Monodisperse crosslinked pHEMA particle-supported chiral Mn(III)salen catalyst for asymmetric epoxidation of olefin

Article abstract of DOI:10.1002/aoc.1873

Monodisperse crosslinked poly(hydroxyethyl methacrylate) particles (pHEMA) were synthesized for immobilization of the chiral Mn(III)salen homogeneous catalyst by axial coordination. The pHEMA-Mn(III)salen catalyst was subsequently characterized by FT-IR, UV and scanning electron microscopy. The results showed that, the heterogeneous Mn(III)salen catalysts also exhibited high activity and enantioselectivity compared to the homogeneous catalyst for the disubstituted cyclic indene and 6-cyano-2,2-dimethylchromene. Moreover, the catalysts were easily separated from the reaction systems and could be renewed several times without significant loss of catalytic activity. Meanwhile, the enantiomeric excess (ee) value remained at 80% in the eighth cycle. The pHEMA support, immobilized by Mn(III)salen, probably acted as a mediator of the reaction between the substrate and the oxidant, and enhanced the stability of the Mn(III)salen compound. Copyright

Full text of DOI:10.1002/aoc.1873

Full Paper
Received: 16 October 2011
Revised: 14 November 2011
Accepted: 13 December 2011
Published online in Wiley Online Library: 8 February 2012
(wileyonlinelibrary.com) DOI 10.1002/aoc.1873
Monodisperse crosslinked pHEMA particle-
supported chiral Mn(III)salen catalyst for
asymmetric epoxidation of olefin
a,b
a
a
c
a
Saili Wei , Yuhai Tang *, Ruiqin Zhao , Guojin Xu and Yuansuo Zheng
Monodisperse crosslinked poly(hydroxyethyl methacrylate) particles (pHEMA) were synthesized for immobilization of the
chiral Mn(III)salen homogeneous catalyst by axial coordination. The pHEMA-Mn(III)salen catalyst was subsequently character-
ized by FT-IR, UV and scanning electron microscopy. The results showed that, the heterogeneous Mn(III)salen catalysts also
exhibited high activity and enantioselectivity compared to the homogeneous catalyst for the disubstituted cyclic indene
and 6-cyano-2,2-dimethylchromene. Moreover, the catalysts were easily separated from the reaction systems and could be
renewed several times without significant loss of catalytic activity. Meanwhile, the enantiomeric excess (ee) value remained
at 80% in the eighth cycle. The pHEMA support, immobilized by Mn(III)salen, probably acted as a mediator of the reaction
between the substrate and the oxidant, and enhanced the stability of the Mn(III)salen compound. Copyright © 2012 John
Wiley & Sons, Ltd.
Keywords: pHEMA; chiral pHEMA–Mn(III)salen; asymmetric epoxidation; olefins; recycling
an excellent medium applied in chromatographic separation
and immobilization of bioactive molecules because: (i) pHEMA
Introduction
Manganese(III)salen compounds have been reported as highly
active and enantioselective catalysts in the epoxidation of
possesses abundant hydroxyl groups on its surface, so it can
be modified to meet the requirement of different chromato-
[
1]
[7]
unfunctionalized olefins in homogeneous phase. Currently,
the heterogenization of these homogeneous catalysts on effec-
tive supports is the object of intense research in order to make
graphic applications; (ii) it has been reported that pHEMA
has excellent biocompatibility because its physical properties
can be easily tailored, and it has been used extensively in
medical applications such as contact lenses, keratoprostheses,
[
2]
them recyclable as well as economical. More importantly, the
immobilization of Mn(III)salen compounds also effectively
restrains the formation of inactive l-oxo-Mn(IV) dimers by isolat-
ing the catalytic sites. In previous work, various carriers have
been used to prepare heterogeneous Mn(III)salen catalysts,
[
8]
orbital implants, and tissue engineering. However, as far as
we know, no research of immobilized catalyst onto porous
pHEMA as carrier has yet been reported.
In our team, the method of ’single-step seed swelling
and polymerization’ was used to prepare monodisperse
[
3]
[4]
[5]
such as polymeric resins, clays, zeolites, and MCM-41
[6]
[9]
silica materials. These heterogeneous catalysts usually display
activities and enantioselectivities for the asymmetric epoxida-
tion of unfunctionalized olefins.
crosslinked and porous pHEMA particles. Then, the chiral
Jacobsen catalyst was axially immobilized onto insoluble
pHEMA via a hydroxyl group, which was much easier to
handle and recycle. The preparation of heterogeneous chiral
Mn(III)salen catalyst was subsequently characterized by FT-IR,
UV and scanning electron microscopy. It was found that
the resulting system showed comparable and even higher
enantioselectivities than homogeneous catalysts for the
asymmetric epoxidation of unfunctionalized olefins.
