π‐Facial hydrogen bonding in the chiral resolving agent 2,2,2‐trifluoro‐1‐(9‐anthryl)‐ethanol and its racemic modification

Abstract

Abstract2,2,2‐Trifluoro‐(9‐anthryl)‐ethanol (TFAE) has been extensively used, in its pure enantiomeric forms, as a chiral solvating agent in nuclear magnetic resonance spectroscopy (NMR). It has also played an important role in the development of chiral stationary phases in liquid chromatography (LC). X‐ray crystallography of the enantiomeric and racemic crystals shows, in both cases, the formation of an intermolecular hydrogen bond between the OH and the π‐face of one of the rings of the anthracene aromatic system.1 Few examples of such hydrogen bonding have been published previously, and those that have are not as clear cut as in this case. An explanation for the hydrogen bonding is sought using molecular modelling via the PM3 analytically derived molecular electrostatic potentials. Using NMR and dynamic lineshape analysis, the barrier to rotation about the aryl‐carbon bond is estimated, indicating the CCF3 bond to be perpendicular to the anthracene axis in nonpolar solution. This conformation is identical to the conformation in the crystal. Evidence is also presented to support the formation of intermolecular π‐facial hydrogen bonding in TFAE solutions. It is thought that such hydrogen bonding may be implicated in chiral recognition using this compound. © 1994 Wiley‐Liss, Inc.