DABCO tribromide immobilized on magnetic nanoparticle as a recyclable catalyst for the chemoselective oxidation of sulfide using H2O 2 under metal- and solvent-free conditions

Article abstract of DOI:10.1016/j.catcom.2013.08.025

Magnetic nanoparticle-supported 1,4-diazabicyclo[2.2.2] octane tribromide (MNPs-DABCO tribromide) as a bromine source was readily prepared and characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectrosc

Full text of DOI:10.1016/j.catcom.2013.08.025

DABCO tribromide immobilized on magnetic nanoparticle as a recyclable
catalyst for the chemoselective oxidation of sulfide using H2O2 under metal-
and solvent-free conditions
Amin Rostami, Yahya Navasi, Darush Moradi, Arash Ghorbani-Choghamarani
PII:
DOI:
Reference:
S1566-7367(13)00334-8
doi: 10.1016/j.catcom.2013.08.025
CATCOM 3629
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
12 June 2013
31 July 2013
26 August 2013
Please cite this article as: Amin Rostami, Yahya Navasi, Darush Moradi, Arash
Ghorbani-Choghamarani, DABCO tribromide immobilized on magnetic nanoparti-
cle as a recyclable catalyst for the chemoselective oxidation of sulfide using H2O2
under metal- and solvent-free conditions, Catalysis Communications (2013),
doi:
10.1016/j.catcom.2013.08.025
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DABCO tribromide immobilized on magnetic nanoparticle as a
recyclable catalyst for the chemoselective oxidation of sulfide using
H2O2 under metal- and solvent-free conditions
Amin Rostami*,a,b, Yahya Navasia, Darush Moradia, Arash Ghorbani-Choghamaranic
aDepartment of Chemistry, Faculty of Science, University of Kurdistan, Zip Code
66177-15175, Sanandaj, Iran.
bResearch Centre for Medicinal Plant Breeding and Improvement, University of
Kurdistan, Sanandaj, Iran.
cDepartment of Chemistry, Faculty of Science, Ilam University, Ilam, Iran.
(E-mail: a_rostami372@yahoo.com, Tel: +989183730910; Fax: +988716624004)
ABSTRACT
Magnetic nanoparticle-supported the 1,4-diazabicyclo[2.2.2] octane tribromide
(MNPs-DABCO tribromide) as a bromine source was readily prepared and
characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), fourier
transform infrared spectroscopy (FT-IR), Energy dispersive X-ray spectroscopy
(EDX), the scanning electron microscopy (SEM), and vibrating sample magnetometer
(VSM) techniques. The catalytic activity of MNPs-DABCO tribromide was
investigated in the chemoselective oxidation of sulfides to sulfoxides using 30% H2O2
as an oxidant at room temperature under solvent-free conditions. The heterogeneous
catalyst could be recovered easily and reused 15 times without significant loss of its
catalytic activity.
Keywords: Magnetic nanoparticles, 1,4-Diazabicyclo[2.2.2] octane tribromide,
Nanocatalyst, Hydrogen peroxide, Sulfides, Sulfoxides

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1. Introduction
Catalyst recovery and reusability are the two most important features for many
catalytic processes; most heterogeneous systems require a tedious workup of the final
reaction mixture to recover the catalyst [1].
Magnetic nanoparticles (MNPs) have recently appeared as a new type of catalyst
support because of their easy preparation and functionalization, large surface area
ratio, facile recovery and recyclability via magnetic force as well as low toxicity and
price [2]. These fascinating features have made MNPs a promising alternative to other
catalyst supports [3-5].
In order to decrease the toxicity of molecular bromine, a wide variety of bromine
source reagents were designed and applied in organic synthesis [6-8]. The main
disadvantage of reagents based on bromine is their separation from the products which
needs solid-liquid or liquid-liquid techniques in many reactions. This drawback can be
overcome by immobilizing these reagents or catalysts on magnetic nanoparticle
(MNPs), which can be easily removed from the reaction mixture by magnetic
separation.
Sulfoxides are valuable synthetic intermediates for the synthesis of chemically and
biologically important and significant molecules [9]. The main synthetic route for the
preparation of these valuable materials is oxidation of the corresponding sulfides.
Although a wide variety of reagent systems available for this key transformation, the
major disadvantages of these methods include removal or recovery of the expensive
catalyst, metal residues in the products, and the formation of large amounts of toxic
waste [10-14]. Hence there is a need to develop new catalysts which can overcome
these drawbacks.

