Immobilized molybdenum acetylacetonate complex on expanded starch for chemoselective oxidation of sulfides to sulfoxides with t-BuOOH at room temperature

Article abstract of DOI:10.1080/10426507.2017.1322084

We explored a highly efficient protocol for the oxidation of alkyl aryl sulfides to sulfoxides with high selectivities catalyzed by a molybdenum acetylacetonate complex immobilized on expanded corn starch (ECS-MoO₂(acac)₂) in the pre

Full text of DOI:10.1080/10426507.2017.1322084

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Immobilized molybdenum acetylacetonate
complex on expanded starch for chemoselective
oxidation of sulfides to sulfoxides with t-BuOOH at
room temperature
Mei Hong & Jun-Min Yan
To cite this article: Mei Hong & Jun-Min Yan (2017): Immobilized molybdenum acetylacetonate
complex on expanded starch for chemoselective oxidation of sulfides to sulfoxides with t-
BuOOH at room temperature, Phosphorus, Sulfur, and Silicon and the Related Elements, DOI:
10.1080/10426507.2017.1322084
To link to this article: http://dx.doi.org/10.1080/10426507.2017.1322084
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May 2017.
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Date: 29 May 2017, At: 00:09

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Immobilized molybdenum acetylacetonate complex on expanded starch for chemoselective
oxidation of sulfides to sulfoxides with t-BuOOH at room temperature
Mei Hong,*Jun-Min Yan
College of Chemical Engineering, Nanjing Forestry University, 159 Longpan Road, 210037,
Nanjing, China.
E-mail: hongmei-613@njfu.edu.cn
Abstract
We explored a highly efficient protocol for the oxidation of alkyl aryl sulfides to sulfoxides with
high selectivities catalyzed by a molybdenum acetylacetonate complex immobilized on expanded
corn starch (ECS-MoO2(acac)2) in the presence of 70% t-BuOOH solution (water). We obtained
predominantly the monooxygenated product. The resulting products were obtained in good to
excellent yields within a reasonable time. The catalyst could readily be separated from the
reaction mixture and reused for several runs without significant loss in catalytic efficiency.
Keywords: Molybdenum (VI) acetylacetonate complex; expanded corn starch; selective
oxidation; sulfide; sulfoxide
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Introduction
Chemoselective oxidation of organic sulfides represents one of the most fundamentally
important reactions in the domain of organic chemistry, which is fascinating from both chemical
and biological perspectives. 1 Sulfoxides are indispensable as they appear in numerous natural
products and drug molecules, act as chiral auxiliaries and C–C bond forming reagents.2 They
also have widespread utilization as convenient intermediates or reagents in organic syntheses.
Indeed, a number of drugs in therapeutic areas such as antiulcerative, cardiotonic,
antihypertensive as well as psychotonics and vasodilators contain a sulfoxide functionality.3
Because of this, numerous reagents and methods have been developed in an effort to produce
sulfoxides as selectively and inexpensively as possible. Furthermore, with the growing emphasis
on environmentally benign processes, more and more chemists have sought out ways to bring
about this transformation with minimal waste as possible. As a result, a vast number of useful
transition metal based catalysts have emerged over the years for the oxidation of sulfides by
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H2O2.
Although H2O2, being easily available and environmentally benign most of the
oxidation processes require a large excess of H2O2 (4–8.0 equiv. with respect to sulfide) to attain
high yields of the sulfoxides.9, 10 In that context, t-BuOOH appears to be a perfect alternative as it
is inexpensive, has high thermal stability and is safe to handle even in large quantities for the
efficient and selective production of sulfoxides.11 In addition, the probability of decomposition
by trace metallic impurities is less than that of H2O2.12 The by-product of oxidation, t-BuOH can
be easily removed by distillation or rotary evaporation, thus avoiding the need for aqueous work
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up. So far, some interesting transition metal catalysts have been reported for sulfide oxidation,
including La,11 Bi,13 Fe,14 V15 and Mn16 in the presence of t-BuOOH. However, most of them
present disadvantages, such as difficulty in recovery of the catalyst,13 high cost,16 the
requirement of a promoter or a co-catalyst,13 high temperature,14 long reaction time.14 Therefore,
simple, convenient and environmentally benign methods for the oxidation of sulfides to
sulfoxides using t-BuOOH as a green oxidant are still highly desired. The use of heterogeneous
catalysts in the liquid phase has the inherent advantages of easier handling, separation, recovery
and recycling, and enhanced stability.
