Alumina-supported monoperphosphoric acid for selective oxidation of sulfides to sulfoxides

Article abstract of DOI:10.1080/17415993.2010.544041

Selective oxidation of aromatic and aliphatic sulfides to the corresponding sulfoxide was achieved by using POCl₃/H₂O₂ and (alumina-supported phosphorus oxychloride)/H₂O₂. A versatile procedure for th

Full text of DOI:10.1080/17415993.2010.544041

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Alumina-supported monoperphosphoric
acid for selective oxidation of sulfides
to sulfoxides
Moslem M. Lakouraj a & Vahid Hasantabar a
a Department of Organic Chemistry, Faculty of Chemistry ,
University of Mazandaran , Babolsar , 47416 , Iran
Published online: 28 Jan 2011.
To cite this article: Moslem M. Lakouraj & Vahid Hasantabar (2011) Alumina-supported
monoperphosphoric acid for selective oxidation of sulfides to sulfoxides, Journal of Sulfur
Chemistry, 32:1, 93-98, DOI: 10.1080/17415993.2010.544041
To link to this article: http://dx.doi.org/10.1080/17415993.2010.544041
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Journal of Sulfur Chemistry
Vol. 32, No. 1, February 2011, 93–98
Alumina-supported monoperphosphoric acid for selective
oxidation of sulfides to sulfoxides
Moslem M. Lakouraj* and Vahid Hasantabar
Department of Organic Chemistry, Faculty of Chemistry, University of Mazandaran, Babolsar 47416, Iran
(Received 23 June 2010; final version received 27 November 2010)
Selective oxidation of aromatic and aliphatic sulfides to the corresponding sulfoxide was achieved by
using POCl3/H2O2 and (alumina-supported phosphorus oxychloride)/H2O2. A versatile procedure for
the oxidation of sulfides to sulfoxides without any over-oxidation to sulfones has been reported. It is
noteworthy that the reaction is clean and tolerates oxidatively sensitive functional groups and the sulfur
atom is selectively oxidized.
O
S
I or II
S
I = H2O2/POCl3, Et2O, 0°C
II = H2O2/(alumina-POCl3), MeOH, r.t.
Keywords: sulfide; sulfoxide; hydrogen peroxide; phosphorus oxychloride; oxidation; peracid
1. Introduction
The convenient synthesis of sulfoxides is of high interest in organic chemistry because of the
importance of sulfoxides as synthetic intermediates for the construction of various chemically
and biologically active molecules (1).
To date, the most important methods of sulfoxide preparation involve the oxidation of sulphides
and various oxidizing agents have been introduced for this purpose (2–13). Unfortunately, many
of these reagents are not satisfactory for medium- to large-scale operations for several reasons.
The main reasons are the low content of the effective oxygen that is available for the oxida-
tion, generation of environmentally unfavorable by-products and high cost. Another reason is the
undesirable over-oxidation of sulfoxides to sulfones during the oxidation.
Indeed, selectiveoxidationofsulfidestosulfoxidesremainsachallengingtaskandconstitutesan
active research on its own, particularly with the emphasis on “green chemistry”. In this direction,
the use of aqueous hydrogen peroxide as an oxidizing agent is highly preferred in view of an
*Corresponding author. Email: lakouraj@umz.ac.ir
ISSN 1741-5993 print/ISSN 1741-6000 online
© 2011 Taylor & Francis
DOI: 10.1080/17415993.2010.544041
http://www.informaworld.com

