Gas-Phase Smiles Rearrangement Reactions of Deprotonated 2-(4, 6-Dimethoxypyrimidin-2-Ylsulfanyl)-N-Phenylbenzamide and Its Derivatives in Electrospray Ionization Mass Spectrometry

The negative ions of deprotonated 2-(4, 6-dimethoxypyrimidin-2-ylsulfanyl)-N-phenylbenzamide and its derivatives are studied by electrospray ionization tandem mass spectrometry (ESI-MS/MS). Upon collisional activation, the [M − H]− ions dissociate in two competitive pathways that can be considered as the gas-phase Smiles rearrangement reactions, giving rise to the characteristic fragment ions [M − H − C7H4OS]− and [M − H − C13H8NSR]− (R = substituent). Theoretical computations were invoked to shed light on the reaction mechanisms of the representative Compound 1 by the semiempirical PM3 method. These theoretical calculations show that the formation of [M − H − C13H8NSR]− (R = H for Compound 1) is more favorable. Furthermore, it is found that the intensities of the two product ions are strongly influenced by the position and the nature of the substituents. For the para-substituted compounds, the ln[(M − H − C7H4OS−)/(M − H − C13H8NSR−)] values are well correlated with the σp− substituent constants. In addition, the dependence of the intensity ratios of these two ions, ln[(M − H− C7H4OS−)/(M − H − C13H8NSR−)](R = CH3), on the collision energy can be used to distinguish the positional isomers.