High-valence oxomolybdenum complexes have proven to be excellent catalysts for the reduction
and oxidation of organic compounds.17, 18 As the homogeneous Mo-complexes have some well
known disadvantages (difficulties with separation and recyclability), many attempts to
immobilize the Mo-complexes have been performed.19, 20 Recently, we disclosed the excellent
performance of molybdenum acetylacetonate complex immobilized on expanded corn starch
(ECS-MoO2(acac)2) in stillingia oil epoxidation using t-BuOOH as the oxidant.21 Encouraged by
these exciting results, we decided to explore further their catalytic activity in oxidation
processes. Interested in developing a system according to the green chemistry principles, in the
present study, we report an efficient method for the selective oxidation of sulfides to sulfoxides
using t-BuOOH as the oxidant under mild reaction conditions in ethanol as green solvent with
ECS-MoO2(acac)2 (Scheme 1). More importantly, the catalysts could be separated and efficiently
reused for four cycles without a significant loss in catalytic activity. To the best of our
knowledge, this is the first report on the oxidation of sulfides by molybdenum complex using t-
BuOOH as the oxidant.
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Results and Discussion
To improve the yield of the reaction and find the optimized conditions, several reactions were
carried out in the same conditions with changing parameters. We investigated the oxidation of
methyl phenyl sulfide (thioanisole) as a model substrate and the trials were performed with
different amounts of molybdenum acetylacetonate supported on expanded corn starch, reaction
times, solvents and various oxidants at room temperature. It can be seen that the conversion
increased from 86% to 97% when the amount of ECS-MoO2(acac)2 increased from 0.5 mol% to
1 mol% after 6 h. The selectivity of sulfoxide increases with the addition of catalyst from 0.5
mol% to 1.5 mol% (Table 1, entry 1 and entry 2). When the amount of catalyst is increased from
1.5 mol% to 2.5 mol% (Table 1, entries 3-5), the selectivity of sulfoxide reduces from 97 to 90%.
The conversion of sulfide almost did not change. It indicated that the excess catalyst promote the
over-oxidation of sulfoxide. According to these results, 1.5 mol% of ECS-MoO2(acac)2 was
selected as the optimum amount of catalyst.
To analyze the solvent effect, the reaction was carried out in different solvents like 9:1 (v/v)
mixture of n-hexane/methanol, methanol, ethanol, toluene and acetonitrile following the same
protocol. The results listed in Table 2 show that the solvent has a remarkable influence on the
reaction outcome. The oxidation carried out in ethanol gives the highest yield of sulfoxide. A
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conversion of 94% was obtained with a sulfoxide selectivity of 97% (Table 2, entry 3). In the
case of other solvents a lower selectivity of sulfoxide was obtained.
Table 3 shows the results for the thioanisole oxidation as a function of reaction time using ECS-
MoO2(acac)2 as catalyst. It can be observed that the sulfoxide selectivity increased with the
reaction time up to 6 h (Table 3, entry 3), and then it decreased by prolonging the reaction time
to 20 h. In all cases the conversion of thioanisole remained almost constant.
To examine the effect of oxidant on the oxidation of thioanisole, the reaction was performed with
different oxidants by using the same reaction conditions (Table 4). When 2 equiv. of 30% H2O2
(water) was used as oxidant, the reaction gave the corresponding sulfoxide in 36% yield and 36%
selectivity (Table 4, entry 1). In the presence of 2 equiv of 70% t-BuOOH (water) the yield was
72% with 73% selectivity (Table 4, entry 2). The effect of the concentration of t-BuOOH used on
the conversion and product selectivity was studied at various t-BuOOH/substrate molar ratios in
the presence of 1.5 mol% catalyst. The selectivity pattern changed with varying amounts of t-
BuOOH in the oxidation of thioanisole. With 1.5 equiv. of t-BuOOH, desired yields and
selectivity of sulfoxide were achieved (Table 4, entry 4). Nevertheless, a further increase in this
ratio significantly reduced the selectivity of the reaction products.
