148873-79-4Relevant articles and documents
Conformational Studies of Selectively Methylated (1,3)(1,4)Cyclophanes. Steric Effects in the Relative Conformational Ground States
Lai, Yee-Hing,Yap, Angeline Hui-Tin,Novak, Igor
, p. 3381 - 3385 (2007/10/02)
Three selectively methylated (1,3)(1,4)cyclophanes, namely, 12,15-dimethyl-, 12,16-dimethyl-, and 12,13,14,15-tetramethyl(1,3)(1,4)cyclophanes, were synthesized from their corresponding dithia(1,3)(1,4)cyclophanes by the photochemical desulfurization method.Dynamic 1H NMR spectroscopy was employed to estimate the relative conformational barrier in each cyclophane.The results show that there is an increase of about 13 kJ mol-1 going from the parent (1,3)(1,4)cyclophane to its 12,15- and 12,16-dimethyl derivatives.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 the 12,13,15,16-tetramethyl derivative was unexpectedly found to be similar to those at the two dimethylated derivatives.This however could be explained by a decrease in the transition-state energy due to inward bending of the (1,4)-bridged ring accompanied by a decrease in conformational ground-state energy due to outward bending of the methyl groups.
