Mono- and Digold(I) Complexes with Mesoionic Carbenes: Structural Characterization and Use in Catalytic Silver-Free Oxazoline Formation

Abstract

Triazolylidenes are a prominent class of mesoionic carbenes that have found use as supporting ligands in homogeneous catalysis in recent years. We present here the syntheses of three new mononuclear gold(I) chlorido and two new dinuclear gold(I) chlorido complexes. The ligands in the aforementioned complexes are derived from either the corresponding monotriazolium or the bitriazolium salts. All complexes have been characterized by 1 H and 13 C{ 1 H} NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction studies. Structural characterization delivers a delocalized bonding situation within the triazolylidene ligands and a linear coordination at the gold(I) centers. The gold(I) centers in all cases are bound to one triazolylidene-C donor and a chlorido ligand. Additionally, for the digold(I) complexes large Au−Au distances were observed, ruling out the existence of aurophilic interactions in these digold complexes in the solid state. All of the gold(I) complexes were tested as (pre)catalysts for the cyclization reaction of propargylic amides to form oxazolines. We show here that the steric bulk of the substituents on the triazolylidene ligands plays a decisive role in the catalytic efficiency of the gold(I) complexes. Copper(II) triflate is shown as a viable alternative to silver(I) salts as an additive for the oxazoline formation. Mechanistic studies show the detection of a gold(I) triazolylidene vinyl complex as an intermediate in the catalytic synthesis of oxazoline with these complexes. These results thus establish copper(II) triflate as an alternative to silver(I) salts as an additive in gold(I) triazolylidene catalysis. Furthermore, it also shows that steric tuning of triazolylidene ligands can indeed be utilized for increasing the catalytic efficiency of the corresponding complexes.

Directed Design of a AuI Complex with a Reduced Mesoionic Carbene Radical Ligand: Insights from 1,2,3‐Triazolylidene Selenium Adducts and Extensive Electrochemical Investigations

“…A reversible reduction on the MIC moiety is extremely rare. A heteroleptic triazolylidene gold complex and a cationic homoleptic triazolylidene gold complex showed low chemical reversibility of the reduction at −40 °C, which was detected through a proportionally small re‐oxidation wave [15] . Quasi‐reversible or reversible reduction have been observed in chelating ligands containing triazolylidenes (for example, with additional pyridyl substituents) [16] .…”

“…To investigate the synthetic potential of PhDippTrz and to explore if the reversibility of reduction also remains when incorporating these triazolylidenes in transition‐metal complexes, a gold chloride complex with the PhDippTrz ligand was synthesized (Scheme 3). Gold triazolylidene complexes have recently found use in homogeneous catalysis, in photochemistry, and they have also been investigated for their biological activity [15, 18] . The synthetic strategy was deprotonation of the corresponding triazolium salt and coordination through a weak base route, introduced by Nolan and co‐workers for normal NHCs [19] .…”

Triple the fun: tris(ferrocenyl)arene-based gold(i) complexes for redox-switchable catalysis

“…Uncovered by Hashmi and co-workers in 2004, 98 the catalytic synthesis of oxazolines has quickly developed into a standard reaction for gold(I) complexes. [99][100][101][102][103][104] The groups of Sarkar and Heinze, among others, have established the transformation of 5 to 6 as a platform to perform and study RSC using gold(I) catalysts. [105][106][107][108][109] Rendering the gold(I) centres reversibly more Lewis-acidic and hence more catalytically active 110 through oxidation of a connected ferrocenyl moiety is one way of obviating the sometimes problematic use of silver salts for activation of gold(I) precatalysts by halide abstraction and enables temporal control over the activity of the catalyst.…”

Cycloisomerization of π‐Coupled Heteroatom Nucleophiles by Gold Catalysis: En Route to Regiochemically Defined Heterocycles

“…2). However, in their case, the process stops after the formation of alkylidene oxazoles 139 without further isomerization to oxazoles, thus demonstrating the crucial role played by different gold catalysts in bringing divergence in product selectivity [255] . Carbophilic activation of exactly same substrate by a T‐shaped gold→stilborane complex equally undergoes 5‐ exo ‐ dig cyclization to produce dihydrooxazole [256] .…”