Conformational Studies of Selectively Methylated (1,3)(1,4)Cyclophanes. Steric Effects in the Relative Conformational Ground States

Article abstract of DOI:10.1021/jo00091a027

Three selectively methylated <2.2>(1,3)(1,4)cyclophanes, namely, 12,15-dimethyl-, 12,16-dimethyl-, and 12,13,14,15-tetramethyl<2.2>(1,3)(1,4)cyclophanes, were synthesized from their corresponding dithia<3.3>(1,3)(1,4)cyclophanes by the photochemical desulfurization method.Dynamic 1H NMR spectroscopy was employed to estimate the relative conformational barrier in each <2.2>cyclophane.The results show that there is an increase of about 13 kJ mol^-1 going from the parent <2.2>(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.