Highly efficient epoxidation of alkenes with sodium periodate catalyzed by reusable polystyrene-bound ruthenium(III) salophen

Article abstract of DOI:10.1007/BF03246558

In this paper, highly efficient epoxidation of alkenes catalyzed by ruthenium(III) salophen chloride, [Ru(salophen)Cl], supported on functionalized chloromethylated polystyrene, PS, is reported. The PS was modified with 1,4-diaminobenzene, 4-aminophenol and 4-aminothiophenol, and [Ru(salophen)Cl] was attached to the supports via axial ligation. The prepared catalysts were used for efficient epoxidation of alkenes with NaIO₄ at room temperature. These new heterogenized catalysts were characterized by elemental analysis, FT-IR spectroscopy, scanning electron microscopy and transmission electron microscopy. The heterogeneous catalysts were reusable in the oxidation reactions and were reused several times.

Full text of DOI:10.1007/BF03246558

J. Iran. Chem. Soc., Vol. 8, No. 4, December 2011, pp. 1019-1029.
JOURNAL OF THE
Iranian
Chemical Society
Highly Efficient Epoxidation of Alkenes with Sodium Periodate Catalyzed by Reusable
Polystyrene-Bound Ruthenium(III) Salophen
a,
a,
a
a
b
c
M. Moghadam *, V. Mirkhani *, S. Tangestaninejad , I. Mohammadpoor-Baltork , H. Kargar , I. Sheikhshoaei and
d
M. Hatefi
a
Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran
Department of Chemistry, Payame Noor University,P.O. Box 19395-3697 Tehran, Islamic Republic of Iran
b
c
Department of Chemistry, Shahid Bahonar University, Kerman, Iran
d
Department of Chemistry, Faculty of Science, Vali-e-Asr University, 77176 Rafsanjan, Iran
(
Received 20 February 2011, Accepted 21 April 2011)
In this paper, highly efficient epoxidation of alkenes catalyzed by ruthenium(III) salophen chloride, [Ru(salophen)Cl],
supported on functionalized chloromethylated polystyrene, PS, is reported. The PS was modified with 1,4-diaminobenzene, 4-
aminophenol and 4-aminothiophenol, and [Ru(salophen)Cl] was attached to the supports via axial ligation. The prepared catalysts
were used for efficient epoxidation of alkenes with NaIO at room temperature. These new heterogenized catalysts were
4
characterized by elemental analysis, FT-IR spectroscopy, scanning electron microscopy and transmission electron microscopy.
The heterogeneous catalysts were reusable in the oxidation reactions and were reused several times.
Keywords: Ruthenium(III) salophen, Alkene epoxidation, Sodium periodate, Polystyrene, Heterogeneous catalysts
INTRODUCTION
heterogeneous counterparts offer many advantages such as
easy separation and recovery of the catalyst from the reaction
media, higher stability of the catalytic species and catalyst
protection against destruction. Several methods such as non-
covalent immobilization in zeolites, clay or siloxane
membranes [27-32], covalent grafting onto inorganic supports
such as silica or MCM-41 [33-37], co-polymerization of a
functionalized salen monomer into an organic polymer [38],
attachment or build-up of a salen structure to a performed
polymer [39-44], and axial ligation to surface bound ligands
[45-47] have been used for immobilization of Schiff base
complexes (salen or salophen) on the supports.
The use of supported Co(salen) as a highly efficient
catalyst for hydrolytic kinetic resolution of racemic epoxides
and enantioselective parallel synthesis have been reported by
Jacobsen group [48,49]. Polystyrene is one of the most
popular polymeric supports used in synthetic organic
Ruthenium compounds are versatile electron-transfer and
energy-transfer compounds [1-22]. Due to these unique
properties, ruthenium complexes have a wide range of
applications in various research areas, such as artificial
photosynthesis, photomolecular devices, probes for biological
macromolecules, oxidation catalysts and organic synthesis [8-
2
2].
Salen and salophen ligands and their metal complexes have
found extensive applications in the fields of synthesis and
catalysis [23-26]. These homogeneous catalysts cannot be
recovered and are degraded in the reaction media, which limit
their practical applications. On the contrary, their
*Corresponding authors. E-mail: moghadamm@sci.ui.ac.ir;
mirkhani@sci.ui.ac.ir

