Mass Spectra of New Heterocycles: XXX. Study of 2-(Alkylsulfanyl)pyridines by Electron Ionization Mass Spectrometry

The behavior of a representative series of previously unknown 2-(alkylsulfanyl)pyridines, prepared starting from isothiocyanates, acetylene or allene carbanions, and alkylating agents via the intermediate formation and aromatization of 6-(alkylsulfanyl)-2,3-dihydropyridines, under electron ionization (70 eV) has been studied for the first time. All studied compounds formed stable molecular ions (M^+•, I_rel 21–100%). In most cases, the primary fragmentation pathway involved the loss of a hydrogen atom to give an [M – H]⁺ ion. Moreover, for 3-ary(hetaryl)-substituted pyridines, except for 6-(vinyloxymethyl)-2-(methylsulfanyl)-3-phenylpyridine and 6-methyl-2-(methylsulfanyl)pyridine, this is the main fragmentation pathway. The other significant primary fragmentation pathways of the molecular ions of the synthesized compounds were associated with the fragmentation of the alkylsulfanyl group to form [M – Me]⁺, [M – SH]⁺, [M – SCH]⁺, [M – SCH₂]^+•, [M – SMe]⁺, or [M – SEt]⁺ ions, depending on the nature and position of substituents in the pyridine ring. The introduction of a substituent (R, OR, SR), where R = Alk > Me, into the pyridine molecule gives rise to a competing fragmentation pathway for the M^+• ion, associated with the loss of an alkene molecule. The fragmentation pathways of the molecular ions of 2-(alkylsulfanyl)pyridines with an OR substituent in the pyridine ring depends on whether the charge is localized on the oxygen or the sulfur atom. In the case of longer chain alkyl substituents (R = Bu, MeCHOEt), McLafferty rearrangement occurs along with simple bond cleavage. The fragment ions formed from the molecular ions of 3-aryl(hetaryl)pyridines are stabilized through the rearrangement into polycyclic aromatic structures that undergo little further fragmentation.