A highly efficient reusable homogeneous copper catalyst for the selective aerobic oxygenation sulfides to sulfoxides

Article abstract of DOI:10.1016/j.tetlet.2018.01.066

Readily available copper showed efficient activity and great selectivity for the homogeneous catalysis of oxidation of sulfides to sulfoxides using molecular oxygen as the oxidant. The reaction proceeds under mild conditions in the presence of a catalytic amount of TEMPO. Importantly, the catalysts could be conveniently recovered and reused. And this methodology was proved to be applicable for the transformation of various aromatic and aliphatic sulfides into the corresponding sulfoxides with high conversion and high selectivity.

Full text of DOI:10.1016/j.tetlet.2018.01.066

Accepted Manuscript
A highly efficient reusable homogeneous copper catalyst for the selective aero-
bic oxygenation sulfides to sulfoxides
Cheng Ren, Runxing Fang, Xiaochun Yu, Shun Wang
PII:
S0040-4039(18)30101-1
https://doi.org/10.1016/j.tetlet.2018.01.066
TETL 49653
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
1 December 2017
15 January 2018
22 January 2018
Please cite this article as: Ren, C., Fang, R., Yu, X., Wang, S., A highly efficient reusable homogeneous copper
catalyst for the selective aerobic oxygenation sulfides to sulfoxides, Tetrahedron Letters (2018), doi: https://doi.org/
10.1016/j.tetlet.2018.01.066
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A highly efficient reusable homogeneous
copper catalyst for the selective aerobic
oxygenation sulfides to sulfoxides
Cheng Rena, Runxing Fanga, Xiaochun Yua, * and Shun Wanga, *
Leave this area blank for abstract info.

1
Tetrahedron Letters
A highly efficient reusable homogeneous copper catalyst for the selective aerobic
oxygenation sulfides to sulfoxides
Cheng Rena, Runxing Fanga, Xiaochun Yua,  Shun Wanga, *
aCollege of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, Zhejiang 325035, P. R. China.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Readily available copper showed efficient activity and great selectivity for the homogeneous
catalysis of oxidation of sulfides to sulfoxides using molecular oxygen as the oxidant. The
reaction proceeds under mild conditions in the presence of a catalytic amount of TEMPO.
Importantly, the catalysts could be conveniently recovered and reused. And this methodology
was proved to be applicable for the transformation of various aromatic and aliphatic sulfides into
the corresponding sulfoxides with high conversion and high selectivity.
Available online
Keywords:
Aerobic oxidation reaction
Sulfide
2009 Elsevier Ltd. All rights reserved.
Sulfoxide
Copper catalyst
Introduction
them. Heterogeneous catalysts have attracted much attention due
to their advantages, easy separation from the products and
Developing highly efficient catalytic oxygenation of sulfides
to sulfoxides has attracted increasing attention in the last decade.
It is because the associated products, sulfoxides are key
intermediates for the synthesis of chemical and biologically
important molecules,1 and equally important, organic sulfoxides
are widely used as effective ancillary ligands in synthetic
chemistry, especially in asymmetric synthesis.2
recyclability. Now several heterogeneous materials catalyzed or
mediated methods for oxidation of sulfides into sulfoxides are
available, such as dye-sensitized titanium dioxide/TEMPO,22
titanium dioxide /amine,23 porous conjugated polymers,24 solid-
supported Rose Bengal,25 metal−organic frameworks,26
MoS2/Ta3N5,27 molybdenum-doped α-MnO2,28 Au/MnO2-x,29
supported TEMPO derivatives with Mn(NO3)2-Co(NO3)2,30 SiO2-
supported Ru complex,31 and so on. These catalysts could be
recovered and reused, but there was still quite a bit of room for
improvement of their catalytic activities, selectivities, and in
particular the range of suitable substrates.
A number of promising strategies for selective oxidation of
sulfides to sulfoxides have been developed in the past few years.
Different oxidants, such as H2O2,3 organic hydroperoxide,4
halogen oxysalt,5 oxone,6 manganese,7 persulfate,8 peracid,9
nitrate10 and so on, have been used. The use of the greenest O2 is
more desirable and advantageous over the existing methods from
the standpoint of green chemistry. Moreover, using O2 as oxidant
proves extremely challenging due to the difficulty of O2
activation and its subsequent highly reactive nature once
activated.
In this report, we present a highly efficient and practical
process for the selective synthesis of sulfoxides through aerobic
oxidation of sulfides with O2 in which copper acts as a reusable
homogeneous catalyst.
Results and discussion
A variety of catalytic systems for the aerobic oxygenation of
sulfides into sulfoxides have been reported to date, such as
Diphenyl sulfide was chosen as a model substrate for
optimizing our oxidation reactions. The results were summarized
in Table 1. Firstly, copper salt showed good ability in this
transformation in the presence of pyridine (50 mol%) and
TEMPO (5 mol%), as the reaction did not work without copper.
We have thus screened various copper catalysts, including
Cu(OAc)2.H2O, CuSO4, CuSO4.5H2O, Cu(NO3)2.3H2O, CuO,
CuCl, CuBr, CuI, Cu powder, Cu2O, CuCl2.2H2O, (Table 1,
Entries 1-12). It was found that CuSO4 performed the best. There
was a dramatic decrease in catalytic activity when the reaction
PyHBr3/TBN/H2O,11
N-hydroxyphthalimide,12
flavin(cat.)
14
combined with HCO2H/Et3N,13 tetra-O-acetylriboflavin,
palladium dimer [Pd(PBut2H)(μ-PBut2)]2,15 palladium(II)–
porphyrin complex,16 Pt(II) complexes,17 Fe(NO3)3-FeBr3,18
Fe2O3 with isovaleraldehyde,19 vitamin B2 derivative,20 boron-
dipyrromethene,21 etc. And some catalytic systems offered the
target products in high yields with high selectivities, however,
the recovery and reuse of the catalysts were very difficult for
———
 Corresponding author. e-mail: xiaochunyu@wzu.edu.cn; shunwang@wzu.edu.cn.

