Ionic liquid-functionalized graphene oxide as an efficient support for the chiral salen Mn(iii) complex in asymmetric epoxidation of unfunctionalized olefins

Article abstract of DOI:10.1039/c4cy01290a

Imidazolium-based ionic liquid (IL)-functionalized graphene oxide (GO) was prepared and used to support chiral salen Mn(iii) complexes. Technologies of characterization suggested that the intact chiral complex was covalently appended on flat planes and ed

Full text of DOI:10.1039/c4cy01290a

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ARTICLE TYPE
Ionic liquid-functionalized graphene oxide as an efficient support for
chiral salen Mn(III) complex in asymmetric epoxidation of
unfunctionalized olefins
a
a
a
a
a
a
Weiguo Zheng, Rong Tan,* Shenfu Yin, Yaoyao Zhang, Guangwu Zhao, Yaju Chen, and Donghong
Yin*
a,b
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Imidazolium-based ionic liquid (IL)-functionalized graphene oxide (GO) was prepared and used to
support chiral salen Mn(III) complex. Technologies of characterization suggested that intact chiral
complex was covalently appended on flat planes and edge of GO sheet through an imidazolium-based IL
linker. The flexible layer-structure, as well as active role of IL linker in overall reaction, makes the novel
heterogeneous catalyst efficient and universal in the enantioselective epoxidation of unfunctionalized
olefins using NaClO as an oxidant. Remarkable enhancement of reaction rate with excellent conversion
1
0
(
99%) was observed over a wide range of alkenes either bulky or less (styrene, α-methylstyrene, indene,
1
5
1,2-dihydronaphthalene, 6-cyano-2,2-dimethylchromene, and 6-nitro-2,2-dimethylchromene). The
enantiomeric excess (ee) for epoxides was also encouraging (in the range of 73–93%), except for styrene
(
ee, 40%) and α-methylstyrene (ee, 44%). More importantly, the catalyst was perfectly stable and could
be reused for several times without significant loss of activity and enantioselectivity.
prepared this type of complexes by covalent linkage of polyether
chains-modified limidazolium IL moieties with chiral salen
Introduction
15
ligand at two sides of 5,5’-positions. Remarkable enhancement
of reaction rate was observed over the catalysts in the water-
dichloromethane biphasic system due to the inherent phase-
transfer capacity originated from polyether chain. More
importantly, limidazolium-based IL moiety with special
2
2
3
3
0
5
0
5
Optically active epoxides are key intermediates in organic
chemistry because they can undergo regioselective ring-opening
reactions or functional group transformations, giving rise to a
wide variety of biologically and pharmaceutically important
4
4
5
5
6
0
5
0
5
0
1, 2
compounds. Enantioselective epoxidation of olefins catalyzed
by chiral salen Mn(III) complex is one of the most efficient
‘
ionophilicity’ and polarity played positive effect on stabilizing
the chiral salen Mn(III) complex and preventing the degradation
of catalyst during the epoxidation with NaClO. Unfortunately, the
homogeneous complexes were difficult to separate from the
reaction system. Inevitable leaching loss of catalyst resulted in
gradual decrease in activity during reuse. An ideal solution to this
problem is attaching the homogeneous phase-transfer catalyst to
an insoluble support, so that the catalysts can be easily separated
from the reaction mixture by simple filtration or centrifugation.
Recently, layered multifunctional materials, such as zinc
3-10
strategies for obtaining enantiopure epoxides. Aqueous sodium
hypochlorite (NaClO) used as an oxygen source has been drawn
much attention in the transition, since it is reasonably stable,
readily available, environment friendly and inexpensive. Despite
of high yield and enantioselectivity of epoxides, long reaction
time (3-24 h) is often required due to the limited mass transfer in
the aqueous/organic biphase system, resulting in relatively lower
5, 8, 11, 12
turnover frequency (TOF) value.
Chiral salen Mn(III)
complexes with built-in phase-transfer capability have been
16
17,
phosphonate-phosphate or layered double hydroxides (LDHs),
13, 14
developed for the biphasic condition.
Recently, we have
18
have drawn considerable attention in the field of heterogeneous
catalysis due to the tailorable interlayer spaces. Apart from
facilitating separation, location of catalytic sites in the flexible
interlayer regions renders all active centres accessible to reactants
during the reaction, which in turn allows the heterogeneous
catalytic reactions to be carried out under pseudo-homogeneous
conditions. GO, a new layered material, has attracted increasing
a
*
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;
b
Technology Center, China Tobacco Hunan Industrial Corporation,
1
9-21
attention for the scientists all over the world.
The ultrahigh
NO. 426 Laodong Road, Changsha, Hunan, 410014, China.
2
-1
specific surface area (theoretical 2630 m g ), especially the
unique surface properties (multiple oxygenated functional groups
on the basal planes and edge), provides GO sheet with a great
Fax: +86-731-8872531; Tel: +86-731-8872576; E-mail:
yiyangtanrong@126.com (R. Tan) or yindh@hunnu.edu.cn (D. Yin)
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capability for loading catalytically active molecules through
Shimadzu SEM (Superscan SSX-550, Japan). X-ray diffraction
(XRD) patterns were recorded on a Philips X'PERT-Pro-MPD
diffractometer using Cu Kα radiation (λ= 1.542 Å). A continuous
22-24
covalent or non-covalent methods.
Lately, we have
2+
intercalated the Mn ions into the layered GO sheets through
2+
o
o
coordination of Mn ions with the oxygenated functional groups 60 scan mode was used to collect the 2θ from 5 to 80 . Fourier
25
5
0
5
0
5
0
5
of GO sheet. The layered GO sheet with flexible interlayer
spaces indeed favoured reagents' diffusion towards catalytic
active sites, resulting in high efficiency and good universality of
transform infrared (FT-IR) spectra were obtained as potassium
-1
bromide pellets with a resolution of 4 cm and 32 scans in the
-1
range 400–4000 cm using an AVATAR 370 Thermo Nicolet
spectrophotometer. X-ray Photoelectron Spectroscopy (XPS) data
2+
the heterogeneous Mn /GO nanocomposite in the epoxidation of
2
alkenes with aqueous H O . The promising initial work 65 were obtained with an ESCALab220i-XL electron spectrometer
2
1
1
2
2
3
3
encouraged us to employ the GO sheet as an efficient support for
chiral salen Mn(III) complex to provide an efficient and reusable
catalyst for the asymmetric epoxidation of olefins with aqueous
NaClO. Considering the positive effect of limidazolium-based IL
from VG Scientific using 300 W Al Kα radiation. The base
-7
pressure was about 3×10 Pa. Ultraviolet–visible light (UV-vis)
spectra were recorded on UV-vis Agilent 8453
spectrophotometer. The samples were fully dispersed in
a
on stabling and activating chiral salen Mn(III) complex, We thus 70 dichloromethane under sonication. The suspension was poured
decided to utilize an imidazolium-based IL linker to covalently
attach the chiral salen Mn(III) complex to GO sheet. The IL
linker with specific miscibility was also expected to fine tune the
surface properties of the GO sheet, improving compatibility of
the heterogeneous catalyst in the aqueous/organic biphase 75 were heated from room temperature up to 900 C with 10 K/min
reaction.
