Hydroxylation of aromatics with hydrogen peroxide catalyzed by cationic complexes of platinum(II). Evidence for the intermolecular oxidation of platinum aryls

The catalytic hydroxylation of a variety of aromatic substrates is reported using 70% hydrogen peroxide as primary oxidant and a series of (P-P)Pt(CF3)X complexes (P-P = different tetraaryldiphosphines; X = CH2Cl2,-OH,-OPh) as catalysts. The reactivity observed increases with the presence of electron-releasing substituents at the aromatic ring, selectively producing ortho and para products. Good amounts of products are obtained with the most activated substrates phenol, anisóle, m-cresol, and 1,3-dimethoxybenzene, and in all cases an interesting ortho selectivity (up to 95%) is observed. A mechanistic study carried out on the last substrate suggests the involvement of an electrophilic metalation of the aromatic ring to produce platinum-aryl intermediates followed by oxygen transfer from a platinum-hydroperoxy species. This seems to represent a rare example in which a soluble transition-metal complex catalyzes the direct hydroxylation of an aromatic ring via electrophilic metalation under mild conditions.