Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
5280205 | Tetrahedron Letters | 2009 | 4 Pages |
Abstract
Tetranitroazacalix[4]arenes have been synthesized by the nucleophilic aromatic substitution of 1,5-difluoro-2,4-dinitrobenzene with 1,3-diaminobenzenes. An X-ray crystal structure analysis revealed that the azacalixarenes adopt a non-symmetrical 1,3-alternate conformation, and the dinitrobenzene rings strongly conjugate with the bridging nitrogen atoms. In the 1H NMR spectrum (CDCl3, 30 °C), the tetraisopropyl derivative 3b displays a pair of diastereotopic methyl signals of the isopropyl groups in agreement with the frozen 1,3-alternate conformation on the NMR time scale. The free energy of activation (ÎG298â ) for the macrocyclic inversion was determined to be 87.5 kJ molâ1 by temperature-dependent NMR spectroscopy.
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Authors
Hisatoshi Konishi, Shun Hashimoto, Terunobu Sakakibara, Shingo Matsubara, Yusuke Yasukawa, Osamu Morikawa, Kazuhiro Kobayashi,