Experimental and Theoretical Investigation of Ion Pairing in Gold(III) Catalysts.

The ion pairing structure of the possible species present in solution during the gold(III)-catalyzed hydration of alkynes: [(ppy)Au(NHC)Y]X and [(ppy)Au(NHC)X]X [ppy = 2-phenylpyridine, NHC = NHC = 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene; NHC = NHC = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene X = Cl, BF, OTf; Y = HO and 3-hexyne] are determined. The nuclear overhauser effect nuclear magnetic resonance (NMR) experimental measurements integrated with a theoretical description of the system (full optimization of different ion pairs and calculation of the Coulomb potential surface) indicate that the preferential position of the counterion is tunable through the choice of the ancillary ligands (NHC, NHC, ppy, and Y) in [(ppy)Au(NHC)(3-hexyne)]X activated complexes that undergo nucleophilic attack. The counterion can approach near NHC, pyridine ring of ppy, and gold atom.

Monitoring of the Pre-Equilibrium Step in the Alkyne Hydration Reaction Catalyzed by Au(III) Complexes: A Computational Study Based on Experimental Evidences.

The coordination ability of the [(ppy)Au(IPr)] fragment [ppy = 2-phenylpyridine, IPr = 1,3-(2,6-di-isopropylphenyl)-imidazol-2-ylidene] towards different anionic and neutral X ligands (X = Cl, BF, OTf, HO, 2-butyne, 3-hexyne) commonly involved in the crucial pre-equilibrium step of the alkyne hydration reaction is computationally investigated to shed light on unexpected experimental observations on its catalytic activity. Experiment reveals that BF and OTf have very similar coordination ability towards [(ppy)Au(IPr)] and slightly less than water, whereas the alkyne complex could not be observed in solution at least at the NMR sensitivity. Due to the steric hindrance/dispersion interaction balance between X and IPr, the [(ppy)Au(IPr)] fragment is computationally found to be much less selective than a model [(ppy)Au(NHC)] (NHC = 1,3-dimethylimidazol-2-ylidene) fragment towards the different ligands, in particular OTf and BF, in agreement with experiment.

The mechanism of the gold(I)-catalyzed Meyer-Schuster rearrangement of 1-phenyl-2-propyn-1-ol 4- cyclization.

With the aim of rationalizing the experimental counterion- and solvent-dependent reactivity in the gold(i)-catalyzed Meyer-Schuster rearrangement of 1-phenyl-2-propyn-1-ol, a computational mechanistic study unraveled the unexpected formation of a gold-oxetene intermediate via commonly unfavorable 4-endo-dig cyclization triggered by the counterion in low polarity solvents.