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 (ΔE_2.30) and singlet-triplet spittings (ΔE_S-T) as well as their electron affinities (EA_2.30) 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 EA_2.30 and only slightly lowers ΔE_S-T of both meta-benzyne moieties. However, ΔE_2.30 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 ΔE_2.30 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 ΔE_2.30 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 ΔE_2.30 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 EA_2.30 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.