INTERACTION OF ACHIRAL HOST AND GUEST IN A CHIRAL SOLVENT AS STUDIED BY CIRCULAR-DICHROISM SPECTROSCOPY - COMPLEXATION OF ESTERS AND ETHERSWITH CALIX[4]RESORCARENE IN LIMONENE

When dissolved in limonene as a chiral hydrocarbon solvent, calix[4]re sorcarene exhibits induced circular dichroism (CD). The Co intensities become weaker in the presence of increasing amounts of an ester or et her as a guest. This is due to the formation of an achiral host-guest complex, which competes with the chiral host-solvent interaction, Anal ysis of the CD data allowed characterization of the host-guest complex es in terms of binding constants and stoichiometries. The major drivin g force of the complexation is host-guest hydrogen-bonding. Dimethyl d icarboxylates and diacetoxyalkanes form expectedly more stable complex es than the corresponding monoesters; diesters are capable of multiple host-guest hydogen-bonding. The affinities of monoesters are remarkab ly dependent on the chain-lengths of the alkyl moieties; this provides further evidence for the importance of guest-host CH-pi or van der Wa als interactions. Diesters haring an appropriate chain-length exhibit a 2:1 (host to guest) stoichiometry. The complexation of higher homolo gs of ethyleneglycol dimethyl ether shows a similar 2:1 or 3:1 stoichi ometry. These results strongly suggest that the host molecules cluster around a diester or an oligoether guest in a highly cooperative manne r via host-host hydrogen-bonding. The potentiality of the present CD m ethod for the evaluation of achiral host-guest interaction, especially that in a hydrocarbon solvent, is discussed.