2
Tetrahedron Letters
was carried out in air (table 1, Entry 13). As expected, the
phenanthroline and L-proline were employed as the ligands,
respectively, no oxidation product was detected and only intact
substrate was recovered (Entries 16-19, Table 1). Exploration on
the effects of other amines, such as TEA, DABCO, and TEMED
on the reaction showed that pyridine was the most suitable ligand
for this transformation (Table S1, SI).
complete loss of activity was observed in a N2 atmosphere (table
1, Entry 14). No reaction occured in the absence of TEMPO
(table 1, Entry 15).
We subsequently investigated the effect of different ligands on
observed reactivity. When 2,2’-bipyridine, 4,4’-bipyridine, 1,10-
Table 1. Screening of reaction condition.
Sel. (%)b
Entrya
[Cu]
Ligand(50mol%)
Conv. (%)b
2a
99
-
3a
1
-
1
2
Cu(OAc)2.H2O
-
pyridine
pyridine
71
N.R.
99
3
CuSO4
pyridine
91
100
98
-
9
-
4
CuSO4.5H2O
Cu(NO3)2.3H2O
CuO
pyridine
72
5
pyridine
65
2
-
6
pyridine
N.R.
5
7
CuCl
pyridine
100
-
-
8
CuBr
pyridine
N.R.
N.R.
48
-
9
CuI
pyridine
-
-
10
11
12
13c
14d
15e
16
17
18
19
Cu2O
pyridine
100
97
-
-
Cu powder
CuCl2.2H2O
CuSO4
pyridine
35
3
-
pyridine
N.R.
51
pyridine
98
-
2
-
CuSO4
pyridine
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
CuSO4
pyridine
-
-
CuSO4
1,10-Phenanthroline
2,2-Bipyridine
4,4-Bipyridine
L-Proline
-
-
CuSO4
-
-
CuSO4
-
-
CuSO4
-
-
aThe reaction was conducted in anoxic conditions. Unless otherwise noted, reaction conditions: Sulfide (0.5 mmol) in CH3OH (1 mL), pyridine (50 mol%),
TEMPO (5 mol%), [Cu] (5 mol%), O2, 65oC, 24h .
bDetermined by GC Agilent 7890A and GC-MS.
cThe reaction was conducted under air condition.
dThe reaction was conducted under N2 condition.
eNo TEMPO was added in the reaction.
Further screening of copper, TEMPO and pyridine loading,
were summarized in Table S2 in supporting information. It was
also found that the reaction could give good conversion at room
temperature with longer reaction time (Table S3, SI). Through
solvent screening (Table S4, SI), we found the reaction could
take place only in MeOH with good conversion. The oxidation of
1a with O2 gave diphenyl sulfoxide 2a in 85% isolated yield
under the following conditions: CuSO4 (5 mol%), pyridine (20
mol%), and TEMPO (4 mol%) in MeOH (0.5 M) with stirring at
65°C under O2 (1atm) condition for 24 h.
electron-withdrawing substituent at the para-position such as
NO2-, CH3CO-, and NC- (Table 2, Entries 8-10) showed a
negative effect on the reaction. In particular, a much longer
reaction time (16 h) was needed for them to complete the
reaction. Meanwhile, we also observed the steric effects of the
substrates on the reaction time. For achieving a satisfied
conversion more hindered ortho-substituted sulfides such as 2-
chlorophenyl methyl sulfide (Table 2, Entry 11), and 2-
bromophenyl methyl sulfide (Table 2, Entry 13) required longer
reaction times than corresponding para-substituted sulfides such
as 4-chlorophenyl methyl sulfide (Table 2, Entry 3) and 4-
bromophenyl methyl sulfide (Table 2, Entry 4). The ethyl and
cyclopropyl substitution at sulfur did not slow down the reaction
rate (Table 2, Entries 17 and 18).
To explore the generality of this procedure, a variety of
sulfides bearing various functional groups were evaluated by
performing the reaction under the given conditions. Results
presented in Table 2 illustrated that CuSO4/TEMPO/pyridine
catalyst system worked well for various aromatic and aliphatic
sulfides. Methyl phenyl sulfide gave a much higher yield (Table
2, Entry 2, 92% isloated yield) in 8 hr. Different substituted-
phenyl groups in the methyl aryl sulfides 1c-p gave excellent
yields of the sulfoxide product (Table 2, Entries 3-16). A strongly
It should be noted that the sulfoxidations of 4-
(methylthio)phenol (Talbe 2, Entry 7), allyl phenyl sulfide (Table
2, Entry 19), 2-(phenylthio)ethanol (Talbe 2, run 20),
dihexylsulfane (Talbe 2, Entry 21) and tetrahydrothiophene
(Talbe 2, Entry 22) were inferior, giving low conversion of the