Recently, there has been a growing interest in polymer-
based porous carbon materials since they exhibit some
advantages over conventional inorganic supports, such as
higher stability in acid and basic media. Conventional
inorganic supports such as alumina and silica are dissolved
at high pH values, and the former is also attacked even at
very low pH values. In addition to higher stability, both the
porous texture and surface chemistry of carbon materials
can be modified by appropriate methodologies. In this regard,
the porous texture and surface chemical properties of poly
*
Correspondence to: Y. Tang, Institute of Analytical Sciences, School of Science,
Xi’an Jiaotong University, Xi’an, Shaanxi 710061, China. E-mail: weisaili.1983@
stu.xjtu.edu.cn
(
hydroxyalkyl methacrylate) can be easily tailored to meet
specific requirements.
Poly(hydroxyalkyl methacrylate) (pHAMA), with plentiful
hydroxyl groups, shows active chemical properties and has
been used in fiber processing, coating, light-curing resins,
adhesives, paper processing, rubber industries and so on.
Among these compounds, poly(2-hydroxyethyl methacrylate)
a Institute of Analytical Sciences, School of Science, Xi’an Jiaotong University,
Xi’an, Shaanxi, 710061, China
b State Key Laboratory of Loess and Quaternary Geology, IEE, CAS, Xi’an,
Shaanxi, 710075, China
(pHEMA) is most widely used. In addition, porous pHEMA is
c Medical School, Xi’an Jiaotong University, Xi’an, Shaanxi, 710061, China
Appl. Organometal. Chem. 2012, 26, 114–120
Copyright © 2012 John Wiley & Sons, Ltd.

pHEMA-supported Mn(III)salen catalyst for epoxidation of olefin
Experimental
Materials and Instruments
1
,2-Diaminocyclohexane, 3,5-di-tert-butyl-hydroxybenzaldehyde,
and titanium(IV)tetrachloride were obtained from Alfa Aesar
chemical Co. Benzoyl peroxide (BPO), polyvinyl alcohol (PVA),
2
-hydroxyalkyl methacrylate (HEMA) and ethylene glycol
dimethacrylate (EDMA) were laboratory-grade reagents from
local suppliers. The (R,R)-(À)-N,N′-bis(3,5-di-tert-butylsalicylidene)-
1,2-cyclohexanediamino-manganese(III)chloride (known as Jacobsen
catalyst, Mn(III)salen), was synthesized according to the literature
[10]
procedure (Scheme 1). All other chemicals were reagent grade
and used as received.
The UV–visible and FT-IR spectra of some samples were
recorded from 200 to 800 nm on a UV-3310 spectrophotometer
À1
and from 400 to 4000 cm
on a Nicolet AVATAR330 FT-IR
spectroscopy (KBr), respectively. The heterogeneous chiral Mn
III)salen catalyst was analyzed by SEM using a JEOL-JSM-6360
(
LV microscope operating at 20 kV. The observed surface was
obtained by coating the cryogenic fracture with a thin gold
layer. The chiral column was a Daicel Chiralcel OD-H manufac-
tured by Daicel Chemical industries Ltd, and the associated
high-performance liquid chromatographic analysis (HPLC)
was carried out on a Shimadzu instrument (system controller:
LC-10AT VP; UV–visible detector: SPD-10A VP). The reaction
products were analyzed by HPLC (Shimadzu), containing UV
detection wavelength: 254 nm; eluent agent: n-hexane/i-PrOH,
À1
9
9/1, v/v; flow rate: 0.8 ml min ; column pressure: 2.0–3.0 MPa,
ꢀ
and column temperature: 25 C).
Synthesis of pHEMA (Scheme 2)
The polystyrene seed microspheres (PSt) were prepared by the
dispersion polymerization method. A 100 ml three-necked flask
was charged with 15 ml styrene, 1 g polyvinyl pyrrolidone (PVP)
as stabilizer and 0.3 g BPO as initiator in 40 ml ethanol. The
mixture was dissolved by ultrasound until the solution became
clear and transparent. After bubbling dry nitrogen gas for
1
5 min beneath the liquid surface, the nitrogen pipe was main-
tained above the surface to maintain the nitrogen ambience.
