Chiral oxo-vanadium (+)-pseudoephedrine complex immobilized on magnetic nanoparticles: A highly efficient and recyclable novel nanocatalyst for the chemoselective oxidation of sulfides to sulfoxides using H2O2

Article abstract of DOI:10.1016/j.molcata.2014.12.010

The first chiral oxo-vanadium (+)-pseudoephedrine complex supported on magnetic nanoparticles Fe₃O₄ [VO(pseudoephedrine)@MNPs] as novel magnetically interphase nanocatalyst was designed and characterized using the most common techniq

Full text of DOI:10.1016/j.molcata.2014.12.010

Accepted Manuscript
Title: Chiral oxo-vanadium (+)-pseudoephedrine complex
immobilized on magnetic nanoparticles: A highly efficient and
recyclable novel nanocatalyst for the chemoselective oxidation
of sulfides to sulfoxides using H O
2
2
Author: Amin Rostami Bahareh Atashkar
PII:
S1381-1169(14)00570-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.12.010
MOLCAA 9372
To appear in:
Journal of Molecular Catalysis A: Chemical
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Revised date:
Accepted date:
16-5-2014
13-9-2014
7-12-2014
Please cite this article as: Amin Rostami, Bahareh Atashkar, Chiral oxo-vanadium
+)-pseudoephedrine complex immobilized on magnetic nanoparticles: A highly
(
efficient and recyclable novel nanocatalyst for the chemoselective oxidation of
sulfides to sulfoxides using H2O2, Journal of Molecular Catalysis A: Chemical
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Chiral oxo-vanadium (+)-pseudoephedrine complex immobilized on magnetic
nanoparticles: a highly efficient and recyclable novel nanocatalyst for the
chemoselective oxidation of sulfides to sulfoxides using H
2
O
2
a,b,*
a
Amin Rostami a_rostami372@yahoo.com a.rostami@uok.ac.ir, Bahareh Atashkar
a
Department of Chemistry, Faculty of Science, University of Kurdistan, Zip Code 66177-
1
5175, Sanandaj, Iran
b
Research Centre of Nanotechnology, University of Kurdistan, P.O. Box 416, Sanandaj, Iran
Corresponding author: Tel.: +989183730910; Fax: +9188716624004.
Graphical Abstract
Chiral oxo-vanadium (+)-pseudoephedrine complex immobilized on magnetic nanoparticles
Fe
3
4
O : a highly efficient and recyclable novel nanocatalyst for the chemoselective oxidation
of sulfides to sulfoxides using H
2
O
2
Highlights

The first chiral VO(Pseudoephedrine)@MNPs was easily synthesized and
characterized.



It catalyzed highly efficient chemoselective oxidation of sulfides.
The catalyst allows for greener method while keeping high yields and low times.
The catalyst was easily reused up to 20 runs without significant loss of activity.
ABSTRACT
The first chiral oxo-vanadium (+)-pseudoephedrine complex supported on magnetic
nanoparticles Fe
3
4
O [VO(Pseudoephedrine)@MNPs] as novel magnetically interphase

nanocatalyst was designed and characterized using the most common techniques.
VO(Pseudoephedrine)@MNPs catalyzed the chemoselective oxidation of sulfides to
sulfoxides using hydrogen peroxide as a green oxidant in high yields with 20-27%
enantiomeric excesses under solvent-free conditions at room temperature. The catalyst was
recycled up to 20 times with little loss of activity and enantioselectivity.
Keywords
sulfides, sulfoxides, hydrogen peroxide, oxo-vanadium (+)-pseudoephedrine complex,
magnetically catalyst
1
.Introduction
Compare to heterogeneous catalysis, in homogeneous catalysis, the chemo-, regio-, and
enantioselectivity of the catalyst is better, but the difficulty of catalyst separation from the
final product creates economic and environmental barriers. Magnetic nanoparticles (MNPs)
have recently considered as ideal supports for the heterogenization of homogeneous catalysts
due to their easy preparation and functionalization, large surface area ratio, facile recovery
and recyclability via magnetic force, as well as low toxicity and price [1-4].
Recently, there has been increasing interest in the synthesis of vanadium (V) Schiff base
complexes derived from chiral and achiral amino alcohols and their catalytic activity. These
complexes have been used successfully as catalyst in enantioselective oxidation of organic
sulfides [5-6], asymmetric alkynylation of aldehydes [6], epoxidation of cyclooctene [7] and
stereoselective synthesis of functionalized cyclic ethers [8-9]. However, the main
disadvantage of a catalyst based on chiral vanadium complexe is their separation from the
products, which needs solid-liquid or liquid-liquid techniques in many reactions. This
drawback can be overcome by immobilization of these catalysts on MNPs.

