TY - JOUR
T1 - Conformational Studies of Selectively Methylated [2.2](1,3)(1,4)Cyclophanes. Steric Effects in the Relative Conformational Ground States
AU - Lai, Yee Hing
AU - Yap, Angeline Hui Tin
AU - Novak, Igor
PY - 1994/6/1
Y1 - 1994/6/1
N2 - 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.
AB - 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.
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U2 - 10.1021/jo00091a027
DO - 10.1021/jo00091a027
M3 - Article
AN - SCOPUS:0343440728
SN - 0022-3263
VL - 59
SP - 3381
EP - 3385
JO - Journal of Organic Chemistry
JF - Journal of Organic Chemistry
IS - 12
ER -