Highly selective and efficient oxidation of sulfide to sulfoxide catalyzed by platinum porphyrins

Article abstract of DOI:10.1142/S1088424615501126

Two platinum porphyrins, meso-tetramesitylporphyrinatoplatinum and meso-tetrakis(pentaflourophenyl) porphyrinatoplatinum, are explored for catalytic application in the selective oxidation of sulfide to sulfoxide by iodosylbenzene. The obtained overall turnover number of 90,000 in the oxidation of thioanisole in the presence of meso-tetrakis(pentaflourophenyl) porphyrinatoplatinum indicates the pronounced catalytic activity of the platinum porphyrins. Perfect selectivity toward sulfoxide or sulfone also was achieved via stoichiometric control of reactants.

Full text of DOI:10.1142/S1088424615501126

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2015; 19: 1279–1283
DOI: 10.1142/S1088424615501126
Published at http://www.worldscinet.com/jpp/
Highly selective and efficient oxidation of sulfide to sulfoxide
catalyzed by platinum porphyrins
Tahereh Alemohammada, Saeed Rayatib◊ and Nasser Safari*a
a Department of Chemistry, Shahid Beheshti University, P.O. Box 1983963113, G. C., Evin, Tehran, Iran
b Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran 15418, Iran
Received 25 November 2015
Accepted 23 December 2015
ABSTRACT: Two platinum porphyrins, meso-tetramesitylporphyrinatoplatinum and meso-
tetrakis(pentaflourophenyl) porphyrinatoplatinum, are explored for catalytic application in the selective
oxidation of sulfide to sulfoxide by iodosylbenzene. The obtained overall turnover number of 90,000 in
the oxidation of thioanisole in the presence of meso-tetrakis(pentaflourophenyl) porphyrinatoplatinum
indicates the pronounced catalytic activity of the platinum porphyrins. Perfect selectivity toward
sulfoxide or sulfone also was achieved via stoichiometric control of reactants.
KEYWORDS: sulfides, selective oxidation, platinum porphyrins, high turnover number.
existence of a six-coordinated peroxo-type species as an
INTRODUCTION
oxidant which is less reactive than a metal oxo species
Oxidation of sulfides is very important from both
and therefore can control the reactivity and selectivity of
synthetic and mechanistic investigation points of view.
the catalyst [34–36].
Sulfoxides are valuable compounds owning to their
Recently, the formation of PtIV(porp+·)(OH)2 species
pharmaceutical, industrial and synthetic applications
as the active oxidizing species in the catalytic oxidation
[1–3]. Oxidation of sulfides to sulfoxides as the main
of alkenes by platinum porphyrins has been reported by
route to get sulfoxides has been the subject of extensive
us [37]. Electrochemical formation of this species has
studies [4–11]. Over-oxidation of sulfides to sulfones is
also been reported and well-documented by Kadish and
the major problem of the oxidation. Therefore, selective
co-workers [38, 39]. As a part of our ongoing interest
oxidation of sulfides to sulfoxides under mild and green
on catalytic activity of platinum porphyrins in oxidation
conditions has remained a challenge thus far [12–20]. On
reactions, we examined their catalytic activity in the
the other hand, since sulfide and sulfoxide have different
oxidation of various sulfides.
nucleophilic tendency, some groups use sulfide oxidation
to get some clue about the active oxidizing species of a
catalytic system [21, 22].
Metalloporphyrins particularly manganese and iron
porphyrins have been investigated extensively as models
RESULTS AND DISCUSSION
Catalytic oxidation of thioanisole (as a model
substrate) with iodosylbenzene in the presence of meso-
tetramesitylporphyrinato platinum (PtTMP) has been
of CPY-450 enzymes to oxidize organic substrates
including sulfides [23–29]. While a high-valent metal
studied. Blank experiment (Entry 1, Table 1) revealed that
oxo species is responsible for oxidation of substrate in
the presence of the catalyst is essential for an effective
catalytic reaction.
To note, selectivity of the reaction can be controlled
by thioanisole to oxidant molar ratio (Entries 2 and 3,
Table 1). In the 1:1 molar ratio of the oxidant to
the presence of metal porphyrins and related macrocycles
[24, 30–33] but there are some reports that confirm the
◊SPP full member in good standing
thioanisole, sulfoxide is the sole product while in the 3:1
(oxidant/thioanisole) molar ratio, sulfone was exclusively
*Correspondence to: Nasser Safari, email: n-safari@sbu.ac.ir,
tel: +98 21-2990-2886, fax: +98 21-2243-1671
Copyright © 2015 World Scientific Publishing Company

