Effects of Cyclopentadienyl Ligand Structure on Rhodium(III)‐Catalyzed Ortho ‐C─H Arylation of Aryl Ketones

Ketones are attractive innate directing groups for C─H bond functionalization, but their weak coordination to transition metals often necessitates harsh conditions. We report a cyclopentadienyl‐ligand‐controlled rhodium(III)‐catalyzed ortho ‐arylation of aryl ketones with arylboronic acid pinacol esters. Comparison of electronically and sterically differentiated ligands identified an electron‐rich, anisotropic 3,4,5‐trimethoxyphenyl‐substituted cyclopentadienyl ligand (Cp TMP *) as the most effective catalyst platform at 80 °C. A preformed cationic rhodium(III) complex containing Cp TMP * afforded the model biaryl ketone in 65% isolated yield, whereas the conventional pentamethylcyclopentadienyl rhodium(III) catalyst was substantially less effective under the same conditions. The reaction accommodates electron‐rich, electron‐deficient, and halogenated arylboronic acid pinacol esters, together with aryl ketones bearing varied alkyl substituents, providing the corresponding biaryl ketones in yields of up to 79%.