Fluorine displacement addition-elimination mechanism

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 fluorines in fluorinated C60 can be displaced by a variety of nucleophiles. Furthermore, fluorinated C60 derivatives are likely to be synthetically useful because they are both more reactive, and more soluble than other halogenated derivatives (Taylor et al. 19925). They thus react readily with water, which negates their use as lubricants (Taylor et al. 1992 a). Mass spectrometry indicates that the products contain numerous hydroxy groups and epoxide links (probably from elimination of either HF or H20 from adjacent groups). An addition-elimination mechanism is probably involved since the normal SN2 process is ruled out because backside attack (Taylor et al. 19926) is impossible. 

In one method the polypeptide is reacted with Sanger s reagent, 2,4-dinitrofluo-robenzene (DNFB). The nucleophilic nitrogen of the N-terminal amino acid displaces the fluorine in a nucleophilic aromatic substitution reaction. (This reaction follows an addition—elimination mechanism see Section 17.11.) The polypeptide is then hydrolyzed to its individual amino acid components. Because the bond between the nitro-... 

This displaces the third fluorine and all that is left is to hydrolyse the ester to the free acid with aqueous base (Chapter 12). Every single reaction in this quite complicated sequence is one that you have met earlier in the book, and it forms a fitting climax to this section on the addition-elimination mechanism for aromatic nucleophilic substitution. We now need to mention two other less important possibilities. 

B Displacement of fluorine and halogen from unsaturated carbon - addition-elimination mechanism... 

The Introduction of Fluorine Into Aromatic Rings.— The direct fluorination (F2-N2) of nitrobenzene, benzotrifluoride, benzoic acid, and toluene, usually in trifluoroacetic acid at 5 C, has been described by Russian workers. Where direct comparisons can be made, the isomer distributions of the monofluoroarenes obtained are qualitatively similar to those formed in acetonitrile during concurrent American work, sununarized in Vol. 1 (p. 189). However, the Russians favour a homolytic mechanism, the preference for attack at the /wc/o-position in nitrobenzene being attributed to the electrophilic character of the attacking fluorine atom. Electrophilic displacement of a proton by F, via the usual addition-elimination mechanism, was preferred in the American paper, and this view has been emphasized in a recent review of the field by the same author. ... 

The nucleophilic terminal amino function of the protein (i.e., the first amino acid residue or monomer of the protein the rest of the amino acids have their amino functions involved in the polymer linkage or nonnucleo-philic amide linkages) will displace the fluorine atom by means of an addition-elimination mechanism. This occurs with the rate-limiting formation of... 

Besides fluorine the influences of another substiments such as Cl, CF3, NO2, CN on fluorine displacement in 2,4,6-trifluoropyrimidine were studied. The results (Table 8) can be satisfactory rationahzed in terms of bimolecular additional-elimination SjvAr mechanism through Meisenheimer type complexes [157]. 

The electron-deficient double bond in lH-pentafluoropropene reacts easily with nucleophiles such as azide anion to form primarily an addition product l-azido-l,2,3,3,3-pentafluoropropane. In the basic medium, unsaturated l-azido-2,3,3,3-tetrafluoropropene is also formed, either by elimination of hydrogen fluoride from the addition product, or by direct displacement of fluorine by SN2 mechanism [90]. 

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