Unexpected Anion Effect in the Alkoxylation of Alkynes Catalyzed by N-Heterocyclic Carbene (NHC) Cationic Gold Complexes

The intermolecular alkoxylation of alkynes is the oldest application of cationic gold(I) catalysts; however, no systematic experimental data about the role of the anion are available. In this contribution, the role of the anion in this catalytic reaction as promoted by a N‐heterocyclic carbene‐based gold catalyst, [(NHC)AuX] (X=BARF−, BF4−, OTf−, OTs−, TFA−, or OAc−) is analyzed, through a combined experimental (NMR spectroscopy) and theoretical (DFT calculation) approach. The most important factor seems to be the ability to the proton from the methanol during the nucleophilic attack, and such ability is related to the anion basicity. On the other hand, too high coordination power or basicity of the anion worsens the catalytic performance by preventing alkyne coordination or by forming too much free methoxide in solution, which poisons the catalyst. The intermediate coordinating power and basicity of the OTs− anion provides the best compromise to achieve efficient catalysis. Goldilocks and the six anions: The role of the anion in the intermolecular alkoxylation of alkynes catalyzed by [(NHC)AuX] is analyzed with a combined experimental and theoretical approach. The anion plays a role in the ion of the proton from the methanol during the nucleophilic attack, in the coordination of alkyne, and in the deactivation of the catalyst. Intermediate coordinating power and basicity of the OTs− anion gives the best catalytic performance.