Metal-free chemoselective oxidation of sulfides to sulfoxides by hydrogen peroxide catalyzed by in situ generated dodecyl hydrogen sulfate in the absence of organic co-solvents

Article abstract of DOI:10.1002/adsc.200505312

The mild oxidation of sulfides to sulfoxides with an aqueous solution of H₂O₂ (35%) catalyzed by in situ generated dodecyl hydrogen sulfate as Bronsted acid surfactant in the absence of any organic cosolvents and under metal-free con

Full text of DOI:10.1002/adsc.200505312

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DOI: 10.1002/adsc.200505312
Metal-Free Chemoselective Oxidation of Sulfides to Sulfoxides
by Hydrogen Peroxide Catalyzed by in situ Generated Dodecyl
Hydrogen Sulfate in the Absence of Organic Co-Solvents
H. Firouzabadi,* N. Iranpoor,* A. A. Jafari, E. Riazymontazer
Chemistry Department, College of Sciences, Shiraz University, Shiraz 71454, Iran
Fax: (þ98)-711-228-6008, (þ98)-711-228-0926, e-mail: firouzabadi@chem.susc.ac.ir, iranpoor@chem.susc.ac.ir
Received: August 6, 2005; Accepted: January 12, 2006
SupportingInformation for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The mild oxidation of sulfides to sulfoxides
with an aqueous solution of H2O2 (35%) catalyzed by
in situ generated dodecyl hydrogen sulfate as Brønst-
ed acid surfactant in the absence of any organic co-
solvents and under metal-free conditions is describ-
ed. The method shows high chemoselectivity and
the process is highly green for solid sulfoxides which
crystallize out immediately after their formation
from the reaction mixture. Liquid sulfoxides can be
isolated under high vacuum by bulb-to-bulb distilla-
tion or by extraction with Et2O or EtOAc. Yields
of the products are excellent.
reactions in aqueous solutions.[5,6] Kobayashi and his
co-workers have extensively studied the use of transi-
tion metal salts as water-tolerant Lewis acids and also
Lewis acids surfactants for the promotion of organic re-
actions in aqueous media.[7] However, from the view-
points of practicability and applicability, surfactant-aid-
ed organic reactions are still at their preliminary stages.
Recently, we have successfully applied a micellar aque-
ous solution of sodium dodecyl sulfate (SDS) for the
ringopeningof epoxides with various types of nucleo-
philes[8a] and hetero-Michael additions.[8b]
Organosulfur compounds are versatile intermediates
in organic synthesis and are useful for the preparation
of biologically and medically important products.[9] In
particular, sulfoxides and sulfones have been extensive-
ly used for carbon-carbon bond formingreactions, [10] re-
arrangements,[11] and eliminations.[12] There are many
reagents available for oxidation of sulfides to sulfox-
ides.[13] From the standpoint of green chemistry, nowa-
days, environmental benign systems for these useful ox-
Keywords: aqueous hydrogen peroxide; Brønsted
acid surfactants; dodecyl hydrogen sulfate; oxidation;
sulfides; sulfoxides
Use of aqueous media as a reaction solvent has attracted idations are desirable. The use of aqueous hydrogen per-
much attention in synthetic organic chemistry for sever- oxide as a cheap and a clean oxidizer which is trans-
al reasons.[1] In comparison with organic solvents, water formed into harmless water after performingthe oxida-
is cheap, safe and reduces the use of harmful organic sol- tion is an excellent alternative to toxic oxidants.[14] For
vents and leads to the development of environmentally sulfoxidations conducted in the presence of hydrogen
friendly chemical processes.[2] In addition, reactions in peroxide, organic co-solvents such as CH3CN, CH3OH,
aqueous media illustrate unique reactivities and selec- CH3CH2OH, CH3CO2H, acetone, hexafluoro-2-propa-
tivities that are not usually observed in organic media.[3] nol and phenol should be added to the reaction mixture
However, organic solvents are still used instead of water in order to overcome the insolubility and compatibility
for mainly two reasons; first, most organic substrates are of sulfides with aqueous H2O2.[15] In this article, we re-
not soluble in water, and as a result, water cannot func- port a green process in which aqueous hydrogen perox-
tion as a reaction medium. Second, many reactive sub- ide has been used as an oxidizingagent in the presence of
strates, reagents, and catalysts are sensitive towards wa- in situ generated dodecyl hydrogen sulfate, a Brønsted
ter and are decomposed or deactivated in aqueous me- acid surfactant, for the chemoselective oxidation of sul-
dia. A possible new way to improve the solubility of sub- fides to their sulfoxides in the absence of any organic co-
strates is the use of surface-active compounds that can solvents and any metal catalysts.
form micelles[4] or vesicular structures. The use of micel-
lar and vesicle-formingsurfactants as catalysts is wide-
First, we studied the oxidation of 2 mmol of diphenyl
sulfide as a model compound with 8 mmol of aqueous
spread and has been investigated in detail for different H2O2 (35%) as the oxidant. The reaction was allowed
434
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Adv. Synth. Catal. 2006, 348, 434 – 438

