Reactivity of organic σ,σ,σ,σ,σ-pentaradicals

Max, Joann P.; Ma, Xin; Ding, Duanchen; Milton, Jacob; Nash, John J.; Kenttämaa, Hilkka I.

doi:10.1016/j.ijms.2018.10.026

Cited by 9 publications

(31 citation statements)

References 63 publications

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“…As the cyclohexyl radical is generated near the protonated nitrogen atom of the 2,4-didehydroquinolinium cation, the incipient radical might abstract the hydrogen atom from the protonated nitrogen atom (Scheme 1) to generate the 2,4-didehydroquinolinium radical cation, which is not feasible for triradicals 1-3 that generate the cyclohexyl radical far away from the protonated nitrogen atom. Based on a previous publication, [47] the 2,4-didehydroquinolinium radical cation reacts with cyclohexane through predominant "addition-H" reaction (49 %) and abstraction of two H atoms (39 %), similarly as 4. This may explain why the triradical 4 shows different reactivity from the triradicals 1-3.…”

Section: Reaction Efficienciesmentioning

confidence: 99%

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

Ding

Feng

Chapman

et al. 2021

Chemistry A European J

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Examination of the reactions of σ-type quinoliniumbased 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.

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Section: Reaction Efficienciesmentioning

confidence: 99%

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

Ding

Feng

Chapman

et al. 2021

Chemistry A European J

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“…meta -Benzyne derivatives have received increasing interest − as their radical reactivity has been demonstrated to be tunable from nonexistent to predominant by minor structural changes, such as the addition of substituents to the system. , They are also important intermediates in many organic reactions and bioorganic processes. ,− However, the examination of the chemical properties of meta -benzynes in condensed phases is challenging either due to the difficulty in generating pure meta -benzynes or due to their high reactivities under such conditions. Therefore, many gas-phase reactivity studies have been carried out to explore the reactivity of protonated meta -benzyne analogues by using ion trap mass spectrometry. − …”

Section: Introductionmentioning

confidence: 99%

Protonated Ground-State Singlet meta-Pyridynes React from an Excited Triplet State

Feng

Jiang

et al. 2021

J. Org. Chem.

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The gaseous 2,6-didehydropyridinium cation and its derivatives transfer a proton to reagents for which the reaction for their singlet ground states is too endothermic to be observed. These reactions occur from the lowest-energy excited triplet states, which has not been observed (or reported) for other meta-benzyne analogues. Quantum chemical calculations indicate that the (excited) triplet states are stronger Brønsted acids than their (ground) singlet states, likely due to unfavorable three-center, four-electron interactions in the singlet-state conjugate bases. The cations have substantially smaller (calculated) singlet–triplet (S–T) splittings (ranging from ca. −11 to −17 kcal mol–1) than other related meta-benzyne analogues (e.g., −23.4 kcal mol–1 for the 3,5-isomer). This is rationalized by the destabilization of the singlet states (relative to the triplet states) by reduced (spatial) overlap of the nonbonding molecular orbitals due to the presence of the nitrogen atom between the radical sites (making the ring more rigid). Both the singlet and triplet states are believed to be generated upon formation of these biradicals via energetic collisions due to their small S–T splittings. It appears that once the triplet states are formed, the rate of proton transfer is faster than the rate of intersystem crossing unless the biradicals contain heavy atoms.

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“…The Journal of Organic Chemistry respectively. 10 Quite different results were obtained in this study (Table 2). The exact reason for this discrepancy is not known at this time, but the values reported here were found to be highly reproducible, which was not true for the previously reported values.…”

Section: Chart 1 2-pyridyl Cationmentioning

confidence: 69%

“…The EA v and ΔE S−T values of the 4,6-didehydroquinolinium (q13) and 4,7-didehydroquinolinium (q14) cations have been calculated before. 10 In the same study, their reaction efficiencies with cyclohexane were determined to be 2 and 4%, Branching ratios of primary reactions were determined by dividing the abundance of the product ion by the sum of the abundances of all product ions. d Calculated at the CASPT2/CASSCF (12,11)/cc-pVTZ//UB3LYP/cc-pVTZ level of theory.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…The related monoradicals have been reported to react with these reagents in the gas phase via simple radical pathwaysH or I atom abstraction, respectively. 10,11,14 A kinetic reactivity study was carried out, and the results were compared with those obtained for related monoradicals 11 and those previously reported 8,11,14 for (iso)quinolinium-based biradicals. The experiments were complemented by quantum chemical calculations.…”

Section: ■ Introductionmentioning

confidence: 99%

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Effects of the Distance between Radical Sites on the Reactivities of Aromatic Biradicals

Ding

Jiang

et al. 2020

J. Org. Chem.

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Coupling of the radical sites in isomeric benzynes is known to hinder their radical reactivity. In order to determine how far apart the radical sites must be for them not to interact, the gasphase reactivity of several isomeric protonated (iso)quinoline-and acridine-based biradicals was examined. All the (iso)quinoliniumbased biradicals were found to react slower than the related monoradicals with similar vertical electron affinities (i.e., similar polar effects). In sharp contrast, the acridinium-based biradicals, most with the radical sites farther apart than in the (iso)quinolinium-based systems, showed greater reactivities than the relevant monoradicals with similar vertical electron affinities. The greater distances between the two radical sites in these biradicals lead to very little or no spin−spin coupling, and no suppression of radical reactivity was observed. Therefore, the radical sites can still interact if they are located on adjacent benzene rings and only after being separated further than that does no coupling occur. The most reactive radical site of each biradical was experimentally determined to be the one predicted to be more reactive based on the monoradical reactivity data. Therefore, the calculated vertical electron affinities of relevant monoradicals can be used to predict which radical site is most reactive in the biradicals.

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Reactivity of organic σ,σ,σ,σ,σ-pentaradicals

Cited by 9 publications

References 63 publications

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

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

Protonated Ground-State Singlet meta-Pyridynes React from an Excited Triplet State

Effects of the Distance between Radical Sites on the Reactivities of Aromatic Biradicals

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