Transmetalation from Silver Complexes

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... 

Direct C-H activation at abnormal carbene positions was achieved by heteroatom-directed cyclometalation reactions using pyridyl-functionalized azolium salts and [IrCp Cl2]2 (Cp = pentamethylcyclopentadienyl Scheme 3.4). Cyclometalation proceeded well with both triazolium salt 20 and imidazolium salt 22 and yielded iridium complexes 21 and 23 in good yields. Transmetalation from silver was not efficient in either case. The imidazolium precursor underwent activation and oxidation of the exocyclic C2-bound CH3 group, whereas triazolium salt 20 formed a mixture of compounds upon reaction with Ag20, probably due to poor selectivity and competitive coordination of the pyridyl nitrogen. 

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

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/ ... 

Although catalytic amounts of Ag20 have been found to promote the palladium-catalyzed coupling of aryl boronic acids and terminal alkynes, the authors in this case do not attribute the desired reactivity to the formation, and subsequent transmetalla-tion, of a silver acetylide. Rather, it is proposed that the Ag20 activates the alkynylpalladium complex to allow transmetallation from the boronic acid, and that any competing formation of the silver acetylide results in a homocoupling of the alkynes (Scheme 1.55).123... 

Recently, Peris and coworkers reported the synthesis of highly stable orthometallated Cp -Ir NHC complexes and a catalytic application in the deuteration of organic molecules. Complexes (227) were synthesized from imidazolium salt (225) and [IrCp Cl2]2 as metal precursor either by a one-step procedure with sodium iodide to minimize the mixture I versus Cl or by transmetallation from the corresponding silver carbene affording (226) followed by the C-H activation of the phenyl ring by the iridium... 

A CHELATING RHODIUM A-HETEROC YCLIC CARBENE COMPLEX BY TRANSMETALLATION FROM A SILVER-NIIC INTERMEDIATE... 

Youngs and coworkers used variation 3 of this ligand containing additional methylene groups between the NHC and pyridine moiety. They tried to synthesize a Rh complex by transmetallation from the dinuclear silver complex 33. However, only the binuclear Rh complex 34 was obtained [11] (Scheme 9.4). 

NHC complexes can be synthesized in a wide variety of ways from a metal precursor and the imidazohum salt (Eq. 4.19 and Eq. 4.20) a free NHC (Eq. 4.21) or by transmetallation from the silver NHC complex, often conveniently available under mild conditions from the... 

Several mono-NHC complexes were obtained. For example, transmetallation from a silver-NHC complex afforded the preparation of an Rh-NHC complex, in which the NHC ligand was obtained from caffeine. Deprotonation of chiral ferrocenyl imidazolium salts allowed the preparation of chiral Rh(l)-NHC complexes that could also be P,C and S,C chelate. Dicyclophane imidazolium carbene ligands also provided an efficient way of obtaining chiral Rh-NHC catalysts. ... 

Pyridine-bridged bisimidazolium salts also afforded the preparation of dimetallic Rh(l) species by transmetallation from a silver(l)-NHC complex and deprotonation with a weak base. A pincer pyridine-bis-NHC Rh(lll) complex 330 was obtained from the reaction of the corresponding bisimidazolium salt and [Rh(/r-Cl)(cod)]2 in the presence of... 

Figure 3.1 Abnormal imidazolylidene gold(I) complexes prepared by transmetalation from a silver intermediate.

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... 

Rather than utilizing complete transmetallation reactions, mixed metal silver(i) aryl complexes 8-11 were obtained from the reaction of soluble silver salts with arylgold(i) complexes possessing ancillary phosphine or arsine ligands, as shown in Scheme l.29,30... 

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. ... 

Other transmetallation reactions of silver halides or their complexes with phosphine ligands (e.g. [AgIP(Bu-w)3]) with alkylhthium and alkylmagnesium reagents confirm the fleeting stability of the alkylsilver compounds. In each case the composition of the AgR intermediates has been derived from the analysis of the products formed in their thermal decomposition. So far, representatives of the class of simple alkylsilver complexes have not been isolated in pure form. 

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). ... 

This restricts possibilities of obtaining complexes of the corresponding ligand with metals unaWe to act as templates. One solution is transmetallation, namely, the treatment of a kinetically labile complex with a metal ion to form inert complexes. Thus it is possible to synthesise complexes of nickel(II), copper(II), iron(II), cobalt(II) and cobalt(in) with the unstable ligand LI 6 from the corresponding [Pb(L16)]X2 or [Ag(L16)]X, themselves assembled with the use of lead(II) or silver(I) as templates [46] (Eq. 1.8). 

Another methodology is based on silver transmetallation [35]. In the first step, a silver-carbene complex is formed from the imidazolium salt and silver(I) oxide under reflux in dichloromethane. Subsequently, this complex is treated with a rhodium salt at room temperature to form the rhodium- carbene complex in high yields. In a few cases, also enetetramines have been used as precursors, delivering electron-rich carbenes by cleavage of the central C=C bond [36]. 

The coupling of anilines with arylboronic acids has been described in a process catalysed by palladium acetate without the need for ligands, bases, or salts. The mechanism shown in Scheme 17, where S represents solvent, involves in situ formation of the dia-zonium salt from the aniline, and formation of an arylpalladium alkoxo complex which allows the transmetallation step with arylboronic acids. It has been shown that a free amine may be used as a DG in a palladium-catalysed reaction promoted by soluble silver salts. The latter aid the formation of intermediates, (136), which may undergo cyclopalladation followed by transmetallation with an arylboronic acid and reductive elimination. Related work has shown thatbiaryl-2-amines may react with aryl iodides in the presence of palladium acetate and silver acetate to give mono- or di-arylated species such as (137). 

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