Studies in Organic Mass Spectrometry. Part 20: A Hidden Ortho Effect in the Electron Ionisation Mass Spectra of Some 2′-Alkyl Substituted 2- and 3-Thiophenecarboxanilides

The electron-ionisation-induced amide-bond cleavage of some 2′-methyl- and 2′-ethyl-substituted 2- and 3-thiophenecarbox-anilides, which yields formally anilylium ions having relative intensities apparently in contrast with the Stevenson–Audier rule, has been investigated by mass-analysed ion kinetic energy (MIKE) spectrometry and compared to that of the 3′- and 4′-isomers. It has been shown that, in the case of the 2′-methyl and 2′-ethyl derivatives, the amide-bond cleavage is anchimerically assisted through the hidden migration of a benzyl hydrogen to the nitrogen. Analysis of the MIKE and collision-induced decomposition (CID) MIKE spectra of model compounds indicates that this cryptic ortho effect produces a stable ortho quinoide or aminotropylium structure.