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www.rsc.org/greenchem

1463-9262(2010)12:9;1-U

ISSN 1463-9262

Cutting-edge research for a greener sustainable future

www.rsc.org/greenchem Volume 12 | Number 9 | September 2010 | Pages 1481–1676

COMMUNICATIONLuque, Varma and BaruwatiMagnetically seperable organocatalyst for homocoupling of arylboronic acids

CRITICAL REVIEWDumesic et al.Catalytic conversion of biomass to biofuels

Green ChemistryD

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Highly atom-economic, catalyst- and solvent-free oxidation of sulfides into sulfones

using 30% aqueous H2O2

Marjan Jereb* Faculty of Chemistry and Chemical Technology, Aškerčeva 5, 1000 Ljubljana, Slovenia.

E-mail: marjan.jereb@fkkt.uni-lj.si; Fax: ++386-1-241-9220; Tel: ++386-1-241-9248 † Electronic supplementary information (ESI) available. Highly atom-efficient oxidation of sulfides into sulfones under solvent- and catalyst-free reaction conditions using a 30% aqueous solution of H2O2 at 75 °C is reported. A structurally diverse set of phenyl alkyl-, phenyl benzyl-, benzyl alkyl-, dialkyl-, heteroaryl alkyl- and cyclic sulfides were transformed into sulfones regardless of the agreggate state and electronic nature of the substituents. In spite of the heterogeneous reaction mixtures throughout the work, no difficulties with stirring and reaction progress were noted. In numerous cases, only 10 mol% excess of H2O2 was used, thus contributing considerably to the high atom economy of the process. Some solid substrates required a variable excess of hydrogen peroxide; however, the reactions were performed strictly without organic solvents. The transformation was demonstrated to be amenable for scale-up with both liquid and solid sulfides. In addition, isolation and purification of the crude products can be simply done with only filtration and crystallization. Key words: solvent free, uncatalyzed, sulfides, sulfones, atom economy, hydrogen peroxide

Introduction

Modern chemistry has been inseparably linked with green chemistry.1 It has been still gaining importance in all aspects of chemistry. Sustainable development in all stages from the development to the final production cycle has become a practical need in an over-polluted world.2 Solvent-free reaction conditions represent one of the most notable contributions to the protection of the environment and people's health; further benefits include enhanced cost efficiency, waste reduction and operational simplicity.3 High atom economy is one of the most essential contributions to the overall process efficiency.4 Uncatalyzed reactions of neat reactants are of fundamental importance in chemistry and one of the most desired transformations. In spite of the increasing number of studies with neat reactants, exploitation of the intrinsic reactivity of neat reactants remains undervalued and should receive greater attention.3c

A 30% aqueous solution of H2O2 is a non-toxic, environmentally benign, cheap and effective oxidizer, and its only residue is water. Besides oxygen, it is one of the most 'green' oxidants and its oxidative power and positive properties have been highlighted in several publications.5 Sulfones represent an important class of compounds due to their properties and reactivity. The strong inductive effect of the sulfone moiety and the broad possibilities of the subsequent transformations make them very attractive in the field of asymmetric organocatalysis.6 Numerous sulfones and their derivatives exhibit different biological activities and have been a subject of an extensive investigation.7 Phenyl and heteroaryl substituted sulfones are important reactants in the Julia-type olefination reaction;8 in addition, the Grignard reagents of sulfones are useful intermediates for the synthesis of

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variously functionalized sulfones.9 1,2-Diarylethenyl sulfones serve as masked diarylethynes, intermediates for carbon-rich materials and molecular wires.10 Good hydrolytic stability in basic and acidic media, resistance to oxidation and corrosion, excellent thermal stability and good electrical properties make polymeric sulfones attractive functional materials.11 Oxidation of sulfides into sulfones has been extensively studied, and H2O2 was frequently utilized in combination with different catalysts i.e. MoO3,

12 CH3ReO3,13

dioxo-molybdenum(VI) complex,14 NH4Cl,15 polyoxometalate-cored dendrimers,16 silica-vanadia catalyst,17 silica sulfuric acid,18 heterogeneous TiO2,

