Spin‐Spin Coupling between the Biradical Moieties in Aromatic Tetraradicals Increases their Reactivity

Abstract

The reactivity of a carbon‐centered σ,σ,σ,σ‐type singlet tetraradical containing two meta‐benzyne moieties, the 2,4,5,7‐tetradehydroquinolinium cation, was examined in the gas phase toward dimethyl disulfide, cyclohexane and allyl iodide. The major reactions were thiomethyl radical abstraction, H atom abstraction and allyl radical abstraction, respectively. The reactivity of this tetraradical was found to be greater than that of the relevant meta‐benzyne biradicals. The reactivity of individual meta‐benzynes has been found previously to be controlled by their (calculated) distortion energies (ΔE2.30) and singlet‐triplet spittings (ΔES‐T) as well as their electron affinities (EA2.30) at the TS geometry for hydrogen atom abstraction reactions. The addition of another meta‐benzyne moiety to a meta‐benzyne to generate the above tetraradical does not change EA2.30 and only slightly lowers ΔES‐T of both meta‐benzyne moieties. However, ΔE2.30 is significantly decreased for both meta‐benzyne moieties, which explains the higher reactivity of the tetraradical. A similar finding was made previously for an isomeric tetraradical, the 2,4,6,8‐tetradehydroquinolinium cation, that also contains two meta‐benzyne moieties. The decrease in ΔE2.30 is rationalized by a stabilizing coupling between one radical site in each meta‐benzyne moiety for both tetraradicals. Interestingly, the more reactive meta‐benzyne moiety in the two tetraradicals was found to be different: that in the pyridine ring (2,4−) is more reactive for the 2,4,5,7‐tetraradical while that in the benzene ring (6,8−) is more reactive for the 2,4,6,8‐tetraradical. This difference is rationalized by the uniquely small ΔE2.30 value for the 6,8‐moiety in the 2,4,6,8‐tetraradical, which makes this moiety (and this tetraradical) unusually reactive. On the other hand, the ΔE2.30 values for the meta‐benzyne moieties in the 2,4,5,7‐tetraradical are greater and differ only slightly from each other. The slightly greater EA2.30 for the 2,4‐moiety in this tetraradical partially rationalizes the greater reactivity of this moiety when compared to the 5,7‐moiety in the tetraradical.