Alkylation with sulfoxonium salt

Dianions of alkyl-substituted sulfoxonium ylides 91a,b,e,f were prepared from 83 also by a two-step procedure via 2,4-dialkylated disulfones 90a-g. Deprotonation of 83 and alkylation yielded a family of bis-sulfones 90a-g, which upon a second deprotonation by treatment with an appropriate base were converted into the corresponding salts 91a,b,e,f (Scheme 11) <1996CB161>. 

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. 

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. 

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). 

C-Alkylation of )8-Ketoester Enolates. Treatment of 1 with MeOTf in MeN02, followed by anion exchange with sodium tetraphenylborate gave the sulfoxonium salt 32 (eq 10). Reaction of this sulfoxonium salt with the 8-ketoester 33 provided a 3 1 mixture favoring the C-alkylated product 34 over the C -aIkylated material 35 (eq 11). ... 

Sulfoxonium salts, alkylation with — 15, 695 Suifoxyiic acid esters... 

However, sulfoxides are also good nucleophiles that can be alkylated by methyl halides to form the corresponding sulfoxonium salts [37]. This reaction normally takes place at room temperature with methyl iodide, but apparently requires a higher temperature when a less reactive halide like CH2B1CI is involved. 

Methylation of DMSO with methyl iodide yields trimethylsulfoxonium iodide (Scheme 14). This is an unusual reaction since other sulfoxides and halides form the products of O-alkylation. By treatment with strong bases, the trimethylsulfoxonium salt is converted into the sulfoxonium ylide (25) (Scheme 14). 

Alkylation of phenoxatellurin with methyl iodide requires heating in a sealed tube to give the corresponding 10-methyltelluronium salt (94). This product is an alkylating agent in its own right and in DMSO solution an equilibrium is established between the salt, phenoxatellurin, and trimethyl-sulfoxonium iodide (95) (Scheme 24) <84JHC413>. 

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