Solvent effects on the conformations and hydrogen bond structure of partially methylated p-tert-butylcalix[4]arenes

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Abstract
The effect of the solvent on the conformations of unsubstituted p-tert-butylcalix[4]arene (1) and its methyl ethers 2–6 has been investigated by 1H NMR spectroscopy. The conformational distribution of the 1,2-dimethyl ether 4 and of the tetramethyl ether 6 is strongly influenced by the solvent used. The exact geometry of the cone conformation of the 1,3-dimethyl ether 3 and of the 1,2-dimethyl ether 4 changes from distinct C2 symmetry in CCl4 to close to C4 symmetry in CS2. It seems that inclusion of a small solvent molecule (e.g. CS2) in the cone conformation can take place. Spectra recorded at temperatures up to 125°C in CDCl2CDCl2 showed that the mono- and 1,3-di-methyl ethers are fixed in the cone conformation, whereas the unsubstituted calix[4]arene and the tetramethyl ether are flexible. These observations support a concerted mechanism for the cone-to-cone interconversion in 1, in which two or more phenol rings rotate simultaneously. The hydrogen bonding in partially methylated calix[4]arenes was investigated by IR spectroscopy. In all calix[4]arenes with neighbouring hydroxy groups, a strong cooperativity effect of 80% or more was observed. The exact geometry of the cone conformation affects the strength of the hydrogen bonds, because it influences the O–H ⋯ O angle in the calix[4]arene. The effect of the solvent on the geometry of the cone conformation is translated in differences of up to 79 cm–1 in the OH-stretch frequencies for spectra recorded in CCl4 and in CS2.

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