Fe3O4/PEG-SO3H as a heterogeneous and magnetically-recyclable nanocatalyst for the oxidation of sulfides to sulfones or sulfoxides

Article abstract of DOI:10.1039/c7nj02513k

We present below a sulfonated-polyethylene glycol-coated Fe₃O₄ nanocomposite (Fe₃O₄/PEG-SO₃H) as a greatly effective and ecological nanocatalyst for the selective oxidation of sulfides to sulfoxides or sulfones with brilliant yields under solvent-free conditions by employing 30% hydrogen peroxide as the oxidant. A number of sulfides containing alcohol, ester, and aldehyde functional groups were fruitfully and selectively oxidized without altering the desired characteristics. The magnetic nanocatalyst (Fe₃O₄/PEG-SO₃H) can be conveniently and swiftly retrieved through the utilization of an external magnetic tool and recycled for more than 10 reaction runs without significantly decreasing its catalytic behavior.

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Fe3O4/PEG-SO3H as heterogeneous and magnetically recyclable
nanocatalyst for oxidation of sulfides to sulfones or sulfoxides
Saeideh Mirfakhraei,a Malak Hekmati,a Fereshteh Hosseini Eshbala,b Hojat Veisi*,b
aDepartment of Pharmaceutical Chemistry, Faculty of Pharmaceutical Chemistry, Pharmaceutical Sciences
Branch, Islamic Azad University (IAUPS), Tehran, Iran.
bDepartment of Chemistry, Payame Noor University, Tehran, Iran
E-mail: hojatveisi@yahoo.com
Abstract. We detail below of sulfonatedꢀpolyethylene glycol coated Fe3O4 nanocomposite
(Fe3O4/PEGꢀSO3H) as a greatly effective and ecological nanocatalyst for the selective
oxidation of sulfides to sulfoxides or sulfones, in brilliant yields under solventꢀfree
conditions, whilst employing 30% hydrogen peroxide as an oxidant. A number of sulfides
containing alcohol, ester, and aldehyde functional groups are fruitfully and selectively
oxidized without altering the desired characteristics. The magnetic nanocatalyst (Fe3O4/PEGꢀ
SO3H) can be conveniently and swiftly retrieved through the utilization of an external
magnetic tool and recycled for greater than 10 reaction runs without significantly decreasing
its catalytic behavior.
Keywords: Magnetic, Nanocatalyst, Sulfide, Sulfone, Sulfoxide
1. Introduction
Immobilization of homogeneous catalysts on solid supports provides intriguing paths for the
advancement of innovative and stimulating catalysts due to their simple separation from the
reaction vessel, permitting the salvage of the solid and later its reapplication. The greatest
band of evolution in the surface immobilization strategy is the covalent fastening of the
organic compound or ligand to the solid surface [1]. The addition of organic moieties with
pendant fastened chains, either to the external component or the interior surface, is a
procedure applied to alter the physical and chemical traits of natural or synthesized
substances. It can enhance the capabilities of the new multifunctional substancesꢀ considered
as “inorganicꢀorganic hybrids”ꢀ to work in various academic or technological applications
[2]. An unambiguous benefit of this form of hybrids is the optimal blend of both organic and
inorganic characteristics in a single nanomaterial [2].
The selective oxidation of sulfides to sulfoxides or sulfones has been an endeavor for a long
time and is a valuable conversion used in organic chemistry [1,2]. The outcomes resulted in
scientists attempting to validate the chance of formulating an innovative method for the
oxidation of sulfides. After the earliest recorded procurement of sulfoxides by Maercker in
1865, a large portion of procedures have been advanced for this transformation of sulfides to
sulfoxides [3], and a number of other oxidants, including highꢀvalence metal salts,
concentrated HNO3 [4], mꢀchloroperoxybenzoic acid, sodium metaperiodate [5], halogens,
and nitrogen pentoxide, have been vastly applied for the oxidation of sulfides [6ꢀ10].
