H2O2 as Terminal Oxidant

A large number of catalysts have been reported in the literature for the H2O2 oxidation of sulfides to sulfoxides. These include various metal complexes (of transition metals and lanthanides), flavins, and henzdhiazoles. 

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. 

Tungsten-based catalytic systems for H2O2 oxidations of sulfides have attracted considerable interest, and some early reports include the use of H2WO4 [20]. More recently, various tungsten-catalyzed methods have been used [21-25]. 

Polyoxometalates WZnMn2(ZnW9034)2 were employed to oxidize sulfides to sulfones in moderate selectivity (85-90% selectivity with 10-15% of sulfone) [23]. 

Choudary and coworkers [25] subsequently extended the tungsten system to the use of layered double hydroxides (LDHs) with water as solvent and 30% H2O2 as the oxidant. Under these conditions the new catalysts LDH-W04 gave good turnover rates however, the selectivity of the oxidation of thioanisole was moderate, with a sulfoxide sulfone ratio of 88 12 (Eq. (8.5)). Other sulfide oxidations also gave a moderate selectivity (Eq. (8.5)) 

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The addition of acetic acid (0.5 equiv. to the substrate) to the catalyst system led to increased activity (doubling of yield) by maintaining the selectivity with 1.2 equiv. H2O2 as terminal oxidant. Advantageously, the system is characterized by a certain tolerance towards functional groups such as amides, esters, ethers, and carbonates. An improvement in conversions and selectivities by a slow addition protocol was shown recently [102]. For the first time, a nonheme iron catalyst system is able to oxidize tertiary C-H bonds in a synthetic applicable and selective manner and therefore should allow for synthetic applications [103]. 

NH4CIO4, NELtPFe) have been tested and the best results were obtained in acetonitrile as solvent in the presence of Ai-methylimidazole using H2O2 as terminal oxidant (Scheme 92). Under these conditions chromene derivatives could be converted into the corresponding epoxides 177 in yields between 55 and 98% and ee values of 88-95%. 

SCHEME 177. Coupled catalytic system for olefin dihydroxylation with H2O2 as terminal oxidant... 

The reactivity patterns for the alkane functionalization by iron(II) mononuclear complexes using H2O2 as terminal oxidant suggest that two reaction pathways are mainly associated with this type of chemistry one involving uncontrolled hydroxyl radicals, in particular produced by Haber-Weiss... 

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