Kinetic and mechanism of the aqueous selective oxidation of sulfides to sulfoxides:

Article abstract of DOI:10.1142/S1088424613500776

The selective oxidation of sulfides with hydrogen peroxide to give sulfoxides was carried out in aqueous solution by water-soluble manganese porphyrin as mimics of cytochrome P450-like catalyst. Different factors that influencing the selective oxidation of sulfides, for example, catalyst, amount of catalyst, solvent and reaction temperature were investigated. MnTE4PyP (meso-tetrakis(N-ethylpyridinium-4-yl) manganese porphyrin) was efficient and selective catalyst for oxidation of various sulfides. The reaction showed first-order dependence in both [sulfide] and [H₂O₂], and a fractional order respect to catalyst. Oxygen atom transfer mechanism involves high-valence intermediate was proposed, which was supported by kinetic orders and spectrophotometric evidences. Copyright

Full text of DOI:10.1142/S1088424613500776

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2013; 17: 1104–1112
DOI: 10.1142/S1088424613500776
Published at http://www.worldscinet.com/jpp/
Kinetic and mechanism of the aqueous selective oxidation
of sulfides to sulfoxides: insight into the cytochrome
P450-like oxidative metabolic process
Xian-Tai Zhou, Gang-Qing Ren and Hong-Bing Ji*
School of Chemistry and Chemical Engineering, Key Laboratory of Low-Carbon Chemistry & Energy
Conservation of Guangdong Province, Sun Yat-sen University, 510275 Guangzhou, P. R. China
Received 17 February 2013
Accepted 2 June 2013
ABSTRACT: The selective oxidation of sulfides with hydrogen peroxide to give sulfoxides was carried
out in aqueous solution by water-soluble manganese porphyrin as mimics of cytochrome P450-like
catalyst. Different factors that influencing the selective oxidation of sulfides, for example, catalyst,
amount of catalyst, solvent and reaction temperature were investigated. MnTE4PyP (meso-tetrakis(N-
ethylpyridinium-4-yl) manganese porphyrin) was efficient and selective catalyst for oxidation of various
sulfides. The reaction showed first-order dependence in both [sulfide] and [H2O2], and a fractional order
respect to catalyst. Oxygen atom transfer mechanism involves high-valence intermediate was proposed,
which was supported by kinetic orders and spectrophotometric evidences.
KEYWORDS: metalloporphyrins, oxidation, sulfide, sulfoxide, kinetic.
sulfoxidation pertaining to the use of metalloporphyrins
INTRODUCTION
in combination with hydrogen peroxide is limited [8].
Theselectiveoxidationoforganicsulfidestosulfoxides
Baciocchi’s group reported series works on the efficient
without any over-oxidation to sulfones is a challenging
oxidation of sulfides with hydrogen peroxide in C2H5OH,
research interest in synthetic organic chemistry, partly
in which the pentafluoro-substituted iron porphyrins
because of the importance of sulfoxides as intermediates
were used as catalyst [9]. Most recently, Simonneaux and
in biologically active compounds [1]. Among all methods
co-authors reported the first example on the asymmetric
described so far, the oxidation of sulfides by various metal
oxidation of sulfides catalyzed by chiral iron porphyrin
catalysts is one of the most attractive routes. To this end,
using hydrogen peroxideas oxidant inmethanol and water.
copper, iron oxide nanoparticles, polyoxometalate, silica-
Sulfonate groups were introduced into iron porphyrins
vanadia and Mo(VI) have been used as the catalysts [2].
with the aim of water-solubility of catalyst [8c].
