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Aerobic Direct C(sp2)‑H Hydroxylation of 2‑Arylpyridines by Palladium Catalysis Induced with Aldehyde Auto-Oxidation
Herein we present
a Pd-catalyzed direct C–H hydroxylation
of 2-arylpyridines using molecular oxygen (O<sub>2</sub>) as the sole
oxidant. The key aspects of the method include: (a) the activation
of molecular oxygen with a nontoxic and inexpensive aldehyde; (b)
an efficient association of the in situ-generated acyl peroxo radical
with palladium catalysis; and (c) convenient operating conditions.
On the basis of the results obtained in a series of control experiments,
a Pd<sup>II</sup>/Pd<sup>IV</sup> catalytic cycle is implicated for
the transformations. Furthermore, the method offers an easy access
to a broad range of substituted 2-(pyridin-2-yl)Âphenols in good isolated
yields