Anion cation complexing agents

The foregoing sections have dealt with the nucleophilic properties of anionic species whose salts were solubilized in apolar solvents with the aid of cation-complexing agents. Apart from the nucleophilic character of the anion, however, its basic strength is also expected to increase with decreasing extent of solvation. This section will deal with hydrogen-abstraction reactions of crown ether-solubilized bases. 

Figure 5.13 A representation of the differing actions of phase transfer agents. Organic cations will form an ion pair in order to transfer an anion, while complexing agents such as crown ethers transfer the whole inorganic salt across the phase boundary...

Solid-liquid solvent-free phase-transfer catalysis (PTC) is specific for anionic reactions including base-catalysed isomerisation7. Usually, a catalyst (typically a tetraalky-lammonium salt or a cationic complexing agent) is added to an equimolar mixture of an electrophile and a nucleophile, one of which serves as both a reactant and the... 

The first observation of the NMR spectrum of Na- in a metal solution without added cation-complexing agents has recently been reported (65a). The rationale behind this observation was that the solvent HMPA by itself appeared to fulfill many of the requirements generally sought from macrocyclic complexing agents high solubility via cation complexation, stability to electron reduction, weak anion solvation, etc. The NMR spectrum of Na- in fluid Na-HMPA solutions, shown in Fig. 26, exhibits precisely the same chemical shift as that observed for Na-in solutions of Na in anhydrous methylamine and ethylamine in the presence of 2,2,2-cryptand. The metal anion here is truly "gas-like in... 

One of the general features of the reactivity of enolate anions is the sensitivity of both the reaction rate and the ratio of C- versus O-alkylation to the degree of aggregation of the enolate. For example, addition of HMPA frequently increases the rate of enolate alkylation reactions. Use of dipolar aprotic solvents such as DMF and DMSO in place of the THF also leads to rate acceleration. These effects can be attributed, at least in part, to dissociation of the lithium enolate aggregates. Similar effects are observed when crown ethers or similar cation-complexing agents are added to reaction mixtures. 

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