Multistage\nTransformation of Xylene Compounds Enabled\nby Pyridinium Photocatalyst through Solvent and Light Manipulation

Selective oxidation of xylenes represents\nan attractive\nway to\nproduce a variety of high-value oxygenated products. Herein, a mild\nand selective C(sp<sup>3</sup>)–H aerobic oxidation protocol\nfacilitated by a pyridinium<i>-</i>based photoredox catalyst\nhas been developed, enabling multistage photo-oxygenation of xylenes\nwith ambient air as the sole oxidant. It was found that all xylene\nsubstrates achieved nearly complete conversion at ambient temperature,\nwherein <i>para-</i> and <i>meta-</i>xylenes underwent\ntransformation into their corresponding dicarboxylic acids as final\nproducts, while <i>ortho-</i>xylene was converted to phthalic\nanhydride due to the spatial proximity of the two methyl groups. Significantly,\nthe catalytic outcomes can be manipulated by the reaction conditions.\nUpon solvent and light modulation, tolualdehyde and toluic acid were\nalso selectively obtained in high yields. Compared with existing photocatalytic\nsystems for xylenes that mainly focus on the formation of tolualdehyde\nor toluic acid, the present findings showcase the great potential\nof pyridinium<i>-</i>based photocatalysts in achieving multistage\ntransformations and deep oxidation of xylenes and offer a straightforward\norganocatalytic system for the metal-free oxidation of methyl aromatics\nunder mild conditions.