As models of hemes or cytochrome P450, metallo-
Another kind of water-soluble metalloporphyrins
porphyrins are effective catalysts for the selective
could be prepared by introducing pyridinium group on
oxidation of hydrocarbon and other organic compounds
the phenyl rings. This kind of cationic metalloporphyrins
under mild conditions [3]. The sulfoxidation of sulfides
could be immobilized on supports and used commonly
catalyzed by metalloporphyrins is a subject of current
as heterogeneous catalyst in oxidations [10]. Cationic
interest as a direct route for selectively producing
metalloporphyrins could also be used directly in homo-
sulfoxides or sulfones [4], in which various oxidants,
geneous oxidation in aqueous solution. We developed
for example, NaOCl [5], NaIO4 [6] and Oxone [7] were
efficient procedures for the oxidation of alcohols,
used. Although hydrogen peroxide is widely accepted as
oximes, and oxidative coupling reaction of amines
a “green” and effective oxidant in organic oxidations, the
catalyzed by pyridinium group substituted water soluble
metalloporphyrins [11]. To our knowledge, direct selective
oxidation of sulfides to sulfoxides catalyzed by cationic
manganese porphyrins is still unknown.
*Correspondence to: Hong-Bing Ji, email: jihb@sysu.edu.cn,
tel: +86 20-84113658, fax: +86 20-84113654
Copyright © 2013 World Scientific Publishing Company

KINETIC AND MECHANISM OF THE AQUEOUS SELECTIVE OXIDATION OF SULFIDES TO SULFOXIDES 1105
Table 2. Oxidation of thioanisole with H2O2 in various solventsa
The aim of this work was to develop an
efficient procedure for preparing sulfoxides
catalyzed by cationic metalloporphyrins in
aqueous solution with hydrogen peroxide
as oxidant. Particular attention was devoted
to identify the role of oxo-metal species in
the oxidation process. The study is also
intended to gain insight into the effects of
reactants on the reaction rate and present a
reasonable mechanism of the sulfoxidation.
Besides achieving the mimicking oxidation
of sulfides to sulfoxides procedures, the
study could also provide the understanding
on the cytochrome P450 to promote
oxidative metabolism kinetic process in
living organisms.
Selectivity, %
Entry
Solvent
Conv., %
sulfoxide
sulfone
1
2
Cyclohexane
Ethyl acetate
EtOH
12
28
33
35
39
95
78
98
90
92
46
38
35
2
10
8
3
54
62
65
98
1
4
MeOH
5
CH3CN
6
H2O
7b
8c
H2O/CH3CN
H2O/CH3CN
99
99
1
aThioanisole (1 mmol), H2O2 (30 wt%, 2 mmol), MnTE4PyP (1 × 10-3 mmol),
b
c
solvent (10 mL), 60 min, 60 °C. H2O/CH3CN = 4/1, v/v. H2O/CH3CN =
1/1, v/v.
RESULTS AND DISCUSSION
slight higher activity no matter cationic or anionic group
on the phenyl rings. For the catalysts with same central
ion, it also could be known that the metalloporphyrins
with ethyl pyridine group on the outside ring showed
Catalysis of various catalysts for the aqueous
oxidation of sulfides
For the initial studies on the catalytic activity of
different water-soluble metalloporphyrins catalysts in
the oxidation of sulfides in aqueous solution, thioanisole
was selected as model substrate to assess their catalytic
properties. The results are summarized in Table 1.
The reaction was examined at 60 °C. As shown
in Table 1, in the presence of water-soluble metallo-
porphyrins catalyst, all the reactions proceeded with high
conversions (entries 1–9). Compared with cobalt and iron
porphyrin catalysts, manganese porphyrins presented
higher activity than methyl pyridine and sulfonatophenyl
(such as entries 1–3).
It is interesting to note that the cationic metallo-
porphyrins show much higher selectivity toward
sulfoxide, while sulfone is the main product when the
reaction catalyzed by anionic metalloporphyrins (entries
1–2 and entry 3 as example). It is likely due to the
presence of stronger electron withdrawing substituents
on the porphyrin ligand, which results in the increment
of the redox potential of catalyst. For catalyst with
higher redox potential, high valent metal-oxo
intermediate is more easily reduced to initial
valence. Therefore, the reaction could be well-
Table 1. Oxidation of thioanisole catalyzed by various catalysts in
aqueous solutiona
controlled in the stage of sulfoxide based on
the low probability that the over-oxidation of
sulfoxide by metal-oxo intermediate. Therefore,
manganese porphyrin (MnTE4PyP), the most
selective among the studied catalysts, was
chosen for further investigations.