Herein, imidazole-modified GO sheet was synthesized and
used to immobilize the chiral salen Mn(III) complex through the
N-alkylation of imidazole groups on GO sheet with the methyl
2
chloride group (-CH Cl) at 5 position of chiral salen ligand. 80 layer chromatography (TLC) was conducted on glass plates
Chiral salen Mn(III) complex was thus appended covalently on
the flat planes and edge of GO sheet through an imidazolium-
based IL linker. It has proved that the novel catalyst was efficient
and universal in the enantioselective epoxidation of
unfunctionalized olefins with NaClO. Remarkable enhancement 85 capillary column (HP19091G-B213, 30 m×0.32 mm×0.25 μm).
of reaction rate with high TOF values was observed over a wide
range of alkenes either bulky or less. Furthermore, the
heterogeneous catalyst can be quantitatively recovered from the
reaction mixture by centrifugation for steady reuse. Thus, the
problems associated with the mass transfer and separation of
catalyst in aqueous/organic biphase reactions can be well solved.
into a 1 cm quartz cell for UV-vis absorption with
dichloromethane as the reference. Termogravimetric and
differential thermogravimetric (TG-DTG) curves were obtained
on a NETZSCH STA 449C thermal analyzer. Samples (ca. 0.01 g)
o
under air flow using alumina sample holders. Elemental analyses
were carried out on Vario EL III Elemental analyses made in
1
Germany. H NMR spectra of samples were recorded on a
Varian-500 spectrometer with TMS as an internal standard. Thin
coated with silica gel GF254. Mn ion content was measured by the
method of complexometry with ethylenediamine tetraacetic acid
30
(EDTA). The conversion and ee values were measured by a
6890N gas chromatograph (Agilent Co.) equipped with a chiral
Preparation of asymmetric chiral salen Mn(III) complex
Asymmetric chiral salen Mn(III) complex of (R,R)-N-(3,5-di-
tert-butylsalicylidene)-N’-[3-tert-butyl-5-chloromethylsalicylide-
ne]-1,2-cyclohexanediaminato] manganese(III) chloride (as
31
90 shown in Chart 1) was synthesized as described earlier.
Experimental
N
N
N
N
Mn
Cl
Mn
Cl
t-Bu
O
O
t-Bu
2
ClH C
O
O
t-Bu
Materials and reagents
t-Bu
t-Bu
t-Bu
t-Bu
(
±)-1,2-diaminocyclohexane
and
high-purity
graphite
neat complex
asymmetric chiral salen Mn(III) complex
(
99.9999%, 200 mesh) were purchased from Alfa Aesar. Indene
Chart 1 The structures of neat and asymmetric chiral salen Mn(III)
4
4
5
0
5
0
and 1,2-dihydronaphthalene were obtained by TCI. Pyridine-N-
oxide (PyNO) was bought from Aldrich. Other commercially
available chemicals were laboratory grade reagents from local 95 Preparation of catalyst 1
complexes.
suppliers. All of the solvents were purified by standard
26
The preparation of IL-functionalized GO supported chiral salen
Mn(III) complex (denoted as catalyst 1) was outlined in Scheme
procedures. Styrene and indene were passed through a pad of
neutral alumina before use. 6-cyano-2,2-dimethylchromene and
1
.
6
-nitro-2,2-dimethylchromene were synthesized according to the
27
literature procedures. GO was prepared by the oxidation of
high-purity graphite powder with H SO /KMnO according to the 100 stirred in oxalyl chloride (100 mL) at 80 C for 24 h to activate
method of Hummers and Offeman.
chloromethyl-2-hydroxybenzaldehyde were prepared according
Synthesis of imidazole-modified GO sheet: GO (0.25 g) was
o
2
4
4
28
3-tert-Butyl-5-
the carboxylic units by forming the corresponding acyl chlorides.
After removing excess oxalyl chloride, the mixture was washed
with anhydrous THF and dried at room temperature under
vacuum to afford the acyl chlorinated GO as a brown solid. The
05 acyl chlorinated GO (0.15 g) was treated with 1-(3-
aminopropyl)imidazole (50 mL) at 80 C for 18 h under N
29
to the described procedures. [(R,R’)-(N,N’-bis(3,5-di-tert-
butylsalicylidene)- 1,2-cyclohexanediaminato] manganese(III)
chloride (denoted as neat complex, as shown in Fig. 1) was
1
3
o
prepared according to the described procedures.
2
. The
reaction mixture was filtrated through 0.2μm PTFE filter. The
solid residue was then washed with dichloromethane and diethyl
5
5
Methods
Surface structure of the samples was measured using a
2
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-1
ether to remove the unreacted 1-(3-aminopropyl)imidazole. The
sheets as brown solid. FT-IR (KBr): νmax/cm 3430, 3162, 2920,
imidazole-modified GO sheets were obtained as light brown solid. 35 2810, 2363, 1650, 1556, 1224, 1151, 1110, 960, 871, 588, 457.
-
1
FT-IR (KBr): νmax/cm 3329, 2881, 1658, 1556, 1224, 1110, 873,
47, 662, 620. The imidazole content in the modified GO sheet is
The content of amino group in the modified GO sheet is 0.73
mmol/g, which was determined according to the content of
nitrogen element analyzed by elemental analyses.
7
5
0.56 mmol/g, which was determined according to the content of
nitrogen element analyzed by elemental analyses.
Synthesis of catalyst 2: The amino-modified GO sheet (0.5 g)
40 was added to a solution of the asymmetric chiral salen Mn(III)
complex (0.63 g, 1 mmol) in dry toluene (100 mL). The resulting
suspension was refluxed for 24 h under nitrogen protection. After
being filtrated and washed with dry toluene and diethyl ether, the
black crude product was extracted thoroughly in dichloromethane
45 using a Soxlet apparatus. The residue was dried at room
temperature under vacuum for 24 h to give the catalyst 2. FT-IR
O
HOOC
C
Cl
O
C
O
H2N
N N
O
Cl
C
Cl
O
HO
HO
80 oC, 18 h, N2
OH
OH 80 oC, 24 h
O
O
O
COOH
C
N
N
Cl
N
N
NH
C
-1
O
(KBr): νmax/cm 3430, 3266, 3008, 2951, 2867, 1658, 1610, 1524,
N
O
N
O
NH
Mn
Cl
OH
ClH2C
t-Bu
1389, 1224, 1164, 1036, 985, 569, 417; UV-vis (CH
2 2
Cl ):
HO
OH
λ
max/nm 331, 420, 502; Mn ion content: 0.61 mmol/g.
