Anions naked

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... 

Other measures of nucleophilicity have been proposed. Brauman et al. studied Sn2 reactions in the gas phase and applied Marcus theory to obtain the intrinsic barriers of identity reactions. These quantities were interpreted as intrinsic nucleo-philicities. Streitwieser has shown that the reactivity of anionic nucleophiles toward methyl iodide in dimethylformamide (DMF) is correlated with the overall heat of reaction in the gas phase he concludes that bond strength and electron affinity are the important factors controlling nucleophilicity. The dominant role of the solvent in controlling nucleophilicity was shown by Parker, who found solvent effects on nucleophilic reactivity of many orders of magnitude. For example, most anions are more nucleophilic in DMF than in methanol by factors as large as 10, because they are less effectively shielded by solvation in the aprotic solvent. Liotta et al. have measured rates of substitution by anionic nucleophiles in acetonitrile solution containing a crown ether, which forms an inclusion complex with the cation (K ) of the nucleophile. These rates correlate with gas phase rates of the same nucleophiles, which, in this crown ether-acetonitrile system, are considered to be naked anions. The solvation of anionic nucleophiles is treated in Section 8.3. 

The facile solubility of inorganic salts in DMSO is considered to be due to the strong solvation of the cations resulting in the formation of naked anions . 

Both of the above-mentioned catalyst types get the anions into the organic phase, but there is another factor as well. There is evidence that sodium and potassium salts of many anions, even if they could be dissolved in organic solvents, would undergo reactions very slowly (dipolar aprotic solvents are exceptions) because in these solvents the anions exist as ion pairs with Na or and are not free to attack the substrate (p. 443). Fortunately, ion pairing is usually much less with the quaternary ions and with the positive cryptate ions, so the anions in these cases are quite free to attack. Such anions are sometimes referred to as naked anions. 

It should be noted that the three-dimensional polyether cages (the cryptands) are usually most effective at producing naked anions . With these, the metal ion is completely encapsulated by the polyether network and thus better charge separation is achieved. In the case of the crowns, such complete encapsulation does not normally occur and hence the counter anion is more readily able to associate directly with the com-plexed metal cation. In such cases, the use of the term naked is somewhat of a misnomer. 

It needs to be noted that phase transfer catalysis has implications for energy conservation for example, reactions which normally require heat may proceed at room temperature in the presence of naked anions. 

For those applications involving the activation of an inorganic anion (that is, generation of a naked anion), the cryptands, rather than the crowns, tend to be the reagents of choice. Such reagents are thus also ideal for applications involving phase-transfer catalysis of the type discussed previously. 

If the styryl substituent retained its donor nature in the anion-radical state, an increase, not a decrease in the value of the nitrogen HFC constant (a(N)) would have been observed. Experiments show that fl(N) values for anion-radicals of nitrostilbenes decrease (not increase) in comparison with the fl(N) value for the anion-radical of nitrobenzene (Todres 1992). Both naked anion-radicals and anion-radicals involved in forming complexes with the potassinm cations obey such regularity. In the cases of potassinm complexes with THF as a solvent, a(N) = 0.980 mT for PhNOj anion-radical and fl(N) = 0.890 mT for PhCH=CHCgH4N02-4 anion-radical. In the presence of 18-crown-6-ether... 

On one-electron reduction followed by one-electron oxidation, cis isomers of 2- and 4-nitrostilbenes turn into the neutral nitrostilbene molecules, but in the trans forms. On oxidation of the naked anion-radicals, the neutral trans forms are the only products (cis trans conversion degrees were 100%). In the case of the coordination complexes, the trans isomers are formed only up to 40% (Todres 1992). Scheme 3.45 describes these transformations. 

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). 

In solid-liquid systems anhydrous solid salts or bases form the inorganic phase. The phase transfer catalyst brings the required anions into the nonpolar medium by ion exchange with the solid phase or by solubilizing the salt. An appealing term naked anions is often used here. The system is recommended when traces of water should be avoided. 

Sodium or potassium ions can also participate in the phase-transfer process when they are converted to lipophilic cations by complexation or by strong specific solvation. A variety of neutral organic compounds are able to form reasonably stable complexes with K+ or Na + and can act as catalysts in typical phase-transfer processes. Such compounds include monocyclic polyethers, or crown ethers (1), and bicyclic aminopolyethers (cryptates) (2). They can solubilize inorganic salts in nonpolar solvents and are particularly recommended for reactions of naked anions. Applications of these compounds have been studied.12,21-31... 

In contrast, sodium p -nitrophenoxide is similar to the allyllithium compounds. The counter cation reduces the p character of the oxygen and prevents the delocalization of the negative charge over the aromatic system. The valence orbitals are centered toward Na+, whereas for the naked anion they are more disposable or, in other words, more diffuse (Scheme 22). Therefore, in the latter case, unfavorable out-of-phase overlap with the leaving group is increased, and rear-side attack of the nucleophile is promoted, leading to inversion (Table XI). 

Cation exchange from the metal cation to the onium carbanion improves the intrinsic reactivity of the latter due to formation of the naked anion . At the same time, the... 

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 heavier elements of Group 13, in particular thallium, are able to form discrete naked clusters with alkali metals. Table 13.7.1 lists some examples of Tl - naked anion clusters, and Fig. 13.7.3 shows their structures. 

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