Carbene transfer reactions from silver complexes

The carbene transfer reaction from silver NHC complexes has developed into a standard procedure for the synthesis of NHC complexes. This versatile procedure was introduced by Lin et al. in 1998 [102]. It is based on the preparation of silver NHC complexes which are obtained in good yield by the in situ deprotonation of azolium salts with silver oxide (Fig. 9). Depending on the counter ions present in the azolium salt and the steric demand of the N,N -substituents, complexes 25a-25c... 

The two carbene units can be embedded in a (macro)cyclic ring system known as a cyclophane. A standard procedure for the synthesis of such a system starts with a,a -dibromoxylene and potassium imidazolide [368]. Cychsation can be achieved by reacting the bis-imidazole compound with a second equivalent of a,a -dibromoxylene (see Figure 3.116). The cyclic bis-imidazolium cyclophane can then be reacted with paUadium(II) acetate to form the palladium complex [369,370]. The silver(I) and gold(I) complexes are accessible from the reaction with silver(I) oxide [371] and the usual carbene transfer reaction to gold(I) [372]. 

In spite of the fact that silver(i) X-heterocyclic carbene complexes were widely employed as carbene-transfer reagents for the synthesis of other transition metal carbene complexes, their synthesis could also be achieved by the reaction of silver salts with relatively more labile carbene metal complexes, albeit rare. Complexes 71a-71c were reported to be synthesized from the reaction of the corresponding pentacarbonyl(carbene)chromium(i) complexes with silver(i) hexafluorophosphate in CDC13 under inert atmosphere (Scheme 17).117... 

Even a rod-like backbone derived from acetylene is conceivable and has been realised by Liu et al. [367] starting from l,4-dichlorobutin-2. The usual reaction with methyl imidazole and subsequent reaction with silver(l) oxide and carbene transfer to gold(I) [Au(SEtj)Cl] makes the dimeric silver(I) and gold(l) complexes accessible featuring argentophilic and aurophilic interactions, respectively. 

A very interesting amido functionahsed carbene was prepared by Legault et al. [116] from A-mesitylimidazole and 0-(2,4-dinitrophenyl)hydroxylamine, an electrophilic ami-nation reagent [117]. The exo-amino group is subsequently acylated to afford a zwitterionic amido functionalised carbene (see Figure 4.38). Reaction with silver(l) acetate and sodium carbonate [a rare variant of the silver(I) oxide method] yields the silver(l) carbene complex as a dimer with a Ag-Ag bond. The silver(l) carbene complex can be used as a carbene transfer reagent to synthesise the homoleptic monomeric copper(Il) carbene complex. 

Switching from palladium to rhodium, we encounter some very interesting chemistry. Zeng et al. [302] reacted the tiidentate PCP phosphino functionalised imidazolium salt with silver(I) oxide and subsequently transferred the carbene to rhodium(I) (see Figure 3.100). Careful selection of the rhodium precursor complex and reaction conditions enables tetrahedral, square bipyramidal and octahedral rhodium(I) and rhodium(III) complexes to be formed. As the authors explained, the activation of the C-Cl bond in methylene chloride in an oxidative addition reaction on rhodium(I) resulting in a rhodium(in) complex requires an electron rich rhodium(I) complex. The presence of a NHC ligand is advantageous in this respect. 

N-Heterocyclic carbene complexes can be generated from the stable, free carbenes or by reaction of carbene precursors. Three examples of the s)mthesis of N-heterocyclic carbene complexes from carbene precursors are shown in Equations 13.14-13.16. The reaction of an imidazolium salt with a complex containing a basic ligand forms N-heterocydic carbene complexes (Equation 13.14), as does transfer of the carbene from a silver-carbene complex (Equation 13.15), - or reaction of an imidazolium-2-carboxylate, with accompanying decarboxylation (Equation 13.16). ... 

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