Metal-Free Chemoselective Oxidation of Sulfides to Sulfoxides
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to proceed for 24 h. Analysis of the mixture revealed the curred with absolute chemoselectivity for the sulfoxide
formation of the sulfoxide in only 5% yield. Then we de- formation. In none of the reactions we have presented
cided to run the reaction in acidic media. For this pur- in this article was the formation of sulfones as over-oxi-
pose, Brønsted acids with different acidic strengths, dation products detected in the reaction mixtures. As il-
such as acetic acid, trichloro- and trifluoroacetic acids, lustrated in Table 2, various functional groups, including
¼
¼
À
À
sulfuric and hydrochloric acids were employed. None C C, C O, OH and CN werecompatible and the sulf-
of the acids were effective and after a longreaction
time most of the startingmaterial remained intact.
oxides were obtained in almost excellent yields. In sever-
al cases, the reaction was complete in less than 10 min
Then, a similar reaction was studied in the presence of (entries 8–11). However, in the cases of diphenyl sulfide
5 mol % of sodium dodecyl sulfate (SDS).The results and benzyl phenyl sulfide (entries 19 and 20) longer re-
of this study were also not promisingand after 24 h
only 20% formation of the correspondingsulfoxide
action times were required. To further determine the se-
lectivity of the method, the oxidation of diphenyl sulfide
was observed. However, we decided to run the reaction in the presence methyl phenyl sulfide was studied. We
in a mixture of 5 mol % of sodium dodecyl sulfate(SDS) have observed that the selectivity of the method was ex-
and 10 mol% of the above-mentioned protic acids (Ta- cellent and diphenyl sulfide was transformed into its
ble 1, entries 4–7). Acetic acid as aweak acid in the pres- sulfoxide quantitatively without formation of sulfone
ence of SDS was not able to conduct this oxidation and whereas the formation of a trace amount of methyl phe-
the reaction proceeded in only 30% after 24 h (Table 1, nyl sulfoxide was detected in the reaction mixture
entry 3). However, in the presence of strongacids, the
rate of the reaction was enhanced and the reaction was
(Scheme 1).
The presented protocol follows an environmentally
completed after 2 h (Table 1, entries 4–6). By consider- friendly green process in which the solid sulfoxides pre-
ation of these observations, we may conclude that pro- cipitate out from the aqueous reaction mixture as soon
ton transfer from strongacids to dodecyl sulfate anion
as they are formed. In the case of liquid sulfoxides,
is a facile and favoured process which generates dodecyl bulb-to-bulb distillation under high vacuum resulted in
hydrogen sulfate in the reaction media. However, this in the isolation of the pure sulfoxides. Another alternative
situ generated Brønsted acid surfactant shows dual ac- is to isolate the liquid sulfoxides by extraction with di-
tivity. One action is to wrap the sulfide molecule by its ethyl ether or ethyl acetate and the evaporation of the
non-polar tail and its second function is to bringthe
H2O2 molecule close to the sulfur atom. After this proc-
solvent.
In conclusion, we have developed a metal-free and or-
ess, the reactionproceeds via activation of hydrogenper- ganic solvent-less protocol for a highly selective, effi-
À
oxide by hydrogen bonding between R OSO3H and cient and green process for the chemoselective oxidation
H2O2 in the vicinity of the sulfur atom (Figure 1). So- of sulfides into their sulfoxides by aqueous solution of
dium dodecyl sulfate (SDS) also wraps the sulfide mol- H2O2 catalyzed by the in situ generation of dodecyl hy-
ecule but is not able to form the required hydrogen drogen sulfate. The reactions were highly selective for
bondingfor its activation.
the conversion of sulfides into their sulfoxides without
¼
In order to show the general applicability of the proto- sulfone formation. Functional groups such as C C,
col, a series of structurally diverse sulfides was convert- C O, OH, or CN tolerate the reaction conditions and re-
ed to their sulfoxides in high yields. All the reactions oc- main intact. The operationalsimplicity, high yields of the
¼
Table 1. Optimization of the oxidation of diphenyl sulfide to its sulfoxide usingaqueous solution of H O2.
2
Entry
Surfactant (5 mol %)
HX (10 equiv. %)
Time [h]
Yield [%]
1
2
3
4
5
6
7
8
9
None
SDS
SDS
SDS
SDS
SDS
SDS
SDS
None
None
None
None
CH3CO2H
CCl3CO2H
CF3CO2H
H2SO4
HCl
HCl
HCl
CCl3CO2H
24
24
24
2
2
2
5
20
30
95
94
93
95
80[a]
10
5
2
24
24
5
10
[a]
2 mmol H2O2 (0.25 mL) was used.
Adv. Synth. Catal. 2006, 348, 434 – 438
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asc.wiley-vch.de
435

