Arylic C–X Bond Activation by Palladium Catalysts: Activation Strain Analyses of Reactivity Trends

We have quantum chemically explored arylic carbon–substituent bond activation via oxidative insertion of a palladium catalyst in C(6)H(5)X + PdL(n) model systems (X = H, Cl, CH(3); L(n) = no ligand, PH(3), (PH(3))(2), PH(2)C(2)H(4)PH(2)) using relativistic density functional theory at ZORA-BLYP/TZ2P. Besides exploring reactivity trends and comparing them to aliphatic C–X activation, we aim at uncovering the physical factors behind the activity and selectivity. Our results show that barriers for arylic C–X activation are lower than those for the corresponding aliphatic C–X bonds. However, trends along bonds or upon variation of ligands are similar. Thus, bond activation barriers increase along C–Cl < C–H < C–C and along Pd < Pd(PH(3)) or Pd(PH(2)C(2)H(4)PH(2)) < Pd(PH(3))(2). Activation strain analyses in conjunction with quantitative molecular orbital theory trace these trends to the rigidity and bonding capability of the various C–X bonds, model catalysts, and ligands.