Chemoselective oxidation of sulfides to sulfoxides with urea hydrogen peroxide (UHP) catalyzed by non-, partially and fully β-brominated meso-tetraphenylporphyrinatomanganese(III) acetate

Article abstract of DOI:10.1016/j.inoche.2013.11.036

Selective oxidation of sulfides to sulfoxides with urea hydrogen peroxide in the presence of the manganese complex of non-, partially and fully brominated meso-tetraphenylporphyrin, (MnTPPBr_x(OAc) (x = 0, 2, 4, 6 and 8)) is reported. Although, the maximum conversion was achieved in the case of MnTPPBr₄(OAc), little difference was found between the catalytic activity of MnTPP(OAc), MnTPPBr₂(OAc) and MnTPPBr₄(OAc). MnTPPBr₈(OAc) showed an unusually very low catalytic efficiency compared to the other manganese porphyrins. The presence of small amounts of acetic acid was shown to have significant effect on the total conversion and the oxidative stability of the catalyst.

Full text of DOI:10.1016/j.inoche.2013.11.036

Inorganic Chemistry Communications 40 (2014) 82–86
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Chemoselective oxidation of sulfides to sulfoxides with urea hydrogen
peroxide (UHP) catalyzed by non-, partially and fully β-brominated
meso-tetraphenylporphyrinatomanganese(III) acetate
a,
a
b,
Saeed Rayati ⁎, Fatemeh Nejabat , Saeed Zakavi ⁎
a
Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran 15418, Iran
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Selective oxidation of sulfides to sulfoxides with urea hydrogen peroxide in the presence of the manganese com-
Received 13 October 2013
Accepted 26 November 2013
Available online 4 December 2013
plex of non-, partially and fully brominated meso-tetraphenylporphyrin, (MnTPPBr
is reported. Although, the maximum conversion was achieved in the case of MnTPPBr
found between the catalytic activity of MnTPP(OAc), MnTPPBr (OAc) and MnTPPBr
x
(OAc) (x = 0, 2, 4, 6 and 8))
(OAc), little difference was
(OAc). MnTPPBr (OAc)
4
2
4
8
showed an unusually very low catalytic efficiency compared to the other manganese porphyrins. The presence
of small amounts of acetic acid was shown to have significant effect on the total conversion and the oxidative
stability of the catalyst.
Keywords:
Oxidation
Sulfides
Urea hydrogen peroxide (UHP)
Manganese porphyrin
© 2013 Elsevier B.V. All rights reserved.
Organic sulfoxides are useful and active intermediates in both labora-
tory and industry, organic synthesis and therefore the chemoselective
oxidation of organic sulfides to the corresponding sulfoxides has been
the subject of various studies for the past two decades [1–3]. An addition-
al challenge in this respect is to make such processes environmentally
friendly, by strategies such as using nontoxic solvents, green oxidants
and energy-efficient catalytic methods [2–5]. Metalloporphyrins as
model catalysts of cytochrome P450 have been used extensively for bio-
mimetic oxidation of organic compounds [6–10]. On the other hand, in
the past two decades great interest has been focused on using clean pro-
cedures for oxidation reactions catalyzed by metalloporphyrins [11–13].
In this regard, hydrogen peroxide and its derivatives such as UHP (urea
hydrogen peroxide) and polyvinylpyrrolidone-supported hydrogen per-
mixture. The released urea upon the reaction of UHP with organic
substrates is then available to form strong hydrogen bonds with the
water molecules formed in the oxidation reaction. Herein, a green and
simple method for selective oxidation of sulfides to sulfoxides with
x
UHP catalyzed by (MnTPPBr (OAc) (x = 0, 2, 4, 6 and 8)), in the
presence of imidazole (ImH) in ethanol is reported. Also, the influence
of different parameters on the efficiency of the catalysts was investigated.
The free base porphyrins and the manganese complexes were
prepared and purified as reported previously [16].
Oxidation of methyl phenyl sulfide with UHP catalyzed by
x
MnTPPBr (OAc) (x = 0, 2, 4, 6 and 8) gave methyl phenyl sulfoxide
as the major product. In a search for suitable reaction conditions to
achieve the maximum conversion and highest selectivity for sulfox-
ide, the effect of different parameters including solvent, tempera-
ture, amount of oxidant and ImH and the presence of acetic acid
(HOAc) was studied.
2 2
oxide (PVP–H O ) as cheap and environmentally friendly oxidants
which only produce water and oxygen as side products are attractive ox-
idants for oxidation of organic compounds [2–5,11–14]. Also, UHP has
the advantage that the vacuum dried reagent may be used as a nearly
water-free peroxide source. It is noteworthy that in the oxidation of or-
ganic compounds catalyzed by manganese porphyrins, the presence of
Oxidation of methyl phenyl sulfide with UHP was carried out in the
presence of MnTPPBr
highest conversion was observed in the presence of MnTPPBr
However, little difference was found between the catalytic activity of
MnTPPBr (OAc) with no, two and four bromine atoms. Also, MnTPPBr
8
x
(OAc) (x = 0, 2, 4, 6 and 8) (Table 1) and the
4
(OAc).
2
H O leads to the formation of a high-valent Mn-oxo species as the active
oxidant [11,12]. In the oxidation of sulfides, higher oxidizing ability of the
high valent manganese oxo species compared to the corresponding
manganese(III) species, i.e. (porphyrin)Mn(III)(oxidant)(axial base) is
possible to direct the reaction towards the formation of sulfone as the
major product [15]. In other words, further oxidation of sulfoxide to
sulfone may be prevented by the removal of water from the reaction
x
showed very low catalytic activity. Along with our previous work [16],
the present results confirmed that the partially brominated Mn-porphy-
rins have a higher catalytic activities compared to the fully ß-brominated
one.
The reaction was performed in dichloromethane, methanol, ethanol
and the mixture of dichloromethane and the non-chlorinated solvents
(Table 2). Although the highest conversion was achieved in mixture of
⁎
Corresponding authors. Tel.: +98 21 22850266; fax: +98 21 22853650.
E-mail addresses: rayati@kntu.ac.ir, srayati@yahoo.com (S. Rayati).
CH Cl :MeOH or methanol, ethanol is a more convenient solvent for
2
2
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.11.036

