Interaction of achiral host and guest in a chiral solvent as studied by circular dichroism spectroscopy. Complexation of esters and ethers with calix[4]resorcarene in limonene

Abstract

Abstract When dissolved in limonene as a chiral hydrocarbon solvent, calix[4]resorcarene exhibits induced circular dichroism (CD). The CD intensities become weaker in the presence of increasing amounts of an ester or ether as a guest. This is due to the formation of an achiral host-guest complex, which competes with the chiral host-solvent interaction. Analysis of the CD data allowed characterization of the host-guest complexes in terms of binding constants and stoichiometries. The major driving force of the complexation is host-guest hydrogen-bonding. Dimethyl dicarboxylates and diac-etoxyalkanes form expectedly more stable complexes than the corresponding monoesters; diesters are capable of multiple host-guest hydogen-bonding. The affinities of monoesters are remarkably dependent on the chain-lengths of the alkyl moieties; this provides further evidence for the importance of guest-host CH-π or van der Waals interactions. Diesters having an appropriate chain-length exhibit a 2:1 (host to guest) stoichiometry. The complexation of higher homologs of ethyleneglycol dimethyl ether shows a similar 2:1 or 3:1 stoichiometry. These results strongly suggest that the host molecules cluster around a diester or an oligoether guest in a highly cooperative manner via host-host hydrogen-bonding. The potentiality of the present CD method for the evaluation of achiral host-guest interaction, especially that in a hydrocarbon solvent, is discussed.