Moghadam et al.
O
[
Ru(salophen)Cl@amine-PS]
NaIO , CH CN/ H O
hours. The content of ruthenium(III) salophen on the polymer,
which was calculated from its ruthenium content in the
heterogenized catalyst, was determined by ICP.
4
3
2
Scheme 1. Epoxidation of alkenes with NaIO in the presence
4
General Procedure for Alkene Epoxidation with
of [Ru(salophen)Cl@amine-PS]
NaIO Catalyzed by [Ru(salophen)Cl@amine-PS]
4
To a mixture of alkene (1 mmol), [Ru(salophen)@PS] (600
mg) in CH CN (10 ml) was added a solution of NaIO (2
3
4
chemistry because of its inexpensiveness, ready availability,
mechanical robustness, chemical inertness and facile
functionalization.
mmol) in H O (10 ml). The reaction mixture was stirred
2
magnetically at room temperature. The progress of the
reaction was monitored by GC. At the end of the reaction, the
In continuation of our research on the preparation of
polystyrene-bound catalysts [46,47,50-53], here, we report the
preparation, characterization and investigation of catalytic activity
of [Ru(salophen)Cl] supported on chloromethylated polystyrene
modified with 1,4-diaminobenzene, 4-aminophenol and 4-
aminothiophenol (Scheme 1).
reaction mixture was diluted with Et
The catalyst was thoroughly washed with Et
2
O (20 ml) and filtered.
O and combined
2
washings and filtrates were purified on a silica-gel plate or a
1
silica-gel coluRu. IR and H NMR spectral data confirmed the
identities of the products.
Catalyst Reuse and Stability
EXPERIMENTAL
The reusability of each catalyst was investigated in the
multiple sequential epoxidation of cyclooctene as described
above. At the end of each reaction, the catalyst was separated
from the reaction mixture by simple filtration, washed with
Alkenes were obtained from Merck chemical company and
passed through a column containing active alumina to remove
the peroxidic impurities. Salophen ligand was prepared and
metallated according to the literature [54,55]. FT-IR spectra
were obtained with potassium bromide pellets in the range
Et O and dried before being used in the next run.
2
RESULTS AND DISCUSSION
-1
4
00-4000 cm with a Nicolet Impact 400D spectrometer.
Scanning electron micrograph of PS was taken on a Philips
XL 30 SEM instrument. Gas chromatography experiments
Preparation and Characterization of the Catalysts
[
Ru(salophen)@DAB-PS], [Ru(salophen)@AP-PS]
(
GC) were performed with a Shimadzu GC-16A instrument
and [Ru(salophen)@ATP-PS]
using a 2 m column packed with silicon DC-200 or Carbowax
Scheme 2 shows the preparation route for [Ru(salophen)@
DAB-PS], [Ru(salophen)@AP-PS] and [Ru(salophen)@ATP-
PS]. First, amines such as 1,4-diaminobenzene (DAB), 4-
aminophenol (AP) or 4-aminothiophenol (ATP) were reacted
with chloromethylatedpolystyrene, (cross-linked with 2%
2
0m. In the GC experiments, n-decane was used as internal
1
standard. H NMR spectra were recorded on a Bruker-Avance
AQS 300 MHz spectrometer.
Preparation
of
Polystyrene
Supported
-1
divinylbenzene, 4-5% Cl content, 1.14-1.40 mmol g Cl), to
Ruthenium(III) Salophen, Ru(Salophen)Cl@amine-
PS
To a solution of ruthenium(III) salophen (0.45 g, 1 mmol)
in acetonitrile (50 ml) was added amine-modified polystyrene
afford the amine-modified PS (DAB-PS, AP-PS and ATP-PS).
In the final step, [Ru(salophen)Cl] was attached to these
supports via axial ligation.
The prepared catalysts were characterized by elemental
analysis, FT IR spectroscopy, scanning electron microscopy
(
3 g). The mixture was vigorously stirred at 80 °C for 24 h.
After cooling, the red-brown resins were collected by
filtration, washed thoroughly with acetonitrile, methanol, and
ether successively, and dried at room temperature for several
(
SEM) and transmission electron microscopy (TEM). The
nitrogen content of the supports, measured by CHNS analysis,
showed values of 2.25, 1.15 and 1.20% for DAB-PS, AP-PS
1
020

