An Oxygen‐peri‐Bridged Quinolinium Cation and Its Monoradical Counterpart

2017 
Abstract: Generation of a σ,σ,π-triradical was attempted by collision-activated dissociation (CAD) of two iodine atoms and an NO● radical from 2,4-diiodo-5-nitroquinolinium cation in an ion trap mass spectrometer. However, the reactivity of the product was consistent with a monoradical rather than a triradical. The mechanism of its formation was explored. In the first CAD step, an iodine atom is lost to generate the 2-iodo-5-nitro-4-dehydroquinolinium cation. Quantum chemical calculations suggest that in the second CAD step, the radical site at C-4 adds to the adjacent NO2 group followed by loss of NO● to generate the 2-iodo-4-oxyl-5-dehydroquinolinium cation. Calculations also suggest that in the third CAD step, this cation cyclizes to form an oxygen-peribridged 2-iodoquinolinium cation that then loses the remaining iodine atom to generate a distonic 4,5-oxygen-peribridged 2-dehydroquinolinium cation. For the analogous 4-oxyl-5-dehydroquinolinium cation, the cyclization is calculated to be exoergic by 8.7 kcal mol-1 with a free energy barrier of 14.0 kcal mol-1. The cyclized product is the first experimentally observed oxygen-peribridged naphthalene derivative. It displays low reactivity. However, it's monoradical counterpart is highly reactive and undergoes typical radical reactions.
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