Chiral and achiral sulfoxides are valuable intermediates for the synthesis of fine
chemicals, pharmaceuticals and biologically active molecules [10-14]. The main synthetic
route for the preparation of these valuable materials is via oxidation of the corresponding
sulfides.
Concerning the green oxidant, hydrogen peroxide is one of the most powerful candidates,
since it is inexpensive, readily available, high atom efficiency, and water is expected as the
only by-product to be generated from the reaction. Oxidation of sulfides with H
2
2
O is slow;
hence extensive catalysts have been undertaken for the activation of H for this reaction.
2
O
2
Although reported catalysts display considerable progress, the separation and reuse of the
catalysts are difficult [15-17].
In continuation of our studies on preparation and applications of magnetic nanocatalysts
[
18-20], herein, we report the synthesis, characterization and catalytic properties of new
chiral VO(Pseudoephedrine)@MNPs in the chemoselective oxidation of sulfides to
sulfoxides using H as a green oxidant.
2
O
2
2
. Experimental
.1. General remarks
2
The materials were purchased from Merck and Fluka and were used without any additional
purification. All reactions were monitored by thin layer chromatography (TLC) on gel F254
plates. Melting points were obtained in open capillary tubes and also were measured on an
Electrothermal 9100 apparatus. The X-ray diffraction (XRD) data were collected on an
X’Pert MPD Philips diffractometer with Cu Ka radiation source (λ=1.54050Å) at 40 kV
voltage and 40 mA current. The scanning electron microscopy (SEM) image was obtained by
VEGATESCAN. The thermogravimetric analysis (TGA) was carried out on a Bähr STA 503
°
instrument (Germany) under air atmosphere, heating rate 10 C/min. The magnetic
measurements were carried out in an Alternating Gradient Force Magnetometer (AGFM,

Meghnatis Daghigh Kavir Co., Made in Iran) at room temperature. UV-Vis spectra were
recorded on a JASCOV-550 UV-Vis spectrophotometer.
2
.2. Preparation of large-scale the magnetic Fe
3
O
4
nanoparticles (MNPs)
FeCl ·6H O (4.865 g, 0.018 mol) and FeCl ·4H
3
2
2
2
O (1.789 g, 0.0089 mol) were added to
1
2
00 mL deionized water and sonicated until the salts dissolved completely. Then, 10 mL of
5% aqueous ammonia was added quickly into the reaction mixture in one portion under N
2
atmosphere at room temperature followed by stirring about 30 min with mechanical stirrer.
The black precipitate was washed with doubly distilled water (five times).The product stored
in a refrigerator to use.
2
.3. Preparation of Fe
3
O
4
@SiO
2
core-shell (SMNPs)
The synthesized MNPs Fe
3
O
4
(1gr) suspended in 2-propanol (200 mL) and sonicated for
2
0 min. PEG (5.36 g), water (20 mL), ammonia solution (28 wt.%,10 mL) and tetraethyl
orthosilicate (1.2 mL) were respectively added into the suspension, and continuously reacted
for 24h under stirring at room temperature. The iron oxide nanoparticles with a thin layer of
silica (Fe
and water and dried under vacuum.
.4. Preparation of the N-(3-trimethoxysilane)propyl pseudoephedrine (TMSP-
Pseudoephedrine) ligand
+)-Pseudoephedrine hydrochloride (0.403 g, 0.002 mol) was dissolved in 25 mL of
O/ethanol (1:1). Then 0.380 mL (0.002 mol) of (3-chloropropyl) trimethoxysilane
CPTMS), and sodium bicarbonate (0.168 g, 0.004 mol) were added and the mixture was
3
O
4
@SiO ) were separated by an external magnet, washed three times with ethanol
2
2
(
H
2
(
refluxed for 24h. After this time, the yellow-orange solution was obtained. The concentrated
product stored in a refrigerator to use.
2
.5. Preparation of the VO(TMSP-Pseudoephedrine) complex