ꢀ
The polymerization was performed at 70 C for 24 h under stir-
ring. The suspension of polymer microspheres were filtrate
loaded on to a nylon sieve bag (120 mm) and washed with
ethanol repeatedly until the percolation liquid was clarified.
The polystyrene seed microspheres were dried under vacuum
for 24 h to obtain a 3 mm monodisperse particle size (Fig. 1).
Scheme 2. Route for preparation of the pHEMA–Mn(III)salen compound.
The method of ’single-step seed swelling and polymerization’
was used for the preparation of monodisperse crosslinked and
porous pHEMA particles. The polymerization procedure was
performed as follows. HEMA as monomer, ethylene glycol
dimethacrylate (EDMA) as crosslinker and toluene containing
BPO as initiator were emulsified in 100 ml water containing
0
.1% (w/w) of sodium n-dodecyl sulfate and 1% (w/w) PVP with
an ultrasonic disrupter. The mass ratio of HEMA to EDMA was
:1. The emulsion was added dropwise to the dispersion of
the PSt seed particles (50% w/w). The mixture was stirred for
0 h at room temperature so that all emulsified organic phase
3
1
was adsorbed by polymer seeds. After bubbling dry nitrogen
gas for 20 min into the solution, the temperature was increased
ꢀ
Scheme 1. Structure of Jacobsen catalyst.
to 70 C and polymerization was carried out for 24 h. The
Appl. Organometal. Chem. 2012, 26, 114–120
Copyright © 2012 John Wiley & Sons, Ltd.
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X. Guojin et al.
of 0.1g pHEMA–Mn(III)salen compound in 100 ml concentrated
À1
H SO for 2 h, the Mn(III)salen content was found to be 6.8mg g
.
2
4
Catalysis Experiments (Scheme 3)
Enantioselective epoxidation reactions were performed accord-
[
11]
ing to the procedure established by Pietikänen
First, 2 mmol unfunctionalized olefins (styrene, 1-octene,
-cyano-2,2-dimethylchromene, indene and a-methylstyrene)
and 0.4 mmol NH OAc (20 mol% based on the molarity of the
olefins) were added to 4 ml CH Cl containing 0.2 mmol catalyst
10 mol% based on the molarity of the olefins) under stirring. A
as follows.
6
4
2
2
(
À1
solution of Na
2
HPO
4
(0.05 mol l ) was added to a pre-cooled
or m-CPBA as oxidant. The pH value
of the mixed solution was accurately adjusted to 11.3 by slowly
2 2
solution of either NaClO, H O
Figure 1. Micrograph morphology of PSt particles.
À1
À1
instilling HCl (1molL ) or NaOH (1mol L ) solution. The mixture
resulting gel beads were washed with hot water, methanol and
acetone three to five times successively, followed by drying un-
der vacuum. Finally, unreacted monomer and other pore agent
impurities in the pHEMA microspheres were removed by ex-
was pre-cooled to a specified temperature. After the reaction
was over, the catalyst was filtered for recycling. The CH Cl2
2
phase was separated, washed with water and brine, dried over
anhydrous MgSO , and concentrated under reduced pressure.
4
ꢀ
traction with toluene at 120 C for 48 h. The product was dried
The residue was purified by flash chromatography (SiO , petroleum
2
ꢀ
at 60 C under vacuum for 3 h. (Fig. 2). FT-IR (KBr) v: 3475, 2946,
ether/CH Cl , 2/1, v/v) to produce the epoxides.
À1
2
2
2
893, 1728, 1497, 1403, 1276, 1175, 784 cm
.
Results and Discussion
Synthesis of pHEMA–Mn(III)salen
FT-IR Spectra
The resulting pHEMA powder (1.10 g) was then refluxed with
IR spectra of Na@pHEMA and pHEMA–Mn(III)salen are com-
pared in Fig. 3. As shown in the figure, the FT-IR spectrum of
Na@pHEMA revealed one peak characteristic of alcoholic
an aqueous solution of sodium hydroxide (100 ml, 13.3 mmol
À1
g
) for 3 h. The solid was filtrated and washed with distilled
ꢀ
water until pH 7, then dried at 120 C in an oven. The white
powder was obtained under vacuum, designated as
Na@pHEMA.
