A Possible Method for the Derivatization of Cyclic Sulfides with Polydialkylsiloxanes for Subsequent Study by “Soft” Ionization Mass Spectrometry

In developing methods for the preliminary chemical modification of nonpolar sulfur-containing compounds for their subsequent effective study by “soft” ionization [electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI)] mass spectrometry, a rather unexpected possibility of implementing S-alkylation of analytes with polydialkylsiloxanes in the presence of trifluoromethanesulfonic acid (CF₃SO₃H) as a superacid co-alkylating agent has been found. The ESI and MALDI mass spectra show that the corresponding S-alkylsulfonium salts are formed in both cases. However, in using polydimethylsiloxane, it is not possible to choose reaction conditions that would provide reproducible product yields. At the same time, in the case of polydiethylsiloxane, the reaction proceeds with an almost quantitative yield, regardless of the structure of the initial thiabicycloalkane. Subsequent studies of derivatization products are best performed using MALDI mass spectrometry, which allows the detection of target signals with high signal-to-noise ratios. In the case of ESI, the spectra are sometimes noticeably noisy with extraneous peaks, obviously due to hydrolysis and condensation products. A rational mechanism of the described chemical transformation is proposed, including indispensable activation by the trifluoromethanesulfonic acid accompanied by the rupture of the Si–C bond and the release of an alkyl cation modifying the sulfide. The effects of the structure and regio-isomerism on the collision-activated dissociation mass spectra of S-ethylated bicyclic sulfonic cations are considered.