Study on the Gas-Phase Reactivity of Charged Pyridynes

Abstract

The reactivities of three isomeric, charged ortho-pyridynes, the 1,2-, 2,3-, and 3,4-didehydropyridinium cations, were examined in the gas phase using Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry. The structures of selected product ions were probed using collision-activated dissociation (CAD) experiments in a linear quadrupole ion trap (LQIT) mass spectrometer. Mechanisms based on quantum chemical calculations are proposed for the formation of all major products. The products of the reactions of the charged ortho-pyridynes in the gas phase were found to closely resemble those formed upon reactions of neutral ortho-arynes in solution, but the mechanisms of these reactions exhibit striking differences. Additionally, no radical reactions were observed for any of the charged ortho-pyridynes examined, in contrast to previous proposals that orthobenzyne can occasionally react via radical mechanisms. Finally, the relative reactivities of those charged gaseous ortho-pyridynes that yielded similar product distributions were found to be affected mainly by the (calculated) vertical electron affinities of the dehydrocarbon sites, which suggests that the reactivity of these species is controlled by polar effects.

Gas-Phase Reactivity of Quinoline-Based Singlet Oxenium Cations

“…This reaction has been previously reported for organic compounds not undergoing radical reactions, such as ortho-benzynes, and it involves nucleophilic addition of the carbon−carbon double bond of allyl iodide to the cation followed by a homolytic C−I bond cleavage. 25,27,28 Upon reactions with cyclohexane, no single hydrogen atom abstraction products were detected for 1−4 (Table 1), in contrast to what has been reported for many carbon-centered radicals. 24,27,28 Instead, abundant product ions were formed that formally correspond to the addition of two hydrogen atoms to the oxenium cations.…”

“…However, these product ions are likely formed upon an initial hydride anion abstraction followed by a proton abstraction (see discussion below), as previously reported for some aromatic even-electron cations. 24,27,28 Based on the above observations, it can be concluded that 1−4 do not contain unpaired electrons.…”

“…However, CAD of radical cations of quinolines with either a methoxy or nitro group in the benzene moiety (Chart ) resulted in the fast elimination of a methyl or nitric oxide radical, respectively, thus generating cations with the m / z ratio of the desired oxenium cations ( 1 – 4 ; for examples, see Scheme and Figure ). The structures of the ions were probed via their reactivity toward various reagents and via verification of the structures of some of their product ions via CAD experiments, as discussed below. ,,, …”

“…Similarly to dimethyl disulfide, the most common radical reactions reported for allyl iodide (abstraction of one iodine atom from an allyl iodide molecule or consecutive abstraction of two iodine atoms from two allyl iodide molecules ,, ) were not observed for the cations believed to be 1 – 4 (Table ). Instead, abstraction of an allyl moiety was found to be the dominant reaction.…”

“…The structures of the ions were probed via their reactivity toward various reagents and via verification of the structures of some of their product ions via CAD experiments, as discussed below. 24,25,27,28 Structural Elucidation of 1−4. The reagents dimethyl disulfide, allyl iodide, and cyclohexane (Table 1) were chosen to probe the structures of the ions thought to be 1−4 via ion− molecule reactions as these reagents are known to react with carbon-centered organic mono-and biradicals (but not evenelectron cations) via transfer of one or two SCH 3 moieties, iodine atoms, or hydrogen atoms, respectively.…”