Reusable chiral salen Mn(III) complexes with phase transfer capability efficiently catalyze the asymmetric epoxidation of unfunctionalized olefins with NaClO

Article abstract of DOI:10.1016/j.apcata.2014.11.043

A series of chiral salen Mn(III) polymers with build-in phase transfer capability was prepared by bridging the chiral salen Mn(III) units with polyethylene glycol (PEG)-based di-imidazolium ionic liquid (IL) side by side. Technologies of characterization

Full text of DOI:10.1016/j.apcata.2014.11.043

Applied Catalysis A: General 491 (2015) 106–115
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Reusable chiral salen Mn(III) complexes with phase transfer capability
efficiently catalyze the asymmetric epoxidation of unfunctionalized
olefins with NaClO
a
a,∗
a
a
a
Yaju Chen , Rong Tan , Yaoyao Zhang , Guangwu Zhao , Weiguo Zheng ,
a
a,b,∗∗
Rongchang Luo , Donghong Yin
Institute of Fine Catalysis and Synthesis, Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education),
Hunan Normal University, Changsha, Hunan 410081, China
a
b
Technology Center, China Tobacco Hunan Industrial Corporation, NO. 426 Laodong Road, Changsha, Hunan 410014, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 August 2014
Received in revised form
A series of chiral salen Mn(III) polymers with build-in phase transfer capability was prepared by bridg-
ing the chiral salen Mn(III) units with polyethylene glycol (PEG)-based di-imidazolium ionic liquid (IL)
side by side. Technologies of characterization suggested the alternation of intact chiral salen Mn(III)
unit with PEG-based dicationic imidazolium IL moiety in the rigid one-dimension polymers. Amphi-
pathic nature of PEG-based di-imidazolium IL moiety allowed the obtained catalysts to undergo inherent
phase transfer catalysis in asymmetric epoxidation of unfunctionalized olefins with NaClO, which in
turn increased the reaction rate of epoxidation in water–dichloromethane biphasic system. Decreasing
total length of polyether chain leads to an increase in built-in phase transfer capability of correspond-
ing complex, which further enhances the catalytic performance. 91–97% of conversion was obtained in
the enantioselective epoxidation of styrene, ␣-methylstyrene, indene, 1,2-dihydronaphthalene, 6-cyano-
2,2-dimethylchromene, and 6-nitro-2,2-dimethylchromene catalyzed by the complex where number of
ethylene oxide unit is 3 within 60 min, which is significant higher than that observed for the neat complex
2
6 November 2014
Accepted 30 November 2014
Available online 9 December 2014
Keywords:
Chiral salen Mn(III) complex
Phase transfer catalysis
Enantioselective epoxidation
Unfunctionalized olefins
Aqueous/organic biphasic system
(56–74%) and the ICP (62–85%). High enantiomeric excess (ee) for the epoxides (in the range of 67–93%)
was also achieved, except for styrene (ee, 34%) and ␣-methylstyrene (ee, 41%). Furthermore, the efficient
phase transfer catalysts could be easily recovered by solvent precipitation and be recycled for seven times
without significant loss of the activity and enantioselectivity.
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
is an attractive oxygen source for the transformation, since it is rea-
sonably stable, readily available and environment friendly. Some
pioneering contributions to the employment are known in the
field of asymmetric epoxidation [15–19]. For example, Kureshy
et al. have reported the catalytic asymmetric epoxidation of non-
functionalized alkenes in dichloromethane catalyzed by dimeric
or polymeric chiral salen Mn(III) complexes using aq. NaClO as an
oxidant. The biphasic systems provided high activity (>99% of con-
version) and enantioselectivity (in the range of 33–93%), but often
require long reaction time (6–24 h) due to the limited mass transfer
Optically active epoxides are key intermediates in organic
chemistry because they can undergo stereospecific ring-opening
reactions or functional group transformations, giving rise to a
wide variety of biologically or pharmaceutically important com-
pounds [1–3]. Enantioselective epoxidation catalyzed by chiral
salen Mn(III) complex is one of the most efficient strategies for
obtaining enantiopure epoxides [4–14]. Aqueous NaClO (aq. NaClO)
[
15,18]. The reaction rate can be speeded up by using phase transfer
catalysts (PTC) that improves the compatibility of aqueous/organic
biphasic system [5]. Amphiphilic groups, such as tertiary amino
alkyl groups [20] or phosphonium groups [21] have thus been
∗
Corresponding author. Tel.: +86 731 8872576; fax: +86 731 8872531.
Corresponding author at: Institute of Fine Catalysis and Synthesis, Key Labora-
∗
∗
tory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of
Education), Hunan Normal University, Changsha, Hunan, 410081, China.
Fax: +86 731 8872531.
ꢀ
introduced into the 5- and/or 5 -positions of chiral salen ligand to
achieve the chiral salen Mn(III) complex with built-in phase trans-
fer capability. Excellent catalytic performance regarding yield (99%
of conversion) and enantioselectivity (in the range of 36–99% ee
E-mail addresses: yiyangtanrong@126.com (R. Tan), yindh@hunnu.edu.cn
(
D. Yin).
http://dx.doi.org/10.1016/j.apcata.2014.11.043
926-860X/© 2014 Elsevier B.V. All rights reserved.
0

Y. Chen et al. / Applied Catalysis A: General 491 (2015) 106–115
107
ꢀ
ꢀ
value) was indeed obtained over the PTC in asymmetric epoxidation
of unfunctionalized olefins with NaClO. Unfortunately, recovery
and reuse of PTC from reaction mixture are often troublesome.
PEG with low molecular weight has emerged as the alterna-
tive modifier to develop the PTC of chiral salen Mn(III) complex
for water/oil biphasic systems, not only due to the phase transfer
capability, but also because the selective solubility profile of PEG
may facilitate the recovery of catalyst through solvent precipitation
Neat complex of [(R,R )-(N,N -bis(3,5-di-tert-butylsalicylidene)-
1,2-cyclohexanediaminato] manganese(III) chloride was prepared
according to the described procedures [30]. The chiral salen
ꢀ
ꢀ
Mn(III) complex of [(R,R )-(N,N -bis(3-tert-butyl-5-chloromethyl
salicylidene)-1,2-cyclohexane diaminato]manganese(III) chloride
was synthesized by a previously described procedure [31].
2.2. Methods
[
22–24]. Recently, we have reported a series of chiral salen Mn(III)
complexes functionalized by PEG-modified imidazolium IL (PEG-
IL) moiety for the enantioselective epoxidation of unfunctionalized
olefins with NaClO [25]. Remarkable enhancement of reaction rate
was observed over the catalysts in the biphasic system due to the
built-in phase transfer capacity originated from the amphiphilic
PEG chain, as well as the positive effect of the imidazolium based-
IL moieties. However, leaching loss of catalyst was also inevitable
due to the amphiphilic nature of complex, although PEG-IL mod-
ifier potentially enabled the separation of catalysts by changing
solvent. Interestingly, reduced leaching was observed over the cata-
lyst which contains more number of ethylene glycol units. The facts
encouraged us to anticipate that properly increasing the molec-
ular weight of catalyst would lower its solubility, aiding catalyst
recovery. Previous report shows that increasing the number of
active sites for catalyst results in higher reactivity and turnover
FT-IR spectra were obtained as potassium bromide pellets with
a resolution of 4 cm and 32 scans in the range 400–4000 cm
−1
−1
using an AVATAR 370 Thermo Nicolet spectrophotometer. The
sample concentration used for pellet preparation is ca. 1 wt%.
