Gas phase intramolecular cyclisation reactions of ionized aromatic compounds in mass spectrometry

Abstract

Five novel gas-phase cyclisation reactions of suitably substituted aromatic compounds upon protonation by fast atom bombardment (FAB), chemical ionization (CI) and electrospray ionisation (ESI) in mass spectrometry have been explored. The [M+H]+ ions of N- [2-(benzoyloxy) phenyl]-benzamide eliminates benzoic acid with concomitant cyclisation, producing protonated 2-phenylbenzoxazole. A novel electrophilic cyclisation utilizing the protonated nitro group as electrophile, followed by competitive eliminations of two OH radicals and H2O from the [M+H]+ ions of 2-nitrophenyl phenyl ether afford heterocyclic product ions. Analogous cyclization of protonated 2-nitro-N phenylaniline followed by eliminations of H2O and OH radical yield heterocyclic fragments. The protonated 2-nitrophenyl-phenyl sulfide also undergoes cyclisation in gas-phase analogous to 2-nitrophenyl-phenyl ether. In addition, a novel ortho effect of nitro group involving cyclisations due to oxygen transfers from the nitro group to the sulphur atom occurs in protonated 2-nitrophenyl phenyl sulfide which resulted in the expulsions of SO and SO2 loss. Moreover, the sulfide undergoes oxidation at the emitter electrode in the ESI source during ionization in acidic solution. The mechanisms proposed for each cyclisation was supported by high-resolution data, chemical substitution, deuterium-labeling, tandem mass spectrometry experiments and theoretical calculations by density functional theory.