Imidazolylidene

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

With the enthusiasm currently being generated by the (so-called) stable carbenes (imidazolylidenes) [7], it is surprising that there are few reports of imidazolium-based ionic liquids being used to prepare metal imidazolylidene complexes. Xiao et al. have prepared bis(imida2olylidene)palladium(II) dibromide in [BMIMJBr [8]. All four possible conformers are formed, as shown in Scheme 6.1-3. 

Despite the planar conformation of nitrogens, several possibilities are available for the introduction of chirality. It is possible to prepare benzim-idazolylidenes, triazolylidenes, imidazolylidenes or unsubstituted-backbone imidazolinylidenes with a stereogenic center on one or two N-substituents (carbenes I and II). The other possibility is to relay the imidazolinylidenes backbone stereogenecity via the N-substituents or to combine stereogenic N-substituents with a chiral backbone (carbenes III or IV). It is at least possible to prepare bis-carbenes of type V with one (or two) stereogenic link between the two carbenes (Fig. 7). 

Since then, it has been observed that the dimerization of imidazolylidenes and triazolinylidenes does not occur at room temperature. In contrast, the dimerization of imidazolinylidenes is a fast and irreversible process but is prevented by using bulky nitrogen substituents. The benzimidazolylidenes are in equilibrium with the dimer, this equilibrium being affected by steric... 

In 2004, Bolm et al. reported the use of chiral iridium complexes with chelating phosphinyl-imidazolylidene ligands in asymmetric hydrogenation of functionalized and simple alkenes with up to 89% ee [17]. These complexes were synthesized from the planar chiral [2.2]paracyclophane-based imida-zolium salts 74a-c with an imidazolylidenyl and a diphenylphosphino substituent in pseudo ortho positions of the [2.2]paracyclophane (Scheme 48). Treatment of 74a-c with t-BuOLi or t-BuOK in THF and subsequent reaction of the in situ formed carbenes with [Ir(cod)Cl]2 followed by anion exchange with NaBARF afforded complexes (Rp)-75a-c in 54-91% yield. The chela-... 

At the origin of this abundance of publications on iV-heterocyclic carbenes is the structural ligand diversity now available (Fig. 1.2). This developing area is noteworthy as most early developments were mainly focused on imidazolylidene and imidazo-lidinylidene NHC-types. 

The main synthetic routes leading to the formation of NHC complexes are depicted in Scheme 1.2. The methodologies given are shown with imidazolidinylidene and imidazolylidene ligands, however, they are applicable to other NHCs [16, 17]. 

Scheme 1.3 Predominant routes to imidazolylidene and imidazolidinylidene precursors...

A one-electron oxidation of the Ru(II) imidazolylidene complex 42, generating the paramagnetic carbene complex 43, has been noted (75) ... 

The activity of catalytic systems based on imidazolylidene carbenes depends on many factors, among which the most important are likely to be the electronic effects of the ligand and the parameters of complexation. Therefore, the dependence of the performance of such systems on, e.g., the choice of precatalyst is not well understood, as in the following example (Equation (34)) in which two similar ligands behave in exactly the opposite way in the systems based on the presynthesized complex or in situ generation of the catalyst 454... 

The most selective catalyst in this series, complex 32 c, with very bulky substituents at the oxazoline and the imidazolylidene moiety, was used to hydrogenate a range of trisubstituted alkenes with enantiomeric excesses of greater than... 

Scheme 29.10 Carbenoid imidazolylidene ligands based on the JM-PHOS ligand (32).

Dimethyl (l,3-dimethyl-2-imidazolylidene)malonate (471, R = R1 = Me) was prepared in 24% yield in the reaction of 1,3-dimethyl-2-methylthioimidazolidinium iodide and dimethyl sodiomalonate in dioxane (70ACS3102). 

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-... 

This review will focus on the use of chiral nucleophilic A-heterocyclic carbenes, commonly termed NHCs, as catalysts in organic transformations. Although other examples are known, by far the most common NHCs are thiazolylidene, imida-zolinylidene, imidazolylidene and triazolylidene, I-IV. Rather than simply presenting a laundry list of results, the focus of the current review will be to summarize and place in context the key advances made, with particular attention paid to recent and conceptual breakthroughs. These aspects, by definition, will include a heavy emphasis on mechanism. In a number of instances, the asymmetric version of the reaction has yet to be reported in those cases, we include the state-of-the-art in order to further illustrate the broad utility and reactivity of nucleophilic carbenes. 

Nolan and co-workers have extended the scope of transesterification reactions to include phosphonate esters as phosphorylating agents [137]. In this publication the authors use dimethyl methylphosphonate 273 and benzyl alcohol with a variety of imidazolylidene carbenes (Table 23). The nse of molecnlar sieves to absorb methanol leads to increased conversion however, longer reaction times lead to decreased... 

Trifluoromethylation can be achieved with the use of imidazolylidene carbene 1 [159], Song and co-workers found this transformation is tolerant of both electron-rich and electron-poor aldehydes (Table 26). Even enolizable aldehydes undergo trifluoromethylation in 81% yield (entry 3). Selective reaction occurs with an aldehyde in the presence of a ketone in the substrate (entry 5). The use of activated ketones as acceptors leads to tertiary alcohols in good yields (entries 7 and 8). 

Most electrophilic carbenes, such as 2/f-imidazolylidenes and 3H- and 4i/-triazolylidenes, in nitrobenzene gave rise to deoxygenation processes involving the intermediacy of the ylide 42, which decomposed to ni-trosobenzene and 43 (Scheme 12). However, the azolones 43 are too unstable to be detected or trapped in the reaction conditions. 

Another strategy was successfully implemented by synthetic deprotonation of the acidic C2 group of the imidazolium cation by basic ligands of metal complexes, forming carbenes (Scheme 12). When Pd(OAc)2 was heated in the presence of [BMIM]Br, a mixture of palladium imidazolylidene complexes formed. The palladium carbene complexes have been shown to be active and stable catalysts for the... 

Although such catalyst systems are known to have rather high productivities for the reaction, the addition of several equivalents of 1,3-dialkylimidazolium salts per equivalent of palladium leads to complete deactivation of the catalyst, which was attributed to the formation of highly stable palladium imidazolylidene complexes (Scheme 18). 

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