The ultraviolet–visible light (UV–vis) spectra were recorded on a
UV–vis Agilent 8453 spectrophotometer. The solution of samples
in dichloromethane (ca. 1.0 mM) was poured into a 1 cm quartz
cell for UV–vis spectroscopy with dichloromethane as the ref-
erence. The termogravimetric and differential thermogravimetric
(
TG-DTG) curves were obtained on a NETZSCH STA 449C ther-
mal analyzer. Samples were heated from room temperature up to
◦
7
00 C under flowing air using alumina sample holders. The sam-
1
ple weight was ca. 10 mg and the heating rate was 10 K/min.
H
NMR spectra of samples were recorded on a Varian-500 spectrom-
eter with TMS as an internal standard. Thin layer chromatography
TLC) was conducted on glass plates coated with silica gel GF254.
Mn ion contents were measured by the method of complexometry
with ethylenediamine tetraacetic acid (EDTA) [32]. The molecular
weight of catalysts was obtained by gel permeation chromatograph
[
26,27]. Therefore, it is desirable to use the PEG-IL and chiral salen
(
Mn(III) complex as building blocks to fabricate polymeric analog for
the biphasic system. We envisaged that the polymeric chiral salen
Mn(III) catalyst containing PEG-IL moiety would not only behave
as a PTC in the aqueous/organic biphasic system, but also could be
facilely recovered from reaction system for efficient reuse.
(
GPC). Analyses were performed on an Alltech Instrument (Alltech,
America) equipped with an Alltech ELSD 800 detector. Mass spec-
trometry analyses were performed using an API 3000 tandem mass
spectrometer (Applied Biosystems, USA) with electrospray inter-
face (ESI-MS/MS in methanol). The optical rotation of catalysts was
measured in dichloromethane on a WZZ-2A Automatic Polarime-
ter. The conversions and the ee values were measured by a 6890 N
gas chromatograph (Agilent Co.) equipped with the chiral capillary
column (HP19091G-B213, 30 m × 0.32 mm × 0.25 ␮m) and the FID
detector.
Herein,
PEG-bridged
di-imidazole
was
synthesized
and N-alkylated with the chiral salen Mn(III) complex of
ꢀ
ꢀ
[
(R,R )-(N,N -bis(3-tert-butyl-5-chloromethyl
salicylidene)-1,2-
cyclohexanediaminato] manganese(III) chloride. The successful
N-alkylation gave the PEG-IL functionalized chiral salen Mn(III)
polymers (PICP), in which chiral salen Mn(III) units were bridged
by PEG-based di-imidazolium IL side by side. It has proved that the
catalysts could achieve the process of phase transfer catalysis in
asymmetric epoxidation of unfunctionalized olefins using NaClO
as an oxidant. Remarkable enhancement of reaction rate with high
turnover frequency (TOF) values was observed over a wide range
of alkenes. More importantly, the catalysts could be quantitatively
recovered from the reaction mixture by solvent precipitation for
efficient reuse. Thus, the problems associated with phase transfer
limitation in the water–oil biphasic reaction and the separation
of catalyst can be well resolved. In addition, the total length of
polyether chain significantly affected the phase transfer capability,
and further affected the catalytic performance of PICP in the
water–dichloromethane biphasic system.
2
.3. Preparation of PICP-n (n = 3, 8, 12) (where n is the average
numbers of ethylene oxide unit in various PEG chains)
The preparation of PICP-n (n = 3, 8, 12) was outlined in Scheme 1.
2.3.1. Synthesis of PEG-bridged di-imidazole
Sodium ethylate (0.68 g, 10 mmol) in anhydrous ethanol (10 ml)
was dropwise added into the anhydrous ethanol solution (20 ml)
of imidazole (0.68 g, 10 mmol). The resulting mixture was stirred
◦
at 60 C for 8 h. Chloride-terminated PEG-n (5 mmol, n = 3, 8, 12,
which is the corresponding average numbers of ethylene oxide
unit in the PEG chains of PEG-200, PEG-400 and PEG-600) was
then slowly added into the stirring mixture. The mixture was
2
. Experimental
◦
2.1. Materials and reagents
stirred at 70 C for another 12 h, resulting in the formation of some
white precipitate. After filtration, the filtrate was collected and
concentrated in vacuum. The gummy residue was dissolved in
dichloromethane (100 ml) and filtrated again to completely remove
the insoluble salt of sodium chloride. The filtrate was washed with
saturated sodium chloride and deionized water for several times.
The organic phase was dried over anhydrous Na SO and fur-
L-(+)-Tartaric acid (>99% ee) and (± )-1,2-diaminocyclohexane
(
(
purity, 99%) were purchased from Alfa Aesar. Pyridine N-oxide
purity, 98%) was bought from Aldrich. Indene (purity, >99%)
and 1,2-dihydronaphthalene (purity, >99%) were obtained by TCI.
Other commercially available chemicals were laboratory grade
reagents from local suppliers. All solvents were purified by
standard procedures. Styrene and indene were passed through
a pad of neutral alumina before use. Chloride-terminated PEG
was prepared according to the reported procedures [28]. 6-
Cyano-2,2-dimethylchromene and 6-nitro-2,2-dimethylchromene
were synthesized according to the literature procedures [29].
2
4
◦
ther dried at 40 C in vacuum to obtain the sticky brownish red
products of PEG-bridged di-imidazole. PEG-bridged di-imidazole
(n = 3): 1H NMR (CDCl , 500 MHz) ıH, ppm: 7.47 (s, 2H, ring
3
NCHN), 7.05 (s, 2H, ring C NCH), 6.96 (s, 2H, ring N
4.13 (t, J = 5.2 Hz, 4H, O CH2 CH2 Nring), 3.72–3.76 (t, J = 5.2 Hz,
4H, O CH2 CH2 Nring), 3.54–3.72 (m, 8H, CH2 C2H4 CH ).
C CH),
O
O
2

1
08
Y. Chen et al. / Applied Catalysis A: General 491 (2015) 106–115
Cl
Cl
O
NaOC H /C H OH
n
2
o
5
2 5
N
N
N
NH
N
N
60 C, 8 h
o
O
C H OH, 70 C, 12 h
2
5
n
PEG-bridged di-imidazole
N
O
N
Mn
Cl
O
N
ClH C
O
CH Cl
N
O
N
O
2
2
N
n
N
Mn
Cl
t-Bu
t-Bu
*
N
Cl
Cl
CH2
*
m
dry toluene, reflux, 48 h
t-Bu
t-Bu
PICP-n (n = 3, m = 8; n = 8, m = 6; n =12, m = 4)
ICP (n = 0, m = 10)
Scheme 1. Synthesis of PICP-n (n = 3, 8, 12) and ICP.
−1
FT-IR (KBr): ꢀmax/cm 2879, 1657, 1511, 1443, 1361, 1242, 1100,
giving the imidazolium based IL functionalized chiral salen Mn(III)
9
45, 880, 831, 710, 667, 623.
polymer (denoted as ICP). The structure of ICP is shown in Chart 1.
