Crystal Chemistry of VAPOL

Ligands employed in enantioselective catalysis are capable of displaying rich phase behavior that
can significantly impact their properties, both structural and physical. To illuminate these issues
in a model system, the solid-state structure and properties of the vaulted biaryl ligand VAPOL
were investigated. Racemic VAPOL and solvates with toluene and ethyl acetate were structurally
characterized. In addition, two polymorphs of (S)-VAPOL were found, and the crystal packing in
these and a very stable CH2Cl2 solvate was elucidated. In contrast to BINOL, the unsolvated forms
of the ligand lack classical hydrogen-bonding motifs. Remarkably, the melting point of racemic
VAPOL is 86 °C higher than that of (S)-VAPOL form I, and there is a 2.0 kcal/mol difference in
stability at room temperature, favoring the racemate. These values are at the upper end of those
observed in the literature for differences between a racemic/chiral pair.