Reactions of Isomeric Triplet and Singlet Aryloxenium Cations in the Gas Phase

ABSTRACT Aryloxenium cations are proposed to act as intermediates in many organic reactions. However, the reactivity of only two such cations with a triplet electronic ground state has been reported. One was studied in solution where only reactions with solvents could be explored. Another one was examined in the gas phase by using tandem mass spectrometry where isolation of the cation is possible, making the study of its reactions with reagents other than solvents feasible. In this preliminary study, the 2‐amino‐4‐pyridyloxenium cation was found to exclusively undergo radical reactions, which have not been reported for singlet aryloxenium cations. Herein, the study on the reactivity of the triplet pyridyloxenium cation has been expanded to include experimentation on additional reactions and a more detailed computational examination of the reaction mechanisms. The results are compared to those obtained for the isomeric singlet 3‐amino‐4‐pyridyloxenium and 3‐amino‐2‐pyridyloxenium cations. The triplet cation exhibits radical‐like behavior toward most of the studied reagents, with reactions being initiated by (radical) abstraction of atoms, or groups of atoms, or radical addition to C=C bonds. In contrast, the isomeric singlet pyridyloxenium cations are unreactive toward or react very slowly with many of the same reagents. Their reaction efficiencies are substantially lower than those of the previously studied singlet quinolyloxenium cations due to their lower electrophilicity. The mechanisms of the above reactions were explored computationally. Important information was obtained for the relative reactivity and the most reactive sites in different types of aryloxenium cations.