O
O
O
S
Catalyst
S
S
H2O2, Solvent (H2O/CH3CN=1/1, v/v), 60oC
+
Selectivity, %
Entry
Catalyst
Conv., %
sulfoxide
sulfone
Effect of solvents on the aqueous oxidation
of sulfides
1
2
3
4
5
6
7
8
9
MnTM4PyP
MnTE4PyP
MnTPPS
95
98
96
86
89
97
89
96
92
99
99
15
95
96
5
1
1
Table 2 shows the effect of solvents on
oxidation of thioanisole with H2O2 over water-
soluble manganese porphyrin (MnTE4PyP).
Nonpolar solvent, cyclohexane is unfavorable
for the oxidation, in which only 12% thioanisole
was converted (entry 1). It could be explained
by the poor solubility of catalyst in nonpolar
solvent. As shown in Table 2, the conversion
of thioanisole increased with the increasing
polarity of solvent (entries 2–6). It is interesting
that the selectivity toward sulfone was higher
85
5
FeTM4PyP
FeTE4PyP
FeTPPS
4
95
7
CoTM4PyP
CoTE4PyP
CoTPPS
93
99
8
1
92
aThioanisole (1 mmol), H2O2 (30 wt%, 2 mmol), catalyst (1 × 10-3 mmol),
solvent (10 mL, H2O/CH3CN = 1/1, v/v), 60 min, 60 °C.
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2013; 17: 1105–1112

1106 X.-T. ZHOU ET AL.
than that of sulfoxide in strong polarity solvent,
especially in water (entry 6). The interpretation for this
phenomenon may be related to the nature of solvation
effect of solvent. The electronic cloud density of sulfur
atom in sulfoxide is increased in water, which the
sulfoxide tends to be further oxidized by the metal-oxo
intermediate. However, when little amount of acetonitrile
was added in the water solution, the selectivity
toward sulfoxide was remarkably enhanced (entry 7).
Increasing the amount of acetonitrile continually (H2O/
CH3CN = 1/1, v/v), thioanisole could be converted to
sulfoxide in stoichiometry (entry 8). It indicates that the
mixed medium is efficient for the oxidation of sulfide
with hydrogen peroxide and water-soluble manganese
porphyrin. One hand, the mass transfer is significantly
accelerated by the excellent solubility of catalyst
in water. And on the other side, sulfoxide could be
preserved against from the over-oxidation in the presence
of acetonitrile.
Fig. 2. Effect of temperature on the selective oxidation of
thioanisole
Effect of temperature on the aqueous oxidation
of sulfides
The effect of reaction temperature on the conversion
of thioanisole, and the selectivity toward sulfoxide for
the MnTM4PyP-catalyzed system were studied, and the
results are presented in Fig. 2.
Effect of catalyst amount on the aqueous
oxidation of sulfides
As shown in Fig. 2, the conversion of thioanisole
increased greatly with the rising temperature from
30 to 60 °C, then kept unchanged at temperature over
60 °C. When the oxidation was conducted under 60 °C,
an excellent selectivity toward sulfoxide was obtained.
The selectivity toward sulfoxide decreased while the
temperature was over 60 °C. Evidently, sulfoxide could
be further oxidized to sulfone at higher temperature. The
optimal reaction temperature for the selective oxidation
of sulfide was 60 °C.
The influence of MnTM4PyP catalyst amount on
the aqueous oxidation of thioanisole in the presence of
hydrogen peroxide is summarized in Fig. 1.
As shown in Fig. 1, with increases in the amount of
catalyst, the conversion of thioanisole and the selectivity
toward sulfoxide increased greatly. As the amount of
catalyst was increased continually, the selectivity toward
sulfoxide declined apparently though the conversion
could be up to over 98%. It indicates that the excess
catalyst promote the over-oxidation of sulfoxide with
metal-oxo intermediate. Therefore, the optimal amount
of catalyst for the aqueous oxidation of thioanisole was
0.1% (based on substrate).
