PARTIAL OR SELECTIVE FLUORINATION

Whereas the previous chapter dealt largely with the synthesis of highly fluorinated systems, here we will be concerned with methods available for introducing mainly one, or two, fluorine atoms at specific points in a molecule, although many of these processes can, of course, be applied to already partly fluorinated systems in order to introduce more fluorine. 

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Another way of applying the selective extraction method directly on the initial solution is to produce a solution of low acidity. This can be achieved by using the hydrofluoride method for fluorination and decomposition of raw material. As was discussed in Paragraph 8.2.2, the raw material is fluorinated by molten ammonium hydrofluoride yielding soluble complex fluorides of ammonium and tantalum or niobium. The cake obtained following fluorination is dissolved in water, leading to a solution of low initial acidity that is related for the most part to the partial hydrolysis of complex fluoride compounds. The acidity of the solution is first adjusted to ensure selective tantalum extraction. In the second step, the acidity of the raffinate is increased to provide the necessary conditions for niobium extraction. 

The a-selectivity for carbon radical addition to propadiene (la) is retained on substituting chlorine or fluorine for hydrogen in radicals of the type CX3 (X=F, Cl), no matter whether the reaction is conducted in the liquid or in the gas phase (Table 11.4) [14, 49-51]. /3-Selective addition to allenes becomes progressively more important for the CC13 radical with an increase in number of methyl substituents [14, 47]. For example, treatment of optically active (P)-(+)-2,4-dimethylpenta-2,3-diene [(P)-(lc)] with BrCCl3 affords a 59 41 mixture of a- and /3-monoadducts [47]. The a-addition product consists of a 20 80 mixture of E- and Z-stereoisomers, whereas the product of /3-addition exclusively exhibits the Z-configuration. The fraction of 2,4-dimethylpenta-2,3-diene (P)-(lc) that was recovered from this reaction mixture had completely retained its optical activity. These results indicate that the a-and the /3-CCl3 addition proceed under kinetic control. If one of the addition steps were reversible, at least partial racemization would inevitably have taken place. 

Various side reactions are possible. The solvent acetonitrile can also react as a nucleophilic agent to form, after hydrolysis, acetamides [R CH(NHAc)R2].23 Methanol as solvent results in a methoxylation product [R CH(OMe)R2].22 The selectivity of partial electrochemical fluo-rination depends on many different parameters, e.g. the type of solvent, the supporting electrolytes, the electrode materials, the electrochemical potential etc. The electrochemical potential can be measured by cyclic voltametry. Partially fluorinated compounds are particularly used in medical fields or as pesticides. 

Partially proton-exchanged Na faujasite X, in turn, is the best catalyst for selective monochlorination with tert-butyl hypochlorite.258 NaX, NaY, and NaKL zeolites used in the chlorination of toluene with sulfuryl chloride undergo rapid deactivation because of the accumulation of polychlorinated toluenes in the pores of the catalysts and dealumination.259, 260 Direct electrophilic fluorination of aromatics can be effected by using Selectfluor in the presence of triflic acid.261 Electrophilic fluorination may also be carried out by R2NF and R3N+FA reagents.262 Elemental fluorine may also act as a powerful electrophile in acidic media (sulfuric acid, trifluoroacetic acid, or formic acid), but monosubstituted aromatics give isomeric mixtures.263-265... 

The use of other novel surfactant systems, such as chiral, fluoro, or functional surfactants should aid in development of more selective procedures in many areas of separation science due to the specific chiral, fluorine, or binding interactions possible. We have recently managed partial optical resolution using chiral mixed micel-... 

The reaction may proceed as homo- or cross-dehydrodimerization [105] and takes place with a wide range of substituted substrates such as higher alcohols, ethers, silanes, and partially fluorinated alcohols and ethers, but also with ketones, carboxylic acids, esters, amides, and amines [106]. Besides the formation of 1,2-diols from saturated alcohols, unsaturated substrates are also dimerized under hydrogen to form l,n-diols other than the 1,2-isomers [107]. The regio-selectivity of the diols is controlled by the formation of the most stable radical, which then dimerizes. 

Nucleophilic fluorination using [18F]fluoride ion is a typical displacement reaction that is generally performed in a polar aprotic solvent such as acetonitrile, dimethyl sulfoxide, or dimethylformamide [100], In polar aprotic solvents, the nucleophilicity of anions including [18F]fluoride is enhanced by the selective solvation of the cations. Conversely, solvation in protic solvents likely reduces an anion s nucleophilicity by interaction with the partial... 

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