Experimental study of the transformation of 2-aminophenylacetic acid hexafluorosilicate into 2-oxindol under mild conditions

Abstract

Currently, ammonium hexafluorosilicates are being actively studied as new anti-caries agents, which, according to the results of researches on rats, show a more significant caries-preventive effectiveness compared to sodium fluoride with a simultaneous significant improvement of the biochemical indicators of the dental pulp and the absence of hepatotoxic effects. Previously, we’ve synthesized 2-, 3-, 4-aminophenylacetic acid hexafluorosilicates (2-, 3-, 4-APhHFS); in an attempt to recrystallize the 2-APhHFS salt from ethanol, a red crystalline product was obtained.
 The purpose of the research – considering the potential prospects of 2-, 3-, 4-APhHFS, it is expedient to establish the composition and structure of the recrystallization product of 2-APhHFS. Mass spectra were recorded on MX-1321 and VG 7070 spectrometers (VG Analytical), X-ray structural analysis was carried out on an Xcalibur E diffractometer.
 As follows from the comparison of the external inspection of the original 2-APhHFS and its recrystallization product (RP) samples, they differ significantly: light brown amorphous 2-APhHFS and red crystalline RP. The characteristics of the mass spectra of 2-APhHFS and RP are also different. The structure of RP as 2-oxindole was objectively established by X-ray diffraction data.
 The recrystallization process of 2-APhHFS to obtain 2-oxindole does not require heating of the reaction mixture, i.e., it is carried out under extremely mild conditions. It is possible that it's connected with the participation of the protonated form of 2-aminophenylacetic acid in the composition of 2-APhHFS in chemical transformations.
 Thus, caution should be exercised in experiments with both solutions and, apparently, solid samples of 2-APhHFS during salt purification procedures or other manipulations. The high lability of 2-aminophenylacetic acid hexafluorosilicate limits the possibilities of using standard purification methods (sublimation, recrystallization) for this compound.