HN
O
HO
Toluene, N , reflux, 24 h
N
O
C
HN
2
HOOC
C
N
Cl
imidazole-modified GO sheets
O
C
O
C
NH2
H2N
N
N
O
Cl
Cl
O
HO
80 oC, 24 h
HO
80 oC, 18 h, N2
OH
OH
N
O
N
O
O
Mn
H2
C
t-Bu
O
Cl-
N
N
O
Cl
N
Mn
O
C
Cl
N
H2C
N
O
COOH
t-Bu
H2N
Cl-
Cl
NH
C
NH
C
N
NH2
O
O
N
N
NH
OH
NH
OH
Mn
ClH2C
O
O
t-Bu
Cl
HO
OH
HO
OH
HN
HO
HN
N
O
N
Mn
N
O
HO
Toluene, N2, reflux, 24 h
t-Bu
O
N
C
H2N
O
C
Cl-
C
Cl
H2
HN
HN
Cl-
amino-modified GO sheets
NH2
N
N
H2C
Catalyst 1
N
N
Mn
O
O
Cl
t-Bu
N
N
N
N
O
Mn
H
2
C
Mn
t-Bu
O
O
CH2
Cl
O
t-Bu
N
H
HN
NH
Cl
H
N
Scheme 1 Synthesis of catalyst 1.
C
O
O
H
OH
HO
1
1
2
0
5
0
Synthesis of catalyst 1: The imidazole-modified GO sheet
0.5 g) was mixed with the asymmetric chiral salen Mn(III)
HN
HO
O
(
N
N
O
C
Mn
H2
C
N
H
N
N
O
t-Bu
O
NH
Cl
H
N
Mn
complex (0.63 g, 1 mmol) in toluene (100 mL). After refluxing
for 24 h under nitrogen protection, the black slurry was filtrated
and washed with dry toluene and diethyl ether. The obtained solid
was extracted thoroughly in dichloromethane using a Soxlet
apparatus, and then dried under vacuum at room temperature to
C
O
t-Bu
H2
Cl
Catalyst 2
5
0
Scheme 2 Synthesis of catalyst 2.
Preparation of IL-complex
-1
give the catalyst 1. FT-IR (KBr): νmax/cm 3330, 2950, 2910,
The other control catalyst of GO-free counterpart (denoted as
IL-complex), in which imidazolium-based IL was covalently
bonded on chiral salen Mn(III) complex at 5-position, was also
prepared through N-alkylation of 1-methylimidazole with the
2
4
861, 1658, 1614, 1556, 1224, 1110, 837, 746, 660, 619, 571,
18; UV-vis (CH Cl ): λmax/nm 322, 401, 498; Mn ion content:
5
5
2
2
0.47 mmol/g.
Preparation of catalyst 2
2
benzyl chloride group (–CH Cl) of asymmetric chiral salen
Mn(III) complex. The structure of IL-complex was shown in
Chart 2.
For comparison, the catalyst of IL-free counterpart (denoted as
the catalyst 2), in which chiral salen Mn(III) complex was
directly anchored to the flat planes and edge of GO sheet through
an alkyl linker, was also prepared according to process outlined
in Scheme 2.
6
0
N
N
2
5
Mn
Cl
Cl-
O
O
t-Bu
H3C
N
N
Synthesis of amino-modified GO sheet: Acyl chlorinated GO
t-Bu
t-Bu
sheet (0.15 g) were dispersed in ethylenediamine (50 mL) at 80
C for 18 h under N . The obtained slurry was filtrated through
2
Chart 2 The structures of IL-complex.
o
3
0
0.2μm PTFE filter, and then successively washed with
dichloromethane (5×20 mL) and diethyl ether (5×20 mL) to
remove the unreacted ethylenediamine. The residue was dried at
room temperature under vacuum to give the amino-modified GO
Catalyst testing
6
5
The selected catalyst (4 mol% on substrate, based on Mn
content in catalyst), unfunctionalized alkene (1 mmol) and PyNO
(
2 mmol, 0.19 g) were added into dichloromethane (2 mL) under
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o
stirring at 0 C. Buffered NaClO as an oxidant (2 mmol, pH=11.5)
GO sheet was activated upon treatment with oxalyl chloride and
further reacted with an excess of 1-(3-aminopropyl)imidazole to
conjugate imidazole units at the edge of GO sheet through amide
was then added in four equal portions. The progress of the
epoxidation reaction was monitored by gas chromatography.
After the reaction, the heterogeneous catalyst was recovered by 60 bond (O=C-N). Moreover, the chemically reactive epoxy groups
5
0
5
0
5
0
centrifugation, washed with dichloromethane, and successively
reused for subsequent epoxidation. The reaction solution was
separated by separatory funnel. The aqueous phase was extracted
with dichloromethane for several times. The extract was
on the basal planes of GO can also undergo nucleophilic ring-
opening reaction with the attack of amine group in 1-(3-
aminopropyl)imidazole. N-alkylimidazole group was thus
decorating the edge and flat planes of GO sheet, forming the
combined with organic phase, and dried over anhydrous sodium 65 imidazole-modified GO sheet. Successive N-alkylation of the N-
1
1
2
2
3
sulfate. Further purification of the dichloromethane phase by
flash column chromatography afforded the epoxides. The
conversion and ee value were measured by a 6890N gas
chromatograph (Agilent Co.) equipped with the chiral capillary
alkylimidazole group on GO sheet with the benzyl chloride group
(–CH Cl) at 5-posotion of asymmetric chiral salen Mn(III)
2
complex resulted in the covalently appending chiral salen Mn(III)
complex on the edge and basal planes of GO sheet through
column (HP19091G-B213, 30 m×0.32 mm×0.25 μm) and the 70 imidazolium-based IL linker. Removal of the unanchored
FID detector. Nitrogen was used as the carrier gas with a flow of
homogeneous species by Soxhlet extraction gave catalyst 1 as
black powder.
