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

Abstract

Zwitterionic imidazolium salts bearing 3-sulfonatopropyl, and 2-pyridyl, 2-picolyl respectively. These complexes proved to be active, selective and recyclable catalysts for the intramolecular cyclization of γ-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.

Gold(I)‐Catalysed Cyclisation of Alkynoic Acids: Towards an Efficient and Eco‐Friendly Synthesis of γ‐, δ‐ and ϵ‐Lactones

“…High regioselectivity for the exo cyclisation was also observed for most substrates. Exceptions were found when internal alkynes bearing a methyl group were used, as in the case of 2b and the β‐alkynoic malonate derivative 2d , for which the endo ‐dig‐cyclisation reaction was competitive with the exo ‐cyclisation, as previously reported ,. Thus, products 3b and 3d were isolated in 94 and 98% yields with an exo : endo ratio of 7:1 and 4:1, respectively.…”

Metal-Catalyzed Intra- and Intermolecular Addition of Carboxylic Acids to Alkynes in Aqueous Media: A Review

“…On the other hand, although the 5-exo-dig cyclization was also the preferred reaction pathway with internal alkynes, mixtures containing the corresponding 5-and 6-membered ring lactones were in some cases obtained. This initial work with 10b was subsequently extended to other Au(III) and Au(I) complexes containing related zwitterionic NHC ligands bearing 3-sulfonatopropyl, and 2-pyridyl, 2-pycolyl or pyridylethyl substituents (10a,c and 11a-c in Figure 3) [45]. All of them proved to be catalytically active, even with Au loadings of only 0.1 mol %, showing in general performances similar to that of 10b.…”

“…In this regard, some differences were observed between the Au(I) and Au(III) species, the recyclability of the latter being much more effective due to their higher stability in the aqueous medium (the Au(I) derivatives 11a-c undergo with time partial decomposition into catalytically inactive Au(0) nanoparticles; a process also observed with 10a-c but which takes place much more slowly). This initial work with 10b was subsequently extended to other Au(III) and Au(I) complexes containing related zwitterionic NHC ligands bearing 3-sulfonatopropyl, and 2-pyridyl, 2-pycolyl or pyridylethyl substituents (10a,c and 11a-c in Figure 3) [45]. All of them proved to be catalytically active, even with Au loadings of only 0.1 mol %, showing in general performances similar to that of 10b.…”

The intermolecular hydro-oxycarbonylation of internal alkynes: current state of the art

“…the cycloisomerization of alkynoic acids. [58][59][60][61][62][63] However, we would like to stress that these intramolecular reactions are favoured from a thermodynamic point of view and do not represent a notable synthetic problem. 52,64 Another challenging aspect related with the use of internal alkynes concerns the control of the regio-and stereoselectivity of the addition process since, starting from non-symmetrically substituted substrates R ), up to four isomeric enol esters can be formed (obviously difficult to separate).…”