−
1
FT-IR (KBr): ꢀmax/cm 3125, 2952, 2864, 1618, 1543, 1439, 1388,
342, 1311, 1266, 1232, 1206, 1162, 1095, 932, 868, 827, 777, 752,
710, 660, 622, 571, 476, 416. UV–vis (CH Cl ): 502 (1240), 406
1
2.3.2. Synthesis of PICP-n (n = 3, 8, 12)
ꢀ
ꢀ
[
(R,R )-(N,N -bis(3-tert-butyl-5-chloro-methyl
,2-cyclohexanediaminato] manganese(III) chloride
.1 mmol) was mixed with PEG-bridged di-imidazole (5 mmol)
salicylidene)-
2
2
(
9161), 317 (27,513) nm. Mn ion content: 0.983 mmol/g (theoreti-
1
5
(3.2 g,
28
cal value: 1.34 mmol/g); ␣D = +607 (C = 0.02, CH Cl ). The average
2
2
molecular weight of ICP is around 7700 g/mol (MW = 7700, m = 10)
based on GPC analysis.
in dry toluene (20 ml). The mixture was refluxed for 48 h under
nitrogen atmosphere. After removal of the solvent in vacuum, the
mixture was washed with ether for several times. The gummy
◦
residue was dried in vacuum at 40 C to provide PICP-n as the
brown and viscous liquid (where n is the average numbers of
ethylene oxide unit in various PEG chains, n = 3, 8, 12). The PICP
containing the average numbers of ethylene oxide unit in PEG
moiety of 3, 8 and 12 were denoted as PICP-3, PICP-8 and PICP-12,
2
.5. Catalyst testing
PICP-n (4 mol%, based on Mn ion content in PICP-n), unfunction-
alized alkenes (0.5 mmol) and pyridine N-oxide (0.095 g, 1 mmol)
were added into dichloromethane (2 ml) under stirring. Buffered
NaClO (1 mmol, pH = 11.5) as an oxidant was then added in four
−
1
respectively. PICP-3: FT-IR (KBr): ꢀmax/cm
3125, 2948, 2865,
1
1
4
618, 1543, 1439, 1386, 1339, 1309, 1266, 1235, 1205, 1164,
100, 1066, 1039, 932, 866, 827, 783, 750, 661, 622, 571, 483,
◦
equal portions at 0 C. The progress of epoxidation was moni-
16. UV–vis (CH Cl ): 502 (2080), 406 (13,714), 317 (28,604) nm.
2
2
tored by GC. After the reaction, volatile solvents were evaporated
under in a vacuum. PICP-n was precipitated out from reac-
tion system by the addition of n-hexane (5 ml), washed with
ether (3 × 5 ml), dried in vacuum, and finally recharged with
fresh solvents and reaction substrates for the next catalytic cycle.
Supernatant was decanted and separated by separatory funnel.
Mn ion content: 0.814 mmol/g (theoretical value: 1.05 mmol/g);
28
␣
D
= +619 (C = 0.02, CH Cl ). The average molecular weight of
2 2
the PICP-3 is around 7600 g/mol (MW = 7600, m = 8) based on GPC
−1
analysis; PICP-8: FT-IR (KBr): ꢀmax/cm 3125, 2946, 2865, 1618,
1
8
4
543, 1441, 1390, 1348, 1308, 1235, 1206, 1100, 1030, 933, 867,
28, 783, 774, 661, 622, 571, 486, 416. UV–vis (CH Cl ): 502 (1921),
2
2
Aqueous phase was extracted with CH Cl2 for several times. The
2
06 (12,489), 317 (29,397) nm. Mn ion content: 0.695 mmol/g
extract was combined with organic phase. The collected organic
phase was dried over anhydrous sodium sulfate and concen-
trated in vacuum. Further purification of the residue by flash
column chromatography afforded pure epoxides. The conversions
and ee values were measured by a 6890 N gas chromatograph
(
theoretical value: 0.866 mmol/g); ␣D28 = +560 (C = 0.02, CH Cl ).
2 2
The average molecular weight of the PICP-8 is around 6700 g/mol
(
ꢀ
1
MW = 6700, m = 6) based on GPC analysis; PICP-12: FT-IR (KBr):
max/cm 3123, 2950, 2867, 1618, 1543, 1440, 1388, 1350, 1308,
264, 1237, 1205, 1100, 934, 829, 779, 662, 622, 571, 487, 416.
−
1
(Agilent Co.) equipped with a chiral capillary column (HP19091G-
UV–vis (CH Cl ): 502 (2317), 406 (14,157), 317 (26,112) nm. Mn
2
2
B213, 30 m × 0.32 mm × 0.25 ␮m) and a FID detector. Nitrogen
ion content: 0.575 mmol/g (theoretical value: 0.741 mmol/g);
−1
was used as the carrier gas with a flow of 30 ml min . The
28
␣
D
= +632 (C = 0.02, CH Cl ). The average molecular weight of
◦
2
2
injector temperature is 250 C, and the detector temperature
is also 250 C. The retention times of the corresponding chiral
epoxides (tabsolute configuration) are as follows: (a) styrene epox-
ide: the column temperature is 90 C, tR = 15.2 min, t = 15.7 min;
the PICP-12 is around 6100 g/mol (MW = 6100, m = 4) based on GPC
analysis.
◦
◦
S
◦
2.4. Preparation of the PEG-free counterpart
(b) ␣-methylstyrene epoxide: the column temperature is 80 C,
t = 16.3 min, tR = 16.5 min; (c) indene epoxide: the column tem-
S
◦
◦
−1
For comparison, the polymeric analog of PEG-free counterpart,
perature was programmed from 80 to 180 C with 8 C min
,
in which chiral salen Mn(III) unit alternates with ethyl-based dica-
tionic IL, was also prepared according to a similar preparation
procedure to that of PICP-n, as shown in Scheme 1. During the
procedure, 1,2-dichloroethane was used instead of the chloride-
terminated PEG to bridge imidazole moiety through nucleophilic
substitution. The obtained ethyl-bridged di-imidazole was readily
tSR = 11.5 min, tRS = 11.9 min; (d) 1,2-dihydronaphthalene epox-
ide: the column temperature was programmed from 80 to
◦
◦
−1
180 C with 6 C min , tSR = 13.4 min, tRS = 13.6 min; (e) 6-cyano-
ꢀ
2,2 -dimethylchromene epoxide: the column temperature was
programmed from 80 to 200 C with 4 C min , tSS = 24.0 min,
tRR = 24.3 min; (f) 6-nitro-2,2 -dimethylchromene epoxide: the col-
umn temperature was programmed from 80 to 200 C with
4 C min
tRR = 30.8 min.
◦
◦
−1
ꢀ
◦
N-alkylated with the chloromethyl groups ( CH Cl) at two sides
2
ꢀ
ꢀ
ꢀ
◦
−1
◦
of 5,5 -positions in (R,R )-(N,N -bis(3-tert-butyl-5-chloromethyl
and retained at 200 C for 5 min, tSS = 30.3 min,
salicylidene)-1,2-cyclohexane diaminato] manganese(III) chloride,

Y. Chen et al. / Applied Catalysis A: General 491 (2015) 106–115
109
N
O
N
O
N
O
N
O
Mn
Cl
Mn
Cl
*
N
H
C
2
N
N
CH2
*
10
t-Bu
t-Bu
N
Cl
Cl
t-Bu
t-Bu
t-Bu
neat complex
t-Bu
ICP
Chart 1. Structures of ICP and neat complex.
3
. Results and discussion
.1. Preparation of PICP-n (n = 3, 8, 12)
PEG is a fascinating phase transfer reagent with tunable solu-
(Fig. 1B, 0.5 g sample in 5 ml dichloromethane). As expected, all
the PICP-n (n = 3, 8, 12) are soluble not only in dichloromethane
(Fig. 1B, a–c), but also in water (Fig. 1A, a–c), at room tempera-
ture. The amphiphilic nature enables PICP-n to behave as efficient
PTC in the water-dichloromethane biphasic reaction. Interestingly,
although polymeric analog, the PEG-free counterpart of ICP is prac-
tically insoluble in water (Fig. 1A-d). We can thus speculate that the
water-solubility of PICP-n is directly related with the PEG moiety.
