Host-Guest Complexation. 32. Spherands Composed of Cyclic Urea and Anisyl Units

Article abstract of DOI:10.1021/ja00335a049

Five new spherands and three new hemispherands containing cyclic urea units have been designed and synthesized, and their binding properties have been examined.Formally, these macrocycles are composed by attaching to one another the following units: 2,6-disubstituted anisyl, or A; 2,6-disubstituted 4-methylanisyl, or A'; N,N-disubstituted tetrahydro-2-pyrimidinone, or U; 2,6-disubstituted 1-(allyloxy)-4-methylbenzene, or A'a; 2,6-disubstituted 1-(benzyloxy)-4-methylbenzene, or A'b; 2,6-disubstituted 4-methylphenol, or A'h; 1,2-disubstituted benzene, or B; and CH2 groups.Host structures and key intermediates are visualized by line formulas in which adjacent letters indicate bonded units. The macroring acts as a semirigid support structure for convergently arranged carbonyl and methoxy groups.Key ring-closing reactions involve urea NH group substitution of ArCH2Br in (CH2)4O-NaH under high dilution.Treatment of U(A'UH)2 with (BrCH2)2B gave U(A'UCH2)2B (1, 7percent), of A(AUH)2 with (BrCH2)2B gave A(AUCH2)2B (2, 11percent), of A(AUH)2 with (BrCH2)2A' gave A(AUCH2)2A' (3, 32percent) of A'b(A'UH)2 with (BrCH2)2A'a gave A'b(A'UCH2)2A'a (4, 41percent), deallylation of which with Pd-EtOH-TsOH gave A'b(A'UCH2)2A'h (5, 25percent), of U(A'CH2Br)2 with (HU)2A' gave U(A'CH2U)2A' (6, 60percent), and of A'b(A'CH2Br)2 with (HU2)A'b gave A'b(A'CH2U)2A'b (7, 41percent), which was hydrolytically debenzylated to give A'h(A'CH2U)2A'h (43percent), methylation of which produced A'(A'CH2U)2A' (8, 71percent).These compounds were generally purified as their NaBr complexes, which were decomplexed by crystallizing the free host from MeOH-H2O mixtures by MeOH evaporation (a phase-transfer process).Crystal structures of complexes A(AUCH2)2A'*NaBr*H2O (3*NaBr*H2O) and A(AUCH2)2A'*CsClO4*H2O (3*CsClO4*H2O) are discussed.Association constants (K_a) between host and guest to give complexes were determined by extracting picrate salts (guests) from D2O into CDCl3 in the absence and presence of hosts at 25 deg C.The rates of extraction were essentially instantaneous on the human time scale.The free energies for complexation for the seven hosts with picrate salts of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NH4⁺, CH3NH3⁺, and t-BuNH3⁺ were determined.These -ΔG⁰ values (kcal mol^-1) ranged from a high of ca. 18.3 (U(A'UCH2)2B (1)) for Li⁺ to a low 7.2 (U(A'CH2U)2A' (6)) for (CH3)3CNH3⁺.Interesting structural recognition factors (K_a^G/K_a^G') for a host distinguishing between two similar guests (G and G') are as follows: Li⁺/Na⁺, U(A'UCH2)2B (1), 30; Na⁺/Li⁺, A'b(A'CH2U)2A'b (7), 3400; Na⁺/K⁺, A(AUCH2)2B (2), 2300; K⁺/Na⁺, A(AUCH2)2A' (3), 3; K⁺/Rb⁺, A'b(A'UCH2)2A'a (4), 110; Rb⁺/Cs⁺, A'b(A'UCH2)2A'a (4), 25; Cs⁺/Rb⁺, U(A'UCH2)2B (1), 2; NH4⁺/CH3NH3⁺, A(AUCH2)2A' (3), 9; CH3NH3⁺/NH4⁺, U(A'UCH2)2B (1), 1.7; CH3NH3⁺/t-BuNH3⁺, A'b(A'UCH2)2A'a (4), 4600; t-BuNH3⁺/CH3NH3⁺, U(A'UCH2)2B (1), 6.Correlations between the structures of hosts and guests and their free energies of binding are interpreted interms of the principles of complementarity and of preorganiz...