Regrettably, a large portion of the procedures utilize hazardous, toxic reagents [11] or
complex reaction methods [12] are convoyed by overoxidation of sulfoxides to the expected
sulfones or other undesired byꢀproducts [13,14]. To deal with these restrictions and
accounting the ecoꢀsustainability, green chemistry, and especially atom economy, a great
amount of research has been focused on enhancing innovative and effective catalytic systems,
based on the employment of aqueous 30% H2O2 as a final ecological oxidant due to its
extraordinary advantages such as its low cost, ecoꢀfriendliness, efficientꢀoxygen content, and
the formation of water as a side product [15].
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Using a homogeneous catalyst in such a process will require the expensive catalyst to be
removed from the reaction vessel and a chromatographic method for this retrieval is
unavoidable [16]. We are therefore reliant on manufacturing catalytic arrangements that are
based on organic material and metal as well as supported catalysts for the activation of
hydrogen peroxide in this procedure [17], which can simply be retrieved from the reaction
vessel and reemployed. Despite the advancements made over the years, most of the methods
mentioned in academic literature describe fruitless efforts at controlling the reaction for
selective and excellent yielding procurement of sulfoxide or sulfone. All the while, a
selection of vanadium (V) Schiff base complexes has received noticeable praise for their
great functional compatibilities and selectivity in the oxidation of sulfides [18].
Quite recently, a magnetically solid acid composite was prepared with the assistance of PEG.
Initially a PEG layer was coated on the Fe3O4 NP’s surface and then sulfonatedꢀ polyethylene
glycol coated Fe3O4 nanocomposite (Fe3O4/PEG) was produced as a significantly effective
solid acid magnetic nanocatalyst by chlorosulfonic acid and utilized as a magnetic
nanocatalyst (Fe3O4/PEGꢀSO3H) in the PaalꢀKnorr reaction [19]. Following through with our
investigation to produce selective, efficient and ecological methods and catalysts in organic
synthesis [20], we disclose the study of the selective oxidation of sulfides to sulfoxides or
sulfones under mild conditions, utilizing sulfonatedꢀpolyethylene glycol coated Fe3O4
nanocomposite (Fe3O4\PEGꢀSO3H) (Scheme 1).
Experimental
Typical experimental procedure for the preparation of sulfoxides
In a 25 ml round bottom flask, 30% (w/w) H2O2 (1.2 equiv, 0.5 g) and the Fe3O4/PEGꢀSO3H
catalyst (10 mg) were successively added to sulfide (1 mmol) and the mixture was stirred
magnetically at room temperature for desired time. The progress of the reaction was
monitored by TLC (EtOAc/nꢀhexane, 1/10). After completion of the reaction, the catalyst
was separated from the product by an external magnet (within 5 s) 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 80ꢀ98% yields. Further purification was achieved by chromatography on
silica gel with nꢀhexane/ethylacetate. All reaction products were identified by GC
chromatogram, 1H NMR, IR spectra and melting point as compared with authentic samples.
Typical experimental procedure for the preparation of sulfones
In a 25 ml round bottom flask, 30% (w/w) H2O2 (3 equiv, 1.2 g) and the Fe3O4/PEGꢀSO3H
catalyst (20 mg) were successively added to sulfide (1 mmol) and the mixture was stirred
magnetically at room temperature for desired time. The progress of the reaction was
monitored by TLC (EtOAc/nꢀhexane, 1/10). After completion of the reaction, the catalyst
was separated from the product by an external magnet (within 5 s) 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 80ꢀ98% yields. Further purification was achieved by chromatography on
silica gel with 1:1 nꢀhexane/ethylacetate. All reaction products were identified by GC
chromatogram, 1H NMR, IR spectra and melting point as compared with authentic samples.
Procedure for reusing the catalyst
2

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After the reaction time, the catalyst Fe3O4/PEGꢀSO3H was separated by an external magnet
and washed several times with EtOH, water and acetone. Then, the recovered dried catalyst
under vacuum was used for another run.