H. Firouzabadi et al.
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Figure 1. Catalytic role of in situ generated dodecyl hydrogen sulfate, a Brønsted acid surfactant, for the activation of hydrogen
peroxide by ROSO3H and solubilization of the lipophilic organic sulfide by its hydrocarbon tail.
Scheme 1.
pure product as a colorless oil; yield: 0.258 g(92%) (see Ta-
products, high chemoselectivity, metal-free process and
ble 2).
avoidingthe use of any organic-solvents while using
2-(Diphenylmethanesulfinyl)-ethanol (i) [Ph2CHS(O)CH2-
aqueous media are the advantages of the presented
method.
1
CH2OH]: H NMR (250 MHz, CDCl3, TMS): d¼2.57–2.62
(m, 1H), 2.68–2.75 (m, 1H), 3.83–3.90 (m, 2H), 4.39 (s, 1H,
OH), 5.23 (s, 1H), 7.33–7.42 (m, 6H), 7.52–7.59 (m, 4H); 13
C
NMR (63 MHz, CDCl3,TMS): d¼52.0, 52.9, 68.9, 126.3,
126.8, 126.9, 1127.4, 1128.0, 134.1, 135.4; MS (EI, 70 eV),
m/e¼260 [Mþ]; anal. calcd. for C15H16O2S: C 69.20, H 6.19;
found: C 69.12, H 6.15.
Experimental Section
Sulfoxidation of Methyl Phenyl Sulfide by Aqueous
H2O2 (35%); Typical Procedure
Acknowledgements
A 10-mL flask was charged with 1 mL (8.0 mmol) of aqueous
H2O2 (35%), 0.029 g(0.10 mmol) of SDS, and 0.248 g
(2 mmol) of methyl phenyl sulfide. To the resultingmixture,
20 mL (0.2 mmol) of aqueous HCl solution were added and stir-
red at room temperature. The progress of the reaction was fol-
lowed by TLC. After complete disappearance of the reactant
(10 min), the excess amount of H2O2 was destroyed by the ad-
dition of asaturatedaqueoussolutionof Na2SO3 to thereaction
mixture. The product was extracted with EtOAc which was
separated and dried over anhydrous Na2SO4. Evaporation of
the solvent under diminished pressure resulted in the almost
The authors are grateful to Iran TWAS Chapter Based at ISMO,
Shiraz University Research Council and Ministry of Science,
Research and Technology of Iran for the support of this work.
436
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Adv. Synth. Catal. 2006, 348, 434 – 438

Metal-Free Chemoselective Oxidation of Sulfides to Sulfoxides
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1
2
3
4
a
b
c
5
92
95
91
93
2 h
5 h
d
10
5
6
7
8
e
f
5
92
95
88
85
3 h
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