Host-Guest Complexation. 44. Cavitands and Caviplexes Composed of Eight Anisyl Groups

The syntheses, crystal structures, and binding properties of cavitands 1 and 2 are reported. Macrocycle 1 is composed of eight anisyl units bonded to one another in their 2,6-positions ((C₆H₃OCH₃)_g) and was prepared in 1.4% yield by Fe(acac)₃oxidation of the dilithium organometallic formed by treatment of 3,3'-dibromo-2,2'-dimethoxy-l, 1 '-biphenyl (4) with H-BuLi. Macrocycle 2 is composed of eight 4-methylanisyl units bonded to one another in their 2,6-positions ((CH₃C₆H₂OCH₃)₈). It was prepared by Fe(acac)₃oxidation of the dilithium organometallic formed by treatment of 3,3'-dibromo-2,2',2,2'-tetramethoxy-5,5',5,5'-tetramethylquaterphenyl (9) with sec-butyllithium. The yield was maximal at 4.7% when the ring closure was carried out in the presence of CsBr. The crystal structures of 1, 2, and 2-C₂H₅OH reveal that each of the eight oxygens is anti to its two neighboring oxygens to provide an (up-down)₄conformational arrangement. In 1, four of the methyls attached to the oxygens are turned inward (those on rings A, B, E, and F), and four are turned outward (those on rings C, D, G, and H). The resulting cavity is small and egg-shaped, with one half lined with methyls and the other half with oxygen's unshared electron pairs. In 2, two of the methyls attached to oxygens are turned inward (those on rings A and E), and six are turned outward (those on rings B, C, D, F, G, and H). Additionally, a para methyl carbon of an adjacent host is located 3.3-3.5 A from the oxygens of rings B, D, F, and H. The cavity of 2 is larger than that of 1 and is shaped like a tangerine, with one quarter lined with methyls and three quarters lined with oxygen's unshared electron pairs. In 2'C₂H₅OH, three of the methyls attached to oxygens are turned inward (those on rings A, E, and H) and the other five are turned outward. The C₂H₅OH molecule is external to the cavity and overlies ring E, and its hydroxyl hydrogen bonds the oxygen of ring E. The crystal structure of 2•CsC10₄shows the Cs⁺guest to be ligated by the eight oxygens located at the apices of a square antiprism. The squares are 2 A apart and have sides ranging from 4.3 to 4,7 A. An oxygen from C10₄” completes the coordination about Cs⁺. The free energies were determined at 25 °C in CDCl₃saturated with D₂0 for 1 and 2 binding the picrate salts of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NH₄⁺, CH₃NH₃⁺, and ?-BuNH₃⁺. The -AG° values are maximal for 1 and 2 complexing Cs⁺(11.8 and 13.9 kcal mol”¹, respectively) and minimal for 1 and 2 complexing /-BuNH₃⁺(3.2 and 4.0 kcal mol”¹, respectively). The relationships between structure and binding are discussed in terms of the principle of preorganization.