Polymer-anchored Cu(II) complex as an efficient catalyst for selective and mild oxidation of sulfides and oxidative bromination reaction

Article abstract of DOI:10.1007/s10562-012-0942-x

A new polymer-anchored Cu(II) complex has been tested for the oxidation of sulfides and in oxidative bromination reaction with hydrogen peroxide as oxidant. Sulfides have been selectively oxidized to corresponding sulfoxides in excellent yields and in presence of KBr as bromine source, organic substrates have been selectively converted to mono bromo substituted compounds. The polymer-anchored Cu(II) catalyst could be easily recovered by simple filtration and reused more than six times without appreciable loss of its initial activity. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-012-0942-x

Catal Lett (2013) 143:225–233
DOI 10.1007/s10562-012-0942-x
Polymer-Anchored Cu(II) Complex as an Efficient Catalyst
for Selective and Mild Oxidation of Sulfides and Oxidative
Bromination Reaction
•
Sk. M. Islam Anupam Singha Roy
•
•
Paramita Mondal Noor Salam Sumantra Paul
•
Received: 26 September 2012 / Accepted: 16 November 2012 / Published online: 1 December 2012
Ó Springer Science+Business Media New York 2012
Abstract A new polymer-anchored Cu(II) complex has
been tested for the oxidation of sulfides and in oxidative
bromination reaction with hydrogen peroxide as oxidant.
Sulfides have been selectively oxidized to corresponding
sulfoxides in excellent yields and in presence of KBr as
bromine source, organic substrates have been selectively
converted to mono bromo substituted compounds. The
polymer-anchored Cu(II) catalyst could be easily recovered
by simple filtration and reused more than six times without
appreciable loss of its initial activity.
the incorporation of metal-based catalysts onto or into inert
supports by different methods, such as alumina [4],
amorphous silicates [5], polymers [6], zeolites [7] and
MCM-41 [8]. Application of polymer supported catalysts
in oxidation reactions has been received attention in recent
years due to their potential advantages over the homoge-
neous one [9–13]. The activity of polymer supported Schiff
base complexes of transition metal ions varies with the type
of Schiff base ligands, coordination sites and metal ions
used in their formation.
Selective oxidation of organic sulfides to sulfoxides is
one of the key reactions in the domain of organic oxidation
chemistry especially because of the potential use of sulf-
oxides as synthetic intermediates for construction of many
chemically and biologically active molecules [14, 15]. The
main synthetic route for the preparation of these valuable
materials is via oxidation of the corresponding sulfides.
There are several reagents available for this key transfor-
mation which is typically achieved using stoichiometric or
catalytic amounts of both organic and inorganic reagents
[16–18]. The use of ‘green oxidants’ such as molecular
oxygen and hydrogen peroxide is attractive, since they
are readily available, inexpensive, and environmentally
benign, with formation of water as the only by-product
[19]. Oxidation of sulfides with H2O2 is slow; hence
extensive studies have been undertaken to develop new
catalysts for this reaction [20–24]. Disadvantages of this
sulfoxidation reaction include difficulties in stopping the
oxidation at the sulfoxide stage, utilization of environ-
mentally unfavorable reagents, solvents, and catalysts, the
preparation of complex catalysts, removal or recovery of
the expensive catalyst, metal residues in the products, and
the formation of large amounts of toxic waste. Hence there
is a need to develop new catalysts which can overcome
these drawbacks.
