Wp. Vanhoorn et al., THEORETICAL-STUDY OF THE CONFORMATIONAL ISOMERISM OF 2,4,6-SUBSTITUTED 1,3,5-TRIMETHOXYCALIXARENES, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 102(33), 1998, pp. 6676-6681
For 2,4,6-trisubstituted 1,3,5-trimethoxycalix 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).