19 tetra-(tetraalkylammonium)octamolybdate,20 immobilized molybdenum heteropolyacid,21 TaC and NbC,22 sodium tungstate/PTC/phenylphosphonic acid,23 MoO2Cl2,

24 Cp'Mo(CO)3Cl,25 tantalum(V) and niobium(V),26 cyanuric chloride,27 [SeO4{WO(O2)}2]

2–,28 composite oxide catalyst LiNbMoO6,29

carbon-based solid acid,30 1,3,4-triazo-2,4,6-triphosphorine-2,2,4,4,6,6-tetrachloride,31 H3BO3,

32 and others. Some

of the other utilized oxidizers were H5IO6,33,34,35 ozone,36 an aqueous NaOCl,37

HOF·CH3CN,38 oxygen/2-methylpropanal,39 NaIO4,40 Oxone,41 and others.

As a rule, all oxidations were performed in different organic solvents. Oxidation also worked well in the supercritical carbon dioxide as an environmentally benign reaction medium.42,43 The uncatalyzed oxidation of sulfides in acetonitrile could be promoted by ultrasound,44 and it could be performed in T-shape micromixer as well.45 The oxidation of sulfides into sulfoxide could be accomplished with 30% aqueous H2O2 under solvent-free reaction conditions;46 in addition, sulfoxidation in the presence of a chiral aluminium(salalen) catalyst was highly enantioselective.47 Hydrogen peroxide exhibited surprisingly low reactivity in the sulfoxidation of sulfides,48 while similar observations were made in sulfoxidation with ozone;49 further oxidation to sulfones is even more demanding. We were interested if 30% aqueous hydrogen peroxide alone is able to oxidize sulfides into sulfones under solvent- and catalyst-free conditions. We report on uncatalyzed sulfonation of neat sulfides with 30% aqueous H2O2. Results and discussion Initially, the reactivity of thioanisole 1a with 2.2 equivalents of 30% aqueous H2O2 was examined at 75 °C. Full conversion to the phenyl methyl sulfone 2a was observed over two hours, and 2a was isolated in a good yield (Table 1, entry 1). Thianisole and H2O2 form two separable phases at room temperature, but reaction took place surprisingly well at the elevated temperature. Further, more challenging sulfides 1b-1e with increasing hydrophobicity were tested. Sulfones 2b-2e were isolated in good yields; however, the reaction times were growing with the hydrophobicity (entries 2–5). In the case of 1e, the best reaction progress was noted when consecutive portions of 1 equivalent of H2O2 were added over several hours. Methoxy substituted thioanisoles 1f-1h were effectively oxidized into the corresponding sulfones 2f-2h in high yields (entries 6–8), and 4-methoxy derivative 1f was the most reactive. 4-Methylphenyl ethyl sulfide 1i furnished sulfone 2i in a 65% yield (entry 9). 4-Fluoro- and 2-fluorophenyl substituted sulfides 1j and 1k were transformed into their sulfones 2j and 2k with 2.2 equivalents of H2O2 in three hours at 75 °C. 2,4-Difluorothioanisole 1l was much less reactive and required 8 hours for complete conversion into 2l (entry 12). Chlorothioanisoles 1m and 1n required 3 equivalents of H2O2 for fast and efficient conversion into 2m and 2n (entries 13 and 14), much less reactive was 1o giving 2o (entry 15). 4-Acetylthioanisole 1p exhibited surprisingly high reactivity in this oxidation, while 4-nitrothianisole 1q required 5 equivalents of H2O2 and 8 hours for full

