Uncharged Catalysts The Crown Ethers

In general, then, most of the catalytic activity attributed to tertiary amines has been shown to be or can probably be explained by in situ onium salt formation. The catalytic activity of the phosphorylated sulfoxides bespeak the need for further exploration in this area [34]. 

The macrocyclic polyethers were prepared by Pedersen a decade ago and shown to complex a variety of cationic substrates [39]. Among these substrates are alkali metal cations, alkaline earth cations and ammonium ions [40]. It has been shown that hindered esters could be saponified by KOH in toluene solution in the presence of dicyclo-hexyl-18-crown-6 [41] or cryptate [42]. Potassium hydroxide is both solubilized in toluene and activated by the crown ether [43]. Sam and Simmons, however, first clearly demonstrated the potential of crowns as phase transfer reagents by solubilizing potassium permanganate in benzene solution (see Eq. 1.15) and using the solubilized reagent in a variety of oxidation processes [44]. 

Phase transfer catalysis utilizing crown ethers differs slightly from the approach which utilizes quaternary ions [45]. Probably the most notable point is that crown ether catalyzed reactions are frequently of the so-called solid-liquid type. In these reactions, 

The three crown ethers which have been most widely used are dibenzo-18-crown-6 [39], dicyclohexyl-18-crown-6 [39] and 18-crown-6 [46]. The dibenzo crown (7) owes its popularity to the fact that a straightforward and efficient preparation of this stable and readily handled substance was published early and a detailed procedure is available in Organic Syntheses [43]. The difficulties with this compound are two-fold. First, it is not one of the better cation complexing crowns [40, 47], 

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