Computational study of a new Heck reaction mechanism catalyzed by palladium(II/IV) species

In this theoretical study on the Heck reaction we explore the feasibility o f an alternative pathway that involves a Pd-II/Pd-IV redox system. Usually, the catalytic cycle is formulated based on a Pd-0/Pd-II mechanism, We perf ormed quantum chemical calculations using density functional theory on a mo del system that consisted of diphosphinoethane (DPE) as a bidentate ligand and the substrates ethylene and phenyl iodide to compare both mechanisms. A ccordingly, the Pd-II/Pd-IV mechanism is most likely to occur in the equato rial plane of an octahedral Pd-IV complex. The energy profiles of both reac tion pathways under consideration are largely parallel. A major difference is found for the oxidative addition of the C-I bond to the palladium centre ,This is a rate-determining step of the Pd-II/Pd-IV mechanism, while it is facile for a Pd-0 catalyst. The calculations show that intermediate ligand detachment and reattachment is necessary in the course of the oxidative add ition to Pd-II. Therefore, we expect the Pd-II/Pd-IV mechanism to be only f easible if a weakly coordinating ligand is present.