For comparison, the IL-free counterpart of catalyst 2, in which
chiral salen Mn(III) complex was directly anchored to GO sheet
-1
3
0 mL min . The retention times of the corresponding chiral
epoxides are as follows: (a) styrene epoxide: the column
o
temperature is 90 C, t
methylstyrene epoxide: the column temperature is 80 C, t
min, t =16.5 min; (c) indene epoxide: the column temperature
R S
=15.2 min, t =15.7 min; (b) α-
o
S
=16.3 75 through alkyl linker, was also prepared according to a modified
31
R
preparation procedure. The synthesis route was outlined in
Scheme 2. During the procedure, ethylenediamine was used to
modify the surface of GO sheet through the amidation of
carboxylic acid group and nucleophilic ring-opening reaction of
o
o
-1
was programmed from 80 to 180 C with 8 C min , tSR=11.5
min, tRS=11.9 min; (d) 1,2-dihydronaphthalene epoxide: the
column temperature was programmed from 80 to 180 C with 6
o
o
-1
C
min ,
t
SR=13.4 min,
t
RS=13.6 min; (e) 6-cyano-2,2’- 80 epoxy group. The decorative amino groups (–NH
edge and flat planes of GO sheet were readily alkylated with the
benzyl chloride group (–CH Cl) at 5-position of asymmetric
2
) located at the
dimethylchromene epoxide: the column temperature was
programmed from 80 to 200 C with 4 C min , tSS=24.0 min,
RR=24.3 min; (f) 6-nitro-2,2’-dimethylchromene epoxide: the
column temperature was programmed from 80 to 200 C with 4
C min and retained at 200 C for 5 min, tSS=30.3 min, tRR=30.8 85 Removal of the unanchored homogeneous species by Soxhlet
o
o
-1
2
t
chiral salen Mn(III) complex, resulting in covalently appending
chiral salen Mn(III) complex on GO sheet through alkyl chain.
o
o
-1
o
min.
extraction gave catalyst 2 as black powder.
Interestingly, catalyst 1 exhibits better dispersion than the IL-
free counterpart of catalyst 2 in both water (Fig. 1A) and
dichloromethane (Fig. 1B). Stable suspension is observed over
the catalyst 1 (Fig. 1A-b and Fig. 1B-b), suggesting better
compatibility of heterogeneous catalyst 1 in the aqueous/organic
biphase reaction. We thus suspected that high dispersion was
related to the special solubility of the imidazolium-based IL
Results and discussion
Preparation and characterization of catalysts
9
0
GO with unique layered structure, as well as the ultrahigh
specific surface area, is an excellent candidate for catalyst support.
In particular, the abundant reactive oxygen functional groups on
the surface provide GO sheet with large capability of loading
3
4
4
5
5
5
0
5
0
5
modifier. The high dispersion of catalyst
1 with easy
catalytic molecules through covalent or non-covalent approaches. 95 accessibility of active sites enables the heterogeneous catalyst to
In general, GO sheet requires further modification to confer high
dispersion in a range of media for catalytic reactions.
modifiers in support systems, can fine tune the surface properties
of support and transfer the properties of ILs to materials,
which are expected to improve the compatibility of the
heterogeneous catalyst in aqueous/organic biphase reaction.
Furthermore, ILs derived from N,N-dialkylimidazolium were
found to activate and stabilize the chiral salen Mn(III) complex
behave as pseudo-homogeneous catalyst in aqueous/organic
biphasic reaction.
3
2, 33
ILs, as
34, 35
Fig. 1 Photographs of neat complex (a), catalyst 1 (b) and catalyst 2 (c)
3
6, 37
during the process of the epoxidation.
Thus, we decided to
1
00
in water (A) and in dichloromethane (B).
covalently immobilize chiral salen Mn(III) complex on
imidazolium-based IL-functionalized GO sheet to produce an
efficient and reusable catalyst for asymmetric epoxidation of
unfunctionalized alkenes with NaClO. Given that single point
attachment fashion with a flexible spacer group minimizes local
Characterization of Samples
XRD
XRD patterns are used to study the change of GO in structure
steric restriction arisen from support, a strategy that we 105 during the loading process. Fig. 2 shows powder XRD patterns of
38
designed here is to covalently append chiral salen Mn(III)
complex onto GO sheet through an imidazolium-based IL linker
at 5-position of salen unit.
pristine GO material, catalysts 1 and 2. GO displays a strong (002)
o
peak centered at 10.5 , suggesting the layered structure with an
average interlayer distances of ca. 0.85 nm (Fig. 2a). While, the
sharp diffraction peak disappears, accompanied by the
The preparation of catalyst 1 is shown in Scheme 1. The layered
4
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Catalysis Science & Technology
o
appearance of a new weak peak at 2θ = 25.98 , after the
organometallic moieties, and GO sheet backbone doesn’t take
introduction of bulky imidazolium IL-tagged chiral salen Mn(III) 45 part in the grafting.
complex moiety (Fig. 2b). A reasonable explanation for these
observations might be that regularly aggregated GO sheet are
3
9
5
0
5
exfoliated to single or fewer layer during the modification. It is
well known that individual GO platelets are interlinked via a
network of hydrogen bonds mediated by oxygen-containing
functional groups, as well as the van der Waals forces between
40
layers. Intercalation of bulky imidazolium IL-tagged chiral
complex into the interlayer spacing may weaken the interlinked
driven forces between GO layers, enabling facile exfoliation of
the layered GO material. Delamination of the nanosheet is also
observed in catalyst 2, since it displays the XRD pattern similar
to catalyst 1 (Fig. 2c vs b). The featured structure of exfoliated
nanosheet makes all active sites accessible, which thus allows the
heterogeneous catalyst to behave as pseudo-homogeneous
catalyst in the reaction.
Fig. 3 SEM images of GO (A), catalyst 1 (B) and catalyst 2 (C).
1
FT-IR
50
The anchoring behavior of chiral salen Mn(III) complex on GO
sheet was investigated by FT-IR analysis in detail. Fig. 4 shows
the FT-IR spectra of catalysts 1 and 2, as well as pristine GO
sheet and neat chiral salen Mn(III) complex for comparison.
Pristine GO shows the skeletal vibrations of non oxidized
1
-1 41, 42
5
6
6
7
7
8
8
9
5
0
5
0
5
0
5
0
graphitic domains at 1621 cm
, as well as the characteristic
vibration bands of oxygen-containing groups at round 3300, 1735,
-1
1
224, and 1050 cm in the FT-IR spectrum, which are associated
2
5.98
with the stretching vibration modes of COO-H/O-H, carboxylic
a
C=O, C-OH, and epoxy C-O groups presented on the surface of
b
c
24, 43, 44
GO, respectively (Fig. 4a).
