THE CONFORMATIONAL DISTRIBUTIONS AND INTERCONVERSIONS OF PARTIALLY METHYLATED CALIX[4]ARENES

Citation
Wp. Vanhoorn et al., THE CONFORMATIONAL DISTRIBUTIONS AND INTERCONVERSIONS OF PARTIALLY METHYLATED CALIX[4]ARENES, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 102(7), 1998, pp. 1130-1138
Citations number
33
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Chemistry Physical
ISSN journal
1089-5639
Volume
102
Issue
7
Year of publication
1998
Pages
1130 - 1138
Database
ISI
SICI code
1089-5639(1998)102:7<1130:TCDAIO>2.0.ZU;2-H
Abstract
The conformational distributions and Cone --> Inverted Cone (Cone') in terconversions of monomethoxy-, 1.2-dimethoxy-, 1,3-dimethoxy-, and tr imethoxycalix[4]arenes 3a-6a were investigated by molecular modeling, The calculated conformational distributions are generally in accordanc e with NMR data obtained for the p-tert-butyl derivatives 3b-6b. Caref ul examination of all possible inversion pathways showed that the high inversion barriers of Cone --> Cone' interconversion are caused by th e cooperative barriers of hydrogen bonds rupture and the steric barrie r of relating a methoxy group through the annulus. This cooperative ef fect is so strong that the Cone --> Cone' conversion is blocked at roo m temperature. The order of the calculated barriers for the Cone --> I nverted Cone interconversions is monomethoxy-(35.1 kcal mol(-1)) > 1,2 -dimethoxy-(32.2 kcal mol(-1)) approximate to 1,3-dimethoxy-(30.3 kcal mol(-1)) > trimethoxycalix[4]arene (27.0 kcal mol(-1)), values that a re in good agreement with the available qualitative experimental resul ts. The rate-limiting step involved in all cases is the rotation of a methoxy-bearing ring.