94 M.M. Lakouraj and V. Hasantabar
effective oxygen content (47%) (14), simplified operation after reaction, cleanness that produces
only harmless water by reaction (15), safety in storage and operation and the low cost of production
transportation (16). However, aqueous hydrogen peroxide is a moderate oxidant and requires the
use of a catalyst (13, 17–19). These features have already stimulated the development of useful
procedures to promote the H2O2 oxidation.
Solid and polymer supports have been extensively studied as a reagent and a catalyst for many
reactions and have found industrial application in several processes (17, 18). More recently, solid
and polymer supports have attracted much attention because of their enhanced selectivity, milder
reaction conditions and associated ease of manipulation (19, 20). As the study continued on the
activation of hydrogen peroxide, we have now found that various sulfides are readily oxidized to
the corresponding sulfoxides in excellent yields under mild conditions by alumina-supported and
-unsupported monoperphosphoric acid which is generated in situ by the treatment of phosphorus
oxychloride and hydrogen peroxide.
2. Results and discussion
We disclosed that H2O2/POCl3 and H2O2/alumina-supported POCl3 systems can be used for the
gentle, efficient and selective oxidation of a wide range of sulfides at mild reaction conditions.
The route for the synthesis of sulfoxides is shown in Scheme 1.
O
S
I or II
S
I = H2O2/POCl3, Et2O, 0°C
II = H2O2/(alumina-POCl3), MeOH, r.t.
Scheme 1. Oxidation of sulfide to sulfoxide.
In order to determine what organic solvent would be most suitable, we studied the oxidation of
methyl phenyl sulfide in the presence of common organic solvents such as THF, acetone, Et2O,
1,4-dioxane, CH3CN and MeOH for both reaction conditions (I and II). The results showed that
Et2O and MeOH are generally the best solvents both in terms of yield and reaction period for
conditions I and II, respectively.
To explore the role of phosphorus oxychloride, we performed a set of preliminary experiments
on methyl phenyl sulfide as a model substrate using 30% aqueous hydrogen peroxide in the
presence of different amounts of phosphorus oxychloride in Et2O at 0 ◦C. The results are depicted
in Table 1. It is found that a 1:1.7:1.7 ratio of sulfide/H2O2/POCl3 is required to achieve a
quantitative yield of corresponding sulfoxides.
It is proposed that the treatment of H2O2 with POCl3 produces monoperphosphoric acid (21).
We presume that the electrophilicity of monoperphosphoric acid is higher than H2O2 which can
easily transfer active oxygen to the sulfide (Scheme 2). The by-product of the oxidation reaction
of sulfides with this oxidizing system is H3PO4 which is separated from the reaction mixture by
neutralization with Na2CO3.
The yield of oxidation reaction was low at higher temperatures due to the thermal decomposition
of the in situ-generated monoperphosphoric acid. To overcome this drawback, we decided to
immobilize the monoperphosphoric acid on alumina. Practically, on treatment of alkali alumina
with POCl3, a non-hygroscopic free-flowing powder was obtained, which could be easily used
even at higher temperatures for the activation of H2O2 (Scheme 3). Then, we attempted a set

Journal of Sulfur Chemistry 95
Table 1. Effect of POCl3 on the H2O2 oxidation of methyl phenyl sulfide.a
Entry
POCl3 (mmol)
H2O2 (mmol)
Yield (%)b
1
2
3
4
5
6
7
8
1
1
1
1
1.5
1.6
1.7
1.8
0.5
1
2
20
68
58
50
83
91
97
93
3
1.5
1.6
1.7
1.8
Notes: aReactions were performed with methyl phenyl sulfide (1 mmol), within 10 min at 0 ◦C.
bIsolated yields.
O
O
P
S
H2O2
–HCl
P
O
HO
O
H
Cl
Cl
–
–H2PO4
OH
Cl
O
S
OH
–
H2PO4
+
H3PO4
S
Scheme 2. Mechanistic pathway for the oxidation of sulfide using H2O2/POCl3.
of similar experiments on methyl phenyl sulfide using 30% aqueous hydrogen peroxide in the
presence of different amounts of alumina-supported POCl3 in MeOH at room temperature.
O
O
P
S
O
O
P
POCl3
r.t.
H2O2
r.t.
O
Al
O
Al
O
n
Al
O
Al
O
n
Al
Al
n
ONa
ONa
O
H
Cl
O
O
O
Al
Al
O
O
O
P
n
S
Al
O
Al
O
O
+
n
S+
O
+
O
P
H
OH
O
–
Scheme 3. Proposed mechanism for the oxidation of sulfide using H2O2/alumina-POCl3.
A ratio of 1:1:0.6 g of sulfide/H2O2/(alumina-supported POCl3) was found to be optimum for
complete conversion of sulfides to sulfoxides. The results are summarized in Table 2.

