Free NHCs

Since Arduengo s first isolation of a free NHC (lAd) L substituents been isolated and characterised. Despite the early assumption compounds... 

The carbene formation can be monitored by C H NMR, as the carbene carbon atom of free NHCs has a signal significantly shifted downfield. Typically, the signal for the O atom is found between 200 and 250 ppm for the free carbene and between 130 and 160 ppm for the corresponding salt... 

In situ derived systems, in general, performed similarly to preformed complexes, in telomerisation of butadiene with MeOH, Tables 4.1 and 4.2 [68,70,71,77,78], In situ systans may be generated from free NHC or from imidazolium salt in combination with an appropriate Pd(0) or Pd(ll) source. Typically, 2-4 equivalents of imidazolium salt relative to Pd have been nsed [68,70,77], In situ catalysts derived from mono- and bis-Fc-snbstituted (Fc = ferrocenyl) imidazohnm and benzimidazolium salts (64-68) (Table 4.2) showed interesting telomerisation activities ascribed to the steric bulk of the Fc substituents [70]. Unsymmetrical salts 65 and 66 bearing A -Fc and A -Me... 

Metal complexes of stable carbenes are now known for almost all the metals of the periodic table. This chapter is divided into an overview of the NHC-main-group metal adducts that are generally synthesized by adding the free NHC to an appropriate metal precursor and a discussion of the various synthetic approaches toward transition metal complexes. 

The in situ complexation of the ligand has the advantage of not having to prepare and isolate the free NHC. In cases where the carbene is hardly stable, not yet accessible at all, or difficult to handle, this approach offers the only chance to prepare the desired complex. 

Electron-rich olefins are nucleophilic and therefore subject to thermal cleavage by various electrophilic transition metal complexes. As the formation of tetraaminoethylenes, i.e., enetetramines, is possible by different methods, various precursors to imidazolidin-2-ylidene complexes are readily available. " Dimerization of nonstable NHCs such as imidazolidin-2-ylidenes is one of the routes used to obtain these electron-rich olefins [Eq. (29)]. The existence of an equilibrium between free NHC monomers and the olefinic dimer was proven only recently for benzimidazolin-2-ylidenes. In addition to the previously mentioned methods it is possible to deprotonate imidazolidinium salts with Grignard reagents in order to prepare tetraaminoethylenes. " The isolation of stable imidazolidin-2-ylidenes was achieved by deprotonation of the imidazolidinium salt with potassium hydride in THF. ... 

The survey on published methods for generating NHC metal complexes shows that a broad variety of different approaches exists. In general, the preparation by cleavage of dimeric metal precursors or exchange of other ligands with free NHC is the most convenient and general approach. In most examples the maximum number of NHC ligands on the metal is achieved by this method. Nevertheless, the necessity to prepare the free NHCs is a limitation. 

In cases where the free NHC cannot be synthesized the complex formation has to be accomplished in situ from a ligand precursor, e.g., the imidazolium salt in the case of imidazolin-2-ylidenes. By this method, it is often possible to prepare complexes which do not have the maximum number of NHC ligands attached to the metal center. 

In C-NMR spectra, the signals for the carbene carbon are usually shifted upheld by about 20-30 ppm upon complexation of the free NHC to a transition metal. Cr-NMR data of [LCr(CO)s] complexes underline that NHC are a special case of carbene ligands because of their lack of tt-acceptor ability. Photoreactions of metal complexes containing NHCs by laser flash and continuous photolysis show that NHCs are quite inert ligands in photolysis reactions. He I and He II photoelectron spectra of platinum(O)- and palladium(O) bis(imidazolin-2-ylidene)... 

For nickel(O) complexes prepared from Ni(r -cod)2 and an excess of the free NHC, it was shown that they exhibit outstanding catalytic activity in the Kumada-Corriu reaction at room temperature toward unreactive substrates like aryl chlorides and even aryl fluorides.Again, an essential element of these catalysts is the need for sterically demanding NHC ligands as observed for the palladium catalysts. 

Two general routes are normally employed for the generation of free 77-heterocyclic carbenes 5. These are (1) the deprotonation of azolium salts of types 1 and 2 or (2) the reductive desulfurization of thiones 3 and 4 (Fig. 4) [1]. Most of the free NHCs are obtained by deprotonation of the azolium salts at the C2 position of the heterocycle with a suitable base like NaH, KOf-Bu, or DMSO/NaH in THF [8] or liquid ammonia... 

