Ind. Eng. Chem. Res. 2010, 49, 2533–2536
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A Simple and Environmentally Benign Method for Sulfoxidation of Sulfides with Hydrogen Peroxide Fenglan Liu, Zaihui Fu,* Yachun Liu, Chunli Lu, Youyu Wu, Fang Xie, Zhengpei Ye, Xiaoping Zhou, and Dulin Yin Key Laboratory of Resource Fine-Processing and AdVanced Materials of Hunan ProVince and Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), College of Chemistry and Chemical Engineering, Hunan Normal UniVersity, Changsha 410081, People’s Republic of China
The development of a simple, efficient, and environmentally benign method for the sulfoxidation of sulfides with aqueous hydrogen peroxide is of importance for the large-scale industrial production of sulfoxides. This article first discloses that various aryl and alkyl sulfides, without help of any catalyst and extra solvent, can be directly oxidized by 30% aqueous hydrogen peroxide to the corresponding sulfoxides with good to excellent yield at 60 °C. Furthermore, the present method has outstanding advantages, with regard to reaction rate and sulfoxide yield, compared with a series of organic solvents-mediated reactions. Sulfoxides are important intermediates in organic chemistry, because of their application in fundamental research and other extended usage,1-3 especially that the chiral sulfoxides are versatile intermediates for the preparation of biologically and medicinally important products.4-6 The most widely used method for the preparation of sulfoxides is the oxidation of the corresponding sulfides. Several methods have been reported for these transformations in the literature.7-15 Some traditional oxidants such as trifluoroperacetic acid,16 an MeNO2 solution in dilute HNO3/H2SO4,17 iodic acid,18 and other hypervalent iodine reagents19-22 are applied frequently to the oxidation of sulfides. However, most of these reagents perform unsatisfactorily for medium- to large-scale synthesis, because of low effective oxygen content, leading to the formation of environmentally unfavorable byproducts and high cost. Aqueous hydrogen peroxide is a particularly attractive oxidant because it is inexpensive, environmentally friendly, easy to handle, and produces only water as a byproduct, which reduces purification requirements. In recent years, some transition-metal catalysts such as vanadium,15,23,24 rhenium,25 iron,26-31 manganese,8,29,32,33 and titanium34 have been widely used to catalyze the selective oxidation of sulfides to sulfoxides, utilizing hydrogen peroxide as the oxidizing agent in the presence of various solvents. However, some of these catalysts are expensive or difficult to be prepared. Moreover, some of these solvents are volatile or toxic, which is not compatible with the environment. Recently, Conte and co-workers15 have developed a highly efficient sulfoxidation of sulfides with H2O2 over vanadium-based catalysts using ionic liquids (ILs) as green solvents. However, the expensive cost and high viscosity of ILs hamper its commercialized application. Mahamuni and co-workers35 have reported a novel and green approach for the sulfoxidation of thioanisole with hydrogen peroxide promoted by ultrasound wave without any catalyst and organic solvent. However, this method is complicated in the experimental procedure and its applicability extended to other sulfides is not reported. Therefore, it is highly desirable to develop a simple, less-expensive, safer, and highly efficient sulfoxidation method for large-scale industrial production of sulfoxides. * To whom correspondence should be addressed. Tel.: + 86 731 88872576. Fax: +86 731 88872531. E-mail address:
[email protected].
