Effects of Amino-Substitution on the Gas-Phase Reactivity of Quinoline-Based Singlet Oxenium Cations

The possibility of generating aryloxenium cations with triplet ground states was explored by comparing the gas-phase reactivities of amino-substituted quinolyloxenium cations to the corresponding unsubstituted cations studied previously. The amino-substituted cations were calculated to have a singlet electronic ground state by at least 3 kcal mol^-1. None of them undergo radical reactions. However, all cations abstract hydride from cyclohexane by the oxygen atom, the only reaction that initially involves this atom. This is followed by proton transfer from the cyclohexyl cation or by addition of the cyclohexyl cation to the pyridine nitrogen atom in the initial collision complex. All cations form stable adducts with dimethyl disulfide (one exception), allyl iodide and water. Based on computations, the most favorable addition site for hydride (after oxygen atom) in most cations is the carbon atom in the benzene ring bearing the greatest positive charge. This was also true for the addition of dimethyl disulfide to two of the unsubstituted cations. However, the favored addition site for two amino-substituted cations is the carbon atom next to the carbonyl group. Another difference between the unsubstituted and amino-substituted cations is that the latter have drastically reduced reactivity due to the presence of the electron-donating amino group.