Imidazolium cation, deprotonation carbene formation

Scheme 5.2-3 Formation of a Pd-carbene complex by deprotonation of the imidazolium cation.

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. 

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],... 

With respect to the ionic hquid s cation the situation is quite different, since catalytic reactions with anionic transition metal complexes are not yet very common in ionic liquids. However, the 1,3-dialkyhmidazolium cation can act as a hgand precursor for the dissolved transition metal. Its transformation under the reaction conditions into a ligand has been observed in three different ways (i) formation of metal carbene complexes by oxidative addition of the imidazolium cation (ii) formation of metal-carbene complexes by deprotonation followed by coordination of the imidazolylidene on the metal center (iii) dealkylation of the imidazolium cation and formation of a metal imidazole complex. These different ways are displayed in a general form in Scheme 5.3-2. 

However, the formation of the metal-carbene complex was not observed in pure, halide-free [BMIM][Bp4], indicating that the formation of carbene depends on the nucleophilicity of the ionic liquid s anion. To avoid the formation of metal-carbene complexes by deprotonation of the imidazolium cation under basic conditions the use of 2-methyl-substituted imidazolium is frequently suggested. However, it should be mentioned here that strong bases can also abstract a proton to form the vinyl imidazolidene species which may also act as a strong ligand to electrophilic metal centers. 

If ILs are to be used in metal-catalyzed reactions, imidazoHum-based salts may be critical due to the possible formation and involvement of heterocyclic imidazo-lylidene carbenes [Eqs. (2)-(4)]. The direct formation of carbene-metal complexes from imidazolium ILs has already been demonstrated for palladium-catalyzed C-C reactions [40, 41]. Different pathways for the formation of metal carbenes from imidazolium salts are possible either by direct oxidative addition of imidazolium to the metal center in a low oxidative state [Eq. (2)] or by deprotonation of the imidazolium cation in presence of a base [Eq. (3)]. It is worth mentioning here that deprotonation can also occur on the 4-position of the imidazolium [Eq. (4)]. The in-situ formation of a metal carbene can have a beneficial effect on catalytic performances in stabilizing the metal-catalyst complex (it can avoid formation of palladium black, for example). However, given the remarkable stability of this imidazolylidene-metal bond with respect to dissociation, the formation of such a complex may also lead to deactivation of the catalyst This is probably what happens in the telomerization of butadiene with methanol catalyzed by palladium-phosphine complexes in [BMIMj-based ILs [42]. The substitution of the acidic hydrogen in the 2-position of the imidazolium by a methyl group or the use of pyridinium-based salts makes it possible to overcome this problem. Phosphonium-based ILs can also bring advantages in this case. 

Efforts have been made to explain the high rate acceleration of Mizoroki-Heck reactions in ionic liquids. The formation of the dialkylimidazol-2-ylidene palladium complex under conditions similar to those employed for the Mizoroki-Heck reaction has been studied. The C2-H proton of the imidazolium cation exhibits high acidity and can be deprotonated to form a carbene species, behaving as a good ligand for transition metals. Therefore, in the presence of a palladium salt and a base, [bmim][Br] formed the dimeric carbene complex 89, which further evolved to the monomeric c/x-90 and trans-9Q complexes. Each of these exists as an anti and a syn rotamer owing to the sterically demanding (V-alkyl substituents (Scheme 35 only the anti-90 rotamers are represented). 

The reactivity of the imidazolium cation mainly stems from the relatively higher acidity (piTa = 21-23) of the H2 hydrogen of the imidazolium nueleus, which has been found to be roughly intermediate between the acidities of acetone (pA a = 19.3) and ethyl aeetate (p a = 25.6). In faet, it is well known that deprotonation at the C2 position of the imidazolium salt generates 7V-heterocyclic carbene ligands.Not surprisingly, the formation of metal-carbene eomplexes has been observed in Pd-eatalyzed Heck-type reactions performed in ILs (Seheme 10). In these cases, the side-reaetion has a benefieial effect since the carbenes are most probably stabilizing the catalytically active species. 

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