We report a highly efficient Friedel–Crafts reaction of 3-alkyl or 3-aryl 3-hydroxyoxindoles with a variety of aromatic and heteroaromatic compounds to unsymmetrical 3,3-diaryloxindoles or 3-alkyl-3-aryloxindoles, which are interesting medicinal targets and useful building blocks for the synthesis of natural products. Hg(ClO4)2⋅3 H2O was identified as a powerful catalyst for this reaction, and is significantly more efficient than other screened metal perchlorate hydrates and Brønsted acids such as HOTf and HClO4. The high catalytic property of Hg(ClO4)2⋅3 H2O originates from the unprecedented dual activation effects of aromatic mercuration, which could generate a strong protic acid to facilitate the generation of a carbocation at the C3-position of oxindoles and simultaneously form the more reactive nucleophilic reaction partner.
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