The origin of an unusually large ¹⁹F chemical shift difference between the diastereomeric α-cyano-α-fluoro-p-tolylacetic acid (CFTA) esters of 3′,4′,5,7-tetra-o-methylepicatechin

During a study on the relationship between the ¹⁹F chemical shift difference for the diastereomeric α-cyano-α-fluoro-p- tolylacetic acid (CFTA) esters of chiral secondary alcohols and the absolute configurations of the alcohols, an unusually large ¹⁹F chemical shift difference has been observed for the CFTA esters of 3′,4′,5,7- tetra-O-methylepicatechin. This large chemical shift difference has been reproduced by ab initio calculations of molecular geometry and GIAO calculations of ¹⁹F chemical shifts on the stable conformations of the diastereomeric α-cyano-α-fluorophenylacetic acid (CFPA) esters of cis-flavan-3-ol as model systems. The origin of the large chemical shift difference has been further investigated using NBO analysis. This revealed that the interaction between π(C=C) of the aryl group in the chiral auxiliary and σ*(C-F) is a very important determinant of the ¹⁹F chemical shifts and this interaction depends on the torsion angle between the plane of aryl group and the C-F bond.