Actually, the water-solubility of catalyst decreases as the length of
polyether chain increases (Fig. 1A-a vs A-b vs A-c). Whereas, the
length of polyether chain does not have significant effect on the
solubility of catalyst in dichloromethane (Fig. 1B-a vs B-b vs B-c).
Notably, the PICP-n (n = 3, 8, 12) are almost insoluble in n-hexane
or ether. It is suggested that the PTC of PICP-n (n = 3, 8, 12) could be
recovered from the biphasic reaction by simple phase separation
techniques via change of solvents.
3
bility [22]. Catalyst containing the PEG moiety often behaves as a
PTC for biphasic reaction and also can be facile recovered by sol-
vent precipitation [33,34]. The inherent advantages encouraged us
to introduce the PEG moiety into the framework of chiral salen
Mn(III) complex to fabricate a reusable PTC of chiral salen Mn(III)
complex for asymmetric epoxidation of unfunctionalized olefins in
dichloromethane using NaClO as an oxidant. ILs derived from N,N-
dialkylimidazolium are found to stabilize the chiral salen Mn(III)
complex during the process of epoxidation and/or accelerate the
reaction [9,25]. Therefore, PEG-modified imidazolium IL was chose
to modify the chiral salen Mn(III) complex for the desirable fabri-
cation. Given that increasing the number of active sites on catalyst
may result in higher reactivity, we attempted to use PEG-modified
imidazolium IL and chiral salen Mn(III) unit as the building blocks
to prepare a ABAB-type polymeric matrix resemble PEG.
3.2. Characterization of samples
3.2.1. FT-IR
The synthesis route for the PICP-n (n = 3, 8, 12) was outlined in
Scheme 1. At first, PEG-bridged di-imidazole with various length of
PEG chain was obtained by nucleophilic substitution of chloride-
terminated PEG with imidazolinium sodium salt under alkaline
The polymerization behavior of chiral salen Mn(III) complex
unit with PEG-based dicationic IL in PICP-n (n = 3, 8, 12) was fur-
ther confirmed by FT-IR analysis. Fig. 2 shows the FT-IR spectra
of PICP-n (n = 3, 8, 12), as well as ICP and neat complex for com-
condition. Successive N-alkylation of the PEG-bridged di-imidazole
parison. Obviously, the FT-IR spectrum of neat complex shows the
ꢀ
−1
with the chloromethyl groups ( CH Cl) at 5- and 5 -positions
C
H vibrations (in the range of 2995–2750 cm ) associated with
2
ꢀ
ꢀ
in (R,R )-(N,N -bis(3-tert-butyl-5-chloromethyl salicylidene)-1,2-
cyclohexane diaminato) manganese(III) chloride resulted in the
polymerization of chiral salen Mn(III) complex with PEG-bridged
di-imidazole through forming imidazolinium-based IL, as shown in
Scheme 1. PEG-based dicationic IL thus alternates with the chiral
complex unit in the polymeric matrix, affording the PICP-n (n = 3, 8,
t-butyl groups and the C C skeletal stretching vibrations (in the
−
1
range of 1500–1250 cm ) associated with benzene rings in salen
ligand. Furthermore, four characteristic vibration bands at around
−
1
1612, 1535, 569, and 413 cm are also observed, which are asso-
ciated with the stretching vibration modes of C N, C O, Mn O,
and Mn N, respectively (Fig. 2a) [9,25,35]. While the characteristic
−
1
1
2) as brown and viscous liquid. For comparison, a PEG-free coun-
bands appear as red shift from 1612, 1535, 569, and 413 cm to
−
1
terpart of ICP, in which chiral salen Mn(III) unit alternates with
ethyl-based dicationic IL, was also prepared according to a prepa-
ration procedure similar to that of PICP-n, except for the use of
1618, 1543, 571 and 416 cm , respectively, in the FT-IR spectra of
PICP-n (n = 3, 8, 12), compared with those of neat complex (Fig. 2b-
d vs 2a). It is mainly due to the electron-deficient substitutes of the
ꢀ
1
,2-dichloroethane instead of the chloride-terminated PEG during
imidazolium cations at the 5- and 5 -positions in salen ligands [36].
the preparation, as shown in Scheme 1.
Fig. 1 shows the solubility of PICP-n (n = 3, 8, 12) and ICP in
water (Fig. 1A, 0.5 g sample in 5 ml water) and in dichloromethane
The presence of imidazolium IL moiety in the polymeric matrixes is
−
1
also evident from the a newly formed band at 622 cm associated
with stretching vibration mode of C C in imidazolium cation
N
Fig. 1. Photographs of PICP-3 (a), PICP-8 (b), PICP-12 (c) and ICP (d) in water (A) (0.5 g sample in 5 ml water) and in dichloromethane (B) (0.5 g sample in 5 ml dichloromethane).

1
10
Y. Chen et al. / Applied Catalysis A: General 491 (2015) 106–115
a
exhibits the characteristic peaks at around 324, 427 and 515 nm,
which is due to charge transfer transition of salen ligand, metal-
to-ligand charge-transfer transition and d-d transition of complex,
respectively [31,38]. As for the PICP-n (n = 3, 8, 12), the correspond-
ing characteristic peaks shift to 317, 406 and 502 nm, respectively
413
1612
569
100
1
b
1535
b'
(
Fig. 3a vs b-d). The blue shift of characteristic peaks should also be
c
due to the electronic effect arisen from the electron-deficient imid-
azolium cations at the 5- and 5 -positions of salen ligand [25]. The
ꢀ
d
e
PEG-free counterpart of ICP shows similar tendency in the UV–vis
spectrum (Fig. 3e), as compared with PICP-n, suggesting the sim-
ilar active sites between ICP and PICP-n. Therefore, PEG moiety
interspersed in PICP-n does not change the electronic properties
of active site for chiral salen Mn(III) complex.
6
22
416
3.2.3. Thermal analysis
571
1
618
1
543
Thermal analysis has been used to monitor the decomposition
profiles of typical PICP-8, as well as neat complex and ICP for com-
parison. The results obtained are depicted in Fig. 4. Neat complex
shows three distinct steps of weight loss in the combined TG–DTG
curves, when heated from room temperature to 800 C under air
flow (Fig. 4A). The first loss in weight is centered at 110 C, which is
4000
3500
3000
2500
2000
1500
1000
500
-
1
Wavenumber (cm )
◦
Fig. 2. FT-IR spectra of neat complex (a), PICP-3 (b), PICP-3 after seven runs (b’),
PICP-8 (c), PICP-12 (d), and ICP (e).
◦
due to a release of chloride anions as hydrogen chloride. A second
◦
large weight loss appears at 216 C, which is assigned to the cleav-
(
Fig. 2b-d) [9]. The results indicate the successfully bridging chiral
age of the tert-butyl groups [39]. The successive decomposition of
residual salen ligand moieties occurs at 349 C and extends up to
597 C with the residue (ca. 12.3 wt%) amounting to manganese
oxides. Fig. 4B displays the decomposition behavior of PICP-8, in
which seven distinct steps of weight losses are observed. Interest-
ingly, the release of chlorine anions in PICP-8 exhibits two weight
loss steps in combined TG-DTG curves, one at 122 C and the other
at 141 C. As mentioned, the former weight loss is due to the release
salen Mn(III) unit with PEG moiety through the formation of imid-
azolium IL, as shown in Scheme 1. An additional distinct FT-IR band
◦
◦
−1
at around 1100 cm attributable to C
indicates the presence of PEG moiety in the polymers (Fig. 2b-d)
37]. It is the amphipathic PEG chain that responsible for the built-
O C stretching vibration
[
in phase transfer capability of PICP-n. The FT-IR spectrum of ICP is
similar to that of PICP-n (Fig. 2e vs 2b-d), except for the absence
of the characteristic C
related to the absence of the PEG moiety in the framework of ICP,
as shown in Chart 1.