Highly Efficient Epoxidation of Alkenes with Sodium Periodate
-1
g for [Ru(salophen)@DAB-PS], [Ru(salophen)@AP-PS] and
H N
XH
2
[
Ru(salophen)@ATP-PS], respectively (Table 1).
X=O, NH, S
The most informative evidence, which confirmed the
anchoring of the Ru(salophen)Cl to the functionalized PS, was
obtained by FT IR spectra of [Ru(salophen)@DAB-PS],
o
DMF, 120 C
CH Cl
Et N, 72 h
CH
2
2
3
[
Ru(salophen)@AP-PS] and [Ru(salophen)@ATP-PS] (Fig. 1
NH
and Table 1). The azomethene (C=N) stretching band of
-1
homogeneous [Ru(salophen)Cl] appeared at 1607 cm (Fig.
1
1
A), while in the supported catalysts this band was observed at
XH
602, 1609 and 1601 cm- for [Ru(salophen)@DAB-PS],
1
[
Ru(salophen)@AP-PS]
and
[Ru(salophen)@ATP-PS],
respectively (Fig. 1B-1D and Table 1). These observations
clearly confirmed the attachment of Ru(salophen)Cl to amine-
modified PS. The scanning electron micrographs of the
catalysts (Fig. 2) show the morphology of the
N
N
O
Ru
Cl
CH CN, 48 h
3
O
[
[
Ru(salophen)@DAB-PS],
Ru(salophen)@ATP-PS] used in this study. The TEM image
[Ru(salophen)@AP-PS]
and
of the prepared catalysts shows how the Ru(salophen)Cl is
distributed on the polystyrene (Fig. 3).
CH2
HX
The Effect of Catalysts Amount in the Epoxidation of
Cyclooctene
In order to optimize the catalysts amount, different
quantities of each catalyst were used in the epoxidation of
N
O
N
Ru
Cl
O
cyclooctene (0.5 mmol) with NaIO (1 mmol). The best results
4
were obtained with 300 mg of each catalyst (Table 2).
Scheme 2. Preparation of [Ru(salophen)Cl@amine-PS]
The Effect of Solvent on the Epoxidation of
Cyclooctene Catalyzed by [Ru(salophen)@DAB-PS],
[
Ru(salophen)@AP-PS] and [Ru(salophen)@ATP-
and ATP-PS, respectively. Based on these values, the amount
of nitrogen in DAB-PS, AP-PS and ATP-PS was calculated to
be 1.61 (0.80 for coordinated nitrogens), 0.82 and 0.86 mmol
per gram of each support, respectively. The Ru content of the
catalysts, measured by ICP, was 0.105, 0.111 and 0.122 mmol
PS]
In order to choose the reaction media, different solvents
were checked in the epoxidation of cyclooctene with NaIO4.
From among the different mixtures of acetonitrile, acetone,
methanol, ethanol (single-phase systems), dichloromethane,
Table 1. Characteristic of the Prepared Catalysts
-1
Ru content (mmol g-1)
C=N band (cm-1)
Catalyst
N content (mmol g )
[
[
[
[
Ru(salophen)Cl]
-
-
1607
1602
1609
1601
Ru(salophen)@DAB-PS
Ru(salophen)@AP-PS]
Ru(salophen)@ATP-PS]
1.60
0.82
0.86
0.105
0.111
0.122
1
021

Moghadam et al.
Fig. 1. FT IR spectra of: (A) [Ru(salophene)Cl], (B) [Ru(salophene)@DAB-PS], (C) [Ru(salophene)@AP-PS] and
(D) [Ru(salophen)@ATP-PS].
chloroform and carbon tetrachloride (two-phase systems with
Bu NBr as phase transfer catalyst), the 1:1 mixture of
H O (UHP), tert-BuOOH and n-Bu NIO were used for the
epoxidation of cyclooctene. The results, which are
2
2
4
4
4
acetonitrile, water was chosen as the reaction medium because
the higher catalytic activity was observed in this experimental
condition (Table 3). The higher catalytic activity in
acetonitrile/water mixture is attributed to the polarity of the
summarized in Table 4, showed that NaIO was the best
4
oxygen source because this oxidant, which is inert in the
absence of the catalyst, affords better oxidation conversion in
CH CN/H O.
3
2
solvent and solubility of NaIO in this medium.
4
Epoxidation of Alkenes with NaIO Catalyzed by
[Ru(salophen)@DAB-PS], [Ru(salophen)@AP-PS]
and [Ru(salophen)@ATP-PS]
The supported catalysts were used for the epoxidation of
olefins with NaIO4 under optimized conditions. The
4
The Effect of Oxidant on the Epoxidation of
Cyclooctene Catalyzed by [Ru(salophen)@DAB-PS],
[
Ru(salophen)@AP-PS] and [Ru(salophen)@ATP-
PS]
Single oxygen donors such as NaIO
4
, Oxone, H
2
O
2
, urea-
[Ru(salophen)@DAB-PS],
[Ru(salophen)@AP-PS] and
1
022

