Lariat ethers cation- interactions

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. 

The Na or K cation interaction has been experimentally probed by using synthetic receptors that comprise diaza-18-crown-6 lariat ethers having ethylene side arms attached to aromatic donors. (Adapted from Meadows et al., 2001)... 

Meadows, E. S. De Wall, S. L. Barbour, L. J. Gokel, G. W. Alkali Metal Cation-Interactions Cbserved by using a Lariat Ether Model System. J. Am. Chem. Soc. 2001, 123, 3092-3107. 

Gokel, G. W., Barbour, L. J., Ferdani, R., Hu, J. X., Lariat ether receptor systems show experimental evidence for alkali metal cation-pi interactions. Acc. Chem. Res. 2002, 35, 878-886. 

The two center structures show the complex that forms between iV,iV -bis(2-phenylethyl)-4,13-diaza-18-crown-6 and KI <2000PNAS6271>. The K+ ion is bound in the center of the macroring, as expected for any 18-crown-6 macrocycle. The twin sidearms of the bibracchial lariat ether turn inward in this complex and the arenes serve as apical donors. The top center structure shows the symmetrically bound cation and illustrates that the iodide anion is excluded from the cation s solvation sphere. The bottom center structure shows the superimposition of the two benzene rings upon each other and upon the K+ ion. Note in the bottom center structure that the iodide anion is not illustrated. The ideal sandwich of arene-cation-arene confirms the cation-pi interaction between benzene and K+. 

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. 

Before proceeding with a discussion of how lariat ethers bind various cations, we will consider how simple monocyclic crown ethers (12-crown-4, 15-crown-5, 18-crown-6, etc.) interact with Na+, K+, Ca2+, and NH4+. The binding of many other ions would doubtless be of interest, but those ions noted here are biologically relevant and of special interest to us. 

Early studies in the lariat ether area required confirmation of the design principles. There were several key questions concerning side-arm participation. The first was simply whether or not the side arm(s) interacted with the ring-bound cation in accord with the design. Second, if the side-arm interaction occurred, to what extent was binding enhanced Third, was there evidence that this arrangement was more flexible than the comespond-ing cryptand ... 

Lariat ethers proved to be an effective vehicle for demonstrating alkali metal cation-7i interactions. When either Na" or is bound in the macroring, interactions are possible with neutral side arms. Indole, benzene, and phenol were studied, because these arenes are present on the side chains of tryptophan, phenylalanine, and tyrosine, respectively.Additional studies were reported of cation-x interactions between neutral double and triple bonds with these same alkali metal cations." Lariat ethers were also explored as ion-regulated DNA cleavage agents. Monomeric propargylic sulfone-armed lariat ethers were found to cleave DNA. Modest increases in DNA cleavage efficiency were observed in the presence of Na-" and... 

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