Sulfoxonium salts

The additional electronegative oxygen atom in the sulfoxonium salts stabilizes these ylides considerably, relative to the sulfonium ylides. ... 

Even sulfoxides, in which nucleophilicity is decreased by the additional oxygen, can be alkylated by methyl iodide. These sulfoxonium salts have useful synthetic applications as discussed in Section 2.5.1. 

Although radical cations are generated in some electron-irradiated monomers (e.g., vinyl ethers or epoxies), efficient cationic polymerization is not observed. Under certain conditions (addition of iodonium, sulfonium, or sulfoxonium salts, cationic polymerization with the use of electron beam irradiation can be induced. Several studies on radiation cross-linking of elastomers support the concept of ionic mechanism. ... 

Nucleophilic substitutions at optically active sulfur sites in sulfinic esters, sulfoxides, sulfoxonium salts, sulfilimines proceed with inversion. Much of the previous literature is cited by Cinquini et at. (1967), who gives an interesting Walden cycle in (124). The actual inversion step is presumed to be the attack of water on the sulfoxonium intermediate, which is isolable (Hogeveen et al., 1966). 

We have mentioned cations in this chapter several times and now we will gather the various ideas together. Cations are stable on the sulfur atom itself, as you have just seen in sulfonium and sulfoxonium salts. They are stable on adjacent carbon atoms since the sulfur atom contributes a lone pair to form a C=S+ n bond, and they are stable on the next carbon atom along the chain since sulfur contributes a lone pair to form a C-S+ o bond in a three-membered ring. 

The formation and reactions of sulfoxonium ylids demonstrate how sulfoxides occupy a useful and interesting part of the middle ground between sulfides and sulfones—they are weakly nucleophilic, like sulfides (and can be alkylated with methyl iodide to give sulfoxonium salts as we have just seen), but at the same time they stabilize anions almost as well as sulfones. However, sulfoxides are perhaps the most versatile of the three derivatives because of a good deal of chemistry that is unique to them. There are two reasons why this should be so. 

The generally accepted mechanism6 of Pummerer rearrangement is the one in which there is an initial attack on the sulfoxide oxygen atom by an electrophilic species, e.g., protonation or acylation. Acylation is followed by proton abstraction by a base from the a-carbon atom of the sulfoxide to form an ylide, which rapidly eliminates an acetate anion to form the a-sulfonium salt. Addition of acetate anion to the sulfonium intermediate completes the formation of the a-functionalized sulfide. Ylide formation from sulfoxonium salts is well recognized, and this aspect... 

An intramolecular cyclization of -hydroxysulfoxides effected by At-chloro-succinimide, A bromosuccinimide, or sulfuryl chloride gives 1,2-oxathietane 2-oxides, for example, 496, via a sulfoxonium salt. These cyclic sulfoxonium intermediates have been suggested in the conversion of /3-hydroxy-sulfoxides to /3-chlorosulfones, and to a,j3-unsaturated sulfones, in the conversion of /3-ethoxycarbonylsulfoxides to alkenes and of /3-hydroxysulfides to /3-hydroxysulfoxides, and in the conversion of 3-hydroxy- 3-vinylsulfoxides (e.g., 497) to useful terpene building blocks. The yields of the products from 497 vary somewhat depending on the stereochemistry about the two chiral centers. The more highly substituted 1,2-oxathietane 2-oxides are the most stable, because of... 

Sulfoxides (1) can react as nucleophiles, this being demonstrated by their ability to form sulfoxonium salts (16) with strong acids (Scheme 10). The nucleophilicity of sulfoxides is... 

Sulfoxonium ylides are useful in the preparation of cyclopropane derivatives by reaction with a,p-unsaturated carbonyl compounds (see Chapter 10, p. 189). Sulfoxonium salts (26) are intermediates in the Moffatt oxidation (Scheme 15a) (see Chapter 5, p. 66), and they also undergo several other useful reactions (Schemes 15b-15d). The Pummerer rearrangement involves an oxosulfonium ylide intermediate (see Chapter 5, p. 68). 

Dimethyl sulfoxide (23) (see Chapter 5, p. 66) is sufficiently nucleophilic to react with methyl iodide to form the corresponding sulfoxonium salt, which on treatment with a powerful base affords the sulfoxonium methylide (33) (Scheme 14). The reaction in applicable to other alkyl halides. 

Sulfones and sulfoxides are not affected by borohydrides in alcohol media [RJl] except in the presence of transition metal salts such as FeClg, C0CI2, or TiCl4 [CH3, G02, KT2, LZl, W4] (Figure 5.8). The sulfides obtained are not reduced under these conditions but are desulfurized in the presence of the NP+ salts [G02]. Another possibility is to treat the sulfoxides with NaCNBHg in MeOH. This reduction takes place via the sulfoxonium salt [PSl] (Figure 5.8). 

The conversion of dihydrothiazine oxides into dihydrothiazine dioxides is discussed later (Section VII,B,1). Treatment of the sulfoxide 208a with triethyloxonium tetrafluoroborate yielded the salt 216, which was converted into the (S)-oxide 217 by sodium hydroxide. The formation of the same sulfoxonium salt 218, of unestablished configuration at sulfur, was observed by NMR spectroscopy when the sulfoxides 212 and 213 were treated with acetyl chloride in [ Hs] methyl cyanide at — 35°C. ° ... 

Similar considerations apply to the sulfoxonium and sulfonium ylides, which are formed by deprotonation of the corresponding positively charged sulfur-containing cations. The additional electronegative oxygen atom in the sulfoxonium salts stabilizes these ylides considerably, relative to the sulfonium ylides. ... 

C.iii. Other DMSO Oxidations. Albright and Goldman developed a reagent using DMSO and acetic anhydride that formed an active sulfoxonium complex. 12 jj (jje initial work, yohimbine was oxidized to the ketone (yohimbinone) in 85% yield, at ambient temperatures in 24 h. 2 As with DCC, DMSO initially reacts with acetic anhydride to form sulfoxonium salt 66, which then reacts with the alcohol. Acetate is the leaving... 

Dimethyl sulfide is a useful precursor to sulfur ylids, reacting with iodomethane to give trimethylsulfonium iodide (609). When this salt was treated with n-butyllithium, deprotonation of the hydrogen on the a-carbon led to dimethylsulfonium methylid (610). Dimethyl sulfoxide also reacts with alkyl halides to give a sulfoxonium salt. When DMSO reacted with iodomethane, trimethylsulfoxonium iodide (611) was formed. As with the dimethyl sulfide derivative, treatment with a strong base such as -butyllithium generated the corresponding ylid, dimethylsulfoxonium methylid (612). 

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