◦
◦
O C stretching vibration. The difference is
of chloride anions balanced with Mn(III) cation [39], while the sec-
ond should be attributed to the loss of chloride counteranion in
imidazolium IL moiety [40]. The third weight loss at 210 C accounts
◦
for the cleavage of the tert-butyl groups, which is consistent with
that of neat complex (Fig. 4B vs A). The fourth weight loss at 346 C,
3
.2.2. UV–vis
◦
UV–vis analysis provided more direct evidence for the poly-
which was absent in the combined TG-DTG curves of ICP (Fig. 4B
vs D), is logical to assign to the decomposition of the polyether
chain parts in PICP-8, since PEG-bridged di-imidazole shows sim-
ilar major weight loss in the temperature range (Fig. 4B vs C). The
merization through forming IL. Fig. 3 shows the UV–vis spectra of
PICP-n (n = 3, 8, 12), ICP and neat complex. Obviously, neat complex
◦
fifth weight loss is at 397 C, followed by an additional weight loss at
324
◦
◦
4
50 C that extended up to ca. 501 C. The two steps that were well
distinguished in the DTG curve can be assigned to the successive
cleavage of salen ligand moieties in PICP-8. Obviously, the cor-
responding decomposition temperature increases compared with
that of neat complex (Fig. 4B vs A), because of the mutual stabiliza-
tion of salen ligand part and the PEG-based IL moieties. It is indirect
proof for the successful bridging chiral salen ligand with PEG-based
IL moieties. Complete decomposition of the ILs moiety in PICP-8
4
27
5
15
a
◦
occurs at 625 C, leaving non-removable residue belonged to the
formed manganese oxide.
502
b
b'
c
3.3. Catalytic performances
Asymmetric epoxidation of alkenes catalyzed by chiral salen
Mn(III) complex using aq. NaClO as an oxidant have been exten-
sively investigated in recent years because aq. NaClO is inexpensive,
safe, and environmentally benign. Although high yield and enan-
tioselectivity, the catalytic efficiency (in terms of TOF), are often
limited by poor mass transfer in the water-dichloromethane
biphasic system [15–19]. Complexes of PICP-n were designed as
PTC for the asymmetric epoxidation of alkenes using aq. NaClO as
an oxidant. We proposed that PICP-n with built-in phase trans-
fer capacity could circumvent the mass transfer problem and
enhance catalytic efficiency in the biphasic system. The results
3
17
c'
d
d'
e
4
06
3
00
350
400
450
500
550
600
Wavelength (nm)
Fig. 3. UV–vis spectra of neat complex (a), fresh PICP-3 (b), PICP-3 after seven runs
b’), fresh PICP-8 (c), PICP-8 after seven runs (c’), fresh PICP-12 (d), PICP-12 after
seven runs (d’) and ICP (e).
(
◦
of the enantioselective epoxidation of different substrates at 0 C,

Y. Chen et al. / Applied Catalysis A: General 491 (2015) 106–115
111
1
00
A
100
80
60
40
20
0
B
b
8
6
4
2
0
0
0
0
0
b
a
a
0
100 200 300 400 500 600 700 800
0
100 200 300 400 500 600 700 800
o
o
Temperature( C)
Temperature( C)
C
100
80
60
40
20
0
D
1
00
b
8
6
4
2
0
0
0
0
0
b
a
a
0
100 200 300 400 500 600 700 800
0
100 200 300 400 500 600 700 800
o
o
Temperature( C)
Temperature( C)
Fig. 4. TG-DTG curves of neat complex (A), PICP-8 (B), PEG-bridged di-imidazole (C), and ICP (D) ((a) thermogravimetric curves; (b) differential thermogravimetric curves).
using PICP-n (n = 3, 8, 12) as catalysts and aq. NaClO as an oxi-
dant in dichloromethane, are summarized in Table 1. To investigate
the built-in phase transfer capability originated from PEG moiety,
PEG-free counterpart (denoted as ICP) was prepared as a control
[42]. The catalytic performance of PICP-n (n = 3, 8, 12) was also
compared with that of other reported catalytic systems, such as
chiral macrocyclic salen Mn(III) complex [15] and mesoporous sil-
ica supported chiral complexes [12] under similar conditions. As
expected, PICP-n gave significantly higher catalytic activity (in
terms of TOF value) than the reported catalysts either homoge-
neous (Table 1, entry entries 3–5 vs entry 6) or heterogeneous
(Table 1, entries 3–5 vs 7–9). It is easy to understand that PICP-
n with built-in phase transfer capacity circumvent mass transfer
restriction associated with aqueous/organic biphasic system and
give higher catalytic efficiency.
Furthermore, we noticed that various length of polyether chain
in PICP affected the catalytic activity of corresponding complex
in water–dichloromethane biphasic epoxidation. The conversion
of styrene decreased with increasing the total length of polyether
chain. PICP-3 showed the highest catalytic activity, giving styrene
epoxide in almost quantitative conversion (96%) with 34% ee value
within 60 min (Table 1, entry 3). As the numbers of ethylene oxide
unit in PEG chain increased from 3 to 8, and further increased to 12,
the conversion decreased accordingly (Table 1, entries 3–5). In the
case of PICP-12, only 82% conversion of styrene was obtained in
the epoxidation (Table 1, entry 5). A possible explanation might
be that the length of polyether chain directly affects the inher-
ent phase transfer capability of corresponding complex, since the
water-solubility of catalysts decrease as the total length of PEG
chain increases, as shown in Fig. 1A (A-a vs A-b vs A-c). How-
ever, there is no change in enantioselectivity with lengthening PEG
chain.
catalyst by N-alkylation of ethyl-bridged di-imidazole with the
ꢀ
chloromethyl groups ( CH Cl) at 5- and 5 -positions of chiral salen
2
Mn(III) complex. Furthermore, the traditional chiral salen Mn(III)
complex (as shown in Chart 1) was also employed for comparison.
As expected, neat chiral salen Mn(III) complex with limited
water-solubility was less efficient in the water-dichloromethane
biphasic system. Only 65% conversion of styrene was obtained
within 60 min (Table 1, entry 1). Although still poor water-
solubility, ICP offered higher conversion (74% of styrene) than
neat complex (Table 1, entry 2 vs 1) due to the positive effect of
imidazolium IL moieties on activating chiral salen Mn(III) complex
[
9,25] and/or some synergetic interaction between neighboring
sites [26,27,41]. However, it was far less active than the catalysts of
PICP-n (n = 3, 8, 12) (Table 1, entry 2 vs entries 3–5). Using aq. NaClO
as an oxidant, the PICP-n gave excellent conversions (in the range of
8
2–96%) with 34% enantioselectivity (ee values) in the asymmetric
◦
epoxidation of styrene at 0 C within 60 min (Table 1, entries 3–5).
Enhanced catalytic efficiency over PICP-n should arise from the
PEG chains interspersed in their framework. Amphipathic nature
of PEG chain endows the PICP-n with desirable built-in phase
transfer capacity in the water–dichloromethane biphasic reaction.
