Oxidative Insertion as Frontside Sn 2 Substitution: A Theoretical Study of the Model Reaction System Pd + CH₃Cl

The potential energy surface of the model reaction system Pd + CH₃Cl has been explored using density functional theory based on the local density approximation (LDA) and its nonlocal extension NL-SCF. Oxidative insertion (OxIn) of Pd into the C-Cl bond has the lowest activation barrier (ΔE^‡= —1.5 kcal/mol relative to the separated reactants; NL-SCF) and leads to exothermic production of CH₃PdCl (ΔE_r = -7.7 kcal/mol). The “straight” Sn 2 substitution is not competitive as it leads to the highly endothermic formation of PdCH₃⁺ + Cl^- (ΔE_r = 145.2 kcal/mol). However, in combination with a concerted rearrangement of the Cl^- leaving group from C to Pd, the substitution process (S_N2/Cl-ra) leads to the exothermic formation of CH₃PdCl via a still high but much lower energy barrier (ΔE^‡ = 29.6 kcal/mol). Furthermore, radical mechanisms proceeding via single electron transfer (SET) have been considered. Solvent effects, estimated using a simple electrostatic continuum model, tend to favor the straight Sn 2 substitution because of the charge separation in the products, but oxidative insertion remains dominant. In order to explain the intrinsic preference of the Pd atom to react via oxidative insertion, a detailed analysis of the bonding mechanism between Pd and CH₃Cl has been carried out. It is argued that oxidative insertion in organometallic chemistry corresponds to frontside S_N2 substitution in organic chemistry, in spite of obvious differences. Finally, possible effects of ligands are discussed.