Oxidation of sulfides with urea-hydrogen peroxide catalyzed by iron-salen complexes

Article abstract of DOI:10.1080/17415993.2011.593634

The oxidation of sulfides to sulfoxides with hydrogen peroxide in good yield and high selectivity has been catalyzed very effectively by a series of Schiff base salen-type iron complexes in a monophasic medium (CH ₂Cl₂/CH₃OH) at ambient temperature. The results of the present study show that the reaction rate strongly depends on both catalyst steric effects and steric and electronic properties of sulfides. In the system, the electronic spectral studies are applied to explain the formation of a reactive (salen) Fe=O intermediate.

Full text of DOI:10.1080/17415993.2011.593634

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Oxidation of sulfides with
urea–hydrogen peroxide catalyzed by
iron–salen complexes
Mojtaba Bagherzadeh a & Maryam Zare a
a Chemistry Department , Sharif University of Technology ,
Tehran, PO Box 11155-3615, Iran
Published online: 22 Jun 2011.
To cite this article: Mojtaba Bagherzadeh & Maryam Zare (2011) Oxidation of sulfides with
urea–hydrogen peroxide catalyzed by iron–salen complexes, Journal of Sulfur Chemistry, 32:4,
335-343, DOI: 10.1080/17415993.2011.593634
To link to this article: http://dx.doi.org/10.1080/17415993.2011.593634
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Journal of Sulfur Chemistry
Vol. 32, No. 4, August 2011, 335–343
Oxidation of sulfides with urea–hydrogen peroxide catalyzed by
iron–salen complexes
Mojtaba Bagherzadeh* and Maryam Zare
Chemistry Department, Sharif University of Technology, Tehran, PO Box 11155-3615, Iran
(Received 13 March 2011; final version received 31 May 2011)
The oxidation of sulfides to sulfoxides with hydrogen peroxide in good yield and high selectivity has been
catalyzed very effectively by a series of Schiff base salen-type iron complexes in a monophasic medium
(CH2Cl2/CH3OH) at ambient temperature. The results of the present study show that the reaction rate
strongly depends on both catalyst steric effects and steric and electronic properties of sulfides. In the
=
system, the electronic spectral studies are applied to explain the formation of a reactive (salen) Fe
intermediate.
O
O
Fe (sal)Cl / UHP
R
R
S
S
R'
R'
ImH, CH2Cl2/CH3OH, r.t.
Six examples
Conversion: 27–97%
Selectivity: 96–100%
Fe(sal)cl: six iron–salen catalysts
Keywords: catalytic oxidation; iron-Schiff base; sulfide; urea–hydrogen peroxide; nitrogen donor ligands
1. Introduction
The coordination chemistry of transition metal complexes with N2O2 Schiff base ligands has
already been studied and some of these complexes are known to catalyze important biochem-
ical reactions, e.g. in catalytic oxidation reactions such as hydrocarbons (1, 2) and oxidation
of sulfides to sulfoxides (3), aziridination of alkenes (4), Diels–Alder reaction (5), hydrolytic
kinetic resolution of epoxides (6), alkylation of aldehydes (7), and electrochemical reduction
processes (8). In the area of bioinorganic chemistry, Schiff base complexes are interesting syn-
thetic models for the investigation of the similar metal-containing sites in metallo-proteins and
metallo-enzymes (9–13).
Furthermore, many metalloporphyrin and metallosalen complexes of vanadium, molybdenum,
titanium, iron, chromium, and manganese have been used as catalysts in the oxidation of sulfides
(14–17). The oxidation of sulfides is the major method for the synthesis of sulfoxides (18, 19).
*Corresponding author. Email: bagherzadehm@sharif.edu
ISSN 1741-5993 print/ISSN 1741-6000 online
© 2011 Taylor & Francis
DOI: 10.1080/17415993.2011.593634
http://www.informaworld.com