Furthermore, the ether groups in PEG chain was reported to trans-
port the real oxidant HClO from aqueous to organic phase through
the formation of hydrogen bonds [25,41]. Mass transfer restriction
associated with aqueous/organic biphasic system was thus well
resolved. Slightly lower ee value (34%) upon PICP-n (n = 3, 8, 12) and
Remarkably enhanced catalytic efficiency over the PICP-3
was also observed in the case of the ␣-methylstyrene, indene,
1,2-dihydronaphthalene, 6-cyano-2,2-dimethylchromene and 6-
nitro-2,2-dimethylchromene, as shown by TOF in Table 1 (entries
12, 15, 18, 21 and 24). Notably high TOF values were obtained over
PICP-3 in the epoxidation of the substrates. The TOF values are
ꢀ
ICP should be due to the variation of electronic property of the 5,5 -
substituents in salen ligand (Table 1, entries 2–5). In most cases,
complex bearing electron-deficient substituents, such as imidaz-
ꢀ
olium cations, at the 5,5 -positions affords slight lower ee value

1
12
Y. Chen et al. / Applied Catalysis A: General 491 (2015) 106–115
Table 1
a
Results of the enantioselective epoxidation of unfunctionalized olefins over different chiral salen Mn(III) complexes .
b
b
c
TOFd × 10−3/s–1
Entry
Catalyst
Substrate
Product
Conv. /%
ee /%
Yield /%
1
2
3
4
Neat complex
ICP
PICP-3
65
74
96
86
39
34
34
34
60
68
90
80
4.51
5.13
6.67
5.97
PICP-8
5
PICP-12
82
34
77
5.69
6
7
8
9
Polymer 1ae Ref. [15]
99
66
79
82
74
33
25
31
30
45
95
1.83
0.38
0.46
0.47
5.14
f
g
CAT2-41 Ref. [12]
ns
f
g
CAT2-48 Ref. [12]
ns
f
g
CAT2-15 Ref. [12]
ns
1
0
Neat complex
72
1
1
ICP
85
41
79
5.90
1
1
2
3
PICP-3
Neat complex
97
72
41
70
91
66
6.74
5.00
1
4
ICP
78
67
71
5.42
1
1
5
6
PICP-3
Neat complex
93
66
67
80
86
61
6.46
4.58
1
7
ICP
73
77
67
5.07
1
1
8
9
PICP-3
Neat complex
93
56
77
95
87
49
6.46
3.89
2
0
ICP
62
92
56
4.31
2
2
1
2
PICP-3
Neat complex
91
56
93
95
85
50
6.32
3.89
2
3
ICP
63
93
59
4.38
2
4
PICP-3
91
93
84
6.32
a
Catalyst (4 mol% of substrate, based on Mn ion content), substrate (0.5 mmol), pyridine N-oxide (1.0 mmol), NaClO (1.0 mmol, pH = 11.5, added in four equal parts), CH2Cl2
◦
(
2 ml), 60 min, 0 C.
b
Determined by GC.
c
Isolated yields.
d
−1
TOF is calculated by the expression of [product]/[catalyst] × time (s ).
e
Catalyst (5 mol% of substrate, based on Mn ion content), styrene (0.625 mmol), pyridine N-oxide (0.12 mmol), NaOCl (1.5 mmol, pH = 11.5, added in four equal parts),
◦
CH2Cl2 (1 ml), 3 h, 0 C.
f
Catalyst (2 mol% of substrate, based on Mn ion content), styrene (1 mmol), phenylpyridine N-oxide (0.38 mmol), NaOCl (2 mmol, pH = 11.5, added in four equal parts),
◦
CH2Cl2 (3 ml), 24 h, 0 C.
g
Not supplied.
significantly higher than those of ICP (Table 1, entry 12 vs 11,
entry 15 vs 14, entry 18 vs 17, entry 21 vs 20, entry 24 vs 23), and
also those of neat complex in corresponding epoxidations (Table 1,
entry 12 vs 10, entry 15 vs 13, entry 18 vs 16, entry 21 vs 19,
entry 24 vs 22). Excellent conversions (in the range of 91–97%)
were obtained with all the alkenes used in this work but the high-
est chiral induction (93%) was observed for the electron deficient
6-cyano-2,2-dimethylchromene and 6-nitro-2,2-dimethylchrom-
ene (Table 1, entries 21 and 24). The relatively bulkier olefins, such
as indene and 1,2-dihydronaphthalene, show the moderate ee val-
ues (67% and 77%, respectively) (Table 1, entries 15 and 18), while,
the ee values were not encouraging in the case of the terminal-
olefins, such as styrene and ␣-methylstyrene (Table 1, entries 3
and 12).

Y. Chen et al. / Applied Catalysis A: General 491 (2015) 106–115
113
1
00
100
a
a
b
A
B
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
90
80
70
60
50
40
30
20
b
c
d
e
10
10
20
30
40
50
60
10
20
30
40
50
60
Reaction time (min)
Reaction time (min)
Fig. 5. The conversion vs reaction time plot for asymmetric epoxidation of styrene (A) catalyzed by PICP-3 (a), PICP-8 (b), PICP-12 (c), ICP (d) and neat complex (e), and the
conversion vs reaction time plot for PICP-3 (B) with typical styrene (a) and 6-nitro-2,2-dimethylchromene (b) as substrates.
In order to understand the process of phase transfer catalysis
in the aqueous/organic biphasic system, we have used kinetics to
study the reaction rates of asymmetric epoxidation of styrene over
the catalysts of neat complex, ICP and PICP-n (n = 3, 8, 12). The time-
dependent plot of corresponding catalyst is shown in Fig. 5A. It is
observed from the profile that conversion of styrene increased lin-
early up to 40 min, after which a significant increase is not observed.
Therefore, the initial rate constants Kobs were determined from the
data in this time range. Neat complex, ICP, PICP-12, PICP-8 and
PICP-3 give Kobs values of 1.32, 1.51, 1.66, 1.70, and 1.83 M/min,
respectively. The order of reaction rates over the all tested catalysts
is thus as follows: PICP-3 > PICP-8 > PICP-12 > ICP> neat complex,
viewing from the kinetics point. The results demonstrate the built-
in phase transfer capacity of PICP-n arisen from the PEG moiety.
It circumvents the mass transfer restriction associated with aque-
ous/organic biphasic system, enhancing the reaction rate. And also,
the inherent phase transfer decreased with increasing the total
length of polyether chain of corresponding complex. Furthermore,
the kinetic profiles for PICP-3 with typical styrene and 6-nitro-
enantioselectivity depending on the solubility of catalyst, as well as
the coordinative ability of solvent. PICP-3 is difficultly soluble in n-
hexane and ether, the problem associated with mass transfer is still
encountered. Poor conversion and ee value were indeed obtained
when the reaction was performed in n-hexane (22% of conversion
with 14% ee value) or in ether (26% conversion with the 15% ee
value) (Table 2, entries 1 and 2). If the catalyst can be dissolved
completely in reaction medium, conversion and ee value should
be enhanced. In the case of dichloromethane, 96% conversion with
34% ee value could be obtained within 60 min (Table 2, entry 3).