3. Results and Discussion
Polyethylene glycol (PEG) is used to alter Fe3O4 NPs and the newly synthesized Fe3O4/PEG
is sulfonated by chlorosulfonic acid to make polyethylene glycol sulfonic acidꢀfunctionalized
magnetic Fe3O4 nanoparticles (Fe3O4/PEGꢀSO3H) as solid acid magnetic nanocatalyst. The
content of acid sites of Fe3O4@PEG400ꢀSO3H nanocomposite was estimated by back
titration using HCl (0.337 N). 10 mL of NaOH (0.130 N) was added to 0.1 g of this
composite and stirred for 30 min. The catalyst was separated and washed with deionized
water. The excess amount of NaOH was titrated with HCl (0.337 N) in the presence of
phenolphthalein as indicator. The acid site content was 1.35 mmol gꢀ1. Also, the ionꢀexchange
pHꢀanalysis demonstrated a loading of 1.42 mmol SO3H gꢀ1. The BET surface area, pore size
and pore volume of this composite are 32.89 m2·gꢀ1, 22.1 nm and 0.09 m3·gꢀ1, respectively.
Using 30% H2O2 as a green oxidant, the selective oxidation of sulfides to sulfoxides or
sulfones was used to investigate the catalytic activity of the heterogeneous and magnetically
recyclable solid acid nanocatalyst, Fe3O4/PEGꢀSO3H (Scheme 1).
OSO3H
HO3SO
HO3SO
OSO3H
OSO3H
OSO3H
PEG
HO3SO
Fe3O4 NPs
OSO3H
HO3SO
HO3SO
OSO3H
OSO3H
O
S
O
O
Fe3O4/PEG-SO3H
H2O2, solvent-reff, r.t.
S
or
S
R1
R2
R1
R2
R1
R2
R1, R2 = alkyl and aryl
Scheme 1. Fe3O4/PEGꢀSO3H catalyzed the oxidation of using H2O2.
We detail the chemoselective oxidation of sulfides to sulfoxides or sulfones with H2O2 in the
presence of a catalytic proportion of Fe3O4/PEGꢀSO3H by monitoring the amount of H2O2
and catalyst and controlling these quantities (Scheme 1). Initially, to ensure optimal reaction
conditions we determined the effect of various quantities of hydrogen peroxide and catalyst
on the oxidation of diphenyl sulfide, a relatively inert substrate, as a model compound with
respect to duration and resulting product yield (Table 1). Table 1 shows that even after 12 h,
the reaction had not completed due to there being no catalyst (entries 4 and 10). When
Fe3O4/PEGꢀSO3H was included, the times were reduced substantially and the yield had
increased to 98%. H2O2 (1.2 equiv) in the presence of the nanocatalyst (10 mg) was
discovered to be optimal for the entire transformation of sulfides to sulfoxides. The
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application of excess H2O2 (3.0 equiv) in the presence of Fe3O4/PEGꢀSO3H (20 mg) resulted
to the respective sulfone in a clean reaction.
Table 1
Optimization of the amounts of H2O2 and Fe3O4/PEGꢀSO3H for the selective oxidation of
diphenyl sulfide to diphenyl sulfoxide or diphenyl sulfone.a
Entry H2O2 Catalyst Time
Yield (%)b
Sulfoxideb
Sulfonec
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
65
70
96
75
45
(equiv)
0.5
(mg)
10
(min.)
30
1
2
3
4
5
6
7
8
9
10
30
70
96
70
75
ꢀ
1.0
1.2
1.2
1.2
10
10
0
10
3
24 h
30
5
2.4
3.0
3.0
3.0
10
10
20
10
0.0
30
30
10
60
ꢀ
ꢀ
ꢀ
ꢀ
3.0
12 h
aReaction conditions: sulfide (1 mmol), 30% H2O2, Fe3O4/PEGꢀSO3H, rt.
