Epoxidation of alkenes with NaIO4 catalyzed by an efficient and reusable natural polymer-supported ruthenium(III) salophen catalyst

Article abstract of DOI:10.1007/s13738-015-0774-x

In the present study, preparation, characterization, and catalytic activity of Ru(salophen)Cl supported on chitosan were investigated. The prepared heterogeneous catalyst was characterized by diffuse reflectance UV-vis and FT-IR spectroscopic techniques, scanning electron microscopy, and neutron activation analysis. In this catalytic system, the effects of different solvents were studied in the epoxidation of cis-cyclooctene and CH₃CN/H₂O was found to be a better solvent. Also, the effects of oxygen donors such as NaIO₄, H₂O₂, H₂O₂/urea(UHP), tert-BuOOH, NaClO, and Bu₄NIO₄ were studied in the epoxidation of cis-cyclooctene and NaIO₄ was selected as an oxidant. The catalytic activity of this new heterogeneous catalyst in the epoxidation of cyclic and linear alkenes using NaIO₄ as an oxidant in CH₃CN/H₂O at room temperature was studied. The obtained results led us to conclude that [Ru(salophen)Cl@ chitosan] is an efficient catalyst for the epoxidation of alkenes with NaIO₄. The catalyst can be readily recovered simply by filtration and reused several times without any significant loss in its catalytic activity.

Full text of DOI:10.1007/s13738-015-0774-x

J IRAN CHEM SOC
DOI 10.1007/s13738-015-0774-x
ORIGINAL PAPER
Epoxidation of alkenes with NaIO catalyzed by an efficient
4
and reusable natural polymer‑supported ruthenium(III) salophen
catalyst
1
2
1
1
M. Hatefi Ardakani · M. Moghadam · S. Saeednia · Z. Pakdin‑Parizi
Received: 28 April 2015 / Accepted: 2 November 2015
©
The Author(s) 2015. This article is published with open access at Springerlink.com
Abstract In the present study, preparation, characteriza-
tion, and catalytic activity of Ru(salophen)Cl supported
on chitosan were investigated. The prepared heteroge-
neous catalyst was characterized by diffuse reflectance
UV–vis and FT-IR spectroscopic techniques, scanning
electron microscopy, and neutron activation analysis. In
this catalytic system, the effects of different solvents were
Introduction
Transition metal Schiff base complexes have been widely
used as homogeneous or heterogeneous catalysts in vari-
ous reactions such as aliphatic and aromatic hydroxylation,
olefin epoxidation, and alcohol oxidation [1–27]. Although
homogeneous catalysts exhibit high catalytic activity in
many organic reactions, recovery and recycling of these cata-
lysts are difficult because of oxidation of ligand or formation
of dimericperoxo-species and µ-oxospecies [28]. Immobili-
zation of homogeneous catalysts on a solid support enables
easy recovery and recycling of homogeneous catalysts and
avoids their disadvantages with respect to handling and reus-
ability of the catalyst [29]. Recently, ruthenium complexes
have been developed as suitable catalysts in organic synthe-
sis [30–35]. Also, Schiff base complexes of ruthenium(III)
are used as potential catalysts for olefin epoxidation [36].
Chitosan is a linear copolymer in which 2-amino-2-deoxy-
d-glucopyranose(glucosamine) is linked to 2-acetamido-
2-deoxy-d-glucopyranose(N-acetylglucosamine) by a β-(1,4)
glycosidic bond (Scheme 1). Chitosan is produced by N-dea-
cetylation of chitin, which is easily obtained from crab or
shrimp. This natural polymer has many good properties such
as biocompatibility, biodegradability, and non-toxicity, which
make it an environmental-friendly material. Furthermore, it
has high percentage of nitrogen and optical chiral sites in its
molecular framework [37]. Recently, the novel applications
of chitosan as natural support have been reported [37–42].
