Of sulfides and sulfoxides

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°. 

The oxidative imination of sulfides and sulfoxides via nitrene transfer processes leads to N-substituted sulfilimines and sulfoximines. This reaction is interesting as chiral sulfoximines are efficient chiral auxiliaries in asymmetric synthesis, a promising class of chiral ligands for asymmetric catalysis and key intermediates in the synthesis of pseudopeptides [169]. However, very few examples of such iron-catalyzed transformations have been described. 

Different metal complexes have shown the ability to catalyze these imination reactions, such as rhodium, copper, and iron.22 In 2005, Bolm found that the disilver(I) complex described in Section 6.2.2 catalyzes the imination of sulfides and sulfoxides... 

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. 

Puri and co-workers [128,129] found that sulfur is introduced to the carbon matrix by interactions with oxygen-containing functional groups or by addition to unsaturated sites. Thus, the substitution reactions with quinone and phenolic groups lead to thioquinone and thiophenol structures, and the addition reactions result in the formation of sulfide and sulfoxide groups. Moreover, Chang showed the evidence of thiolactones in a number of carbonaceous materials after reaction with various sulfur-containing materials [126]. 

OXIDATION OF SULFIDES AND SULFOXIDES 10.1. Catalytic reactions with dioxygen... 

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). 

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