C(spⁿ)−X (n=1–3) Bond Activation by Palladium

We have studied the palladium-mediated activation of C(spⁿ)−X bonds (n = 1–3 and X = H, CH₃, Cl) in archetypal model substrates H₃C−CH₂−X, H₂C=CH−X and HC≡C−X by catalysts PdL_n with L_n = no ligand, Cl⁻, and (PH₃)₂, using relativistic density functional theory at ZORA-BLYP/TZ2P. The oxidative addition barrier decreases along this series, even though the strength of the bonds increases going from C(sp³)−X, to C(sp²)−X, to C(sp)−X. Activation strain and matching energy decomposition analyses reveal that the decreased oxidative addition barrier going from sp³, to sp², to sp, originates from a reduction in the destabilizing steric (Pauli) repulsion between catalyst and substrate. This is the direct consequence of the decreasing coordination number of the carbon atom in C(spⁿ)−X, which goes from four, to three, to two along this series. The associated net stabilization of the catalyst–substrate interaction dominates the trend in strain energy which indeed becomes more destabilizing along this same series as the bond becomes stronger from C(sp³)−X to C(sp)−X.