The present paper describes the preparation, charac-
terization, and catalytic activity of ruthenium(III) salo-
phen supported on chitosan in the epoxidation of alk-
enes using sodium periodate at room temperature. As
far as we know, our study is the first to use chitosan as
support for Ru(salophen)Cl complex in heterogeneous
catalysis.
studied in the epoxidation of cis-cyclooctene and CH CN/
3
H O was found to be a better solvent. Also, the effects of
2
oxygen donors such as NaIO , H O , H O /urea(UHP),
4
2
2
2
2
tert-BuOOH, NaClO, and Bu NIO were studied in the
4
4
epoxidation of cis-cyclooctene and NaIO was selected as
4
an oxidant. The catalytic activity of this new heterogene-
ous catalyst in the epoxidation of cyclic and linear alkenes
using NaIO as an oxidant in CH CN/H O at room temper-
4
3
2
ature was studied. The obtained results led us to conclude
that [Ru(salophen)Cl@ chitosan] is an efficient catalyst for
the epoxidation of alkenes with NaIO . The catalyst can be
4
readily recovered simply by filtration and reused several
times without any significant loss in its catalytic activity.
Keywords Ru(III)salophen · Chitosan · Alkene
epoxidation · Sodium periodate · Heterogeneous catalyst
*
M. Hatefi Ardakani
m.hatefi@vru.ac.ir
1
2
Department of Chemistry, Faculty of Science, Vali-e-Asr
University of Rafsanjan, 77188-97111 Rafsanjan, Iran
Catalysis Division, Department of Chemistry, University
of Isfahan, 81746-73441 Isfahan, Iran
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J IRAN CHEM SOC
Scheme 1 Chemical structure of the chitosan, n: degree of the acety-
lation
Scheme 2 Proposed structure of [Ru(salophen)Cl@chitosan]
Experimental
Chitosan was obtained from Sigma-Aldrich (viscomet-
ric average molecular weight is about 250,000, degree of
deacetylation ≥75 %). Other materials were purchased
from Fluka and Merck chemical companies. FT-IR spectra
−1
obtained as KBr pellets were in the range of 400–4000 cm
when observed using Nicolet-iS10 spectrometer. Diffuse
reflectance spectrum was recorded on a Shimadzu UV-265
instrument using optical-grade BaSO as reference. Scanning
4
electron micrographs of the catalyst and support were taken
on a SEM Philips XL30 instrument. Gas chromatography
experiments (GC) were performed with Philips GC-PU 4600
instrument using a 2-m column packed with silicon DC-200
and FID detector. n-Decane was used as internal standard.
The salophen ligand was prepared by refluxing the ethanolic
solutions of 1,2-phenylenediamine and salicylaldehyde in a
Fig. 1 UV–Vis spectrum of a Ru(salophen)Cl and b [Ru(salophen)
Cl@chitosan]
1
:2 molar ratio and metallated with RuCl ·3H O [43, 44].
3
2
were separated by filtration, washed with distilled water,
ethanol, and acetone successively, and dried at 80 °C for
Preparation of the catalyst, [Ru(salophen)Cl@chitosan]
8
h to yield the chitosan-supported ruthenium(III) salophen.
A mixture of chitosan (2 g) in acetic acid (2 %, 100 ml)
was stirred electromagnetically at 323 K for 0.5 h. Then
distilled water (100 ml) was added to form a colloidal
solution. The pH of the solution was adjusted to 8 by the
addition of an aqueous solution of Na CO (1 %). Then,
General procedure for catalytic epoxidation of alkenes
with NaIO catalyzed by [Ru(salophen)Cl@chitosan]
4
A mixture of alkene (1 mmol), catalyst (800 mg, containing
0.05 mmol Ru(salophen)Cl) in CH CN (5 ml), and NaIO
2
3
Ru(salophen)Cl (0.45 g, 1 mmol) was dissolved in CH CN
3
3
4
(
50 ml) and was added drop-wise to the first solution. After
(2 mmol) in H O(5 ml) was prepared and stirred magneti-
2
stirring for 24 h in these conditions, the light-brown solids
cally at room temperature. The progress of the reaction was
1
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J IRAN CHEM SOC
monitored by GC. At the end of the reaction, the reaction
mixture was diluted with Et O (20 ml) and filtered. The cat-
2
alyst was thoroughly washed with Et O, and the combined
2
washings and filtrates were purified on silica gel plates or a
1
silica-gel column. IR and H NMR spectral data confirmed
the identities of the products.
