Comparing Nickel- and Palladium-Catalyzed Heck Reactions

The possible use of Ni in Heck reactions was investigated with use of the density function theory method. It was found that the mechanisms of the Ni- and Pd-catalyzed Heck reactions are quite similar to each other. Nevertheless, oxidative addition and olefin insertion occur with lower energy barriers in the Ni system than in the Pd system. Because β-hydride elimination is more efficient in the Pd system than in the Ni system, there is a poorer selectivity to vinylation over Michael addition in the Ni system than in the Pd system. In addition, catalyst regeneration through HX removal is considerably harder to achieve with the Ni system than with the Pd system. Therefore, either a very strong base should be used for the Ni catalysis or a reductive pathway should be designed to remove HX from the Ni complex. Compared to the Pd system, oxidative addition of an alkenyl or aryl chloride is not much harder than oxidative addition of an alkenyl or aryl iodide in the Ni system. Therefore, the Ni-catalyzed Heck rea...