[AuCl(NHC)(ppy)]Cl (1) and [AuCl(PPh3)(ppy)]OTf (2) complexes [ppy = 2-phenylpyridine, NHC = 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene] successfully catalyze the hydration of alkynes in γ-valerolactone (GVL), under acid-free conditions. The solution structure, reactivity, and catalytic...

The coordination ability of the [(ppy)Au(IPr)]2+ fragment [ppy = 2-phenylpyridine, IPr = 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene] towards different anionic and neutral X ligands (X = Cl−, BF4−, OTf−, H2O, 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 BF4− and OTf− have very similar coordination ability towards [(ppy)Au(IPr)]2+ 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)]2+ fragment is computationally found to be much less selective than a model [(ppy)Au(NHC)]2+ (NHC = 1,3-dimethylimidazol-2-ylidene) fragment towards the different ligands, in particular OTf− and BF4−, in agreement with experiment. Effect of the ancillary ligand substitution demonstrates that the coordination ability of Au(III) is quantitatively strongly affected by the nature of the ligands (even more than the net charge of the complex) and that all the investigated gold fragments coordinate to alkynes more strongly than H2O. Remarkably, a stabilization of the water-coordinating species with respect to the alkyne-coordinating one can only be achieved within a microsolvation model, which reconciles theory with experiment. All the results reported here suggest that both the Au(III) fragment coordination ability and its proper computational modelling in the experimental conditions are fundamental issues for the design of efficient catalysts.

L–Au–X [L = 1,3-bis­(2,6-di-isopropylphenyl)-imidazol-2-ylidene {NHCiPr}, tris­(3,5-bis­(trifluoromethyl)­phenyl)­phosphine {PArF}, bis­(imino)­acenaphtene-1,3-bis­(2,6-di-isopropylphenyl)­dihydroimidazol-2-ylidene {BIAN}, 1,3-bis­(2,6-di-isopropyl-phenyl)­dihydroimidazol-2-ylidene {NHCCH2 }, bis­(tert-butylamino)­methylidene {NAC}, 2-(di-tert-butylphosphino)­biphenyl {JohnPhos}, tricyclohexylphosphine {PCy3}, triphenylphosphine {PPh3}, tris­(2,4-di-tert-butylphenyl)­phosphite {POR3}; X– = Cl–, OTf–, OTs–] catalysts were tested in the hydration of alkynes in neat and acid-free conditions. The overall catalytic evidence confirms that not only the counterion as previously observed by us but also the ligand play a crucial role. As a matter of fact, only complexes bearing NHC ligands showed appreciable catalytic activity. A complete rationalization of the ligand and counterion effects enabled us to develop a highly efficient methodology for the hydration of inactive diphenylacetylene in solvent-, silver-, and acid-free conditions. Thus, it was possible to reduce the catalyst loading to 0.01 mol % (with respect to diphenylacetylene) leading, to the best of our knowledge, to the highest TON (3400) and TOF (435 h–1) values found at 120 °C. The favorable catalytic conditions allowed us to reach for the first time very low E-factor (0.03) and high EMY (77) values for this substrate.

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