Treatment of pristine GO with
1
-(3-aminopropyl)imidazole results in the changes of carboxylic
1
0
20 30 40 50 60 70 80
-theat, degrees
and epoxy groups, as shown in the FT-IR spectrum of imidazole-
modified GO (Fig. 4b). After imidazole modification, the band
corresponding to carboxylic C=O group disappears with the
2
a
b
c
-1
formation of a new band at 1658 cm , which is the characteristic
stretching vibration of C=O in amide (-CONH-) group (Fig. 4b vs
10
20
30
40
50
60
70
80
a).
It
provides
a
convincing
proof
that
1-(3-
2
-theat, degrees
aminopropyl)imidazole moiety gets anchored on the edge of the
GO sheet through an amidation process. Furthermore, the
Fig. 2 XRD patterns of GO (a), catalyst 1 (b) and catalyst 2 (c) (The
inset is the magnified XRD patterns of GO (a), catalyst 1 (b) and catalyst
-1
2
0
characteristic band at 1050 cm , assigned to the C-O stretching
vibration of the epoxy group, also almost disappears in the FT-IR
spectrum of imidazole-modified GO. This could be interpreted as
evidence that epoxy groups interact with amine groups during the
2
(c)).
SEM
modification.
Actually,
amine
group
in
1-(3-
SEM images provide direct information on microstructure and
morphology of pristine GO sheet, catalysts 1 and 2, as shown in
Fig. 3. Obviously, pure GO support has a typical layered structure
in the SEM image (Fig. 3A). GO nanosheets agglomerate with
each other through a network of hydrogen bonds and van der
aminopropyl)imidazole can induce the nucleophilic ring-opening
2
3
3
4
5
0
5
0
45, 46
reaction of epoxide,
which makes the successful
functionalization of N-alkyl imidazole groups on the flat planes
of GO sheet. Additional new characteristic bands at 1556 and
-1
40
1110 cm associated with the ring stretching of imidazole
Waals forces between layers. While, the interlayer interactions
of GO nanosheets are disturbed by the introduction of IL-tagged
chiral salen Mn(III) complex moiety. Exfoliated or at least
loosened lamellar structure is observed in the SEM image of
catalyst 1, as shown in Fig. 3B. It is logical to deduce that the IL-
tagged chiral salen Mn(III) complex are not only decorating the
edge of GO sheet, but also intercalated into the interlayer spacing.
Bulky organometallic moieties on the flat planes of GO sheet
prevent the restacking of the nanosheet, thus providing accessible
void for reagents. Catalyst 2, in which chiral salen Mn(III)
complex is direct anchored on the surface of GO sheet through a
alkyl chain, also exhibits intercalated and exfoliated morphology
in SEM image (Fig. 3C). Notably, the basal nanosheet of GO
support is intact in catalysts 1 and 2. It means that chemically
reactive oxygen moieties provide the anchoring sites for the
47
moiety further confirm the presence of imidazole moiety on the
planes and edge of GO sheet (Fig. 4b). It is imidazole modifier
that is responsible for the covalent anchoring chiral salen Mn(III)
complex on GO sheet by forming an imidazolium-based IL linker,
as shown in Scheme 1. Grafting the complex onto the imidazole-
modified GO sheet gives new characteristic bands at around 1614,
-1
5
71 and 418 cm , which are assigned to the stretching vibrations
of C=N, Mn–O and Mn–N in the chiral salen Mn(III) framework
15
(
Fig. 4c). Obviously, the structure of the complex is maintained
in the immobilized state. Notably, the characteristic vibrations of
C=N, Mn–O and Mn–N slightly shift from 1608, 565 and 416
-
1
-1
cm to 1614, 571 and 418 cm , respectively, after grafting,
compared with those of the neat complex (Fig. 4c vs e). The
observations indicate that chiral salen Mn(III) complex is
covalently anchored on GO sheet through the imidazolium-based
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Page 6 of 10
IL linker. Election-deficient effect of the imidazolium cation
results in red shift of the characteristic bands.
The FT-IR spectrum of catalyst 2 is similar to that of catalyst
the amidation process at edge of GO. Furthermore, the
modification also results in a significant decrease in the intensity
of C-O-C peak (from 58.5% to 16.5%) accompanied by the
increase of C-OH group (from 12% to 68%). It demonstrates the
15, 48
1
(Fig. 4d vs. c), except for the absence of the characteristic
−1
5
0
5
stretching vibration of imidazole ring (at 1556 and 1110 cm ). 50 nucleophilic ring-opening reaction of epoxide group with the
The difference is related to the absence of imidazolium-based IL
moiety in the framework of catalyst 2. Furthermore, the
characteristic band derived from the stretching vibration mode of
C=O in –CONH– group (at around 1658 cm ) in the FT-IR
spectrum of catalyst 2 provides an evidence that chiral salen 55 survey (Fig. 5a) and high-resolution C1s spectra (Fig. 5b), as
amino group in 1-(3-aminopropyl)imidazole during the
modification. The results indicate that the edge and flat planes of
GO sheet are modified by N-alkylimidazole.
−1
Amino-modified GO shows similar tendency in the XPS
1
Mn(III) complex is directly anchored onto amino-functionalized
GO sheet by forming amide group (Fig. 4d), as shown in Scheme
compared with imidazole-modified GO sheet. It confirms the
successful modification of N-alkyl chain on the edge and flat
planes of the GO sheet through an amidation process and
nucleophilic ring-opening reaction, respectively, as shown in
2
. Notably, compared with catalyst 2, a significant feature
observed for catalyst 1 is the presence of imidazolium-based IL
linker that positively affects catalytic performance of catalyst 1 60 Scheme 2. Furthermore, we noticed that an additional Cl 2p peak
in the aqueous/organic biphasic epoxidation.
1
50
at 198.8 eV is obvious in the XPS survey spectrum of amino-
modified GO. The Cl 2p peak should be probably due to that the
hydrogen chloride arisen from amidation process was captured by
terminal amino groups on GO sheet. It provides an indirect
evidence for the presence of terminal amino groups on GO sheet,
as well as the modification through an amidation process.
a
1
735
1621
3
300
1050
1110
1
224
6
5
1
556
1
658
b
c
d
1224
1
556
GO
1
658
571
GO
(b)
(a)
O1s
418
C-C/C=C(284.6eV)
1
614
1110
C-O-C(286.7eV)
1
224
C1s
1
658
417
C-OH(285.7eV)
O-C=O(289.4eV)
5
69
1
610
1224
e
1
608
416
500
5
65
C-C/C=C(284.6eV)
C-N(285.4 eV)
imidazole-modifiedGO
imidazole-modifiedGO
4
000
3500
3000
2500
2000
1500
1000
-
1
Wavenumber (cm )
C1s
C-OH(285.7eV)
Fig. 4 FT-IR spectra of pristine GO sheet (a), imidozale-modified GO (b),
catalyst 1 (c), catalyst 2 (d) and neat complex (e).