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conversion into 2q (entries 16 and 17). Both sulfides are solid and reaction took place well anyway. The trifluoroethyl group reduced reactivity in comparison with the ethyl group (entries 18 and 9), and sulfone 2r was obtained in a moderate yield. Phenyl cyclopropylmethyl sulfide 1s smoothly yielded 2s with 10 mol% excess of H2O2 only (entry 19). The branching of the alkyl substituent on the sulfur atom in 1t and 1u did not hamper the oxidation into 2t and 2u (entries 20 and 21). Phenyl 2-bromoethyl sulfide 1v was converted into sulfone 2v in good yield (entry 22). Oxidation of ethyl 2-(phenylthio)acetate 1w proceeded with lower selectivity, and 2w was obtained in low yield (entry 23). Oxidation of phenylthioacetonitrile 1x was surprisingly fast in comparison with 1w, and 2x was isolated in good yield (entry 24). Prolonged reaction time led to the less selective reaction and a lower yield of 2x. Sulfides with unsaturated alkyl substituents were also examined in solvent-free oxidation. Allyl phenyl sulfide 1y was smoothly converted into 2y (entry 25). The more sensitive phenyl vinyl sulfide 1z was oxidized into 2z in a short reaction time; however, the yield was somewhat lower (entry 26). Further, aryl and benzyl propargyl sulfides 1aa and 1bb could be selectively oxidized into 2aa and 2bb in good yields (entries 27 and 28), and prolonged reaction time deteriorated the reaction outcome. Benzyl ethyl sulfide 1cc and benzyl butyl sulfide 1dd were efficiently oxidized into sulfones 2cc and 2dd proving benzyl sulfides to be suitable substrates for this oxidation. Heterocyclic sulfones exhibit different biological activities and are important in medicinal chemistry. 2-Methylthiopyridine 1ee and 1-methyl-2-methylthioimidazole 1ff were converted into sulfone derivatives 2ee and 2ff; however, a little higher excess of H2O2 was needed (entries 31 and 32). The reactivity of both substrates was relatively low. Further, the reactivity of alkyl substituted sulfides was examined. Dimethyl sulfide 1gg and 2.2 equivalents of 30% aqueous H2O2 were stirred in a tightly closed conical vial. Two phases turned into one after 30 minutes of stirring, and full conversion into sulfone 2gg took place in 16 hours (entry 33). Ethyl hexyl sulfide 1hh as highly nonpolar compound was surprisigly reactive with aqueous H2O2 giving sulfone 2hh in high yield (entry 34). Cyclohexyl ethyl sulfide 1ii and cyclohexyl butyl sulfide 1jj were efficiently converted into sulfones 2ii and 2jj in high yields (entries 35 and 36). 2-Hydroxyethyl butyl sulfide 1kk was selectively oxidized into sulfone 2kk, and no oxidation of hydroxy group took place (entry 37). The oxidation method was finally tested on several solid and substantially hydrophobic starting sulfides. Reactions were performed at 75 °C, and some of the reagents melted below the reaction temperature, while others remained solid. No difficulties due to the heterogeneous reaction mixtures were noted. Solid substrates could be considered to be demanding substrates in such a reaction, and this is particularly true for the substrates with melting points above 75 °C. A variable excess of H2O2 was needed in these reactions; however, reactions were conducted without organic solvent. Phenyl benzyl sulfide 1ll was oxidized into sulfone 2ll with a three-fold excess of aqueous H2O2, which was added consequtively in four portions (entry 38). 4-Methoxybenzyl phenyl sulfide 1mm was efficiently oxidized into sulfone 2mm by using a three-fold excess of H2O2 in 5 hours at 75 °C (entry 39). 4-Methoxybenzyl 4-methylphenyl sulfide 1nn similarly furnished its sulfone 2nn in a good yield (entry 40). 4-Methylthiobenzophenone 1oo and 4-methyl-4'-methylthiobenzophenone 1pp yielded the corresponding sulfones 2oo and 2pp in high yields using a three-fold excess of H2O2

in 8 and 9 hours, respectively (entries 41 and 42). Finally, cyclic sulfides were tested in sulfonation reaction. Oxidation of benzo[b]thiophene 1qq produced sulfone 2qq in moderate yield (entry 43).

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Thiochroman-4-one 1rr as more polar and liquid substrate led to its sulfone 2rr in 75% in short reaction time (entry 44). Dibenzothiophene 1ss remained solid throughout the reaction, and oxidation to sulfone 2ss surprisingly proceeded succesfully in a heterogeneous mixture with a nine-fold excess of H2O2 added in several portions (entry 45). Oxidation of 1ss and its derivatives is one of the most important tasks in oil refining industries for the removal of sulfur. High environmental standards require maximum sulfur content in ppm level,50 and this process has been intesively studied.51,

