Ether sidearms, bibracchial lariat

Alcohol sidearms, bibracchial lariat ethers, 34 Alkenes... 

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. 

Gatto, V.J. Arnold, K.A. Viscariello. A.M. Miller. S.R. Gokel, G.W. Novel synthetic access to 15- and 18-membered ring diaza-bibracchial lariat ethers (BiBLEs) and a study of sidearm-macroring cooperativity in cation binding. Tetrahedron Lett. 1986, 327-330. 

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... 

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