Stable perfluorinated carbanions

Inductive effects of ortho substituents are important in governing the acidity of a C-H bond in substituted benzenes [106], and a variety of data indicate that the acidifying influence of fluorine falls off in the order ortho meta para this is illustrated by Table 4.13 [107]. 

Also relating to these data is the observation that substitution accompanies metallation in the reaction of 1,2,3,4-tetrafluorobenzene with butyl lithium [108]. 

Studies on the influence of halogen on the acidity of hydrogen in 1-chloro-2-fluoroethene showed that the kinetic acidity of the hydrogen a- to chlorine is greater than that for hydrogen a- to fluorine, in accordance with lower repulsions for chlorine a- to the carbanion centre [79]. 

Similarly, the pKa values for a range of halogenated ethenes [26] (Table 4.15) demonstrate that a-halogen substituents facilitate vinyl carbanion formation in the same order as in the haloform series, i.e. Br Cl F. 

Calculations suggest that negative hyperconjugation is also a factor in vinylic systems although the vinyl anions are thermodynamically unstable relative to the formation of ethyne and fluoride ion [111]. 

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Generation and Reactions of Unusual Carbanions. Both a- and -halo carbanions are generally labile species and their generation and reactions require extremely low temperatures. TASF(Me) has been used to prepare several stable and isolable perfluorinated carbanions (eq 12) or alkoxides. As compared to the corresponding metal salts, the TAS+ counterion has little coordination to the fluorines of the anion, and this presumably slows the decomposition of the TAS salts to carbenoids or alkenes. 

In organo-fluorine chemistry many reactions are known where more or less stable ionic or radical intermediates are formed, for example, carbocations, carbanions. and partially fluorinated and perfluorinated radicals and carbenes. While some of these species are short lived, others are surprisingly stable and isolable. In the latter case electronic and steric arrangements often kinetically stabilize the intermediate. 

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