Mass Spectra of New Heterocycles: XXX. Investigation of 2-(Alkylsulfanyl) Pyridines by Electron Ionization

The behavior of a representative number of previously unknown 2-(alkylsulfanyl)pyridines obtained from isothiocyanates, acetylene or allene carbonions and alkylating agents through the synthesis and aromatization of 6- (alkylsulfanyl)-2,3-dihydropyridines under electron ionization (70 eV) has been studied for the first time. All the studied compounds form stable molecular ions (M⁺,I_rel21 - 100%), the common direction of primary decay for most compounds is the formation of the [M - H]⁺ ion. Moreover, for 3-ary(heteroaryl)-substituted pyridines, except for 6-(vinyloxymethyl)-2-(methylsulfanyl)-3-phenylpyridine and 6-methyl-2-(methylsulfanyl)pyridine, this pathway is the main one. Other significant directions of primary fragmentation of M⁺ of the studied compounds are associated with the degradation of the alkylsulfanyl group, resulting in ions [M - M]⁺,[M - SH]⁺,[M - SCH]⁺,[M - SCH]⁺,[M - SM]⁺, or [M - Et]⁺ depending on the nature and position of substituents in the pyridine ring. The introduction of a substituent (R, OR, SR) into the pyridine molecule, in which R = Alk > Me, causes the appearance of a competing direction of decomposition of M⁺, associated with the release of an alkene molecule. Fragmentation of molecular ions of 2-(alkylsulfanyl)pyridines containing the OR substituent is determined by the localization of the charge either on the oxygen atom or on the sulfur atom. With an increase in the length of the alkyl substituent (R = Bu, MeCHOEt), along with a simple rupture of bonds, the occurrence of the McLafferty rearrangement is noted. Fragment ions formed during the fragmentation of molecular ions of 3-aryl(heteryl)-containing pyridines are stabilized through rearrangement into polycyclic aromatic structures, which are then practically not destroyed.

2-(alkylsulfanyl)pyridines, electron ionization, mass-spectra, molecular ions, fragmentation, McLafferty rearrangement