Sulfoxidation transition metal peroxides

Neumann and co-workers have used sandwich-type polyoxometalates as catalysts for such reactions.305 The sandwich consists of transition metal ions between two Keggin anions, as in Ki2WZnMn(II)2(ZnW9034)2. In some cases, quaternary ammonium salts were used as the cations to take the catalyst into organic solvents. Sulfides were oxidized to sulfoxides with 30% aqueous hydrogen peroxide in 85-90% yields, with some sulfone also being formed. The system was more than 99% selective in the conversion of cyclooctene to its epoxide. The system also shows good selectivity between double bonds (6.57), probably the result of the bulky anion and the increased electron density in alkyl-substituted double bonds. 

Sulfides react faster with hydrogen peroxide and alkyl hydroperoxides than do alkenes. For this reason, transition metal catalysts are rarely necessary, but these reactions are acid catalyzed and first order in both sulfide and peroxide. The acid (HX) can be as weak as alcohol or water but the "effectiveness (of the oxidation) is determined by the pXa of the acid. Sulfides also react faster with peroxides than do ketones (see the Baeyer-Villiger reaction, sec. 3.6). Formation of the sulfone in these reactions is straightforward, but requires more vigorous reaction conditions. It is usually easy to isolate the sulfoxide from oxidation of a sulfide. Direct conversion of a sulfide to a sulfone requires excess peroxide and vigorous reaction conditions (heating, long reaction times, more concentrated peroxide). 

High-valent transition metal complexes, e.g. complexes of Mo, V and Ti, catalyze numerous oxidations of organic substrates by alkyl hydroperoxides, such as epoxidation of alkenes, oxidation of tertiary amines to the corresponding N-oxides, of sulfides to sulfoxides. It is now generally understood that the active intermediates in these transformations consist of metal-alkyl peroxide complexes M—OOR in their triangular form (7a) resulting... 

Transition Metals as Catalysts Oxidation of various sulfides to sulfones by H2O2 mediated by TiCl3 was reported by Watanabe [19]. Quantitative yields of sulfoxide were obtained within 5-15 min with no overoxidation to sulfone. However, a sevenfold excess of hydrogen peroxide and 2 equivalents of TiCls per mole of substrate were employed. 

Peroxides, high-valent transition metal 0x0 species, or peroxo radicals are the typical oxygen donor reactive species, 0 , in sulfoxidation... 

One more example is related to the Unalool oxidation with hydrogen peroxide using a [Ln(OH)(H20)(naphthalenedisulfonate)], where Ln=Nd, Pr, La [104], though the nature of the active sites is not quite clear, since the MOF contains no transition metal ions, like Ti, Fe, Co, and so on. Similarly, oxidation of methylphenylsulfide to sulfoxide and sulfone with H O is catalyzed by scandium ions in a [Sc2(bdc)3] MOF [105]. Among other oxidation reactions, the use of a Cu-MOF ([Cu3(btc)J) in the conversion of phenols to higher phenols deserves attention and the nature of activity is not so elusive [106]. 

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