Intramolecular aromatic substitution and amino‐Claisen rearrangement in substituted N‐(2‐propynyl)anilines on electron impact

Abstract

AbstractN‐(2‐Propynyl)anilines undergo amino‐Claisen rearrangement to a minor extent in the ion source, losing a molecule of HCN under electron impact conditions. However, metastable molecular ions with energies closer to threshold undergo Claisen rearrangement giving rise to more abundant [M − HCN]+· ions in the first field‐free region. Loss of a hydrogen from the molecular ion gives rise to the base peak in the mass spectrum of N‐(2‐propynyl)aniline. The hydrogen that is expelled for the formation of the [M − H]+ ion is observed to be from the amino nitrogen, propargylic carbon and the ortho‐carbon of the ring. The last process leads to a cyclic fragment involving intramolecular aromatic substitution. Double oxygen migration from the nitro group to the triple bond, due to the ortho effect, yields an abundant ion at m/z 105 in N‐(2‐propynyl)‐o‐nitroaniline. The proposed fragmentation pathways and ion structures are substantiated by high‐resolution data, B/E and B2/E linked‐scan spectra, collisionally activated dissociation–B/E linked‐scan spectra and deuterium isotopic labelling.