Urea-Substituted Tetramethylcyclopentadienyl Ligands for Supramolecularly Accelerated RhIII-Catalyzed ortho-C-H Olefination of Benzoic Acid Derivatives

The design and synthesis of air-stable and conveniently crystallizable Rh-III-cyclopentadienyl catalysts substituted with a urea moiety, which are able to accelerate the C-H olefination of benzoic acid derivatives, is reported. Through kinetic studies and NMR titration experiments, the catalysts' substrate recognition ability mediated by hydrogen bonding was identified to be the reason for this effect. Introduction of pyridone-phosphine ligands capable of forming additional H-bond interactions increased the catalytic performance even further. By unveiling a proportionality between reaction rate and relative complex formation enthalpy the hypothesis of a supramolecular catalyst preformation was supported. Its application to a variety of substrates proved the catalyst system's advantages, generally increasing the yields when compared to the results obtained with widely used [RhCp*Cl-2](2).