3
corresponding sulfoxides. However, when the dosage of TEMPO
was increased to 10 mol%, the reaction was greatly improved,
and the yields of the target products (Table 2, Entries 7 and 19-22)
could be increased greatly even without affecting the oxidation-
susceptible functional groups such as hydroxyl group. On the
other hand, for allyl phenyl sulfide (Talbe 2, Entry 19) and
dihexylsulfane (Talbe 2, Entry 21), their oxidation reaction
should be carried out under room temperature, otherwise,
byproducts, diphenyl disulfide or dihexyldisulfide were largely
produced.
Table 2. Copper/pyridine/TEMPO catalyzed oxidation of various sulfides into sulfoxides.
Sel.(%)b
Entrya
R1
R2
t/hr.
Conv. (%)b
Yield(%)c
2
3
1
Ph
Ph
24
8
99
91
98
96
96
97
97
>99
99
95
93
99
98
>99
>99
>99
95
94
94
94
97
96
100
9
2
4
4
3
3
-
85(2a)
92(2b)
93(2c)
93(2d)
91(2e)
94(2f)
95(2g)
93(2h)
89(2i)
86(2j)
94(2k)
90(2l)
96(2m)
99(2n)
96(2o)
91(2p)
88(2q)
90(2r)
92(2s)
58(2t)
90(2u)
88(2v)
2
Ph
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
Et
99
3
4-ClC6H4-
4-BrC6H4-
4-CH3OC6H4-
4-CH3C6H4-
4-HOC6H4-
4-NO2C6H4-
4-CH3COC6H4-
4-NCC6H4-
2-ClC6H4-
2-FC6H4-
2-BrC6H4-
2-CH3OC6H4-
3-CH3OC6H4-
3-ClC6H4-
Ph
8
>99
99
4
10
8
5
>99
>99
97
6
10
24
16
16
16
12
12
12
8
7d
8
99
1
5
7
1
2
-
9
>99
99
10
11
12
13
14
15
16
17
18
>99
>99
>99
>99
>99
>99
99
-
8
-
10
12
12
48
48
72
24
5
6
6
6
3
4
-
Ph
>99
98
19d,e
Ph
Allyl
20d
21de
22d
Ph
-(CH2)2OH
n-C6H13-
63
n-C6H13-
95
-(CH2)4-
92
aUnless otherwise noted, the reaction conditions: Sulfide (0.5 mmol), CuSO4 (5 mol%), pyridine (20 mol%), TEMPO (4 mol%), O2 (1atm), CH3OH (1ml), 65oC.
bDetermined by GC Agilent 7890A and GC-MS.
cThe reaction was conducted under N2 condition. Isolated yields of sulfoxides.
d10 mol% of TEMPO was used.
eThe reaction temperature is 25oC.
It is worth noting that, the oxidizable moiety such as olefins,
aliphatic alchols, phenols remained unaffected by
CuSO4/TEMPO catalyst (Table 2, Entries 7, 19-20), revealing the
high chemical selectivity of catalyst system. Meanwhile, under
the optimized conidition 2a was synthesised on gram scale (10
mmol) with 84% isolated yield(1.70g).
of ligand, it could also catalyze the reaction efficiently (Table 3,
Entry 2), and possibly the solid was the complex of CuSO4 with
ligand. With this in mind, we synthesized the complex A of
copper and pyridine directly through mixing CuSO4 and pyridine.
The thus-obtained solid was washed with petroleum, and also
showed excellent catalytic activity (Table 3, Entry 1).
Interestingly, during the process of purifying the product after
the reaction, it was observed that when ethyl acetate was added to
the mixture, the blue solid precipitate appeared which could be
recovered conveniently through filtration. When the solid was
used in a new reaction, we found that even without the addition
Next, we investigated the recyclability and reusability of the
catalyst using 1a as a model substrate as shown in Table 3. It is
noteworthy that there was an obvious decrease in the yield of
diphenyl sulfoxide when we used the recovered complex without
the addition of pyridine (Table 3, Entries 3, 4). We speculate that

