Water-Soluble Gold(I) and Gold(III) Complexes with Sulfonated N-Heterocyclic Carbene Ligands: Synthesis, Characterization, and Application in the Catalytic Cycloisomerization of γ-Alkynoic Acids into Enol-Lactones

Zwitterionic imidazolium salts bearing 3-sulfonatopropyl, and 2-pyridyl, 2-picolyl, and 2-pyridylethyl substituents have been synthesized and employed as precursors for the preparation of novel water-soluble Au(I)- and Au(III)-NHC complexes of general composition [AuCl(NHC)] and [AuCl3(NHC)] (NHC = N-heterocyclic carbene), respectively. These complexes proved to be active, selective, and recyclable catalysts for the intramolecular cyclization of gamma-alkynoic acids under biphasic toluene/water conditions, leading to the desired enol-lactones in high yields under mild conditions (r.t.). Remarkably, despite the well-known ability of gold complexes to promote the hydration of C C bonds, the competitive hydration process was not observed, even during the cycloisomerization reactions of 1,6-diynes.