Competing benzyl cation transfers in the gas-phase fragmentation of the protonated benzyl phenylalaninates

• Dissociation of the [M + H]+ ions were investigated for benzyl phenylalaninates.
• Possible pathways were proposed for the competing benzyl cation transfers.
• The benzyl cation transfers were supported by high resolution MS, MSn spectra, deuterium labeling experiments and theoretical calculations.
• Benzyl cation is more favorable to migrate to the phenyl ring rather than to the amino N prior to the dissociation reaction of losing (H2O + CO).

In this study, the competing benzyl cation transfer reactions have been explored by investigating the gas phase chemistry of the protonated benzyl phenylalaninates. Protonation at the carboxylic O atom results in the breakage of the ester CO bond to afford the benzyl cation, which undergoes the competing migration to the amino N atom or the phenyl ring C atom. Both the amino and the phenyl ring hydrogen atoms can be activated to be mobile due to the electrophilic attack of the transferring benzyl cation, and migration of the activated hydrogen atom to the carboxylic hydroxyl leads to (H2O + CO) elimination of the precursor ion. Interestingly, it is much more preferred for the benzyl cation to transfer to the phenyl ring via the amino N, leading to the stepwise benzyl cation transfer, albeit the amino N atom contains more nucleophilic affinity. The mechanistic processes have been confirmed by the MS3 spectra data, along with D-labeling experiments and theoretical calculations.

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