Keywords Oxybromination ꢀ Sulfide oxidation ꢀ
Polymer-anchored Cu(II) catalyst ꢀ Potassium bromide ꢀ
Hydrogen peroxide
1 Introduction
In the recent past, there has been an increasing interest in
developing environmental friendly greener processes
which are also economically viable. However, some
problems such as excessive use of oxidants, corrosion,
difficulty in recovery and separation of the catalyst from
reaction mixtures are associated with homogeneous cata-
lysts and make such systems environmentally unsuitable
[1]. In recent years, the design and synthesis of catalyti-
cally active supported metal complexes have received
considerable interest. Many effective and recyclable het-
erogeneous catalysts have been studied for the liquid phase
oxidation [2, 3]. Various approaches have been focused on
Sk. M. Islam (&) ꢀ A. S. Roy ꢀ P. Mondal ꢀ N. Salam ꢀ S. Paul
Department of Chemistry, University of Kalyani, Kalyani, Nadia
741235, WB, India
e-mail: manir65@rediffmail.com
123

226
M. Islam et al.
Bromoaromatics are important synthetic intermediates
CH3
Cl
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
[25]. The reported methods involve an electrophilic bro-
mination of aromatic rings using brominating agents other
than molecular bromine, such as N-bromosuccinimide [26],
a hexamethylenetetraminebromine complex [27] and an
alkyl bromide/sodium hydride/DMSO combination [28];
whereas the others are based on oxidative bromination
[29–32]. In oxidative halogenation, halide ions can be used
as a halogen source together with a suitable oxidant; thus,
this method represents more ecologically benign and eco-
nomically attractive pathway to haloaromatics than the
classical electrophilic substitutions. Main advantages of
oxyhalogenation, which makes it safer and greener, are the
use of low-cost and easy-to-handle metal halides as halo-
genating agents and atom economy, that is, a full utiliza-
tion of halogen atoms instead of using only half of the
available amount. Only a few examples of the use of
bromide or chloride salts as halogen source have appeared,
all reporting low substrate conversions and/or low selec-
tivities for individual products.
N
N
Cu Cl
HN
S
H2
C
P
NH
Fig. 1 Polymer-anchored Cu(II) complex
and methanol (100 mL) was heated under reflux for 2 h
after cooling the precipitate was filtered off, washed with
methanol, crystallized from methanol to give white
crystals.
2.3 Synthesis of Polymer-Anchored Cu(II) Catalyst
It was readily prepared in two steps [35]. The chlorome-
thylated polystyrene (2 g) was added to solution of
4-acetylpyridine thiosemicarbazone (1.07 g in 20 mL) in
DMF and then refluxed for 24 h. After cooling to room
temperature, the light yellow coloured polymer beads were
filtered, washed thoroughly with methanol and dried in
vacuum. The polymer supported Schiff base (1 g) was kept
in contact with methanol (10 mL) in a round bottom flask.
To this, methanolic solution (10 mL) of CuCl2 (1 % w/v)
was added over a period of 45 min and refluxed for 6 h.
Then green coloured copper loaded beads were filtered
carefully, washed with methanol and dried in vacuum.
In this paper, we report the synthesis and characteriza-
tion of a polymer-anchored Cu(II) catalyst. The catalytic
activity of the complex was tested in oxidation of sulfides
and oxidative bromination reaction of aromatic com-
pounds. 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 six times without any significant loss in its activities.
2.4 General Experimental Procedure for Oxidation
Reaction of Sulfides
2 Experimental
A typical experimental procedure for the oxidation reaction
of sulfides using polymer-anchored Cu(II) catalyst is
described as follows: A mixture of sulfide (5.0 mmol),
catalyst (5.55 9 10-3 mmol), and 30 % hydrogen perox-
ide (10 mmol) in acetonitrile (10 mL) is stirred at room
temperature (Scheme 1). At the end of specified time, the
contents were analyzed by Varian 3400 gas chromatograph
equipped with a 30 m CP-SIL8CB capillary column and a
Flame Ionization Detector. Peak position of various reac-
tion products were compared and matched with the reten-
tion times of authentic samples. Identity of the products
was also confirmed by using an Agilent GC–MS.
2.1 Synthesis of Polymer-Anchored Cu(II) Complex
Polymer-anchored Cu(II) catalyst (Fig. 1) was synthesized
according to our reported method [33]. Catalyst was readily
prepared in three steps. At first, Schiff base, 4-Acetylpyr-
idine Thiosemicarbazone was prepared then it was reacted
with chloromethyl polystyrene to prepare polymer-
anchored Schiff base ligand. Finally, polymer-anchored
Schiff base ligand reacted with copper chloride salt in
ethanol to produce polymer-anchored copper catalyst.