S. Rayati et al. / Inorganic Chemistry Communications 40 (2014) 82–86
83
Table 1
Oxidation of methyl phenyl sulfide with UHP catalyzed by MnTPPBr
x
(OAc) (x = 0, 2, 4, 6 and 8) in the presence of ImH in CH
3
2 2
OH:CH Cl
(9:1) at room temperature.a
Sulfoxide selectivity%
Catalyst
Conversion%
Sulfoxide%
Sulfone%
None
MnTPP(OAc)
MnTPPBr
MnTPPBr
MnTPPBr
MnTPPBr
Trace
44
46
48
20
5
–
–
–
2
–
–
–
–
44
44
48
20
5
100
96
100
100
100
2
4
6
8
(OAc)
(OAc)
(OAc)
(OAc)
a
The molar ratios for catalyst:ImH:sulfide:UHP are 1:10:20:60. Reaction time: 5 min.
Table 2
Effect of various solvents (in different molar ratios) on the oxidation of methyl phenyl sulfide in the presence of ImH and MnTPPBr
at room temperature.a
Sulfone%
4
Entry
Time
Solvent
CH Cl
Conversion%
Sulfoxide%
Selectivity (sulfoxide)%
1
2
3
4
5
6
7
5
5
2
2
3.6
26.6
21.0
34.7
33.0
12.0
5.4
3.6
26.6
21.0
34.7
33.0
12.0
5.4
–
–
–
–
–
–
–
100
100
100
100
100
100
100
CH Cl
CH Cl
CH Cl
MeOH
CH Cl
2
2
:MeOH (1:1)
:MeOH (9:1)
:MeOH (1:9)
5
5
5
15
20
2
2
2
2
2
2
:ethanol (1:9)
Ethanol
a
4
The molar ratios for MnTPPBr :ImH:MePhS:UHP are 1:10:20:40.
Table 3
The effect of various conditions on the oxidation of methyl phenyl sulfide catalyzed by MnTPPBr
in the presence of ImH in ethanol.a
Conversion%
4
Entry
Time (min)
Temperature
HAC/cat
ImH/cat
UHP/cat
Sulfoxide%
Sulfone%
Sulfoxide selectivity%
1
2
3
4
5
6
7
8
9
20
20
5
10
10
5
5
5
5
5
25
25
25
25
25
25
25
25
25
0
–
10
10
30
30
30
30
30
30
30
30
30
30
30
40
60
60
60
60
40
40
40
40
40
60
60
60
5.4
6.2
5.4
6.2
–
100
100
100
90
41
82
80
87
100
85
77
–
–
–
19.0
26.7
96.2
56.7
67.6
42.2
40.0
42.5
80.4
90.8
73.6
19.0
24.2
39.1
46.7
54.2
37.4
40.0
35.9
61.6
55.1
40.3
–
–
2.5
57.1
10.0
13.4
4.7
0
6.8
18.8
35.7
33.3
45
45
30
15
55
30
30
45
55
1
1
1
1
0
1
2
3
10
10
10
0
0
0
60
55
a
See the footnotes of Table 1.
environmentally friendly conversion of sulfides to sulfoxides [17–19]
and therefore in further optimization of the catalytic system ethanol
was used as solvent (Table 3). UHP is very insoluble in dichloromethane,
due to the strong hydrogen bonds between H O and urea. Solvents
2 2
such as methanol or ethanol increase the solubility of UHP and conse-
quently hydrogen peroxide.
The selective oxidation of sulfides to sulfoxides has been an impor-
tant challenge in synthetic organic chemistry [1–3,20]. Accordingly,
the optimum conditions for the formation of sulfoxide as the major
product have been investigated. The results for the oxidation of methyl
phenyl sulfide in various conditions are summarized in Table 3. As
the results show, in the absence of HOAc, very low conversions
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
conversion
sulfoxide
sulfone
10
30
40
50
ImH/Catalyst
Fig. 1. Effect of molar ratio of the ImH:MnTPPBr
MnTPPBr :ImH:MePhS:UHP are 1:X:20:40.
4
on the oxidation of methyl phenyl sulfide with UHP in ethanol and 10 μl HOAc at room temperature. The molar ratios for
4

