Conformational studies of selectively methylated [2.2](1,3)(1,4)cyclophanes. Steric effects in the relative conformational ground states

Abstract

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 of 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. © 1994 American Chemical Society.