A magnetically recyclable Fe3O4@SiO2/Mn(III) chiral salen complex as a highly selective and versatile heterogeneous nanocatalyst for the oxidation of olefins and sulfides

Article abstract of DOI:10.1039/c4ra09973g

In this study, a new magnetically recoverable heterogeneous Mn(iii) nanocatalyst (MHMC) was developed by covalent grafting of a homogeneous chiral Mn(iii) salen complex (HMC) onto the surface of a Fe₃O₄/SiO₂ core-shell. The structural and chemical nature of MHMC was characterized by different techniques. The catalytic activity of the MHMC and comparison with its homogeneous analogue were studied for the enantioselective epoxidation of various alkenes to epoxide compounds using tert-butyl hydroperoxide (TBHP) as an oxidant. The MHMC showed higher enantioselectivity and comparable catalytic reactivity to its homogeneous analogue with benefits such as facile recovery and recycling of the heterogeneous catalyst. The results showed that a higher enantioselectivity and yield were observed in the presence of N-methylmorpholine N-oxide (NMNO). The catalytic activity of this complex was also tested in the oxidation of sulfides using aqueous 30% H₂O₂ in excellent yields. The catalyst was reused for several runs without significant loss in activity and selectivity.

Full text of DOI:10.1039/c4ra09973g

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Graphical abstract:
An efficient system for selective epoxidation of alkenes using tert-BuOOH catalyzed by a new
magnetically recoverable Mn(III) salen complex was developed. The magnetically recoverable
heterogeneous chiral Mn (III) complex showed high selectivity and comparable catalytic
reactivity with its homogeneous analogue with benefits such as facile recovery and recycling of
the heterogeneous catalyst. Catalytic activity of this complex was also tested in the oxidation of
sulfides using aqueous 30% H2O2 in excellent yields.

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Magnetically Recyclable Fe3O4@SiO2/Mn (III) Chiral Salen complex as a
Highly Selective and Versatile Heterogeneous Nanocatalyst for the
Oxidation of Olefins and Sulfides
Ali Allahresani, Mohammad Ali Nasseri*
Department of Chemistry, Faculty of Sciences, University of Birjand, P. O. Box 97175-615, Birjand, Iran
Keyword: magnetically recoverable heterogeneous Mn (III) nanocatalyst, alkenes, sulfides, oxidation.
In this study, a new magnetically recoverable heterogeneous Mn (III) nanocatalyst (MHMC) was developed
by covalent grafting of homogeneous chiral Mn (III) salen complex (HMC) on the surface of Fe3O4/SiO2
coreꢀshell. The structural and chemical nature of MHMC was characterized by different techniques. The
catalytic activity of the MHMC and comparison with its homogeneous analogue were studied for the
enantioselective epoxidation of various alkenes to epoxide compounds using tertꢀbutyl hydroperoxide
(TBHP) as an oxidant. The MHMC showed higher enantioselectivity and comparable catalytic reactivity with
its homogeneous analogue with benefits such as facile recovery and recycling of the heterogeneous catalyst.
The results showed that higher enantioselectivity and yield were observed in the presence of Nꢀ
methylmorpholine Nꢀoxide (NMNO). Catalytic activity of this complex was also tested in the oxidation of
sulfides using aqueous 30% H2O2 in excellent yields. The catalyst was reused for several runs without
significant loss in activity and selectivity.
Introduction
Chiral Mn (III) salen complexes as versatile and useful catalysts have been used for oxidative
transformations [1]. In homogeneous systems, recycling of catalysts and separation of products from reaction
system is difficult. Because of the inherent advantages of heterogeneity, including efficient product
purification and easy catalyst recovery, the heterogenization of chiral Mn (III) salen complexes has been
received much attention [2ꢀ6]. Several different strategies have been focused on the covalent binding of
manganese (III) salen complexes onto insoluble solid supports such as mesoporous materials, zeolites,
polymer supports, organic polymerꢀinorganic hybrid materials, clays, MCMꢀ41, SBAꢀ15 and activated silica
[7ꢀ15]. Compared to the reactions performed in the homogeneous phase, the covalent attachment procedures
*
Corresponding author. Tel./fax: +98 5612502065
Eꢀmails: manaseri@birjand.ac.ir; malinasseri@yahoo.com (M. A. Nasseri).
