Counterion Effect in the Reaction Mechanism of NHC Gold(I)-Catalyzed Alkoxylation of Alkynes: Computational Insight into Experiment

Experimental data suggest that anions that provide a compromise between the hydrogen-bond acceptor and the coordinating powers rather than poor coordinating anions unexpectedly increase the efficiency of L-Au-X (L = ligand, X = anion) catalyzed alkyne alkoxylation reactions, where the nucleophilic attack is the rate-determining step. No systematic computational studies about the role of the anion in the different steps of the catalytic cycle are available yet. In this paper, the remarkable anion influence on the catalytic efficiency of [NHCAuX] (X = BF4-, OTf-, OTs-, TFA(-), and OAC(-)) complexes in the intermolecular addition of methanol to 2-butyne process has been analyzed through a density functional theory (DFT) approach. The role of the anion has been considered in all the steps of the reaction mechanism: pre-equilibrium, nucleophilic addition, and protodeauration. In the nucleophilic attack step, the anion acts (i) as a template, holding the methanol in the right position for the outer-sphere attack; (ii) as a hydrogen-bond acceptor, enhancing the nucleophilicity of the attacking methanol; (iii) as catalyst deactivator, by either its strong coordinating and/or basicity power, preventing the alkyne coordination or forming free alkoxide, respectively. In the protodeauration step, the anion acts as a proton shuttle, lowering the activation barrier. DFT calculations support intermediate coordinating and basicity power anions as the most efficient catalysts.