Palladium-Catalyzed Activation of Carbon–Halogen Bonds: Electrostatics-Controlled Reactivity

We have quantum chemically studied the palladium-mediated activation of C(spⁿ)−X bonds (n=1–3; X=F, Cl, Br, I) in the archetypal model substrates H₃C−CH₂−X, H₂C=CH−X, and HC≡C−X by a model bare palladium catalyst, using relativistic density functional theory at ZORA-BLYP/TZ2P. The bond activation reaction barrier decreases, for all sp-hybridized carbon centers, when the substituent X of the substrate is changed from X=F to I. Activation strain and energy decomposition analyses reveal that the enhanced reactivity along this series originates from (i) a less destabilizing activation strain due to an intrinsically weaker C(spⁿ)−X bond; and (ii) an increasingly more stabilizing electrostatic interaction between the catalyst and the substrate. The latter is a direct consequence of the more diffuse electron density and higher nuclear charge of the X atom in the C(spⁿ)−X bond when going from X=F to I, which, in turn, engages in a more favorable electrostatic attraction with the nucleus and electrons, respectively, of the palladium catalyst.