Aqueous oxidation of various sulfides catalyzed
by MnTM4PyP
To evaluate the scope of the catalytic system,
various sulfides were subjected to the reaction system
using 0.1%.mol (based on substrate) of water-soluble
manganese porphyrins (Table 3). As shown in Table 3,
most substrates could be smoothly converted to corres-
ponding sulfoxides with high conversion rate and
excellent selectivity.
It can be observed that the efficiency of the oxidation in
thiscatalyticsystemisrelatedwiththeelectronicpropertyof
substrates. Compared with the electron-donating groups at
p-position, the electron-withdrawing groups seemed more
unfavorable to the formation of sulfoxides. It indicated the
electron-withdrawing effect retarded the combination of
substrate with metal-oxo intermediate (entries 1–5). The
influence of steric effects could further be observed from
the oxidation of diphenyl sulfide, in which the conversion
was 82% after much longer reaction time (entry 6).
Comparing with thioanisole, methyl benzyl sulfides could
beconvertedunderthesameconditionswithlongerreaction
Fig. 1. Effect of the amounts of catalyst on the selective
oxidation of thioanisole
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2013; 17: 1106–1112

KINETIC AND MECHANISM OF THE AQUEOUS SELECTIVE OXIDATION OF SULFIDES TO SULFOXIDES 1107
Table 3. Selective oxidation of various sulfides catalyzed by MnTE4PyP in aqueous
solutiona
Selectivity, %
Entry
1
Substrate
Time, min
60
Conv., %
98
sulfoxide
99
sulfone
1
S
S
90
90
99
97
97
98
3
2
2
3
S
H3CO
S
4
90
89
98
2
Cl
S
5
6
120
120
25
82
96
98
4
2
O2N
S
S
7
8
120
60
96
96
99
4
1
>99
S
S
9
60
99
62
99
99
1
1
O
S
OH
10
120
a Sulfide (1 mmol), H2O2 (30 wt%, 2 mmol), catalyst (1 × 10-3 mmol), solvent (10 mL, H2O/
CH3CN = 1/1, v/v), 60 °C.
time (entry 7). Sulfoxidation of the linear chain di-n-butyl
evaluated. The rate constant obtained for the variations of
sulfide smoothly proceeded with high conversion and
selectivity (entry 8). Cyclic sulfide, that is, 1,4-thioxane,
could also be efficiently sulfoxidated to the corresponding
sulfioxide stoichiometrically. In addition, the catalytic
system exhibited specific selective oxidation performance
towards the sulfide contains hydroxyl group, and no
products from hydroxyl group oxidation were detected
(entry 10).
sulfide, catalyst and H2O2 are tabulated in Table 4.
(1) Effect of [sulfide]. The kinetics of oxidation of
sulfide has been investigated under pseudo first order
conditions, keeping the concentrations of H2O2 always
in large excess (nearly 10-folds) over that of [sulfide].
The oxidation followed first-order kinetics with respect
to [sulfide], as evidenced by the linear plot of log[sulfide]t
vs. time up to 80% conversion of [sulfide]0 (Fig. 3) (r >
0.9988). Further, effect of varying (2.0 × 10-3~6 × 10-3
M) on the rate of oxidation has been studied at a constant
[MnII] (0.5 × 10-5 M), [H2O2] (5.0 × 10-2 M). The pseudo
first order rate constant kobs were found to be independent
of [sulfide]0, indicating that the rate of the oxidation was
first order in sulfide (Table 4).
