In their recent publication in Scientific Reports, Vermeeren, Sun, and Bickelhaupt for the first time reveal the physical origin of why metal-mediated activation of arylic C–X bonds is more facile than that of the corresponding aliphatic bonds.
The authors report an extensive and detailed study, conducted with the ADF suite of programs, on the activation of arylic C–H, C–Cl, and C–C bonds by various palladium-based model catalysts, using state-of-the-art relativistic density functional theory. They were able to trace the enhanced reactivity of arylic C–X activation to the character of the C–X antibonding σ* orbital. The latter is the substrate’s LUMO which accepts charge from the catalyst’s metal dπ-type orbitals when it oxidatively adds and dissociates. For C–X bonds involving an sp2 carbon atom, and thus for arylic C–X bonds, this σ* LUMO is at lower orbital energy, and thus a better acceptor orbital, than for C–X bonds involving an sp3 carbon atom. Making use of the canonical energy decomposition analysis of ADF, it was shown that the lower lying arylic C–X σ* orbital leads to a more stabilizing orbital interaction with the catalyst, and thus to a more stable transition state and a lower activation barrier.
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Pascal Vermeeren, Xiaobo Sun, and F. Matthias Bickelhaupt, Arylic C–X Bond Activation by Palladium Catalysts: Activation-Strain Analyses of Reactivity Trends, Scientific Reports 2018.Key conceptsADF bonding analysis catalysis