Aminopalladation mechanistic pluralism probed by helium nanodroplet ion spectroscopy

We employed mass spectrometry and infrared ion spectroscopy of mass-selected ions in ultracold helium nanodroplets to identify and structurally characterize transient species generated during the intramolecular aminopalladation step of a palladium(II) chloride-catalyzed reaction of O-allyl tosylcarbamate. We also investigated how addition of a quaternary ammonium salt affected the mechanistic landscape of the reaction. By recording mass spectra of reaction mixtures under different conditions, we identified signals with m/z ratios and isotope patterns consistent with previously proposed intermediates for both syn- and anti-aminopalladation pathways. Addition of tetramethylammonium (TMA) acetate reduced the intensity of the anti-aminopalladation (anti-AP) exclusive intermediate. The mass-selected anti-AP intermediate, characterized by helium nanodroplet spectroscopy, was consistent with previously proposed structures, while the other intermediate of interest corresponded to two isomeric species, one exclusive to the syn-AP step and another present in both proposed reaction pathways. Reaction monitoring under conditions similar to those used in synthesis revealed the evolution of the isomer mixture over time, which was interpreted as the consumption of the syn-AP exclusive intermediate and the formation of the intermediate common to both pathways. These results provide experimental evidence that both syn- and anti-aminopalladation pathways are feasible and could contribute to the outcome of the reaction.