A theoretical study of the heck reaction:: N-heterocyclic carbene versus phosphine ligands

The reaction mechanism of the Heck reaction has been studied using theoretical chemistry. Our main focus has been the comparison between phosphine ligands and N-heterocyclic carbenes (NHC) and between the normal C2-NHC versus the abnormal C5-NHC. Our overall free-energy landscape shows that oxidative addition involves a significant energy barrier, while the phosphine system is relatively favored. In the olefin insertion and beta-hydride elimination steps, we considered both the neutral and ionic pathways. For the ionic pathway, the rate-determining step in addition to oxidative addition is olefin migration. For the neutral pathway, the second rate-determining step in addition to oxidative addition is also olefin insertion for NHC systems, while for the phosphine system the energy profile is much flatter. For the phosphine system, our result suggests that the neutral pathway is favored. Our study demonstrates that the catalytic capacity of (PdL2)-L-0 involving the abnormal C5-NHC ligand is comparable to the regular C2-NHC ligand. For the mechanisms examined in this study, there is no obvious advantage of using NHC ligands.