Selective oxidation of sulfides and oxidative bromination of organic substrates catalyzed by polymer anchored Cu(II) complex

Article abstract of DOI:10.1016/j.tetlet.2011.10.138

A new polymer-anchored Cu(II) complex has been synthesized and characterized. The catalytic performance of the complex has been tested for the oxidation of sulfides and in oxidative bromination reaction with hydrogen peroxide as the oxidant. Sulfides have been selectively oxidized to the corresponding sulfoxides in excellent yields and in the presence of KBr as the bromine source, organic substrates have been selectively converted to mono bromo substituted compounds using polymer-anchored Cu(II) catalyst. This catalyst showed excellent catalytic activity, high selectivity, and recyclability. The polymer-anchored Cu(II) catalyst could be easily recovered by filtration and reused more than five times without appreciable loss of its initial activity.

Full text of DOI:10.1016/j.tetlet.2011.10.138

Tetrahedron Letters 53 (2012) 127–131
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Selective oxidation of sulfides and oxidative bromination of organic
substrates catalyzed by polymer anchored Cu(II) complex
a,
⇑
a
a
b
a
c
S. M. Islam , Anupam Singha Roy , Paramita Mondal , Kazi Tuhina , Manir Mobarak , John Mondal
a
Department of Chemistry, University of Kalyani, Kalyani, Nadia 741235, WB, India
Department of Chemistry, B.S. College, Canning S-24PGS, WB, India
Department of Material Science, IACS, Kolkata 700032, WB, India
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 April 2011
Revised 19 October 2011
Accepted 22 October 2011
Available online 6 November 2011
A new polymer-anchored Cu(II) complex has been synthesized and characterized. The catalytic perfor-
mance of the complex has been tested for the oxidation of sulfides and in oxidative bromination reaction
with hydrogen peroxide as the oxidant. Sulfides have been selectively oxidized to the corresponding sulf-
oxides in excellent yields and in the presence of KBr as the bromine source, organic substrates have been
selectively converted to mono bromo substituted compounds using polymer-anchored Cu(II) catalyst.
This catalyst showed excellent catalytic activity, high selectivity, and recyclability. The polymer-anchored
Cu(II) catalyst could be easily recovered by filtration and reused more than five times without appreciable
loss of its initial activity.
Keywords:
Oxidative bromination
Polymer-anchored Cu(II) catalyst
Sulfide oxidation
Ó 2011 Elsevier Ltd. All rights reserved.
Hydrogen peroxide
Sulfoxides and sulfones are useful synthetic intermediates for
the synthesis of several important organic compounds. There
systems suffers from being wasteful in the bromine employed as
one-half ends up as hydrogen bromide. In large-scale operations
this is an environmental as well as an economical problem. In oxy-
1
are also many biologically important compounds which contain a
sulfoxide or sulfone moiety. Sulfoxides and sulfones are generally
prepared via oxidation of the corresponding sulfides; however, it
is often very difficult to stop the oxidation of the sulfoxide. In
the oxidation of sulfides, other oxidizable functional groups, such
as alcohols or olefins, present in sulfides sometimes react as
1
0,11
bromination,
2 2
HBr as a bromine source and H O /TBHP as oxi-
dants were thought to be a possible solution to overcome these
difficulties and met with partial success, since HBr is highly toxic
and corrosive and is as harmful as molecular bromine to the envi-
ronment. The oxidative bromination of aromatics is an attractive,
mild alternative to the more widely used bromine in glacial acetic
acid. Recently the oxidative bromination of aromatic compounds
has been described using potassium bromide, hydrogen peroxide,
1
well-producing undesirable compounds. A large number of meth-
ods have been developed to overcome this drawback; however,
most of these reactions require a stoichiometric amount of oxidant,
resulting in undesirable waste.2 Recently aqueous 30% hydrogen
peroxide has been used as an attractive and environmental friendly
12–15
and various metal-oxo compounds as catalysts.
Homogeneous catalysts have been used in organic reactions
3
16–18
oxidant for the oxidation of sulfides, because it is inexpensive,
since a very long time.
