An Oxygen‐ peri ‐Bridged Quinolinium Cation and Its Monoradical Counterpart

A Portable Reagent Inlet System Designed to Diminish the Impact of Air and Water to Ion-Molecule Reactions Studied in a Linear Quadrupole Ion Trap

A portable reagent inlet system for a linear quadrupole ion trap (LQIT) mass spectrometer was designed to diminish the impact of air and water on gas-phase ion-molecule reactions. Compared to the traditional reagent mixing manifolds that has been extensively used for decades, the portable system is much simpler and has fewer junctions and a smaller inner space. These changes reduce the amount of air and water introduced into the mass spectrometer with the reagent. Furthermore, unlike the traditional manifolds, the portable system can be easily attached to or detached from the LQIT mass spectrometer. Finally, the price of the portable system is only 1/10 of that of a traditional manifold as estimated in 2022. Therefore, the portable system has several advantages over the traditional reagent mixing manifolds.

Gas-Phase Reactivity of Phenylcarbyne Anions

Gas-phase reactivities of the phenylcarbyne anion and its four derivatives were studied using a linear quadrupole ion trap mass spectrometer. The phenylcarbyne anions were calculated to have a triplet ground state (singlet-triplet splittings of 4-9 kcal mol^-1), with the exception of the 4-cyanophenylcarbyne anion that has a singlet ground state (singlet-triplet splitting of -1.9 kcal mol^-1). Only the phenylcarbyne anions with a triplet ground state react with acetone and dimethyl disulfide via radical mechanisms. On the other hand, only the phenylcarbyne anion with a singlet ground state abstracts H₂O and H₂C═C═O from acetic acid via electrophilic addition of the reagents to the anion. Finally, two hydroxy-substituted phenylcarbyne anions (with triplet ground states) partially tautomerize with the assistance of reagent molecules to the more stable distonic phenylcarbene anions. This occurs via abstraction of a proton from the reagent by the phenylcarbyne anion to generate a neutral (triplet) phenylcarbene and a reagent anion, which is followed by proton abstraction from the hydroxyl group of the neutral phenylcarbene by the reagent anion to generate the distonic phenylcarbene anion in an excited triplet state. Experiments performed on deuterated hydroxy-substituted phenylcarbyne anions verified the mechanism. The reactivities of the distonic phenylcarbene anions were found to be quite different from those of the phenylcarbyne anions. For example, they were found to abstract CH₂ from acetonitrile, which is initiated by C-H insertion─typical singlet carbene reactivity.

Spin-Spin Coupling Controls the Gas-Phase Reactivity of Aromatic σ-Type Triradicals

Examination of the reactions of σ-type quinolinium-based triradicals with cyclohexane in the gas phase demonstrated that the radical site that is the least strongly coupled to the other two radical sites reacts first, independent of the intrinsic reactivity of this radical site, in contrast to related biradicals that first react at the most electron-deficient radical site. Abstraction of one or two H atoms and formation of an ion that formally corresponds to a combination of the ion and cyclohexane accompanied by elimination of a H atom ("addition-H") were observed. In all cases except one, the most reactive radical site of the triradicals is intrinsically less reactive than the other two radical sites. The product complex of the first H atom abstraction either dissociates to give the H-atom-abstraction product and the cyclohexyl radical or the more reactive radical site in the produced biradical abstracts a H atom from the cyclohexyl radical. The monoradical product sometimes adds to cyclohexene followed by elimination of a H atom, generating the "addition-H" products. Similar reaction efficiencies were measured for three of the triradicals as for relevant monoradicals. Surprisingly, the remaining three triradicals (all containing a meta-pyridyne moiety) reacted substantially faster than the relevant monoradicals. This is likely due to the exothermic generation of a meta-pyridyne analog that has enough energy to attain the dehydrocarbon atom separation common for H-atom-abstraction transition states of protonated meta-pyridynes.

Light-driven selective aerobic oxidation of (iso)quinoliniums and related heterocycles

Selective C1–H/C4–H carbonylation of N-methylene iminium salts, catalyzed by visible-light photoredox and oxygen in the air, has been reported. A ruthenium complex acts as a chemical switch to conduct two different reaction pathways and to afford two different kinds of products. In the absence of the ruthenium complex, the Csp²–H bonds adjacent to the nitrogen atoms are oxidized to α-lactams by the N-methyleneiminium substrates themselves as photosensitizers. In the presence of the ruthenium complex, the oxidation reaction site of quinoliniums is switched to the C4 region, resulting in the formation of 4-quinolones. The use of two transformations directly introduces oxygen into the nitrogen heterocyclic skeletons under an air atmosphere.

The selective C1–H/C4–H carbonylation of N-methyleneiminium salts catalyzed by visible-light photoredox reactions and oxygen in the air has been reported.

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

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.