Electron-Deficient Phenanthrenequinone Derivative for Photoactivated Hydrogen Atom Transfer Mediated Oxidation of Secondary Alcohols

In 2000, Fukuzumi and co-workers reported a seminal study on the photochemical oxidation of benzylic alcohols with visible-light-excited 9,10-phenanthrenequinone (PQ) under argon atmosphere (J. Am. Chem. Soc. 2000, 122, 8435). We optimized the reaction conditions they reported and were able to oxidize 1-(4-methoxyphenyl)ethanol quantitatively to 4'-methoxyacetophenone in only 15 min with 10 mol % PQ as a photocatalyst under oxygen. However, we observed a significant decrease in the oxidation rate with more electron-deficient benzylic alcohols as starting materials. To improve the photooxidation performance, we designed a high-yielding synthetic route for a novel, more electron-deficient PQ derivative, 3,6-bis(trifluoromethyl)-9,10-phenanthrenequinone (PQ-CF3). Its efficiency as a photocatalyst in the fast oxidation of secondary alcohols was demonstrated not only with several electronically diverse benzylic alcohols but also with aliphatic substrates. The comprehensive mechanistic studies based on Hammett plot construction, kinetic isotope experiments, and DFT computations suggest that the mechanistic pathway of the alcohol oxidation is dependent on the electronic properties of both the catalyst and the substrate. As the key mechanistic discovery, we showed that the newly developed PQ-CF3 operates as a highly efficient hydrogen atom transfer (HAT) catalyst.