Partially O-Alkylated thiacalix[4]arenes: Synthesis, molecular and crystal structures, conformational behavior

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Authors

DVOŘÁKOVÁ Hana LANG Jan VLACH Jiří SÝKORA Jan ČAJAN Michal HIML Michal POJAROVÁ Michaela STIBOR Ivan LHOTÁK Pavel

Year of publication 2007
Type Article in Periodical
Magazine / Source Journal of Organic Chemistry
MU Faculty or unit

Faculty of Science

Citation
Field Organic chemistry
Keywords ALTERNATE CONFORMATION; CONE INTERCONVERSION; INFINITE CHANNELS; SOLID-STATE; X-RAY; COMPLEXATION; THIACALIXARENES; CONFORMERS; H-1-NMR; NMR
Description NMR spectroscopy, X-ray diffraction analysis, and quantum chemical calculations were used for conformational behavior study of partially alkylated thiacalix[4]arenes bearing methyl (1), ethyl (2), or propyl (3) groups at the lower rim. The conformational properties are governed by two basic effects: (i) stabilization by intramolecular hydrogen bonds, and (ii) sterical requirements of the alkoxy groups at the lower rim. While the monosubstituted derivatives la and 3a adopt the cone conformation in solution, distally disubstituted compounds 1b, 1'b, 2b, 2'b, 3b, and 3'b exhibit several. interesting conformational features. They prefer pinched cone conformation in solution, and, except for 3'b, they form also 1,2-alternate conformation, which is flexible and undergoes rather fast transition between two identical structures. The crystal structures of the compounds 1b, 2b, 2'b, and 3b revealed yet quite rare 1,2-alternate conformation forming molecular channels held together by pi-pi interactions. Different channels-with hexagonal symmetry, 0.26 nm wide-are formed in the crystal structure of the pinched cone conformation of 3b. An uncommon hydrogen bonding pattern was found in dimethoxy and dipropoxy derivatives 1'b and 3'b that adopt distorted cone conformations in crystal. Trialkoxy-substituted compounds 1c and 3c adopt the partial cone conformation in solution. A higher mobility of methyl derivative 1c enables also existence of the cone conformer.
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