Silver NHC complex

A significant area of research over the previous year has involved the synthesis of NHC-silver compounds. A series of six benzyl- and / -methoxy-benzyl-substituted NHC-silver complexes has been synthesised from silver acetate. The complexes showed high antibacterial activity (compared to their respective precursors) and promising antitumour IC50 values ( 2.5-... 

Hintermair, U., Englert, U. and Leitner, W, Distinct reactivity of mono- and bis-NHC silver complexes carbene donors versus carbene-halide exchange reagents, Organometallics 30 (14), 3726-3731 (2011). 

The first report of a NHC Ag-catalyzed reaction appeared only in 2005 when Peris and Fernandez disclosed the diboration of alkenes in the presence of a bis-NHC silver complex (Scheme 11.3). The menthol-derived NHC ligand 7 showed only limited scope in this reaction but provided a proof-of-concept for NHC-Ag-based catalysis. Subsequent studies by the same groups examined chiral NHCs 8-10 (Figure 11.3) in this reaction, but enantiomeric excess remained very low (9% ee at best). ... 

The first NHC-silver complex [(IMes)2Ag][OTf] was reported in 1993, and was obtained by the reaction of two equivalents of free IMes with AgOTf. Although this reaction proceeded cleanly and in good yield, it required the... 

Silver acetate provided access to a new class of compounds ionic organometal-lic polymers based on l,2,4-tiiazolin-3,5-diylidenes [Eq. (12)]. Using just 1 equiv of Ag(OAc) leads to the generation of a conventional bis(NHC)-Ag complex. 

Triethylamine in THF can be used as the external base to deprotonate triazolium salts. The resulting NHCs were complexed in situ, e.g., to [(/7 -cymene)RuCl2]2, [(/ -cod)RhCl]2, and [(/ -C5Me5)RhCl2]2. Sodium carbonate in water/ DMSO deprotonates imidazolium iodides in the presence of mercury(II) dichloride to give [Hg(NHC)2][Hgl3Cl]. " A pyridine-functionalized imidazolium salt was deprotonated by lithium diisopropylamide (LDA) in THF and attached in situ to [(p -cod)Pd(Me)Br] [Eq.(17)]. After abstraction of the bromide anion with silver(I) a tetranuclear ring is formed. 

Chiral silver complexes bearing bidentate NHC ligands (24) have been synthesized. They are used in alkene metathesis and allylic alkylation reactions high diastereos- (g) electivity is observed induced by the chiral backbone on the prochiral biphenyl.27 Ruthenium-based complexes obtained from transmetalation with a Grubbs-Hoveyda complex exhibited high activities and enantioselectivities in ring-opening metathesis/ ... 

Note Silver complexes of NHC typically experience the largest coordination chemical shift in the C-NMR spectrum. 

This methane based tris-NHC ligand system behaves very similar to its boron based analogue. The most obvious difference is of course its neutral charge compared with the monoanionic borate ligand. As a consequence, the corresponding silver complex [ Agj HC(MeBenzIm)3 )2](BF )3, although supposedly isostructural with the borate complex... 

Note The difference is actually not in the use of the carbene transfer agent (the silver complex) versus the free NHC, but in the number of copper(I) ions added. 

In the same manner, dimethylsulfide substitution proved to be a practical procedure for the preparation of NHC-gold complexes. Good yields were obtained for the synthesis of monocarbene Au(I) complexes (104) from [(Me2S)AuCl] and one equivalent of free NHC (Scheme 17) iHios jjjg steric hindrance of NHCs was found to be of critical importance, since NHCs with smaller nitrogen substituents such as Mes, wo-propyl or n-butyl led to a lower yield and formation of a byproduct identified as the biscarbene complex [(NHC)2Au]+ Cl. Fortunately, the silver-mediated transmetallation was an efficient alternative route for these NHCs (see Section 2.6). When the same reaction was performed with two equivalents of in situ generated NHCs, the cationic biscarbene complex (105) [(NHC)2Au]X (X = Cl, Br, orPFe) was obtained as the only product. ... 

Lithium(l) NHC adducts are extremely rare, but some examples have been isolated as stable crystalline solids when a ftmctional group is involved for their stabilization, the amino group, for instance. Since these showed to be effective in transmetallation processes, they represent an interesting alternative to the expensive silver complexes. As an example, Arnold reported the synthesis of covalently tethered lanthanide carbene complexes (214) using adduct (213) as source ofNHC (equation 39). Using (213) and the same methodology, unusual cerium complexes (215) and (216) were isolated and characterized (equation 40). ... 

A recent system in which NHCs were effectively combined with polymeric materials was reported by Hunstad, Cannon, Youngs, and Wooley. In this case, NHC-Ag complexes were used to deliver silver cations (Ag ) to the core of amphiphilic polymer nanoparticles when subsequently released, the cations would exhibit... 

Figure 31.6 Left NHC-Ag complex 29 Right Schematic representation of its use as a silver cation-releasing antimicrobial therapeutic. The dashed arrow represents the release of Ag+ from the decomposition of 29.

An alternative solution to the same problem uses mixed vinylalanes that contain silicon in the a-position. Since these derive from a diisobutylaluminum (DIBAL)-based hydroalumination of silylalkynes, the resulting species are diisobutyl(alkenyl)-alanes. These undergo transmetalation in the presence of an NHC-Cu complex, derived from CuCb and, importantly, a bidentate, dimeric silver NHC precursor (illustrated below). Silyl moieties can be TMS or TBS, although the latter, bulkier group in the starting alane requires room temperature for 1,4-addition to ensue. Both 5- and 6-membered enones can be used representative examples are shown below. The resulting vinylsilane in each product can be easily converted to alternative functionality, such as a ketone via oxidation with MCPBA), a halide e.g., an iodide, with NIS), and an alkene yia protodesilylation with TFA). ... 

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