Racemization of (S)-profen thioesters by strong neutral bases in nonpolar organic solvents: Implication for ion-pair kinetic basicity

The racemization of (S)-profen 2,2,2-trifluoroethyl thioesters in isooctane with trioctylamine as base was carried out, in which the Hammett equation log(k*_int) = 3.584ο - 3.745 was successfully applied to describe the electron-withdrawing effect of the substituents to the α-phenyl moiety of the thioesters. A combination of neutral strong organic bases with different nonpolar solvents was employed to determine the second-order interconversion constants for the racemization of (S)-naproxen 2,2,2-trifluoroethyl thioester, in which solvent hydrophobicity was found to have less effect on the racemization. Implication for ion-pair kinetic basicity scale for the neutral strong bases in isooctane was further discussed.