Synthesis of NHC-Metal Complexes

Another very useful alternative is to promote the deprotonation of the imida-zohum salt by the addition of external bases, usually under refluxing conditions [32]. Once the carbene is generated, it reacts with the metal complex to produce the corresponding metal-carbene complex. In general, polar aprotic solvents, for example, THF, are used, and the reaction is run at room temperature. Carbenes can also displace a phosphine ligand in the Wilkinson complex [33]. Other phosphine-rhodium complexes can be likewise employed [34]. 

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]. 

Although the most important venues to imidazohum salts and their corresponding free carbene were already discussed in the previous section, the preparation 

The pure -isomers could be isolated by crystallization. In order to obtain the Rh complex, the carbene was reacted with [RhCl(COD)]2 and AgPFg. 

5-Dlsubstltuted NHCs and Their Metal Complexes Substituents at positions 4 and 5 of the imidazolium ring by fusion with other heterocyclic rings or by simple alkyl groups modify the electronic and steric properties and may therefore exert a strong influence on the reactivity of the carbene catalyst. 

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Widely applicable is the last pathway shown in Scheme 4, the transmet-allation reaction . Although other reagents have been proposed [68], the use of Ag20 as introduced by Lin [69,70] is currently the most important pathway for the synthesis of NHC metal complexes. The imidazofiiun halide forms a silverhafide-NHC complex which then transfers the carbene ligand to the metal precursor. Scheme 4 shows a general representation of possible pathways for the synthesis of transition metal NHC complexes. 

Scheme 4 Synthesis of NHC-metal complexes from carboxylate-NHC adduct...

Another alternative to generating and handling free carbenes is to mask the C2-position of the imidazol-2-ylidene with various thermally labile groups, such that heating these adducts in the presence of transition metal precursors gives the desired NHC-metal complexes. A number of researchers have deployed these species as latent NHCs for the synthesis of NHC-metal complexes. 

Figure 6.48 Synthesis of transition metal complexes with carbohydrate functionalised NHC ligands.

Since the synthesis of NHC TM complexes was initially achieved by in situ formation of NHC from azolium salt in the presence of base followed by coordination to a metal center, a simpler procedure would be useful. In 1996, Arduengo reported the synthesis of Hg(IDM)2 (128) from Hg(OAc)2 and IDM HCl salt. The mechanism of this reaction is depicted in Scheme 21. Dissociation of one acetate ligand generates [Hg(OAc)]+ (126) and the mildly basic acetate anion drives the deprotonation of the imidazohum salt. Thus, the newly formed NHC coordinated promptly (126) and led to [(IDM)Hg(OAc)]+ (127). 

The transmetallation process was found competent for several metals - for instance, Crabtree and coworkers reported the transmetallation of [Ag(NHC)2](AgCl2) to Rh(I) and Ir(i) 155 Owing to its mild reaction conditions, this procedure for the synthesis of NHC-containing complexes has found... 

Metal complexes of NHCs are extremely important as catalysts in reactions of importance to the field of organic synthesis. Section 10-1-1 has already addressed the electronic and steric properties of NHC ligands that enhance the catalytic activity of NHC-metal complexes, and section 10-2-3 discussed methods of synthesis. Very little chemistry, however, occurs at Ccarbene. Instead, NHCs act mainly as supporting ligands, so that useful chemistry can take place at other positions on the catalyst complex. We will see applications of the use of NHC-metal complexes for synthesis in Chapters 11 and 12. 

An alternative method for the synthesis of [(NHC)M] complexes is to bind ligands to a transition metal that can subsequently be converted into an NHC ligand. Using 2-azidophenyl isocyanide" " or 2-nitrophenyl isocyanide" as synthons for the unstable 2-amino isocyanide, diprotic benzannulated NHC-metal complexes were obtained, and easily alkylated to give A,A -disubstituted benzimidazolylidenes. This methodology was also applied to ruthenium-based catalysts (Scheme 3.6)." ... 