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In continuation of our studies on application of N-bromo reagents and magnetically
separable catalysts in organic reactions [15-17], herein, we report the magnetic
nanoparticle-supported DABCO tribromide as a novel, eco-friendly, and reusable
organic-inorganic hybrid catalyst for the chemoselective oxidation of sulfides to
corresponding sulfoxides using H2O2 as a green oxidant. Two major novelties of this
work are: (1) this is the first report of a magnetic-nanoparticle-supported DABCO
tribromide for use as bromine source reagent or catalyst and (2) this is the first time
that a magnetically recyclable nanocatalyst has been used for the oxidation of sulfide
to sulfoxide in the absence of any transition-metal.
2. Experimental
2.1. General procedure for the oxidation of sulfides
The sulfide (1 mmol) was added to a mixture of 30% H2O2 (2.4 equiv, 1 g) and
MNPs-DABCO tribromide (10 mg), and the mixture was stirred at room temperature
for the time specified. The progress was monitored by TLC (EtOAc/n-hexane, 1/10).
After completion of the reaction, the catalyst was separated from the product by an
external magnet (within 5 seconds) and the mixture was washed with Et2O (2×5 mL)
and decanted. The combined organics were dried over anhydrous Na2SO4 and then
evaporation of diethyl ether under reduced pressure gave the pure products in 80-97%
yields.
3. Results and discussions
3.1. Preparation and Characterization of MNPs-DABCO tribromide
DABCO tribromide successfully was immobilized over MNPs by using the surface
modification strategy as descripted in Scheme 1.

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<Scheme 1>
Magnetic Fe3O4 nanoparticles were prepared by chemical coprecipitation method
and coated with 3-chloropropyltrimethoxysilane (CPTMS) by covalent bonds. The
reaction of the supported CPTMS with DABCO in dry acetone under reflux
conditions for 36 h produced the DABCO functionalized magnetic nanoparticles
(MNPs-DABCO) [18]. Ultimately, the reaction of DABCO group with bromine in
CHCl3 led to the corresponding DABCO tribromide supported on magnetic
nanoparticles (MNPs-DABCO tribromide) [19-20]. Experimental details were
provided in the Supplementary data.
The catalyst has been characterized by XRD, TGA, EDX, FT-IR, SEM and VSM
techniques. The position and relative intensities of all peaks in the XRD pattern of
MNPs-DABCO tribromide confirm well with standard XRD pattern of Fe3O4,
indicating retention of the crystalline cubic spinel structure during functionalization of
MNPs (Supplementary data, Fig. S1) [21].
One indication of bond formation between the nanoparticles and the catalyst can be
inferred from TGA. The TGA curves of the MNPs-CPTMS and MNPs-DABCO
tribromide are presented in Fig. 1. The weight loss at temperatures below 200 ºC is
due to the removal of physically adsorbed solvent and surface hydroxyl groups [22].
Organic groups have been reported to desorb at temperatures above 260 °C. The
weight loss of CPTMS-modified magnetite NPs appears about 2.5% at 270-480ºC
which is contributed to the thermal decomposition of the 3-chloropropyl silane
groups. For MNPs-DABCO tribromide, there is a well-defined mass weight loss of
7.5 % between 250 and 500 ºC related to the breakdown of the DABCO tribromide
moieties. On the basis of this result a loading of 0.22 mmol/g was obtained.
<Figure 1>

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EDX spectrum shows the elemental composition (Fe, Si, Br and Cl) of the MNPs-
DABCO tribromide (Supplementary data, Fig. S2).
Successful functionalization of the MNPs can be inferred from FT-IR techniques.
The FT-IR spectra for the MNPs (black), MNPs-CPTMS (green), MNPs-DABCO
(red), and MNPs-DABCO tribromide (blue) samples are shown in Figure 2. The FTIR
spectrum for the magnetite nanoparticles alone shows a stretching vibration at 3381
cm-1 which incorporates the contributions from both symmetrical and asymmetrical
modes of the O-H bonds which are attached to the surface iron atoms. The presence of
an adsorbed water layer is confirmed by a stretch for the vibrational mode of water
found at 1626 cm-1. The absorption bands at 574 cm-1 and 623 are attributed to the Fe-
O bonds. The band formation between MNPs and 3-chloropropylsilica group is
confirmed by Fe-O-Si absorption band that appear at 999 cm-1. The presence of
DABCO moiety on the surface of MNPs can be inferred from both FT-IR spectra of
MNPs-DABCO and MNPs-DABCO tribromide. There are four characteristic peaks
include CH2 starching and bending bonds at 2954 cm−1 and 1462 cm−1 respectively,
C-N+ starching at 1615 cm−1 and C-N starching at 1386 cm−1 [18, 23-24]. Compared
with the spectrum of MNPs-DABCO, the MNPs-DABCO tribromide exhibits a
characteristic broad band in the region 1107-991 cm-1. Br3 exhibits a signal between
180 and 200 cm-1 which is not detectable with the FT–IR spectrum [25].
<Figure 2>
The SEM image of MNPs-DABCO tribromide was confirmed that the catalyst was
made up of uniform nanometer-sized particles less than 26 nm (Supplementary data,
Fig. S3).
The magnetic property of MNPs and MNPs-DABCO tribromide were
characterized by VSM (Supplementary data, Fig. S4). As expected, the bare MNPs,

