Disulfides reaction with enolate anions

Methanol-0-4 methyl nitrite, and dimethyl disulfide have been examined as potential chemical probes for distinguishing between alkoxides and enolates in the gas phase.171 Methanol-0-d proved to be unsuitable and methyl nitrite reacts too slowly in contrast, the reactive ambident behaviour of dimethyl disulfide results in elimination across the C—S bond on reaction with alkoxides ( hard bases ) and attack at sulfur by enolates ( soft bases ). This probe has been applied to investigation of the anionic oxy-Cope rearrangement. The dianionic oxy-Cope rearrangement is a key step in a squarate ester cascade involving stereoinduced introduction of two alkenyllithium reagents cis to each other.172... 

With dimethyl disulfide as the electrophile the cis/trans-ratio is 90 10 even in tetrahydrofuran. Competition experiments show that this reaction is much faster than the usual alkylations, which afford mainly frans-compounds68). With the sulfur electrophile a single electron transfer (SET) seems likely generating a cyclopropyl radical as a reactive species, which naturally displays a different selectivity compared to the enolate anion. 

Pyrazolones also condense with aldehydes in aldol-type processes, or react with other electrophiles, such as carbon disulfide, in each case reaction presumably proceeding via the enol tautomer, or its anion. In basic solution isoxazol-3-ones alkylate either on oxygen or nitrogen, and the choice of base can influence the ratio. ... 

Jones and colleagues have prepared 1,4-dicarbonyl compounds by conjugate additions of enolate and related anions to a,P-unsaturated sulfoxides [80,81]. For example, the lithium enolate of acetone dimethylhydrazone (83), in the presence of dimethyl sulfide-copper(I) bromide complex, underwent conjugate addition to 2-phenylsulfinyloct-l-ene (82). Quenching the reaction mixture with dimethyl disulfide gave the doubly protected 1,4-diketone derivative (84), which, on sequential hydrolysis with copper(II) acetate and trifluoroacetic acid gave the dodecane-2,5-dione (85) as the product in 54% yield from (82) (Scheme 5.27). Other examples of the addition of enolate-type species to a,p-unsaturated sulfoxides have also been reported [82.83]. 

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