Sulfides and Sulfoxides

Nitro ilkenes are gener illy prepared by the subsdnidon reacdon of fi-nirro sulfides and sulfoxides with a variety of carbon nucleophiles via an addidon-eliminadon sequence. This method is partiadarly usefiil for the preparadon of cyclic nitroalkenes fEq. 4.100. ... 

Since diorganyl sulfides and sulfoxides can be mutually interconverted electrochemi-cally [222] (for the oxidation of sulfides to sulfoxides and further to sulfones, see Sect. 9.2.3), sulfoxides take an intermediate position between sulfides and sulfones (Eq. 27). 

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]. 

This procedure, with slight modifications depending on the physical properties of the sulfide and sulfoxide in question, has been used to prepare a variety of sulfoxides as illustrated by examples provided in Table I In the case of very insoluble sulfides, co solvents such as methanol or dioxane may be employed Very... 

Under similar conditions, diphenyl sulfide, C6H5-S-C6H5, was found to be unreactive, indicating that the reactions listed in Table XII take place inside the pore structure, which is not accessible to bulky molecules such as diphenyl sulfide. The selectivity to sulfoxides (Table XII) is the result of a competition between sulfides and sulfoxides for the catalytic site dimethyl sulfide competes effectively, and the selectivity to sulfoxide is 97% with only 3% sulfone produced. The other reactant molecules give larger amounts of sulfones. However the reaction of dimethyl sulfide was carried out at 298 K, whereas the other sulfides reacted at the reflux temperature of acetone the temperature difference may explain part of the differences shown in Table XII. 

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. 

Bach and Korber reported in 1998that iron(I I) chloride could be used in combination with tert-butyloxycarbonyl azide (BocN3) for nitrene transfer to sulfides and sulfoxides [170], Whereas usually 1 equiv. of FeCl2 was employed in the imination of sulfoxides, catalytic amounts of this iron salt (25mol%) could also be applied (Table 3.13). 

Bolm and coworkers discovered that inexpensive and easy to handle iron(III) acetylacetonate [Fe(acac)3] was also capable of catalyzing nitrene transfer to sulfides and sulfoxides using various sulfonyl amides in combination with iodosylbenzene (Scheme 3.58) [174]. 

This catalytic system proved to be highly efficient. Thus, a low catalytic amount of Fe(acac)3 (5-10 mol%) permits the successful imination of a broad variety of substituted sulfides and sulfoxides. The best results were obtained with nosyl sulfonamide as nitrogen source (Figure 3.11). Moreover, as previously observed with FeCl2, this catalyst also iminates sulfides more readily than sulfoxides. 

The sulfides and sulfoxides can be interconverted. Oxidation of the sulfides with either photochemically-generated singlet oxygen or preferably with tetrabutylammonium periodate ( ) selectively oxidizes the sulfides to the sulfoxides and reduction of the sulfoxides with LiAlH4 converts them to the sulfides. (For these and other analytical methods, cf. Reference 7) Reduction of the sulfoxides with LiAlH4 showed that the resultant sulfides were identical to the native sulfides isolated from the aromatic fraction of the oil. 

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