On the ground of the optimization studies, ECS-MoO2(acac)2 (1.5 mol%) and 1.5 equiv 70% t-
BuOOH (water) in EtOH at room temperature were the best reaction conditions for the oxidation
of methyl phenyl sulfide to methyl phenyl sulfoxide. In an effort to establish the scope of the
protocol, we have assessed the catalytic activity of ECS-MoO2(acac)2 under the optimized
conditions, using a series of various types of aliphatic and aromatic sulfides. As shown in Table
5, aromatic substrates with various substituents including –CH3, –OCH3, –Cl, –CN and –NO2
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could be tolerated and the sulfoxides were obtained in good to excellent yields (Table 5, entries
1-5). Whereas, a strong electron-withdrawing group at the para-position showed a negative effect
on the reaction. In particular, a much longer reaction time (15 h) was needed for 4-
nitrothioanisole to complete the reaction (Table 5, entry 4), presumably due to the strong electron
withdrawing effects of the nitro group. Not only aryl sulfides but also linear alkyl ones (Table 5,
entries 7 and 8) could be oxidized to the corresponding sulfoxides in high yields. On the other
hand, bulky sulfides such as diphenyl sulfide (Table 5, entry 6) react smoothly under the present
conditions. The chemoselectivity of the method was also notable. Sulfides having a carbon-
carbon double bond (Table 5, entry 9) and hydroxyl group (Table 5, entry 10) were cleanly
transformed into the corresponding sulfoxides in excellent yield without formation of any
epoxide and carbonyl by-products.
Lastly, recyclability of the catalyst being an attractive feature, we studied the recyclability for the
oxidation of thioanisole to the corresponding sulfoxide. The reaction mixture was centrifuged
after the first run and the obtained solid residue was washed with ethanol, dried under vacuum
and reused for four runs. The sulfoxide yields of the reused catalysts were 90%, 90%, 88% and
87%, respectively, when the catalyst was used for 1, 2, 3, 4 cycles under the optimized reaction
conditions (Figure S 1 Supplemental Materials). These results suggested that the catalyst
developed in this study can be recycled without significant loss in catalytic activity for at least 4
times. To further assess the recyclability of catalysts, the hot filtration test (separation of catalyst
at the reaction temperature) was performed for ECS-MoO2(acac)2 by filtering off the reaction
solution after 30% on conversion of the catalytic oxidation at room temperature, and leaving
them to react for 10 h. No sulfide conversion was observed after the filtration step for ECS-
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MoO2(acac)2. ICP analysis of the used catalyst (after four runs) showed 0.4% leaching of
molybdenum.
Experimental
Materials and Methods
All chemicals and solvents were purchased from Sigma-Aldrich and used without further
purification. The resulting reaction mixtures were analyzed by gas chromatography-mass
spectroscopy (GC-MS) and GC. The GC-MS (Agilent 7890A, Agilent 5975C MSD) and GC
(Agilent 7890A) instruments were equipped with a DB-5 capillary. The ICP analyses were
carried out by a Perkin-Elmer optima 2000 DV spectrometer. Melting points were obtained with
a Shimadzu DSC-50 thermal analyzer. 1H and 13C NMR spectra were obtained on a Bruker RX-
500 Advance spectrometer in CDCl3 using tetramethylsilane as internal standard. The
Supplemental Materials contains sample characterization data for the products obtained.
Typical sulfide oxidation procedure
A mixture of methyl phenyl sulfide (1 mmol), 70% t-BuOOH (1.5 equiv.), ECS-MoO2(acac)2
o
catalyst (1.5 mol%) in ethanol (3 mL) was stirred at 25 C. Completion of the reaction was
indicated by TLC (n-hexane/ethylacetate 10 : 1) and gas chromatography (GC). After
completion, ethyl acetate was added to it. The reaction mixture was then centrifuged and
decanted to separate the molybdenum compound. The aqueous phase was extracted with ethyl
acetate 3-4 times. After drying with anhydrous sodium sulfate, the organic residue was analyzed
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by GC and then purified by column chromatography on silica gel with ethyl acetate/hexane to
give methyl phenyl sulfoxide. The separated catalyst was washed with EtOH and dried under
1
vacuum. H NMR (500 MHz, CDCl3) δ (ppm): 7.65-7.63 (m, 2H), 7.55-7.47 (m, 3H), 2.72 (s,
13
3H); C NMR (125 MHz, CDCl3) δ (ppm): 145.8, 131.2, 129.5, 123.6, 44.1; EI-MS, m/z: [M]+
139.99.
Conclusions
In conclusion, we have explored a new method for the selective oxidation of various aromatic
and aliphatic sulfides to the corresponding sulfoxides with 70% t-BuOOH (water) as the oxidant
catalyzed by ECS-MoO2(acac)2. The described method in this paper clearly indicate that the
methodology involves mild, environmentally friendly and selective synthesis of sulfoxides from
both aromatic and aliphatic sulfides. The resulting products were obtained in good yields. This
catalyst can be considered as heterogeneous version of MoO2(acac)2 and easily prepared from
commercially available starting materials. The properties of high activity, high stability, easy
recoverability, and reusability of the catalyst render it potentially valuable catalyst in industrial
applications.
