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 beta-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 reaction may be applied to alkenyl or aryl chloride relatively easily. Also, because beta-hydride elimination is more difficult in the Ni system than in the Pd system, the Ni-catalyzed Heck reaction may be applied to aliphatic halides. For an olefin with an electron-donating substituent, the Ni-catalyzed coupling should slightly favor the Markovnikov-type product, if the steric effect is not significant. For an olefin with an electron-withdrawing substituent, the Ni-catalyzed coupling should provide the anti-Markovnikov-type product as the major product. In addition, it was found that phosphine and pyridine ligands can reasonably well reduce the free energy in the HX removal step. Therefore, they appear to be promising ligands for the Ni-catalyzed Heck reactions. Finally, we found that the solvation effects, cation pathway, and anionic pathway in the Heck reactions did not change the general trends for the reactivities of the Ni and Pd catalysts.