THEORETICAL-STUDY OF THE CONFORMATIONAL ISOMERISM OF 2,4,6-SUBSTITUTED 1,3,5-TRIMETHOXYCALIX[6]ARENES

Citation
Wp. Vanhoorn et al., THEORETICAL-STUDY OF THE CONFORMATIONAL ISOMERISM OF 2,4,6-SUBSTITUTED 1,3,5-TRIMETHOXYCALIX[6]ARENES, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 102(33), 1998, pp. 6676-6681
Citations number
34
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Chemistry Physical
ISSN journal
1089-5639
Volume
102
Issue
33
Year of publication
1998
Pages
6676 - 6681
Database
ISI
SICI code
1089-5639(1998)102:33<6676:TOTCIO>2.0.ZU;2-E
Abstract
For 2,4,6-trisubstituted 1,3,5-trimethoxycalix[6] arenes 1, two compet ing interconversion pathways have been postulated in the literature fo r the Cone/1,2,3Alternate exchange, viz the ''tert-butyl through the a nnulus'' and ''lower rim through the annulus'' pathways. Both pathways were compared with molecular modeling with the conjugate peak refinem ent method. One variable-size atom (Sx) was introduced to represent th e lower-rim substituents R, abstracting the ''O-CH2-rigid group'' moti fs to one ''O-CH2-Sx'' group. Both the postulated mechanisms of Cone - -> 1,2,3Alternate isomerization are plausible. For large lower-rim sub stituents (Sx greater than or equal to approximate to 6 Angstrom), the ''tert-butyl through the annulus'' mechanism is preferred over the '' Sx through the annulus'' mechanism. The calculated upper free energy b arrier for the isomerization process is 17.5 kcal mol(-1), reasonably close to the experimental value of approximately 21 kcal mol(-1) (van Duynhoven et al. J. Am. Chem. Soc. 1994, 116, 5814).