Multistage Transformation of Xylene Compounds Enabled by Pyridinium Photocatalyst through Solvent and Light Manipulation

Selective oxidation of xylenes represents an attractive way to produce a variety of high-value oxygenated products. Herein, a mild and selective C(sp³)-H aerobic oxidation protocol facilitated by a pyridinium-based photoredox catalyst has been developed, enabling multistage photo-oxygenation of xylenes with ambient air as the sole oxidant. It was found that all xylene substrates achieved nearly complete conversion at ambient temperature, wherein para- and meta-xylenes underwent transformation into their corresponding dicarboxylic acids as final products, while ortho-xylene was converted to phthalic anhydride due to the spatial proximity of the two methyl groups. Significantly, the catalytic outcomes can be manipulated by the reaction conditions. Upon solvent and light modulation, tolualdehyde and toluic acid were also selectively obtained in high yields. Compared with existing photocatalytic systems for xylenes that mainly focus on the formation of tolualdehyde or toluic acid, the present findings showcase the great potential of pyridinium-based photocatalysts in achieving multistage transformations and deep oxidation of xylenes and offer a straightforward organocatalytic system for the metal-free oxidation of methyl aromatics under mild conditions.