À1
groups at 3475 cm (stretching C-O). Ester groups were identified
-
1
À1
by peaks at 1728 cm (C O stretching) and 1175 cm
(C-O
stretching), respectively. Other peaks represented C-C and C-H
A mixture of Jacobsen catalyst (1.0 mmol) and Na@pHEMA
À1
vibrations of -CH
3
and -CH
2
groups. The peak at 519 cm of
(
2.0 g) was added to ethanol (60 ml) and stirred for 8 h under
refluxing. The suspension was then filtrated, and washed
thoroughly with ethanol and CH Cl to produce pHEMA–Mn
III)salen as a little buff powder. The CH Cl filtrate was
detected by UV–visible spectrometer until no peak signal of
Jacobsen catalyst (with CH Cl as reference) was observed.
The loading amount of Mn(III)salen in heterogeneous chiral Mn
2
2
(
2
2
2
2
(
III)salen compound was estimated quantitatively by atomic absorp-
2 2
Scheme 3. Enantioselective epoxidation of alkenes in CH Cl . Substrate:
styrene, 1-octene, 6-cyano-2,2-dimethylchromene, indene, a-methylstyrene.
tion spectrometry on a WFX-1E2 type spectrometer. After digestion
Figure 3. FT-IR spectrum of Na@pHEMA and pHEMA-Mn(III)salen. Blue
line, Na@pHEMA; red line, pHEMA–Mn(III)salen.
Figure 2. SEM morphology of monodisperse crosslinked pHEMA particles.
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Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 114–120

pHEMA-supported Mn(III)salen catalyst for epoxidation of olefin
pHEMA–Mn(III)salen was associated with the Mn-N bond.
Briefly, the IR spectra of pHEMA–Mn(III)salen also presented an
absorption band at 1578 cm , which was assigned to C N
Na@pHEMA. The band at 238 nm is assigned to the p–p*
transition of the benzene ring of salicylaldehyde, while the
band at 325 nm is attributed to the n–p* transition of the
azomethine chromophore. The band at 432 nm is ascribed to
À1
vibrations characteristic of the imines present in the Mn(III)salen
compound. It was proved again that the Mn(III)salen was
[
13]
the manganese d–d transition.
The 14 nm shift (from 252
[
12]
successfully immobilized on to the Na@pHEMA.
to 238 nm) and the 10 nm shift (from 442 to 432 nm) observed
from the anchored Mn(III)salen can be attributed to interactions
À
between the oxygen anion (O ) of Na@pHEMA and the central
UV–Visible Spectra
metal of the Mn(III)salen. This confirmed that Mn(III)salen
compound has been anchored on to the support. To verify this
point, NaOEt was added to the solution of Mn(III)salen, and the
mixture was checked by the UV–visible spectrum ( Fig. 4B). In
the NaOEt–Mn(III)salen solution, all the characteristic bands
appeared in their spectra. But the NaOEt–Mn(III)salen solution
exhibited a red shift from 238 and 432 nm to 247 and
In the UV–visible spectra (Fig. 4), the pHEMA–Mn(III)salen
compound displayed absorptions at 252, 325, 442 nm(Fig. 4A),
which were close to 238, 325, and 432 nm (Fig. 4 C)
corresponding to the characteristics of Mn(III)salen. It identified
that the Mn(III)salen was successfully anchored into the
4
38 nm, which indicated that an interaction existed between
–
the Mn(III)salen complex and oxygen anion (O ) of NaOEt.
The UV–visible spectra of the pHEMA–Mn(III)salen complex
was very similar to that of NaOEt–Mn(III)salen solution. This
proved the above point again and further confirmed that Mn
(III)salen compound had been anchored on to the pHEMA.
SEM Spectra
The SEM morphology of the pHEMA particles and pHEMA–Mn
(III)salen particles are shown in Fig. 5, which indicates that
the diameter of the particles of the heterogeneous catalysts
was in the submicron range. In Fig. 5(A), the structure of
pHEMA particles was loose. Various caves, holes, pores and
channels of different shapes could be clearly observed. How-
ever, Fig. 5(B) shows a hairy surface of pHEMA particles, which
further confirms the immobilization of chiral Mn(III)salen on the
pHEMA supports.
Figure 4. UV–visible spectrum of pHEMA–Mn(III)salen and Jacobsen
catalysis. Line A, pHEMA–Mn(III)salen; line B, NaOEt–Mn(III)salen; line C,
Jacobsen catalysis.
