Conformational Preferences of a Polar Biaryl: A Phase- and Enantiomeric Purity-Dependent Molecular Hinge

The favored (R S *,M*) diastereoisomer of 2-aryl-pyridine 1 in the solution state results from intramolecular dipole−dipole interactions. In the crystalline state, intermolecular interactions dominate, and the conformation switches reversibly to (R S *,P*). Only racemic 1 exhibits this switching property: enantiomerically pure 1 exists as the (R S *,M*) diastereoisomer in both the solution and crystalline state.