Metal complexes with M-heterocyclic carbene ligands were known long before the first stable NHCs were isolated. Wanzlick [5] and Ofele [6] demonstrated as early as 1968 that NHC complexes can be obtained by in situ deprotonation of azolium salts in the presence of a suitable metal complex without prior isolation of the free NHC ligand (Fig. 1). In these cases a ligand of the metal complex precursor (acetate or hydride) acted as a base for the deprotonation of the imidazolium cation. This method has been successfully transferred to other metal precursors containing basic ligands like [Pd(OAc)2] [97] and [(cod)lr(p-OR)2lr(cod)] [98, 99]. Alternatively, an external base such as NaOAc, KOf-Bu or MHMDS (M = Li, Na, K) can be added for the deprotonation of the azolium salt [100]. In general, the in situ deprotonation of azolium salts appears as the most attractive method for the preparation of NHC complexes as it does not require the isolation of the reactive free carbene or its enetetramine dimer. 

Most recently, Hedrick and co-workers have illustrated the use of alcohol adducts 298 as a sufficient catalyst/initiators for ROP, therefore eliminating the need for external alcohol [153], These adducts undergo carbene formation at room temperature in THF. Additional advantages of these adducts, compared to free NHCs, is that they are not moisture sensitive and they provide the opportunity to synthesize more complex polymers (Eq. 28a). Star polyesters can be generated in one step (Eq. 28b). 

Use of Carbene Adducts or Protected Forms of Free NHC Carbenes. . 87... 

Abstract The manuscript describes the methods that are most often used in the preparation of N-heterocyclic carbene (NHC) complexes. These methods include (1) insertion of a metal into the C = C bond of bis(imidazolidin-2-ylidene) olefins (2) use of carbene adducts or protected forms of free NHC carbenes (3) use of preformed, isolated free carbenes (4) deprotonation of an azolium salt with a base (5) transmetallation from an Ag-NHC complex prepared from direct reaction of an imidazolium precursor and Ag20 and (6) oxidative addition via activation of the C2 - X (X = Me, halogen, H) of an imidazolium cation. 

Use of carbene adducts or protected forms of free NHC carbenes... 

During the early attempts to synthesize free NHC, Wanzlick and colleagues tried to prepare l,3-diphenylimidazolidin-2-ylidene (2) by thermal elimination of chloroform from 1, but they rather obtained the dimeric electron-rich olefin 3 (Scheme 1) [15-17]. Wanzlick postulated that the carbene 2 could be formed as an intermediate during the formation of 3, and proposed the existence of an equilibrium between 2 and 3. Evidence supporting this equilibrium came later with the works performed by the research groups of Denk, Hahn, Lemal, and Cavell [18-21]. 

Isolation of novel free carbenes is not always trivial, mainly due to difficulties with decomposition and the need to handle free NHCs under inert atmosphere conditions. In this context, the use of protected forms of free NHC carbenes has appeared as a useful alternative to the preparation of NHC complexes. N-Heterocyclic rings containing alkoxide or trichloromethyl groups, such as shown in Scheme 7, can be considered as NHC-adducts in the sense that they can readily eliminate alcohol or chloroform to unmask the carbene, which would then coordinate to the metal. 

The reaction of triazolium and benzimidazolium salts with sodium methoxide yields the corresponding methoxy-triazoles and benzimidazoles [30,31], which can be also used as triazolilydene and benzimidazo-lilydene precursors. Notably, adduct formation does not occur for certain unsaturated imidazolium salts with a C = C backbone. For the latter, reaction with KOBu results in direct deprotonation to the free NHC (Scheme 8, also shows the reaction of a dihydroimidazolium salt with KOBu ) [32],... 

The preparation of the first free stable NHCs by Arduengo [8] broke with the idea that these compounds were too unstable to be used as ligands in the preparation of transition metal complexes. Although in the beginning it was thought that the stability of free NHCs is mainly due to steric effects, it is now assumed that electronic effects play a more important role [37]. Arduengo was the first to propose that both the nitrogen lone pairs and the C = C (in... 

The most widely used method for the preparation of free NHCs is the deprotonation of an azohum salt with NaH or KOBuf [10,14,37]. In the case of N,N -methylene-bridgcd bisimidazolium salts, the preparation of the free dicarbenes is only possible by the use of potassium hexamethyldisilazide (KHMDS) in toluene [14,41]. Other strong bases deprotonate the methylene bridge breaking the bisazol unit [42],... 

---