Mechanistic and kinetic studies
To explore the kinetic behavior over the aqueous
oxidation of sulfide (using thioanisole as model
compound), factors influencing the rate of oxidation such
as effects of (i) [sulfide], (ii) [MnII], and (iii) [H2O2] effect
have been studied. Rate and activation parameters were
(2) Effect of [H2O2 ]. At a constant [sulfide] (4.0 ×
10-3 M) and [MnII] (0.5 × 10-5 M), the kinetic runs
were carried out with various initial concentrations of
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2013; 17: 1107–1112

1108 X.-T. ZHOU ET AL.
similar experimental conditions, an increase in [MnII]0
increased the kobs values (Table 4). A plot of rate
constant kobs (min-1) against [MnII] yields a straight line
with a non-zero intercept (Fig. 5). Observed reaction
order of [MnII] can be obtained from linear regression
of log kobs vs. log [MnII] (order ~0.698) at 323 K. This
indicates that the reaction is of fractional order with
respect to [MnII]. The fractional order in the rate with the
catalyst concentration should be result from formation
of manganese hydroperoxo intermediate. Then an oxo
species in a high valence oxidation state was generated
via heterolytic cleavage of oxygen-oxygen bond of the
hydroperoxo intermediate.
Table 4. Effect of the concentration of sulfide, manganese
porphyrin catalyst and H2O2 on the reaction rate at 323 K
[Sulfide] × 103, M [MnII] × 105, M [H2O2] × 102, M kobs × 103,
min-1
[Sulfide] variation
2.0
0.5
0.5
0.5
0.5
0.5
5.0
5.0
5.0
5.0
5.0
5.38
5.40
5.45
5.58
5.43
3.0
4.0
5.0
6.0
[H2O2] variation
(4) Effect of temperature. The present oxidation study
was carried out in the range (313–333 K) and the results
are shown in Table 5. The kobs values increased with an
increase in the temperature. The plot of log kobs vs. 1/T
was a straight line (Fig. 6). From Arrhenius plot, the value
of energy of activation (Ea) was calculated. The calculated
activation energy from the slope was 56.1 kJ/mol.
4.0
0.5
0.5
0.5
0.5
0.5
1.0
3.0
5.0
7.0
9.0
1.13
3.29
5.45
7.48
9.87
4.0
4.0
4.0
4.0
[MnII] variation
4.0
4.0
4.0
4.0
4.0
0.2
0.4
0.6
0.8
1.0
5.0
5.0
5.0
5.0
5.0
3.03
4.83
6.08
7.82
9.53
Fig. 4. The linear plot of kobs vs. [H2O2] confirms the first order
reaction with respect to [H2O2]
Fig. 3. Plot of log [sulfide]t vs. time at 323 K
hydrogen peroxide, which yielded rate constants whose
values depended on [H2O2]. The pseudo first order rate
constants kobs (min-1) thus obtained were found to increase
with [H2O2] (Table 4) over a range of [H2O2] used (1.0 ×
10-3~5.0 × 10-3 M). This shows the reaction obeys first
order with respect to [H2O2]. This was confirmed by the
linear plots of kobs (min-1) vs. [H2O2] which yielded a
straight line passing through the origin (Fig. 4).
(3) Effect of [MnII]. The reaction rates were measured
at constant [sulfide] (4.0 × 10-3 M), [H2O2] (5.0 × 10-2 M)
but various [MnII] (0.2 × 10-5~1.0 × 10-5 M). Under
Fig. 5. The linear plot of rate constant kobs (min-1) vs. [MnII] with
nonzero intercept confirms that the reaction is fractional order
with respect to [MnII]
Copyright © 2013 World Scientific Publishing Company
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KINETIC AND MECHANISM OF THE AQUEOUS SELECTIVE OXIDATION OF SULFIDES TO SULFOXIDES 1109
Table 5. Effect of temperature variation
a
on pseudo first order rate constant kobs
Entry
T, K
kobs × 104, min-1
1
2
3
4
5
313
318
323
328
333
2.98
3.71
5.45
7.78
10.39
a[Sulfide] = 4.0 × 10-3 M, [MnII] = 0.5 ×
10-5 M, [H2O2] = 5.0 × 10-2 M
Fig. 8. Profile of oxidation of thioanisole catalyzed by
MnTE4PyP in the presence of hydrogen peroxide
The initial characteristic absorption peaks of
MnTE4PyP were at 463 and 493 nm. By adding hydrogen
peroxide and substrate into the solution, in situ UV-vis
measurement revealed a new absorption peak at 558 nm,
while the characteristic absorption peak of MnTE4PyP
was weakened gradually. These evidence suggested the
generation of high-valent oxidant active species oxo-
Mn(IV) in the aqueous solution [13].
