Silver-Free Catalysis with Gold(I) Chloride Complexes

Abstract

Gold(I) chloride complexes are stable, widespread precatalysts that generally require activation by halide abstraction to display useful catalytic activity. Chloride scavenging is typically performed in situ by using silver salts. This procedure, apart from mandating the use of an additional metal, often negatively impacts the reaction outcome, because Ag additives are not catalytically innocent (silver effect). Therefore, both the development of alternative chloride scavengers and the design of self-activating gold(I) chloride complexes endowed with special ligands have lately been the subject of intense research efforts. This review describes recent advances in the field of silver-free Au(I) catalysis employing gold(I) chloride complexes, with an emphasis on approaches emerged in the last decade.

Silver‐Free Au(I) Catalysis Enabled by Bifunctional Urea‐ and Squaramide‐Phosphine Ligands via H‐Bonding

“…Having prepared a library of complexes, some of which were found to H‐bond with the chloride ligand ( 7 a , 7 b , 8 b , 9 b ) or the chloride anion ( 4 , 6 ) in the solid state, we performed catalytical tests without silver additives to see whether such H‐bonding interactions would prove strong enough in solution to render the metal center catalytically active. The cyclization of N ‐propargyl benzamide 12 a was chosen as benchmark reaction, as it is a model transformation for silver‐free Au(I) catalysis,[ 4b , 6 , 8 , 9 , 10 , 11 , 13 ] and, to further aid comparison, the same conditions employed by Gabbaï were used (Scheme 2 ). [11] Importantly, this reaction under Au(I) catalysis affords exclusively methylene oxazoline 13 a , [25] whereas isomerized oxazole 14 a is obtained in the presence of Au(III) or acidic additives,[ 26 , 27 ] being thus ideally suited to discriminate a pure Au(I) manifold from catalytic activity due to impurities or degradation/disproportionation products.…”

Protic Ferrocenyl Acyclic Diamino Carbene Gold(I) Complexes

“…Also silver-free activation by e. g. in situ replacing the chlorido ligand using alkali metal salts, copper(II) triflate or methyl triflate is possible. [36][37][38][39][40] Alternatively, introducing ferrocenyl (Fc) as redox-active moiety in carbene [41][42][43][44] (Scheme 1) or phosphane ligands [45] and oxidizing the ferrocene to ferrocenium can initiate catalysis. The reversible on/off switching of the catalyst leads to redox-switchable gold catalysis.…”

Spotlight on Ligand Effects in 1,2,3-Triazolylidene Gold Complexes for Hydroamination Catalysis: Synthesis and Catalytic Application of an Activated MIC Gold Triflimide Complex and Various MIC Gold Chloride Complexes

“…Mostly gold chloride complexes have been applied in gold­(I)-based homogeneous catalytic studies. Most of these complexes, with few exceptions, need to be activated through a silver salt or other halide scavenger. , We recently introduced a further activation strategy, which is the oxidation of a gold chloride complex with a ligand bearing a ferrocenyl unit. − This approach has also been applied to related gold complexes by other groups. ,, A common trait in these methods is the required additive in the catalytic application. In recent years, next to the classical relatively inert gold chloride complexes, more and more defined, stable, “activated” gold complexes have been reported.…”

“…In recent years, next to the classical relatively inert gold chloride complexes, more and more defined, stable, “activated” gold complexes have been reported. In most of them, the halide is substituted with a better leaving group. ,− One of those weakly coordinating ligands is bistriflimide (NTf 2 ). This ligand was introduced by Gagosz and co-workers in phosphine gold complexes .…”