Substituent Effects on the Reactivity of the 2,4,6‐Tridehydropyridinium Cation, an Aromatic σ,σ,σ‐Triradical

Abstract

2,4,6-Tridehydropyridinium cation (7) undergoes three consecutive atom or atom group abstractions from reagent molecules in the gas phase. By placing a π-electron-donating hydroxyl group between two radical sites, their reactivity can be quenched by enhancing their through-space coupling via a favorable resonance structure. Indeed, 3-hydroxy-2,4,6-tridehydropyridinium cation (8) abstracts only one atom or group of atoms from reagents. On the other hand, an electron-withdrawing cyano group between two of the radical sites (9) destabilizes the analogous resonance structure and diminishes through-space coupling between the radical sites, resulting in abstraction of three atoms, just like 7. However, the cyano-substituent also increases acidity to the point that 9 reacts pre-dominantly via proton transfer instead of undergoing radical reactions. Therefore, acidic triradicals may undergo nonradical, barrierless proton transfer reactions faster than radical reactions, which are usually accompanied by barriers. Examination of the analogous cyano-substituted mono-and biradicals revealed behavior similar to that of the corresponding unsubstituted species, with the exception of substantially greater reactivities due to their greater (calculated) vertical electron affinities. Finally, the 3-cyano-2,6-didehydropyridinium cation with a singlet ground state (S-T splitting: –11.9 kcal mol−1) was found to react exclusively from the lowest-energy triplet state by fast proton transfer reactions.

Spin–Spin Coupling Between Two meta‐Benzyne Moieties In a Quinolinium Tetraradical Cation Increases Their Reactivities

“…The analogous tri‐ and tetraradicals containing meta ‐benzyne moieties should be even more interesting due to their complex electronic structures, which have inspired some theoretical studies on the triradicals . However, due to extreme difficulties in studying these species experimentally, the reactivity of only three such triradicals (2,4,6‐tridehydro‐, 3‐hydroxy‐2,4,6‐tridehydro‐, and 3‐cyano‐2,4,6‐tridehydropyridinium cations) and one tetraradical (2,4,6‐tridehydropyridine radical cation) has been studied thus far and only in the gas phase . Unlike the triradicals, the tetraradical was found not to undergo radical reactions .…”

“…Briefly, its tetraiodo precursor was protonated in an atmospheric‐pressure chemical ionization source, the ions were isolated in the LQIT by ejecting all unwanted ions, and the iodine atoms were cleaved off by collision‐activated dissociation. Previous studies have demonstrated that the radical sites created in this manner in aromatic systems do not undergo rearrangement . The structures of all radical species were verified by using well‐established structurally diagnostic reactions, as described previously .…”

“…Also, the reactivities of the radicals were compared to those of several isomeric radicals to rule out isomerization reactions. Reaction efficiency (i.e., percentage of gas‐phase collisions that result in product formation) was determined for all reactions as described in the literature . The efficiencies correlate inversely with reaction barriers: the greater the reaction efficiency, the lower the barrier for the reaction.…”

“…Te traradical 1 was generated in al inear quadrupole ion trap (LQIT) mass spectrometer by using methods described previously (additional details are provided in the Supporting Information). [3][4][5][7][8][9][10] Briefly,i ts tetraiodo precursor was protonated in an atmosphericpressure chemical ionization source, the ions were isolated in the LQIT by ejecting all unwanted ions, and the iodine atoms were cleaved off by collision-activated dissociation. Previous studies have demonstrated that the radical sites created in this manner in aromatic systems do not undergo rearrangement.…”

“…However, due to extreme difficulties in studying these species experimentally, the reactivity of only three such triradicals (2,4,6‐tridehydro‐, 3‐hydroxy‐2,4,6‐tridehydro‐, and 3‐cyano‐2,4,6‐tridehydropyridinium cations) and one tetraradical (2,4,6‐tridehydropyridine radical cation) has been studied thus far and only in the gas phase . Unlike the triradicals, the tetraradical was found not to undergo radical reactions . Due to resonance, this tetraradical is best described as a highly electrophilic carbocation with a delocalized, and therefore unreactive, meta ‐benzyne moiety.…”