Activation of H−H, C−H, C−C, and C−Cl Bonds by Pd(0). Insight from the Activation Strain Model

Abstract

To achieve more insight into palladium-catalyzed H−H, C−H, C−C, and C−Cl bond activation and the mutual competition between these processes, several mechanistic pathways for oxidative addition of Pd(0) to H2 (H−H), CH4 (C−H), C2H6 (C−C and C−H), and CH3Cl (C−Cl) were studied uniformly at the ZORA-BP86/TZ(2)P level of relativistic nonlocal density functional theory (DFT). Oxidative addition is overall exothermic for all model reactions studied, with 298 K reaction enthalpies (ΔHr,298) of −35.7 kcal/mol (C−Cl) through −9.7 kcal/mol (C−H in CH4). The lowest barrier pathway is the direct oxidative insertion of Pd into the C−X or H−H bond (X = H, CH3, Cl), with 298 K activation enthalpies (ΔH ⧧298) that increase in the order H−H (−21.7 kcal/mol) < C−Cl (−6.0 kcal/mol) ≈ C−H (−5.0 and −4.1 kcal/mol for CH4 and C2H6) < C−C (9.6 kcal/mol). The “straight” SN2 substitution resulting in PdCH3+ + X- or PdH+ + H- is highly endothermic (144−237 kcal/mol) and thus not competitive. Only in the case of Pd + CH3Cl is a thi...