Silver carbene

Conditions Copper salt 4 mol %, silver carbene 4 mol % Et2Zn 1.5 equiv, Et20, -78 °C, 16 h... 

In 2007, Fernandez et al. demonstrated that transition-metal complexes with heterobidentate S/C ligands based on imidazopyridin-3-ylidene and thioether functionalities could be readily prepared from the corresponding azolium salts by reaction with Ag20 and transmetalation of the resulting silver carbenes with appropriate metal sources. The cationic Pd(allyl)(carbene-S) complexes have proven to be active catalysts in the test reaction, reaching enantioselectivities of... 

An extension of the research on silver complexes with Lewis base-functionalized mono(A-heterocyclic carbene) ligands has been made toward the better-studied and stronger coordinating phosphine systems. The reaction of a diphenylphosphine-functionalized imidazolium salt with silver oxide in dichloromethane affords a trinuclear silver carbene complex 50, as confirmed by electrospray-ionization mass spectrometry.96,97 Metathesis reaction of 50 in methanol using silver nitrate gives 51 in 33% yield. The crystal structures of 51 were found to be different when different solvents were used during crystallization (Scheme 12).97 One NO3- anion was found to be chelated to... 

Youngs and co-workers extended the chemistry toward synthesizing water-soluble silver carbene complexes 68a-68c (Figure 28) by using hydroxyl-containing pincer-type iV-heterocyclic carbenes as ligands.1 4 It is interesting... 

Pyridine-functionalized N-heterocyclic carbene Rh and Ir complexes have also been described as active precatalysts for C=0 bond TH. For example, Peris and coworkers observed the formation of metal hydrides by C—H oxidative addition of a pyridine-N-substituted imidazolium salt such as N-"Bu-N -(2-pyridylmethyl-imidazolium) hexafluorophosphate in the reaction leading to M-pyNHC complexes, that is [lr(cod)H(pyNHC)Cl] (58) [54]. Transmetallation from silver carbene... 

Iridium 2-pyridinylmethyl imidazolylidene C,N-chelates were obtained by transmetallation of the silver carbene complexes and tested for catalytic activity in the TH of benzophenone and nitroarenes by isopropanol [55]. The neutral monodentate complexes [(L-KC)Ir(COD)Clj [61a,b L = l-methyl-3-(6-mesityl-2-pyridinylmethyl)-2-imidazolylidene, l-mesityl-3-(6-mesityl-2-pyridinylmethyl)-2-... 

Scheme 21 Catalytically active silver carbene complex 17...

Deprotonation with aluminum alkys, 9, 272 mononuclear carbonyl iridium complexes, 7, 302 for palladium cyclopentadienyl complexes, 8, 390 in Ru and Os half-sandwich preparations, 6, 569 in silver carbene synthesis, 2, 206 Desulfurization... 

Chavan and coworkers provide evidence that the Wolff rearrangement is facilitated by the formation of silver nanoclusters, which initiate electron transfer to the diazo compound providing 8. While the precise fate of this species remains to be firmly established, they suggest a multicycle process involving the intermediacy of a silver carbene 10 (Scheme 8.2).10 12 Decomposition of the silver carbene to the free carbene 14 precedes rearrangement to ketene 13, which is then trapped with water to provide the carboxylic acid 15 (Scheme 8.2). 

On the basis of these results, a mechanism (Scheme 8.10) involving the intermediacy of a silver-carbene 54 was proposed in which the insertion product arises from the formation of the halonium ylide 55, followed by a 1,2 shift (55 —> 26, or 51 or 52). Alternatively, if the substrate and thus the halonium ylide 56 contain a (3-hydrogen, this could be removed by an intramolecular deprotonation with concomitant loss of halide resulting in formation of the olefin 57 and the a-haloacetate 53. At this stage, no independent evidence has been obtained to support this pathway thus this mechanism is purely speculative (see text below). Indeed, although the pathway has been depicted as involving metal-free intermediates, it is quite likely that this is not the case, but this awaits independent experimental verification. 

