Mechanistic studies of visible light-induced CO release from a 3-hydroxybenzo[g]quinolone

Organic compounds that can be triggered using light to release CO in biological environments are of significant current interest to probe the role of CO in biology and as potential therapeutics. We recently reported that a 3-hydroxybenzo[g]quinolone (5) can be used as a CO delivery molecule to produce anticancer and potent anti-inflammatory effects. Herein we report mechanistic studies of the visible light-induced CO release reaction of this compound. In wet CH₃CN under aerobic conditions, 5 releases 0.90(2) equivalents of CO upon illumination with visible light (419 nm) to give a single depside product. Performing the same reaction under an ¹⁸O₂ atmosphere results in quantitative incorporation of two labeled oxygen atoms in the depside product. Monitoring via ¹H NMR and UV-vis during the illumination of 5 in CH₃CN using 419 nm light revealed the substoichiometric formation of a diketone (6) in the reaction mixture. H₂O₂ formation was detected in the same reaction mixtures. DFT studies indicate that upon light absorption an efficient pathway exists for the formation of a triplet excited state species (5b) that can undergo reaction with ³O₂ resulting in CO release. DFT investigations also provide insight into diketone (6) and H₂O₂ formation and subsequent reactivity. The presence of water and exposure to visible light play an important role in lowering activation barriers in the reaction between 6 and H₂O₂ to give CO. Overall, two reaction pathways have been identified for CO release from a 3-hydroxybenzo[g]quinolone.