Naked fluoride anion

The application of cesium fluoride as base in the synthesis of crown compounds from phenols and the ditosylates of polyethylene glycols was first described by Reinhoudt [44]. This method uses the high basicity of weakly solvated ( naked ) fluoride anions under aprotic conditions and is based on the formation of very stable H—F bonds (approx. 569 kJ/mol, H—Cl approx. 432 kJ/mol for comparison) [8],... 

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. 

Since non-bound or non-coordinated nucleophiles are even more reactive, crown-ethers [138] and cryptands (polyaminoethers) [139,140] have been used to chelate the alkali metal cations, notably the potassium ion of K[ F]F. This allows the [ F]fluoride anion to be less tightly paired with the cation and therefore to be more reactive, which has been coined the naked ion effect. In practice, the crown-ether (e.g. 18-crown-6) or better the polyaminoether Kryptofix-222 (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) is added to the aqueous K[ F]F/K2C03 solution which is then concentrated to dryness [139,140]. The complex (KP FIF-K ) can be further dried, if needed, by one or more cycles of addition of dry acetonitrile and azeotropic evaporation. 

As well as increasing anion nucleophilicity, crown or cryptand complexation can enhance the basicity of the anion. Table 3 exemplifies this effect with 1-bromooctane where base-promoted elimination to 1-octene competes with nucleophilic substitution. Being small and poorly solvated, naked fluoride is a strong and hard base which causes, in the case of certain substrates (e.g. Scheme 6), the elimination product to predominate. As the naked anions increase in size they display less basic characteristics but retain high nucleophilic reactivity (74JA2250). 

More recently, a new class of covalent-linked calix[4]pyrrole-anthraquinone compounds (29-31) have been introduced by Sessler s group [48], These are considered to be powerful naked eye sensors for fluoride, chloride and dihydrogen phosphate ions in dichloromethane. The most pronounced colour change was observed upon the addition of the fluoride anion into a solution of the receptor 30 in dichloromethane. The addition of bromide, iodide or nitrate anions did not lead to significant colour changes. 

As far as chemical sensors are concerned, colorimetric chemosensors for anions based on calix[4]pyrrole (16-18, 22-31) showed strong binding to the fluoride anion. Receptors (29-31) are the first naked-eye detectable chemosensors that are able to discriminate between different anionic substrates as a result of detectable colour changes. On the other hand, the fluorescence of the receptors (16-18, 22-28) is quenched significantly in the presence of anionic guests. 

As we have seen, the search for naked fluoride is quite intensive. The main idea is to create large cations which will not be strongly attached to the fluoride anion. Crown ethers were one solution and TASF was another. It sprang naturally from DAST chemistry, and indeed it was Middleton again who was one of the first to prepare it from SF4 (equation 95). It is a crystalline compound and proved to be a good source for organic-soluble... 

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... 

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]. 

The quest for naked fluoride has its counterpart on the cationic side with the search for truly uncoordinated lithium cations, which are not deactivated by interactions with solvent or anion and thus limited in their Lewis acidity. 

The role of calcite and fluorite used in ball-milling process was proposed. When the minerals are mechanically crushed, the newborn solid surface is activated with the naked ionic species in situ generated. The naked carbonate and fluoride anions act as a strong base capable of deprotonating the active methylene compounds, with the consequent formation of a carbanion stabilized via the coordination with calcium cation, which combines with a carbonyl compound, eventually leading to the Knoevenagel product. 

The concept of naked anions, i.e. anions solubilized in non-solvating media by the crown complexation of their counter-cations, has been applied further this year. Naked fluoride ion has been used as a catalyst for Michael additions,such as the cyanoethylation of active methylene compounds, and also as a base to mediate the acylation (and protection) of the indole nitrogen of tryptophan in peptides. The selective cleavage of protected amino-acids from oxyacyl resins [equation (18)] is a... 

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