Conformational Studies by Dynamic NMR. 97. Structure, Conformation, Stereodynamics and Enantioseparation of Aryl Substituted Norbornanes

The structure of a 1,7,7-triaryl norbornane (compound 3) has been determined by X-ray diffraction
and was found essentially equal to that predicted by molecular mechanics calculations. Restricted
rotation of the aryl groups also has been observed by dynamic NMR spectroscopy in this compound
and in a number of analogously substituted norbornanes. The aryl−norbornane bond rotation
barriers were measured by line shape analysis of the 13C NMR spectra obtained at temperatures
lower than −100 °C and were found to cover the range 6.0 to 7.9 kcal mol-1. An exception was the
rotation involving the o-anisyl group in compound 5, which occurs near ambient temperature since
the corresponding barrier is much higher (14.4 kcal mol-1). In one case (compound 4) configurational
enantiomers could be separated by chiral HPLC and the corresponding CD spectra recorded.