Gold carbene species

At the beginning of the new millennium, Hashmi et al. presented a broad research study on both intramolecular and intermolecular nucleophilic addition to alkynes and olefins [18]. One of the areas covered by these authors was the isomerization of co-alkynylfuran to phenols [19]. After that, Echavarren and coworkers identified the involvement of gold-carbene species in this type of process, thus opening a new branch in gold chemistry [20]. And subsequently, Yang and He demonstrated the initial activation of aryl —H bonds in the intermolecular reaction of electron-rich arenes with O-nucleophiles [21, 22]. 

The mechanism of intermolecular Au(I)-catalysed cyclopropanation of alkenes with 1,6-enynes, leading to biscyclopropylated derivatives (64) in a highly stereospecific manner, has been explored experimentally and theoretically. The results are consistent with the participation of cyclopropyl gold carbene species (65) as a key intermediate. 

It is important to note in an overview of this chemistry that the formation of most of the products can be mechanistically explained as proceeding via cyclopropyl gold carbene intermediates. The involvement of these species has been previously proposed in enyne cyclizations with other metals and their structures have been later determined by calculations. ... 

The easiest reactions are those in which the nucleophile is the gold-activated species. Examples of this are Au(I)-catalyzed carbene and nitrene transfers (equations 142 and 143) that convert olefins into cyclopropanes or aziridines, respectively. In the carbene transfer, ethyl diazoacetate is the source of carbene and the active NHC-gold cationic catalyst is generated by chloride abstraction with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate NaBAT4. The cyclopropanation is competitive with other carbene insertions with active C H or N H bonds present in the substrate. For the aziridinations of olefins, nitrene formation is accomplished by the oxidation of sulfonamides with PhI(OAc)2 and the catalyst of choice is a gold-(I) triflate with a terpyridine ligand. 

Colloidal gold and fine copper powder also catalyze diazoalkane polymerizations. The reaction appears to proceed by formation of alkylidene or carbene species that are bound to the surfaces of metals. " The initiations are completed by additions of diazoalkanes to the bound carbenes followed by liberations of nitrogen. Termination may take place by chain transfer, perhaps to a monomer, or to the solvent. " ... 

Note Since this article was written, a number of other studies have appeared on the generation and reactions of gas phase reactive metallic species, including the use of decarboxylation reactions to produce organometallic ions (105-107] and the lithium acetate enolate anion (108j silver and silver hydride cluster ions (109) gold carbenes [110, 111] and metal-oxo cations 112]. 

Organometallic complexes of copper, silver, and gold are ideal precursors for carbene complexes along with some C- and N-coordinated species. Their reactivity pattern, in particular in oxidative addition reactions, was the most comprehensively studied. 

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