Figure 5. SEM morphology of the pHEMA–Mn(III)salen compound.
Appl. Organometal. Chem. 2012, 26, 114–120
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X. Guojin et al.
Catalytic Performance
heterogeneous catalysts for asymmetric epoxidation. The
asymmetric epoxidations of styrene and other substrates were
tested at pH 11.3. NH OAc as the auxiliary catalyst was used
4
for the asymmetric epoxidation of olefins. The results are
listed in Table 1.
Despite Mn(III)salen complexes having been widely employed
in homogeneous media as versatile catalysts for hydrocarbon
[
14,15]
oxidation,
they are subject to inactivation via m-oxo and
[
16]
m-peroxodimerization.
Immobilization of these complexes
2 2
The effects of different oxidants, such as NaOCl, H O and
into a solid support not only diminishes catalyst inactivation
due to reduction in mobility, but also provides an easy recov-
ery from the reaction medium, which allows catalyst recycling
immediately. Heterogeneous systems also offer improved
lifetime and may affect catalytic activity by providing an envi-
ronment that is able to drive the substrates to the active
complex region.
The catalytic activity of the system was investigated with a
monosubstituted styrene and 1-octene, disubstituted 6-cyano-2,2-
dimethylchromene, indene and a-methylstyrene as substrates in
m-CPBA, were also investigated in the epoxidation of indene
using pHEMA–Mn(III)salen as the catalyst. Rather low activi-
ties of indene epoxides were obtained in the presence of
2 2
m-CPBA and H O . Meanwhile, a significant increase in the
yield and enantioselectivity of the epoxide was obtained by
adding NaOCl. The results showed that NaOCl was the best
oxygen source.
The heterogeneous Mn(III)salen catalysts also exhibited high
activity and enantioselectivity for asymmetric epoxidation of
disubstituted cyclic olefins. The pHEMA–Mn(III)salen catalyst
gave a 94% ee value for the asymmetric epoxidation of indene,
compared to a 93% ee value of the homogeneous Mn(III)salen
Cl catalysts (entries1 and 2). Similarly, the 80% ee value
obtained from the asymmetric epoxidation of 6-cyano-2,
CH
ganic substrate phase and an aqueous solution of either NaOCl,
or m-CPBA. After each experiment, the catalyst was filtered
2 2
Cl . The catalyst system possessed two different phases: an or-
2 2
H O
and subsequently used by washing only with dichloromethane
and drying under vacuum without further purification.
2-dimethylchromene was slightly higher than its corresponding
homogeneous 77% ee value (entries 7 and 8). The pHEMA
support immobilized by Mn(III)salen probably acted as a medi-
ator of the reaction between the substrate and the oxidant,
and enhanced the stability of the Mn(III)salen compound.
However, for the asymmetric epoxidation of single-substituted
Asymmetric Catalytic Epoxidation
Five typical substituted unfunctionalized olefins – a monosubstituted
styrene and 1-octene, disubstituted 6-cyano-2,2-dimethylchromene,
indene and a-methylstyrene
–
were tested against the
a
Table 1. Asymmetric epoxidation of unfunctional olefins
b
d
c
e
f
Entry
Substrate
Oxidant /catalyst
ee (%)
Yield (%)
Time (h)
Config.
1
2
3
4
5
6
7
8
NaClO/pHEMA–Mn(III)salen
93
93
90
92
86
89
80
77
94
95
94
96
90
92
85
88
5.5
4.5
5.5
4.5
5
1R,2S
1R,2S
1R,2S
1R,2S
1R,2S
1R,2S
1R,1R
1R,1R
NaClO/Mn(III)salen
H
H
2
O
O
2
/pHEMA–Mn(III)salen
2
2
/Mn(III)salen
m-CPBA/pHEMA–Mn(III)salen
m-CPBA /Mn(III)salen
4
NaClO/pHEMA–Mn(III)salen
NaClO/Mn(III)salen
5
4.5
9
NaClO/pHEMA–Mn(III)salen
65
71
65
68
6
5
R
R
1
0
NaClO/Mn(III)salen
11
12
13
14
15
16
17
18
19
20
21
NaClO/pHEMA–Mn(III)salen
70
72
92
51
92
90
90
87
84
82
80
75
80
90
65
88
87
85
83
80
78
75
6.5
5
R
R
R
R
R
R
R
R
R
R
R
NaClO/Mn(III)salen
NaClO/pHEMA–Mn(III)salen (1st)
NaClO/pHEMA–Mn(III)salen (2nd)
NaClO/pHEMA–Mn(III)salen (2nd)
NaClO/pHEMA–Mn(III)salen (3 rd)
NaClO/pHEMA–Mn(III)salen (4th)
NaClO/pHEMA–Mn(III)salen (5th)
NaClO/pHEMA–Mn(III)salen (6th)
NaClO/pHEMA–Mn(III)salen (7th)
NaClO/pHEMA–Mn(III)salen (8th)
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5.5
g
a
Reaction conditions: substrate (2 mmol), catalyst (0.2 mmol, 10 mol%), co-catalyst NH
temperature was 0 C.
