Nitrogen lariats

The nitrogen lariats are readily prepared from the appropriately N-substituted diethanolamines (see Eq. 4.2 and Sect. 4.4). 

Although the principal application for 9 has been in the synthesis of cryptands (see Chap. 8), this material has also served as precursor to a number of nitrogen based lariat ethers , sometimes referred to as crown complexanes . Binding constants for such compounds have been measured for a few examples in a few cases , but... 

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. 

We believe that the nitrogen-pivot compounds are more flexible than the carbon-pivot structures. ITiis conclusion is based on cation binding data and on solution studies involving C-NMR relaxation time and other NMR techniques (54,55). In part as a result of this, the cation binding affinities for the nitrogen-pivot lariats (31) generally exceed those of the carbon-pivot (29,30) compounds. 

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... 

Another control experiment was run to further confirm the significance of the cation-pi interaction in these bibracchial lariat ether model complexes. In this case, a diaza-18-crown-6 derivative was prepared in which a 2-phenylethyl pi-donor sidearm was attached to one nitrogen and a 2-methoxyethyl sigma donor was attached to the other <2002CC1808>. The structure is illustrated as 14, above. The solid-state structure of the 14 KI complex showed the typical apical solvation of the ring bound cation. In this case, however, one apex was solvated in the pi-fashion (benzene) and the other by the oxygen sigma donor. 

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... 

From these comments, it is evident that the crab-like cyclization reaction is a convenient method for preparation of a great variety of aza-crowns in good yields. Some of the polyaza products included /V-pivot lariat polyaza-crowns, polyaza-crowns and cyclams with one or two unsubstituted ring nitrogen atoms, peraza-cage compounds, and polyaza-crowns containing sulfur atoms. This new method could become the method of choice for preparation of the functionalized polyaza- and peraza-crowns. 

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 . 

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+.

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. 

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. 

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