48 a provided a higher ee value of 92% compared
with its homogeneous ee of 86%, and comparable sub-
strate conversion was obtained. Similar results were also
achieved by [bmim][PF6]/1-MCM-48 b with an ee value
of 83% (entry 10). It is noteworthy that such remarkably
enhanced ee values were obtained with no obvious
decrease in olefins conversion. This may be due to the
combination of solution chemistry (ionic liquid) and
the spatial effect originated from the heterogeneous
carrier media MCM-48, which possesses well defined
pore size and 3-D topological structure. Recently, it
was reported25 that Mn(III) salen complexes immobi-
lized on MCM-48 via multi-step grafting presented a
The synthesized heterogeneous catalysts were evaluated
in the epoxidation of unfunctionalized olefins with
R
R
H
H
(S,S)-a: R-R=-(CH2)4-
(S,S)-b: R=Ph
N
N
Mn
Cl
t-Bu
O
O
t-Bu
t-Bu
t-Bu
[bmim][PF6]
1
Mn
CH3
CH3
N
N
PF6
Mn
PF6
R1
R3
O
R2
R4
R1
R3
R2
R4
N
N
*
*
Mn
(CH2)3
OMe
(CH2)3
OMe
Si
Si
Mn
O
O
O
O
MCM-48
Figure 1. The asymmetric epoxidation of olefins over the chiral Mn(III) salen complexes immobilized on ionic liquid modified MCM-48.

L.-L. Lou et al. / Tetrahedron Letters 47 (2006) 6513–6516
Table 1. Asymmetric catalytic epoxidation of unfunctionalized olefinsa
6515
Entry
Substrate
Catalyst
Conv. (%)b
ee (%)c
1
2
3
4
5
6
7
8
Styrene
Styrene
Styrene
Styrene
a-Methylstyrene
a-Methylstyrene
1-Phenylcyclohexene
1-Phenylcyclohexene
1-Phenylcyclohexene
1-Phenylcyclohexene
Indene
a
>99
50
89
52
>99
99
96
95
82
83
48(S)
41(S)
64(S)
48(S)
50(S)
>99(S)
86(1R,2R)
92(1R,2R)
78(1R,2R)
83(1R,2R)
88(1S,2R)
87(1S,2R)
[bmim][PF6]/1-MCM-48 a
b
[bmim][PF6]/1-MCM-48 b
a
[bmim][PF6]/1-MCM-48 a
a
[bmim][PF6]/1-MCM-48 a
b
9
10
11
12
[bmim][PF6]/1-MCM-48 b
b
97
97
Indene
[bmim][PF6]/1-MCM-48 b
a Reaction conditions: reaction temperature 273 K, reaction time 2 h, solvent: CH2Cl2, oxidant: m-CPBA/NMO, substrate/catalyst/m-CPBA/
NMO = 1:0.01:2:5.
b Conv.% determined by GC with chiral column using toluene as internal standard.
c Ee% determined by GC with RESTEK RT-BetaDEXse chiral column.
Table 2. The recycling studies of immobilized catalysts [bmim][PF6]/
1-MCM-48 a and [bmim][PF6]/1-MCM-48 b in the epoxidation of
1-phenylcyclohexenea
48 for the asymmetric epoxidation of unfunctionalized
olefins. These heterogeneous catalysts were stable, recy-
clable and exhibited comparable activity and remark-
Recycling Catalyst
number
Conv. (%) ee (%)
ably higher enantioselectivity (e.g., >99% and 92% ee
for a-methylstyrene and 1-phenylcyclohexene, respec-
tively) than those obtained by the homogeneous
counterparts.
1st
[bmim][PF6]/1-MCM-48 a 95 92(1R,2R)
[bmim][PF6]/1-MCM-48 b 83
[bmim][PF6]/1-MCM-48 a 93
[bmim][PF6]/1-MCM-48 b 80
[bmim][PF6]/1-MCM-48 a 93
[bmim][PF6]/1-MCM-48 b 82
83(1R,2R)
90(1R,2R)
81(1R,2R)
91(1R,2R)
83(1R,2R)
2nd
3rd
Acknowledgments
a Reactions were performed in CH2Cl2 at 273 K for 2 h with m-CPBA/
NMO as oxidant and in the presence of 1 mol % of the catalysts.
We gratefully acknowledge the support of this research
by the National Science Foundation of China (Nos.
29973016 and 20233030), and the Ministry of Educa-
tion, China.
high ee value of 99% for the epoxidation of a-methyl-
styrene due to the restrain of free rotation of the inter-
mediate by the topological structure of MCM-48, but
the conversion was much lower (only 34%).