96 M.M. Lakouraj and V. Hasantabar
Table 2. Effect of alumina-POCl3 on the H2O2 oxidation of methyl phenyl sulfide.a
Entry
Alumina-POCl3 (g)
H2O2 (mmol)
Yield (%)a,b
1
2
3
4
5
6
0.6
0.6
0.6
0.3
0.4
0.5
1
2
3
1
1
1
99
93
90
65
87
96
Notes: aReactions were performed with methyl phenyl sulfide (1 mmol), at room temperature. bIsolated yields.
To extend the scope of the reaction and to generalize the procedure, we investigated the
sulfoxidation of a variety of organic sulfide including diallyl, dibenzyl, alkyl benzyl, dialkyl
and heterocyclic sulfides under the optimized reaction conditions (Table 3).
To evaluate the effect of alumina supported POCl3, we studied the oxidation of methyl phenyl
sulfide in the presence of H2O2 and alumina alone. The results showed that conversion is restricted
to 30–40% over 24 h. We have also studied the effect of H3PO4 on the possible activation of H2O2
in the oxidation of sulfide and the conversion was less than 5% within 24 h.
All the reactions occurred with complete selectivity for sulfoxide formation, no over-oxidation
products such as sulfones were detected in the reaction mixtures. The chemoselectivity of the
procedure was noteworthy. Under these conditions, various functional groups including olefin,
alcohol, ester, nitrile and aldehyde groups were tolerated (Table 3, entries 5, 6, 9, 10 and 12).
These substrates selectively underwent oxidation at the sulfur atom without undergoing further
structural changes in their functional groups.
Finally, the results of sulfoxidation were compared with those of previously reported oxidation
methods in Table 4.
On comparison of the available reagents for such oxidation, the in situ-generated monop-
erphosphoric acid and the alumina-supported oxidizing agents possess certain features that
over-oxidation can be avoided, mild reaction condition are used and the reagents are envi-
ronmentally safe and easy to handle. The inherent advantages such as cost, smoothness, high
yields of pure product and its environmental compatibility justify the use of in situ-generated
monoperphosphoric acid as an efficient reagent for selective oxygenation of the thioethers.
Table 3. Oxidation of sulfides to sulfoxides with H2O2 in the presence of alumina-POCl3.a
(II), yield
Entry
R1
R2
(I), yield (%)b,c
(%)b/time (min)
1
2
3
4
5
6
7
8
9
Ph
Ph
Me
PhCH2
PhCH2
−
97
98
89
96
90
98
96
91
95
88
95
94
99/20
98/25
97/30
98/15
95/15
98/120
96/180
95/10
97/300
93/18
96/240
97/320
PhCH2
−
CH2CH2CH2CH2
=
=
CH2 CHCH2
CH2 CHCH2
Ph
4-BrC6H4
n-Bu
CH2CH2OH
PhCH2
n-Bu
Ph
Me
4-BrC6H4
Ph
CH2COOEt
CH2CH2CHO
4-BrC6H4CH2
CH2CN
10
11
12
Notes: aThe products were characterized by comparison of their spectroscopic and physical data with those reported in the
literature (23). bYields refer to pure isolated products. cThe reactions were completed within 10 min.