However, these homogeneous catalysts
easy to handle, safely stored, and produces only water as a by-
product. Since the oxidation of sulfides with aqueous hydrogen
peroxide in the absence of any catalyst is slow, several transition
metal (Ti, Fe, V, Cu, and Mn) compounds have been used as
catalysts for this reaction.4 Aryl halides are important synthetic
intermediates for a variety of transformations including carbon–
carbon bond formation via cross-coupling reactions such as
Stille–Suzuki, Heck, and Sonogashira and carbon–heteroatom bond
formation via aromatic functionalization protocols.5 A variety of
methods for the bromination of aromatics have been reported in
face the problem of separation from the reaction mixture and its
reuse. Homogeneous catalyst can be recovered and reused if it is
heterogenized by supporting on an insoluble support. A variety
of supports have been tested, including inorganic carriers such as
19
20
21
22
molecular sieves, silica, zeolites, clays, as well as poly-
2
3
24
mers and resins. In recent years, chloromethylated polystyrene
cross linked with divinylbenzene is one of the most widely used
macromolecular supports for immobilization of homogeneous cat-
25
alysts. Activity and selectivity of heterogeneous metal complexes
2
6
were controlled by polymeric supports.
Heterogenization or
6
–9
the literature.
The classical direct bromination of aromatic
immobilization of homogeneous complexes on polymeric supports
has developed as a promising strategy for combining the advanta-
ges of homogeneous and heterogeneous catalysts due to their easy
separation from the products by filtration and the possibility of
⇑
Corresponding author. Tel.: +91 33 2582 8750; fax: +91 33 2582 8282.
E-mail address: manir65@rediffmail.com (S. M. Islam).
2
7
recycling them by continuous operation.
0
040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.10.138

1
28
S. M. Islam et al. / Tetrahedron Letters 53 (2012) 127–131
In this Letter, we report the synthesis and characterization of a
as commodity chemicals and, in some cases, as pharmaceuticals.
To test the catalytic activity of the present catalyst, first oxidation
of sulfides were conducted with hydrogen peroxide at room tem-
perature. For optimization of reaction conditions, we choose oxida-
tion of diphenyl sulfide as a probe reaction. The effects of solvent,
polymer-anchored Cu(II) catalyst. The catalytic activity of the com-
plex was tested in oxidation of sulfides and oxidative bromination
reaction of aromatic compounds. This catalyst shows high catalytic
activity and selectivity in the above two reactions. This polymer-
anchored catalyst can be separated and reused for more than five
times without any significant loss in its activities.
Polymer-anchored Cu(II) catalyst can be prepared in two steps
which is shown in Scheme 1. Polymer-anchored ligand was pre-
pared according to the literature method.28 Then polymer-an-
2 2
temperature, and amount of H O were examined and the results
are presented in Table 1. The oxidation of diphenyl sulfide with
30% hydrogen peroxide using polymer-anchored Cu(II) complex
as a catalyst was examined in several solvents (Table 1). The less
polar solvents which are immiscible with aqueous hydrogen per-
oxide were not effective (runs 1–3). In the polar solvents, diphenyl
sulfide was efficiently oxidized into the corresponding sulfoxide.
The oxidative ability of this reaction system in more polar solvents
chored ligand (1.0 g) was reacted with CuCl
15 mL) at 70 °C to obtained the brown colored catalyst.
Due to insolubilities of the polymer-anchored ligand and copper
2
(0.10 g) in ethanol
(
complex in all common organic solvents, their characterization
was limited to their physicochemical properties, elemental analy-
sis, SEM, TGA, FT-IR, diffuse reflectance UV–vis and atomic absorp-
tion spectroscopy which confirm the immobilization of copper
onto polymer-anchored ligand. Elemental analyses of the ligand
and complex support the formulation of the complex. Atomic
absorption spectroscopy suggested 4.77% of Cu in the complex.
like CH
that in less polar solvents like CH
and tBuOH (run 9). The best condition to prepare the sulfoxide
selectively is the reaction in CH CN at room temperature (run 4).