References and notes
1. Bianchini, C.; Barbaro, P. Top. Catal. 2002, 19, 17–32.
2. Song, C.-E.; Lee, S.-G. Chem. Rev. 2002, 102, 3495–
3524.
3. McMorn, P.; Hutchings, G. J. Chem. Soc. Rev. 2004, 33,
108–122.
4. Li, C. Catal. Rev.—Sci. Eng. 2004, 46, 419–492.
5. Kim, G.-J.; Shin, J.-H. Tetrahedron Lett. 1999, 40, 6827–
6830.
6. Zhou, X.-G.; Yu, X.-Q.; Huang, J.-S.; Li, S.-G.; Li, L.-S.;
Che, C.-M. Chem. Commun. 1999, 1789–1790.
7. Kim, G.-J.; Kim, S.-H. Catal. Lett. 1999, 57, 139–143.
8. Xiang, S.; Zhang, Y.; Xin, Q.; Li, C. Chem. Commun.
2002, 2696–2697.
9. Kureshy, R. I.; Ahmad, I.; Khan, N. H.; Abdi, S. H. R.;
Singh, S.; Pandia, P. H.; Jasra, R. V. J. Catal. 2005, 235,
28–34.
10. Zhang, H.; Xiang, S.; Li, C. Chem. Commun. 2005, 1209–
1211.
11. Welton, T. Chem. Rev. 1999, 99, 2071–2083.
12. Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000,
39, 3772–3789.
The homogeneous catalyst b showed high activity and
enantioselectivity for the epoxidation of indene, a ee
value of 88% was collected (entry 11). Immobilizing
the Mn(III) salen complex b on the 1-MCM-48 with
[bmim][PF6] led to a similar result for indene epoxida-
tion, an ee value of 87% was achieved with no decrease
in indene conversion.
The used catalysts [bmim][PF6]/1-MCM-48 a and
[bmim][PF6]/1-MCM-48 b were reused for the epoxida-
tion of 1-phenylcyclohexene and the results are pre-
sented in Table 2. The results showed that the activity
and enantioselectivity had no obvious decrease for at
least 3 runs. At the end of each cycle, dichloromethane
was removed under reduced pressure and the residue
was stirred with hexane. Then the solid catalyst was fil-
tered, dried and reused. No leaching of ionic liquid and
Mn(III) salen complex was observed by NMR and ICP.
13. Song, C. E.; Roh, E. J. Chem. Commun. 2000, 837–838.
14. Mehnert, C. P.; Cook, R. A.; Dispenziere, N. C.;
Afeworki, M. J. Am. Chem. Soc. 2002, 124, 12932–12933.
15. Mehnert, C. P. Chem. Eur. J. 2005, 11, 50–56.
16. Mehnert, C. P.; Mozeleski, E. J.; Cook, R. A. Chem.
Commun. 2002, 3010–3011.
3. Conclusions
In summary, the Mn(III) salen complexes were immobi-
lized by imidazolium-based ionic liquid modified MCM-

6516
L.-L. Lou et al. / Tetrahedron Letters 47 (2006) 6513–6516
17. Riisager, A.; Wasserscheid, P.; Hal, R. van; Fehrmann, R.
J. Catal. 2003, 219, 452–455.
18. Gruttadauria, M.; Riela, S.; Meo, P. L.; D’Anna, F.;
Noto, R. Tetrahedron Lett. 2004, 45, 6113–6116.
22. Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.;
Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D.
H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.;
Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834–
10843.
19. Breitenlechner, S.; Fleck, M.; Muller, T. E.; Suppan, A. J.
¨
Mol. Catal. A: Chem. 2004, 214, 175–179.
20. Huang, J.; Jiang, T.; Gao, H.; Han, B.; Liu, Z.; Wu, W.;
Chang, Y.; Zhao, G. Angew. Chem., Int. Ed. 2004, 43,
1397–1399.
23. Zhang, W.; Loebach, J. L.; Wilson, S. R.; Jacobsen, E. N.
J. Am. Chem. Soc. 1990, 112, 2801–2803.
24. Irie, R.; Noda, K.; Ito, Y.; Matsumoto, N.; Katsuki, T.
Tetrahedron Lett. 1990, 31, 7345–7348.
21. Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J.
C.; Beck, J. S. Nature 1992, 359, 710–712.
25. Yu, K.; Lou, L.-L.; Ding, F.; Wang, S. J.; Wang, Z. L.;
Liu, S. X. Catal. Commun. 2006, 7, 170–172.