2.2 Synthesis of 4-Acetylpyridine Thiosemicarbazone
2.5 General Procedure for the Oxidative Bromination
Reaction
The Schiff base ligand 4-Acetylpyridine thiosemicarbazone
was prepared by mixing 4-acetylpyridine and thiosemi-
carbazide in a stoichiometric ratio [34]. A mixture of
4-acetylpyridine (0.10 mol), thiosemicarbazide (0.10 mol)
A 50 mL two-necked round bottomed flask was charged
with 5.55 9 10-3 mmol of catalyst, the substrate (2 mmol)
123

Efficient Catalyst for Oxidation of Sulfides and Oxidative Bromination Reaction
227
Scheme 1 Oxidation of
sulfides
O
S
Polymer-anchored
Cu(II) catalyst
30% H2O2, room temp.
acetonitrile
O
O
S
S
R1
R2
R1
R2
R1
R2
Sulfide
Sulfoxide
Sulfone
R
1 = R2 = aryl or alkyl group
and KBr (2.2 mmol) in acetic acid (5 mL). 30 % H2O2
(5 mmol) was then, added drop wise to the reaction mix-
ture and the contents allowed to stir at room temperature
(Scheme 2). The reaction mixture was monitored by thin
layer chromatography (TLC). After the specified time of
the reaction, the catalyst was filtered and solid was washed
with ether. The combined filtrates were washed with sat-
urated sodium bicarbonate solution. The organic extract
was dried over anhydrous sodium sulfate and solvent
evaporated under reduced pressure. The products were
analyzed by Varian 3400 gas chromatograph equipped with
a 30 m CP-SIL8CB capillary column and a Flame Ioni-
zation Detector. The products were purified by column
groups. The lowering in frequency of the above peaks indi-
cates the coordination of metal centre with the ligand This
suggests that the ligand acts as bidentate chelating agent
coordinated through ‘N’ (azomethine) and ‘S’ atom [37, 38].
The pyridine ring vibrations are quite similar to those of the
phenyl ring which was assigned at 1600, 1570, 1460 and
770 cm-1 [39, 40]; these bands are existed in polymer-
anchored copper complex; this mean that the pyridine rings
not participated in the complexation. The diffuse reflectance
(DR)-UV–vis spectra of polymer-anchored ligand shows
two absorption bands at the range 225–265 nm which
assigned to p?p* transition [41] and the second appeared at
315 nm due to intraligand n?p* transition [42, 43], a band
centered at 420 nm may be assigned to charge transfer
transition. These absorptions also present in the spectra of the
copper complex but they are shifted. This shift in the spectra
of the complex attributed the complexation behavior of the
ligand and supports the coordination of the ligand to metallic
ion. The spectrum of the polymer-anchored complex
exhibits band around at 620–780 nm (sh.) that is assigned to
d$d transition which corresponds to the square-planar
geometry [44, 45]. The scanning electron micrographs of
polymer-anchored Schiff base ligand and the supported
catalyst clearly show the morphological change which
occurred on the surface of polymer-anchored ligand after
loading of the metal. Also presence of copper metal along
with sulfur and chlorine can be further proved by energy
dispersive spectroscopy analysis of X-rays (EDAX) which
suggests the formation of metal complex with the polymer-
anchored ligand. Thermal stability of the complex was
investigated using TGA. Polymer-anchored Schiff base
ligand decomposed in the temperature range 370–380 °C
and the heterogeneous complex decomposed at 390–400 °C,
so after complexation thermal stability of the polymer-
anchored copper complex improved by 20–30 °C.
1
chromatography and confirmed by H NMR.