To the solution of the TMSP-Pseudoephedrine ligand (0.002 mol) in 25 mL of
H
2
O/ethanol (1:1), vanadyl acetylacetonate (0.265 g, 0.001 mol) was added and the mixture
was refluxed for 24h. The dark-green solution was obtained. The concentrated product stored
in a refrigerator to use.
2
.6. Preparation of the VO(Pseudoephedrine)@MNPs
Fe @SiO (1 g) was added to the solution of VO(TMSP-Pseudoephedrine) complex
0.001 mol) in 25 mL of H O/ethanol (1:1) and then the mixture was reflux for 24h. The final
sample was separated by magnetic decantation and washed two times with dry CH Cl , EtOH
and CH Cl respectively to remove the unattached complex. The product stored in a
refrigerator to use.
.7. General procedure for the oxidation of sulfides to sulfoxides
The sulfide (1 mmol) was added to a mixture of 30% H (1 g, 2.4 equiv) and
3
O
4
2
(
2
2
2
2
2
2
2
O
2
VO(Pseudoephedrine)@MNPs (40 mg, 1.4 mol%), and then the mixture was stirred at room
temperature for the time specified. The progress was monitored by TLC (EtOAc/n-hexane,
1
/2). After completion of the reaction, the catalyst was separated from the product by an
external magnet (within 5 seconds), washed with Et O (2×5 mL) and decanted. The
combined organics were washed with brine (5 mL) and dried over anhydrous Na SO
evaporation of Et O under reduced pressure gave the pure products in 80-97% yields and 9-
2
2
4
.The
2
1
5% ee.
3
. Results and discussions
3
.1. Preparation and Characterization of VO(Pseudoephedrine)@MNPs
To prepare the catalyst, firstly (+)-Pseudoephedrine hydrochloride was reacted with
CPTMS to give TMSP-Pseudoephedrine, subsequently this compound was allowed to react
with VO(acac) to produce VO(TMSP-Pseudoephedrine) complex. Ultimately, the resulting
2

oxo-vanadium complex were subjected with SMNPs [21], to prepare the corresponding
VO(Pseudoephedrine)@MNPs (Scheme 1) [22].
The catalyst has been characterized by UV-Vis spectrophotometer, SEM, XRD, TGA, fourier
transform infrared spectroscopy (FT-IR), Energy-dispersive X-ray spectroscopy (EDX) and
AGFM. UV-Vis spectroscopy was applied to the characterization of VO(TMSP-
Pseudoephedrine) complex. UV-Vis spectrum of the VO(acac)
d–d transitions] and 706 nm [ligand-to-metal charge transfer (LMCT)] (Figure 1a) [23].
UV-Vis spectrum of the VO(TMSP-Pseudoephedrine) complex (Figure 1b) shows three
2
shows two bands at 585 nm
[
bands at 265-312 nm (π–π* transition of phenyl ring on ligand), 320-420 nm (may be
assigned as a ligand-to-metal charge transfer (LMCT) transition originating from the oxygen
and nitrogen on pseudoephedrine ligand to the empty d orbital at the vanadium (V) center)
and 420-650 nm (d–d transition) [24].
The SEM image of the VO(Pseudoephedrine)@MNPs confirmed that the catalyst was
made up of uniform nanometer-sized particles less than 33 nm (Figure 2).
The XRD pattern of the VO(Pseudoephedrine)@MNPs is shown in Figure 3. Weak broad
bands (2θ=11.5-23º) appeared in XRD pattern which could be attributed to the amorphous
silane shell formed around the magnetic cores [22]. The interlayer spacing (dhkl), calculated
using the Bragg equation, agrees well with the data for standard magnetic Fe
3
4
O [25] (Table
1
).
One indication of bond formation between the nanoparticles and the catalyst can be
inferred from TGA. The TGA curve of the VO(Pseudoephedrine)@MNPs shows the mass
loss of the organic functional group as it decompose upon heating (Figure 4). The weight loss