1

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generally lead to a decrease in catalyst performance, but the heterogenized catalysts are stable against active
phase leaching and could be recycled for several times [16ꢀ19]. From economic point of view, the separation
of ultra scaled heterogeneous catalysts from reaction system via routine methods such as free sedimentation,
centrifugation and filtration is difficult, timeꢀconsuming and costly. To overcome this issue, the use of
magnetic nanoparticles (MNPs) which can be easily separated by applying a simple external magnet without
any significant loss of activity put forward a solution to this problem. Furthermore, magnetic nanoparticles
are widely studied in the fields of biomedicine, magnetic resonance imaging (MRI), biotechnology,
separation and catalysis [20ꢀ24].
On the other hand, selective oxidation of organic compounds is a fundamental reaction in the synthesis of
biologically active intermediates such as sulfoxides, sulfones and epoxides [25]. Sulfoxide are well known as
valuable intermediates for the synthesis of various useful compounds, including antibacterial, antifungal,
antiulcer, and antiꢀhypertensive agents [26]. Sulfones which are also synthesized by overꢀoxidation of
sulfoxides, exhibit different biological activities and have been the subject of extensive investigation [27ꢀ29].
Also, asymmetric epoxidation of alkenes into unique labile threeꢀmembered ether rings as an important
fundamental organic transformation is of great attention in chemical and pharmaceutical industries [30ꢀ32].
Now, in continuation of our ongoing research on the application of heterogeneous catalyst in organic
synthesis [33ꢀ35] and development of novel catalytic oxidation methods using environmentally benign
oxidants, such as hydrogen peroxide (H2O2) and tertꢀbutyl hydroperoxide (TBHP), we now report design and
characterization of the covalent attachment of HMC onto Fe3O4/SiO2 NPs and its catalytic properties in the
enantioselective epoxidation of alkenes using TBHP as an oxidant in dichloromethane. We also described
selective oxidation of sulfides to sulfoxides and solfones using H2O2 as oxidant in water as a green media. By
using this catalyst, the corresponding products were obtained in excellent yields and MHMC which is
recovered from the reaction mixture by using external magnetic fields simplified the isolation of products.
Results and Discussion
The immobilization of HMC on Fe3O4/SiO2 is illustrated in Scheme 1.
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OH
OH
HO
HO
HO
OH
OH
OH
Silica
Fe3O4
A
OH
OH
HO
HO
HO
HO
HO
OH
OH
OH
OH
HO
HO
HO
HO
OH
OH
N
N
Silica
Fe3O4
Mn
HO
OH
Dry toluene, N2
Ref, 24 h
O
O
O
OH
OH
OH
HO
HO
HO
HO
OH
OH
N
N
OH
HO
OH
B
Mn
O
O
C
HO
OH
Scheme 1. Schematic illustration for the preparation of MHMC catalyst.
Characterizations of catalyst
MHMC nanoparticle was simply synthesized by covalent grafting of HMC on Fe3O4/SiO2 (Scheme 1). The
magnetic properties of the Fe3O4, Fe3O4/SiO2 and MHMC were studied by a vibrating sample magnetometer
(VSM) at 300 k. As shown in Fig. 1, all of the nanoparticles have superparamagnetism at 300 K. Also, no
hysteresis phenomenon was observed in Fig. 1. The saturation magnetization values for Fe3O4, Fe3O4/SiO2
and MHMC catalyst were 70.495, 38.30 and 34.30 emu/g, respectively. Decreasing in the saturation
magnetization values of Fe3O4/SiO2 and MHMC catalyst is depending on the immobilization of SiO2 and
HMC onto Fe3O4, respectively. However, despite the considerably decrease of the magnetization of MHMC
compared to Fe3O4, MHMC can still be separated from the solution by using an external magnetic field on
the sidewall of the reactor.
Fig.1 Magnetization curves of (a) Fe3O4 Fig. 2 XRD pattern of (A) Fe3O4, (B) Fe3O4/SiO2
(b) Fe3O4/SiO2 (c) MHMC at 300 K.
and (C) MHMC
The XRD pattern of Fe3O4, Fe3O4/SiO2 and MHMC were determined by powder Xꢀray diffraction (XRD). As
shown in Fig. 2(a), the XRD pattern of Fe3O4 indicates a crystallized structure at 2θ: 30.2◦, 35.4◦, 43.3◦,
53.6◦, 57.5◦ and 63.1◦ which are assigned to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0)
crystallographic faces of magnetite. The XRD pattern of Fe3O4/SiO2 and MHMC show a broad peak at 2θ =
3

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15–27◦ which is depend on amorphous silica Fig. 2(b, c). The size of these particles could be estimated from
the Debye–Scherrer equation (1) by determining the width of the (3 1 1) Bragg reflection (1).
