J. Am. Chem. Soc. 135, 14548–14551 (2013)
To achieve this they designed a structure containing one biaryl unit linked to a substituted imidazole. The reduced steric demand of the five-membered imidazole ring means that rotation around the central axis would be expected to be facile; however, the presence of a pentafluorobenzyl group on N1 stabilized the chiral ground-state conformations. They obtained the enantiopure compound using a deracemization approach, which involved coordination of the racemate to a chiral palladium complex and subsequent heating in the presence of KPF6. This was then treated with 1,2-bis(diphenylphosphino)ethane, which regenerated the free ligand with an enantiomeric excess of 98%. This compound proved to be configurationally stable over a period of months, and this stability was attributed to π-stacking between the pentafluorophenyl ring and the biaryl unit. This claim was supported by X-ray crystallographic studies — showing π-stacking in the solid state — and by comparison with a sterically similar non-fluorinated analogue. This analogue showed a significantly decreased barrier to rotation and had a half-life of a mere 22 minutes at 75°C, as opposed to 8.7 hours for the original compound.
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