3 Results and Discussion
3.1 Characterization of Polymer-Anchored Cu(II)
Catalyst
Due to insolubilities of the polymer-anchored ligand and
copper complex in all common organic solvents, their
characterization were limited to their physicochemical
properties, elemental analysis, SEM, TGA, FT-IR, diffuse
reflectance UV–Vis and atomic absorption 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 2.34 % (0.37 mmol g-1
)
of Cu in the complex. In polymer supported Schiff base
ligand, the sharp C–Cl peak (due to –CH2Cl groups) at
1,264 cm-1 in the starting polymer was absent [36]. The
polymer-anchored ligand shows broad band around
3,320–3,450 cm-1 for –NH (secondary amine) stretching
vibrations. The polymer-anchored ligand also exhibited
peaks at 1,638 and 1,152 cm-1 due to m(C=N) and m(C=S)
3.2 Oxidation of Sulfides
The efficiency of the polymer-anchored Cu(II) catalyst in
the oxidation of organic sulfides using H2O2 as oxidant at
room temperature has been explored.
Br
Polymer-anchored
Cu(II) catalyst
In order to optimize the reaction conditions, several
reactions were performed using methyl phenyl sulfide
(MPS) as model substrate in the presence of different
solvents as shown in Table 1. The nature of the solvent was
KBr/H2O2/AcOH
R
room temp.
R
Scheme 2 Oxidative bromination reaction
123

228
M. Islam et al.
Table 1 Effect of different solvents on oxidation of methyl phenyl sulfide with polymer-anchored Cu(II) catalyst
O
Polymer-anchored
Cu(II) catalyst
O
O
S
S
S
Ph
Me
Ph
Me
Solvent, 30% H2O2
Ph
Me
1
1a
1b
Entry
Solvent
Time (min)
Conversion (%)a
Selectivity (%)a,b
1a
TOF (h-1
)
1b
1
2
3
4
5
6
7
MeOH
240
210
120
120
90
67
73
9
78
64
89
81
96
56
79
22
36
11
19
4
151
188
41
EtOH
Chloroform
Dichloromethane
CH3CN
17
84
16
72
77
505
29
H2O
300
120
44
21
H2O:CH3CN
324
Conditions: MPS (5 mmol); 30 % aq H2O2 (10 mmol); solvent (10 mL); 5.55 9 10-3 mmol of catalyst at room temperature
a
Conversion and selectivity were determined by GC
b
Products were characterized by GC–MS
Table 2 Optimization of reaction conditions for oxidation of methyl phenyl sulfide with polymer-anchored Cu(II) catalyst
O
Polymer-anchored
Cu(II) catalyst
O
O
S
S
S
Ph
Me
Ph
Me
CH3CN, 30% H2O2
Ph
Me
1
1a
1b
Entry
Molar ratio Cu:MPS
H2O2 (mmol)
Time (min)
Conversion (%)a
Selectivity (%)a,b
TOF (h-1
)
1a
1b
1
2
3
4
5
6
7
1:900
1:900
1:900
1:900
1:600
1:300
1:900
5
240
90
89
56
84
98
78
81
72
69
99
96
81
93
89
95
31
1
200
336
505
353
467
243
649
5
10
10
10
10
10
90
4
150
60
19
7
60
11
5
60
Conditions: MPS (5 mmol); acetonitrile (10 mL); at room temperature
a
Conversion and selectivity were determined by GC
b
Products were characterized by GC–MS
found to play a crucial role in determining the activity and
selectivity of the catalyst in the oxidation reactions. The
data in Table 1 clearly shows that both methanol and eth-
anol fully miscible with sulfides and H2O2 are highly
effective as a solvent. Reactions were also conducted in
solvents such as chloroform or dichloromethane which