Results and discussion
Preparation and characterization of catalyst,
[
Ru(salophen)Cl@chitosan]
The supported catalyst, [Ru(salophen)Cl@chitosan],
seemed to have been formed by interaction between
the amino groups of chitosan and ruthenium atoms of
Fig. 3 Scanning electron micrograph of
Ru(salophen)Cl@chitosan]
a chitosan and b
[
Ru(salophen)Cl (Scheme 2) [45]. The presence of a large
number of amine groups on the chitosan explains the uptake
of metal cations on the free electronic doublet of nitrogen
at near neutral pH [46]. The new heterogeneous catalyst
[Ru(salophen)Cl@chitosan] was characterized by FT-IR
spectroscopy, diffuse reflectance UV–Vis spectroscopy,
scanning electron microscopy (SEM), and neutron acti-
vation analysis (NAA). The Ru content of catalyst, deter-
−
1
mined by NAA, was found to be about 0.065 mmol g .
The UV–Vis spectra of Ru(salophen)Cl and [Ru(salophen)
Cl@chitosan] were recorded in the range of 300–600 nm. In
Fig. 2 FT-IR spectrum of a chitosan and b [Ru(salophen)Cl@chi-
tosan]
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J IRAN CHEM SOC
this region, Ru(salophen)Cl showed a sharp peak and a shoul-
der at 325 and 397 nm which can be attributed to π → π* and
n → π* transitions [25] (Fig. 1a). These peaks were observed
with only a slight shift at 318 and 370 nm in the UV–Vis
spectra of [Ru(salophen)Cl@chitosan] (Fig. 1b). This con-
firmed the presence of metallocomplex on the support.
with water, a 1:1 mixture of acetonitrile/water was found
to have the highest catalytic activity. The results are shown
in Table 2.
Catalytic alkene epoxidation with NaIO in the
4
presence of [Ru(salophen)Cl@chitosan]
The FT-IR spectrum provided further evidence for
attachment of Ru(salophen)Cl to the support. The FT-IR
spectrum of [Ru(salophen)Cl@chitosan] shows a band at
The catalytic activity of this catalyst was investigated in
the epoxidation of different alkenes such as linear, cyclic,
and phenyl-substituted ones. The epoxidation reaction was
performed at room temperature with NaIO in CH CN/H O
−1
1
560 cm which is not observed in the FT-IR spectrum
of pure chitosan. This band belongs to azomethine (C=N)
stretching vibration in the salophen ligand (Fig. 2).
The SEM images of the chitosan and [Ru(salophen]Cl@
chitosan] showed a clear change in morphology of the chi-
tosan after introducing to salophen complex (Fig. 3).
4
3
2
(Scheme 3). In the epoxidation of cyclooctene with NaIO4
in the presence of [Ru(salophen)Cl@ chitosan] after 24 h,
the conversion was 73 % with 100 % selectivity. Cyclohex-
ene was oxidized with yields of 68 and 85 % epoxide selec-
tivity. The cyclohexene-1-one as a by-product was detected
in the reaction mixture. In the epoxidation of styrene and
α-methylstyrene, the conversion was 65 and 58 %, and
the epoxide selectivities were 78 and 81 %, respectively.
In the oxidation of styrene and α-methylstyrene, benzal-
dehyde and acetophenone were produced as by-products,
respectively.
Catalytic activity
First, in order to achieve the suitable reaction conditions,
the reaction parameters were optimized in the epoxidation
of cyclooctene.
Effects of oxidants on the epoxidation of cyclooctene
catalyzed by [Ru(salophen)Cl@chitosan]
In the case of indene, the selectivity was 100 %. Lin-
ear alkenes such as 1-heptene and 1-dodecene were con-
verted to their corresponding epoxides with 100 % selectiv-
ity (Table 3). Blank experiments in the absence of catalyst
were also investigated. The obtained results showed that
the alkenes were not epoxidized in the absence of catalyst
or oxidant.
Different oxygen donors such as NaIO , H O , H O /
4
2
2
2
2
urea (UHP), tert-BuOOH, NaClO, and Bu NIO were
4
4
used for the epoxidation of cyclooctene in the presence
of [Ru(salophen)Cl@chitosan]. The results showed that
NaIO is the best oxidant because of its good oxidation con-
4
version and inertness in the absence of catalyst (Table 1).