The effect of temperature was investigated for the oxidation of di-
phenyl sulfide and the results are shown in Table 1. When reaction
temperature increases from room temperature (run 4) to 40 °C
(run 10) there was a slight increase in conversion but further in-
crease in temperature (conversion kept constant on run 11) had
3
CN (run 4), MeOH (run 6) and EtOH (run 7) is greater than
3
CH CN (run 5), iPrOH (run 8),
2
3
The polymer-anchored ligand shows a peak around 3448-3425,
À1
1
685, 1598, and 1450 cm for –NH (secondary amine),
t
(C@O),
t
a-
sym (COO) and
t
sym (COO) stretching vibrations. The lowering in
no effect on conversion. Amount of H
on the conversion and selectivity of diphenyl sulfide oxidation
reaction. Conversion increased when amount of H increased
from 5 mmol (run 12) to 10 mmol (run 4) but there was no signif-
icant change in conversion when amount changed to 15 mmol (run
13) in the same reaction condition. The selectivity of sulfoxide de-
2 2
O has a significant effect
frequency of the above peaks indicates the coordination of metal
center with the ligand. The polymer-anchored Cu(II) complex also
2 2
O
À1
29
À1
29
exhibited peaks at 548 cm
(Cu–N), 652 cm
15 cm (Cu–Cl). The diffuse reflectance (DR)-UV–vis spectra
of the copper complex shows that a broad band centered at
40 nm may be assigned to charge transfer transition, while the
(Cu–O) and
À1
30
3
4
creased with the increase of the amount of H
ther oxidation of sulfoxide.
2 2
O . This is due to fur-
2
2
shoulder at around 600 nm may be attributed to B1g? A1g transi-
tion in a square-planar stereochemistry.31 The scanning electron
micrographs of polymer-anchored ligand and the supported cata-
lyst clearly show the morphological change which occurred on
the surface of polystyrene after loading of the metal. The catalyst
is thermally stable up to 430 °C.
To examine the reactivity of the catalyst further, oxidation of
other sulfides also has been investigated (Table 2). Substrate scope
is extended to methyl phenyl sulfide, ethyl phenyl sulfide, dibutyl
sulfide, and diethyl sulfide etc. The sulfoxides were selectively ob-
tained in all cases. A series of substrates, aryl alkyl, aryl allyl, and
dialkyl sulfides, could be oxidized to the corresponding sulfoxides.
The reactivity and conversion were dependent on the nature of the
Oxidation of sulfides is the most straightforward method for the
synthesis of sulfoxides and sulfones, both of which are important
P
CH
2
NH2
DMF
-HCl
P
CH Cl
NH
2
+
OH
C
O
OH
C
O
(
1)
(
2)
CuCl2
70 °C
Ethanol
P
CH
2
NH
Cl
Cu
O
OH
2
C
O
(3)
Scheme 1. Preparation of polymer-anchored Cu(II) complex.

S. M. Islam et al. / Tetrahedron Letters 53 (2012) 127–131
129
Table 1
The salt effect was also studied for the oxidative bromination
reaction of salicylaldehyde. The catalyst was employed using
H O as an oxidant and MBr (M = Li, Na, K) as a bromine source
2 2
Effect of different solvents on oxidation of diphenyl sulfide with polymer-anchored
Cu(II) catalysta
Run Solvent
Time
h)
Conversionb
(%)
Selectivity of sulfoxideb,c
(%)
(Table 3). Among the three salts, KBr has been found to be the most
efficient bromine source and a monoselective product is obtained.
The use of sodium bromide did not produce regioselective prod-
ucts. LiBr, though efficient, is less selective than KBr. Influence of
temperature was also studied but there was no effect on the
conversion of salicylaldehyde. With increase of temperature the
conversion of salicylaldehyde was almost the same. A wide range
of solvents have been employed in this reaction including, carbon
tetrachloride, hexane, methanol, acetonitrile, and acetic acid. The
best results were obtained when acetic acid was used as a solvent
compared to others. The role of hydrogen peroxide was confirmed
by conducting a blank experiment where the formation of bromo
compound was not observed. All substrates selectively converted
to their monobrominated products (Table 4). Substrates like phe-
nol, resorcinol, anisole and N,N-dimethylaniline showed excellent
para-selectivity. The activated aromatic substrates, like phenol,
resorcinol, aniline, anisole and N,N-dimethylaniline, have shown
high conversion into their respective monoselective products.
The inactive substrates, like benzene, have shown low conversion
and required longer reaction time. On the other hand, deactivated
aromatic ring did not show any conversion in this reaction condi-
tion. 4-substituted aromatics selectively converted to 2-bromo
derivatives and vice-versa.
