Coupled Light-Driven p-Xylene Oxidation and H2O2 Production with Organic Photocatalysts in Propylene Carbonate

The aerobic photocatalytic oxidation of p-xylene to p-tolualdehyde (p-TALD) is reported here in propylene carbonate as a green solvent under visible light irradiation using organic photocatalysts, such as 9,10-anthraquinone (AQ) and 3,6-di-tert-butyl-9-mesityl-10-phenylacridinium (Acr+-1). Optimized conditions achieved up to 80–84% p-xylene conversion in 24 h, with p-TALD as the main product (up to 44% yield, 60% selectivity). The photooxidation is coupled to dioxygen reduction for H2O2 production, reaching concentrations up to 27 mM. Mechanistic investigation by laser flash photolysis and electron paramagnetic resonance spectroscopy revealed a non-innocent role of propylene carbonate, which promotes reductive quenching of 3*AQ, generating the radical anion AQ•– and dianion AQ2– reduced species of the photocatalyst. Upon aerobic oxidation, these are responsible for generation of reactive oxygen species such as superoxide radical anion (O2•–) and hydrogen peroxide (H2O2). The oxidation of p-xylene proceeds through a radical chain mechanism initiated by hydrogen atom transfer. The photocatalytic conditions were extended to other substrates bearing activated C–H bonds, demonstrating broad synthetic applicability.