STUDIES IN ORGANIC MASS-SPECTROMETRY .20. A HIDDEN ORTHO EFFECT IN THE ELECTRON IONIZATION MASS-SPECTRA OF SOME 2'-ALKYL SUBSTITUTED 2-THIOPHENECARBOXANILIDES AND 3-THIOPHENECARBOXANILIDES

The electron-ionisation-induced amide-bond cleavage of some 2'-methyl- and 2'-ethyl-substitute 2- and 3-thiophenecarboxanilides, which yield s formally anilylium ions having relative intensities apparently in co ntrast with the Stevenson-Audier rule, has been investigated by mass-a nalysed 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 anchimeri cally assisted through the hidden migration of a benzyl hydrogen to th e 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.