Carbanion as a Route to Methyl Ketones

On treatment with a strong base such as sodium hydride or sodium amide, dimethyl sulfoxide yields a proton to form the methylsulfinyl carbanion (dimsyl ion), a strongly basic reagent. Reaction of dimsyl ion with triphenylalkylphosphonium halides provides a convenient route to ylides (see Chapter 11, Section III), and with triphenylmethane the reagent affords a high concentration of triphenylmethyl carbanion. Of immediate interest, however, is the nucleophilic reaction of dimsyl ion with aldehydes, ketones, and particularly esters (//). The reaction of dimsyl ion with nonenolizable ketones and 

Powdered sodium amide reacts with dimethyl sulfoxide to generate the sodium salt under the same conditions, with the evolution of ammonia, and is comparable to sodium hydride in its reactivity. 

5 to 2 M solution of methylsulfinyl carbanion in dimethyl sulfoxide is prepared under nitrogen as above from sodium hydride and dry dimethyl sulfoxide. An equal volume of dry tetrahydrofuran is added and the solution is cooled in an ice bath during the addition, with stirring, of the ester (0.5 equivalent for each 1 equivalent of carbanion neat if liquid, or dissolved in dry tetrahydrofuran if solid) over a period of several minutes. The ice bath is removed and stirring is continued for 30 minutes. The reaction mixture is then poured into three times its volume of water, acidified with aqueous hydrochloric acid to a pH of 3-4 (pH paper), and thoroughly extracted with chloroform. The combined extracts are washed three times with water, dried over anhydrous sodium sulfate, and evaporated to yield the jS-ketosulfoxide as a white or pale yellow crystalline solid. The crude product is triturated with cold ether or isopropyl ether and filtered to give the product in a good state of purity. 

oj-(Methylsulfinyl)acetophenone is prepared by the reaction of methylsulfinyl carbanion (0.07 mole) and 5.25 g (0.035 mole) of ethyl benzoate yielding approx. 5 g (78.6%) of the white solid. Recrystallization from ethyl acetate gives colorless needles, mp 86-87 . 

Methylsulfinyl -pentyl ketone is prepared by the reaction of the reagent (0.1 mole) and 7.2 g (0.05 mole) of ethyl caproate yielding the crude product as a pasty crystalline mass. The material is dissolved in hot isopropyl ether (20 ml), petroleum ether (30 ml) is added, and the solution cooled to -15°. Filtration yields approx. 6 g of white plates, mp 40-44°. Recrystallization from isopropyl ether affords pure product, mp 45-47°. 

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