Solvent‑, Silver‑, and Acid-Free NHC-Au‑X Catalyzed Hydration of Alkynes. The Pivotal Role of the Counterion

• Published in: ACS catalysis Vol. 6; no. 11; pp. 7363 - 7376
• Main Authors: Gatto, Mattia, Belanzoni, Paola, Belpassi, Leonardo, Biasiolo, Luca, Del Zotto, Alessandro, Tarantelli, Francesco, Zuccaccia, Daniele
• Format: Journal Article
• Language: English
• Published: American Chemical Society 04.11.2016
• Subjects: DFT; gold catalysis; green chemistry; hydration; counterion effect
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.6b01626

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 alkynes. 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 °C) temperature and to reduce the catalyst loading up to 0.01 mol % with respect to the substrate, leading to high TON (104) and TOF (103 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.