52, 53 Thioxanthen-9-one 1tt was anticipated to be a 'difficult' substrate under our reaction conditions, due to its high melting point and low solubility. It was oxidized into sulfone 2tt in a good yield with a nine-fold excess of H2O2 added in consecutive portions (entry 46). 1,3-Dithiane 1uu dissolved immediately in aqueous H2O2 and yielded disulfone 2uu in 3 hours at 75 °C (entry 47). The crude product was washed with water and crystallized from acetone giving 59% of 2uu as white crystalls. The reaction was demonstrated to be amenable for scale-up with liquid and solid substrates. After 3 hours of stirring of neat thionisole 1a (20 mmol) and 5 mmol of 30% H2O2 at 75 °C, 20 mmol of H2O2 was added. Two phases at the beggining of the reaction slowly turned into one. After 17 h of stirring 19 mmol of H2O2, and after 47 h additional 10 mmol of H2O2 were added. Full conversion into sulfone 2a was reached in 65 h, and the product already began to partly crystallize during the reaction. Upon cooling 2a was filtered off as white crystalls. The product gave a clean 1H NMR spectrum without further purification. The yield (91%) is significantly higher than on a 0.3 mmol scale (68%). As a highly volatile substrate, dimethyl sulfide 1gg was tested. The oxidation was performed in a tightly closed Ace pressure tube; 30% aqueous H2O2 (66 mmol) and 1gg (30 mmol) formed two separated phases, which turned into one after 30 minutes of heating. The mixture was stirred for 33 h at 75 °C. The tube was cooled and 3 mmol of H2O2 were added, and the mixture was further stirred for 25 h (full conversion in 58 h). Upon cooling, 2gg was filtered off (1.63 g). The mother liquor was concentrated in the air furnishing additional 0.84 g, in total (88%) of dimethyl sulfone 2gg. The yield is in comparison with 1 mmol scale (79%) considerably higher. Scaling-up on solid substrates was tested on benzyl phenyl sulfide 1ll and on 4-methxoxybenzyl 4-methylphenyl sulfide 1nn in a similar way as exemplified on the case of 1ll. 2 mmol of 1ll and 4.4 mmol of H2O2 were stirred at 75 °C and after 3, 8 and 18 h 2 mmol of H2O2 were added each time. 1nn melted soon after starting of heating, and two phases were present for approx. 2 h, and then a white solid crystallized. Full conversion in the heterogeneous reaction mixture was achieved in 27 h. The crude product was washed with water, filtered off and purified by crystallization from methanol giving 2nn (0.43 g, 78%) as white solid. Conclusions In this paper, we describe an environmentally friendly protocol for the oxidation of variously substituted sulfides into sulfones at 75 °C using 30% aqueous H2O2 under solvent- and catalyst-free reaction conditions. In numerous cases, only 10% excess of H2O2 was utilized, thus contributing to the high atom economy of the oxidation. The transformation worked well with the both liquid and solid sulfides in spite of a heterogenous reaction mixture, and no difficulties with stirring were noted. Highly nonpolar, hydrophobic sulfides were also surprisingly efficiently oxidized into sulfones, although a variable excess of aqueous H2O2 was needed. Oxidation worked well with sulfides bearing electron-withdrawing- as well as electron-donating groups. Unsaturated i.e. allyl-, vinyl- and propargyl sulfides were efficiently oxidized into their sulfones.

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Solid sulfides may be considered to be demanding substrates for this reaction; this is particulary true for sulfides with melting points above 75 °C. The latter were also oxidized under these conditions. The feasibility of the scale-up was demonstrated on liquid and solid substrates. Since numerous sulfones are solid, they could be isolated by filtration and purified by crystallization. This is an important, multiple advantage of this method, since the extraction, chromatography and organic solvents could be greatly avoided. Experimental

Representative procedure of the non-catalyzed oxidation of sulfides into sulfones using 30%

aqueous solution of H2O2 under SFRC (small scale)

To thioanisole 1a (0.3 mmol, 37 mg) a 30% aqueous solution of H2O2 (0.66 mmol, 75 mg) was added and the mixture was stirred at 75 °C until the complete consumption of the starting material (TLC check). The crude reaction mixture was diluted with 5 mL of ethyl acetate, dried over anhydrous Na2SO4, and filtered off and solvent evaporated. The crude product could be crystallized; however due to the relatively great losses on such a scale, filtration over short pad of silica gel gave pure phenyl methyl sulfone 2a in higher yield (32 mg, 68%). Representative procedure of the non-catalyzed oxidation of liquid sulfide into sulfone using 30%

aqueous solution of H2O2 under SFRC (scale-up)