4
OAc (0.4 mmol, 20 mol%), NaClO (5.8 mmol). The reaction
ꢀ
b
c
Oxidant: NaClO (5.8 mmol), H
2
O
2
(5.8 mmol), m-CPBA (5.8 mmol).
Yield of the isolated epoxide.
d
Determined by HPLC over a chiral OD-H column after comparing the retention times with that of the racemic epoxide sample.
e
Monitored by thin-layer chromatography every other half hour.
f
Absolute configurations of major enantiomers which were determined by WXG-5 polarimeter.
g
After the first run, pHEMA–Mn(III)salen was filtered and directly added to a second run, without a washing step.
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Appl. Organometal. Chem. 2012, 26, 114–120

pHEMA-supported Mn(III)salen catalyst for epoxidation of olefin
styrene and 1-octene, the polymer-immobilized Mn(III)salen
catalyst led to a 65% ee value of styrene epoxide and 70%
ee value of 1-octene epoxide – lower than that of the homoge-
neous catalyst. The possible reason is that Mn(III)salen immobi-
lized on to pHEMA supports led to a low selectivity of styrene
epoxide with phenyl aldehyde and acetophenone as
by-products.
Since catalysts had been used to perform reactions, much
effort has been devoted not only to develop superior catalysts
but also to find methods for their reuse or use in continuously
operated reactors. With this aim in mind, the recycling of
pHEMA–Mn(III)salen was investigated with the a-methylstyrene
as an example.
Table 2. Effect of temperature on the epoxidation of indene by
pHEMA–Mn(salen)
a
ꢀ
b
Entry
Temperature ( C)
ee (%)
Yield (%)
Time (h)
1
2
3
4
5
À9
À5
0
95
93
93
87
82
67
81
94
95
98
7
6
5.5
5
15
25
4.5
a
Reaction conditions: indene (2 mmol), catalyst (0.2 mmol, 10 mol%),
NH OAc (0.4 mmol, 20 mol%), NaClO (5.8 mmol).
Monitored by thin-layer chromatography every other half hour.
4
b
Blank experiments with Na@pHEMA as catalyst showed
no significant turnover frequencies. After the first run,
pHEMA–Mn(III)salen was filtered and directly added to a
second run, leading to 51% ee conversion (65% epoxide
yield). With isolation of the catalyst after the second run
followed by washing with dichloromethane, a subsequent
run led to 92% ee value (88% epoxide yield). This regenera-
tion process could be repeated four times, and the values
of ee were nearly constant (decreasing only 2% between the
first and the fourth times). After the eighth run, the ee value
still remained at 80% (75% epoxide yield). This proved
the stability of the catalysts under our experimental condi-
tions. Thus the reuse of catalyst was possible after isolating
and washing the catalyst. This washing step was presumably
effective in removing the inorganic waste from the catalyst,
which began to block access to the active site. Even though
the yield increasing from 82% to 94%, and ee values increased
from 68% to 93% (entries 3 and 4). The increases in activity
and ee values might be due to the presence of the partial
interaction between Mn(III)salen and NH OAc. Meanwhile, the
4
ee value of 93% corresponded to the ee value of 94% for
the homogeneous case. Here, the axial immobilization of Mn
(III)salen compounds contributed to the coordination of
NH OAc, leading to corresponding ee values with a homoge-
4
neous result.
To the best of our knowledge, the heterogeneous
pHEMA–Mn(III)salen catalyst showed more activity than the
corresponding homogeneous catalyst in the epoxidation of
olefins. Despite the limitations of heterogeneous catalytic
systems, such as slow reactant diffusion to the catalytic site,
pHEMA probably acted as a mediator of the reaction between
the substrate (organic phase) and the oxidant dissolved in the
aqueous phase. The substrate and the oxidant were brought
together by the pHEMA–Mn(III)salen system, which promoted
the diffusion of these compounds to the catalytic site where
the reaction took place.
the recycled heterogeneous catalyst showed
a slightly
decreasing activity, the selectivity of epoxide was not
changed during the reuse process. The data showed that
the supported catalysts exhibited excellent catalytic activity
and enantioselectivity and could be reused several times
without significant loss of its catalytic activity.