1280 T. ALEMOHAMMAD ET AL.
Table 1. Optimization of the molar ratio for {catalyst/thioanisole/PhIO} in catalytic thioanisole
oxidation by PtTMP as catalysta
Entry Molar ratio Conversion, %b
Sulfoxide, %
Sulfones, %
Selectivity to sulfoxide, %
1
2
3
4
5
6
7
8
0:20:60
—
100
85
—
—
100
—
—
—
—
—
—
—
1:20:60
—
—
1:20:20
85
100
1:50:50
90
90
100
1:60:60
91
85(90)c
91
85(90)c
100
100(100)a
1:100:100
1:150:150
1:200:200
70
70
100
65
65
100
a The reactions were all triplicated at room temperature, in CH2Cl2 and the average data are reported
with 4% error. GC injections were carried out after 30 min. b GC yield is based on starting thioanisole.
cNumbers in parentheses are for 24 h after starting of the reaction.
Table 2. Sulfoxide yield (%) of thioanisole
Table 3. Yield (%) of various sulfides oxidation by two different
oxidation in different solventsa
Pt-porphyrins using PhIO as oxidanta
Time, min
Solvent
Catalyst
Entry
1
Sulfide
30
60
90
PtTMP PtTPFPP
CH2Cl2
85
55
88
70
90
75
S
85
86(14)b
CHCl3
Ph
Me
CH3COCH3
CH3CN
Traceb
Trace
58
50
80
S
2
3
60
13
80(12)b
25
Trace
80
Trace
80 (6.3)c
S
CH2ClCH2Cl
a PtTMPandPhIOwereusedascatalystandoxidant
respectively and the molar ratio for {catalyst/
S
4
5
82
60
100
b
thioanisole/PhIO} is {1/100/100}. The catalyst
was dissolved gradually in acetone. c6.3% sulfone
was observed.
37(18)b
S
6
100
100
S
achieved. These results confirm that the over-oxidation of
sulfoxide depends on the oxidant to sulfide molar ratio.
The effect of different catalyst concentrations was also
explored and the results are presented in Table 1 (Entries
4–8). The obtained conversion for “1:100:100” molar ratio
is just 6% less than that of “1:60:60” molar ratio, while the
molar ratio in the former is almost half of the latter. So, the
conditions given in Entry 6 were chosen as the optimized
conditions.
The influence of different solvents on the oxidation
of thioanisole was studied with the results presented in
Table 2. The lowest catalytic activity in the acetone and
acetonitrile may possibly be due to their low solubility
in these solvents because the catalyst was dissolved
overtime in acetone and showed catalytic activity.
In order to find the effect of the other oxidants
on the oxidation reaction, catalytic oxidation of
thioanisole with urea hydrogen peroxide (UHP),
hydrogen peroxide (H2O2) and tert-butyl hydroperoxide
a The reactions were carried out at room temperature, in CH2Cl2.
The molar ratio for {catalyst/substrate/PhIO} is {1/100/100}.
GC injections were carried out after 30 min. Numbers in
b
parentheses are the yield of sulfone.
(TBHP) has been studied. No oxidation product was
achieved in the presence of these oxidants, probably
because these oxidants can only oxidize PtII(porp) to
PtIV(porp)(OH)2 that is inactive in oxidation reactions
while oxidants such as iodosylbenzene(PhIO) or meta-
chloroperbenzoic acid (m-CPBA) can oxidize PtII(porp)
by three electrons to PtIV(porp+·)(OH)2 which is a mild
oxidant [37].
A variety of sulfides were oxidized by two platinum
porphyrins, meso-tetrakis(pentaflourophenyl) porphyrin-
atoplatinum (PtTPFPP) and meso-tetramesitylporphyrin-
atoplatinum (PtTMP) which have very different
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HIGHLY SELECTIVE AND EFFICIENT OXIDATION OF SULFIDE TO SULFOXIDE CATALYZED BY PLATINUM PORPHYRINS 1281
Table 4. Oxidation of thioanisole using PhIO catalyzed by some metal porphyrinsa
Catalyst
Conversion, % Sulfoxide, % Sulfone, % Selectivity to sulfoxide, %
PtTMP
85
53
78
85
43
62
—
10
16
100
80
Fe(TMP)Cl
Mn(TMP)Clb
80
a The reactions were carried out at room temperature, in CH2Cl2 as solvent. The molar
ratio for {catalyst/substrate/PhIO} is {1/100/100} and GC injections were carried out after
30 min. b Imidazol used as co-catalyst with 20 fold of Mn(TMP)Cl.
O
S
electronic density on the porphyrin ring (Table 3). The
CH2Cl2
results show that both reactivity and selectivity of the
catalysts depend on electronic properties of platinum
porphyrins. The higher conversion was achieved in the
presence of PtTPFPP. This may be due to the electron
withdrawing effects of the fluorine atoms which caused
higher activity of the PtIV(porp+·)(OH)2 species. Sulfone
formation in some cases confirmed the higher activity of
PtTPFPP compared to PtTMP [40].
PtTPFPP + PhSMe + PhIO
100000 100000
rt
Ph
Me
1
several days
Conversion = 90%
TON = 90000
