Unraveling the Anion/Ligand Interplay in the Reaction Mechanism of Gold(I)-Catalyzed Alkoxylation of Alkynes

In this work DFT calculations have been performed to investigate the anion/ligand interplay in the reaction mechanism of alkoxylation of alkynes promoted by gold(I) catalysts of general formula [L-Au-X] (L = NHC, P(tBu)3and X = OTs-, OTf-, BF4-, TFA-) on the basis of available experimental data. The observed catalytic efficiency trend in this series of compounds strictly depends on the specific anion/ligand combination used, thus suggesting that it cannot be estimated by evaluating the properties of L and X separately. Similarly to [NHC-Au-X], for the [P(tBu)3-Au-X] catalyst series, we demonstrate that the anion effect in the reaction mechanism can be predicted on the basis of its coordinating/proton acceptor properties. A comparison between the P(tBu)3/OTs-and NHC/OTs-settings shows that the anion/ligand interplay has a crucial role in the nucleophilic attack step of the reaction mechanism. A charge-displacement (CD) analysis reveals that the activation of the unsaturated hydrocarbon multiple bond (alkyne) by the [L-Au]+fragment depends both on the ligand-withdrawing ability at the outer region of the CC bond and on the counterion affinity for the cationic fragment, both affecting in the opposite way the electrophilic character of the alkyne at the transition state.