Cyclization of N-Arylcyclopropanecarboxamides into N-Arylpyrrolidin-2-Ones under Electron Ionization and in the Condensed Phase

RationaleMass spectrometry is known as an excellent method to predict the behavior of organic compounds in solution. The behavior of organic compounds in the gas phase inside the ion source of a mass spectrometer allows their intrinsic properties to be defined, avoiding the influence of intermolecular interactions, counter ions and solvent effects.MethodsArylpyrrolidin‐2‐ones were obtained by condensed‐phase synthesis from the corresponding N‐arylcyclopropanecarboxamides. Electron ionization (EI) with accurate mass measurements by high‐resolution time‐of‐flight mass‐spectrometry and quantum chemical calculations were used to understand the behavior of the molecular radical cations of N‐arylcyclopropanecarboxamides and N‐arylpyrrolidin‐2‐ones in the ion source of a mass spectrometer. The geometries of the molecules, transition states, and intermediates were fully optimized using DFT‐PBE calculations.ResultsFragmentation schemes, ion structures, and possible mechanisms of primary isomerisation were proposed for isomeric N‐arylcyclopropanecarboxamides and N‐arylpyrrolidin‐2‐ones. Based on the fragmentation pattern of the N‐arylcyclopropanecarboxamides, isomerisation of the original M+• ions into the M+• ions of the N‐arylpyrrolidin‐2‐ones was shown to be only a minor process. In contrast, this cyclization proceeds easily in the condensed phase in the presence of Brønsted acids.ConclusionsBased on the experimental data and quantum chemical calculations the principal mechanism of decomposition of the molecular ions of N‐arylcyclopropanecarboxamides involves their direct fragmentation without any rearrangements. An alternative mechanism is responsible for the isomerisation of a small portion of the higher energy molecular ions into the corresponding N‐arylpyrrolidin‐2‐one ions. Copyright © 2016 John Wiley & Sons, Ltd.