NHC-Au-X (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, X- = BArF-, BF4-, SbF6-, OTf-, NTf2-, ClO4-, OTs-, TFA(-)) catalysts were tested in the hydration of allcynes. A complete rationalization of the counterion effect enabled us to develop a highly efficient methodology under solvent-, silver-, and acid-free conditions. Thus, it was possible to use room or mild (60 degrees C) temperature and to reduce the catalyst loading up to 0.01 mol % with respect to the substrate, leading to high TON (10(4)) and TOF (10(3) h(-1)) values. The favorable catalytic conditions allowed us to reach very low E factor (0.03-0.06) and high EMY (94-97) values. Finally, the absence of solvent permits easy separation of the liquid product from solid catalyst and ionic additives by distillation, giving products with high purity that are uncontaminated by metals. This opens the way to catalyst recycling (up to four times) without loss of activity. The overall catalytic and kinetic evidence, supported by computational results, confirms that the anion plays a crucial role in all steps of the reaction mechanism: pre-equilibrium, nucleophilic attack, and protodeauration. As a matter of fact, only the two complexes bearing OTf- and NTf2- counterions showed catalytic activity; all others are completely inactive. Protodeauration is the rate-determining step under these aprotic and apolar conditions, and in our calculations, the first anion-mediated proton transfer takes place easily in one step, leading to a gold enol complex. Different pathways have been computationally explored for the conversion of gold enol to ketone product by modeling different experimental conditions
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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