Dehydrohalogenation fluoride ions

As previously reported, the radical addition of CF2Br2 on glycals (initiated by sodium dithionite) affords difluorobromomethylated compounds. These latter molecules are easily dehydrohalogenated in the presence of TBAF. Under such conditions, these difluorovinyl compounds can add a fluoride ion (from TBAF). The subsequent elimination of the acetate moiety yields trifluoromethyl unsaturated compounds. The double bond can then be reduced (Figure 6.37). The same kind of reaction occurs in the presence of DAST with gcm-difluor-omethylene compounds, which are obtained by addition of an ylide onto an ulose (Figure 6.37). 

Reduced solvation of commonly used E2 bases (HO , RO ) in dipolar non-HBD solvents may elevate their reactivities to such an extent that E2 reactions of quite inert substrates occur [306]. Halide ions in dipolar non-HBD solvents are sufficiently strong bases to promote dehydrohalogenations of haloalkanes [73, 74]. Even the fluoride ion is the most efficient in this reaction [307, 308, 600] the elimination rates decrease in the order F > Cl > Br > I . 

Among the fluoride ion promoted reactions which occur in dipolar non-HBD solvents are alkylations of alcohols and ketones, esterifications, Michael additions, aldol and Knoevenagel condensations as well as eliminations for a review, see reference [600]. In particular, ionic fluorides are useful in the dehydrohalogenation of haloalkanes and haloalkenes to give alkenes and alkynes (order of reactivity R4N F > K ([18]crown-6) F > Cs F K F ). For example, tetra-n-butylammonium fluoride in AjA-dimethylformamide is an effective base for the dehydrohalogenation of 2-bromo-and 2-iodobutane under mild conditions [641] cf Eq. (5-123). 

Dehydrohalogenation with fluoride ion Acetylene derivs. and allenes from a,/ -ethylenehalides... 

Dehydrohalogenation of polyvinylidene fluoride and polyvinylidene chloride was used previously for synthesis of a new form of a hnear chain carbon - carbyne [1,2], We have found that dehydrochlorination proceeded with a decreasing rate and completed in 6 h at 80°C. According to potentiometric data for chloride ions, only 31% of chlorine contained in the initial polymer systan reacted under these conditions (Fig. 4.1). 

Electrophilic nitration of olefins can also be carried out with nitronium salts in pyridinium poly (hydrogen fluoride) (PPHF) solution491 (which also acts as solvent) to give high yields of nitrofluorinated alkanes. In the presence of added halide ions (iodide, bromide, chloride) the related haloalkanes are formed, and these can be dehydrohalogenated to nitroalkenes492 [Eq. (5.183)]. 

---