Intramolecular Electrophilic Aromatic Substitution in Gas-phase Fragmentation of Protonated N-Benzylbenzaldimines

In this study, the gas-phase fragmentations of protonated N -benzylbenzaldimines were investigated by electrospray ionization tandem mass spectrometry (ESI-MS n ). Upon collisional activation, several characteristic fragment ions are produced and their fragmentation mechanisms are rationalized by electrophilic aromatic substitution accompanied by benzyl cation transfer. (1) For N -( p -methoxybenzylidene)-1-phenylmethanimine, concomitant with a loss of HCN, a product ion at m / z 121 was observed. It is proposed to be generated from electrophilic substitution at the ipso -position by transferring benzyl cation rather than cleavage of the C-N double bond. (2) For N -( m -methoxybenzylidene)-1-phenylmethanimine, a product ion at m / z 209 was obtained, corresponding to the elimination of NH 3 carrying two hydrogens from the two aromatic rings respectively. This process can be rationalized by two sequential electrophilic substitutions and cyclodeamination reaction based on the benzyl cation transfer. Deuterium-labeled experiments, density functional theory (DFT) calculation and substituent effect results also corroborate the proposed mechanism. Figure a ᅟ