However, the catalytic activity of PICP-3 was not encouraging in
methanol, acetone, and acetonitrile, although the catalyst can also
be readily dissolved (Table 2, entries 4, 5, and 6). The principal
explanation for this difference is that the solvent containing oxygen
or nitrogen atom with lone electron pair can induce probably coor-
dination with metal center of complex, suppressing the ability of
the formation of the active oxygen transfer species (Mn(V)-oxo) in
the epoxidation of styrene [9,25,43]. Notably, water-soluble PICP-3
is also inactive in water. Only 35% conversion with 3% ee value was
obtained in water (Table 2, entry 7). The lower activity is attributed
to the limited solubility of styrene in the aqueous solution of the
catalyst and oxidant, and the poor enantioselectivity is partially
due to the solvation of water. The solvent effect on the catalytic
performance of PICP-3 also proved to be valid when 6-nitro-2,2-
dimethylchromene (with higher product ee) was performed as the
substrate (Table 2, entries 8–14). Therefore, dichloromethane is a
suitable solvent, in which the catalyst behaves as the efficient PTC
to smoothly catalyze the asymmetric epoxidation.
2
,2-dimethylchromene as substrates is also shown in Fig. 5B. It
is observed from the profiles that conversion of substrates also
increased linearly up to 40 min, after which a significant increase is
not observed. Therefore, the initial rate constants Kobs were deter-
mined from the data in this time range. Epoxidation of styrene
and 6-nitro-2,2-dimethylchromene over PICP-3 give Kobs values
of 1.83 and 1.57 M/min, respectively. Thus, the reaction rates of
styrene is faster that that of 6-nitro-2,2-dimethylchromene, view-
ing from the kinetics point. The difference in reaction rates should
be relevant to the mass transfer in reaction system. Actually, 6-
nitro-2,2-dimethylchromene itself is solid and is insoluble in water.
Since the water-soluble PICP-n (n = 3, 8, 12) are selec-
tively soluble in some organic solvents, various solvents should
affect the built-in phase transfer capacity, and further the cat-
alytic performance of catalyst in the asymmetric epoxidation of
non-functionalized alkenes. Table 2 summarizes the results of
comparative study of the enantioselective epoxidation of styrene
over PICP-3 in various solvents. As shown in Table 2, the epox-
idation results indicated differences in catalytic activity and
3.4. Reusability
Different from the traditional PTC, the catalysts of PICP-n (n = 3,
8
, 12) not only efficiently accelerate the biphasic epoxidation, but
also can be easily precipitated out from the reaction system by
adding sufficient n-hexane to reaction mixture. The supernatant
with the product was separated from catalyst by simple decanta-
tion. Recovered catalyst can be readily recycled by adding fresh
reaction substrates.

1
14
Y. Chen et al. / Applied Catalysis A: General 491 (2015) 106–115
Table 2
a
Results of asymmetric epoxidation of unfunctionalized olefins in different solvents over PICP-3 .
b
b
c
TOFd × 10−3/s−1
Entry
Substrate
Solvent
Conv. /%
ee /%
Yield /%
1
2
3
n-Hexane
Ether
Dichloromethane
22
26
96
14 (R)
15 (R)
34 (R)
17
20
90
1.53
1.81
6.67
4
Methanol
72
12 (R)
68
5.00
5
6
7
8
9
Acetone
Acetonitrile
Water
n-Hexane
Ether
Dichloromethane
80
77
35
20
32
91
27 (R)
21 (R)
3 (R)
47 (R)
51 (R)
93 (R)
75
73
30
14
27
85
5.56
5.35
2.43
1.38
2.22
6.32
1
0
1
1
Methanol
67
39 (R)
62
4.65
1
1
1
2
3
4
Acetone
Acetonitrile
Water
78
72
41
58 (R)
63 (R)
12 (R)
72
67
37
5.42
5.00
2.85
a
PICP-3 (4 mol% of substrate, based on Mn ion content), styrene (0.5 mmol), pyridine N-oxide (1.0 mmol), NaClO (1.0 mmol, pH = 11.5, added in four equal parts), solvent
◦
(
2 ml), 60 min, 0 C.
b
The same as Table 1.
The same as Table 1.
The same as Table 1.
c
d
Fig. 6 shows the results of the recovery and reusability of these
PICP-12, respectively). Therefore, the efficient PTC of PICP-n is
quite stable under the basic reaction condition (pH 11.5, NaClO
buffer). Oxidative decomposition of chiral salen Mn(III) complex,
a main reason for deactivation of the complex in basic epoxidation
[44], did not occur during the reaction. Further evidence for the
stability of catalysts was provided by UV–vis spectra. No signifi-
cant changes of the catalysts took place even after reuse for seven
times (Fig. 3b vs b’, c vs c’, d vs d’). Perfect stability of PICP-n (n = 3,
8, 12) was also proved by the reusability of typical PICP-3 in asym-
metric epoxidation of 6-nitro-2,2-dimethylchromene (Fig. 6D).
Characterization of the recycled PICP-3 (after seven runs) by FT-
IR spectra shows no significant changes of the PICP-3 even after
reuse for seven times (Fig. 2 b vs b’). Manganese content of recycled
catalysts in the asymmetric epoxidation of styrene using NaClO as
an oxidant (Fig. 6A–C). To our delight, PICP-n (n = 3, 8, 12) could be
reused up to seven times with no significant loss in yield and enan-
tioselectivity of styrene epoxide. The leaching tests to the reaction
medium were performed by directly determining the manganese
content in supernatant via chemical analysis. No manganese was
detected in the supernatant, which reveals negligible leaching loss
of Mn species during the reaction. Furthermore, chemical anal-
ysis of the recovered catalysts gives manganese content (0.809,
0
1
.692, and 0.738 mmol/g for recovered PICP-3, PICP-8, and PICP-
2, respectively) almost identical to that of the corresponding fresh
one (0.814, 0.695, and 0.741 mmol/g for fresh PICP-3, PICP-8, and
1
00
A
100
90
80
70
60
50
40
30
20
10
0
B
100
90
80
70
60
50
40
30
20
10
0
C
100
90
80
70
60
50
40
30
20
10
0
D
b
a
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
a
a
a
b
b
b
1
2 3 4 5 6 7
1 2 3 4 5 6 7
1 2 3 4 5 6 7
1 2 3 4 5 6 7
Run times
Run times
Run times
Run times
Fig. 6. Reuse of PICP-3 (A), PICP-8 (B) and PICP-12 (C) in the asymmetric epoxiation of styrene, and reuse of PICP-3 in the asymmetric epoxiation of 6-nitro-2,2-
◦
dimethylchromene (D) with aq. NaClO at 0 C (a: conversion; b: ee value).

Y. Chen et al. / Applied Catalysis A: General 491 (2015) 106–115
115
PICP-3 (0.811 mmol/g) determined by chemical analysis is also
almost identical to that of the fresh one (0.814 mmol/g). The excel-
lent stability of PICP-n in the reaction system should origin from
mild reaction conditions and stable structure of the complex. Short
[4] W. Zhang, J.L. Loebach, S.R. Wilson, E.N. Jacobsen, J. Am. Chem. Soc. 112 (1990)
801–2803.
2
[
[
5] S. Chang, J.M. Galvin, E.N. Jacobsen, J. Am. Chem. Soc. 116 (1994) 6937–6938.
6] Q. Xia, H. Ge, C. Ye, Z. Liu, K. Su, Chem. Rev. 105 (2005) 1603–1662.
[7] C. Baleizão, H. Garcia, Chem. Rev. 106 (2006) 3987–4043.
[8] H. Zhang, Y. Zhang, C. Li, J. Catal. 238 (2006) 369–381.
◦
reaction time (within 60 min) and low temperature (0 C) used in
[
9] R. Tan, D. Yin, N. Yu, H. Zhao, D. Yin, J. Catal. 263 (2009) 284–291.
our studies would prevent or at least slow catalyst degradation.