Acknowledgements
The work was supported by the Natural Science Foundation of Jiangsu Province (China) (No.
BK20140969), the Priority Academic Program Development of Jiangsu Higher Education
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Institutions (PAPD) and the open fund of Jiangsu Key Lab of Biomass Energy and Materials
(JSBEM201603).
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Table 1 Optimization of the catalyst amount on the oxidation of thioanisole as a model reaction
a
catalyzed by ECS-MoO2(acac)2
Catalyst
Entry
Conversionb(%)
Yieldb(%)
Selectivityc (%)
amount(mol%)
1
2
3
4
5
0.5
1
86
97
94
93
93
43
89
91
86
84
50
92
97
92
90
1.5
2
2.5
a Reaction conditions: 3 mL of solvent; 1 mmol of thioanisole; 1.5 mmol of 70% t-
o
b
BuOOH (water); RT (25 C); 6 h; with stirring. Determined by GC on the crude
reaction mixture. c Selectivity = sulfoxide/(sulfoxide + sulfone).
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a
Table 2 Effect of various solvents on oxidation of thioanisole catalyzed by ECS-MoO2(acac)2
Entry
1
Solvent
Conversionb(%) Yieldb(%)
Selectivityc (%)
83
n-
96
80
Hexane/Methanol
(9:1, v:v)
Methanol
Ethanol
2
3
4
5
91
94
97
93
76
91
83
65
84
97
86
70
Toluene
Acetonitrile
a Reaction conditions: 1 mmol of thioanisole; 1.5 mmol of 70% t-BuOOH (water); 1.5
o
b
mol% of ECS-MoO2(acac)2; RT (25 C); 6 h; with stirring. Determined by GC on
the crude reaction mixture. c Selectivity = sulfoxide/(sulfoxide + sulfone).
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a
Table 3 Effect of reaction time on oxidation of thioanisole catalyzed by ECS-MoO2(acac)2
Entry
Time (h)
Conversionb(%) Yieldb(%)
Selectivityc (%)
1
2
3
4
5
2
93
93
94
95
96
78
82
91
78
61
84
88
97
82
64
4
6
8
20
a Reaction conditions: 3 mL of ethanol; 1 mmol of thioanisole; 1.5 mmol of 70% t-
o
b
BuOOH (water); 1.5 mol% of ECS-MoO2(acac)2; RT (25 C); with stirring.
c
Determined by GC on the crude reaction mixture. Selectivity = sulfoxide/(sulfoxide
+ sulfone).
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Table 4 Effect of different oxidants and the concentration of t-BuOOH on the oxidation of
thioanisole catalyzed by ECS-MoO2(acac)2 at room room temperaturea
Entry
Oxidant
Conversionb(%) Yieldb(%)
Selectivityc
(%)
36
1
2
H2O2 (2 equiv.)
t-BuOOH
equiv.)
99
36
72
(2 98
73
3
t-BuOOH
equiv.)
(1 54
38
91
70
97
4
t-BuOOH
(1.5 94
equiv.)
a
Reaction conditions: 3 mL of ethanol; 1 mmol of thioanisole; 1.5 mol% of ECS-
o
b
MoO2(acac)2; RT (25 C); 6 h; with stirring. Determined by GC on the crude
reaction mixture. c Selectivity = sulfoxide/(sulfoxide + sulfone).
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Table 5 Oxidation of sulfides to the corresponding sulfoxides by using 70% t-BuOOH and ECS-
MoO2(acac)2 catalysta
Conversionb
(%)
Selectivityc
(%)
Entry
Substrate
Product
Time (h)
Yieldb(%)
1
2
3
1
2
3
5
5
7
96
97
94
93
95
91
97
98
97
4
4
15
93
89
96
5
6
5
6
7
96
88
94
85
98
97
24
7
8
9
7
8
9
6
6
7
96
97
93
92
94
90
96
97
97
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10
10
6
97
94
97
a
Reaction conditions: 3 mL of ethanol; 1 mmol of sulfide; 1.5 mol% of ECS-MoO2(acac)2; 1.5
o
b
mmol of 70% t-BuOOH (water); RT (25 C); with stirring. Determined by GC on the crude
reaction mixture. c Selectivity = sulfoxide/(sulfoxide + sulfone).
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Cat:
Scheme 1 Selective oxidation of sulfides to sulfoxides.
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