To thioanisole 1a (20 mmol, 2.48 g) a 30% aqueous solution of H2O2 was added (5 mmol, 0.57 g), and the mixture was heated at 75 °C with stirring for 3 h in a round-bottom flask equipped with a reflux condenser. The mixture was cooled and 20 mmol of 30% aqueous solution of H2O2 was added (2.28 g). Two phases at the beginning of the reaction slowly turned into one. The mixture was further heated and stirred, and additional H2O2 was added (19 mmol, 2.17 g after 15 h and 10 mmol, 1.14 g after 40 h). Full conversion was reached in 65 h. The product already began to crystallize in part already during the reaction. The mixture was cooled, and the product 2a (2.87 g, 91%) as white solid was filtered off. The product gave a clean 1H NMR spectrum and could be used without further purification. The yield (91%) is significantly higher than on a 0.3 mmol scale (68%). 2-Fluorophenyl ethyl sulfone (2k). Colorless oil, yield 78%, (0.3 mmol of 1k, 0.66 mmol of H2O2), r.t. = 3 h. 1H NMR: δ 1.31 (t, J = 7.5, 3H), 3.33 (q, J = 7.5, 2H), 7.23–7.28 (m, 1H), 7.33–7.38 (m, 1H), 7.64–7.69 (m, 1H), 7.93–7.98 (m, 1H); 19F NMR: δ -109.6 (m, 1F); 13C NMR: δ 7.1, 50.0 (d, J = 2.6 Hz), 117.1 (d, J = 21.4 Hz), 124.8 (d, J = 3.8 Hz), 126.3 (d, J = 14.9 Hz), 130.8, 136.1 (d, J = 8.4 Hz), 159.6 (d, J = 255.4 Hz); IR (neat) 1473, 1451, 1317, 1264, 1225, 1143, 1123, 1072, 826, 766, 738 cm-1; HRMS: (ESI) calcd for C8H10FO2S 189.0386, found 189.0386 (M+1). 4-Methylphenyl-(2,2,2-trifluoroethyl) sulfone (2r). White crystals, yield 58%, (0.2 mmol of 1r, 0.44 mmol of H2O2 after 0 h and after 1 h), r.t. = 3.5 h, mp 89.5–91.2 °C. 1H NMR: δ 2.48 (s, 3H), 3.88 (q, J = 9.0 Hz, 2H), 7.40 (d, J = 8.1 Hz, 2H), 7.85 (d, J = 8.1 Hz, 2H); 19F NMR: δ -61.9 (t, J = 9.0 Hz, 3F), 13C NMR: δ 21.7, 58.5 (q, J = 31.2 Hz), 121.1 (q, J = 277.9 Hz), 128.5, 130.1, 135.5, 146.1; IR (neat): 1342, 1316, 1303, 1269, 1253, 1248, 1149, 1125, 1081 cm-1; HRMS: (ESI) calcd for C9H10F3O2S 239.0354, found 239.0357 (M+1). Anal Calcd for C9H9F3O2S: C, 45.38; H, 3.81. Found: C, 45.83; H, 3.59.

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4-Methoxyphenyl propargyl sulfone (2aa). White crystals, yield 82%, (0.3 mmol of 1aa, 0.50 mmol of H2O2 after 0 h and 0.25 mmol after 0.5 h), r.t. = 2.2 h, mp 76.6–79.3 °C. 1H NMR: δ 2.37 (t, J = 2.7 Hz, 1H), 3.90 (s, 3H), 3.94 (d, J = 2.7 Hz, 2H), 7.04 (d, J = 8.9 Hz, 2H), 7.91 (d, J = 8.9 Hz, 2H); 13C NMR: δ 48.6, 55.7, 71.9, 76.0, 114.3, 129.0, 131.1, 164.2; IR (neat): 3267, 2950, 1595, 1579, 1497, 1365, 1323, 1305, 1298, 1264, 1246, 1229, 1217, 1171, 1132, 1086, 1016, 881, 834, 826, 804, 756, 697 cm-1;

