Naked fluoride ion

In media such as water and alcohols fluoride ion is strongly solvated by hydro gen bonding and is neither very basic nor very nucleophilic On the other hand the poorly solvated or naked fluoride 10ns that are present when potassium fluoride dis solves m benzene m the presence of a crown ether are better able to express their anionic reactivity Thus alkyl halides react with potassium fluoride m benzene containing 18 crown 6 thereby providing a method for the preparation of otherwise difficultly acces sible alkyl fluorides... 

Representative organic syntheses. A solution of the naked fluoride ion may be generated by dissociation of KF in an acetonitrile or benzene solution containing 18-crown-6 (Liotta Harris, 1974). The considerable nucleophilicity of this anion under these conditions is demonstrated by the fact that it is capable of displacing leaving groups from both sp2 and sp3 hybridized carbons in a number of structural environments. 

The fluoride ion complex of 5-octamethyl-[12]-mercuracarborand-4 26 is an effective source of naked fluoride <2004AGE1854>. Treatment of a tosylated sugar with fluoride 26, in the presence of tetra- -butyl ammonium iodide 27, leads to 49% tosylate replacement. The reaction does not occur without co-reagent 27, so presumably its role is to help release fluoride ion from complex 24. Scheme 3 illustrates the preparation of fluoride reagent 26 by decomplexa-tion of diiodide 25 with AgBp4 forming neutral tetramer 24, followed by treatment with tetramethylammonium fluoride. 

Another interesting application of Jt-conjugated systems is their use as chemosensors. For example, a modified dithienophosphole oxide 160 (see Scheme 49 for the synthesis of its precursor) that incorporates boryl-end caps is a very sensitive and selective sensory material for the fluoride ion (Scheme 126). Treatment of this derivative by fluoride induces a red shift of the emission maximum (160, 452 nm 373, 485 nm) that is visible to the naked eye. This shift in the fluorescence emission can even be detected down to micromolar concentrations of fluoride ion notably, the addition of other halides (CP, Br, P) does not affect the fluorescence in any way <20060L495>. 

Carbon dioxide behaves as a weak fluoride ion acceptor towards naked fluoride ion sources such as tetramethylammonium fluoride, neopentyl(trimethyl)-ammonium fluoride and piperidinium fluoride (pip+F ) in the absence of a solvent and in CH2F2 and CF3CHFCF3 solvents for pip+F [25]. The C02F anion has been characterized in the solid state by 1H 13C MAS NMR spectroscopy and... 

Bis(triphenylstannyl) selenide (56), which is prepared by the reaction of NaHSe with triphenylstannyl chloride [82,83], smoothly liberates a nucleophilic selenolate species with the aid of a fluoride ion [82]. The reaction conditions are neutral, producing the selenolate in a naked form. Therefore, in the reaction with halides, the corresponding symmetrical selenides are obtained in excellent yields without generating by-products (Scheme 42) [82]. When 56 is reacted... 

Since mono-adducts obtained from reacting 11-substituted bridge[10]annulenes with triazolinediones contain a substituted cyclopropane ring, we attempted nucleophilic displacement under stringent reaction conditions. Even with naked fluoride ion the substrates were recovered unchanged... 

The fluorination of haloethyl or alkanesulfonyloxyethyl aromatic compounds in conventional methods usually uses a naked fluoride ion generated from phase-transfer catalyst (i.e., potassium ion complexed by a cryptand, or a tetraalkylammonium salt) to... 

Walton and Ricci found that an acylsilane underwent desilylation by a naked fluoride ion to give an acyl anion, which was then trapped in situ by an alkyl halide (Eq. (5.21)) [27]. 

Because of the strong basicity of the fluoride ion, in nucleophilic Sn2 reactions elimination usually occurs as a dominant side-reaction. On the other hand, the basicity of the naked fluoride can be used for synthetic purposes, e. g. for deprotonation of phosphonium salts to the corresponding ylides in a system based on potassium fluoride with catalytic amounts of 18-crown-6 in acetonitrile. 

Naked enolates without any complexation can be made from silyl enol ethers using fluoride ion, a very selective nucleophile for silicon 49, and a non-metallic cation, usually a tetra-alkylammonium ion. The commonest reagent is TBAF (TetraButylAmmonium Fluoride Bu4N+ F ). In this way the naked enolate 50 was made. It proved to be acylated with acetic anhydride exclusively at oxygen to give the enol acetate 53 and alkylated with Mel at carbon to give the ketone 51 in 84% isolated yield.18... 

Fluoride ion in solvents with which it is not able to form hydrogen bonds is sometimes described as naked , but this term is misleading in DMF, it still has a Gibbs energy of solvation of about —400kJmol ( i60kJmol less negative than in water) and so is still very much less reactive than in the gas phase. 

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