Mass Spectra of New Heterocycles: XXVIII. Electron Impact and Chemical Ionization Mass Spectra of 3- and 5-(1H-Pyrrol-1-yl)- and 3-(1-Methyl-1H-pyrrol-2-yl)thiophen-2-amines

Abstract: The behavior of previously unknown 3- and 5-(1H-pyrrol-1-yl)- and 3-(1-methyl-1H-pyrrol-2-yl)thiophen-2-amines under electron impact (70 eV) and chemical ionization (reactant gas methane) has been studied. The title compounds were synthesized in one pot from lithiated 1-propa-1,2-dien-1-yl-, 1-prop-2-yn-1-yl-, and 1-methyl-2-prop-1-yn-1-yl-1H-pyrroles, isothiocyanates, and methyl iodide. Under electron impact, all compounds formed stable molecular ions (M+·, Irel 100%) which underwent fragmentation along two common pathways, C–N bond cleavage in the amine substituent with charge localization on the sulfur atom and generation of [M – R]+ ions (first pathway) and [M – Me]+ ions (second pathway). An exception was N-butyl-substituted thiophenamine which showed no [M – Me]+ ion in the mass spectrum. The stability of these ions depended on the nature of the R substituent. Thus, the [M – R]+ ion peak (R = Alk) had the highest intensity (Irel 28–91%), whereas the [M – R]+ ions (R = Ar) were the least abundant (Irel 3–8%). In contrast, the intensity of the [M – Me]+ ion peak in the mass spectra of N-aryl-substituted thiophenamines (Irel 28–52%) was higher than the intensity of the corresponding ion peak of N-alkyl analogues (Irel 14–41%), except for N-ethyl-3-(1-methyl-1H-pyrrol-2-yl)thiophen-2-amine, in the spectrum of which the intensity of the [M – Me]+ ion peak was 57%. The [M – R]+ and [M – Me]+ ions derived from all N,N-dimethylthiophen-2-amines had the same m/z value and structure. The third fragmentation pathway was observed only for N-alkylthiophenamines; it involved α-cleavage in the amine substituent with charge localization on the exocyclic nitrogen atom. The fragmentation pathways of ions derived from all thiophenamines studied were proposed on the basis of analysis of tandem mass spectra (MS2). All 3-(1H-pyrrol-1-yl)thiophen-2-amines under chemical ionization with methane as reactant gas easily underwent protonation, charge transfer, and electrophilic addition processes, and the resulting M+·, [M + H]+ and [M + Et]+ gave high-intensity peaks in the mass spectra. © The Author(s) 2024.