The crown polyethers

The macrocycle types discussed so far tend to form very stable complexes with transition metal ions and, as mentioned previously, have properties which often resemble those of the naturally occurring porphyrins and corrins. The complexation behaviour of these macrocycles contrasts in a number of ways with that of the second major category of cyclic ligands - the crown polyethers. 

Following the original paper, reports of the synthesis of new crowns and crown-like molecules proliferated. A typical property of these systems is their ability to form stable complexes with the alkali metal and alkaline earth ions. Prior to the synthesis of the crowns, the coordination chemistry of the above ions with organic ligands had received very little attention. A further impetus to the study of such complexes was the recognition of the important role of Na+, K+, Mg2+ and Ca2+ ions in biological systems. 

The formation of complexes of non-metallic guests by crowns as well as by other types of cyclic hosts is discussed in Chapter 5. 

Effects of chain length on cyclization. The effect of chain length on cyclization of a series of acyclic precursors of benzo-crowns, of which (168) is the first member, has been studied (Mattice Newkome, 1982). In accordance with kinetic data, calculations of the Monte Carlo type indicate that the probability of adoption of a conformation with a small head-to-tail distance makes a significant contribution to the cyclization rate for the small to medium rings. 

Template contributions. Alkali metal ions have been documented to play a template role in a number of crown syntheses. Thus, for example, the presence of K+ has been shown to promote the formation of 18-crown-6 in syntheses such as [4.2] (Green, 1972) intermediates of type (174) are 

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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 ( ). 

While the aforementioned ionophores are microbial metabolites, the crown polyethers, the depicted prototype of which is dicyclohexyl-18-crown-6 (Fig. 2D) are synthetic macrocyclic ethers. The first crown ether synthesized, dibenzo-18-crown-6, is the first multidentate synthetic macrocycle with the ability to form stable complexes with alkali and alkaline earth compounds. The complexing abilities of this crown ether led to the preparation of many others in rapid succession. Different size crown rings have been synthesized containing benzyl, cyclohexyl and naphthyl moieties Optically pure dinaphthyl crown ethers have been used to resolve asymmetric amine salts by enantioselective extraction from an aqueous solution into chloroform... 

Main-group Ions.—The formation of complexes between the alkali metals and large ring systems such as the crown polyethers is considered in Part in. 

Crown polyethers have also been synthesized (Kopolow et al., 1971, 1973) by polymerization of the corresponding vinyl monomer (4 -vinylmonobenzo-15-crown-5) (Scheme 13-4). The polymeric crown ethers were obtained as amorphous solids, softening at 122-128°C and with an average molecular weight of 11,600. The polymeric polyethers, just like the monomeric crown ethers, had the ability to bind alkali metal, particularly potassium, ions. Hence, these have been used for extraction of potassium. The crown polyethers form a complex with KMn04, which is... 

Main-group Ions.— There have been several reports dealing with the formation of complexes between the alkali metals and large ring systems such as the crown polyethers. In view of the particular relevance of these studies to the problems of active and passive transport of these ions through biological membranes, they are discussed in Part III. 

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