Conformational studies by dynamic NMR. 93.1 stereomutation, enantioseparation, and absolute configuration of the atropisomers of diarylbicyclononanes

The structure of 9-(4-methoxyphenyl)-9-(3-methylphenyl) bicyclo[3.3.1]nonane (1) has been determined by single-crystal X-ray diffraction, and the rotation barriers about the two aryl-C9 bonds have been measured by variable temperature NMR spectroscopy in solution. In the case of 9(4-methoxyphenyl)-9-(1-naphthyl)bicyclo[3.3.1]nonane (3), the barrier involving the naphthyl-C9 rotation was found to be so high as to allow the physical separation of the two atropisomers by enantioselective HPLC at ambient temperature: the absolute configuration could be established on the basis of the corresponding CD spectra. It was also observed that the Ar-C9 rotation barriers of monoaryl-substituted nonanes are much lower than those of the corresponding diaryl-substituted nonanes.