Hydrogen Peroxide-Tellurium Dioxide1


[7722-84-1]  · H2O2  · Hydrogen Peroxide-Tellurium Dioxide  · (MW 34.02) (TeO2)

[7446-07-3]  · O2Te  · Hydrogen Peroxide-Tellurium Dioxide  · (MW 159.60)

(selective oxidizing agent for conversion of sulfides to sulfoxides1)

Handling, Storage, and Precautions: see Hydrogen Peroxide. A mixture of 30% H2O2 (2.4 mL, 24 mmol) and TeO2 (0.032 g, 2.0 mmol) or TeO2 (0.32 g, 20 mmol) in methanol (10 mL) can be stored in a refrigerator for a prolonged time without loss of reactivity. For the oxidation of sulfides to sulfoxides, better results are obtained when hydrogen peroxide is added slowly to the mixture of the sulfide and tellurium dioxide in methanol. These operations should be conducted in an efficient fume hood.

Selective Oxidation of Sulfides.

The hydrogen peroxide-tellurium dioxide system is a remarkably selective agent for the oxidation of sulfides to sulfoxides. Sulfides are transformed into sulfoxides in high yields by treatment with hydrogen peroxide in the presence of a catalytic amount of tellurium dioxide at rt. Sometimes a small amount of over-oxidation product, the corresponding sulfone, is produced. This over-oxidation can be suppressed by slow addition of aqueous 30% hydrogen peroxide solution to a mixture of the sulfide substrate and tellurium dioxide in methanol. Even with 1 equiv of TeO2, the resulting sulfoxide is not oxidized further to sulfone. Aromatic sulfides (eq 1) as well as aliphatic sulfides (eq 2) are transformed to sulfoxides in high yield.

Usually, the oxidation of aliphatic sulfides proceeds in better yield and requires shorter reaction times than that of aromatic derivatives. Chemoselective oxidation of a sulfide group by the H2O2-TeO2 system in the presence of a double bond can be achieved (eq 3). Thus, to a mixture of crotyl phenyl sulfide (0.165 g, 1.00 mmol) and tellurium dioxide (0.016 g, 0.10 mmol) in methanol (2.0 mL), aqueous 30% hydrogen peroxide solution (0.136 g, 1.20 mmol) is added slowly at rt. After stirring at rt for 24 h, the reaction mixture is partitioned between methylene chloride and water. The product is purified by column chromatography.

One problem with the catalytic TeO2-H2O2 system is that the reaction time is relatively long. While the use of a stoichiometric amount of tellurium dioxide considerably shortens the reaction time, the addition of a small amount of concentrated HCl (1/100 molar ratio of sulfide) drastically accelerates the oxidation of sulfide (eq 4).

The effect of concentrated HCl is especially pronounced in the oxidation of a carbohydrate sulfide; this substrate is not oxidized by 1 equiv of the TeO2-H2O2 system after prolonged reaction time, but is oxidized to the sulfoxide in high yield in 2 h by using the catalytic TeO2-H2O2-HCl system (eq 5). It is also noteworthy that the acid-labile isopropylidene group in the carbohydrate sulfide is not affected by the TeO2-H2O2-HCl system. Also the carbonyl group in keto sulfides is not affected by the H2O2-TeO2-HCl system (eqs 5 and 6).

The reaction of benzaldehyde, geraniol, or ethylene glycol with H2O2-TeO2 leads mainly to recovery of the starting material. In this regard, the TeO2-H2O2 system appears to be superior to widely used reagents such as hydrogen peroxide, peroxy acids, and periodates, which sometimes cause over-oxidation or affect other functional groups.2 Other recently reported oxidizing agents such as ruthenium porphyrin with heteroaromatic N-oxide,3 (Ph4P)[RuO2(OAc)Cl2] with N-Methylmorpholine N-Oxide,4 sulfonimidoyl chloride with superoxide,5 and Benzyltrimethylammonium Tribromide6 have similar drawbacks.

Oxidizing systems related to the H2O2-TeO2 system include the H2O2-SeO2 system and the H2O2-RSO2H system. Sulfides are selectively oxidized to sulfoxides by treatment with H2O2 in methanol containing a catalytic amount of the sulfinic acid,2a and the H2O2-SeO2 system is known to be an efficient and chemoselective agent for oxidation of sulfides to sulfones7 and a selective oxidizing agent for sulfides to sulfoxides in certain cases.8

The true oxidizing species in the H2O2-TeO2 system is likely to be peroxytellenious acid (H2TeO4), which will be reduced back to H2TeO3 and quickly regenerated by H2O2 during the reaction in the catalytic cycle shown in eqs 7-9.

1. Kim, K. S.; Hwang, H. J.; Cheong, C. S.; Hahn, C. S. TL 1990, 31, 2893.
2. (a) Oae, S. Organic Sulfur Chemistry: Structure and Mechanism; CRC: Boca Raton, FL, 1991; pp 253-281. (b) Hudlicky, M. Oxidations in Organic Chemistry; American Chemical Society: Washington, 1990; pp 250-263. (c) Madesclaire, M. T 1986, 42, 5459.
3. Higuchi, T.; Ohtake, H.; Hirobe, M. TL 1991, 32, 7435.
4. Griffith, W. P.; Jolliffe, J. M.; Ley, S. V.; Williams, D. J. CC 1990, 1219.
5. Kim, Y. H.; Yoon, D. C. SC 1989, 19, 1569.
6. Kajigaeshi, S.; Murakawa, K.; Fujisaki, S.; Kakinami, T. BCJ 1989, 62, 3376.
7. Kim, K. S.; Sohng, J.-K.; Ha, S. B.; Cheong, C. S. TL 1988, 29, 2847.
8. Drabowicz, J.; Mikolajczyk, M. S 1978, 758.

Kwan Soo Kim

Yonsei University, Seoul, Korea

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