Mass spectra of new heterocycles: XXXI. Fragmentation of functionalized 2,3-dihydropyridines by electron ionization

Behavior of previously unknown and unavailable 5,6-di-, 2,5,6-tri-, and 2,2,5,6-tetra-substituted 2,3-dihydropyridines, synthesized from allenic or acetylenic carbanions, isothiocyanates, and alkylating agents, under the action of electron ionization (70 eV) was studied for the first time. Analysis of the mass spectra of the studied 2,3-dihydropyridines revealed the key patterns of their fragmentation under electron impact. All the studied compounds form detectable molecular ions M^+•, the stability of which and the directions of decomposition depend significantly on the nature and position of the substituents in the heterocycle. For 2-R²-6-(methylsulfanyl)-5-methoxy-2,3-dihydropyridines (2-R¹ = H, 2-R² ≠ OMe), the formation of [M - Me]⁺, [M - SH]⁺ and [M - R²]⁺ ions is characteristic. In the presence of bulky substituents in position 2 or 5 (2-R² ≠ Me or 5-OAlk > OMe), the competing processes become the elimination of the Alk radical [M - Alk]⁺ and the alkene molecule [M - C_nH_2n]^+•. The main direction of fragmentation of molecular ions of 2-methoxy-substituted 5-alkoxy-6-(methylsulfanyl)-2,3-dihydropyridines [Alk = Et, Bu, EtOCH(Me)] is the elimination of a methanol molecule from position 2 of the heterocycle. In the case of 2,5-dimethoxy-6-(methylsulfanyl)-2,3-dihydropyridine, instead of forming the [M - MeOH]^+• ion, competition between the abstraction of Me and OMe radicals is observed. Compounds with acetal substituents in position 5 are characterized by decomposition by mechanisms typical of acetal decomposition, including rearrangements similar to the McLafferty rearrangement. For 2,2-dimethyl-5,6-bis(methylsulfanyl)-2,3-dihydropyridine, destruction of the heterocycle with the release of a MeSCN molecule is observed. In the absence of a substituent at position 2, the decomposition of the molecular ion of 5,6-bis(methylsulfanyl)-2,3-dihydropyridine leads to the ions [M - Me]⁺ and [M - SMe]⁺. The degradation products of the dihydropyridine ring, including the ion [M - MeSCN]^+•, were not identified in the mass spectrum of this compound. 5-Phenyl- and 5-(1-Me-pyrrol-2-yl)-6-(methylsulfanyl)-2,3-dihydropyridines under electron ionization conditions form the most stable molecular ions (I_rel 93-100%), the primary decomposition of which occurs in four (when 2-R² = Me) and seven (when 2-R² = CH₂=CHOCH₂) directions. Aromatization of 2-unsubstituted and 2-monosubstituted 2,3-dihydropyridines at high temperature (under chromatographic sample injection conditions) and/or by electron ionization has been discovered and experimentally confirmed. The process of aromatization proceeds according to the four main mechanisms, including the elimination of hydrogen molecules, methanol (from positions 2 and 5), as well as molecules formed from the substituent in position 2, which leads to the formation of 2,3-di-, 2,6-di- and 2,3,6-trisubstituted pyridines.