Oxidation of chalcogenides using the peroxo complex of molybdenum [MoO(O2)2(H2O)(hmpa)], hmpa = hexamethylphosphoramide

Article J. Braz. Chem. Soc., Vol. 20, No. 7, 1256-1261, 2009. Printed in Brazil - ©2009 Sociedade Brasileira de Química 0103 - 5053 $6.00+0.00 Oxidation of Chalcogenides using the Peroxo Complex of Molybdenum [MoO(O2)2(H2O)(hmpa)], hmpa = hexamethylphosphoramide Jitender M. Khurana,* Arpita Agrawal and Sanjay Kumar Department of Chemistry, University of Delhi, Delhi, 110007, India A oxidação de diversos selenetos, sulfetos, teluretos, selenóxidos, sulfóxidos e teluróxidos com o complexo [MoO(O2)2(H2O)(hmpa)] [hmpa = (Me2N)3PO] em tetraidrofurano a 65-70 °C, para produzir as correspondentes selenonas, sulfonas e teluronas em altos rendimentos, é relatada neste trabalho. This work reports the oxidation of a variety of selenides, sulfides, tellurides, selenoxides, sulfoxides and telluroxides with [MoO(O2)2(H2O)(hmpa)] [hmpa = (Me2N)3PO] in tetrahydrofuran at 65-70 °C to produce the corresponding selenones, sulfones and tellurones in high yields. Keywords: chalcogenides, sulfide, selenide, telluride, oxidation, [MoO(O2)2(H2O)(hmpa)] Introduction The organic chemistry of sulfur, selenium and tellurium has been actively investigated.1 The vast chemistry of sulfoxides and sulfones makes them useful synthons and intermediates for construction of chemically and biologically significant molecules.2,3 The chemistry of selenones is of particular interest due to their thermal instability (fragmentation with olefin formation)4 and their ability to stabilise adjacent anionic centres.5 Although a plethora of reagents is known for oxidation of sulfides such as potassium permanganate,6 ortho-iodoxybenzoic acid,7 periodic acid or molecular bromine,8 mercury(II) oxide-iodine9 and benzeneseleninic acid,10 the oxidation of selenides and tellurides11 has received only a scant attention. Many of these procedures employ environmentally unfavourable reagents, solvents or catalysts. Formation of by-products and high cost are other drawbacks of reported procedures. Recently, a large number of transition metal compounds have been used as catalysts for oxidation of sulfides using H2O2.12 One of the major problems with this method is the concomitant decomposition of H2O2, which therefore requires the use of a large excess of H2O2 to achieve quantitative conversion. As an example, the procedure reported by Brahami13 for the oxidation of sulfide to sulfone requires 20 equivalents of H2O2 and 5 equivalent of ZrCl4 per mole of sulfide. These results prompted us to *e-mail: search for a new procedure for the oxidation of sulfides, selenides and tellurides. Peroxo complexes of molybdenum,14 although known for some time, have not been extensively studied as oxidants for organic substrates. So far, aqua(hexamethylphosphoramide) oxodiperoxomolybdenum(VI), [MoO(O2)2(H2O)(hmpa)], has been used only for epoxidation of olefins,15 oxidation of primary aromatic amines to corresponding nitroso derivatives16 in the presence of H2O2 and for alkoylation of organopalladium compounds.17 Our effort has been directed towards the extension of the chemistry of this peroxomolybdenum complex to other organic substrates. The oxidation chemistry of peroxometal complexes, namely MoVI and WVI derivatives, closely resembles that of simple peroxides.18 Therefore we decided to investigate the application of this Mo complex for oxidation of chalcogenides without the aid of hydrogen peroxide. Results and Discussion This is the first report on the oxygenation of sulfides, selenides, tellurides, sulfoxides, selenoxides and telluroxides to the corresponding sulfones, selenones and tellurones in high yields using [MoO(O 2)2(H2O) (hmpa)] in tetrahydrofuran (THF) at 65-70 °C. Firstly, the oxidation of variety of diaryl, aryl alkyl and dialkyl selenides and selenoxides was carried out. The oxidation of the selenides was complete in 5-17 h as monitored by thin layer chromatography (TLC) using 1:3 molar Vol. 20, No. 7, 2009 1257 Khurana et al. ratio of substrate:Mo complex in dry THF at 65-70 °C, whereas selenoxides underwent complete oxidation in 5-8.5 h using 1:1.5 molar ratio of substrate:Mo complex in dry THF at reflux temperature. The optimum reaction conditions were identified using diphenyl selenide and diphenyl selenoxide as the model substrates. The reactions of diphenyl selenide were carried out in various solvents, e.g., acetonitrile, toluene, carbon tetrachloride, methanol, dimethyl formamide and 1,2-dichloroethane at ambient temperature and under reflux. No oxidation was observed in these solvents at ambient temperature, while only a small amount of diphenyl selenone was observed by TLC after 18-20 h of refluxing. THF was the solvent of choice for oxidation, as diphenyl selenide underwent complete oxidation in 14 h under reflux using 1:3 molar ratio of substrate to Mo complex. The reaction was not complete when a lower molar ratio of substrate to Mo complex was used. Selenoxides could be oxidized to the corresponding selenones using 1:1.5 molar ratio of substrate to Mo complex under similar conditions. It is clear from Table 1 that diaryl selenides and selenoxides undergo slower oxidation compared to dialkyl selenides and selenoxides. Aryl alkyl selenides did not undergo any oxidation. All results are listed in Table 1. Subsequently, the oxidation of various diaryl, alkyl aryl, phenyl benzyl and dialkyl sulfides and sulfoxides to the corresponding sulfones was achieved successfully in high yields using [MoO(O2)2(H2O)(hmpa)] in THF at 65-70 °C. The oxidation of sulfides and sulfoxides was much faster as compared to selenides and selenoxides, and were completed in less then 3 h. Similarly to selenoxides (Table 1), sulfoxides required half the molar ratio of Mo complex (1:1.5) when compared to sulfides for complete oxidation. These results are listed in Table 2. The scope of the reagent was extended for the oxidation of tellurides and telluroxides, for which not many oxidizing agents are known. We report herein a convenient procedure for oxidation of aromatic and aliphatic tellurides and telluroxides with [MoO(O2)2(H2O)(hmpa)] in THF at 65-70 °C to the corresponding tellurones. The oxidations were complete in 1.5-3 h. The reactions of tellurides and telluroxides were faster than those of selenides and selenoxides, but of similar order if compared to sulfides and sulfoxides. Telluroxides required half the molar ratio of complex when compared to tellurides. All results are listed in Table 3. The oxidations of chalcogenides are summarised in Scheme 1. No deselenized, desulfurized or detellurized products were obtained in any of these reactions. No dehalogenation was observed in substrates containing halogen on phenyl ring. Oxidation of selenides, sulfides and tellurides to the corresponding dioxides has been observed to proceed via Table 1. Oxidation of selenides and selenoxides with [MoO(O2)2(H2O)(hmpa)] in THF at 65-70 °C Reaction number Substrates (1) Molar ratio (a) (S:Mo complex) Time / h Yield (2) / (%) 1 2 Dip (...truncated)