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

摘要

> 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 NO ₂ 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‐ peri ‐bridged 2‐iodoquinolinium cation that then loses the remaining iodine atom to generate a distonic 4,5‐oxygen‐ peri ‐bridged 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‐ peri ‐bridged naphthalene derivative. It displays low reactivity. However, it's monoradical counterpart is highly reactive and undergoes typical radical reactions.