Journal of Sulfur Chemistry 97
Table 4. Comparison of our reagents with those reported in the literature.
Substrate
Reagent/conditions
Yield (%)
Reference
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
Ca(OCl)2, wet alumina/CH2Cl2, r.t./40 min
CAN, SiO2 hydrated/CH2Cl2/1 h
90
100
81
97
98
98
98
97
99
88
90
99
94
98
98
95
95
91
95
(23)
(24)
(25)
(19)
(26)
(26)
(27)
–
Fe2O3/O2, RCHO/1 h
H2O2 (35%), Mont.K10/CH3OH, r.t./2.5 h
H2O2 (35%), Amberlyst 15/CH3OH, r.t./3 h
H2O2 (35%), Amberlite IR-400/CH3OH, r.t./3 h
H2O2 (30%)/POCl3, EtOH, 50 ◦C/8 min
POCl3/H2O2 (30%)/Et2O, 0 ◦C/10 min
(POCl3-alumina)/H2O2 (30%)/CH3OH, r.t./20 min
Ca(OCl)2, wet alumina/CH2Cl2, r.t./40 min
CAN, SiO2 hydrated/CH2Cl2/40 min
PhSMe
–
(n-Bu)2S
(n-Bu)2S
(n-Bu)2S
(n-Bu)2S
(n-Bu)2S
(n-Bu)2S
(n-Bu)2S
(n-Bu)2S
(n-Bu)2S
(n-Bu)2S
(23)
(24)
(28)
(29)
(19)
(26)
(26)
(27)
–
Wet silica gel, MMPP/CH2Cl2/1 h
UHP/Ti-beta/acetone, 20 ◦C/2 h
H2O2 (35%), Mont.K10/CH3OH, r.t./2 h
H2O2 (35%), Amberlyst 15/CH3OH, r.t./2 h
H2O2 (35%), Amberlite IR-400/CH3OH, r.t./2 h
H2O2 (30%)/POCl3, EtOH, 50 ◦C/11 min
POCl3/H2O2(30%)/Et2O, 0 ◦C/10 min
(POCl3-alumina)/H2O2 (30%)/CH3OH, r.t./10 min
–
3. Conclusions
Summing up, we have found that POCl3 and alumina-supported phosphorus oxychloride with
commercially available 30% aqueous hydrogen peroxide are efficient and convenient oxidizing
reagents for the mild and selective oxidation of sulfides to their corresponding sulfoxides.
4. Experimental
Chemicals were purchased from Merck, Fluka and Aldrich chemical companies. A number of
sulfides were prepared using the standard synthesis method (22). All sulfoxides were separated
and purified by chromatographic techniques and also identified by comparison of their mp, IR
and NMR with those reported in the literature (23).
4.1. General procedure for the oxidation of sulfide by POCl3/H2O2
The appropriate sulfide (1 mmol), Et2O (2 ml) and 30% H2O2 (0.17 mmol) were mixed in a
round-bottomed flask. Then POCl3 (0.17 mmol) was added dropwise and the mixture stirred
electromagnetically at 0 ◦C. After 10 min, the mixture was quenched by adding 10% aqueous
Na2CO3. The product was extracted with EtOAc (4×5 ml) and the combined extracts were dried
(Na2SO4). The filtrates were evaporated and the corresponding crude sulfoxides were purified by
chromatography on silica gel plate to afford the corresponding pure sulfoxides in 88–98% yield.
4.2. Preparation of alumina-supported POCl3
A50 ml suctionflaskwasequipped witha constant-pressure droppingfunnelscontaining phospho-
rus oxychloride (5.13 g, 0.034 mol) and gas inlet tube for conducting HCl gas over an adsorbing
solution, i.e. water. Basic alumina (10 g) was charged into the flask. Phosphorus oxychloride was
added dropwise over a period of 30 min at room temperature. HCl gas evolved from the reaction
vessel immediately which removed through suction. Then the mixture was shaken for 30 min.

98 M.M. Lakouraj and V. Hasantabar
The white solid was washed with dry dichloromethane (DCM) and dried in vacuo for 4 h. Finally,
alumina-supported POCl3 was obtained in 89% yield (13.61 g) as free-flowing powder. Selected
spectroscopic data: IR (KBr cm−1): 1143 (P O) st, 1259 (P O) st, 650 (P Cl) st).
−
=
−
4.3. General procedure for the oxidation of sulfides by (alumina-supported POCl3)/H2O2
In a round-bottomed flask equipped with a magnetic stirrer, a mixture of sulfides (1 mmol) and
30% H2O2 (1 mmol) in MeOH (2 ml) was prepared. Then alumina-POCl3 (0.6 g) was added and
stirred at room temperature for the appropriate reaction times (Table 3). On completion of the
reaction (monitored by TLC), the mixture was diluted with EtOAc and filtered. The organic layer
was washed with (5%) Na2CO3 solution and then was dried over anhydrous Na2SO4. The solvent
was evaporated under reduced pressure to give the corresponding sulfoxides in pure form.
Acknowledgement
We are grateful to the Research Councils of Mazandaran University for their partial financial support.
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