Substituent Effects in the Unimolecular Fragmentation of Anisole Dication Derivatives^†

The unimolecular fragmentation of metastable anisole dications is studied by tandem-mass spectrometry. Permutations of OH, OCH₃, and NH₂ substituents as well as extensive labeling experiments are used as probes to unravel the mechanisms of the various fragmentation pathways observed. The main channel corresponds to methyl ion elimination, which leads to an energetically favorable charge separation (“Coulomb explosion”). Loss of a neutral CO molecule forms another important branch, and the expelled CO has incorporated a carbon atom from the phenyl ring of anisole. Substitution of the anisole dication with OH, OCH₃, and NH₂ groups results in more specific fragmentations. Ortho- and para-substitutions lead to preferential methyl ion elimination, whereas meta-substitution favors CO loss. The substituent effects are also reflected in pronounced changes in the apparent kinetic isotope effects upon substitution of OCH₃ by OCD₃. On the basis of the various experimental findings and some explorative DFT calculations, possible mechanisms for the skeletal rearrangements prior to CO elimination are proposed.