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 PdI preferentially activate the sp3 C-H bond in toluene, whereas PdII and PdIII 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.