Mechanism of the Arene-Limited Nondirected C–H Activation of Arenes with Palladium: A Combined Experimental and Theoretical Study

Recently, synthetic methods have been discovered which enable the directing group-free C–H activation of arenes with palladium-catalysts and do not require an excess of the arene substrate. By overcoming this long standing challenge, such methods have now become suitable for the functionalization of complex organic molecules. The key to success in several of these transformations has been the use of two complementary ligands, an N-acyl amino acid and an Nheterocycle. Further applications of this design principle will likely require the guidance by a profound mechanistic understanding. This prompted us to engage in a detailed experimental and computational mechanistic study of the dual ligand enabled C–H activation of arenes. Based on comprehensive kinetic experiments, (CID-)MS, (DOSY-)NMR, and DFT calculations we find that a 1:1:1 complex of palladium and the two ligands is indeed the active species that enables a partially rate-limiting concerted C–H activation as part of a Pd⁰/Pd^II-cycle. Our study highlights the importance of catalyst speciation and allows us to rationalize the role of each ligand as well as the observed regioselectivities. These findings are expected to be highly useful for further method development using this powerful class of catalysts.