With hydrogen peroxide as oxidant, profile for the
oxidation of thioanisole catalyzed by MnTE4PyP in
aqueous solution was shown in Fig. 8.
Fig. 6. Arrhenius plot of the rate constants for the oxidation of
sulfide
As shown in Fig. 8, the conversion of thioanisole
increased sharply within the first 20 min, and almost all
of the substrate was oxidized in one hour in this catalytic
system. The salient feature of the present oxidation is
its high selectivity toward sulfoxide during the whole
catalytic process. It indicated that the generation of
sulfoxide was via the reaction of oxo-Mn(IV) and sulfide.
After that, oxo-Mn(IV) was reduced initial valence and
no chance to generate sulfone by the reaction of sulfoxide
and oxo-Mn(IV).
On the basis of the previous observations and results,
a mechanism for the aqueous oxidation of sulfides with
MnTE4PyP as catalyst was proposed:
K1
MnII + H2O2
MnIII-O-O-H
MnIII-O-O-H +H+
MnIV = O + OHi
ꢀꢀꢀꢁ
ꢂ ꢀ ꢀꢀ
(1)
(2)
K2
ꢀꢀꢀꢁ
ꢂ ꢀ ꢀꢀ
Fig. 7. In situ UV-vis spectra of MnTE4PyP in the aqueous
oxidation of thioanisole
O
k3
S
MnIV=O +
MnII
+
(3)
R1
R2
S
R1
R2
In addition, the results from the controlled experiments
as shown in Table 5 have demonstrated that manganese
porphyrin is crucial for the selective oxidation of sulfides.
Generally for the metalloporphyrins-catalyzed oxidations,
high valence intermediate is accepted as the active species
[12]. High valence manganese porphyrin in the catalytic
system was verified from the in situ UV-vis spectroscopy
as shown in Fig. 7.
An oxo species in a high valence oxidation state was
generated via heterolytic cleavage of oxygen-oxygen
bond of the hydroperoxo intermediate. The formation
of sulfoxide was via an oxygen atom transfer process
between oxo-Mn(IV) and sulfide (direct oxygen atom
transfer). However, Baciocchi ever proposed the ET
mechanism (eletctron transfer) towards the oxidation of
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2013; 17: 1109–1112

1110 X.-T. ZHOU ET AL.
a number of aromatic sulfides by H2O2 in aqueous acidic
medium (pH = 3) [8a].
The mechanism for the oxidation of thioanisole by
manganese porphyrin is as follows:
Synthesis
Synthesis of meso-tetrakis(1-methyl-4-pyridyl)-
metalloporphyrin. Pyrrole (20 mL) and 4-pyridinecar-
baldehyde (20 mL) were refluxed in propionic acid
(200 mL) for 60 min to produce meso-tetrakis(4-
pyridyl) porphyrin (T4PyP) with the yield of 24%.
Next, T4PyP (0.25 g) and manganese acetate (1 g)
were refluxed in dimethylformamide (35 mL) for 6 h
under nitrogen atmosphere to form meso-tetrakis(4-
pyridyl) manganese porphyrin (MnT4PyP) with the
yield of 88%. In the following step, MnT4PyP (0.2 g)
and methyl p-toluenesulfonate (12 g) were added to
100 mL dimethylformamide solution. The mixture was
stirred for 12 h at 90 °C, and the solution was cooled
to room temperature. Ethyl ether (20 mL) was added in
the mixture and was kept overnight. After filtration, the
filter cake was washed thoroughly with ethyl ether and
was purified by recrystallization with water/acetone (1:2)
to obtain meso-tetrakis(1-methyl-4-pyridyl) manganese
porphyrin (MnTM4PyP) with 80% yield. The other
metallopophyrins could be synthesized with similar
procedures by replacing other metal ions. The spectral
and analysis data of metalloporphyrins catalysts were
listed as following. [MnTM4PyP]4+. EI-MS: m/z 731.