yielded low conversions of MPS within the range of
9–17 % after 2 h of reaction time. It is interesting to note
that when the reaction was conducted in acetonitrile under
analogous conditions, maximum sulfoxide was observed
along with sulfone (Table 1, entry 5). No appreciable
conversion of was noted in aqueous medium in the present
case probably due to the insolubility of the catalyst as well
as the substrate in water but when a mixed solvent
(H2O:CH3CN) 1:1 (v/v) is used for oxidation the yield is
72 %. As far as the oxidation of MPS is concerned CH3CN
is the best solvent. In a preliminary experiment MPS was
allowed to react with H2O2 (5 mmol) maintain the catalyst:
substrate molar ratio at 1:900 in CH3CN. Under this con-
dition MPS has been oxidized to sulfoxide and sulfone in
the ratio 69:31 (Table 2, entry 1). The significant finding of
the above reaction was the selective formation of sulfoxide
within the initial period of 90 min of starting of the reac-
tion. The initial fast reaction was observed to slow down
123

Efficient Catalyst for Oxidation of Sulfides and Oxidative Bromination Reaction
229
with time and selectivity of sulfoxide decreases. So
selective sulfoxidation of MPS can be achieved by termi-
nating the reaction at specific time in order to prevent over
oxidation of sulfoxide to sulfone. When the reaction was
terminated at 90 min almost only sulfoxide was obtained
(Table 2, entry 2). The conversion of sulfide could be
improved to 84 % by increasing the amount of H2O2 to
10 mmol (Table 2, entry 3). So 1: 2 is the optimal molar
ratio of substrate: oxidant. In this conditions if we increase
the reaction time to 150 min, almost complete conversion
of sulfide was obtained but the selectivity of sulfoxide
decreases. So higher selectivity of sulfoxide can be
obtained at lower conversion.
100
80
60
40
20
0
100
80
60
40
20
0
Selectivity of 1b
Selectivity of 1a
Conversion of MPS
0
30
60
90
120
150
180
Time (min)
Now the effect of catalyst amount was evaluated
(Table 2, entries 3, 5–6). If we increase the amount of
catalyst the rate of the reactions were also increases, but the
corresponding TOF was decrease. So catalyst: substrate
Fig. 2 Conversion/selectivity of oxidation of MPS as a function of
time. Conditions: MPS (5 mmol); 30 % H2O2 (10 mmol); CH3CN
(10 mL); 5.55 9 10-3 mmol of catalyst at room temperature
Table 3 Oxidation of sulfides using 30 % H2O2 catalyzed by polymer-anchored Cu(II) catalyst
Entry
1
Sulfide
Conversion (%)a/time (min)
84 (240)
Selec. Sulfoxide
TOF (h-1)
189
(%)a,b
79
S
2
88 (240)
91 (90)
81
94
198
547
S
3d
S
4
5
6
7
S
S
84 (90)
87 (90)
88 (90)
82 (240)
96
96
93
89
505
523
529
185
CH3
C2H5
S
C6H13
S
8
S
73 (240)
97
164
S
9
79 (240)
68 (240)
96
93
178
153
S
10
OH
Conditions: Sulfide (5 mmol); 30 % H2O2 (10 mmol); CH3CN (10 mL); 5.55 9 10-3 mmol of catalyst at room temperature
a
Conversion and selectivity were determined by GC
b
Products were characterized by GC–MS
123