C-O-C(286.7eV)
N-C=O(287.8eV)
O1s
N1s
2
0
XPS
XPS was employed to gain further insights into surface
chemical changes associated with the reaction shown in Scheme
amino-modifiedGO C-C/C=C(284.6eV)
amino-modifiedGO
C-OH(285.7eV)
O1s
C-O-C(286.7eV)
C1s
1
and 2. Fig. 5 shows the XPS of pristine GO, imidazole-
N1s
2
3
3
4
4
5
0
5
0
5
modified GO sheet and amino-modified GO sheet. As expected,
only C1s and O1s peaks are seen in the XPS survey spectrum for
pristine GO (Fig. 5a). A new N1s (399.5 eV) peak appears,
accompanied by the reduced O1s peak with respect to the C1s
peak, in the XPS survey spectrum of imidazole-modified GO
sheet (Fig. 5a). It suggests that GO was modified by imidazole
group through reaction with oxygenated functional groups. The
deduction also can be drawn from the high-resolution C1s XPS
spectra of GO and imidazole-modified GO sheet (Fig. 5b). Four
typical peaks associated with C-C/C=C (284.6 eV), C-OH (285.7
eV), C-O-C (286.7 eV) and O=C-O (289.4 eV) groups are
C-N(285.4 eV)
N-C=O(287.8eV)
Cl 2p
100 200 300 400 500 600 700 800 900 280
282
284
286
288
290
292
294
BindingEnergy(ev)
BindingEnergy(ev)
Fig. 5 XPS survey spectra of GO sheet, imidozale-modified GO and
amino-modified GO (a) and C1s XPS spectra of GO sheet, imidozale-
modified GO and amino-modified GO (b).
7
0
UV-vis
Fig. 6 shows UV-vis spectra of catalysts 1, 2 and neat chiral
salen Mn(III) complex. It gives more obvious evidence for the
49
obvious in the high-resolution C1s XPS spectra of GO.
Deconvolution of the peak area evaluates the amount of 75 successful grafting, since the characteristic peaks of neat complex
oxygenated functional groups on GO sheets. It shows that there
are 12% of the carbon atoms associated with C–OH, 58.5%
associated with C-O-C, and 20.5% associated with O=C-O on
pristine GO. While, the O=C-O peak for GO weakens obviously
at around 433 and 506 nm show blue shifts after the complex
being grafted. The peak at 433 nm is due to the charge-transfer
transition from metal to salen ligand, and the peak at 506 nm is
15
attributed to d-d transition of complex. As for catalyst 1, the
(
from 20.5% to 7%) upon the imidazole modification. The 80 characteristic peaks associated with metal-to-ligand charge-
reduction of O=C-O group together with the newly formed peaks
associated with O=C-N (287.8 eV, 7.5%) and C-N (285.4 eV)
in C1s XPS spectrum of imidazole-modified GO sheet indicates
transfer transition and d-d transition of complex are significant
shifted to 401 and 498 nm, respectively (Fig. 6b vs a). The
behaviour is mainly attributed to the election-deficient effect of
49
6
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Catalysis Science & Technology
imidazolium cation at 5-position of the chiral salen ligand in the
framework of catalyst 1. Blue shifts of the characteristic peaks
from 433 and 506 nm to 420 and 502 nm, respectively (Fig. 6c vs
15%) is assigned to the cleavage of the tert-butyl groups in salen
ligand, since neat complex shows the similar major weight loss in
the temperature range (Fig. 7A vs. C). Successive pyrolysis of the
a), are also observed in the UV-vis spectrum of catalyst 2, 35 remaining ligand moiety occurs in the range of 368-596 °C,
5
indicating the covalently grafting chiral salen Mn(III) complex on
amino-modified GO sheet.
which is overlapped with the oxidative decomposition of the GO
residue, giving a major weight loss (ca. 32%). The last weight
loss at 702 C (ca. 6.6%), which is absent in the combined TG-
51
o
DTG curves of neat complex and pristine GO sheet (Fig. 7A vs.
B and C), is logical to assign to the complete decomposition of
imidazolium IL moiety. The decomposition extends up to ca. 761
4
0
4
33
o
5
06
C, leaving non-removable residue (ca. 17%) belonged to the
4
01
a
formation of manganese oxide in air at high temperature. The
results provide an indirect proof for the covalent linkage of chiral
45 salen Mn(III) complex on GO sheet through an imidazolium-
based IL linker.
4
98
b
Catalytic performances
4
20
b'
c
Although heterogeneous, IL-tagged catalyst 1 shows good
5
02
5
5
6
6
7
7
8
8
0
5
0
5
0
5
0
5
compatibility in both aqueous and organic solvents. We thus
proposed that the heterogeneous catalyst 1 should behave as a
pseudo-homogeneous catalyst in aqueous/organic biphase system.
The catalytic performance of catalyst 1 in the enantioselective
3
50
400
450
500
550
Wavelength (nm)
Fig. 6 UV-vis spectra of neat complex (a), catalyst 1 (b), the recovered
catalyst 1 after the 7th reuse (b’), and catalyst 2 (c).
o
epoxidation of different substrates at 0 C, using aqueous NaClO
1
0
as oxygen source in dichloromethane, are summarised in Table 1.
Notably, the IL-tagged catalyst 1 significantly accelerated the
aqueous/organic biphasic epoxidation. Using aqueous NaClO as
oxygen source, the novel catalyst gave quantitative conversion of
substrate (99%) with 40% ee value in the asymmetric epoxidation
Thermal analysis
Thermal analysis had been used to monitor the decomposition
profiles of catalyst 1, as well as the neat complex and pristine
GO sheet for comparison. The results obtained are depicted in Fig.
7.
o
of styrene at 0 C within 60 min (Table 1, entry 1). By contrast,
1
5
only 70% conversion with 38% ee value was observed over neat
complex under identical conditions (Table 1, entry 2). The
remarkable enhancement of reaction rate with the employment of
the catalyst 1 might be due to the positive effect of the
imidazolium-based IL linker and/or the synergistic interaction
C
A
B
100
100
100
80
60
40
20
0
b
b
80
60
40
20
0
80
b
18, 52
between active species and the layered GO support.
