OXIDATIVE INSERTION AS FRONTSIDE S(N)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 (Delta E(double dagger) = -1.5 kcal/mol relative to the separated reactants; NL-SCF) and leads to exothermic production of CH3PdCl (Delta E(r) = -7.7 kcal/mol). The ''straight'' S(N)2 substitution is not competitive as it leads to the highly endothermic formation of PdCH3+ + Cl- (Delta E(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(N)2/Cl-ra) leads to the exothermic formation of CH3PdCl via a still high but much lower energy barrier (Delta E(double dagger) = 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 S(N)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(N)2 substitution in organic chemistry, in spite of obvious differences. Finally, possible effects of ligands are discussed.