Palladium‐Catalyzed Cross‐Dehydrogenative Coupling of o‐Xylene: Evidence of a New Rate‐Limiting Step in the Search for Industrially Relevant Conditions

An efficient cross-dehydrogenative coupling of o-xylene under neat conditions, which brings important industrial benefits towards the synthesis of a monomer used in polyimide resins, is reported. The catalyst based on the combination of Pd/N ligand/carboxylate = 1:1:2 does not require a Cu cocatalyst and proceeds at 11bar of O-2 pressure. Evaluation of the deuterium kinetic isotope effect (KIE) provides evidence for three different rate-determining steps, which depend on the reaction conditions (medium, temperature). Under the reported neat conditions, the dissociation of a carboxylate-bridged dimer to generate a more reactive monometallic Pd species is proposed to be the rate-limiting step.