HRMS: (ESI) calcd for C10H11O3S 211.0429, found 211.0426 (M+1). Anal Calcd for C10H10O3S: C, 57.13; H, 4.79. Found: C, 57.32; H, 4.49. Cyclohexyl butyl sulfone (2jj). Colorless oil, yield 87%, (0.3 mmol of 1jj, 0.66 mmol of H2O2), r.t. = 4 h. 1H NMR: δ 0.96 (t, J = 7.4 Hz, 3H), 1.18–1.36 (m, 3H), 1.43–1.60 (m, 4H), 1.71–1.76 (m, 1H), 1.78–1.86 (m, 2H), 1.91–1.98 (m, 2H), 2.12–2.19 (m, 2H), 2.84 (tt, J = 12.2, 3.5 Hz, 1H), 2.88–2.93 (m, 2H); 13C NMR: δ 13.6, 21.9, 23.3, 25.0, 25.1, 25.1, 49.1, 60.7; IR (neat): 2935, 2859, 1453, 1296, 1264, 1126, 1110 cm-1;

HRMS: (ESI) calcd for C10H21O2S 205.1262, found 205.1259 (M+1).

Acknowledgement

Dr. D. Žigon at the Mass Spectroscopy Centre at the ‘Jožef Stefan’ Institute in Ljubljana for HRMS, Mrs. T. Stipanovič and Prof. B. Stanovnik for the elemental combustion analyses and Ministry of Higher Education, Science and Technology (P1-0134) for financial support are gratefully acknowledged. Notes and references

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7 See, for example: (a) O. O. Shyshkina, K. S. Popov, O. O. Gordivska, T. M. Tkachuk, N. V. Kovalenko, T. A. Volovnenko and Yu. M. Volovenko, Chem. Heterocycl. Compd., 2011, 47, 923–945; (b) H. Peng, Y. Cheng, N. Ni, M. Li, G. Choudhary, H. T. Chou, C.-D. Lu, P. C. Tai and B. Wang, ChemMedChem, 2009, 4, 1457–1468; (c) A. S. Newton, P. M. C. Glória, L. M. Gonçalves, D. J. V. A. dos Santos, R. Moreira, R. C. Guedes and M. M. M. Santos, Eur. J. Med Chem., 2010, 45, 3858–3863; (d) M. Radi, E. Petricci, G. Maga, F. Corelli and M. Botta, J. Comb.

Chem., 2005, 7, 117–122; (e) Y. Harrak, G. Casula, J. Basset, G. Rosell, S. Plescia, D. Raffa, M. G. Cusimano, R. Pouplana and M. D. Pujol, J. Med. Chem., 2010, 53, 6560–6571; (f) R. Ettari, C. Bonaccorso, N. Micale, C. Heindl, T. Schirmeister, M. L. Calabrò and M. Zappalà, ChemMedChem, 2011, 6, 1228–1237. 8 See, for example: (a) B. Zajc and R. Kumar, Synthesis, 2010, 1822–1836; (b) P. R. Blakemore, J. Chem. Soc.,

