Hydration of alkynes catalyzed by [Au(X)(L)(ppy)]X in the green solvent gamma-valerolactone under acid-free conditions: the importance of the pre-equilibrium step

[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 gamma-valerolactone (GVL), under acid-free conditions. The solution structure, reactivity, and catalytic properties of (1) and (2) were established by means of multinuclear NMR and computational (DFT) studies. Structural features of 1 during the catalysis, inferred by NMR spectroscopy, clearly indicate that complex 1 retains its square planar structure and no reduction to Au(i) and/or Au(0) nanoparticles was observed. The overall catalytic and kinetic investigations [kinetic isotopic effect (KIE), effect of acid additives, the order of reaction with respect to the catalyst, alkyne and nucleophile and the effect of the temperature] supported by computational results confirm that the pre-equilibrium step of the reaction mechanism is the RDS: water or counterion substitution by 3-hexyne in the first co-ordination sphere of Au(iii) is the key step of the whole process. The description of the mechanism of the hydration of 3-hexyne catalyzed by 1 here reported appears therefore to be of high significance because comprehensive mechanistic studies of the Au(iii)-catalyzed hydration reaction of the CC bond are scarce in the literature and generally lack experimental basis.