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showed the higher magnetic value (saturation magnetization, Ms) of 74.3 emug-1 [26],
the Ms value of MNPs-DABCO tribromide is decreased due to the silica coating and
the layer of the grafted catalyst (56.0 emu g-1). It has been reported that the Fe3O4
nanoparticles with a value of coercivity (Hc) lower than 20 Oe could be called
superparamagnetic. MNPs and MNPs-DABCO tribromide have an Hc of 16.09 and
14.15 Oe, respectively. As a result, the modified MNPs have a typical
superparamagnetic behavior [27-28] and can be efficiently attracted with a small
magnet.
Unfortunately, due to the magnetic properties of MNPs-DABCO tribromide it is
actually impossible to further characterize this material by using solid-state NMR
spectroscopy.
3.2. The catalytic applications of MNPs-DABCO tribromide in the oxidation of
sulfides to sulfoxides
The catalytic activity of MNPs-DABCO tribromide was investigated in the selective
oxidation of sulfides to sulfoxides using 30% H2O2 as a green oxidant (Scheme 2).
<Scheme 2>
In order to optimize the reaction conditions, we evaluated the influence of different
amounts of catalyst and hydrogen peroxide on the oxidation of methyl phenyl sulfide
as a model compound under solvent-free conditions at room temperature (Table 1).
As shown in Table 1, the reaction was incomplete in the absence of a catalyst even
after 24 h. The optimum amount of H2O2 (2.4 equivalent) in the presence of MNPs-
DABCO tribromide (10 mg) is ideal for complete conversion of methyl phenyl sulfide
sulfides to methyl phenyl sulfoxide.
<Table 1>

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In order to generalize the scope of the reaction, a series of structurally diverse
sulfides was subjected to oxidation under the optimized reaction conditions, and the
results are presented in Table 2. The reactions went on well to afford products in short
times and good to high yields. It is important to note that thiantrene was
homoselectively oxidized to thianthrene monosulfoxide without the formation of the
disulfoxide product.
To show the chemoselectivity of this method, the sulfides containing oxidation-
prone and acid-sensitive functional groups such as OH, CHO and COOCH3 were
subjected to the sulfoxidation reaction; these functional groups remained intact during
the conversion of sulfides to sulfoxides (Table 2, entries 8-10).
<Table 2>
The ability to easily recover and recycle of MNPs-DABCO tribromide was
investigated. We have found that this catalyst was rapidly recovered and demonstrated
remarkably excellent recyclability; after the first use of catalyst in oxidation of methyl
phenyl to give methyl phenyl sulfoxide, the catalyst was separated by an external
magnet (Fig. 3), washed thoroughly with ether, and reused as such for subsequent
experiments under similar reaction conditions. As shown in Fig. 4, catalyst was
reusable without any significant loss of activity for the 15th recycling.
<Figure 3>
<Figure 4>
On the basis of the literature [14], a plausible mechanism for this transformation is
outlined in scheme 3.
<Scheme 3>