60
To further elucidate the function of the imidazolium-based IL
moiety in the water/ dichloromethane biphasic reaction, a IL-free
counterpart (catalyst 2) was prepared as a control catalyst by
directly appending the chiral salen Mn(III) complex onto amino-
modified GO sheet. Interestingly, the heterogeneous catalyst 2
offered a slightly higher TOF value than the neat complex (Table
40
20
a
a
a
1
, entry 3 vs 2, entry 7 vs 6, entry 10 vs 9, entry 13 vs 12, entry
0
0
200 400 600 800
0
200 400 600 800
0
200 400 600 800
16 vs 15, and entry 19 vs 18). The observation clearly indicates
that the layered GO support with flexible interlayer space enables
the heterogeneous catalyst to behave as a pseudo-homogeneous
catalyst in the epoxidation. But it does not have a great effect on
boosting the reaction. Slightly higher efficiency over the catalyst
o
o
o
Temperature ( C)
Temperature ( C)
Temperature ( C)
Fig. 7 TG–DTG curves of catalyst 1 (A), pristine GO sheet (B) and neat
complex (C) ((a) thermogravimetric curves; (b) differential
thermogravimetric curves.
2
0
2
is probably due to the improved mass transfer originating from
the hydrophobic alkyl linker. The other control catalyst of GO-
free counterpart (IL-complex) was also employed to confirm the
little effect of GO sheets on enhancing catalytic efficiency.
Despite of similar layered structure, the IL-tagged catalyst 1
was far more efficient than catalyst 2 under identical reaction
conditions (Table 1, entry 1 vs 3). It is logical to deduce that
imidazolium-based IL moiety plays a critical role on accelerating
the aqueous/organic biphasic reaction. The proximity of IL unit to
the active site renders catalytic sites more accessible, thereby
tuning catalyst 1 into an efficient catalyst for this reaction.
Catalyst 1 shows five distinct steps of weight losses in
combined TG-DTG curves upon heating from room temperature
to 900 °C under airflow (Fig. 7A). The first weight loss at 90 °C
is due to the removal of surface-adsorbed water. The second
weight loss at 249 °C (ca. 9.1%) accounts for the pyrolysis of
oxygenated functional groups on the GO surface. Interestingly,
the decomposition temperature of the oxygen functional groups in
catalyst 1 increases compared with that of pristine GO sheet (at
200 °C) (Fig. 7A vs. B), suggesting the mutual stabilization of
GO sheet and the IL moieties. The third weight loss at 325 °C (ca.
2
5
49
3
0
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Page 8 of 10
Furthermore, the high polarity and ionic character of the IL 25 2.35, 1.81, and 1.60 M/min, respectively. The order of reaction
V
moiety exert synergistic effect on stabilizing the active Mn -
salen)-oxo intermediate in the asymmetric epoxidation of
rates over the all tested catalysts is thus as follows: catalyst 1 >
IL-complex > catalyst 2 > neat complex, viewing from the
kinetics point. The results demonstrated that flexible layered
structure of GO sheets allowed the heterogeneous catalysts 1 and
30 2 to behave as pseudo-homogeneous catalysts in the asymmetric
epoxidation. Remarkable enhancement of reaction rate with the
employment of catalyst 1 should be due to the synergistic
(
unfunctionalized olefins, which in turn enhances the catalytic
efficiency of chiral salen Mn(III) catalyst.
5
Table 1 The results of enantioselective epoxidation of unfunctionalized
a
olefins over different chiral salen Mn(III) complexes
b
b
c
d
Conv. ee Yield
(
TOF
-3 -1
Enty
Catalyst
Substrates
Products
%) (%) (%) ×10 (s ) interaction between active species and layered GO support, as
well as the positive effect of imidazolium-based IL linker.
1
2
3
4
5
6
7
Catalyst 1
Neat complex
Catalyst 2
99
40
38
36
39
44
92
64
68
92
91
6.38
4.44
4.72
6.26
6.32
5.21
5.28
H
O
70
76
98
99
100
a
b
(R)
80
60
40
c
IL-complex
Catalyst 1
d
H
O
83
84
99
76
43
42
73
72
75
76
95
68
Neat complex
Catalyst 2
(
R)
20
0
8
9
Catalyst 1
Neat complex
Catalyst 2
6.60
4.72
4.86
H
O
10
20
30
40
50
60
H
Rection time (min)
3
4
5
0
(
1R, 2S)
Fig. 8 The conversion vs. reaction time plot for asymmetric epoxidation
of styrene catalyzed by catalyst 1 (a), IL-complex (b), catalyst 2 (c) and
neat complex (d).
1
0
78
99
66
67
99
58
61
99
57
63
72
84
80
80
92
94
95
93
95
95
70
93
60
61
95
53
58
94
55
59
1
1
1
1
1
1
1
1
1
1
2
3
4
5
6
7
8
9
Catalyst 1
Neat complex
Catalyst 2
6.46
4.17
4.24
6.60
H
O
H
The positive effect of imidazolium-based IL linker on
accelerating the aqueous/organic biphasic epoxidation was also
observed in the case of the α-methylstyrene, indene, 1,2-
dihydronaphthalene, 6-cyano-2,2-dimethylchromene, and 6-nitro-
(
1R, 2S)
Catalyst 1
O
2
,2-dimethylchromene, as shown by TOF values in Table 1
O
NC
H
3.68 45 (Table 1, entries 5, 8, 11, 14, and 17). Notably high TOF values
Neat complex NC
Catalyst 2
H
O
for catalyst 1 were obtained with all the substrates either bulky or
less, suggesting the flexible interlayer spaces of catalyst 1 that
avoided the diffusion constraint. The TOF values were significant
higher than those of catalyst 2 in the corresponding epoxidations
(
3R,4R)
4.03
Catalyst 1
6.53
O
O
O2N
H
Neat complex O2N
O
3.82
4.10
5
0
(Table 1, entry 1 vs. 3, entry 5 vs. 7, entry 8 vs. 10, entry 11 vs.
13, entry 14 vs. 16, entry 17 vs. 19). Excellent conversions (99%)
were obtained with all the alkenes used in this work but the
highest asymmetric induction is observed for the electron-
deficient 6-cyano-2,2-dimethylchromene (ee, 92%) and 6-nitro-
55 2,2-dimethylchromene (ee, 93%) (Table 1, entries 14 and 17).
The relatively bulkier olefins, such as indene and 1,2-
dihydronaphthalene, also showed good ee values (73% and 84%,
respectively) (Table 1, entries 8 and 11), while the ee values were
not encouraging in the case of the terminal olefins, such as
H
(
3R,4R)
Catalyst 2
a
Catalyst (4 mol% of substrate, based on Mn ion content), substrate (1
mmol), PyNO (2 mmol), NaClO (2 mmol, added in four equal parts),
o
1
1
2
0
5
0
CH
2
Cl
2
(2 mL), 60 min, 0 C.
b
Determined by GC.