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336 M. Bagherzadeh and M. Zare
These products have been used, for example, as intermediates for the production of a wide range of
chemically and biologically active molecules (20, 21). More recently, they have been introduced
as ligands in enantioselective metal catalysis (22).
The mechanisms of sulfoxidation reaction catalyzed by cytochrome P450 and horseradish
peroxidase models are of importance in chemical and biological processes. The formation of a
high-valent oxo-iron(IV) porphyrin radical intermediate generally has been accepted to be the
principal oxidizing species in heme-catalyzed reaction (23–25). Recently several mechanistic
studies on iron–salen complexes as model compounds for nonheme enzymes by using them as
catalysts for the oxygenation of organic substrate have been reported in the literature (26–28).
The findings show that the nature of the active catalytic intermediate strongly depends on the
oxygen donor, solvent, and the presence of co-catalyst.
In the present work, steric and electronic properties of sulfides and also catalyst steric effects on
the oxidation of various sulfides to sulfoxides have been investigated by different iron–salen com-
plexes, Fe(sal)Cl, with urea–hydrogen peroxide (UHP) as a cheap and readily available oxidizing
reagent in the mixture of CH2Cl2/CH3OH as the solvent system (Scheme 1). These complexes
have been found to be efficient catalysts for the oxidation of sulfides to sulfoxides with high
selectivity. The electronic spectra of the catalytic system were applied to examine the nature of
active species, as well.
Z
HC
N
O
N
O
CH
I – X : H , Z : C2H4 ,
II – X : H , Z : o-C6H4
III – X : H , Z : c-C6H10 Y : Cl
Y : Cl
Y : Cl
IV – X : t-Butyl , Z : C2H4
V – X : t-Butyl , Z : o-C6H4 Y : Cl
VI – X : t-Butyl , Z : c-C6H10 Y : Cl
Y : Cl
Fe
Y
X
X
X
X
O
O
S
Fe (sal)Cl/UHP
+
R
S
R¢
S
R¢
R
R
R¢
O
Scheme 1. Oxidation of sulfide with UHP catalyzed by Fe(sal) X complexes.
2. Results and discussion
2.1. The effect of solvent on the oxidation of methyl phenyl sulfide with
UHP-catalyzed Fe(salen)Cl
To find the optimized conditions in UHP oxidation of sulfides catalyzed by Fe(salen)Cl [where
salen = N, N-bis(salicylaldehyde)ethylenediimine], methyl phenyl sulfide is used as the model
substrate. The molar ratio 1:20:40 of the catalyst:methyl phenyl sulfide:UHP is found as the
optimum molar ratio for the catalytic system. To obtain the appropriate solvent in UHP oxidation
of sulfides, catalyzed by the iron–salen complexes, the oxidation of methyl phenyl sulfide in
the presence of Fe(salen)Cl is carried out in various solvents. Among the solvents, a mixture
of CH3OH/CH2Cl2 is chosen as the best solvent system for the solubility of both catalyst and
UHP. As UHP is not completely soluble in less polar organic solvents such as dichloromethane,
chloroform, acetonitrile, and acetone, the oxidation yield is low in these solvents (Entries 1–4,
Table 1). Although catalysts, methyl phenyl sulfide, and UHP are completely soluble in DMF,
oxidation yield is low in this solvent (Entry 5, Table 1).

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Journal of Sulfur Chemistry 337
Table 1. The oxidation of sulfide with Fe(salen)Cl/UHP in various solvents.a
Entry
Solvent
CH2Cl2
CHCl3
CH3CN
CH3COCH3
DMF
CH2Cl2:CH3OH
CH3OH
Conversion (%)
Sulfoxide (%)
Sulfone (%)b
1
2
3
4
5
6
7
3
12
32
51
52
85
83
3
12
26
49
50
82
80
–
–
6
2
2
3
3
Notes: The reactions were run for 60 min at room temperature.
aConditions: The molar ratio for catalyst:methyl phenyl sulfide:UHP is 1:20:40; bGC yields based on the
starting methyl phenyl sulfide.
Figure 1. (a) Solid line, absorption spectrum of Fe(salen)Cl; dotted line, absorption spectrum of the mixture
of Fe(salen)Cl and UHP in CH2Cl2/CH3OH at 193 K, and (b) solid line, absorption spectrum of Fe(salen)Cl in
CH2Cl2/CH3OH; dotted line, absorption spectrum of the mixture Fe(salen)Cl and UHP in CH2Cl2/CH3OH; dashed
line, absorption spectrum after the addition of diethyl sulfide at room temperature.