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Conformational Behavior of Rigid and Dancing (1,3)(1,4)Cyclophanes
Author(s) -
Lai YeeHing
Publication year - 1995
Publication title -
journal of the chinese chemical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.329
H-Index - 45
eISSN - 2192-6549
pISSN - 0009-4536
DOI - 10.1002/jccs.199500088
Subject(s) - chemistry , conformational isomerism , ground state , conformational change , crystallography , eclipsed conformation , cyclophane , ring (chemistry) , stereochemistry , crystal structure , molecule , organic chemistry , physics , quantum mechanics
Annelation or substitution of the (1,4)‐bridged ring of [2 2 ](1,3)(1,4)cyclophane 4 , such as in 8 and 9 , respectively, result in two different conformational ground states in each case. Each conformational ground state involves a different destabilizing factor arising from one form of non‐bonding interaction. Only the conformers 8b and 9b were isolated experimentally. Their corresponding precursors 11 and 23 were also found to be conformationally rigid. Only the conformers 11a and 23a were isolated indicating an abrupt change in conformational preference going from the dithia[3 2 ]cyclophanes to the respective [2 2 ]cyclophanes. The selectively methylated [2 2 ]cyclophanes 27 , 31 and 32 were, however, found to be conformationally mobile. There is an increase of about 10–15 kJ mol −1 in conformational energy barrier going from 4 to 27 or 31 . This is consistent with the fact that the presence of the methyl substituents increases the relative conformational ground‐state energy more than it would affect the transition‐state energy. The conformational barrier of 32 was unexpectedly found to be similar to those of 27 and 31 . This could be explained by a decrease in the transition‐state energy due to an inward bending of the (1,4)‐bridged ring accompanied by a decrease in conformational ground‐state energy due to an outward bending of the methyl groups in 32 .