UV-vis (H2O): lmax, nm (log e) 464 (2.48), 559 (1.25). IR
(KBr): n, cm-1 1640, 1009. Anal. calcd. for C44H36N8Mn:
C, 72.22; H, 4.96; N, 15.31. Found C, 71.79; H, 4.92; N,
15.16. [CoTM4PyP]4+. EI-MS: m/z 735. UV-vis (H2O):
d[Sulfide]
−
= k3[O = MnIV ][Sulfide]
dt
Substituting the concentration of above terms in the
rate equation, the rate expression may thus be represented
by Equation 4.
Rate = K1K2k3[MnII][Sulfide][H2O2]
(4)
That is,
Rate = K[MnII][Sulfide][H2O2]
(5)
where, K = K1K2k3.
The rate expression is quite consistent with kinetic
results.
EXPERIMENTAL
All materials are commercial reagent grade and were
purchased from Alfa Aesar or Aldrich without further
purification unless indicated. Pyrrole was redistilled
before use. Other solvents were all of analytical grade.
UV-vis spectrum was recorded on a Shimadzu
UV-2450 spectrophotometer. Mass spectra were obtained
1
on a Shimadzu LCMS-2010A. H NMR and 13C NMR
l
max, nm (log e) 418 (2.47), 539 (0.93). IR (KBr): n,
cm-1 1640, 1005. Anal. calcd. for C44H36N8Co: C, 71.83;
H, 4.93; N, 15.23. Found C, 71.43; H, 4.72; N, 14.96.
[FeTM4PyP]4+. EI-MS: m/z 732. UV-vis (H2O): lmax, nm
(log e) 415 (2.84), 543 (1.23). IR (KBr): n, cm-1 1638,
1002. Anal. calcd. for C44H36N8Fe: C, 72.13; H, 4.95; N,
15.29. Found C, 72.32; H, 4.95; N, 15.31.
spectra were recorded in CDCl3 with a Varian Mercury-
Plus 300 spectrometer. GC analyses were performed on a
Shimadzu GC-2010 plus chromatography equipped with
Rtx-5 capillary column (30 m × 0.25 mm × 0.25 mm).
GC-MS analyses were recorded on a Shimadzu GCMS-
QP2010 equipped with Rxi-5ms capillary column (30 m ×
0.25 mm × 0.25 mm).
Synthesis of meso-tetrakis(N-ethylpyridinium-
4-yl)metalloporphyrin. MnTPyP (0.15 g, 0.221 mmol)
and fresh distilled iodoethane (0.62 mg, 4 mmol) were
addedto20mLdimethylformamidesolution.Themixture
was stirred for 4 h at 45 °C, and the solution was cooled
to room temperature. Next, 20 mL ethyl ether was added,
and the mixture was kept overnight. After filtration, the
filter cake was washed thoroughly with ethyl ether and
recrystallized with water/acetone (1:2) to obtain 0.189 g
(MnTE4PyP) (71.4%). The other metallopophyrins
could be synthesized with similar procedures by
replacing other metal ions. The spectral and analysis
data of metalloporphyrins catalysts were listed as
following. [MnTE4PyP]4+. EI-MS: m/z 787. UV-vis
(H2O): lmax, nm (log e) 463 (2.48), 493 (1.25), 675
(0.87), 769 (0.54). IR (KBr): n, cm-1 1623, 1001. Anal.
calcd. for C48H44N8Mn: C, 73.18; H, 5.64; N, 14.22.
Found C, 74.67; H, 5.51; N, 13.99. [CoTE4PyP]4+.
EI-MS: m/z 791. UV-vis (H2O): lmax, nm (log e) 437
(2.56), 545 (0.87), 615 (0.34). IR (KBr): n, cm-1
1640, 1002. Anal. calcd. for C48H44N8Co: C, 72.81;
H, 5.60; N, 14.15. Found C, 71.67; H, 5.72; N, 15.56.
Procedures of preparing catalysts
The selected water-soluble metalloporphyrins for this
study are shown in Scheme 1, including cationic and
anionic metalloporphyrins. The procedures of synthe-
sizing catalysts are described as follows.
