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.