Lariat ethers cation binding

At this stage it was demonstrated that reduced lariat ethers could bind cations more strongly than the neutral, parent compounds. These structures, when reduced, exhibit cation binding affinities much closer to those of cryptands than do the neutral lariat ethers and behave as if they are more rigid, cryptand-like structures as well. In contrast to the cryptands, they can be switched back to their low-binding, flexible structures on demand using electrochemical or chemical oxidation. 

Cation-Binding Properties of Crown Ethers, Lariat Ethers, Bibracchial Lariat Ethers, and Poly(ethylene glycols) as Potential Phase-Transfer Catalysts... 

Sodium Cation Binding by Nitrogen-pivot. Lariat Ethers... 

For lariat ethers to be effective as polymer-bound phase transfer catalysts, sidearm and macroring cooperation must be intramolecular. It is unlikely that two lariat ethers will be close enough on a polymer backbone or other support for the ring of one compound to interact with the sidearm donors of another. The mechanical attributes of lariat ethers will be independent of spacing but for any advantage in cation binding and anion activation to be realized, the macroring and its attached sidearm must cooperate to envelop the cation, solvate it, and shield it from the counteranion. 

Intramolecularity in lariat ether complexation was demonstrated in three ways. First, when solution cation binding constants were determined. Kg was shown to be independent of cation and macrocycle concentrations (within certain limits). The efficacy of complexation of a single cation by two macrorings [(ML2) complex formation] should be concentration dependent. Second, ammonium cation binding constants were determined for the series (see above) of monoaza-15-crown-5 and -18-crown-6 compounds having (CH2CH20)jiClH3 (n 0 to 8) sidearms (31). 

ESI has been used to study the effect of the basicities of the side arms on the abilities of these lariat ethers to extract metal ions from solution. A series of compounds of structure 9 were studied and the authors conclude that the lariat ether with the least basic side arm binds best to the smaller cations (Table 6). Specifically for Li+, it was found that it is best bound to the lariat ether with the ester side arm. It is proposed that electrostatic repulsion of the lariat ether side arm and the electron-rich jt cloud of the adjacent aromatic ring play a dominant role in determining cation selectivities. The least basic side arm has lower electron density and thus these compounds have smaller cavity sizes and are more selective for smaller ions. ... 

Figure 18. AH-TAS plot for cation binding by lariat ethers.

Gatto, V. J., Arnold, K. A., Viscariello, etal, Syntheses and binding-properties of bibrachial lariat ethers (BiBLEs) - survey of synthetic methods and cation selectivities. J. Org. Chem. 1986, 51, 5373-5384. 

Gustowski, D. A., Gatto, V. J., Mallen, J., Echegoyen, L., Gokel, G. W., Direct correlation of cation binding strengths to Hammett parameters in substituted TV-benzylaza-15-crown-5 lariat ether and A 7V-dibenzyl-4,13-diaza-18-crown-6 BiBLE derivatives. J. Am. Chem. Soc. 1987, 52, 5172-5176. 

Shultz et al (1985) 12-, 15-, and 18-Membered-ring nitrogen-pivot lariat ethers syntheses, properties, and sodium and ammonium cation binding properties. 1 Am Chem Soc 107 6659-6668... 

Other molecules include cryptophanes, for example, 24, hemispherands (an example is 25 " ), and podands The last-named are host compounds in which two or more arms come out of a central structure. Examples are 26 and 27 " and the latter molecule binds simple cations, such as Na, K, and Ca. Lariat ethers are compounds containing a crown ether ring with one or more side chains... 

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