The Essence Of Photocatalytic Oxidation Of Saturated (sp~3) C-H Bonds By Quinone Compounds In Organic Phase

The photochemistry of benzoquinone and its derivates has been studied over decades, and the main contents were focused on the electron transfer and hydrogen abstraction mechanisms involved in the interation between the excited quinone and the C-H bonds of substrates. Here we study the reactions of the excited quinone with saturated C-H bonds of arenes, alkanes and plasmid DNA. It is expected to uncover the universal mechanism for the oxidation of hydrocarbons by non-thermodynamically favored quinones (such as benzoquinone and tetrachlorobenzoquinone). This will help to understand the differences between the photochemical behaviors of quinones in organic systems and bio-organisms.Using isotopic tracer technique, we found that the oxidation of C-H bonds by quinones was reallly initiated by the formation of hydroxyl radical in the excited system instead of the quinone itself. when a hydrocarbon (cyclohexane or toluene) solution of quinone was irradiated with visible light, triplet excited state quinone abstracted a hydrogen atom from the smallest amount of water coordinated to quinone to produce hydroxyl radical, which further initiated the subsequent free-radical reactions in pure organic phase. A rational reaction pathway for the oxygenation of toluene by quinone compounds was proposed. At first, the excited quinone abstracted a hydrogen atom from its coordinated water to give semiquinone radical and hydroxyl radical. The highly reactive hydroxyl radical then abstracted a hydrogen atom from hydrocarbon to afford hydrocarbon radical which could easily combine with molecular oxygen to form hydrocarbon peroxide radical via a thermodynamically favorable process. The generated hydrocarbon peroxide radical can oxidize the semiquinone radical to give quinone and hydrocarbon hydroperoxide. During this reaction process, water was formed, leading to zero consumption of water, and quinone was also regenerated via semiquinone radical as a pivotal medium, which shows a typical catalytic process. Moreover, solvent polarity notably affected the stability of semiquinone radical, which resulted in the final products in different ratios. This new activation mode of C-H bonds dominated by hydroxyl radical and special hydrogen atom transfer from the coordinated water by quinone to the final products can alter the routine understanding for the catalytic behavior of quinone compounds in chemical transformations.We revealed for the first time that the excited quinones first scavenged hydrogen atom from the smallest amount of water coordinated to quinones to produce hydroxyl radicals,as the primary step for initiating the subsequent aerobic transformation reactions. This previously unknown hydrogen transfer process is the essence of quinone-involved photocatalytic oxidation of substrates.