Nucleophilic fluoridation

More general procedures for additions of halogen fluorides to highly fluori-nated olefins involve reactions with a source of nucleophilic fluoride ion, such as an alkali metal fluoride, in the presence of aposttive halogen donor [62 107, lOff, 109, 110, 111] (equations 11 and 12) These processes are likely to occur by the generation and capture of perfluorocarbamonic intermediates Tertiary fluormated carbanions can be isolated as cesium [112], silver [113], or tns(dimethylamino)sul-... 

Kinetic studies also provide other evidence for the SnI mechanism. One technique used F NMR to follow the solvolysis of trifluoroacetyl esters. If this mechanism operates essentially as shown on page 393, the rate should be the same for a given substrate under a given set of conditions, regardless of the identity of the nucleophile or its concentration. In one experiment that demonstrates this, benzhy-dryl chloride (Ph2CHCl) was treated in SO2 with the nucleophiles fluoride ion, pyridine, and triethylamine at several concentrations of each nucleophile. In each case, the initial rate of the reaction was approximately the same when corrections were made for the salt effect. The same type of behavior has been shown in a number of other cases, even when the reagents are as different in their nucleophilicities (see p. 438) as H2O and OH . 

D.P. Cox, J. Terpinski, W. Lawrynowicz, Anhydrous tetrabutylammonium fluoride—A mild but highly efficient source of nucleophilic fluoride-ion, J. Org. Chem. 49 (1984) 3216-3219. 

The coupling of organosilicon compounds with organic electrophiles was not disclosed until 1988 by Hatanaka and Hiyama, when they demonstrated that through the addition of an appropriate silicophilic nucleophile, those desired pentacoordinate species can be generated in situ and transfer an unsaturated group. Nucleophilic fluoride sources were found to be the additive of choice, typically TASK, TBAF, and, in some cases, KF and GsF. These are the fundamental concepts of what is nowadays called the Hiyama reaction. The use of fluoride... 

Bu4NF (TBAF) 261.46 62-63 + + + b high nucleophilic fluoride TBAF (anhyd) KF/ 5... 

Nevertheless, rearrangements can be suppressed, in most cases, when dediazoniation is performed in hydrogen fluoride/pyridine (48 52 w/w) mixture,308,310 since this less acidic medium stabilizes carbocations (such as the phenonium cation) to a far lesser extent and provides more nucleophilic fluoride anions which, however, cannot totally match the anchimeric assistance of the aryl or hydroxy group in tyrosine (5g) and threonine (5h). 

Fluorinated -alkenes and -cycloalkenes have a special relationship with their hydrocarbon analogues, usually exhibiting a chemistry that is complementary. For example, the fluorinated systems are frequently susceptible to nucleophilic attack, in some cases dramatically so, and therefore reactions of nucleophiles with fluorinated alkenes often reveal unique new chemistry. This chapter covers electrochemical reduction, principles governing orientation and reactivity of fluorinated alkenes towards nucleophiles, fluoride ion as a nucleophile and the mirror-image relationship of this chemistry with that of proton-induced reactions, reactions with nitrogen-, oxygen-, carbon- centred nucleophiles etc., and, finally, chemistry of some oligomers of fluorinated -alkenes and -cycloalkenes. 

Two other interesting papers should also be mentioned. Perfluorination of porphyrin was achieved with AgF resulting in a robust catalyst (equation 41), capable of performing around 50 cycles of benzene oxidation or olefin epoxidation with H20281. The second work deals with photolytic reactions of AgF in the presence of Ti02. The formally nucleophilic fluoride behaves as an electrophile and replaces protons in quite a few cases (equation 42)82. 

The following example shows how fluoride ion, normally a poor nucleophile in hydroxylic (protic) solvents, can be a good nucleophile in an aprotic solvent. Although KF is not very soluble in acetonitrile, 18-crown-6 solvates the potassium ions, and the poorly solvated (and therefore nucleophilic) fluoride ion follows. 

The reagent tris(dimethylamino)sulfonium difluorolrimcthylsilicate [TASF(Me)], which is commercially available, albeit expensive, has recently found application in carbohydrate chemistry as a source of nucleophilic fluoride for the substitution of triflates. For example,... 

The addition of tin(II) chloride or tin(II) fluoride, as low-redox-polential reducUtnts. in the dediiizoniation slop of the fluorination of arylamines with polar groups, using hydrogen fluoride in the presence of a nucleophilic fluoride source (e.g., tetrabutylanimonium diliydrogen trifluoride), not only improves the yield of the fluoroaromatic, but al.so allows the fiuorodediazoni-ation process to be performed under milder reaction conditions. ... 

ProMem 11.14 l-Chloro-l,2-diphenyIethaiic tcacUs with the nucleophiles fluoride ion and irieth] amine at the same rate, even though one is charged and one is neutral. Explain... 

Fluorination. Bu NHFj is a convenient source of nucleophilic fluoride which can be... 

Scheme 2.40 The complete nucleophilic replacement of all fluorine atoms can be driven, e.g., by the strong nucleophilicity of the thiolate anion and the lattice energy of the formed NaF aboue), [93] or by removal of the competing expelled nucleophilic fluoride with the volatile l /le3SiF bottom right) [94].

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