Naked anions from crowns

In the absence of die polyether, potassium fluoride is insoluble in benzene and unreactive toward alkyl halides. Similar enhancement of solubility and reactivity of other salts is observed in the presence of crown ethers The solubility and reactivity enhancement result because the ionic compound is dissociated to a tightly complexed cation and a naked anion. Figure 4.13 shows the tight coordination that can be achieved with a typical crown ether. The complexed cation, because it is surrounded by the nonpolar crown ether, has high solubility in the nonpolar media. To maintain electroneutrality, the anion is also transported into the solvent. The cation is shielded from interaction with the anion as a... 

Since X-ray crystallography cannot observe the lone electron directly (Box 2.1), it is questionable whether it is really situated at such a distance from the Cs+ cation. If true, this would represent a very extreme example of the naked anion effect (Section 3.8.2). An alternative explanation localises the electron on the Cs+ cation, which would also account for the observed low conductivity. However, convincing evidence for the separation of cation and electron comes from the nearly isostructural sodide (Na ) and kalide (K ) analogues of [Cs ([18] crown-6) 2] + -e-. In these, species the alkali metal anions are situated in the same localised cavities as their electride analogues. 

The relative inertness of cryptands has made them especially useful for the isolation of otherwise highly reactive or unstable anions. For example, the reaction between RbOs and 18-crown-6 in liquid ammonia permits the isolation of the crystalline ozonide complex [Rb(18-crown-6)] 03-NH3. The use of cryptands is required to isolate complexes derived from the less stable Li03 and Na03 in liquid ammonia crystalline ozonide complexes Li[2.1.1] 03 ([2.2.1] = 4,7,13,18-tetraoxa-l,10-diazabicyclo[8.5.5]eicosane) and Na[2.2.2] 03 ([2.2.2] = 4,7,13, 16,21,24-hexaoxa-l,10-diazabicyclo[8.8.8]hexacosane) can be obtained that contain the bent O3 anion. " The diamagnetic Bi2 anion has been isolated as its [K([2.2.2]crypt)] salt. " Each naked anion (Bi—Bi = 2.8377(7) A) is surrounded by eight [K-crypt]+ cations, and it is notable that the dianion has been stabilized without the bulky substituents usually required for isolation of multiply bonded main-group species (see Figure 15). " ... 

The didehydroazepine (1) or benzazirine intermediates that are generated by the photolysis of phenyl azide have also been intercepted by the naked anions that are produced from various potassium salts when they are used in the presence of crown ethers Potassium acetate and hydroxide both gave the azepinone (2) while potassium halides gave only the o-halo-anilines. 

Liotta and coworkers were among the first to utilize 18-crown-6, 3 for solubilizing inorganic salts in polar and nonpolar aprotic sol vents.In particular, they studied the relative nucleophilicities in a typical 5 2 reaction (Eq. 4) of a series of potassium salts dissolved in acetonitrile by 18-crown-6, 3. The sequence (Na" > MeC02 > CN > F > CP > Br > 1 > SCN ) and the narrow nucleophilicity range found, different from those in polar protic media were attributed to a much weaker solvation of the anion in acetonitrile that does not modify anionic intrinsic reactivity. Because this behavior is typical of the gas phase, the authors claimed the complexes formed by crown ethers with potassium salts in acetonitrile as sources of "naked" anions ... 

While the aforementioned lonophores are Streptomyces metabolites, the crown polyethers. the depicted prototype of which is dlcyclohexyl-18-crown-6, are synthetic W. Although they lack the intricate conformations of the natural lonophores arising from multiple asymmetric carbon atoms, their molecular llgeindlng properties are analogous. While they are less efficient ion carriers, their lack of labile linkages confers Increased chemical stability they find extensive use in org inlc synthesis for solubilizing electrolytes, e.g. K " enolates, in nonpolar solvents thereby providing reactive naked anions ( ). 

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... 

The spectroscopic Na data are analyzed to yield reliably the three corresponding equilibrium constants. It is thus possible to calculate back the relative amounts of each of these entities as a function of the (C)/(E-Na" ") molar ratio. A structure-reactivity correlation is successful - and outstandingly so- for only one of these the rate constant for alkylation of E Na" by C2H5I is strictly proportional to the concentration of (E Na C). Therefore, at least to first approximation, this is the single kinetically active species in the system. The monomeric tight ion pairs E Na, their dimers (E Na )2, the crowned dimers which presumably have a structure of the type (E Na E Na C), are all inert under our conditions. This serves to show that the idea of a "naked anion"(49) is underly naive instead we are dealing with an anion E which continues to benefit from stabilization, albeit reduced, from presence of the neighboring Na cation which, in turn, is bound also to the comple-xant molecule C, either a crown ether or a cryptand. 

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. 

Triarylbenzoamido-crown ethers 71 and 72 [58] showed abilities as naked-eye sensors for F, H2PO4, AcO , citrate, trimesate, and isophthalate in the presence of potassium picrate in DMS0 H20 (1 1 v/v). Upon addition of the aforementioned anions to a solution of 71-K+ or 72-K+ picrate, the color of the solution changed from yellow to green. 

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... 

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],... 

A cyclic transition state model, that differs from the Zimmerman-Traxler and the related cyclic models inasmuch as it does not incorporate the metal in a chelate, has been proposed by Mulzer and coworkers [78] It has been developed as a rationale for the observation that, in the aldol addition of the dianion of phenylacetic acid 152, the high ti-selectivity is reached with naked enolate anions (e.g., with the additive 18-crown-6). Thus, it was postulated that the approach of the enolate to the aldehyde is dominated by an interaction of the enolate HOMO and the n orbital of the aldehyde that functions as the LUMO (Scheme 4.31), the phenyl substituents of the enolate (phenyl) and the residue R of the aldehyde being oriented in anti position at the forming carbon bond, so that the steric repulsion in the transition state 153 is minimized. Mulzer s frontier molecular orbital-inspired approach reminds of a 1,3-dipolar cycloaddition. However, the corresponding cycloadduct 154 does not form, because of the weakness of the oxygen-oxygen bond. Instead, the doubly metallated aldol adduct 155 results. Anh and coworkers also emphasized the frontier orbital interactions as being essential for the stereochemical outcome of the aldol reaction [79]. 

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