Palladium-catalyzed activation of H_nA-AH_nbonds (AH_n= CH₃, NH₂, OH, F)

We have quantum chemically studied activation of HnA-AHn bonds (AHn = CH3, NH2, OH, F) by PdLn catalysts with Ln = no ligand, PH3, (PH3)2, using relativistic density functional theory at ZORA-BLYP/TZ2P. The activation energy associated with the oxidative addition step decreases from H3C-CH3 to H2N-NH2 to HO-OH to F-F, where the activation of the F-F bond is barrierless. Activation strain and Kohn-Sham molecular orbital analyses reveal that the enhanced reactivity along this series of substrates originates from a combination of (i) reduced activation strain due to a weaker HnA-AHn bond; (ii) decreased Pauli repulsion as a result of a difference in steric shielding of the HnA-AHn bond; and (iii) enhanced backbonding interaction between the occupied 4d atomic orbitals of the palladium catalyst and σ∗ acceptor orbital of the substrate.