Sulfides, Sulfoxides, and Sulfones

Although sufides are among the most straightforward of substrates to oxidize, the selective oxidation of sulfides to sulfoxides and to a lesser extent sulfones is often challenging for substrates other than simple dialkyl-, diaryl-, and alkylaryl-sulfides. In transition metal-catalyzed oxidations the relatively low reactivity of electron-deficient sulfides and their insolubility in polar solvents is problematic when H2O2 is employed as the terminal oxidant. 

Katsuki and Saito reported that di-p-oxo titanium complexes of chiral salen ligands serve as efficient catalysts for asymmetric oxidation of a range of sulfides using H 2O2 or UHP as terminal oxidants [149]. Enantioselectivities as high as 94% were achieved [149]. A monomeric peroxo titanium species was proposed as the active oxidant based on MS and NMR studies [150]. 

Mn catalysts that show activity in alkene or alcohol oxidation with H2O2 are potentially active in the oxidation of sulfides also. The Mn-tmtacn catalysts and a number of tn-sifu-formed complexes employing ligands such as 39 are examples of such catalysts (see above). These complexes were found to be highly active in the oxidation of sulfides to sulfoxides. For example, the dinudear manganese complex based on tmtacn (6) performs efficiently in the oxidation of aryl alkyl sulfides and generally results in fiJl conversion within 1 h. Unfortunately, as is often the case, in 

Hydrogen peroxide is a particularly attractive oxidant and holds a prominent position in the development of benign catalytic oxidation procedures. In recent years a number of highly versatile catalytic oxidation methods based on, for example, polyoxometalates [154], methyltrioxorhenium [155] or tungstate [4, 156] complexes in the presence of phase transfer catalysts, all using hydrogen peroxide as the terminal oxidant, have been introduced. Mn-catalyzed epoxidations, aldehyde 

New York, (b) Trost, B.M. and Fleming, I. (eds) (1991) Comprehensive Organic Synthesis, vol. 7, Pergamon Press, Oxford, 

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The 13C NMR data for representative three-membered sulfones and sulfoxides are given in Table 5. The chemical shifts of the sp3-hybridized a-carbon in the parent thiirane70 and the five-membered ring86 sulfide, sulfoxide and sulfone are 18.1, 31.7, 54.3 and 51.1, respectively, whereas those of cyclopropenone, diphenylcyclopropenone and dimethylcyclopropenone are 169.087, 148.788 and 157.9, respectively. 

Ohta and coworkers used a bacterium, Corynebacterium equi IFO 3730, rather than a fungus, to oxidize eight alkyl phenyl and p-tolyl sulfides to their respective sulfoxides (119, 120) of configuration R. Virtually all of the sulfur compounds were accounted for as the sum of uncreacted sulfide, sulfoxide and sulfone. The enantiomeric purities of the sulfoxides obtained were quite good and are shown below in parentheses. The formation of the allyl sulfoxides in high optical purity is noteworthy. The authors believe that the sulfoxides were formed by enantioselective oxidation of the sulfides rather than by enantioselective oxidation of racemic sulfoxides, since the yield of sulfoxides was greater than 50% in five of the ten oxidations reported (see also Reference 34). 

A final, rather different example which fits in appropriately here, in that it involves hydrogen exchange, is the measurement of equilibrium and rate constants for the base-catalyzed isomerization of unsaturated sulfides, sulfoxides and sulfones ... 

In summary, the direct insertion of zinc dust to organic halides is an excellent method for preparing a broad range of polyfunctional organozinc halides bearing various functional groups like an ester" , an ether, an acetate" , a ketone, cyano", halide" , N,N-bis(trimethylsilyl)amino °, primary and secondary amino, amide, phthalimide , sulfide, sulfoxide and sulfone , boronic ester , enone " or a phosphonate . An alternative method is based on transmetalation reactions. 

This early example is one of the numerous synthetic utilizations of the cycloadditions of olefins activated by sulfur atoms at various oxidation levels (vinyl sulfides, sulfoxides and sulfones = -S(0) R, n = 0,1, 2). Most of the work carried out in this field has been pertinently reviewed and discussed in a 1988 Tetrahedron Report with 204 references [485], Some specific aspects are underlined here and recent examples given. 

There has been much recent interest in difluoromethyl phenyl sulfones and sulfoximines regarding their use as difluoromethylating agents. Fluorine, proton, and carbon NMR data for these compounds, along with those of analogous sulfide, sulfoxide, and sulfonic acid are provided in Scheme 4.24. No obvious trends are observed for most of the data, the exception is as the oxidation state of the sulfur increases, the CF2H carbons become shielded. 

Prakash, G. K. S. Hu, J. Olah, G. A. Preparation of tri- and difluoromethylsilanes via an unusual magnesium metal-mediated reductive tri- and difluoromethylation of chlorosilanes using tri- and difluoromethyl sulfides, sulfoxides, and sulfones. J. Org. Chem. 2003, 68, 4457-4463. 

Mohraz, M., Jiam-qi, W., Heilbronner, E., Solladie-Cavallo, A., and Matloubi-Moghadam, F., Some comments on the conformation of methyl phenyl sulfides, sulfoxides, and sulfones, Helv. Chim. Acta, 64, 97, 1981. 

Photoelimination of Sulfur from Sulfides, Sulfoxides, and Sulfones... 

In this chapter we shall discuss some of the rich and varied chemistry of these, and other, organosul-fur compounds. The stabilization of anions by sulfur is where we begin, and this theme runs right through the chapter. We will start with sulfides, sulfoxides, and sulfones. Sulfur has six electrons in its outer shell. As a sulfide, therefore, the sulfur atom carries two lone pairs. In a sulfoxide, one of these lone pairs is used in a bond to an oxygen atom—sulfoxides can be represented by at least two valence bond structures. The sulfur atom in a sulfone uses both of its lone pairs in bonding to oxygen, and is usually represented with two S=0 double bonds. 

Trifluoromethyl-substituted sulfides, sulfoxides, and sulfones can undergo [4-F 2]-cycloaddition reactions with various dienes under thermal conditions (Table 2) sec also formation of 15 and 17. 5... 

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