Dialkyl sulfides, oxidation

The same catalysts have been investigated for the asymmetric oxidation of dialkyl sulfides using PhIO as the oxidant in CH3CN perfluorooctane.[54] Although the conversions (>80%) and selectivities to sulfoxides (>90%) were generally good, and the more heavily fluorinated catalysts could be recycled 4 times with only small drops... 

Sulfur(II)-containing compounds possess the reducing activity and react with hydroperoxides and peroxyl radicals [1-5]. They are employed as components of antioxidant additives to lubricants and polymers [30-35]. Denison and Condit [36] were the first to show that dialkyl sulfides are oxidized by hydroperoxides to sulfoxides and then to sulfones... 

Enantiomerically pure sulfoxides play an important role in asymmetric synthesis either as chiral building blocks or stereodirecting groups [156]. In the last years, metal- and enzyme-catalyzed asymmetric sulfoxidations have been developed for the preparation of optically active sulfoxides. Among the metal-catalyzed processes, the Kagan sulfoxidation [157] is the most efficient, in which the sulfide is enantioselectively oxidized by Ti(OzPr)4/tBuOOH in the presence of tartrate as chirality source. However, only alkyl aryl sulfides may be oxidized by this system in high enantiomeric excesses, and poor enantioselectivities were observed for dialkyl sulfides. 

Enzymes, in particular peroxidases, catalyze efficiently the enantioselective oxidation of alkyl aryl sulfides and also dialkyl sulfides, provided that the alkyl substituents are sterically differentiable by the enzyme. The peroxidases HRP, CPO, MP-11, and the mutants of HRP, e. g. F41L and F4IT, were successfully used as biocatalysts for the asymmetric sulfoxidation (Eq. 14). A selection of sulfides. 

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... 

The hydrotrioxides oxidize also simple dialkyl sulfide such as dimethyl and diethyl sulfides to the corresponding sulfoxides and the latter to sulfones in high yields (Scheme 19) ... 

Enantiopure 2,2,5,5-tetramethyl-3,4-hexanediol was prepared by Yamanoi and Imamoto [46]. A combination of Ti(0-i-Pr)4 with this diol (1 2) gives a chiral catalyst for sulfide oxidation with cumyl hydroperoxide in the presence of 4A molecular sieves in toluene. At -20°C p-tolyl methyl sulfoxide (95% ee) was obtained in 42% yield together with 40% sulfone, A kinetic resolution increased, to some extent, the enantiomeric excess of the product, that is, at lower conversion (20% yield) the enantiopurity of the resulting sulfoxide was only 40% ee. This catalytic system is ineffective for the enantioselective oxidation of dialkyl sulfides. 

Dialkyl sulfides are simultaneously oxidized and a-chlorinated by a combination of S02C12 and KNOs or AgN03 (equation 123)868. 

Several approaches to the enantioselective oxidation of sulfides have been reported,61 including enzymatic approaches,62 use of optically pure oxidants,63 and several modifications of the Sharpless epoxidation procedure.63,64 The success of these procedures is somewhat substrate dependent for example, dialkyl sulfides and more complex substrates can give unpredictable results. 1,3-Dithiane itself is oxidized with only ca. 20% ee optically pure DiTOX has, however, been obtained by resolution.65... 

The typical S-oxidation with BVMOs allows the formation of chiral sulfoxides from organic sulfides. This oxidation has received much interest in organic chemistry due to its use in the synthesis of enantiomerically enriched materials as chiral auxiliaries or directly as biologically active ingredients. This reaction has been studied extensively with CHMO from Adnetohacter showing high enantioselectivi-ties in the sulfoxidation of alkyl aryl sulfides, disulfides, dialkyl sulfides, and cychc and acyclic 1,3-dithioacetals [90]. CHMO also catalyzes the enantioselective oxida-hon of organic cyclic sulfites to sulfates [91]. 

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