ꢁλ
ꢀ = βꢂꢃꢄθ
(1)
λ is the wavelength of the Xꢀray, k is the Scherrer constant, β is the half width of the peak and θ is half of the
Bragg angle. Therefore, the size (D) of the particles could be easily estimated. The average diameter of Fe3O4
NPs was about 13 nm, while the diameter of Fe3O4/SiO2 NPs was about 18 nm which is due to the
agglomeration of Fe3O4 inside nanosphere and surface growth of silica on the shell. The IR spectrum of
Fe3O4 shows important vibration bands in 560–590 and 3400 cm−1, which are due to FeꢀO and OꢀH,
respectively (Fig. 3a). According to the IR spectra of Fe3O4/SiO2 in Fig 3 (b), there are several important
vibration bands in 560–590, 954, and 3400 cm−1, which are due to FeꢀO, SiꢀOH, and OꢀH, respectively. The
vibration bands in 755 and 1100 cmꢀ1 are due to SiꢀOꢀSi. These vibration bands (755, 954 and 1100 cmꢀ1)
confirmed coating of the silica shell on the surface of the Fe3O4 NPs. The IR spectra of HMC showed the
strong band at 1590 cmꢀ1 which is assigned coordination of Mn through C=N functional groups (Fig. 3d). The
new stretching vibration bonds at 2930, 2880 and 1590 cmꢀ1 suggests the grafting of HMC on the Fe3O4/SiO2
0
NPs (Fig. 3c). The TGA curve of Fe3O4/SiO2 shows a weight loss over the range of 90ꢀ160 C of about 3%.
–
This loss can be attributed to the loss of adsorb water and dehydroxylation of internal OH groups. The
second weight loss step is over the range 250ꢀ590 0C which can be ascribed to even further decomposition of
the materials. The total weight losses are approximately 10% (Fig. 4 a). As shown in fig. 4b, the amount of
water decreased after the incorporation of HMC onto the Fe3O4/SiO2, indicating that the HMC has been
successfully immobilized onto the Fe3O4/SiO2.The TGA curve of MHMC shows a weight loss over the
range of 90ꢀ160 0C and 250ꢀ650 0C which is similar to the TGA curve of Fe3O4/SiO2. The total weight losses
are approximately 12.5% (Fig. 4 b). The loading of HMC is 0.21 mmol g ꢀ1 based on the difference in the
weight losses for Fe3O4/SiO2 and MHMC.
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Fig.3 FTꢀIR spectra of fresh (a) Fe3O4 (b)Fe3O4/SiO2 (c)
MHMC and (d) HMC
Fig.4 Thermogravimetric weight loss pattern of
Fe3O4/SiO2NPs (a) and MHMC (b) with temperature
raised of 10 0C/min
The TEM images of Fe3O4 and Fe3O4/SiO2 NPs are shown in Figure 5(a) and 5(b). The results showed the
average product size of Fe3O4 and Fe3O4/SiO2 NPs 15 and 20 nm similar to the results of XRD patterns. The
TEM images of Fe3O4/SiO2 NPs indicate the successful coating of magnetic Fe3O4 NPs (Fig. 5b). Figure 5c
shows the TEM image of MHMC which assigned successful coating of MHC on the Fe3O4/SiO2 NPs.
(C)
Fig. 5 TEM image of Fe3O4 (a), TEM image of Fe3O4/SiO2 (b) and TEM image of MHMC (c)
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The BET specific surface area of the MHMC which is determined at room temperature by nitrogen
physisorption was 18.5 m2/g. The loading of HMC (B) in the MHMC was 0.18 mmol/g based on Mnꢀelement
analysis by AAS.
Catalytic activity studies
Asymmetric epoxidation of unfunctionalized olefins
The catalytic activity of MHMC was investigated for the asymmetric epoxidaton of unfunctionalized olefins.
In an initial attempt, the reaction of αꢀmethylstyrene as a model substrate with TBHP as an oxidant and PNO
0
as an additive was performed at 0 C in the presence of MHMC nanoparticle in dichloromethane (DCM)
media and the effects of different solvents, additives and the amount of catalyst were studied for this reaction.
