Properties, Stability, and Reactivity of Fluorinated Carbanions

All perfluoroalkyl carbanions are stabilized by the negative inductive effect —If) of their fluoro substituents. At the same time, a-fluoro carbanions are destabilized by electronic p-tr repulsion of the lone electron pairs of the fluorine and at the anionic center (+f effect). This is reflected in the relatively weak acidity of CHF, in comparison with the other haloforms - the pfQ values of the various trihalo-methanes are CHF, 30.5, Cl ICI, 22.4, CHBr, 22.7, and Cl I, 68-70 [34]. 

In contrast, trifluoromethyl copper(I) can be generated and reacted, e. g. by thermal decarboxylation of trifluoroacetates in the presence of copper(I) salts at 150 °C, or by reaction of copper powder with perfluoroalkyl iodides. Nevertheless, even for the outwardly stable CFjCu there is evidence of a solvent- and temperature-dependent equilibrium between the trifluoromethyl copper(I) species and a difluorocar- 

The efficiency of the reaction depends critically on the solvating power of the solvent for the coppcr(I) complex - DMF, pyridine, and DM SO give the highest yields. Because the complex is not sensitive to hydrolysis, the reaction also tolerates the presence of free carboxyl, amino, or hydroxy groups. The order of reactivity of the halogens as the aromatic leaving groups is I Br Cl. 

Probably the most efficient way to generate CFf from the view point of atom economy is deprotonation of inexpensive CHFj with a strong base [59]. Unfortunately, this route poses two problems. First, the low boiling point of fluoroform (—82.2 °C) creates - at least on the laboratory scale - the practical problem of handling a gas. The second problem is the need to trap and stabilize the trifluoromethyl anion immediately after its generation, to suppress fragmentation. This second complication, in particular, impeded the apparently straightforward preparative 

4 MOLEKEL 4.2 P. Flukinger, H. P. Liithi, S. Portmann, J. Weber, Swiss Center for Scientific Computing, Manno, Switzerland, 2002. 

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