Effect of Reaction Temperature
Conclusions
The results of the enantioselective epoxidation of indene
catalyzed by the pHEMA–Mn(III)salen complex at different
reaction temperatures are summarized in Table 2. As
expected, the catalytic activity and enantioselectivity were
relevant to the reaction temperatures. A decrease of reaction
Chiral Mn(III)salen compound was axially immobilized on
to pHEMA via a hydroxy group, which efficiently catalyzed
asymmetric epoxidation of olefins with NaClO as oxidant in
the biphasic system. Compared to the homogeneous catalyst,
the heterogeneous Mn(III)salen catalysts also exhibited high
activity and enantioselectivity for the disubstituted cyclic
indene and 6-cyano-2,2-dimethylchromene. Moreover, the
polymer-immobilized Mn(III)salen catalysts can be easily
recycled by simple filtration. These heterogeneous catalysts,
which were recycled at least five times for the asymmetric
ꢀ
temperature from 25 to À9 C led to a decrease of epoxide
yield from 98% to 67%. However, an increase of enantioselec-
tivity with the best ee value of 95% was observed, which was
[
17]
similar to the results reported in Yao et al.
The reason
might be an increase in enantiofacial selectivity in the initial
C-O bond-forming step and suppression of the trans path-
way in the second step at low temperature. After consider-
ꢀ
ing two aspects of the yield and ee value, 0 C was selected
Table 3. Effect of co-catalyst NH OAc on epoxidation of indene by
4
a
pHEMA–Mn(III)salen
in this work.
b
Entry
Catalyst
Mn(III)salen
pHEMA–Mn(III)salen
NH
4
OAc ee (%) Yield (%) Time (h)
Effect of Co-catalyst NH OAc
4
1
2
3
4
P
94
75
93
82
92
82
94
68
4.5
4.5
5.5
5.5
The pHEMA–Mn(III)salen catalyst was active and enantioselec-
tive for the asymmetric epoxidation of indene. In Table 3, the
indene was quantitatively converted into its epoxide with
A
P
A
7
5% ee (without NH OAc) and 93% ee (with NH OAc) catalyzed
4 4
a
Reaction conditions: indene (2 mmol), catalyst (0.2 mmol, 10 mol%),
NaClO (5.8 mmol). The reaction temperature was 0 C.
by homogeneous Mn(III)salen Cl catalyst (entries 1 and 2). In
the absence of NH OAc, the heterogeneous pHEMA–Mn(III)
salen catalyst gave 68% yield. The addition of NH OAc led to
ꢀ
4
b
4
P, NH OAc present (0.4 mmol, 20 mol%); A, absent.
4
Appl. Organometal. Chem. 2012, 26, 114–120
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

X. Guojin et al.
[
[
3] D. Disalvo, D. B. Dellinger, J. W. Gohdes, React. Funct. Polym. 2002, 53, 103.
4] P. Das, I. Kuzniarska-Biernacka, A. R. Silva, A. P. Carvalho, J. Pires,
C. Freire, J. Mol. Catal. A: Chem. 2006, 248, 135.
epoxidation of a-methylstyrene, showed stable activity and
enantioselectivity. It was speculated that the pHEMA supports
immobilized by Mn(III)salen compound probably acted as a
mediator of the reaction between the substrate and the oxidant,
and enhanced the stability of the Mn(III)salen compound. A
follow-up study to prove the speculative theory will be made.
[
5] B. M. Choudary, N. S. Chowdari, M. L. Kantam, P. L. Santhi, Catal. Lett.
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[
[
[
Acknowledgments
[
10] J. F. Larrow, E. N. Jacobsen, Y. Gao, Y. P. Hong, X. Y. Nie, C. M. Zepp,
The authors are grateful to the Chairperson Foundation of
Xi’an Jiaotong University for financial support. The authors
also gratefully acknowledge Prof. Langchong He for helpful
discussions in chiral HPLC separations.
J. Org. Chem. 1994, 59, 1939.
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wileyonlinelibrary.com/journal/aoc
Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 114–120