Herein, for the first time, we report a simple and green method for the selective oxidation of aryl and alkyl sulfides with aqueous H2O2 in the absence of a catalyst and extra solvent. Experimental Section Reagents. Various organic sulfides were purchased from Alfa Aesar, and other commercially available chemicals were laboratory-grade reagents from local suppliers. All the chemicals were used without further purification. General Procedure for the Oxidation of Organic Sulfides. A mixture containing sulfide and 30% aqueous H2O2 (1 equiv, relative to the substrate) was stirred at 35-70 °C. After H2O2 was consumed completely via an inspection with potassium iodide-starch test paper, the reaction mixture was extracted by ethyl ether and dehydrated by anhydrous sodium sulfate. The organic phase then was analyzed using an Agilent 6890N gas chromatography system that was equipped with a HP-5 quartz capillary column (30 m × 0.32 mm × 0.25 µm) and a flame ionization detector, using ultrapure nitrogen as a carrier gas (rate of 1.0 mL/min). Sulfoxide products were identified by gas chromatography-mass spectroscopy (GC-MS) (using a Hewlett-Packard Model 5973/6890 system; electron impact ionization at 70 eV, helium carrier gas, 30 m × 0.25 mm cross-linked 5% PHME siloxane (0.25 µm coating) capillary column, Model HP-5MS) techniques and also by co-injection with authentic samples. For the sake of comparison, we checked the oxidation of thioanisole with 30% aqueous H2O2 at 40 °C in various extra solvents. Results and Discussion The liquid oxidation of thioanisole with H2O2 at 40 °C was performed in the presence of various protonic and aprotic solvents, and the results are summarized in Table 1. Entries 1-5 illustrate that H2O2, without the help of a catalyst, could slowly oxidize thioanisole to the sulfoxide with an excellent selectivity (>99%) in these reaction media. Among these solvents examined, the protonic solvents (i.e., water and especially methanol) were superior to those aprotic solvents, such as acetonitrile, acetone, and dichloromethane, in terms of sulfoxide yield (see entries 1 and 2, versus entries 3-5). Entry 6 demonstrates that the reaction also proceeded effectively
10.1021/ie9017572 2010 American Chemical Society Published on Web 01/25/2010
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Table 1. Oxidation of Thioanisole with Aqueous Hydrogen Peroxide in the Absence of Catalysta entry
solvent
temperature, T (°C)
time (h)
sulfoxide yield (%)
selectivity to sulfoxide (%)
1 2 3 4 5 6b 7b 8b 9b 10b 11b,c 12b,d 13b,e
H2O MeOH CH3CN acetone CH2Cl2 H2O H2O H2O H2O H2O H2O H2O H2O
40 40 40 40 40 40 35 50 60 70 60 60 60
24 24 24 24 24 5 10 2 1 1 1.5 4 1
73 87 37 37 57 93 93 96 93 94 92 97 17
>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 100
a Conditions: substrate (5 mmol), 30% aqueous H2O2 (5 mmol) and solvent (5 mL). b Using only H2O in the oxidant as a solvent. c 1 equiv ) 20% H2O2, relative to the substrate. d 1 equiv ) 10% H2O2, relative to the substrate. e Using methyl phenyl sulfoxide (5 mmol) as a substrate and giving yield and selectivity for methyl phenyl sulfone.
without these extra solvents and provided a much quicker rate (5 h) and higher yield (ca. 93%) than the above solventsmediated reaction. Furthermore, increasing the temperature from 35 °C to 60 or 70 °C could shorten the reaction time drastically from 10 to 1 h in the case of maintaining a high yield (entries 6-10). Entries 11 and 12 show that the use of a low concentration of oxidant could improve the yield at a certain extent but the reaction time was prolonged obviously. Inciden-
tally, the present change of temperature or oxidant concentration had little effect on the selectivity for sulfoxide (>99%). This is because the further oxidation of the product sulfoxide is difficult, under the present conditions, as supported in entry 13. To determine functional group compatibility of the reaction, the sulfoxidation of some representative sulfides with H2O2 was conducted at 60 °C without any extra solvent. As shown in Table 2, these sulfides were effectively oxidized to the corresponding sulfoxides with good to excellent yields. Among the substrates examined, diethyl sulfide was the most active, affording a very high sulfoxide yield in 5 min (entry 1). n-Butyl ethyl sulfide was almost quantitatively oxidized to its sulfoxide with 97% yield, provided that the time was prolonged to 2 h (entry 2). However, the sulfoxidation of di-n-octyl sulfide was quite difficult and required a longer time to obtain a good sulfoxide yield (entry 3). This may be attributed to the fact that a longer chain sulfide with poorer solubility in aqueous H2O2 may lead to the increase in mass-transfer resistance. Compared with the above two short-chain dialkyl sulfides, various aryl alkyl sulfides generally showed a slightly lower activity (entries 4-10), which is likely due to the decrease in the nucleophilicity of these substrates, because of a p-π conjugated effect of their S atoms with aryls. Furthermore, the influence of substrate nucleophilicity on sulfoxidation could be observed upon the aryl alkyl substrates. For example, the sulfoxidation rate of 4-halogenated thioanisoles and especially 2-chloroethyl phenyl sulfide was much slower than that of the unsubstituted counterparts (entries 6 and 7 versus entry 4 and entry 9 versus entry 5), because of
Table 2. Oxidation of Sulfides by 30% Aqueous H2O2 in the Absence of Organic Solventa
a
Conditions: 5 mmol substrate, 5 mmol oxidant at 60 °C. b Sulfoxide yield. c Selectivity to sulfoxide.
Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010 Scheme 1. Proposed Reaction Mechanism
the electron-withdrawing effect of halogen substituents.36 Diphenyl sulfide conjugated to electron-withdrawing groups showed the lowest reactivity and sulfoxide yield (entry 10), because of its lesser nucleophilicity and strong steric hindrance.22 Based on the results presented here, as well as those in previous studies,37-40 it is apparent that the above data are better explained according to the following reaction pathway (see Scheme 1). In this mechanism, a protic solvent HOR (where R ) H or CH3) interacts with a hydrogen peroxide to form a cyclic hydrogen-bonded species (1), then 1 easily reacts with a nucleophilic sulfide to generate another cyclic species (2); finally, 2, as a unstable species, is decomposed to obtain a product sulfoxide and regenerate a solvent HOR by means of proton transfer. Notably, the specific interaction of protic solvents, as described by Dankleff and co-workers,41 is based on their ability to decrease the energy requirements of the displacement on peroxide oxygen by minimizing charge separation, and the reactivity of sulfides is correlated with their relative nucleophilicity and steric effect, as supported by the above experimental results. Conclusion In summary, we have developed a simple, practical, and ecofriendly method for the sulfoxidation of organic sulfides with aqueous hydrogen peroxide without any catalyst and extra solvent. This method can achieve approximatively stoichiometric transformation for some sulfides and give an excellent selectivity for sulfoxides (>98%) under mild reaction conditions. Therefore, it has a potential application for the industrial synthesis of sulfoxides from the corresponding sulfides. Acknowledgment We acknowledge the financial support for this work by the National Natural Science Foundation of China (Nos. 20873040, 20573035). Literature Cited (1) Cremlyn, R. J. An Introduction to Organosulfur Chemistry; John Wiley and Sons: Chichester, England, 1996. (2) Drabowski, J.; Kielbasinski, P.; Mikolajczyk, M. Synthesis of Sulfoxides; John Wiley and Sons: New York, 1994. (3) Pati, S.; Rappoport, Z. The Synthesis of Sulphones, Sulphoxides, and Cyclic Sulphides; John Wiley and Sons: New York, 1994. (4) Legros, J.; Dehli, J. R.; Bolm, C. Applications of Catalytic Asymmetric Sulfide Oxidations to the Syntheses of Biologically Active Sulfoxides. AdV. Synth. Catal. 2005, 347, 19. (5) Carreno, M. C. Applications of Sulfoxides to Asymmetric Synthesis of Biologically Active Compounds. Chem. ReV. 1995, 95, 1717. (6) Ferna´ndez, N.; Khiar, I. Recent Developments in the Synthesis and Utilization of Chiral Sulfoxides. Chem. ReV. 2003, 103, 3651. (7) Hajipour, A. R.; Ruoho, A. E. Benzyltriphenylphosphonium Chlorochromate (BTPPCC): An Efficient and Novel Reagent for Oxidation of Sulfides to the Corresponding Sulfoxides under Non-aqueous Conditions. Sulfur Lett. 2003, 26, 83. (8) Bagherzadeh, M.; Latifi, R.; Tahsini, L.; Amini, M. Catalytic Oxidation of Sulfides to Sulfoxide Using Manganese(III) Complexes with
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ReceiVed for reView November 5, 2009 ReVised manuscript receiVed January 2, 2010 Accepted January 12, 2010 IE9017572