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In summary, we have designed the first MNPs-DABCO tribromide as a source of
bromin that catalyze oxidation of sulfides to sulfoxides using H2O2 with remarkably
high activity and chemoselectivity. The heterogenized catalyst can be readily recycled
by magnetic decantation and used for oxidation for up to 15 times without any
significant loss of activity. Studies to further explore the potential of this powerful
immobilization strategy for the preparation of magnetically recoverable chiral
catalysts are underway.
Acknowledgment
We are grateful to the University of Kurdistan Research Councils for the partial
support of this work.
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Schemes
O
O
(3-chloropropyl) triethoxysilane
ethanol/water, 40 °C, 8 h
NH2OH
Fe3O4 NP
+
Si
Cl
FeCl3.6H2O
FeCl2.4H2O
Fe3O4 NP
N2, rt, 30 min
O
MNPs
MNPs-CPTMS
N
N
O
O
O
Si
N
O
Si
N
Fe3O4 NP
Fe3O4 NP
Br2, CHCl3
Cl
(Br3) (Cl)
O
O
NH
N
acetone, reflux, 36h
MNPs-DABCO
MNPs-DABCO Tribromide
Scheme 1. Synthesis of MNPs-DABCO Tribromide
O
S
MNPs-DABCO tribromide (Cat.), H2O2
Solvent-free, r.t.
R1
R2
S
R1
R2
R1, R2 = Aryl, alkyl
Scheme 2. MNPs-DABCO tribromide catalyzed the oxidation of sulfides to sulfoxides using H2O2

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O
O
Si
N
Fe3O4 NP
(Br3) (Cl)
O
NH
O
O
Si
N
Fe3O4 NP
(Br) (Cl)
O
NH
S
R1
R2
Br2
H2O
Br
S
R2
Br
R1
H2O2
H2O
Br
O
S
+ 2H+
R2
R1
Scheme 3. A possible mechanism for the oxidation of sulfides to sulfoxides using H2O2 catalyzed by
MNPs-DABCO tribromide

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Figures
Fig. 1. TGA profile of the MNPs-CPTMS and MNPs-DABCO Tribromide

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Fig. 2. FTIR spectra of MNPs (black), MNPs-CPTMS (green), MNPs-DABCO (red) and MNPs-
DABCO tribromide (blue)
Fig. 3. A reaction mixture in the absence (left) or presence of a magnetic field (right)

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Fig. 4. The recycling experiment of MNPs-DABCO tribromide for oxidation
of methyl phenyl sulfide (1 mmol) using H2O2 at room temperature for
20 min

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Tables
Table 1
The selective oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide under different
conditions
Entry
1
Catalyst (mg)
Catalyst-Free
H2O2 (equiv.)
2.4
Time/min
24h
Converted yield (%)a
80
2
3
4
5
6
7
8
Fe3O4 NP (5)
2.4
2.4
2.4
2.4
2.4
0.6
1.2
24h
20
5
80
MNPs-DABCO tribromide (5)
MNPs-DABCO tribromide (10)
MNPs-DABCO tribromide (15)
MNPs-DABCO tribromide (30)
MNPs-DABCO tribromide (10)
MNPs-DABCO tribromide (10)
100
100
100
100
100
100
5
3
20
15
a Conversion of sulfide to sulfoxide was determined by GC

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Table 2
MNPs-DABCO tribromide (10 mg) catalyzed selective oxidation of sulfides (1 mmol) to sulfoxides
using 30% H2O2 (2.4 equiv.) under solvent-free conditions at room temperature
Entry
1
Sulfide
Time (min)
20
Sulfoxidea
Yield (%)b
97
S
O
CH3
S
S
S
O
S
2
20
95
O
S
3
4
25
5
97
90
O
S
S
O
O
S
S
O
S
5
70
92
S
S
O
6
7
15
5
80
93
S
S
S
11
11
11
11
O
S
S
O
8
9
5
5
83
82
OH
S
OH
H
O
S
H
O
O
O
S
O
O
10
5
82
S
O
O
1
aAll the products are known and were characterized by IR, H NMR, and by melting point comparisons with those
of authentic samples [10-14].
bIsolated yield

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Graphical Abstract
DABCO tribromide immobilized on magnetic nanoparticle as a recyclable catalyst for
the chemeselective oxidation of sulfide using H2O2 under metal- and solvent-free
conditions
O
O Si
N
Fe3O4 NP
O
S
(Cat.)
(Br3) (Cl)
O
NH
S
R1
R2
R1
R2
H2O2 (30%), Solvent-Free, r.t.
R1, R2 =Aryl, Heterocyclic, Benzylic and Alkyl

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Highlights
 MNPs-DABCO tribromide as a source of bromine was synthesized and
characterized.
 MNPs-DABCO tribromide catalyzed highly efficient the chemoselective
oxidation of sulfides.
 H2O2 was used as a green, commercially available, cheap, and chemically
stable oxidant.
 The catalyst was easily recovered and reused without any significant loss in
catalytic activity.