Oxidation of sulfides and sulfoxides

1 THE FIRST EXAMPLE OF DIRECT OXIDATION OF SULHDES TO SULFONES 

2 SELECTIVE OXIDATION OF SULFIDES TO SULFOXIDES AND SULFONES USING 

1 Oxidation of Benzyl 4-bromobenzyl sulfide to Benzyl 4-broinobenzyl 

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

OXIDATION OF SULFIDES AND SULFOXIDES 10.1. Catalytic reactions with dioxygen 

Henbest and Trocha-Grimshaw have shown that sulfoxides may be oxidized to sulfones in the presence of iridium and rhodium complexes [139, 140]. Oxidations were studied by passing air through a solution of the sulfoxide in hot wopropanol containing 10% water in the presence of the catalyst. Sulfones were formed when chlorides of iridium or rhodium were used while chlorides of ruthenium, osmium and palladium were not effective. Under reaction conditions the chlorides should be converted wholly or partly to metal sulfoxide complexes. 

The related rhodium complex H[RhCl4(Me2SO)2] was similarly effective as a catalyst for the oxidation of dimethylsulfoxide to the sulfone, equation (95). 

This complex was used to examine the catalytic oxidation of two other sulfoxides. Diphenylsulfoxide gave a 65% yield of sulfone in 22 hr and dibenzyl sulfoxide gave a 30% yield of the corresponding sulfone after 48 hr. It appears that the ease of oxidation diminishes with the substituents in the order Me Ph PhCH2. 

Diallylsulfide can also be oxidized selectively to the sulfone without oxidation of the unsaturated side chain [144]. Coordination of the sulfur is apparently strong enough to exclude the olefinic ligand from the reactive center since in the absence of the sulfur, olefins are readily epoxidized by r-BuOOH in the presence of molybdenum complexes. The product of hydroperoxide oxidation of sulfides depends on the reaction conditions [143]. Sulfoxides are obtained at temperatures below 50 °C using excess sulfide whereas sulfones are the predominant product at temperatures above 55 °C using excess hydroperoxide. 

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SCHEME 104. Solvent-free oxidation of sulfides and sulfoxides using (NEl4)3PMoi2O40 on apatite... 

Oxidation of sulfides and sulfoxides using Oxone dispersed on silica gel or alumina was reported . A study of surface mediated reactivity of Oxone compared its reactivity with that of ferf-butyl hydroperoxide. Oxidation of sulfides to sulfones in aprotic solvents mediated by Oxone on wet montmorillonite or clay minerals proceeds in high yields. Interestingly, when Oxone on alumina is applied for selective oxidation of sulfides in aprotic solvents, the product distribution is temperature-dependent and sulfoxides or sulfones are obtained in good to excellent yields (equation 56) . ... 

In 2002, Ichihara and coworkers reported on the utilization of a Keggin-type phos-phomolybdate (NfLOsPMonOzu) on apatite as catalyst for the solvent-free epoxidation of olefins (see Section III.A.3.c) and the oxidation of sulfides and sulfoxides in the presence of urea hydrogen peroxide (Scheme 104)297. Chemoselectivities of the oxidation of the sulfides were good [product ratios (sulfide/sulfoxide) 84/16 up to 91/9] depending on the substrate, temperature and reaction time. 

Oxidation of sulfides and sulfoxides, as discussed above, and alkylation of sulfinate salts are the most common methods used to obtain sulfones for synthetic purposes [71, 109-113], A hydrogen peroxide-urea-phthalic anhydride system was recently proposed as a mild convenient reagent for the efficient preparation of sulfones from organic sulfides [114]. 

Sulfones. The oxidant is useful for oxidation of sulfides and sulfoxides to sulfones in CH2Cl,-CH3COOH at - 10°. 

Oxidation of sulfides and sulfoxides. The oxidation can be controlled by temperature. For example, benzenesulfenylmethyl azide is converted to the sulfoxide (96% yield) at -40°C, but further oxidation at -20°C gives the sulfone (93% yield). 

Oxidation of sulfides and sulfoxides. The oxidation with Oxone is controllable by reaction temperature and time. For example, p-methylthiobenzoic acid is oxidized to the sulfoxide at temperature below 5°C in 5 min, and to the sulfone at room temperature while extending the reaction time to 1 h. Both reactions are carried out in aqueous acetone in the presence of NaHCOj and NaOH. 

The mechanisms discussed below were obtained from DFT calculations [22] using Gaussian 03 [23], The oxidation of sulfides and sulfoxides by M0O2CI2 has been described within a study of several molybdenum-derived catalysts [24], while the reduction of sulfoxides and sulfones is reported for the first time in this work. 

The same system (CPM0O2X formed in situ from CpMo(CO)3X (D) and TBHP) is active in the oxidation of sulfides and sulfoxides, both with HjOj and TBHP, and for X = Cl [15]. The reaction mechanism was investigated with DFT [24] in order to determine whether the same pathways were available for olefin and RjS/RjSO oxidation. In this study, a lower energy path for the conversion of G into the peroxo complex E was found by addition of one equivalent of TBHP, which is equivalent to the reaction of C with two molecules of TBHP. The same applies to H2O2 as oxidant. This was not, however, the cmcial step that determined the catalytic activity of the peroxo complex in epoxidation (Fig. 24.4). 

More recently benzyl triethylammonium permanganate has been reported to be a mild, one-phase oxidizer of sulfides and sulfoxides to sulfones in 63-98% yield. 

Coupled cluster and DFT (density functional theory) computations for the permanganate oxidation of sulfides and sulfoxides indicated a [l,3]-dipolar cycloaddition of permanganate (Scheme 1), as opposed to earlier mechanisms involving attack of the sulfur atom by one of the oxygens or by manganese. The ab initio activation enthalpies are in reasonable agreement with the experimental data, but the ab initio activation entropies are not. " ... 

DFT computations on the oxidations of sulfides and sulfoxides with hypochlorite ion, hypochlorous acid, and alkyl hypochlorites showed that the anion transforms sulfides... 

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