Highly Efficient Epoxidation of Alkenes with Sodium Periodate
A
B
C
D
E
F
Fig. 2. SEM image of: (A) DAB-PS, (B) [Ru(salophene)@DAB-PS], (C) AP-PS, (D) [Ru(salophene)@AP-PS], (E)
ATP-PS and (F) [Ru(salophen)@ATP-PS].
1
023

Moghadam et al.
B
A
C
Fig. 3. TEM image of: (A) [Ru(salophene)@DAB-PS], (B) [Ru(salophene)@AP-PS] and (C) [Ru(salophen)
ATP-PS].
@
a
Table 2. Optimization of the Catalysts Amount in the Epoxidation of Cyclooctene with NaIO
4
b
Epoxide yield (%) after 5 h
Ru(salophen)@DAB-PS] [Ru(salophen)@AP-PS] [Ru(salophen)@ATP-PS]
Catalyst amount (mg)
[
0
2
5
5
61
5
60
6
2
4
3
3
00
50
00
50
7
9
9
71
87
70
89
2
3
3
87
89
a
b
Reaction conditions: cyclooctene (0.5 mmol), oxidant (1 mmol), catalyst, CH CN/H O (5 ml/5 ml). GC
3
2
yield based on the starting alkene.
1
024

Highly Efficient Epoxidation of Alkenes with Sodium Periodate
Table 3. The Effect of Solvent on the Epoxidation of Cyclooctene with NaIO Catalyzed by [Ru(salophen)Cl] Supported
4
on Modified PS at Room Temperaturea
b
Epoxide yield (%) after 5 h
Ru(salophen)@DAB-PS] [Ru(salophen)@AP-PS] [Ru(salophen)@ATP-PS]
Row
Solvent
CH CN/H
[
1
2
3
4
5
6
7
3
2
O (1:1)
CH CN/H O (2:1)
93
73
52
67
59
34
23
15
87
64
39
47
55
25
14
10
89
66
43
54
62
27
19
11
3
2
CH OCH /H O
3
3
2
CH
3
OH/H O
2
CH CH OH/H O
3
2
2
CH Cl /H O
2
2
2
CHCl
3
/H
2
O
8
CCl /H O
4
2
a
b
Reaction conditions: cyclooctene (1 mmol), NaIO (2 mmol), catalyst (600 mg), solvent/H O (10 ml/5 ml). GC yield
4
2
based on the starting alkene.
Table 4. The Effect of Oxidant on the Epoxidation of Cyclooctene Catalyzed by [Ru(salophen)Cl] Supported on Modified
a
PS at Room Temperature
b
Epoxide yield (%) after 5 h
[Ru(salophen)@DA [Ru(salophen)@AP- [Ru(salophen)@AT
Row
Oxidant
Solvent
B-PS]
93
PS]
87
56
77
32
13
19
41
P-PS]
89
1
2
3
4
5
6
NaIO (2 mmol)
CH CN/H O
4
3
2
NaIO (1 mmol)
CH CN/H O
61
58
4
3
2
Oxone
H O
CH
3
CN/H
2
O
83
79
CH CN
49
34
2
2
3
UHP
CH CN
30
21
3
tert-BuOOH
CH
3
CN
23
25
7
(n-Bu) NIO4
CH CN
57
43
4
3
a
b
Reaction conditions: cyclooctene (1 mmol), oxidant (2 mmol), catalyst (600 mg), CH CN/H O (10 ml/10 ml). GC yield
3
2
based on the starting alkene.
[
Ru(salophen)@ATP-PS] were found to be efficient catalysts
Catalyst Reuse and Stability
for the epoxidation of alkenes with NaIO
4
(Tables 5-7). In
The reusability of a heterogeneous catalyst is of great
importance from synthetic and economical points of view. The
homogeneous [Ru(salophen)Cl] cannot be recovered even
once; in contrast, the PS-supported catalysts can be filtered
and reused several times.
three catalytic systems, the cyclohexene was oxidized in high
yield to cyclohexene oxide and 2-cyclohexen-1-one was
obtained in 5-9% yields. In the epoxidation of styrene, the
major product was styrene oxide and only small amounts of
benzaldehyde was produced (6-8%). In the oxidation of α-
methylstyrene, the corresponding epoxide was produced in
high yield and acetophenone was obtained as the minor
product (3-8%). Linear alkenes such as 1-octene and 1-
dodecene were epoxidized in high yields with 100%
selectivity. In the case of α-pinene, the α-pinene oxide was
produced in 76-78% and allylic oxidation products, Verbenol
and Verbenone, were produced in 10-11%.
The
reusability
of
[Ru(salophen)@DAB-PS],
[Ru(salophen)@AP-PS] and [Ru(salophen)@ATP-PS] was
investigated in the multiple sequential epoxidation of
cyclooctene with NaIO
4
. At the end of each reaction, the
catalyst was separated by simple filtration, washed with Et
2
O
and dried carefully before being used in the next run. After
using each catalyst for five consecutive times, the epoxide
yields were 55, 47 and 43% for [Ru(salophen)@DAB-PS],
1
025

