Lariat ethers nitrogen-pivot

The structure shown above is a representative of the class of lariat ethers we have called "nitrogen-pivot" compounds (31). The presence of invertable nitrogen as the point of sidearm attachment makes these compounds more flexible than the corresponding "carbon-pivot" lariat ethers (29,30). The carbon- and nitrogen-pivot lariat ethers have been prepared with a variety of sidechains attached. Examples of compounds which could be linked to existing polymers such as chloromethylated polystyrene or copolymerized with other monomers are shown below. 

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

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... 

Figure 1. Cation binding by 12-, i5-, and 18-membered ring, nitrogen-pivot lariat ethers. Ring sizes triangles, 12 squares, 15 circles, 18. Cations filled symbols, IVa+ open symbols, NH4+.

Gustowski. D.A. Echegoyen, L. Goli. D.M. Kaifer. A. Schultz, R.A. Gokel. G.W. Electrochemically switched cation binding in nitrobenzene-substituted, nitrogen-pivot lariat ethers. J. Am. Chem. Soc. 1984,106. 1633-1635. 

Gokel and coworkers prepared a series of nitrogen-pivot lariat crown ethers (Schultz et al., 1985). These compounds all have side arms connected to a... 

Echegoyen. L. Cokel, G.W. Kim. M.S. Eyring, E.M. Petrucci. S. Mechanism of complexation of sodium cation by nitrogen-pivot lariat 15-crown-5 ethers in methanol at 25°C. J. Phys. Chem. 1987. 91. 3854-3862. 

Abstract. The evolution of lariat ethers from relatively simple, substituted crown ethers into electrochem-ically sensitive ligands is presented. Although nitrogen-pivot lariats were observed to be better binders than the corresponding parent crowns and to retain considerable flexibility after complexation, overall stability constants were not favorable for cation transport applications. This led to the syntheses of nitrobenzene- and anthraquinone-substituted systems capable of reversible redox behavior and drastically enhanced cation binding abilities when reduced. Application of these in enhanced cation transport processes has been demonstrated. 

The combination of sulfur and nitrogen donor atoms in crown ethers appears to provide an ideal binding site for Hg + as several sensors, mainly electrochemical, which use azathiacrown ethers to target the metal have been reported. The selectivity for mercury over other metals is often astonishingly high. " A 15-crown-5 C-pivot lariat ether incorporating a thioether side chain was found to extract 46% of Hg + from aqueous solution into dichloroethane, although it was also able to remove almost 100% of Ag+ under the same conditions. ... 

When the tether is attached to the macroring at carbon, the molecule is said to have a carbon pivot atom. When the side arm is attached as illustrated at the right, the pivot atom is nitrogen. Because the molecular models of these compounds resemble lassos and because the combination of side arm and macroring donors can permit a cation to be roped and tied , we have called such macrocycles lariat ethers . 

The trends are noted to be relatively similar for both the carbon-pivot and nitrogen-pivot types of lariat ethers. Binding strengths and selectivities are dependent on ring size and in general increase as ligand size increases. Strong selectivities are noted for the potassium ion, as in the crown ethers. 

Two-armed nitrogen pivot systems, termed bibracchial lariat ethers (BiBLEs), have recently been synthesized in one-step reactions (79). (Scheme 6). Table 2.6 reports the stability constants of a number of these compounds for the Na", and Ca guest cations. It can be seen that polar donor groups like ester carbonyl in the sidearms strongly favour Ca over either Na" or but less polar groups like ethers favour... 

We have categorized the lariat ethers according to the means by which sidearms are attached (pivot point) and how many sidearms are present The carbon-pivot compounds are more chemically stable than the nitrogen-pivot compounds but diey are inherently less flexible. The ability of nitrogen to undergo facile inversion makes the nitrogen-pivot structures generally more adaptable to a cation than is the case with carbon-pivot derivatives. Examples of lariat ether compounds are shown below. 

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