Relative Reactivities of Three Isomeric Aromatic Biradicals with a 1,4‐Biradical Topology Are Controlled by Polar Effects

Abstract

Unexpectedly, the 5‐dehydroquinoline radical cation was formed in the gas phase from the 5‐iodo‐8‐nitroquinolinium cation upon ion‐trap collision‐activated dissociation. This reaction involves the cleavage of a nitro group to generate an intermediate monoradical, namely, the 8‐dehydro‐5‐iodoquinolinium cation, followed by rearrangement through abstraction of a hydrogen atom from the protonated nitrogen atom by the radical site. Dissociation of the rearranged radical cation through elimination of an iodine atom generates the 5‐dehydroquinoline radical cation. The mechanism was probed by studying isomeric biradicals and performing quantum chemical calculations. The 5‐dehydroquinoline radical cation showed greater gas‐phase reactivity toward dimethyl disulfide, cyclohexane, and allyl iodide than the isomeric 5,8‐didehydroquinolinium cation, which is more reactive than the isomeric 5,8‐didehydroisoquinolinium cation studied previously. All three isomers have a 1,4‐biradical topology. The order of reactivity is rationalized by the vertical electron affinities of the radical sites of these biradicals instead of their widely differing singlet–triplet splittings.

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

“…The precursors 1 b-4 b used to generate triradicals 1-4 were synthesized as described previously. [27,33,35,44] The precursors 5 b and 6 b used to generate triradicals 5 and 6 were synthesized and characterized as described in the Supporting Information.…”

“…All experiments were performed using a linear quadrupole ion trap (LQIT) mass spectrometer as described in the literature. [12,33,38,41,44,45] Briefly, methanol solutions of the radical precursors were injected into an atmospheric pressure chemical ionization (APCI) source of the LQIT to generate protonated radical precursors. These ions were transferred into the linear quadrupole ion trap, isolated, and subjected to sequential collision-activated dissociation (CAD) events to cleave off either one or more iodine atoms and/or a nitro group to produce the desired radical sites.…”

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

“…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. − …”

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

“…This mixture showed major allyl abstraction instead of I atom abstraction upon reaction with allyl iodide. 20 Perhaps not surprisingly, no correlations were found between the reactivities (i.e., reaction efficiencies) of the biradicals studied here and either their ΔE S−T or EA v values (individually) (Figures 2, 3 and S1). This was the case when comparing either the total reaction efficiencies for cyclohexane and allyl iodide or the reaction efficiencies for H atom abstraction from cyclohexane or I atom abstraction from allyl iodide.…”

“…This behavior is not characteristic of metaand para-benzynes as not all meta-and para-benzynes described in the literature show similar behavior. 20,21 It should also be noted that the ions expected to be q11 may, in fact, be a mixture of isomers. When the monoradical precursor of this biradical, the 8-dehydro-2-iodoquinolinium cation, is subjected to collision-activated dissociation (CAD) to generate the second radical site, the H atom at N-1 may migrate to C-8 to ultimately form an ortho-benzyne isomer of q11.…”