Strong Electron-Donating Ligands Accelerate the Protodeauration Step in Gold(I)-Catalyzed Reactions: A Quantitative Understanding of the Ligand Effect

We have conducted a theoretical exploration of the ligand electronic effect in the protodeauration step of a model gold(I) cyclization reaction, for which experimental data are available. The mechanism of the protodeauration is investigated through a density functional theory (DFT) approach, and the electron-donating power of the ligand is quantified through the charge displacement function (CDF). We find that the frequently encountered assumption in the literature that "strong electron donating ligands accelerate the protodeauration" can be set into a quantitative framework by our combined DFT/CDF theoretical approach, which allows us also to rationalize the highest catalytic efficiency of Buchwald phosphine type ligands in this process. We analyze the ligand effect on the gold complex substrate (LAu-S) bond strength, namely the bond to be broken during the protodeauration, and we find that the LAu-S interaction energies linearly correlate with the activation barriers. Finally, energy decomposition analysis (EDA) is used to investigate the LAu-S bond, and we show that changes in the interaction energies are mainly due to changes in the electrostatic component, whose value is in turn modulated by the ligand electron-donating power.