Tetrahedron Letters
Copper–Schiff base complex catalyzed oxidation of sulfides
with hydrogen peroxide
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Prasanta Gogoi, Mukul Kalita, Tirtha Bhattacharjee, Pranjit Barman
Department of Chemistry, National Institute of Technology, Silchar 788010, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A straightforward, efficient, and selective oxidation of sulfide to sulfoxide with 30% H2O2 catalyzed by
copper(II)–Schiff base complex is described. The reactions proceed under mild conditions in acetonitrile
at room temperature to provide a variety of aryl and alkyl sulfoxides in excellent yield.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 25 October 2013
Revised 17 December 2013
Accepted 19 December 2013
Available online 30 December 2013
Keywords:
Copper catalyst
Sulfide oxidation
H2O2
Sulfoxides are a common functionality found in numerous
pharmaceutically active compounds.1 They also have widespread
utilization as convenient intermediates or reagents in organic syn-
theses.2 Indeed, a number of drugs in therapeutic areas such as
antiulcerative, cardiotonic, antihypertensive as well as psychoton-
ics and vasodilators contain sulfoxide functionality.3 Traditional
method for the synthesis of sulfoxides which gives good yields
could be obtained from different catalytic systems based on gold,4
titanium,5 iron,6 vanadium,7 palladium,8 manganese,9 copper,10
cobalt,11 magnesium,12 molybdenum,13 zinc,14 zirconium,15 and
ruthenium.16 Recent developments using non-metal catalysts and
even, a neat condition without any catalyst can give a good yield
and selectivity for common sulfides.17 Therefore, it is of current
interest to develop economically and environmentally more sus-
tainable procedures for the preparation of sulfoxides.20
On the contrary, the Schiff base metal catalyzed preparation of
sulfoxide from sulfide has been widely developed.18 In this Letter,
we wish to describe the protocol in which H2O2, an ideal terminal
oxidant, has been used in the presence of Schiff base–copper com-
plex 1 for the oxidation of sulfides to their sulfoxides; a process
delivering a high yield and desirable reaction time.
To check the feasibility of our envisioned route to oxidation of
sulfides, a series of experiments were carried out. Initially, 2-nitro-
phenylbenzyl sulfide (2b) was treated with 30% H2O2 in the pres-
ence of copper–complex 1 (0.5 mol % based on the sulfide) in
CH3CN (Table 1). After stirring the reaction mixture at room tem-
perature for 6 h, the 2-nitrophenylbenzylsulfide (2b) was com-
pletely converted to the corresponding sulfoxide and sulfone in
Table 1
Oxidation of 2-nitrophenylbenzyl sulfide (2b) using 30% aqueous H2O2 catalyzed
by 1a
Entry
Catalyst (mol %)
Solvent
Time
(h)
Sulfoxideb
(%)
Sulfoneb
(%)
1
2
3
4
5
6
7
8
1 (0.5)
1 (0.05)
1 (1.0)
1 (0.5)
1 (0.5)
1 (0.5)
CuCl2 (0.5)
Cu(C5H7O2)2 (1.0)
[CoL]H2O (0.5)
[NiL]H2O (0.5)
CH3CN
CH3CN
CH3CN
CH3COCH3
CH2Cl2
CH3OH
CH3CN
CH3CN
CH3CN
CH3CN
6
10
6
6
8
8
8
7
8
95
68
90
70
90
84
70
75
78
57
5
10
8
10
8
7
12
10
18
15
9c
10c
8
a
Reaction condition: substrate (2 mmol), complex 1 (0.5 mol %), and 30% H2O2
(5 mmol) were stirred at room temperature in solvent (2 mL).
b
Isolated yield.
c
Where ligand L, N-[(2-benzylthio)-phenyl] salicylaldimine was prepared as
reported in our earlier work.19
good chemoselectivity (95:5) (Table 1, entry 1). On decreasing
the catalyst loading under this condition, the reaction time was
prolonged for the satisfied conversion but the ratio of the sulfoxide
and sulfone was decreased (Table 1, entry 2). However, use of more
than 0.5 mol % catalyst 1 did not improve the yield (Table 1, entry
3). Using 0.5 mol % copper–catalyst 1, the chemoselectivity of 95:5
could be achieved in 6 h (Table 1, entry 1). In comparison to
0.5 mol % catalyst loading possessed the higher catalytic efficiency
due to the short reaction time. Among the other tested catalysts
(entries 7–10), catalyst 1 was found to be the most effective and
the product 3b (Fig. 1, Scheme 1) was obtained in 95% yield. With
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0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.