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

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.