84
S. Rayati et al. / Inorganic Chemistry Communications 40 (2014) 82–86
Table 4
Oxidation of various sulfides with UHP catalyzed by MnTPPBr
high valent manganese oxo species has been shown to be facilitated
in the presence of alcohols [21]. The decreased selectivity for sulfoxide
seems to be due to the higher oxidizing ability of the high valent
4
in the presence of ImH and
a
10 μl of HOAc at 0 °C in ethanol.
Entry
1
Sulfides
Conversion%
Selectivity%
77 (41)
2 2
metal oxo species compared to the six coordinated Mn(III)–H O one.
80.4 (96.2)b
Acceptable conversions and selectivity for sulfoxide may be achieved
by changing the molar ratio between HOAc, ImH and the oxidant.
The presence of nitrogenous base has been shown to have significant
effect on the catalytic activity of manganese porphyrins [22–27]. Oxida-
tion of methyl phenyl sulfide was carried out using various molar ratios
of ImH/catalyst (Fig. 1) and the 1:30 molar ratio was found to be the op-
timized one. Further increase in the molar ratio catalyst to ImH, beyond
the 1:40 one led to a dramatic decrease in the conversion. This observa-
tion may be due to the formation of an inactive six coordinate species,
2
91.4 (89.5)
77 (69)
57 (67)
3
45.2 (33.9)
a
The molar ratios for MnTPPBr
The reaction carried out in EtOH and 15 μl of HOAc at room temperature.
4
:ImH:MePhS:UHP are 1:30:20:60.
b
i.e. Mn(porphyrin)(ImH)
Oxidation of various sulfides with UHP in the presence of catalytic
amounts of MnTPPBr and ImH gave a mixture of sulfoxide and sulfones
as the products (Table 4).
In the absence of MnTPPBr
2
[27].
Table 5
The effect of HOAc on the oxidation of methyl phenyl sulfide with UHP in ethanol at room
temperature.
4
a,b
Entry
HOAc/UHP
Conversion%
Selectivity to sulfoxide%
4
, the oxidation of methyl phenyl sulfide
with UHP did not proceed even after 45 min. Also, in the presence of
acetic acid a conversion of ca. 8.5% was obtained (Table 5). These obser-
vations suggest a catalytic cycle containing the manganese porphyrin as
catalyst and peracetic acid formed by the reaction of hydrogen peroxide
and HOAc [28] as oxidant (Scheme 1, I) [29–31].
1
2
0
0.7
0
8.5
0
100
a
The molar ratios for sulfide:UHP are (20:60).
For reactions performed in the absence of MnTPPBr .
4
b
Increasing the rate of the reaction by addition of HOAc suggests the
formation of a manganese-acylperoxo species (Scheme 1, II) [32–34]
which facilitates the cleavage of the oxygen–oxygen bond to generate
a manganese-oxo intermediate (Scheme 1, III) [35,36]. On the other
(
5.4–26.7%) were obtained and increases in ImH concentration or reac-
tion time have little effect on the conversion (Table 3, Entries1–4). Ad-
dition of 15 μl of HOAc increased the conversion to 96.2%, but the
selectivity for sulfoxide decreased (Table 3, Entry 5). The formation of
4
hand, oxidative stability of MnTPPBr (OAc) has been found to be higher
Scheme 1. Proposed catalytic cycle.

S. Rayati et al. / Inorganic Chemistry Communications 40 (2014) 82–86
85
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