Crystal structures of Mosher's salt and ester elucidated by X-ray crystallography

Abstract

The crystal structure of Mosher's salt 3, prepared from (R)-3,3,3-trifluoro-2-methoxy-2-phenylpropanoic acid [(R)-MTPA, (R)-1] and (R)-1-phenylethylamine [(R)-PEA, (R)-2], was determined using X-ray crystallography. In crystalline salt 3, the bifurcated hydrogen bond (i.e., the methoxy-group-assisted salt bridge) and the aromatic C–H⋯π interaction formed the closest ion pair of the MTPA anion and PEA cation. The crystal packing of salt 3 was also determined. The conformations of the crystalline (R_acid)-MTPA esters were then statistically examined using the Cambridge Structural Database (CSD). For secondary alcohols, ca. 65% of the dihedral angles between the ester carbonyl groups and the trifluoromethyl groups ranged between −30° and 0°. The other conformational properties associated with the trifluoromethyl, methoxy, and phenyl groups were examined in detail. The conformation of the MTPA ester prepared from the tertiary alcohol is also discussed in this paper.