In DCM media, the Lewis acidity of Mn (III) center, which is withdraw electron from the peroxidic oxygen
and susceptible to be attacked by nucleophilic olefin is one of the most important characteristics that
determines catalytic performance in the olefin epoxidation. In order to elucidate the role of the MHMC
catalyst, at first, the reaction was performed in the absence of catalyst and only 8 % yield was obtained
(Table 1, entry1). Also, the reaction was performed in the absence of additive pyridine Nꢀoxide (PNO) and an
obvious decrease in the yield (75%) and enantioselectivity (50%) were observed (not listed). To optimize the
amount of catalyst, the reaction was carried out in the presence of different amount of MHMC (0.4ꢀ2.8 mol
% based on Mn element) at 0 0C in 3 ml DCM. The best result was achieved by using of 2.4 mol% (based on
Mn element) of MHMC (Table 1, entry 7). Decreasing the amount of catalyst resulted in lower yields under
the same conditions (Table 1, entries 2ꢀ6). On the other hand, higher amounts of catalyst did not improve the
yield or reaction time (Table 1, entry 8). For comparison, the homogeneous chiral Mn (III) complex (B) was
also investigated under the same reaction conditions, but shortening the reaction time to 3 h (Table 1, entry
9). The catalytic activity of the Fe3O4 and Fe3O4/SiO2 NPs was also investigated in the epoxidation of αꢀ
methylstyrene with TBHP as an oxidant and low yields were observed (30 and 45%, respectively). Blank
experiment showed that Fe3O4, Fe3O4/SiO2 and MHMC alone are inactive towards the αꢀmethylstyrene
epoxidation.
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Table 1. Optimization of different amount of MHMC nanocatalyst on the reaction of αꢀmethylstyrene with
TBHP as a model reaction.a
Entry
Catalyst amount (mol%)
Yieldb (%)
1
2
3
4
5
6
7
8
9
0
7
0.4
20
32
47
63
81
97
98
0.8
1.2
1.6
2.0
2.4
2.8
homogeneous chiral Mn
(III) complex (2.4 m0l%)
98
a Reaction conditions: substrate (1 mmol), DCM (3 ml), oxidant (2 mmol) and MHMC (2.4 mol% based on Mn) at 0 0C and duration 6 h.
b Determined by GC with a Shimadzu CBP5 column (30 m × 0.32 mm × 0.25 mm).
Moreover, the effect of different solvents was investigated in the reaction under the same conditions. It seems
that, the polarity of solvent has a great influence on catalytic activity of MHMC catalyst (Table 2). By
decreasing the polarity of solvents from water to CHCl3, the yields were increased from 15 to 50% (Table 2,
entries 1ꢀ7). However, the excellent yield of the product was observed in DCM (Table 2, entry 9).
Table 2. The effect of different solvents on the reaction of αꢀmethylstyrene with TBHP catalyzed by MHMC
as a model reaction.a
Entry
Solvent
H2O
Catalyst amount (mol%)
Yieldb (%)
1
2
3
4
5
6
7
8
9
2.4
2.4
2.4
2.4
2.4
2.4
2.4
2.4
2.8
15
20
24
30
35
50
60
75
98
EtOH:H2O
MeOH
EtOH
THF
CHCl3
CH3CN
EtOAc
DCM
a Reaction conditions: substrate (1 mmol), oxidant (2 mmol) and MHMC (2.4 mol% based on Mn) at 0 0C and duration 6 h.
b Determined by GC with a Shimadzu CBP5 column (30 m × 0.32 mm × 0.25 mm).
The effect of different oxidants/additives in the epoxidation of αꢀmethylstyrene
The effect of various oxidants such as NaIO4, mꢀCPBA, urea hydrogen peroxide (UHP), H2O2, Oxone, PhIO,
PhI(OAc)2 and TBHP was investigated in the epoxidation of αꢀmethylstyrene. As can be seen from Table 3,
when the reaction was carried out in the absence of oxidant, no yield was observed even after 12 h (Table 3,
entry 1). This result showed the efficiency of oxidant in the epoxidation of alkenes. As shown in Table 3,
oxidants such as UHP, H2O2, PhIO, PhI(OAc)2 gave low yields and enantioselectivity (Table 3, entries 2ꢀ5),
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while oxone and NaIO4 (For NaIO4, the reaction was carried out in the 2:1 mixture of acetonitrile: water)
gave better yields and enantioselectivity (Table 3, entries 6,7). Finally, mꢀCPBA and TBHP gave excellent
yield, but TBHP showed better enantioselectivity (Table 3, entries 8,9, respectively). In addition, different
equivalents of the oxidant were tested, and the best catalytic activity was obtained with 2 equivalents of the
oxidant which provides moderate source of oxygen for the catalytic reaction (not listed). Consequently, the
optimum molar ratio of olefin to oxidant is 1: 2.
Table 3. The effect of different oxidants on the asymmetric epoxidation of αꢀmethylstyrene catalyzed by
MHMC.a
Entry
Oxidant
ꢀꢀꢀꢀꢀ
Yield (%)
eeb(%)
0
1
2
3
4
0
H2O2
20
25
22
50
PhIO
51
PhI(OAc)2
60
UHP
35
55
5
6
7
8
9
NaIO4
Oxone
TBHP
50
65
97
98
55
57
80
60
m
ꢀCPBA
a Reaction conditions: αꢀmethylstyrene (1 mmol), oxidant (2 mmol), NMNO (0.5 mmol) and MHMC (2.4 mol% based on
Mn) at 0 0C and duration 6 h. b Determined by GC with chiral column (HP 19091GꢀB213, 30 m × 0.25 mm × 0.25 ꢁm).