Scheme 1. Oxidation of thioanisole in large-scale catalyzed by
PtTPFPP
As can be seen from Table 3, the reactivity of the
catalysts also depend on electronic and steric properties
sulfides although electronic effects are much more
impressive (Entries 2 and 3, Table 3). The more electron
density on sulfur, the higher conversion.
The catalytic activity of PtTMP has been compared
with iron and manganese counterparts (Table 4). Pt-
porphyrin shows not only higher activity than Fe- or
Mn-porphyrins but also exhibits perfect selectivity for
sulfoxide product.
Higher selectivity of the platinum porphyrins is due to
the formation of PtIV(porp+·)(OH)2 species [37] which is
responsible for oxidation of sulfides while in the case of
Fe- or Mn-porphyrins, a high-valent metal oxo species
will be formed which is highly reactive [36].
Fig. 1. UV-vis spectra of reaction mixture (a) before starting
the reaction (blue) and (b) after 2 days from the reaction (red)
Stability towards oxidative degradation is another
salient feature of the platinum porphyrins. Higher activity
of platinum porphryins arises from this outstanding
feature of platinum porphyrins compared with iron or
manganese porphyrins. Therefore, as anticipated, the
catalyst shows almost no degradation after the experiment
in contrast to the reports for Mn- or Fe-porphyrins which
undergo degradation very quickly [41, 42].
The turnover number is calculated by the following
equation:
mmolsulfoxide
TON =
mmolcatalyst
(1)
High turnover number is anticipated for this catalytic
system due to outstanding stability of the platinum
porphyrins towards oxidative degradation. Therefore,
thioanisole oxidation catalyzed by PtTPFPP in large-
scale was investigated (Scheme 1) and it should be noted
that the turnover number (TON) of the catalyst could
exceed 90,000, due to its high oxidative stability and
negligible degradation (see Fig. 1). As it is obvious from
Fig. 1, there is just small demetalation of the PtTPFPP
catalyst. To the best of our knowledge, in the oxidation
reaction systems for sulfide to sulfoxide catalyzed by
metalloporphyrins, such a high TON value has been
reported just for ruthenium porphyrins [43].
EXPERIMENTAL
All chemicals and solvents were purchased from
Merck, Fluka or Aldrich chemical companies and used
as recieved. The free bases, meso-tetraphenylporphyrin
(H2TPP), meso-tetramesitylporphyrin (H2TMP) and
meso-tetrakis(pentaflourophenyl)porphyrin(H2TPFPP),
were prepared according to the literature [44]. Free base
porphyrins were metalated with platinum(II) chloride,
PtCl2, as described in the literature to give corresponding
meso-tetraphenylporphyrinato platinum (PtTPP), meso-
tetramesitylporphyrinato platinum (PtTMP) and meso-
tetrakis(penta-flourophenyl)porphyrin (PtTPFPP)
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J. Porphyrins Phthalocyanines 2015; 19: 1281–1283

1282 T. ALEMOHAMMAD ET AL.
Table 5. Spectroscopic data for prepared porphyrins
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402
412
392
510, 540
506, 584, 645
508, 539
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8.83
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red porphyrins are given in Table 5. Iodosylbenzene
was obtained by the hydrolysis of iodosylbenzene
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Stock solutions of the platinum porphyrins (3 × 10-3 M)
were prepared in CH2Cl2. In a 10 mL round-bottom flask,
the reagents were added in the following order: sulfide
(0.3 mmol), catalyst (3 × 10-3 mmol), iodosylbenzene,
PhIO, (0.3 mmol) was then added to the reaction mixture
at room temperature. The mixture was stirred for 30 min
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to sulfoxides or sulfones with 100 percent selectivity
using controlled stoichiometry of the reaction. Platinum
porphyrin degradation under reaction conditions is
negligible, therefore high turnover number in sulfide
oxidation was observed. Adjusted catalytic activity of
the active oxidant PtIV(porp+·)(OH)2 results in perfect
selectivity toward sulfoxide or sulfone via stoichiometric
control of reactants.
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We would like to thank the Graduate Study Councils
of Shahid Beheshti University and also K. N. Toosi
University of Technology for financial support.
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