4
Tetrahedron Letters
it is because the complexes were not very stable, and ligand
pyridine might have partially dissociated from then. Another a
series of three recycle experiments were done where 20 mol% of
pyridine was added (Table 3, Entries 5-7), respectively, and the
results indicated that the copper catalyst could be reused for the
same reaction at least three times while retaining its high
catalytic performance. The recovered solids from the catalyst
system of CuSO4/TEMPO/pyridine, were crystallized and
characterized by single crystal X-ray diffraction, as shown in
figure 1.
Table 3. Catalyst recycling experiments.
Sel.(%)b
Figure 2. Proposed possible mechanism for the oxidation of
sulfides with O2 catalyzed by Cu(II) and TEMPO.
Entry Pyridine(mol%) TEMPO
Conv.(%) b
2a
91
94
98
100
90
90
91
3a
9
1c
2
3
4
5
6
7
-
4
4
4
4
4
4
4
97
87
40
28
99
97
97
Conclusions
-
6
In summary, we have developed a simple and environmentally
friendly method for highly efficient oxidation of sulfides to
sulfoxides with molecular oxygen as the oxidant, and cheap and
readily available CuSO4/pyridine/TEMPO as catalysts. The
system proceeds under mild conditions, and has been proven to
be applicable for the selective oxidation of the aromatic and
aliphatic sulfides with impressive yield. Moreover,
catalyst/product separation is very simple through the
precipitation and filtration. It was demonstrated that the thus-
obtained solid could be reused without considerable loss of
activity during the oxidation reaction.
-
2
-
-
20
20
20
10
10
9
aAll the reaction was conducted with sulfide (0.5mmol), recovered catalyst
(4mg) in CH3OH (1 mL), oxygen atmosphere, 65oC, 24hr.
bDetermined by GC Agilent 7890A and GC-MS.
c4 mg complex A was used.
Acknowledgments
Financial support from the National Natural Science
Foundation of China (Nos. 21302143 and 51572198), Natural
Science Foundation of Zhejiang Province (Nos. LY13B020006,
and LZ17E020002) are greatly appreciated.
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6
Tetrahedron Letters
Highlights
A highly efficient reusable homogeneous copper
catalyst for the selective aerobic oxygenation
sulfides to sulfoxides
Cheng Ren, Runxing Fang, Xiaochun Yu*and Shun
Wang*
College of Chemistry and Materials Engineering, Wenzhou University,
Chashan University Town, Wenzhou, Zhejiang Province 325035, People’s
Republic of China.
The highlights were elucidated as below:
1. CuSO4 acts a reusable homogeneous catalyst.
2. Molecular oxygen as the green oxidant.
3. Mild reaction conditions
4. High convertion and high selectivity.