Kinetics and mechanism of the base-catalyzed oxygenation of 1H-2-phenyl-3-hydroxy-4-oxoquinolines in DMSO/H2O

The oxygenation of 4′-substituted 1H-2-phenyl-3-hydroxy-4-oxoquinolines (PhquinH2) in a DMSO/H2O (50/50) solution leads to the cleavage products at the C2-C3 bond in about 75% yield at room temperature. The oxygenation, deduced from the product compositions, has two main pathways, one proceeding via an endoperoxide leading to CO-release, and the other through a 1,2-dioxetane intermediate without CO-loss. The reaction is specific base-catalyzed and the kinetic measurements resulted in the rate law −ⅆ[PhquinH2]/ⅆt=kOH− [OH−] [PhquinH2] [O2]. The rate constant, activation enthalpy, and entropy at 303.16 K are as follows: kOH−=(2.42±0.03)×103mol−2L2s−1; ΔG‡=73.13±4.02 kJ mol−1; ΔH‡=70.60±4.04 kJ mol−1; ΔS‡=−28±2 J mol−1 K−1. The reaction fits a Hammett linear free energy relationship for 4′-substituted substrates, and electron-releasing groups make the oxygenation reaction faster (ρ=−0.258). The EPR spectrum of the reaction mixtures showed the presence of the organic radical 1H-2-phenyl-3-oxyl-4-oxoquinoline and superoxide ion due to single electron transfer from the carbanion to dioxygen. The pathway via 1,2-dioxetane could be proved by chemiluminescence measurements.