at temperatures below 200 ºC is due to the removal of physically adsorbed solvent and
surface hydroxyl groups [26]. Organic spacers have been reported to desorb at temperatures
above 260 °C. The curve shows a weight loss of about 19% from 260 to 600 ºC, resulting
from the decomposition of VO (N-(3-silane) propyl pseudoephedrine) complex grafting to the
SMNPs surface. The loading of the complex in VO(Pseudoephedrine)@MNPs can be
calculated from TGA, which confirmed a loading of approximately 0.35 mmol/g.
The EDX spectrum shows the kinds of elements present (V, Fe, Si and N) in the
VO(Pseudoephedrine)@MNPs (Figure 5).
Figure 6 shows FTIR spectra for (+)-pseudoephedrine hydrochloride, TMSP-
Pseudoephedrine ligand, VO(acac)
2
, VO(TMSP-Pseudoephedrine) complex and
VO(Pseudoephedrine)@MNP. The FTIR spectrum for the (+)-pseudoephedrine
-1
hydrochloride (Fig. 6a) shows stretching vibrations at 3341 and 3380 cm which incorporates
−
1
the N-H and O-H bonds. Vibrations in the range of 1428-1652 cm are attributed to the
aromatic ring. The FTIR spectrum for the TMSP-Pseudoephedrine (Fig. 6b) shows the bands
-1
at 1110 and 2924 cm assigned to the Si-O and C-H stretching vibrations respectively. In the
-
1
FTIR spectrum of the VO(acac) , the bands at 799 and 997cm assigned to the V-O and V=O
2
-1
respectively and the bands at 1527 and 1556cm assigned to the C=O stretching
vibrations(Fig. 6c) [27]. The complexation reaction between TMSP-Pseudoephedrine and
VO(acac)
pseudoephedrine) complex that show the V-O and V=O vibrations in the same vicinity (Fig.
d).The introduction of VO(TMSP-Pseudoephedrine) complex to the surface of SMNPs is
2
is confirmed by comparing the spectra of VO(acac)
2
and VO(TMSP
6
−
1
confirmed by the band at 1090cm assigned to the Si-O-Si stretching vibration (Fig. 6e) [27].
-1
The bands at low wave numbers (≤700 cm ) come from vibrations of Fe-O bonds of iron
-1
oxide, in which for the bulk Fe
3
O
4
samples appear at 570 and 375 cm but for Fe
3
O
4
-1
nanoparticles at 622 and 582 cm as a blue shift, due to the size reduction [28, 29].

Superparamagnetic particles are beneficial for magnetic separation, the magnetic property
of the MNPs and VO(Pseudoephedrine)@MNPs were characterized by AGFM. The room
temperature magnetization curves of the MNPs and the VO(Pseudoephedrine)@MNPs are
shown in Figures 7a and 7b. As expected, the bare MNPs, showed the higher magnetic value
-1
(
saturation magnetization, Ms=74.3 emug ) [30], and the Ms value of
VO(Pseudoephedrine)@MNPs is decreased due to the silica coating and the layer of the
-1
grafted catalyst (Ms=30.4 emug ). The MNPs and VO(Pseudoephedrine)@MNPs have an
coercivity (Hc) of 1.45 and 1.12 Oe, respectively and the remnant magnetization (Mr) of
-1
~
1.52 and 1.23 emug , respectively. As a result, the VO(Pseudoephedrine)@MNPs have a
typical superparamagnetic behaviour [31-33] and can be efficiently attracted with a small
magnet.
These analysis indicated the successful anchoring of the VO(TMSP pseudoephedrine)
complex on the surface of magnetic nanoparticles. Unfortunately, due to the magnetic
properties of VO(Pseudoephedrine)@MNPs it is actually impossible to further characterize
this material by using solid-state NMR spectroscopy.
3
.2. The catalytic application of VO(Pseudoephedrine)@MNPs in the oxidation of sulfides to
sulfoxides
The catalytic activity of VO(Pseudoephedrine)@MNPs as reusable organic-inorganic
hybrid catalyst was tested in the chemo-, and enantiselective oxidation of sulfides to
sulfoxides using 30% H
2
2
O under solvent-free conditions at room temperature (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 2).
As shown in Table 2, in the absence of a catalyst the reaction was incomplete even after