bIsolated yield
The simplicity of this method has been highlighted by an easy oxidant of a vast number of
alkyl, aryl and aromatic sulphides as shown in Tables 2. The reaction demonstrates
appropriate general utilization for the synthesis of both sulfoxides and sulfones. The intended
products were attained in minimal times and in excellent yields. It is important to understand
that the sulphides were oxidized chemoselectively in the presence of functional groups
impervious to oxidation, such as OH, C=C, C=NH, C=NOH, CHO (Table 2, entries 8, 15, 9,
11, 12, 16). The effect of steric hindrance to the oxidation by an ortho substituted system
(Table 2, entry 5) was studied and the related products was obtained in good yields. The
likely mechanism for the oxidation of sulphide to the respective sulfoxide or sulfone
employing H2O2 in the presence of Fe3O4/PEGꢀSO3H is outlined in Scheme 2.
Table 2
Selective oxidation of sulfides to sulfoxides or sulfones under solventꢀfree condition.
Entry
Substrate
Sulfoxidea
Time (min) Yield
(%)
Sulfoneb
Time (min) Yield
(%)
Ref.
S
1
2
3
3
3
2
96
92
92
10
15
10
96
92
90
25b
S
25b
25b
S
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4
5
5
96
90
20
30
94
90
25b
21
S
NO2
15
S
6
7
8
10
5
90
90
90
30
20
20
85
90
88
25b
25b
25b
S
S
O2N
S
5
Br
S
OH
9
10
12
10
90
92
92
30
20
30
85
90
88
25b
21
S
10
11
S
25b
N
HO
S
12
12
90
30
85
25b
N
13
10
96
25
90
25b
S
S
14
15
10
8
95
90
30
20
90
85
25b
22
S
OH
S
H
16
10
85
20
82
22
O
aReaction conditions: sulfide (1 mmol), 30% H2O2 (1.2 equiv), Fe3O4/PEGꢀSO3H (10 mg), rt.
bReaction conditions: sulfide (1 mmol), 30% H2O2 (3.0 equiv), Fe3O4/PEGꢀSO3H (20 mg), rt.
To investigate the reusability of Fe3O4/PEGꢀSO3H, the catalyst was removed from the
reaction mixture by a magnetic device as noted earlier and reapplied for the proceeding run
after being rinsed with acetone. As expected, the catalyst was capable of being employed in
12 or more cycles for the oxidation of diphenyl sulfide to the respective sulfone. There was
no evidence after 10 minutes of reaction that there was any decrease in conversions. (Fig. 1).
A hot filtration test was also executed to check the leaching of sulfonic acid from the surface;
but after 10 mins of reaction there was an insignificant quantity of reaction progress. The
TEM image from the recycled catalyst after 12 cycles revealed that its nanostructure had been
kept during the reactions and after the multiple runs (Fig. 2). Elemental analysis (CHNS) of
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recovered catalyst following the 12 runs demonstrated that the leaching of active catalytic
locations was less than 5 wt% from the total quantity of catalytic sites. Also, the ionꢀ
exchange pHꢀanalysis for recovered catalyst demonstrated a loading of 1.34 mmol SO3H gꢀ1.
Additionally, this result confirmed by backꢀtitration analysis of the catalyst.
OH
O
O
O
S
H2O
H2O2
O
S
O
S
O
O
R1
R2
O
O
H
H
R1
R2
S
O
O
O
O
S
O
H
H
H
R1
O
O
O
O
S
S
R2
R1
R2
H
O
H2O2
R1
OH
O
O
O
S
S
H2O
R2
Scheme 2. Proposed mechanism for the oxidation of sulfide to the corresponding sulfoxide or
sulfone with aqueous hydrogen peroxide in the presence of Fe3O4/PEGꢀSO3H.
Fig. 1 Reusability of the nanocatalyst
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Fig. 2. TEM images of fresh a); and reused catalyst Fe3O4/PEGꢀSO3H after the 12th run b).