Isolated yields.
c
d
-1
TOF is calculated by the expression of [product] / [catalyst] × time (s ).
In order to further understand the higher catalytic performance 60 styrene and α-methylstyrene (Table 1, entries 1 and 5).
of catalyst 1 in detail, we have used kinetics to study the reaction
rates of asymmetric epoxidation of styrene over the catalysts of
neat complex, IL-complex, catalyst 1 and catalyst 2. The time-
dependent plot of corresponding catalyst is shown in Fig. 8. It is
It is known that the reaction medium plays a crucial role in the
catalytic performance of chiral salen Mn(III) complex in the
asymmetric epoxidation of non-functionalized alkenes.
15, 37
Table
2
summarizes the results of comparative study of the
observed from the profile that conversion of styrene increased 65 enantioselective epoxidation of styrene over catalyst 1 in various
linearly up to 30 min, after which a significant increase is not
solvents. It was found that dichloromethane was the optimal
solvent for the enantioselective epoxidation of styrene with
NaClO, giving excellent yield of styrene epoxide with high ee
value (Table 2, entry 1), whereas n-hexane appeared to be
observed. Therefore, the initial rate constants
Kobs were
determined from the data in this time range. Catalyst 1, IL-
complex, catalyst 2 and neat complex give Kobs values of 2.70,
8
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Catalysis Science & Technology
unsuitable for the reaction (Table 2, entry 2). The difference
should attribute to the better dispersion of catalyst 1 in 50 complex under basic epoxidation conditions, didn’t occur. Short
salen Mn(III) complex, a main reason for deactivation of the
53
o
dichloromethane than in n-hexane. However, the catalytic
performance, especially enantioselectivity of the catalyst 1 was
not encouraging in acetone, acetonitrile, and methanol, although
catalyst 1 can also be well dispersed in these media (Table 2,
entries 3, 4, and 5). The principal explanation for this difference
is that the solvent containing oxygen or nitrogen atom with lone
electron pair can induce probably coordination with metal center
of the chiral salen Mn(III) complex, suppressing the ability of the
formation of the active oxygen transfer species (Mn(V)-oxo) in
reaction time (within 60 min) and low temperature (0 C) used in
our studies should prevent or at least slow catalyst degradation.
Furthermore, the imidazolium-based IL linker moiety plays a
positive effect on stabling the complex.
5
0
5
100
a
80
1
60
37
the epoxidation of styrene. Furthermore, only 34% yield of
styrene epoxide with 12% ee value was obtained in water, where
catalyst 1 can be readily dispersed (Table 2, entry 6). The lower
activity is due to the limited solubility of styrene in the aqueous
solution of catalyst and oxidant, and the poor enantioselectivity is
partially due to the solvation of water.
4
0
b
1
20
0
Table 2 The results of asymmetric epoxidation of styrene in different
1
2
3
4
5
6
7
a
Run times
solvent over catalyst 1
5
5
b
b
c
d
-3
Fig. 9 Reuse of catalyst 1 in asymmetric epoxidation of styrene with
Conv.
%)
ee
Yield
(%)
TOF -1×10
o
Entry
1
Solvent
NaClO as oxidant at 0 C (a: conversion; b: ee value).
(
(%)
(s )
dichloromethane
99
40 (R)
15 (R)
33 (R)
30 (R)
27 (R)
12 (R)
92
29
80
86
76
34
6.38
2.17
5.61
6.06
5.29
2.52
Conclusions
2
3
4
5
n-hexane
acetone
40
89
95
92
57
The chiral Mn(III) salen complex was successfully immobilized
60 on the flat planes and edge of the layered GO materials through
an imidazolium-based IL linker. Benefiting from fascinating
layered structure of the GO sheet, as well as active role of IL
linker in the overall reaction mechanism, the resulting catalyst 1
was efficient and universal in the enantioselective epoxidation of
65 unfunctionalized olefins with aqueous NaClO as an oxidant.
Remarkable enhancement of reaction rate was observed over a
wide range of alkenes either bulky or less (styrene, α-
methylstyrene, indene, 1,2-dihydronaphthalene, 6-cyano-2,2-
acetonitrile
methanol
water
6
a
2
0
Catalyst 1 (4 mol% of substrate, based on Mn ion content), styrene (1
mmol), PyNO (2 mmol), NaClO (2 mmol, added in four equal parts),
o
solvent (2 mL), 60 min, 0 C.
b
Same as in Table 1.
c
dimethylchromene,
70 Furthermore, the heterogeneous catalyst was stable in the water-
CH Cl biphasic reaction system. It could be easily recovered and
and
6-nitro-2,2-dimethylchromene).
Same as in Table 1.
d
2
5
Same as in Table 1.
2
2
Recycling
recycled for seven times without significant loss of the reactivity.
Although designed catalyst
homogeneous catalyst, it can be facilely recovered from the
aqueous/organic biphasic reaction. After the reaction, the
1 behaves as an efficient
Acknowledgements
3
3
4
4
0
5
0
5
The project was financial supported by the National Natural
heterogeneous catalyst 1 was quantitatively recovered from the 75 Science Foundation of China (Grant Nos. 21476069, 21003044),
reaction mixture by centrifugation. The supernatant was then
separated from the lower catalyst by simple decantation. The
recovered catalyst was washed with ethyl acetate and dried at 40
the Scientific Research Fund of Hunan Provincial Education
Department (13B072), the Program for Excllent Talents in Hunan
Normal University, the Hunan Provincial Innovation Foundation
for Postgraduate (CX2013B209) and the Program for Science and
o
C overnight for reuse.
Fig. 9 shows the results of the recovery and reusability of 80 Technology Innovative Research Team in Higher Educational
catalyst 1. To our delight, catalyst 1 could be reused for seven
times with no appreciable decrease in yield and enantioselectivity
of styrene epoxide. Chemical analysis of the manganese content
in the supernatant revealed no detectable leaching of Mn species
during the reaction. The results suggested the excellent stability
and reusability of catalyst 1 in the aqueous/organic biphasic
epoxidation. UV–vis spectra gave a direct evidence of the
stability of catalyst 1 based on the fact that no significant change
in the catalyst occurred even after reuse for seven times (Fig. 6b
vs. b’). These observations suggested that the efficient catalyst 1
was perfectly stable under the basic reaction condition in this
work (pH 11.5, NaClO buffer), and the decomposition of chiral
Institutions of Hunan Province. The authors are also grateful to
Dr Rongchang Luo (Sun Yat-Sen University) for his help with
the characterization with XPS.
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