Abstract
The synthesis and characterization of novel gold(I) complexes containing hydrophilic ferrocenylphosphino sulfonate ligands, i.e., compounds [AuCl{(η5-C5H3PR2(SO3iPr))Fe(η5-C5H5)}] (R = Ph (2a), p-Tol (2b), Cy (2c)), are presented, including a single-crystal X-ray diffraction study on 2a. Complexes 2a–c were checked as catalysts for the intermolecular addition of carboxylic acids to nonactivated internal alkynes using water as a green reaction medium. The best results in terms of activity were obtained with 2a in combination with AgOAc, which was able to promote the selective anti addition of a variety of aromatic, aliphatic, and α,β-unsaturated carboxylic acids to both symmetrical and unsymmetrical internal alkynes at 60 °C, employing metal loadings of only 2 mol %.
Highlights
Enol esters represent an important class of synthons in organic chemistry, commonly employed as intermediates, among others, in cross-coupling [1,2,3], asymmetric hydrogenation [4,5,6], and cyclization reactions [7,8,9], as well as monomers in polymerization and oligomerization processes [10,11,12]
With the complexes [AuCl{(η5 -C5 H3 PR2 (SO3 i Pr))Fe(η5 -C5 H5 )}] (2a–c) in hand, we explored their catalytic potential in hydro-oxycarbonylation reactions of internal alkynes in water
Complex [AuCl(SMe2 )] [38], which were synthesized as previously described in the literature
Summary
Enol esters represent an important class of synthons in organic chemistry, commonly employed as intermediates, among others, in cross-coupling [1,2,3], asymmetric hydrogenation [4,5,6], and cyclization reactions [7,8,9], as well as monomers in polymerization and oligomerization processes [10,11,12]. Among the different methods of synthesis of these valuable molecules, the intermolecular addition of carboxylic acids to alkynes catalyzed by transition metals (hydro-oxycarbonylation reaction) is probably the most straightforward and atom-economical one because the starting materials are widely available and no byproducts are generated in the process. It is worth noting that, despite the growing interest in developing catalytic transformations in environmentally friendly aqueous media [28], efforts devoted to finding catalytic systems able to promote the hydro-oxycarbonylation of alkynes in water have been very scarce, with most of the examples dealing with intramolecular processes [29]. The only protocol for the intermolecular addition of carboxylic acids to internal alkynes in water that can be currently found in the literature was developed by our group, making use of the gold(I) complex [AuCl(PPh3 )] in combination with the chloride abstractor AgOAc [30]. As shown in Scheme 1, the trisubstituted enol ester products were generated with complete Z-stereoselectivity as the result of the anti-addition of the carboxylate anion to the corresponding cationic π-alkyne-Au(I) intermediate
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