from drawbacks such as extended period of time, use of
corrosive acids, hazardous peracids, and toxic metallic
compounds. Consequently, there is a need for mild and
selective methods for the efficient conversion of sulfide to
sulfoxides or sulfones. The use of urea-hydrogen peroxide
adduct under solvent-free conditions appears to be a viable
and safer protocol. Our results for the general oxidation of
sulfides to sulfoxides (entries 1, 3, 5, and 7) and sulfones
(entries 2, 4, 6, and 8) and nitrogen heterocycles to their
corresponding N-oxides (entries 9 and 10)32 are summarized
in Table 2.
carboxylic acid and p-anisaldehyde gives p-methoxyphenol,
whereas aliphatic aldehydes lead to the formation of complex
products. In the case of nitriles, the reaction rate is faster
for aliphatic nitrile, benzylnitrile, as compared to benzonitrile.
The selective oxidation of sulfides to sulfoxides or sulfones
can be achieved by varying the reaction time (Table 2). Alkyl
sulfides get oxidized faster than the aromatic sulfide.
Interestingly, no reaction occurs at room temperature for the
aforementioned examples.33,34
In conclusion, this solvent-free oxidative protocol using
an inexpensive, safe, and easily handled reagent, urea-
hydrogen peroxide adduct, is a simple and efficient protocol
that is applicable to a variety of organic molecules. The
operational simplicity, rapid reaction rates, and formation
of pure products in high yields at a very moderate temper-
ature make this method superior to existing protocols.
Acknowledgment. We are grateful for financial support
to the Texas Research Institute for Environmental Studies
(TRIES).
Table 2. Solid State Oxidation of Sulfides and Nitrogen
Heterocycles Using UHP
OL990522N
(22) (a) Gazdar, M.; Smiles, S. J. Chem. Soc. 1908, 93, 1833. (b) Peak,
D. A.; Watkins, T. I. J. Chem. Soc. 1950, 445.
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R. J. Prakt. Chem. 1926, 113, 40.
(24) (a) Marcker, C. Justus Liebigs Ann. Chem. 1865, 136, 75. (b)
Bordwell, F. G.; Boutan, P. J. J. Am. Chem. Soc. 1957, 79, 717.
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(b) Barnard, D. J. Chem. Soc. 1957, 4547.
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(27) Mel’nikov, N. N. Usp. Khim. 1936, 5, 443.
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W. B.; Stobie, A. Tetrahedron Lett. 1982, 23, 957.
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(32) (a) Muzart, J. Synthesis 1995, 1325. (b) Ohta, A.; Ohta, M. Synthesis
1985, 216. (c) Klein, B.; Berkowitz, J. J. Am. Chem. Soc. 1959, 81, 5160.
(33) General Procedure for UHP Oxidation Reactions. The starting
material (2 mmol) was added to the finely powdered urea-hydrogen
peroxide adduct (376 mg, 4 mmol) in a glass test tube, and the reaction
mixture was placed in an oil bath at 85 °C for the specified time (Tables 1
and 2). After completion of the reaction, monitored by TLC (8:2 v/v, hexane:
EtOAc), the reaction mixture was extracted into ethyl acetate and the
combined extracts were washed with water and dried over anhydrous sodium
sulfate. The solvent was removed under reduced pressure to afford the crude
product, which was purified by chromatography to deliver pure product, as
confirmed by the spectral analysis.
(34) Typical Procedure for Oxidation of Sulfides: Preparation of
Methyl Phenyl Sulfoxide and Sulfone. In a typical experiment, methyl
phenyl sulfide (248 mg, 2 mmol) was added to the finely powdered urea-
hydrogen peroxide adduct (376 mg, 4 mmol) in a glass test tube, and the
reaction mixture was placed in an oil bath at 85 °C for 15 min for complete
conversion of sulfide to sulfoxide with a trace amount (10%) of sulfone.
For complete conversion of sulfide to sulfone, the reaction time was
extended to 1 h (see Table 2). After completion of the reaction, monitored
by TLC (8:2 v/v, hexane:EtOAc), the reaction mixture was extracted into
ethyl acetate and the combined extracts were washed with water and dried
over anhydrous sodium sulfate. The solvent was removed under reduced
pressure to afford the crude product, which was purified by chromatography
on a silica gel column; fractions obtained with pure hexane as eluent afforded
pure product (80% of sulfoxide), as confirmed by the spectral analysis.
a The yield refers to isolated product, and the results in brackets refer to
sulfone. b The percentage yield was obtained from GC-MS; results in
parentheses refer to dioxide.
Our summarized results for the solid-state oxidation of a
variety of organic substrates in Tables 1 and 2 demonstrate
the versatility of the UHP reagent under solvent-free condi-
tions. In general, the reaction rate for hydroxylated benzal-
dehydes is faster when compared to that for hydroxylated
acetophenones. Benzaldehyde upon oxidation affords only
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