Gold-NHC systems

Despite the limited number of examples reported in the literature concerning the antimicrobial activity of gold-NHC systems and the numerous studies of silver species, no research on this subject has been reported with copper analogs. 

In 2009, Metzler-Nolte and coworkers reported the first gold-NHC systems conjugated with amino acids and peptides as antiproliferative reagents [14]. Chloride and bromide species were tested with gold(I) and gold(III) analogs (Figure 7.13). Three cell lines were chosen HeLa (human cervbc epitheloid... 

Trans. 2009 340—352. (b) Hemmert C, Poteau R, dit Dominique FJ-B, Ceroni P, Bergamini G, Gomitzka H. Amide-functionalized bis(NHC) systems anion efect on gold-gold interactions. Eur J Inorg Chem. 2012 3892—3898. 

Interestingly most of the catalytic systems based on Tp M units contain a coinage metal such as copper or silver. This is also observed for NHC-based catalytic systems, with the addition of gold to those metals. This similarity can be explained in terms of a common feature of both Tp M and (NHC)M moieties with those metals they leave just one coordination site for the catalytic reaction to occur. A second coordination site may also accessible in some cases, for example with Cu(ll)-based systems where five-coordinate geometries are available. In the case of (NHC)M systems (M = Cu, Ag, Au), the linear complexes can accept an incoming ligand (reactant) through the transient formation of three-coordinate intermediates (Scheme 2). 

The combination of an (N-heterocyclic carbene) NHC-gold complex and Selectfluor was found to be a highly efficient catalyst system for car-bene-transfer reactions (140L4472). 

Furthermore, it has to be emphasized that this method allows straightforward access to gold complexes carrying unsymmetrically substituted NHC ligands. Apart from this it has to be noted that NHC derivates, which bear CH-acidic protons (the NHOC and eNHC systems) that can potentially lead to problems during the formation of the carbene/attachment to the metal center, can be synthesized easily and under mild conditions. Due the fact that m the NHOC systems an enolization can directly Influence the electronic situation at the carbene carbon atom, these systems are also called switchable carbenes. ... 

Another subfield of enyne cycloisomerization chemistry is best described as cycloaddition reactions. For instance, allenedienes were found to react smoothly in the presence of an IPr-based gold(I) cationic species to produce the formal [4 + 3] cycloadducts (Equation (11.8)). This method is a valuable approach to fused seven-membered ring systems and was applied to a transannular reaction, allowing for the construction of a pentacyclic system. Of note, Toste showed that, by replacing the NHC on gold by a triarylphosphite, an alternative [4 + 2] cycloaddition was observed from the same linear allenedienes. ... 

The analyzed tri-NHCs ligands are interesting not only for a possible improvement in the catalytic efficiency of the derived trimetaUic systems but also for applications in material science for example, the trimetaUic gold(I) complex with ligand 29 could be employed in the development of main-chain organometaUic microporous polymers. ... 

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