Imidazolylidenes, generation

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. 

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... 

Scheme6.1 Generation of active bis-imidazolylidene complexes and catalyst deactivation via formation of a tetrakiscarbene complex...

The main advantage of these coordinating anions is that they stabilize the active species. This is particularly obvious in the case of palladium complexes, whose tendency to decompose into black metal is well documented. Imidazolium-based ionic liquids can generate in situ formation of metal-imidazolylidene carbene complexes by a deprotonation of the imidazolium cation. The ease of deprotonation depends on the nucleophilicity of the anions. In this case, NAILs may act as both solvents and catalyst ligand precursors [13],... 

Heterocyclic imidazolylidene carbenes proved to be very interesting ligands of transition metal complexes. Their electronic properties have very often been compared to those of basic phosphines. They are indeed good a-donors but weak n-acceptors. A number of carbene complexes have been involved in catalytic reactions such as metathesis (Ru) or C-C coupling (Pd, Ni). They can be generated easily by deprotonation of an imidazoUum salt in the presence of an organic base. 

Having synthesized enol ester 51b and 51c, examination of the NHC-catalyzed rearrangement was undertaken. Previously, NHCs such as diaryl imidazolylidene 13 had proven optimal for the preparation of simple pyra-nones. Unfortunately, subjection of enol ester 51b to carbene 13, generated in situ from precatalyst 13 HC1, failed to bring about any reaction, even when the reaction was heated to reflux (Table 2, entry 1). Similar results were achieved when the reaction was conducted in tetrahydrofuran (THF), with only trace amounts of the desired pyranone observed by NMR analysis of the crude reaction mixture (Table 2, entry 2). Thus, the challenges of total synthesis had found our methodology wanting. 

When the free carbenes are stable enough, as is the case with most unsaturated imidazolylidene derivatives, they can be isolated and engaged in substitution reactions. The first such preparation using this method was reported in 1999 by Nolan [16], who prepared RuCl2(indenylidene)(imidazolylidene)(PR3) complexes 7—10 from Af,A/ -bis((mesityl)imidazol-2-ylidene) (IMes) and A5A/ -bis((2,6-diisopropylphenyl)imidazolyl-2-ylidene) (IPr) carbenes in toluene at room temperature (Scheme 14.5). Other NHCs substituted on the NHC backbone [48, 49] and various Af-aryl groups [50] have also been prepared and coordinated onto ruthenium indenylidene moieties using the free or in sif -generated carbenes with complex 6. 

A substantial improvement on the above-described class of Ru-vinyUdene complexes has been achieved by Louie and Grubbs through synthesis of a novel generation of complexes coordinating an A-heterocyclic carbene (NHC) ligand (e.g. imidazolylidene). This new set of Ru complexes, e.g. 15 (IMes = l,3-(2,4,6-trimethylphenyl)imidazoT2-ylidene, R = Cy, R = t-Bu) and 16 (iPrlM = l,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene, R = Cy, R = Ph) has been conveniently produced from the bisphosphane-Ru complex 9 (R = Cy) by reaction with free imidazoline carbene or its precursor salts (Scheme 7). 

The generation of the imidazolylidene from the ionic liquid l-ethyl-3-methyl-imidazolium acetate has been theoretically studied in the liquid and the gas phases and in the presence and in the absence of C02. In the liquid, CO2 appears to facilitate the carbene formation followed by the formation of the carboxylic adduct of the carbene. 

As a versatile alternative to the generation of free carbenes, Hashmi and co-workers developed a methodology for the synthesis of abnormal NHCs where the carbene is constructed directly at the metal center. Thus, a [3 + 2] dipolar cycloaddition of azomethine ylides and gold isonitriles afforded the abnormal saturated imidazolinylidene gold eomplexes 9 [eqn (3.3)]. This methodology circumvents the regioseleetivity issue of C4 vs. C5 eoordination because the carbenic carbon is pre-coordinated to the metal center in the isonitrile precursor. Moreover, this procedure provided aeeess to a new subclass of abnormal carbenes and yielded the first abnormal earbene complexes featuring a saturated heteroeyele, unlike all previous abnormal imidazolylidene complexes, derived from unsaturated heteroeyeles. 

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