Study of fragmentation pathways of lithiated alpha,beta-unsaturated thioesters by electrospray ionization mass spectrometry.

The fragmentation pathways of lithiated alpha,beta-unsaturated thioesters with different substituents were investigated by electrospray ionization tandem mass spectrometry (ESI-MS/MS) in positive ion mode. In mass spectrometry of the alpha,beta-unsaturated thioesters, Ar-CH=CH-CO-S-Ph, loss of PhSLi and elimination of a thiophenol were the two major fragmentation reactions of the lithiated molecules. The elemental compositions of all the ions were confirmed by high-resolution Fourier transform ion cyclotron resonance tandem mass spectrometry (FTICR-MS/MS). The thioesters studied here were para-monosubstituted on the phenyl ring of cinnamoyl and the electron-withdrawing groups favored loss of a thiophenol, whereas the electron-releasing groups strongly favored the competing reaction leading to the loss of PhSLi to form a cinnamoyl cation, Ar-CH=CHCO(+). The intensity ratios of the two competitive product ions were well correlated with the sigma(p) (+) substituent constants. The mechanisms of these two competing routes were further investigated by density functional theory (DFT) calculations.