Weak bases can be used to liberate free carbenes in situ from the imidazolium salt, instead of using a strong base e.g. NaH, KOt-Bu, KHMDS). Despite the fact that the pK of azolium salts is ca. 15-25, this only measures the equilibrium position and not the rate of the deprotonation, which can occur quite quickly. " This is exploited in organocatalysis, where the NHC catalyst is typically added as the corresponding azolium salt, with only a weak base present to deprotonate it. This approach has been used in the synthesis of a number of NHC-metal complexes which do not possess a basic ligand or the ability to oxidatively add the C2-H of the azolium salt. 

The use of NHC complexes of silver is less developed in catalysis however, such species are widely employed as NHC transfer agents in the synthesis of NHC-TM complexes. Peris and Lledos showed that Ag20 readily depro-tonated [IMe-H] and that facile Agl transfer resulted in the formation of a [(IMe)Agl] complex. The presence of a second imidazolium strongly facilitated these processes by H-bonding through the C2-H group. The second imidazolium was first deprotonated by AgOH and then metallated. The overall... 

Even though the oxidative addition pathway of imidazolium salts has been shown to be possible at certain metal complexes under special circumstances, it is far from being generally applicable to the synthesis of NHC complexes [60-67]. 

Figure 4.49 Synthesis of transition metal NHC complexes with Fc functiomJised carbene ligands.

Figure 4.60 Synthesis of Cp-metallated annulated Cp-NHC ligands and their mercury(ll) carbene complexes.

Because free NHCs bind so strongly to a metal, it is not necessary to use an excess amount of these ligands to make NHC-metal complexes. This contrasts with the synthesis of metal phosphine complexes, where often an excess of the ligand is required for success. There are numerous examples of simple combination of the free carbene and a suitable metal complex. Equation 10.25 illustrates one example.43... 

Scheme 31.13 Synthesis of crosslinked polynorbornene-supported NHC-metal complexes through C02-masked NHC monomers reversible reaction of NHCs with CO2 (inset).

The synthesis of polyethylene has also been explored using NHC-metal complexes as polymerization catalysts. Unfortunately, oligomeric or low-molecular-weight polyethylene is often encountered when a variety of... 

A detailed and extensive review of the synthesis and reactivities of NHC - borane complexes has been published. Modification of the boron substituents of NHC - boranes, as well as their involvement in radical reactions (H-dot donors), ionic reductions, transition-metal-assisted group transfer, their catalytic applications, and their characterizations are thoroughly developed. 

Silver carbene complexes act as very efficient NHC transfer reagents for the synthesis of different metal-NHC complexes. On the other hand, as far as we know, there is only one example described to date where a silver-NHC complex was used for asymmetric catalysis. In 2006, Ferndndez and coworkers reported the first and only asymmetric catalysis using a chiral NHC-silver catalyst. Enantioselective diboration of styrenes was realized by using silver complex 111 as the catalyst (Scheme 3.70) [99], However, the diol was obtained in low yield and low enantioselectivity (less than 10% ee). 

In addition to their work with hthium-NHC complexes mentioned earlier, the Arnold group [36] has reported the synthesis of related potassium alkoxide NHCs. These complexes were prepared through the reaction of an alcohol-fimc-tionalized imidazolium iodide salt with excess potassium hydride in THF. Again, however, these are examples of alkali metal complexes in which the NHC fragments are part of larger anionic species. 

Much simpler than the template-controlled generation of /3-functionalized isocyanides is their direct use in the synthesis of NHC complexes. Hydroxyalkylisocyanides such as 2-hydroxyethyl isocyanide are stable molecules but they are also known to become activated on contact with selected metal ions where they isomerizes to Al,0-heterocycles. FehUiammer et al. [40]... 

A further approach for making available a vacant coordination site for olefins at the metal was the synthesis of various bimetallic complexes with a weakly bond metal fragment (30a-c) by Grubbs [188]. These complexes displayed high activity in ROM polymerizations [188]. Herrmann et al. reported a series of analogous NHC complexes (30d-f) [193-195] with improved activity [194,220] and stability [220]. A comparative report on the stability of various metathesis initiators in RCM is given in Ref. [220]. 

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