The choice of additives is crucially important in the asymmetric epoxidation of olefins. The effect of various
additives such as NMNO, PNO, NH4OAc, MI, Py and imidazole was investigated in the epoxidation of αꢀ
methylstyrene in DCM/ TBHP system (Table 4). Usually, additive such as Nꢀmethylmorpholine Nꢀoxide
(NMNO), bind to the Mn (III) center prior to the epoxidation reaction, reported to improve epoxidation yields
and enentioselectivity and our finding is in accordance with those reported in literature [36]. However, some
study reported that NMO, decrease the catalytic activities [37]. The results showed that higher conversion
and ee were observed in the presence of NMNO (Table 4, entry 6). When NH4OAc is used in DCM, low
yield (13%) with only 30% ee is obtained (Table 4, entry 1). Under optimized conditions, the reaction was
carried out in the presence of homogeneous chiral Mn (III) complex (B) and αꢀmethylstyrene is converted to
corresponding epoxide in less than 2.5 h with 97% yield and 20% enentioselectivity (not listed).
Nevertheless, homogeneous chiral Mn (III) complex (B) showed higher activity than the corresponding
heterogeneous analogues, lower enentioselectivity was observed. The higher enentioselectivity in
heterogeneous catalyst may also be attributed to the microenvironment effect and confinement the effect
[38].
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Table 4. The effect of different additives on the asymmetric epoxidation of αꢀmethylstyrene catalyzed by
MHMC.a
Entry
Additive
NH4OAc
Py
Yield (%)
eeb(%)
30
1
2
3
4
5
6
13
50
60
75
97
97
50
MI
64
imidazole
PNO
66
70
NMNO
80
a Reaction conditions: αꢀmethylstyrene (1 mmol), TBHP (2 mmol), additive (0.5 mmol) and MHMC (2.4 mol% based on
Mn) at 0 0C. b Determined by GC with chiral column (HP 19091GꢀB213, 30 m × 0.25 mm × 0.25 ꢁm).
The reactions were also performed at different temperatures (ꢀ40, 40 and 60 0C). Under the same conditions,
0
an obvious decrease in the conversion of the reaction (65%) was observed at ꢀ40 C, but enantioselectivity
did not improve. At 25 0C, the reaction was performed in faster rates (3.5 h) and higher TOF (17.75ꢀ32.74 hꢀ
1) with any drop in the selectivity of product. Despite better yield and faster reaction rate at high temperature
(40 and 60 0C), obvious decreasing in enantioselectivity was observed. Therefore, we employed the
optimized conditions (MHMC, 2.4 mol %), TBHP (2 mmol) and NMNO (0.5 mmol) in 3 ml DCM at 0 0C for
the conversion of several olefins into the corresponding products. Table 5 lists a group of alkenes that were
investigated by MHMC. The catalyst showed excellent activity toward olefins oxidation with an average
isolation yield of 95%. Conditions used: injector temperature, 220 0C; detector temperature, 250 0C, injection
volume, 0.2 µl, and column, isothermal 90 0C for styrene and 80 0C for αꢀmethylstyrene.
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Table 5. Epoxidation of different olefins using TBHP catalyzed by MHMC.a
Entry
Alkene
T(0C)
Time(h)
Conversion (%)
Selectivityb % Yieldc(%) Eed(%) TONe
TOFf×10ꢀ4 (Sꢀ1)
0
6
99
99
100
99
98
98
80 41.25
19.10
32.74
1
25
41.25
80
3.5
0
25
0
6
3.5
6
99
99
95
96
95
97
96
98
94
95
93
95
92
94
96
98
100
100
>99
99
97
96
94
95
94
95
95
96
92
93
92
94
91
93
95
96
70 41.25
68 41.25
75 39.58
19.10
32.74
18.33
27.78
18.33
28.07
18.52
32.41
18.13
31.42
17.94
31.42
17.75
27.20
18.52
28.36
2
3
25
0
4
74
40
6
99
65 39.58
64 40.42
4
5
25
0
4
99
6
>99
99
69
40
25
0
3.5
6
68 40.83
45 39.17
44 39.58
80 38.75
80 39.58
81 38.33
80 39.17
Cl
>99
99
6
7
8
9
25
0
3.5
6
100
100
100
100
>99
>99
25
0
3.5
6
25
0
4
6
45
40
25
4
44 40.83
aReaction conditions: substrate (1 mmol), DCM (3 ml), oxidant (2 mmol), NMNO (0.5 mmol) and MHMC (2.4 mol% based on Mn) .
bSelectivity based on alkenes conversion. cDetermined by GC with a Shimadzu CBP5 column (30 m× 0.32 mm × 0.25 mm).
d ee% was performed by GC with a Agilent HP column (19091GꢀB213, 30 m × 0.25 mm × 0.25 ꢁm).
