Inorganic complexes benzene-soluble

Complexation of inorganic cations such as alkaline or alkaline earth metals by macrocyclic polyethers produces large, lipophilic cationic metal-macrocycle complexes that are readily soluble in nonpolar solvents such as benzene, toluene and haloalkanes. In order to maintain charge balance, the cationic complex has an associated counter anion. In an immiscible two-phase liquid system, such as a mixture of chloroform and water, the anion is necessarily pulled into the organic phase as the cationic complex crosses the phase boundary. A simple illustration of this principle is obtained by addition of a chloroform solution of [18]crown-6 to an aqueous solution of potassium picrate (potassium 2,4,6-trinitrophenolate). The yellow colour of the picrate anion is transported rapidly into the contiguous (physically in contact) chloroform phase upon agitation (Figure 3.43). 

During the complex synthetic procedure, it was necessary to use a support with a high solubility in THF even at -78 °C, in conditions in which PEG is poorly soluble (steps c, d, and f. Fig. 8.49). The extreme solubility of PEG in water would also not allow the complete removal of large quantities of salts during the aqueous removal of organometallic/inorganic salts required in steps c and f. The lipophilic, water-insoluble NCPS resin 8.102 was compatible with these requirements. Moreover, reaction conditions included solvents such as cyclohexane and benzene at temperature below 0 °C, in which NCPS is fully soluble and PEG is not and the purification protocols involved precipitation with methanol, in which NCPS is completely insoluble, in contrast to PEG. 

Complexes between crown ethers and ionic salts are soluble in nonpolar organic solvents, thus allowing many reactions to be carried out under aprotic conditions that would otherwise have to be carried out in aqueous solution. For example, the inorganic compound KMn04 actually dissolves in benzene in the presence of 18-crown-6. The resulting solution of purple benzene is a valuable reagent for oxidizing alkenes. 

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