Moghadam et al.
a
Table 5. Epoxidation of Alkenes with NaIO Catalyzed by [Ru(salophen)@DAB-PS]
4
Row
Alkene
Conversion (%)a
Epoxide yield (%)b
Time (h)
1
2
93
93
86
5
5
95c
3
90d
84
5
4
5
89e
86
81
86
5
5
6
7
88f
77
5
66
55
66
55
10
8
10
a
Reaction conditions: alkene (1 mmol), NaIO
4
(2 mmol), catalyst (0.063 mmol), CH
3
CN/H
2
O (10 ml/10 ml).
b
c
d
GLC yield based on the starting alkene. The by-product is allylic ketone. The by product is benzaldehyde.
e
f
The by-product is acetophenone. In this case, 7% ketone and 4% alcohol were produced.
Table 6. Epoxidation of Alkenes with NaIO
Catalyzed by [Ru(salophen)@AP-PS]a
4
Row
1
Alkene
Conversion (%)a
Epoxide yield (%)b
Time (h)
5
87
87
2
3
87c
88d
82
80
5
5
4
5
82e
82
79
82
5
5
6
7
86f
58
76
58
5
10
8
44
44
10
a
b
e
Reaction conditions: alkene (1 mmol), NaIO (2 mmol), catalyst (0.067 mmol), CH CN/H O (10 ml/10 ml).
4
3
2
c
d
GLC yield based on the starting alkene. The by-product is allylic ketone. The by product is benzaldehyde.
The by-product is acetophenone. In this case, 7% ketone and 3% alcohol were produced.
f
1
026

Highly Efficient Epoxidation of Alkenes with Sodium Periodate
a
Table 7. Epoxidation of Alkenes with NaIO Catalyzed by [Ru(salophen)@ATP-PS]
4
Row
Alkene
Conversion (%)a
Epoxide yield (%)b
Time (h)
1
2
3
89
92c
89d
89
85
82
5
5
5
4
5
85e
84
79
84
5
5
6
89f
78
5
7
8
59
43
59
43
10
10
a
Reaction conditions: alkene (1 mmol), NaIO (2 mmol), catalyst (0.073 mmol), CH CN/H O (10 ml/10
ml). GLC yield based on the starting alkene. The by-product is allylic ketone. The by product is
benzaldehyde. The by-product is acetophenone. In this case, 7% ketone and 4% alcohol were produced.
4
3
2
b
c
d
e
f
Table 8. The results of Catalysts Recovery and the Amounts of Ruthenium Leached in the Epoxidation of
a
Cyclooctene with Sodium Periodate
Run
[Ru(salophen)@DAB-PS]
[Ru(salophen)@AP-PS]
[Ru(salophen)@ATP-PS]
Yield (%)b
Ru leached (%)c Yield (%)b Ru leached (%)c Yield (%)b
Ru leached (%)c
1
2
3
4
93
89
84
79
55
1.5
1.0
0.6
0.4
0
87
82
78
65
47
2.1
1.7
1.0
0.8
0.0
89
85
78
62
43
2.9
2.0
1.1
0.9
5
0
a
b
Reaction conditions: alkene (1 mmol), NaIO (2 mmol), catalyst (600 mg), CH CN/H O (10 ml/10 ml). GLC
4
3
2
c
yield based on the starting alkene. Determined by ICP.
[
Ru(salophen)@AP-PS]
and
[Ru(salophen)@ATP-PS],
CONCLUSIONS
respectively (Table 8 and Fig. 4). The filtrates were collected
for the determination of Ru leaching (measured by ICP). The
results showed that in the first three runs, the ruthenium
catalyst was leached from supports and, therefore, the epoxide
yield decreased (Table 8).
In conclusion, Ru(salophen)Cl supported on amine-
modified polystyrene was prepared by axial ligation of
metallocomplex to the support. Ruthenium(III) salophen,
supported on modified PS, [Ru(salophen)@DAB-PS],
1
027

Moghadam et al.
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