Hydration of Alkynes Catalyzed by L-Au-X under Solvent- and Acid-Free Conditions: New Insights into an Efficient, General, and Green Methodology

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-diisopropylphenyl)dihydroimidazol-2-ylidene {BIAN}, 1,3-bis(2,6-diisopropyl-phenyl)dihydroimidazol-2-ylidene {NHCCH2}, bis(tertbutylamino)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.