2000 Volume 1 Pages 47-56
Fluorination mechanism of 2-hydroxyl-3-phenylcarboxylate with diethylaminosulfur trifluoride (DAST) was investigated by using ab initio MO calculations. This reaction gave two products, 2-fluoro-3-phenylpropiolate and 2-fluoro-3-phenylpropionate, depending on the substituent on the phenyl ring (NO2, H, OCH3). Therefore, two possible mechanisms, the SN2 mechanism and that via the phenonium intermediate, were examined. As the activation energies for the SN2 reaction of the three compounds are almost same each other and very low, the substituent effects on the phenyl ring is too small to distinguish the product distribution observed for the compounds. The stabilization energy of phenonium ions, which are feasible intermediates for the SN1 reaction, are large difference between the three intermediates. As a result, the fluorination is considered to be proceeded on SN1 mechanism. Additionally, it is shown that the ability of leaving group for an intermediate generated from the reaction of alcohol with DAST is as large as mesyl and tosyl groups.
