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.