Hydroxyl Radical Mediated Heterogeneous Photocatalytic Baeyer-Villiger Oxidation over Covalent Triazine/Heptazine-Based Frameworks

The Baeyer-Villiger (B-V) oxidation of ketones to the corresponding lactones/esters is a classic and essential reaction in the chemical industry. However, this oxidation process has not yet been achieved in ambient conditions with the aid of oxygen and heterogeneous photocatalysts. In this study, we developed an organic photocatalytic system using covalent triazine/heptazine-based frameworks (CTF-TB/CHF-TB) to enable the B-V oxidation reaction under mild conditions through a cascade reaction pathway. Experimental data and theoretical calculations showed that heptazine/triazine units can "chelate" and decompose the in situ generated H2O2 into hydroxyl radicals (.OH). Compared to conventional methods that primarily involve metal-activated benzaldehyde at elevated temperatures (e.g., 60 degrees C), the .OH generated in our study can readily cleave the C-H bond of benzaldehyde, forming an active intermediate that drives subsequent sequential processes: O-2 -> H2O2 ->.OH -> Ph-CO.-> Ph-COOO.. By employing this photocatalytic process, a yield of 91% and a selectivity of over 99% were obtained in the oxidation of cyclohexanone to caprolactone at room temperature. This performance is comparable to the state-of-the-art catalysts, and our CHF-TB catalyst demonstrates impressive reusability, maintaining a high yield after 5 consecutive runs. This work presents a straightforward approach for C-H cleavage by organocatalysts to produce E-caprolactone in a mild manner by B-V oxidation.