%ee = [A (major enantiomer) –A (minor enantiomer)] × 100/[A(major enantiomer) + A(minor enantiomer)].
eTON = mmol of converted alkene/mmol Mn;fTOF = TON/time of reaction.
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Oxidation of Sulfides to Sulfoxides
Initially, the oxidation of thioanisole was selected as a model reaction in water with different oxidants
(TBHP, UHP & H2O2) in the presence of MHMC at room temperature (Scheme 2). The results showed that
H2O2 is the best oxidant for oxidation of sulfides as higher conversion was obtained.
O
O
S
O
MHMC (0.003 mmol)
MHMC (0.006 mmol)
S
S
H2O (2 ml), rt. 1h
H2O2( 30 %, 2 eq)
H2O (2 ml), rt. 3 h
H2O2( 30 %, 2 eq)
Ph
Me
Ph
Me
Ph
Me
99% yield
procedure B
procedure A
97% yield
Scheme 2. Catalytic oxidation of sulfides.
Also, different amounts of catalyst were tested and the results showed that the yield and selectivity of the
reaction were affected crucially by the amount of catalyst (Table 6). As the results shown, methyl phenyl
sulfioxide was achieved with excellent yield in the presence of 0.003 mmol catalyst within 1 h using 2.0 eq.
H2O2. Increasing the amount of catalyst and the time of the reaction leads to the further oxidation of
sulfoxide to sulfone. Sulfone’s selectivity increased significantly and reached its maximum to 99% in the
presence of 0.006 mmol catalyst within 3 h. It should be noted that, in the absence of catalyst no significant
sulfoxidation was observed under similar reaction conditions, and only a low conversion was observed in the
presence of 2 eq. of H2O2 after prolonged reaction time (Table 6, entry 7). Also, various H2O2/substrate molar
ratios were studied for the investigation of the effect of oxidant on the yield and selectivity of product in the
presence of 0.003 mmol catalyst. Various amounts of H2O2 affected the conversion of products and the best
conversion and selectivity was achieved with 2 eq. H2O2. Increasing the molar ration of H2O2 leads to a
decrease in the selectivity of corresponding sulfoxide, but H2O2 has less effect compared to the catalyst.
Table 6. The effect of varying H2O2 /catalyst amounts and time on oxidation of thioanisole catalyzed by
MHMC in water.
Entry
H2O2
Procedure Aa
Procedure Bb
(mmol)
(0.003 mmol catalyst)
(0.006 mmol catalyst)
Sulfoxide c%
Sulfone c%
Sulfoxide c%
Sulfone c%
1
2
3
4
5
1
1.5
2
2.5
3
50
0
0
3
12
25
35
ꢀꢀꢀꢀꢀ
20
10
0
0
0
40
55
99
99
100
100
78
97
88
75
65
10
6
3.5
2
0
7d
aReactions were run under air at room temperature for 1 h using 1.0 mmol thioanisole.
bReactions were run under air at room temperature for 3 h using 1.0 mmol thioanisole.
cGC yields based on the toluene as the internal standard. dthe reaction was carried out in the absence of MHMC for 6 h.
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The effect of different solvents (water, EtOAc, CHCl3, THF, DCM, CH3CN, EtOH, MeOH and EtOH/water
(1:1)) was also investigated and results showed that water is the best solvent in the term of conversion and
selectivity. The effect of different PH (2, 3, 4, 5, 6, 7, 8, 9 and 10) was also studied on the conversion and
selectivity of products and no obvious changes in the conversion and selectivity were observed (not listed).
Moreover, the reaction was carried out in the presence of homogeneous chiral Mn (III) complex (B) and
methyl phenyl sulfide is converted to corresponding sulfoxide in 35 min with 97% yield (not listed).
Under optimized conditions, the applicability of catalyst was tested for oxidation of various sulfides (Table
7). Several sulfides undergo selective oxidation to produce the corresponding sulfoxides/sulfones. As the
results shown in Table 7, all substrates could be converted to sulfoxides (procedure A) and sulfones
(procedure B) with high conversion rates, and excellent selectivities were obtained under mild conditions.