The degree of crystallinity and phase investigation of Fe3O4/PEGꢀSO3H (a), and reused
Fe3O4/PEGꢀSO3H (b) after the 12th run were performed by powder Xꢀray diffraction (XRD),
which presented in Figure 3. It can be seen that the fresh catalyst has highly crystalline and
cubic spinel structure which agrees with the standard Fe3O4 XRD pattern (cubic phase). This
pattern is indicative of a crystallized structure at 2θ: 31.3◦, 35.9◦, 43.4◦, 53.8◦, 57.1◦ and
62.5◦, which can be 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 (reference JCPDS card no. 19ꢀ629). The position of peaks
in reused Fe3O4/PEGꢀSO3H catalyst was consistent and do not show any change during the
reaction process, which is indicative that that the crystalline structure of the magnetite was
essentially maintained.
Fig. 3. XRD patterns of fresh a); and reused catalyst Fe3O4/PEGꢀSO3H after the 12th run b).
The DLS measurements were applied to verify the nanostructure of Fe3O4/PEGꢀSO3H that
reused after 12 runs with fresh catalyst. Its demonstrated that the catalyst remained in same
nanometerꢀsized particles as quasiꢀspherical without change with a mean diameter of 18ꢀ25
nm (Fig. 4).
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Fig. 4. DLS images of fresh a); and reused catalyst Fe3O4/PEGꢀSO3H after the 12th run b).
We compared the oxidation of diphenyl sulfide with data from the other academic sources
with the results of the Fe3O4/PEGꢀSO3H to determine its catalytic power. Table 3
demonstrates that the procedures that were previously reported were prone to having at least
one of the following disadvantages: elevated reaction temperatures [17,20ꢀ24], longer
reaction times [17,21ꢀ24], use of a transition metal [16,21ꢀ24], and the necessity of volatile
and/or toxic organic solvents [16ꢀ24].
Table 3: Comparison present methodology with literature in the synthesis of diphenyl
sulfone.
Entry Reaction conditions
Time (min) Yield% Ref.
1
2
3
4
5
6
7
8
Fe3O4/PEGꢀSO3H, solventꢀfree, r.t.
MoO2Cl2, acetone, r.t.
10
33
240
45
210
10
96
91
95
95
90
95
72
96
This work
[16]
[17]
[18]
[21]
[22]
[23]
[24]
H5PMo11Al0.5V0.5O40, CH3CN, reflux
Silica sulfuric acid, CH3CN,r.t.
TaCl5, CH3OH, reflux
Carbonꢀbased solid acid, C2H4Cl2, reflux
Pd Schiff base complex, CH3CN, 50 ºC
Au/CTNꢀsilica, CH3OH, 60 ºC
330
180
4. Conclusions
To conclude, the report has highlighted Fe3O4/PEGꢀSO3H as a heterogeneous magnetic
nanocatalyst can encourage the selective oxidation of a number of sulfides into their
respective sulfoxides and sulfones in the presence of H2O2 as the terminal oxidant. This
acidic magnetic catalyst can effectively replace soluble acids as a result of their benefits,
including high catalytic behavior under mild reaction conditions, good to excellent yields,
control over the degree of oxidation (which offers access to sulfoxides or sulfones), high
chemoꢀselectivity, convenient removal of the catalyst via and external magnetic device an
recyclability of the catalyst for multiple runs without noticeable decreases in the reaction’s
yields.
Acknowledgments
We are thankful to Payame Noor University and Pharmaceutical Sciences Branch, Islamic
Azad University, Tehran, Iran, for partial support of this work.
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Graphical Abstract
Fe3O4/PEG-SO3H as heterogeneous and magnetically recyclable nanocatalyst for
oxidation of sulfides to sulfones or sulfoxides
Saeideh Mirfakhraei, Malak Hekmati, Fereshteh Hosseini Eshbala, Hojat Veisi
OSO3H
HO3SO
HO3SO
OSO3H
OSO3H
OSO3H
PEG
HO3SO
Fe3O4 NPs
OSO3H
HO3SO
HO3SO
OSO3H
OSO3H
O
S
O
O
Fe3O4/PEG-SO3H
H2O2, solvent-reff, r.t.
S
or
S
R1
R2
R1
R2
R1
R2
R1, R2 = alkyl and aryl
10