Modeling of the mechanism of reductive allylation of norbornadiene in the presence of Pd⁰ complexes

Abstract

Hydroallylation of norbornadiene (NBD) with allyl formate (AF) in the presence of the Pd⁰ complexes with the formation of 5-allyl-2-norbornene was modeled by the method based on the DFT-PBE/L11 density functional theory. According to the calculation results, 5-allyl-2-norbornene is formed via two mechanisms. The first assumes that the C-C bond between the NBD and allyl ligand is formed before the cleavage of the formyl C-H bond and elimination of CO₂, whereas following the second mechanism the bond is formed after these processes. For both mechanisms, Pd(AF)(MeCN) is the catalytically active complex and C_NBD-C_All bond formation is the rate-determining step with the Gibbs activation energy equal to 22.8 and 21.3 kcal mol⁻¹for the first and second mechanisms, respectively. High selectivity to 5-allyl-2-norbornene in the absence of phosphine ligands is attributable to the kinetically hindered formation of the second C-C bond needed for the generation of product of oxidative allylation of NBD. The predominance of the exo-substituted product is due to the formation of the thermodynamically stable complex with the bidentate coordination of NBD favored by the endo-coordination of the NBD molecule.