(
1
2
3
4
5
6
7
8
CH
AcOEt
Toluene
CH
CH
2
Cl
2
6
6
6
3
4
3
3
4
4
3
3
3
3
40
51
32
96
86
90
88
66
64
97
97
62
96
23
55
12
94
82
86
89
78
69
91
88
93
75
3
CN
CH
3
2
CN
MeOH
EtOH
i
PrOH
BuOH
t
9
d
1
1
1
1
0
CH
CH
CH
CH
3
CN
3
CN
3
CN
3
CN
e
f
1
2
g
3
a
Conditions: diphenyl sulfide (5 mmol); 30% aq
2 2
H O (10 mmol); solvent
(
10 mL); 50 mg catalyst at room temperature.
b
Conversion and selectivity were determined by GC.
Products were characterized by GC–MS.
Reaction temperature was 40 °C.
Reaction temperature was 50 °C.
c
d
e
f
5
mmol 30% H
2
O
2
was used.
g
1
5 mmol 30% H
2
O
2
was used.
substituent. In the case of allyl sulfides, less oxidation was
observed at the carbon–carbon double bond. Similarly, benzylic
sulfides could be oxidized to the corresponding sulfoxides with
small oxidation products of the benzylic C–H bond.
In order to find out the effect of immobilization of homoge-
neous complex on polymer, the catalytic activity of the present
Table 2
Oxidation of sulfides using 30% H
2
O
2
catalyzed by polymer-anchored Cu(II) catalysta
Conversionb (%)/time (h)
Entry
Sulfide
Selectivity of Sulfoxideb,c (%)
1
2
82 (3)
88 (6)
73
77
S
S
S
3d
4
96, 95, 94, 94, 93 (3)
94
90
88
89
S
CH3
93 (3)
91 (3)
85 (6)
S
S
C H
5
2
5
C H
6
6
13
S
7
87 (3)
86
S
8
9
68 (6)
79 (6)
87
86
S
C H
6
13
Me
S
C H
6
13
10
78 (6)
84
MeO
a
b
c
Conditions: sulfide (5 mmol); 30% aq H
2
O
2
(10 mmol); CH
3
CN (10 mL); 50 mg catalyst at room temperature.
Conversion and selectivity were determined by GC.
Products were characterized by GC–MS and NMR.
Catalytic runs to test recyclability.
d

1
30
S. M. Islam et al. / Tetrahedron Letters 53 (2012) 127–131
Table 3
Effect of different bromine sources on the oxidative bromination of salicylaldehyde using polymer-anchored Cu(II) catalyst
a
Entry
Salt
Conversionb (%)
Selectivityb,c
1
2
3
LiBr
NaBr
KBr
89
79
95
5-Bromo-2-hydroxy benzaldehyde (79) + 3, 5-dibromo-2-hydroxy benzaldehyde (21)
5-Bromo-2-hydroxy benzaldehyde (64) + 3, 5-dibromo-2-hydroxy benzaldehyde (36)
5-Bromo-2-hydroxy benzaldehyde (100)
a
b
c
Conditions: salicylaldehyde (2 mmol); MBr (2.2 mmol); glacial acetic acid (5 mL); 30% aq H
Conversion and selectivity were determined by GC.
Products were characterized by GC–MS and NMR.
2 2
O (2.2 mmol), 50 mg catalyst at room temperature.
Table 4
Oxidative bromination of various organic substrates catalyzed by polymer-anchored Cu(II) catalysta
Entry
Substrate
Conversionb (%) (time/h)
Product selectivityb,c
1d
2
3
4
5
6
7
8
9
Salicylaldehyde
Phenol
Resorcinol
4-Methylphenol
4-Nitrophenol
4-Aminophenol
Anisole
4-Methylanisole
Aniline
4-Nitroaniline
4-Methylaniline
4-Chloroaniline
2-Methylaniline
95, 94, 94, 92, 90 (3.0)
96 (2.5)
95 (2.5)
93 (2.5)
80 (6)
76 (3)
93 (3)
90 (3)
96 (2.5)
88 (3)
5-Bromo-2-hydroxy benzaldehyde (100)
4-Bromophenol (100)
4-Bromo-1,3-dihydroxybenzene (100)
2-Bromo-4-methylphenol (100)
2-Bromo-4-nitrophenol (100)
2-Bromo-4-aminophenol (100)
4-Bromoanisole (100)
2-Bromo-4-Methylanisole (100)
4-Bromoaniline (83) 2-Bromoaniline (10)
2-Bromo-4-nitroaniline (100)
2-Bromo-4-methylaniline (85)
2-Bromo-4-chloroaniline (100)
4-Bromo-2-methylaniline (80)
10
11
12
13
93 (3)
86 (3)
88 (3)
6
-Bromo-2-methylaniline (15)
4-Bromo-2-chloroaniline (91)
-Bromo-2-chloroaniline (9)
4-Bromo-2-nitroaniline (76)
-Bromo-2-nitroaniline (24)
1
1
4
5
2-Chloroaniline
2-Nitroaniline
92 (3)
82 (3)
6
6
16
17
18
N,N-Dimethylaniline
Benzene
Nitrobenzene
92 (3)
10 (6.0)
—
4-Bromo-N,N-dimethylaniline (100)
Bromobenzene (100)
—
a
b
c
Conditions: substrate (2 mmol); KBr (2.2 mmol); glacial acetic acid (5 mL); 30% aq H
Conversion and selectivity were determined by GC.
