Host-Guest Complexation. 25. Effects of Substituents on the Complexing Properties of Chorands

Article abstract of DOI:10.1021/jo00172a053

The syntheses of three benzannelated 18-crown-6 chorands (3,6,and 7) and three dibenzo-20-crown-6 chorands (10b-d) are reported, along with their free energies of association toward Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NH4⁺, CH3NH3⁺, and t-BuNH3⁺ picrates in CDCl3 at 25 deg C.For the 18-membered chorands, as well as previously reported paracyclophane-18-crown-6 hosts and naphtho-18-crown-6 hosts, the average negative free energies of association (-ΔG⁰_av)for Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and NH4⁺ picrates were qualitatively correlated with the degree of coplanarity of β-aryl substituents and the aromatic rings.The results are interpreted in terms of conformational organization of ligating sites in comparision with organization observed in crystalline complexes.Substuents were found to enhance complexation in the dibenzo-20-crown-6 series.This is interpreted as an effect of ligating-site preorganization and free-host destabilization.Substituent effects in chiral bicyclohexyl (13) and meso-bicyclohexyl hosts (14) are explained in similar terms.