More importantly, the possibility of formation of undesired inactive
dimeric ␮-oxo-manganese(IV) dimmers, the other cause for deacti-
vation of the catalytic species, can be avoided because of the severe
restriction of active site (site isolation) in the rigid one-dimension
polymer framework [6,9]. Furthermore, the imidazolium IL linker
moiety plays a positive effect on stabilizing the complex [9,25].
[
10] O.Y. Lyakin, R.V. Ottenbacher, K.P. Bryliakov, E.P. Talsi, ACS Catal. 2 (2012)
1196–1202.
[11] J. Huang, X. Fu, C. Wang, H. Zhang, Q. Miao, Microporous Mesoporous Mater.
53 (2012) 294–301.
12] R. Ji, K. Yu, L. Lou, S. Liu, J. Mol. Catal. A 378 (2013) 7–16.
1
[
[13] R.V. Ottenbacher, D.G. Samsonenko, E.P. Talsi, K.P. Bryliakov, ACS Catal. 4 (2014)
1599–1606.
[
[
[
[
14] L. Ma, F. Su, X. Zhang, D. Song, Y. Guo, J. Hu, Microporous Mesoporous Mater.
84 (2014) 37–46.
15] N.Ch. Maity, S.H.R. Abdi, R.I. Kureshy, N.H. Khan, E. Suresh, G.P. Dangi, J. Catal.
277 (2011) 123–127.
1
4
. Conclusions
16] B.I. Ku z´ niarska, C. Pereira, A.P. Carvalho, J. Pires, C. Freire, Appl. Clay Sci. 53
(
2011) 195–203.
Chiral salen Mn(III) polymers (PICP-n (n = 3, 8, 12)), in which
17] Z. Liu, X. Fu, X. Hu, Q. Miao, J. Organomet. Chem. 713 (2012) 157–162.
chiral salen Mn(III) unit was linked side by side through various
PEG-based di-imidazolium IL bridge, were first prepared and acted
as the phase transfer catalysts in enantioselective epoxidation of
unfunctionalized olefins using aq. NaClO as an oxidant. Remark-
able enhancement of reaction rate with notably high TOF values
was observed over the PICP-n (n = 3, 8, 12), especially over PICP-3,
in the water-dichloromethane biphasic system. Furthermore, the
synthesized PICP-n (n = 3, 8, 12) is quite stable under the basic
reaction condition (pH 11.5, NaClO buffer) and could be facilely
recovered from the reaction system for efficient reuse. The built-in
phase transfer capability, high catalytic efficiency, as well as prefect
stability and reusability render this kind of complexes attractive in
the enantioselective epoxidation of unfunctionalized olefins with
aq. NaClO.
[18] R.I. Kureshy, T. Roy, N.H. Khan, S.H.R. Abdi, A. Sadhukhan, H.C. Bajaj, J. Catal.
86 (2012) 41–45.
2
[
[
19] Y. Zhang, Q. Zhou, W. Ma, J. Zhao, Catal. Commun. 45 (2014) 114–117.
20] R.I. Kureshy, N.H. Khan, S.H.R. Abdi, S.T. Patel, P.K. Iyer, P.S. Subramanian, R.S.
Jasra, J. Catal. 209 (2002) 99–104.
[
[
21] B. Bahramian, V. Mirkhani, M. Moghadam, S. Tangestaninejad, Appl. Catal. A
301 (2006) 169–175.
22] J. Chen, S.K. Spear, J.G. Huddleston, R.D. Rogers, Green Chem. 7 (2005) 64–82.
[23] X. Wang, L. Yin, T. Yang, Y. Wang, Tetrahedron: Asymmetry 18 (2007) 108–114.
[
[
[
24] S. Paul, A.R. Das, Tetrahedron Lett. 54 (2013) 1149–1154.
25] R. Luo, R. Tan, Z. Peng, W. Zheng, Y. Kong, D. Yin, J. Catal. 287 (2012) 170–177.
26] R.I. Kureshy, N.H. Khan, S.H.R. Abdi, S.T. Patel, R.S. Jasra, Tetrahedron Lett. 42
(2001) 2915–2918.
[27] X. Liu, N. Tang, W. Liu, M. Tan, J. Mol. Catal. A 212 (2004) 353–358.
[
[
28] Y. Xu, Y. Bai, J. Peng, J. Li, W. Xiao, G. Lai, J. Organomet. Chem. 765 (2014) 59–63.
29] D. Bell, M.R. Davies, G.R. Geen, I.S. Mann, Synthesis 6 (1995) 707–712.
[30] J.F. Larrow, E.N. Jacobsen, Y. Gao, Y.P. Hong, X.Y. Nie, C.M. Zepp, J. Org. Chem.
59 (1994) 1939–1942.
[
31] L. Lou, K. Yu, F. Ding, X. Peng, M. Dong, C. Zhang, S. Liu, J. Catal. 249 (2007)
02–110.
1
Acknowledgements
[32] H. Ren, Y. Tian, H. Liang, The Handbook of Analytical Chemistry, Chemical
Industry Press, Beijing, 1997, p. 559.
[
[
[
33] F. Alonso, M. Yus, ACS Catal. 2 (2012) 1441–1451.
The project was financially supported by the National Natural
Science Foundation of China (Grant Nos. 21476069, 21003044), the
Scientific Research Fund of Hunan Provincial Education Depart-
ment (13B072), the Program for Excellent Talents in Hunan
Normal University, the Program for Science and Technology Inno-
vative Research Team in Higher Educational Institutions of Hunan
Province.
34] M.R. Nabid, Y. Bide, Appl. Catal. A 469 (2014) 183–190.
35] B.M. Choudary, T. Ramani, H. Maheswaran, L. Prashant, K.V.S. Ranganath, K.V.
Kumar, Adv. Synth. Catal. 348 (2006) 493–498.
36] J. Zou, L. Wang, R. Zeng, Structure Analysis of Organic Compounds, Science
Press, Beijing, 2005, p. 48.
[
[
37] H. Zhi, C. Lü, Q. Zhang, J. Luo, Chem. Commun. (2009) 2878–2880.
[38] F. Bigi, L. Moroni, R. Maggi, G. Sartori, Chem. Commun. (2002) 716–717.
[
39] L. Frunza, H. Kosslick, H. Landmesser, E. Hijft, R. Fricke, J. Mol. Catal. A 123 (1997)
79–187.
40] Y. Kong, R. Tan, L. Zhao, D. Yin, Green Chem. 15 (2013) 2422–2433.
1
[
References
[41] Y. Sun, N. Tang, J. Mol. Catal. A 255 (2006) 171–179.
[
[
[
42] M. Palucki, N.S. Finney, P.J. Pospisil, M.L. Guler, T. Ishida, E.N. Jacobsen, J. Am.
Chem. Soc. 120 (1998) 948–954.
43] B. Gong, X. Fu, J. Chen, Y. Li, X. Zou, X. Tu, P. Ding, L. Ma, J. Catal. 262 (2009)
[1] M. Tokunaga, J.F. Larrow, F. Kakiuchi, E.N. Jacobsen, Science 277 (1997)
9
36–938.
9–17.
[
[
2] H. Yang, L. Zhang, W. Su, Q. Yang, C. Li, J. Catal. 248 (2007) 204–212.
3] O.A. Wong, Y. Shi, Chem. Rev. 108 (2008) 3958–3987.
44] Z. Xu, X. Ma, Y. Ma, Q. Wang, J. Zhou, Catal. Commun. 10 (2009) 1261–1266.