Products were characterized by GC–MS and NMR.
2
O
2
(2.2 mmol), 50 mg catalyst at room temperature.
d
Catalytic runs to test recyclability.
polymer-anchored Cu(II) was compared with its homogeneous
analog in oxidation and oxidative bromination reaction. From the
results it is seen that the present polymer-anchored copper cata-
lyst is more active than the corresponding homogeneous catalyst.
Homogeneous catalyst oxidized diphenyl sulfide with 79% conver-
sion while polymer-anchored copper catalyst performed well with
of salicylaldehyde were carried out under the optimum reaction
conditions. The reaction is stopped after the reaction proceeds
for 2 h. The catalyst was separated from the reaction mixture by fil-
tration and the conversion was determined. The separated filtrate
is allowed to react for another 2 h under the same reaction condi-
tions, but no further increment in conversion is observed in gas
chromatographic analyses. The UV–vis spectroscopy was also used
to determine the stability of the heterogeneous catalyst. The UV–
vis spectra of the reaction solution, at the first run, do not show
any absorption peaks characteristic of copper metal, indicating that
the leaching of copper does not take place during the course of the
oxidation or oxidative bromination reaction. These results suggest
that the catalyst is heterogeneous in nature.
9
2 2
6% conversion with H O at room temperature. In oxidative bro-
mination of salicylaldehyde, homogeneous complex shows 82%
conversion whereas heterogeneous catalyst shows 95% conversion
by GC analysis. Also, the homogeneous catalyst cannot be recov-
2 2
ered or reused. In the absence of the catalyst, H O alone shows
slight activity in oxidation reaction of sulfides but is unable to oxy-
halogenate the substrates to any significant extent. The catalytic
activity of this catalyst was also superior with other reported het-
erogeneous catalysts for the oxidation reaction of sulfides and oxi-
In conclusion we have prepared a polymer-anchored Cu(II)
complex which was used as a heterogeneous catalyst for oxidation
of sulfides and oxidative bromination of aromatic substrates with
3
2
2 2.
dative bromination reaction of organic substrates with H O
The recyclability of the catalyst is important for the catalysis
reaction. The reusability of polymer-anchored Cu(II) complex was
investigated in the oxidation of diphenyl sulfide and oxidative bro-
mination of salicylaldehyde. Catalyst was separated by filtration
after the first catalytic run, washed with solvent and dried under
vacuum then subjected to the second run under the same reaction
conditions. The catalytic run was repeated with further addition of
substrates under optimum reaction conditions and the nature and
yield of the final products were comparable to that of the original
one. It is found that the catalytic activity or selectivity does not
change significantly after five consecutive runs.
2 2
H O as oxygen source at room temperature. The catalyst shows
high catalytic activity and selectivity. This catalyst is air-stable,
inexpensive and easy to make. Leaching test indicates that the cat-
alytic reaction is mainly heterogeneous in nature. The reusability
of this catalyst is high and can be reused five times without signif-
icant decrease in its initial activity.
Acknowledgments
We thank the Department of Material Science, Indian Associa-
tion of Cultivation of Science, Kolkata, for providing the instrumen-
tal support. MI acknowledges the DST, CSIR, and UGC, New Delhi,
India for funding.
To check the leaching of copper into the solution during the
reaction, oxidation of diphenyl sulfide and oxidative bromination

S. M. Islam et al. / Tetrahedron Letters 53 (2012) 127–131
131
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