2
4 hours. The optimum amount of H
2
2
O (2.4 equivalent) in the presence of
VO(seudoephedine)@MNPs(40 mg) is ideal for complete conversion of methyl phenyl
sulfide to methyl phenyl sulfoxide.
In order to generalize the scope of the reaction, a series of structurally diverse sulfides was
subjected to oxidation under the optimized reaction conditions, the results are presented in
Table 3. The reactions went on well to afford products in short times and good to high yields.
The overall products yields were in the range of 80-97% and enantiomeric excess (ee’s) of
9
-15% of the sulfoxide was obtained (Table 3, entries 1-7). When the reaction was carried out
at-20 °C, enantioselectivities improve to 20-27% ee after 300 min (Table 3, entries 10-12).
The influence of the solvents (H O, EtOH, CH Cl and CH CN) and different amounts of
2
2
2
3
hydrogen peroxide and catalyst in the oxidation of benzyl phenyl sulfide on the enantiomeric
excess were evaluated; in all cases the observed enantiomeric excess was low.
To show the chemoselectivity of this method, the sulfides containing oxidation-prone and
acid-sensitive functional groups such as OH, CHO and COOCH were subjected to the
3
sulfoxidation reaction; these functional groups remained intact during the conversion of
sulfides to sulfoxides (Table 3, entries 5-7).
For practical purposes, the ability to easily recover and recycle of the catalyst is highly
desirable. We found that this catalyst rapidly recovered and demonstrated remarkably
excellent recyclability; after the first use of the catalyst in the oxidation of methyl phenyl
sulfide to give methyl phenyl sulfoxide, the catalyst was separated by an external magnet and
was washed thoroughly with ether. It was reused for subsequent experiments under similar
reaction conditions. As shown in Figure 6, the catalyst can be recycled up to 20 runs without
any significant loss of activity and enantioselectivity.
Conclusion

We have designed the first chiral VO(Pseudoephedrine)@MNPs that catalyze the oxidation
of sulfides to sulfoxides using H with remarkably high activity and chemoselectivity.
2
O
2
Also, the enantioselectivity in this reaction was observed. When the reaction was carried out
at room temperature, enantiomeric excess (ee’s) of 9-15% and in -20°C ee’s of 20-27% of
sulfoxides were obtained. The heterogenized catalyst can be readily recovered by using an
appropriate external magnet, which minimizes the loss of catalyst during separation and can
be reused for up to 20 times without significant loss of activity and enantioselectivity. The
other merits of the protocol are the use of a commercially available, eco-friendly, cheap and
chemically stabile oxidant, the mild reaction conditions, operational simplicity, practicability,
short reaction times and good to high yields of products.
Acknowledgment
We are grateful to the University of Kurdistan Research Councils for partial support of
this work.
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Scheme 1. The preparation of the VO(Pseudoephedrine)@MNPs
Scheme 2. VO(Pseudoephedrine)@MNPs catalyzed the oxidation of sulfides to sulfoxides
2 2
using H O
Figure 1. UV-Vis spectra of a) VO(acac)
2
and b) VO(TMSP-Pseudoephedrine) complex
Figure 2. SEM image of the VO(Pseudoephedrine)@MNPs
Figure 3. XRD pattern of the VO(Pseudoephedrine)@MNPs
Figure 4. TGA profile of the VO(Pseudoephedrine)@MNPs
Figure 5. EDX spectrum of the VO(Pseudoephedrine)@MNPs
Figure 6. FTIR Spectra of (a) (+)-Pseudoephedrine hydrochloride,(b) TMSP-Pseudoephedrine, (c)
VO(acac)
2
(d) VO(TMSP-Pseudoephedrine) complex and (e) VO(Pseudoephedrine)@MNPs
Figure 7. Hysteresis loop of(a) the MNPs and (b)
VO(Pseudoephedrine)@MNPs at room temperature (left inset:
the magnified field from -30 to 30 Oe)
Figure 6. The recycling experiment of VO(Pseudoephedrine)@MNPs for the oxidation of
methyl phenyl sulfide (1 mmol) using H at room temperature for 5 min
2
O
2
Table 1. Interlayer spacing (dhkl) for VO(Pseudoephedrine)@MNPs
Sample
dhkl(Å)
1
2
3
4
5
6
Standard magnetic Fe
3
O
4
2.96
2.53
2.53
2.09
2.09
1.71
1.71
1.61
1.61
1.48
1.48
VO(Pseudoephedrine)@MNPs 2.97

