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

Abstract

During a study on the relationship between the (19)F chemical shift difference for the diastereomeric alpha-cyano-alpha-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 alpha-cyano-alpha-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 pi(C=C) of the aryl group in the chiral auxiliary and sigma*(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.