At this point, although there is no independent experimental evidence for the formation of silver carbenes in the reactions described above, and in some cases... 

All the examples described above involved the reaction of diazoacetate derivatives with silver salts to initiate the formation of a putative silver carbene however, other pathways exist. For example, Porcel and Echavarren have reported an intramolecular cyclization of an allylstannane to a pendent alkyne (Scheme 8.22) that involves the intermediacy of a silver carbene.52 As can be seen in Table 8.12, the reactions proceeded in moderate to excellent yield, providing the dienylstannane, while in some cases, reductive destannylation occurred. Several asymmetric reactions were reported with substrate ( )-145d, leading to the formation of the expected adduct in reasonable enantioselectivities (ee = 73-78%) in a preliminary screen with a number of different ligands. 

Mechanistically, most alkyne cyclizations of this type have been interpreted in terms of the catalyst serving as a Lewis acid, activating the n bond to nucleophilic attack,53-57 and while this pathway is feasible, the authors preferred one involving the intermediacy of a silver carbene.52 Control studies ruled out a pathway involving... 

However, an encouraging result was obtained very recently for the 1,4-conjugate addition of dialkyl zinc to a variety of Michael acceptors catalyzed by copper. Alexakis, Roland and coworkers have investigated the addition of diethyl zinc to cycloheptenone and observed an enantiomeric excess of 93% (95% yield) in the presence of Cu(OAc)2 and the silver carbene derivative of imidazolium 1 (Scheme 3) [10]. Silver carbene complexes are efficient transfer agents to copper(II) and therefore the potentially harmful use of a base to generate the catalytic species is avoided. 

An alternative for carbene-ruthenium bond formation is the use of a car-bene transfer agent. Silver carbenes have demonstrated their utility in serving as valuable carbene sources for the preparation of various transition metal... 

One of the most generally used methods to prepare a NHC transition metal complex is the reaction of an azolium salt with silver oxide to form the silver carbene complex [24,25,72], It is so general that it has its own name, the silver(I) oxide (Ag O) method... 

Of course, oxide is a base and thus it falls under the heading of reactions with basic transition metal compounds, but the silver carbene complexes are usually only synthesised because the silver atom coordinates only weakly and can easily be replaced by another metal of choice. It is therefore known as a carbene transfer agent. 

Advantage The silver carbene complex can usually be synthesised using undried solvents and with exposure to air. 

Silver carbene complexes are the most commonly used carbene transfer complexes [83], Other carbene transfer agents include lithium adducts [56], potassium complexes [53], molybdenum carbene complexes [83,84] or chromium carbene complexes [85],... 

Of course, the heteroaromatic sidegroup is usually not strong enongh as a donor ligand to replace chloride from a palladinm(II) centre [31,34], Coordination of the sidearm can be achieved by silver halide abstraction nsing a snitable silver salt [31], using a silver carbene complex as the silver source [31,34,35] or reacting the imidazolium salt directly with [Pd(OAc)2] [36] and therefore in the absence of halide (PF - was nsed as the imidazolium counterion). 

Figure 3.54 Amide deprotonation during the synthesis of amino functionalised silver ( ) carbene complexes.

Note The palladium(II) complex in Figure 3.73 shows no Pd-0 coordination despite the existence of a free coordination site owing to chloride abstraction by the silver carbene complex. Coordination of the weak donor acetonitrile is preferred. 

Direct arylation of methyl imidazole with 2,7-dichloronaphthyridine leads to a potentially tetradentate and practically tridentate bis-carbene ligand on a naphthyridine scaffold [363]. Reaction of the bis-imidazolium salt with silver(l) oxide in the usual way yields a linear trinuclear silver carbene complex with this tris-bridging hgand (see Figure 3.115). 

Liao et al. also synthesised an imidazolium salt carrying two carboxylic acid amide functionalities and generated its silver carbene complex. It is distinguished by absolute linearity of the C-Ag-Cl vector and rather short Ag-C and Ag-Cl bond lengths. 

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