Under given conditions, the catalytic oxidation of sulfides to corresponding sulfoxides showed excellent
efficiency in the terms of conversion (91ꢀ99%), TOF (310ꢀ330 hꢀ1) and selectivity (99ꢀ100). Furthermore, the
presence of either electron withdrawing or releasing groups, present in the phenyl ring of sulfides did not
affect the synthesis of sulfoxide considerably (Table 7, entries 3ꢀ6).
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Table 7. Oxidation of different sulfides using H2O2 catalyzed by MHMC.a
Sulfoxidea
Sulfoneb
Selec. %
Entry
Substrate
Conv.c (%)
98
Select. %
100
TOFd (hꢀ1)
330
Conv.c (%)
99
TOFd (hꢀ1)
S
1
55
100
99
S
2
3
4
5
90
98
99
96
100
100
99
310
323
326
323
91
97
99
95
Cl
50.6
54
S
H3CO
100
99
S
H3C
55
S
99
99
52.8
O2N
S
97
99
320
94
99
6
7
8
52.2
Br
S
53.8
53.8
98
96
100
99
316
320
97
97
100
100
S
a Reaction conditions: sulfide (1 mmol), 30% H2O2 (2 equiv), catalyst (0.003 mmol based on Mn element), 1 h., rt.
b Reaction conditions: sulfide (1 mmol), 30% H2O2 (2 equiv), catalyst (0.006 mmol based on Mn element), 3 h., rt.
cConversion and selectivity were determined by GC, Conversion= [(initial moles of substrate ꢀ final moles of
substrate)/initial moles of substrate] × 100, Selectivity to sulfoxide = [sulfoxide%/(sulfoxide% + sulfone%)] × 100.
d TOF = number of moles of substrate converted per mole of catalyst per hour.
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We also investigated the possibility of reusing of recovered catalysts for new oxidation reactions. Therefore,
the MHMC catalyst was separated by external magnet, washed with EtOH, dried and reused directly for a
subsequent round of reaction without further purification with no significant loss of activity which validates
its recyclability. In each run, no manganese was detected in the filtrates by AAS. As shown in figure 6, it is
obvious that the catalyst worked well for asymmetric epoxidation of αꢀmethylstyrene up to five cycles with
no considerable decrease in reactivity and enantioselectivity (Yield: from 97% to 92%; ee: from excess 80%
to 77%). The decreases in the activity and enantioselectivity of products maybe due to the leaching of silica
or the aggregation of Fe3O4. Also, the reusability of MHMC was also investigated for the oxidation of methyl
phenyl sulfide and no obvious decrease in yields was observed (Fig. 6).
Fig.6 The recycle of the catalyst for oxidation of thioanisol (left) using H2O2 (2 mmol) in water (2 ml) catalyzed by
MHMC (0.003 mmol) at room temperature and asymmetric epoxidation of αꢀmethylstyrene (right), TBHP (2 mmol) and
NMNO (0. 25 mmol) in 3 ml DCM catalyzed by MHMC (2.4 mol% based on Mn) at 00C.
Conclusions
In summary, the novel type of MHMC was synthesized by grafting of the HMC onto Fe3O4/SiO2 NPs. The
new MHMC showed high catalytic activity in the oxidation of organic sulfides and epoxidation of simple
alkenes. In the epoxidation reactions, the influences of oxidants and additives with considering yields and
enantioselectivity on the activity of MHMC were explored at length and TBHP and NMNO were chosen as
the best oxidant and additives, respectively. The MHMC exhibited comparable enantioselectivity than that of
homogeneous counterpart for the asymmetric epoxidation of several alkenes. The MHMC is an efficient
nanocatalyst for environmentally friendly oxidation of various sulfides into both sulfoxides and sulfones at
room temperature in water using 30% H2O2. In addition, no side reactions, short reaction times, mild reaction
conditions, easy and simple work up and excellent selectivity and yields are some advantages of this
nanocatalyst. Moreover, the catalyst is cheap, easily synthesized and recovered by external magnetic field
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and reused without any noticeable loss of activity after several times, thus indicating that the complexes are
stable, which prevents leaching of the active phase and its deactivation.
