Electrophilic addition, to fluoroalkenes

Fluorinated radicals play a significant role in synthetic organo-fluorine chemistry, for example, in electrophilic radical addition to alkenes, single-electron transfer reactions (SET), telomerization of fluoroalkenes with perfluoroalkyl iodides, polymerization to fluoropolymers and copolymers, and thermal, photochemical and radiation destruction of fluorocarbons. Furthermore, such free radicals are of interest for studying structures, reaction kinetics and ESR spectroscopic parameters.38... 

The direction of iodine monofluoride addition to hexafluoropropene and, 1,1-di-fluoroethene suggests an electrophilic reaction mechanism. However, chlorotrifluoroethene gives a mixture of isomers both with iodine monofluoridc and with bromine monofluoride .1 0 The reactions of fluoroalkenes with bromine monofluoride proceed too quickly and for this reason they are conducted in l,l,2-trichloro-l,2,2-trifluoroethane though this hampers the isolation of addition products. 

A variety of interesting electrophilic addition processes have been developed, where additions (where the electrophile is, for example. Cl, Br, I, NO2, -OCH2-, CH2NH2, CH2OH etc.) to fluoroalkenes are achieved by reaction with a series of reagents (Table 7.8). 

Formation of dimer" products seems to be due to increased electrophilicity of the multiple bond of the fluoroalkene. Thus, the initial stage of the reaction of triphenylphosphane with perfluoro(l-methylcyclopentenc) is addition to the multiple bond. The intermediate (7/8) attacks a second molecule of cycloalkene, and difluorotriphenyl-/5-phosphanc is eliminated producing perfluoro[l-methyl-2-(2-melhylcyclopent-l-enyl)cyclopentene] (9) in 75% yield.179... 

F-Teda BF4 is effective for the selective addition of fluorine to steroids in good yield, re-gioselectively and, in many cases, stereoselectively at the 6- and 16-positions, under very mild reaction conditions (Table 7).92 Further, 6 will also efficiently fluorinate silyl and alkyl enolates, enamides, carbanions, a-alkenes and actived aromatic compounds (Table 8). As an extension of this method F-Teda BF+ has been used for the electrophilic fluorination of (fluorovinyl)tin compounds affording terminal fluoroalkenes (see Table 9).88... 

A nucleophilic mechanism can be applied in reductions with complex hydrides of highly fluori-nated aliphatic and alicyclic fluoroalkenes with electron-deficient C = C bonds the hydride anion adds as a strong nucleophilic agent to the more electrophilic carbon atom the intermediate anion can then lose a fluoride ion either from the original C = C bond, or from the allylic position finishing an SN2 displacement of the fluorine. Thus, the reductions of vinylic C-F bonds with hydrides proceed by a nucleophilic addition-elimination mechanism. Displacement of fluorine in highly fluorinated aromatic compounds proceeds by the same mechanism ... 

The effect of monofluorination on alkene or aromatic reactivity toward electrophiles is more difficult to predict Although a-fluonne stabilizes a carbocation relative to hydrogen, its opposing inductive effect makes olefins and aromatics more electron deficient. Fluorine therefore is activating only for electrophilic reactions with very late transition states where its resonance stabilization is maximized The faster rate of addition of trifluoroacetic acid and sulfuric acid to 2-fluoropropene vs propene is an example [775,116], but cases of such enhanced fluoroalkene reactivity in solution are quite rare [127] By contrast, there are many examples where the ortho-para-dueeting fluorine substituent is also activating in electrophilic aromatic substitutions [128]... 

Electrophilic acetylenes 227 (R = Et02C, CHO or CN R = EtC02, Ph or (EtO)2CH) add the elements of hydrogen fluoride on treatment with caesium fluoride under phase-transfer catalysis to yield mainly (Z)- fluoroalkenes 228 dimethyl acetylenedicarboxylate affords a 85 15 mixture of (Z)- and ( )-adducts under these conditions. No addition occurs in homogeneous solutions ... 

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