H2O2 oxidant of sulfides

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. 

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]. 

Stoichiometric oxidation reactions with hydroperoxide 18 were studied, and it was found that 18 oxidizes sulfides to sulfoxides in a highly selective manner [42, 43]. It was later demonstrated that these fiavins can participate as catalysts for the H2O2 oxidation of sulfides to sulfoxides [44, 45]. 

Thus a number of enzymes have been shown to be able to control the oxidation of sulfides to optically active sulfoxides most extensive investigations have concentrated on mono-oxygenases (e.g. from Acinetobacter sp., Pseudomonas putida) and haloperoxidases1 071 (from Caldariomyces fumago and Coral I ina officinalis). A comparison of the methodologies11081 led to the conclusion that the haloperoxidase method was more convenient since the catalysts are more readily available (from enzyme suppliers), the oxidant (H2O2) is cheap and no cofactor recycling is necessary with the haloperoxidases. Typical examples of haloperoxidase-catalysed reactions are described in Scheme 24. 

TABLE 26. CPO-catalyzed oxidations of sulfides using H2O2 or TBHP as oxidants producing sulfoxides (ff)-23 (yields are given and ee values are given in parentheses)... 

In combination with H2O2 (salen)Mn(III) complexes 173a, b, i-n have also been employed by Jacobsen and coworkers as catalysts for the asymmetric oxidation of sulfides to sulfoxides, without a need for additives. From the structurally and electronically different Mn-salen catalysts screened, 173i turned out to be the most active and selective one (equation 58) . While dialkyl sulfides underwenf uncafalyzed oxidation with H2O2, aryl alkyl sulfides were oxidized only slowly compared wifh fhe cafalyzed pathway. Using... 

A solid-phase sulfur oxidation catalyst has been described in which the chiral ligand is structurally related to Schiff-base type compounds (see also below). A 72% ee was found using Ti(OPr-i)4, aqueous H2O2 and solid-supported hgand 91 . More recently, a heterogeneous catalytic system based on WO3, 30% H2O2 and cinchona alkaloids has been reported for the asymmetric oxidation of sulfides to sulfoxides and kinetic resolution of racemic sulfoxides. In this latter case 90% ee was obtained in the presence of 92 as chiral mediator. ... 

WO3-3O % H2O2-CINCHONA ALKALOIDS A NEW HETEROGENEOUS CATALYTIC SYSTEM FOR ASYMMETRIC OXIDATION OF SULFIDES AND KINETIC RESOLUTION... 

The oxidation of sulfides is the most straightforward method for the synthesis of sulfoxides and sulfones.During recent years, very useful procedures involving aqueous hydrogen peroxide (H2O2) as the terminal oxidant are noteworthy due to the effective oxygen-content, low cost, safety in storage and operation and... 

Table 9.2 Oxidation of sulfides RSR to sulfoxides using the H2O2 (20 mmol)/ZrCl4...

V(V), Mo(VI) and Ti(IV) derivatives proved to be both effective and selective in the oxidation of sulfides with alkyl hydroperoxides324. As for H2O2, vanadium is at least twofold more efficient than molybdenum in oxidation and much more selective, as proved by the data obtained with the cyclic disulfide 2-aryl-1,3-dithiolane 80. A large predominance of the tra .y-S-oxide 81 over the c/.v-dcri vati ve 82 is obtained for all the systems investigated (equation 46). However, the diastereoselectivity exhibited by V(V)/TBHP is remarkable315-325. 

The half-life for the oxidation of sulfide in seawater with oxygen was 30 hours at 25°C. If one uses the [H2O2] = 1.0 x 10 7M found (54.55) for surface seawater then the half-life for the oxidation of sulfide by peroxide in seawater would be 2800 hours. At concentrations of H2C>2 > 10- M, the oxidation of H2O2 with H2S becomes competitive with 02. Such concentrations of H2O2 are found in rain waters (5S) thus, peroxide oxidation of H2S may be more important than oxygen oxidation in aerosols or rainwaters. 

One of the simplest methods of oxidation of sulfides to sulfoxides is the use of H2O2 in acetone or preferably in methanol. The oxidation normally proceeds highly selectively under very mild conditions and can be applied to the preparation of various acid-sensitive sulfoxides such as allylic sulfoxides... 

In 1995, Bolm and Bienewald introduced a new, very practical method for the asymmetric catalytic oxidation of sulfides [44]. In the presence of vanadium complex prepared in situ from VO(acac)2 and 23 reactions of various sulfides or dithianes fike 24 with aqueous hydrogen peroxide afforded the corresponding sulfoxides with enantiomeric excesses of up to 85% (Eq. 2). Only traces of the corresponding sulfones were observed. The transformation can easily be carried out in open vessels at room temperature using inexpensive H2O2 as oxidant. 

The TFE- or HFIP-activated H2O2 oxidant promotes not only epoxidation [34, 47, 48], but also Baeyer-Villiger oxidation [48], oxidation of sulfides to sulfoxide [34, 49], and oxidation of thiols to disulfides [5, 50]. The nature of the weaker nucleophilicity and higher acidity of TFE as compared with ethanol is useful as a solvent for Pd-catalyzed asymmetric hydrogenation of trifluoromethylimines [51]. 

Oxidation. A protocol for oxidation of sulfides to sulfoxides with H2O2 in EtOH includes Tf20. The stepwise conversion of arylamines to nitrosoarenes and thence nitro-arenes by H2O2 using heteropolyacids as catalyst has been reported. ... 

Oxidation. For promoting oxidation of sulfides to sulfoxide by H2O2 metal catalysts now include HAUCI4 4H20 andatitanium complex derived from tris(3-t-butyl-2-hydroxy-benzy l)amine. In the latter report good turnover while employing only 0.01 -1 % of the catalyst has been observe. 

Sulfoxides. A convenient and simple method for oxidation of sulfides involves the use of basic H2O2 and acetonitrile in methanol. 

One of the simplest methods of oxidation of sulfides to sulfoxides is die use of H2O2 in acetone or preferably in methanol. The oxidation normally proceeds highly selectively under very mild conditions and can be applied to the preparation of various acid-sensitive sulfoxides such as allylic sulfoxides (11), silyl-substituted vinyl sulfoxides (12) and thietane sulfoxides (13). The oxidation is accelerated in the presence of acetic acid although selective oxidation for 5-monoxides is possible as shown in equations (14), (15) and (16). ° However, the use of excess H2O2 often leads to overoxidation to give sulfones. Many metal salts such as SeO , V20s, TiCla and VO(acac)2 also function as good catalysts. 

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