230
M. Islam et al.
Table 4 Oxidative bromination of various organic substrates catalyzed by polymer-anchored Cu(II) catalyst
Entry
Substrate
Conversion (%)a (time/h)
Product selectivitya,b
1
Salicylaldehyde
Phenol
94 (2.5)
97 (2.0)
94 (2.0)
91 (2.0)
79 (5.0)
77 (3.0)
95 (2.5)
92 (2.5)
96 (2.5)
86 (3.0)
92 (3.0)
87 (3.0)
94 (3.0)
12 (5.0)
–
5-Bromo-2-hydroxy benzaldehyde (100)
4-Bromophenol (100)
2
3
Resorcinol
4-Bromo-1,3-dihydroxybenzene (100)
2-Bromo-4-methylphenol (100)
2-Bromo-4-nitrophenol (100)
2-Bromo-4-aminophenol (100)
4-Bromoanisole (100)
4
4-Methylphenol
4-Nitrophenol
4-Aminophenol
Anisole
5
6
7
8
4-Methylanisole
Aniline
2-Bromo-4-Methylanisole (100)
4-Bromoaniline (85)
9
10
11
12
13
14
15
4-Nitroaniline
4-Methylaniline
4-Chloroaniline
N,N-Dimethylaniline
Benzene
2-Bromo-4-nitroaniline (100)
2-Bromo-4-methylaniline (87)
2-Bromo-4-chloroaniline (100)
4-Bromo-N,N-dimethylaniline (100)
Bromobenzene (100)
Nitrobenzene
–
Conditions: substrate (2 mmol); KBr (2.2 mmol); glacial acetic acid (5 mL); 30 % aq H2O2 (5 mmol), 5.55 9 10-3 mmol of catalyst at room
temperature
a
Conversion and selectivity were determined by GC
b
Products were characterized by GC–MS and NMR
Table 5 Comparison with other reported catalysts
Reaction
Catalyst
Reaction conditions
Conversion/yield (%)
References
This work
Oxidation of MPS
Polymer-anchored Cu(II) 30 % H2O2, CH3CN, room temperature,
catalyst
84 (96 % sulfoxide
selectivity)
90 min
Ps-[Cu(hpbmz)2]
30 % H2O2, CH3CN, room temperature, 2 h. 44.8 (73.4 % sulfoxide
selectivity)
[48]
Preyssler/nano-sized
TiO2)
i-PrOH, 30 % aqueous H2O2, at room
temperature, 60 min
96 (sulfoxide)
[49]
Ru (PVP)/c-Al2O3
30 %H2O2, CH3CN, at room temperature,
120 min
100/98 (sulfoxide)
97 (100 % p-selective)
73 (42 % o-selective)
[50]
Oxidative bromination of Polymer-anchored Cu(II) KBr, H2O2, acetic acid, room temperature,
phenol
This work
[51]
catalyst
2 h
CuZSM-5(30) (5 wt%
Cu)
KBr, H2O2, acetic acid, room temperature,
5 h
15 wt% ZPTA
20 wt% ZPTA
KBr, H2O2, acetic acid, room temperature,
5 h.
93.34 (81.0 % p-selective) [52]
83.75 (79.21 %
p-selective)
ratio 1:900 was optimal and we use it for subsequent
reactions. It is noted here that increasing the reaction
temperature up to 60 °C, a increase in TOF could be
observed without change of selectivity (Table 2, entry 7).
Under optimized reaction conditions in the absence of
catalyst very little conversion of MPS to sulfoxide was
observed. So, it is evident that the catalyst plays a crucial
role in the oxidation reaction of sulfides. A conversion/
selectivity as a function of time was investigated for the
oxidation of MPS. Increased conversion were noted with
increasing reaction time however, the selectivity was
found to be reduced due to the over oxidation to sulfone
(Fig. 2).
Under optimized conditions for sulfoxidation, a series of
various types of sulfides were subjected to the oxidation of
reaction. Aliphatic and aromatic sulfides underwent selec-
tive oxidation to the corresponding sulfoxide under opti-
mized reaction conditions. The results are summarized in
Table 3. The rate of the oxidation of sulfides was observed
to vary depending on the nature of the substrates. Allylic
and vinylic sulfides (Table 3, entries 8–9) were found to be
less oxidized by H2O2 than dialkyl sulfide. The catalyst
123

Efficient Catalyst for Oxidation of Sulfides and Oxidative Bromination Reaction
231
The effect of bromine source was also studied for the
oxidative bromination reaction of Phenol. The catalyst was
employed using H2O2 as an oxidant and MBr (M = Na and
K) as a bromine source (Fig. 3). Among the 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 products. Influence
of temperature was also studied but there was no effect on
the conversion of phenol. With increase of temperature the
conversion of phenol was almost 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. Substrates like phenol, resor-
cinol, anisole and N, N-dimethylaniline showed excellent
para-selectivity. The activated aromatic substrates, like
phenol, resorcinol, aniline, anisole and N, N-dimethylani-
line, have shown high conversion into their respective
monoselective products. The inactive substrates, like ben-
zene, have shown low conversion and required longer
reaction time. On the other hand, deactivated aromatic ring
did not show any conversion at this reaction condition. 4-
substituted aromatics selectively converted to 2-bromo
derivatives and vice versa.