Perkin Trans. 1, 2002, 2563–2585. 9 L. Field, J. E. Lawson and J. W. McFarland, J. Am. Chem. Soc., 1956, 78, 4389–4394. 10 T. Doi, A. Orita, D. Matsuo, R. Saijo and J. Otera, Synlett, 2008, 55–60. 11 J. K. Fink, High Performace Polymers, William Andrew Inc., Norwich, NY, 2008. 12 M. M. Khodaei, K. Bahrami and M. Khedri, Can. J. Chem., 2007, 85, 7–11. 13 S. Yamazaki, Bull. Chem. Soc. Jpn., 1996, 69, 2955–2959. 14 I. Sheikhshoaie, A. Rezaeifard, N. Monadi and S. Kaafi, Polyheron, 2009, 28, 733–738. 15 Q. Xue, Z. Mao, Y. Shi, H. Mao, Y. Cheng and C. Zhu, Tetrahedron Lett., 2012, 53, 1851–1854. 16 S. Nlate, L. Plault and D. Astruc, Chem. Eur. J., 2006, 12, 903–914. 17 F. Gregori, I. Nobili, F. Bigi, R. Maggi, G. Predieri and G. Sartori, J. Mol. Catal. A: Chem., 2008, 286, 124–127. 18 A. Shaabani and A. H. Rezayan, Catal. Commun., 2007, 8, 1112–1116. 19 W. Al-Maksoud, S. Daniele and A. B. Sorokin, Green Chem., 2008, 10, 447–451. 20 C. Yang, Q. Jin, H. Zhang, J. Liao, J. Zhu, B. Yu and J. Deng. Green Chem., 2009, 11, 1401–1405. 21 V. Palermo, G. P. Romanelli and P. G. Vázquez, Phosphorus, Sulfur, Silicon Relat. Elem., 2009, 184, 3258–3268. 22 M. Kirihara, A. Itou, T. Noguchi and J. Yamamoto, Synlett, 2010, 1557–1561. 23 K. Sato, M. Hyodo, M. Aoki, X.-Q. Zheng and R. Noyori, Tetrahedron, 2001, 57, 2469–2476. 24 K. Jeyakumar and D. K. Chand, Tetrahedron Lett., 2006, 47, 4573–4576. 25 C. A. Gamelas, T. Lourenço, A. Pontes da Costa, A. L. Simplício, B. Royo and C. C. Romão, Tetrahedron Lett.,

2008, 49, 4708–4712. 26 M. Kirihara, J. Yamamoto, T. Noguchi, A. Itou, S. Naito and Y. Hirai, Tetrahedon, 2009, 65, 10477–10484. 27 K. Bahrami, M. M. Khodaei and S. Sohrabnezhad, Tetrahedon Lett., 2011, 52, 6420–6423. 28 K. Kamata, T. Hirano, N. Mizuno, Chem.Commun., 2009, 3958–3960. 29 S. Choi, J.-D. Yang, M. Ji, H. Choi, M. Kee, K.-H. Ahn, S.-H. Byeon, W. Baik and S. Koo, J. Org. Chem., 2001, 66, 8192–8198. 30 A. Zali, A. Shokrolahi, M. H. Keshavarz and M. A. Zarei, Acta Chim. Slov., 2008, 55, 257–260. 31 K. Bahrami, M. M. Khodaei and M. S. Arabi, J. Org. Chem., 2010, 75, 6208–6213. 32 A. Rostami and J. Akradi, Tetrahedron Lett., 2010, 51, 3501–3503. 33 D. H. R. Barton, W. Li and J. A. Smith, Tetrahedron Lett., 1998, 39, 7055–7058.

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Graphical abstract

Highly atom-economic, solvent- and catalyst-free oxidation of wide variety of liquid and solid sulfides into sulfones with 30% aqueous H2O2 is reported.

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Table 1 Solvent- and catalyst-free oxidation of sulfides with 30% aqueous H2O2

a

Product t (h) Yield

(%)b

1 2a 2 68 2 2b 3.2 65 3 2c 5.75 64 4 2d 8.25 69 5

2e 11 79

6

2f

1.5

95

7

2g

2.5

87

8

2h

3.75

80

9

2i

3

65

10

2j

3

80

11

2k

3

78

12

2l

8

80

13

2m

1.5

79

14

2n

1.75

93

15

2o

12.5

50

16

2p

2

82

17

2q

8

85

18

2r 3.5 58

19

2s 3.5 85

20 2t 2.5 78 21 2u 2.5 84 22 2v 3 72 23 2w 12 41 24 2x 2.25 68 25 2y 4.5 80 26 2z 0.85 65

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27

2aa 2.2 82

28 2bb 1.75 77

29 2cc 3 80 30 2dd 4 74 31

2ee

11

77

32

2ff

8

50

33 2gg 16 79 34 2hh 0.85 90 35 2ii 3 83 36 2jj 4 87 37 2kk 0.75 79 38 2ll 4 81 39

2mm 5 81

40

2nn 5 82

41

2oo 8 87

42

2pp 9 87

43

2qq

3

46

44

SO2

O

2rr

1.5

75

45

2ss

17

81

46

2tt

17

79

47

SO2

O2S

2uu

3

59

a Reaction conditions: 1 (0.2 or 0.3 mmol), 30% aq. H2O2 (2.2 or more equiv.) stirred at 75 °C. b Isolated yields.

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