Table 2. The selective oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide under different
conditions
Entry
a
Catalyst (mg)
Catalyst-free
H O
2 2
(equiv.)
Time (min)
24h
Conversion (%)
1
2
3
4
5
6
7
8
9
2.4
2.4
2.4
2.4
2.4
2.4
2.4
0.6
1.2
40
50
Fe
O
3 4
NP (40)
24h
60
25
5
VO(Pseudoephedrine)@MNPs (30)
VO(Pseudoephedrine)@MNPs (35)
VO(Pseudoephedrine)@MNPs (40)
VO(Pseudoephedrine)@MNPs (45)
VO(Pseudoephedrine)@MNPs (50)
VO(Pseudoephedrine)@MNPs (40)
VO(Pseudoephedrine)@MNPs (40)
100
100
100
100
100
100
100
5
2
25
20
Table 3. VO(Pseudoephedrine)@MNPs (40 mg) catalyzed selective oxidation of sulfides (1 mmol) to
sulfoxides using 30% H (2.4 equiv.) under solvent-free conditions
2
O
2
a
b
c
Entry
Sulfide
Time (min)
Sulfoxide
Yield (%)
T (°C)
ee (%)
O
S
CH3
S
1
5
97
96
rt
9
O
S
S
2
3
8
rt
rt
10
12
O
S
S
ph
Ph
10
97
85
83
80
O
S
1
5
4
5
6
S
8
5
7
rt
rt
rt
O
S
O
O
S
11
13
OH
H
OH
O
S
S
H
O
O
O
O
O
S
S
S
O
1
0
7
8
10
15
81
80
rt
rt
O
O
S
----

S
9
1
11S
ph
5
11
93
rt
----
11
11
O
O
S
S
Ph
0
300
300
95
85
-20
-20
20
27
O
S
S
1
1
1
2
O
O
O
S
H
S
H
300
80
-20
22
O
O
a
1
The products are known and were characterized by IR, H NMR, and by melting point comparisons with those of
11-15
authentic samples.
b
Isolated yield.
c
Enantiomeric Excess (ee) was determined by polarimeter.
CH3
MeO
Ph
CH3
MeO
MeO
MeO
NaHCO3
EtOH/H O (1:1)
Ph
*
N
*
*
HN
+ 2 MeO
Si
Cl
2
*
2
Si
2
CH3
OH
MeO
CH3
OH
CPTMS
,
reflux, 24h
TMSP-Pseudoephedrine
VO(acac)2
EtOH/H O (1:1)
2
,
reflux, 24h
CH3
CH3
Me
N
MeO
MeO
MeO
Me
N
Ph
O
O
O
Ph
*
SiO2
Fe O NP
Si
*
*
Si
*
SMNPs
O
O
MeO
V
3
4
V
EtOH/H O (1:1)
O
O
O
2
O
N
Si
O
, reflux, 24h MeO
Si
N
Me
*
O
Me
MeO
*
O
H C
3
*
H3C
VO(Pseudoephedine)@MNPs
*
Ph
Ph
VO(TMSP-Pseudoephedrine) Complex
eSchem1

Fig.1
Fig.2
Fig.3
Fig. 4

Fig. 5

giF.6

60
a
5
0
0
4
30
b
2
0
0
0
1
-10
-20
-30
-40
-50
-60
-6000
-4000
-2000
0
2000
4000
6000
Applied field (Oe)
Fig. 7
Fig .8