Experimental and methods
Materials
Tetraethoxysilane (TEOS), FeCl3·6H2O, FeCl2·4H2O, styrene, 4ꢀchlorostyrene, cyclohexene, αꢀmethyl
styrene, indene, cis & trans stilbene, cyclooctene, 1ꢀoctene, mꢀchloroperbenzoic acid (mꢀCPBA), pyridine Nꢀ
oxide (PNO), pyridine (Py), Nꢀmethylmorpholine Nꢀoxide (NMNO), 1ꢀmethyl imidazole (MI), imidazole
(IM), dichloromethane (DCM), ethyl acetate (EtOAc), toluene, ethanol (EtOH), CH3CN, CHCl3, THF,
NaIO4, NH4OAc, TBHP, PhI (OAc)2, H2O2 (30%), (R,R)ꢀ1,2ꢀcyclohexanediamine and oxone were purchased
from Merck Company and used without purification. Graphite powder was obtained from Aldrich Company.
Measurements
The resulting Fe3O4/SiO2 and MHMC were characterized by IR, TEM, TGA, VSM and XRD patterns.
The IR experiments were carried out on a PerkinꢀElmer 783 Infrared spectrophotometer in a KBr pellet,
scanning from 4000 to 600 cmꢀ1 at room temperature. The XRD measurements were carried out by using a
Bruker D8ꢀadvance Xꢀray diffractometer with Cu Kα radiation (k = 1.5406A0). The transmission electron
microscopy (TEM) images measurement were obtained using Philips CM10 instrument. Magnetization
measurements were carried out at 300 K on a vibrating sample magnetometer (VSM Leak shore 7200).
Thermogravimetric analysis (TGA) was performed by heating the samples in an Argon flow at a rate of 100
0
ml minꢀ1 using a Perkin–Elmer Diamond TG/DTA thermal analyzer with a heating rate of 10 C minꢀ1. The
conversion of products was determined by GCꢀ17A Shimadzu with capillary column (Shimadzu, CBP5,
30m× 25mm× 0.25 µm). The ee was determined in a chiral GC column (HP 19091GꢀB213, 30 m × 0.25 mm ×
0.25 ꢁm).
Synthesis of MHMC (C)
The black magnetic Fe3O4 NPs [39], Fe3O4/SiO2 NPs [40], (R,R)ꢀN,N'ꢀBis(2,4ꢀdiꢀhydroxybenzaldehyde)ꢀ1,2ꢀ
cyclohexanediamine [41] and Mn(III) salen complex [42] were prepared as described in literature. The
synthesized homogeneous Mn (III) salen complex (B) has been chemically anchored to the Fe3O4/SiO2 (A).
Briefly, Fe3O4/SiO2 (2g) was dispersed in 100 ml dry toluene for 20 min. Then, 0.5 g of homogeneous Mn
(III) complex was added to the stirring mixture (Scheme 1). The resulting suspension was refluxed for 24 h
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under inert atmosphere. After the completion of the reaction, the MHMC (C) was washed with dry toluene,
EtOH and extracted repeatedly on a Soxhlet extractor with methanol and dichloromethane until the washing
0
becomes colorless. The solid was dried at 70 C under vacuum for 6 h. The MHMC was characterized by
XRD, TEM, FTꢀIR, BET, VSM and TGA.
General procedure for the asymmetric epoxidation of alkenes
In a typical procedure, MHMC as catalyst (2.4 mol% based on Mn element) was dispersed in 3 ml of
dichloromethane for 20 min and cooled to 00C followed by addition of αꢀmethylstyrene as substrate (1
mmol), toluene (internal standard, 40µl), TBHP (2 mmol) as oxidant and NMNO (0.5 mmol) as additive. The
mixture was magnetically stirred at 00C for appropriate times. The progress of the reaction was monitored by
TLC. After the completion of the reaction, the catalyst was separated by external magnet. The solution was
washed with 1 M NaOH (8 ml) and brine (8 ml) and dried over MgSO4. Then, the solution was concentrated
by rotary evaporator to 1 ml. Finally, the conversion and ee values of products were determined by GC. The
catalyst was washed twice with ethanol and reused.
General procedure for the oxidation of sulfides to sulfoxides/sulfones
An appropriate amount of MHMC as catalyst (0.003 & 0.006 mmol based on Mn element for sulfoxide and
sulfone, respectively) was ultrasonicated for 20 min in water (2 ml). Then, sulfide (1.0 mmol), toluene (40 ꢁl,
as the internal standard) and hydrogen peroxide (2.0 mmol 30%, 200 ꢁl) were added to the mixture and the
mixture was stirred at room temperature for the required time (Scheme 2). The progress of the reaction was
monitored by TLC. After the completion of the reaction, the product was extracted with ethyl acetate, washed
with brine, dried over anhydrous Na2SO4, and the yield of products was determined by GC analysis using
toluene as the internal standard. The heterogeneous catalyst was washed with EtOH, dried and used for the
next runs.
Acknowledgments
The authors are grateful to the University of Birjand for financial support.
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