100
80
60
40
20
0
Conversion
Selectivity of p-bromophenol
NaBr
KBr
Bromine source
Fig. 3 Effect of bromine source for the oxidative bromination of
phenol. Conditions: phenol (2 mmol); MBr (2.2 mmol); glacial acetic
acid (5 mL); 30 % H2O2 (5 mmol), 5.55 9 10-3 mmol of catalyst at
room temperature
3.4 Comparison with Homogeneous and Other
Reported Systems
Fig. 4 Recycling efficiency for the oxidation of MPS and oxidative
bromination of phenol with polymer-anchored Cu(II) complex
In order to find out the effect of immobilization of homo-
geneous complex on polymer, the catalytic activity of the
present polymer-anchored Cu(II) complex was compared
with its homogeneous analogue in oxidation and oxidative
bromination reaction (Table 5). From the results it is seen
that the present polymer-anchored copper catalyst is more
active than the corresponding homogeneous catalyst.
Homogeneous catalyst oxidized MPS with 66 % conver-
sion while polymer-anchored copper catalyst performed
well with 84 % conversion with H2O2 at room temperature
at 90 min. In oxidative bromination of phenol, homoge-
neous complex shows 76 % conversion whereas hetero-
geneous catalyst 97 % conversion by GC analysis. Also,
the homogeneous catalyst cannot be recovered or reused. In
the absence of the catalyst, H2O2 alone shows slight
activity in oxidation reaction of sulfides but unable to
oxyhalogenate the substrates to any significant extent. The
catalytic activity of the polymer-anchored catalyst was also
superior with other reported heterogeneous catalysts [48–
52] for the oxidation reaction of sulfides and oxidative
bromination reaction of organic substrates with H2O2.
exhibited excellent chemoselectivity towards sulfur group
of substituted sulfides containing other oxidation prone
functional group Such as –C=C– or –OH (Table 3, entries
8–10). Allylic and vinylic sulfoxides were obtained with
trace amount of epoxide. Conversion and selectivity of
oxidation products of other type of sulfides as a function of
time were given in supporting information. Mechanistic
pathway for the formation of sulfoxide can be explained by
the similar type mechanism [46, 47]. Here, peroxide
replaced chloride from the metal complex and formed
peroxy linked copper complex. This peroxy linked copper
complex reacts with sulfide and formed an active inter-
mediate. Then peroxide attacked the active metal site and
released sulfoxide.
3.3 Oxidative Bromination
We have investigated the use of polymer-anchored copper
complex as catalyst in the reaction of phenol with KBr and
H2O2 at room temperature in acetic acid (Table 4).
123

232
M. Islam et al.
Acknowledgments We thank the Department of Material Science,
Indian Association of Cultivation of Science, Kolkata, for providing
the instrumental support. MI acknowledges DST, CSIR and UGC,
New Delhi, India for funding. ASR acknowledges CSIR, New Delhi,
for providing his senior research fellowship. We acknowledge DST,
Govt of India, for funding the University of Kalyani under purse
programme.
3.5 Catalyst Reusability
Another important issue concerning the application of a
heterogeneous catalyst is its reusability and stability under
reaction conditions. To gain insight into this issue, catalyst
recycling experiments were carried out using oxidation of
MPS and oxidative bromination of phenol. 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 cata-
lytic 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. The results are shown in Fig. 4. It is found that the
catalytic activity or selectivity does not change signifi-
cantly after six consecutive runs.
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