Abstract
Relative stabilities of four conformers and thermodynamic parameters for interconversion among four conformers of 25,26,27,28-tetramethoxycalix[4]arene (1a) and 5,11,17,23-tetra-tert-butyl-25,26,27,-28-tetramethoxycalix[4]arene (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.
| Original language | English |
|---|---|
| Pages (from-to) | 2109-2115 |
| Number of pages | 7 |
| Journal | Journal of the Chemical Society, Perkin Transactions 2 |
| Issue number | 12 |
| Publication status | Published - Dec 1 1992 |
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All Science Journal Classification (ASJC) codes
- Chemistry(all)
Cite this
Relative stabilities of tetramethoxycalix[4]arenes : Combined NMR spectroscopy and molecular mechanics studies. / Harada, Takaaki; Rudziński, Jerzy M.; Shinkai, Seiji.
In: Journal of the Chemical Society, Perkin Transactions 2, No. 12, 01.12.1992, p. 2109-2115.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Relative stabilities of tetramethoxycalix[4]arenes
T2 - Combined NMR spectroscopy and molecular mechanics studies
AU - Harada, Takaaki
AU - Rudziński, Jerzy M.
AU - Shinkai, Seiji
PY - 1992/12/1
Y1 - 1992/12/1
N2 - Relative stabilities of four conformers and thermodynamic parameters for interconversion among four conformers of 25,26,27,28-tetramethoxycalix[4]arene (1a) and 5,11,17,23-tetra-tert-butyl-25,26,27,-28-tetramethoxycalix[4]arene (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.
AB - Relative stabilities of four conformers and thermodynamic parameters for interconversion among four conformers of 25,26,27,28-tetramethoxycalix[4]arene (1a) and 5,11,17,23-tetra-tert-butyl-25,26,27,-28-tetramethoxycalix[4]arene (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.
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M3 - Article
AN - SCOPUS:37049083990
SP - 2109
EP - 2115
JO - Journal of the Chemical Society, Perkin Transactions 2
JF - Journal of the Chemical Society, Perkin Transactions 2
SN - 1470-1820
IS - 12
ER -