Ar
Ar =
Ar =
N
N
CH3
TM4PyP
TE4PyP
N
N
N
N
M
Ar
C2H5
Ar
Ar =
SO3Na TPPS
Ar
M = Mn, Fe, Co
Scheme 1. The structures of water-soluble metalloporphyrins
used as catalyst
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J. Porphyrins Phthalocyanines 2013; 17: 1110–1112

KINETIC AND MECHANISM OF THE AQUEOUS SELECTIVE OXIDATION OF SULFIDES TO SULFOXIDES 1111
[FeTE4PyP]4+. EI-MS: m/z 788. UV-vis (H2O): lmax
,
nm (log e) 417 (3.04), 545 (1.59). IR (KBr): n, cm-1
1625, 1001. Anal. calcd. for C48H44N8Fe: C, 73.09; H,
5.62; N, 14.21. Found C, 72.52; H, 5.95; N, 15.12.
Synthesis of (meso-tetrakis(p-sulfonatophenyl)-
metalloporphyrin. Tetra sodium meso-tetra(sulfonato-
phenyl)porphyrin (TPPS) was synthesized according to
reported procedures [14]. TPPS (0.2 g, 0.18 mmol) and
MnSO4.H2O (0.3 g, 1.8 mmol) were added and stirred
in 20 mL deionized water. The reaction was monitored
by UV-vis (the soret band of the porphyrin was shifted
from 414 nm to 467 nm). So as to wipe off the excess
manganese ion, the reaction solution was passed through
an Amberlite IR-120 column after it was finished. Then
the solution was neutralized by 0.1 moL/L NaOH.
Removal of water under vacuum afforded the crude
MnTPPS, which was dissolved by methanol to eliminate
the salts for three times to give 0.15 g (75.6%) MnTPPS.
The reaction conditions for the synthesis of other MTPPS
are the same as those for MnTPPS. MnTPPS. EI-MS:
m/z 982. UV-vis (H2O): lmax, nm (log e) 467 (2.78), 563
(1.43). IR (KBr): n, cm-1 1654, 1008. Anal. calcd. for
C44H24N4O12S4Mn: C, 53.71; H, 2.46; N, 5.69. Found C,
55.01;H,2.51;N,5.92. CoTPPS.EI-MS:m/z985.UV-vis
(H2O): lmax, nm (log e) 419 (3.14), 542 (1.23). IR (KBr):
n, cm-1 1654, 1006. Anal. calcd. for C44H24N4O12S4Co:
C, 53.50; H, 2.45; N, 5.67. Found C, 54.23; H, 2.72; N,
5.96. FeTPPS. EI-MS: m/z 984. UV-vis (H2O): lmax, nm
(log e) 413 (2.94), 556 (1.43). IR (KBr): n, cm-1 1656,
1005. Anal. calcd. for C44H24N4O12S4Fe: C, 53.66; H,
2.46; N, 5.69. Found C, 52.92; H, 2.95; N, 5.31.
CONCLUSION
In conclusion, an environmental-friendly protocol
has been developed for selective oxidation of sulfides to
sulfoxides with hydrogen peroxide in aqueous solution.
Compared with anionic metalloporphyrins, the cationic
metalloporphyrins showed high selectivity for sulfoxide.
The MnTE4PyP-based catalytic system has proved to
be effective in the oxidation of various sulfides with
hydrogen peroxide as terminal oxidant. The kinetic
parameters were determined, and the reaction scheme
was also presented. The proposed mechanism, involving
high-valence manganese porphyrins formed as a result
of heterolytic cleavage of oxygen–oxygen bond of the
hydroperoxo intermediate was supported by kinetic
orders and spectrophotometric evidences.
Acknowledgements
The authors thank the National Natural Science
Foundation of China (Nos. 21036009 and 21176267),
Guangdong Natural Science Foundation (S2011040-
001776) and Research Fund for the Doctoral Program
of Higher Education of China (20110171120024) for
providing financial support to this project.
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J. Porphyrins Phthalocyanines 2013; 17: 1112–1112