Dianions, active methylene

The sulfenylation of metalloimines is equally applicable to ketones, although using more reactive sulfur electrophiles it is possible to bring about reaction on the unmetallated enamine. ° Sulfenylation of ketone enol silyl ethers also proceeds well with the more reactive sulfur species. Sulfenamides and their derivatives e.g. 11) are particularly suited to the direct sulfenylation of ketones and active methylene compounds such as -diketones, -keto esters and malonates, which undergo facile reaction at room temperature (equation 5). This procedure, however, does not appear to have been exploited for the dehydrogenation of active methylene compounds icf. Section 2.2.4.1). By preparing the dianion (13)... 

The dianions of /S-keto-esters undergo Claisen ester condensation in the expected manner via the y-carbanion. In an extension of a method reported for the preparation of amines from alcohols loc. cit.), the same Japanese authors have described the direct alkylation of active methylene compounds with alcohols (Scheme 42) the method is limited to relatively acidic compounds, with pJ Tg < 11 diethyl malonate (pX 13.3) does not react. 

Tetrachlorocyclopropene (22) and a variety of active methylene compounds are building blocks for hexasubstituted [3]radialenes, as was first reported by Fuku-naga et al. Thus, 22 reacted with malononitrile, dimethyl malonate, and methyl cyanoacetate in glyme in the presence of an excess of NaH to give triafulvene dianion salts 23, which were isolated as tetrabutylammonium salts (Scheme 4.4)... 

Alkylation of a 1,3-dicarbonyl compound at a flanking methyl or methylene group instead of at the doubly activated C-2 position does not usually take place to any significant extent. It can be accomplished selectively and in good yield, however, by way of the corresponding dianion, itself prepared from the dicarbonyl compound and two equivalents of a suitable strong base. For example, 2,4-pentanedione 2 is converted into 2,4-nonanedione by reaction at the more-reactive, less-resonance-stabilized carbanion (1.17). ... 

In the presence of a very strong base, such as amide ion or an organolithium reagent, it is possible to convert dicarbonyl compounds to their dianions. Subsequent alkylation of such dianions leads to alkylation at the more strongly basic enolate site, rather than at the carbon atom between the two carbonyl carbons. The more acidic methylene group activated by two carbonyl substituents is the preferred site in the monoanion, as discussed earlier. The ability to determine the site of monoalkylation by choice of the amount and nature of the basic catalyst has significantly expanded the synthetic utility of enolate alkylations. Scheme 1.7 gives some examples of formation and alkylation of dianions. 

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