Inherent Selectivity of Pd C-H Activation from Different Metal Oxidation States

In investigating potential control factors that would permit a palladium-catalyzed benzylic vs arene C-H activation as previously reported by our group, it was discovered that the oxidation state of the homogenous palladium species influences the selectivity of C-H activation. DFT calculations show that Pd⁰ and Pd^I preferentially activate the sp³ C-H bond in toluene, whereas Pd^II and Pd^III preferentially activate the sp² C-H bond. This selectivity appears to originate from the steric environment created by the ligand framework on the palladium. As the palladium oxidation state increases, the number of ligand sites increases, which decreases the energetic favorability for activation of the weaker, yet more hindered sp³ C-H bond.