Table 2 Effects of solvents on the epoxidation of cyclooctene with
NaIO catalyzed by [Ru(salophen)Cl@chitosan] at room temperature
Effects of solvents on the epoxidation of cyclooctene
catalyzed by [Ru(salophen)Cl@chitosan]
4
Row
Solvent
Epoxide yield (%)
Time (h)
1
2
3
4
5
6
CH CN/H O(1:1)
73
61
27
20
14
11
24
24
24
24
24
24
3
2
Among the mixture of acetonitrile, methanol, ethanol and
acetone (single-phase systems), and dichloromethane (two-
a
CH CN/H O(2:1)
3
2
CH OH/H O
phase system with n-Bu NBr as phase-transfer catalyst)
3
2
4
C H OH/H O
2
5
2
CH COCH /H O
Table 1 Effects of oxidants on the epoxidation of cyclooctene cata-
lyzed by [Ru(salophen)Cl@chitosan] at room temperature
3
3
2
CH2Cl2/H2O
Row Oxidant
Solvent
Epoxide yield (%) Time (h)
Reaction conditions: cyclooctene (1 mmol), NaIO (2 mmol), catalyst
4
(
800 mg, containing 0.05 mmol Ru(salophen)Cl), solvent (5 ml), H O
2
1
2
3
4
5
6
7
NaIO4
CH CN/H O 73
24
24
24
24
24
24
24
3
2
(5 ml)
a
H O
CH CN
25
2
2
3
CH CN (5 ml) and H O (2.5 ml)
3 2
H O /urea
CH CN
38
2
2
3
tert-BuOOH CH CN
17
3
NaOCl
CH CN
41
3
Bu NIO4
CH CN
10
4
3
No oxidant
CH CN
trace
3
Reaction conditions: cyclooctene(1 mmol), oxidant (2 mmol), cata-
lyst (800 mg, containing 0.05 mmol Ru(salophen)Cl), CH CN (5 ml),
Scheme 3 Alkene epoxidation with NaIO4 catalyzed by
3
H O (5 ml)
[Ru(salophen)Cl@chitosan]
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J IRAN CHEM SOC
Table 3 Epoxidation of alkenes
Conversion (%)a
Epoxide selectivity(%)a
Entry
1
Alkene
Time (h)
24
with NaIO catalyzed by
4
[
Ru(salophen)Cl@chitosan]
73
100
2
68b
85
24
3
4
65c
58d
78
81
24
24
5
45
100
24
6
7
a
35
32
100
100
24
24
GLC yield based on the starting alkene
The by-product is allylic ketone
The by-product is benzaldehyde
The by-product is acetophenone
b
c
d
Table 4 The results of catalyst recovery and the amounts of ruthe-
nium leached in the epoxidation of cyclooctene with NaIO4
agitation with magnetic stirring. This new heterogeneous
catalyst has the advantages such as easy preparation, facile
and effective recovery, and recycling of the catalyst. Also,
chitosan is a natural and cheap support and possesses par-
ticular microstructure and excellent function.
Run
Epoxide yield (%)a
Time (h)
Ru leached (%)b
1
2
3
4
5
73
72
71
71
69
24
24
24
24
24
4
2.7
1
Acknowledgments The support of this work by Vali-e-Asr Univer-
sity of Rafsanjan is acknowledged.
0
0
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://crea-
tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
made.
a
GLC yield based on starting alkene
b
Determined by atomic absorption spectroscopy
Catalyst reusability
Since the transition metal complexes are often expensive to
purchase or prepare, we decided to investigate the reusability
of catalyst in the epoxidation reactions. In this manner, the
reusability of [Ru(salophen)Cl@chitosan] was studied in the
epoxidation of cyclooctene. After each experiment, the cata-
lyst was separated from the reaction mixture simply by filtra-
tion, washed thoroughly with CH CN and Et O, and dried
before using it in the subsequent run. The filtrates were used
for the determination of ruthenium leaching by atomic absorp-
tion spectroscopy (AAS). The results showed that in the first
three runs, some Ru is leached from the support (Table 4).
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