Host—Guest Complexation. 42. Preorganization Strongly Enhances The Tendency of Hemispherands To Form Hemispheraplexes

The design and syntheses of four new macrocyclic host compounds (1–4, Chart I) are reported which contain three cyclic urea carbonyls and two anisyl-like oxygens alternately arranged to bind alkali metal or alkylammonium ion guests. The macroring systems are completed with a 1,3-xylyl bridging unit containing a methoxy in its 2- and a methyl in its 4-position (1, 2) or a 3,5-dimethylphenyl group in its 2-position (3, 4). Hosts 1–4 are rigidified further with a trimethylene bridge that spans the two aryl oxygens. Hosts 1 and 2 differ only by the absence in 1 and the presence in 2 of methyl groups in the positions para to the two aryl oxygens. Hosts 3 and 4 are diastereomers, whose interconvertibility by ring inversion is blocked by steric limitations imposed on the system by the two bridging units. The configurational identities of 3 and 4 were established from their crystal structures. Hosts 5–7 (Chart I) were also prepared. Macrocycle 5 resembles 4 except the aryl oxygen trimethylene bridge is replaced by hydrogens. In 6 and 7, the trimethylene bridge is in place, but the 1,3-xylyl bridge is replaced by hydrogens. The crystal structures of the more conformationally mobile compounds 5 and 6 show that the dipoles of the carbonyl groups are better arranged to cancel one another than those of the more rigid hosts, 3 and 4. The crystal structure of 2-(CH₃)₃CNH₃⁺is exactly what was expected from molecular model examination. Free energies and association constants were determined for 1–4 and 7 binding Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NH₄⁺, CH₃NH₃⁺, and (CH₃)₃CNH₃⁺picrates at 25 °C in CDC1₃saturated with D₂0. The resulting -AG° values ranged from 7.3 (7 binding Li⁺) to 16.7 kcal mol^-1(3 binding Na⁺). The -AG° values correlate with the degrees of preorganization of the hosts for binding. Cycle 2 was the strongest binding host. Diastereomer 3, the second strongest complexing host, bound the eight guests 2.25 ± 0.75 (extremes) kcal mol”¹better than diastereomer 4. The crystal structure of 3 shows its binding sites to be better organized for complexation than those of 4. Host 2, containing methyl groups para to the aryl oxygens, bound the eight guests an average of 3.7 kcal mol¹better than 1 without the two methyl groups. This effect is attributed to greater steric inhibitions of solvation of the binding sites of 2 than of 1. Host 7, containing the trimethylene but lacking the xylyl bridge, bound the eight guests with -(AG°)_av= 7,5 kcal mol”¹, which is lower by 4.4-7.1 kcal mol^-1than the hosts containing the latter bridge. Host 3 was found to complex 1 mol of (CH₃)₃C—O—N=0 (1H NMR), The -AO° value of known host 8 was used for comparisons.