Dehalogenation defluorination

Aryl halides can be dehalogenated by Friedel-Crafts catalysts. Iodine is the most easily cleaved. Dechlorination is seldom performed and defluorination apparently never. The reaction is most successful when a reducing agent, say, Br or 1 is present to combine with the I" or Br coming off." Except for deiodination, the reaction is seldom used for preparative purposes. Migration of halogen is also found," both intramolecular and intermolecular." The mechanism is probably the reverse of that of 11-11." ... 

The order of reactivity of the C-X bond (generally I > Br > Cl > F) is consistent with its strength. For instance, the experimentally found dissociation energies for phenyl halides (DPh x) are 528, 402, 339, and 272 kj mol-1 at 298 K for X=F, Cl, Br, and I, respectively [2]. Consequently, catalytic defluorination in the literature is comparatively rare. The different reactivity of the C-X bonds renders possible the selective dehalogenation of compounds containing two dissimilar halides, leaving intact the stronger C-X bond. 

Certain metals can be used for gas-phase defluorinations to give fluoroaromatic compounds. Thus, dehalogenations occur when the compounds in a gas phase are passed at high temperature over fillings or gauze made of copper,124 125 iron,126 131 iron oxides,132 or platinum,133... 

Tris(phenylthio)antimony, 337 Dechlorination (see Reduction reactions) Deconjugation (see Isomerization) Defluorination (see Reduction reactions) Dehalogenation (see Elimination reactions, Reduction reactions) Dehydration (see also Elimination reactions)... 

The same hypercoordinate silylene (79) was generated by the Corriu group in dehalogenation reactions the best results were with the difluoride Ar2SiF2 (Ar = o-Me2NCH2Ph) and lithium metal or lithium naphthalene97. The silylene was trapped by 1,4-addition to 2,3-dimethylbutadiene. Similar defluorination reactions were used to obtain silylenes 87-89 shown in Scheme 25, all trapped with dimethylbutadiene. 

Phenols are present as phenoxide ions in Na/NH3 solutions. Therefore, the transfer of an electron into a rr-antibonding orbital of a phenoxide ion should be slower than the transfer to benzene. Both chlorophenols and fluorophenols are dehalogenated more slowly than chlorobenzenes or fluo-robenzenes [24], Fluorophenoxide ions were more difficult to defluorinate. Thus, 4-fluoro-2-chlorophenol could be converted selectively to 4-fluorophe-nol in Na/NH3 [24], In contrast, chlorofluorobenzenes lost both Cl and F at close to diffusion-controlled rates [24], indicating that using a stoichiometric deficiency of Na would not give any selectivity to fluorobenzene. 

Fluoroanhydride of cyanuric acid 55 was formed in a high yield by the defluorination reaction of perfluoro-l,3,5-triazacyclohexane 61a by action of ferrocene (Scheme 26) [43], Dehalogenation of (NC1CF2)3 61b nnder the action of CISC(O) CF3 was reported (Scheme 26) [44]. 

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