Relative stabilities of four conformers and thermodynamic parameters for interconversion among four conformers of 25,26,27,28-tetramethoxycalixarene (1a) and 5,11,17,23-tetra-tert-butyl-25,26,27,-28-tetramethoxycalixarene (1b) have been determined by 1H NMR spectroscopy. The relative stability of 1a is in the order partial cone (most stable) > cone > 1,2-alternate and 1,3-alternate (undetected) and that for 1b is in the order partial cone (most stable) > cone > 1,2-alternate > 1,3-alternate (least stable). These orders are reproduced well by MM3, in contrast with MM2. In particular, the energy differences between partial cones and cones computed by MM3 (0.27 kcal mol-1 for 1a and 1.50 kcal mol-1 for 1b)† show good agreement with those determined by 1H NMR spectroscopy (0.32 ± 0.13 kcal mol-1 for 1a and 1.2 ± 0.3 kcal mol -1 for 1b). Both the computational and the spectroscopic results suggest that the basic skeletons for cones, 1,2-alternates and 1,3-alternates are relatively rigid (sharp potential energy surfaces) whereas that for partial cones is more or less flexible (flattened potential energy surface). Thus, introduction of the tert-butyl groups into the para-positions destabilizes cones and 1,2-alternates because it is difficult to reduce the increased steric crowding by the conformational change. In 1,3-alternates four phenol units are parallel, so that introduced tert-butyl groups would increase the steric crowding to a lesser extent. The basic skeleton for partial cones changes significantly upon introduction of tert-butyl groups, indicating that the increased steric crowding is relaxed by the conformational change. The finding clearly explains why partial cones frequently appear as the most stable conformer.
|Number of pages||7|
|Journal||Journal of the Chemical Society, Perkin Transactions 2|
|Publication status||Published - 1992|
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