Imidazolium cations functionalized

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. 

Interestingly, in some cases the IL itself can act as the reductive agent. Spherical metal silver NPs were prepared in a hydroxyl-functionalized IL (2) (entry 30, Table 1.1) [17]. In this case, the hydroxyl moiety of the IL plays a reductive role, being oxidized to the corresponding aldehyde. In a similar manner, for Au(III) precursors, the imidazolium cation itself can act as a reducing agent to yield prismatic particles in BMI.PF6 with a very broad size range of diameter 3-20 pm and thickness... 

First attempts to isolate monocarbene-hydrido complexes by oxidative addition of A -(2-pyridyl)imidazolium cations to Pd° with utilization of the chelate effect of the donor-functionalized carbene ligand failed and only the dicarbene complexes such as 29 were isolated [112]. The iridium hydrido complex 30 was obtained in the oxidative addition of an W-(2-pyridylmethyl)imidazolium cation to iridium(I) (Fig. 11) [113]. This reaction proceeds most likely via the initial coordination of the nitrogen donor which brings the imidazolium C2-H bond in close proximity to the metal center. No reaction was observed with Rh under these conditions. 

In accord with calculations performed by Cavell et al. [110], the oxidative addition of C2-X functionalized azolium cations (X = halogen) to metal centers proceeds faster and with a more favorable reaction enthalpy than the oxidative addition of the C2-H substimted imidazolium cations [118, 119]. The former reaction was applied successfully for the preparation of nickel and palladium complexes bearing a variety of different ylidene ligands [119]. 

Alternatively, complexes 54 and 55 with abnormally bound NHC ligands have been obtained by oxidative addition of a C-H bond to Pd° and Pt° while blocking the C2 position of the azolium salt [155] or even without any blocking of the C2 position by oxidative addition of a C5-I bond of a donor-functionalized imidazolium cation (Fig. 19) [156]. 

In an extension of the work to chiral chemistry 244), the imidazolium cation of the ionic liquid was modified to carry a chiral substituent. The high cost of the chiral cation could be justified as the chiral ionic liquid, [MBMIM ] , can be reused. In the ATRP of soluble methyl acrylate, a small effect of the chiral ionic liquid on the polymer tacticity was observed. The use of a chiral ionic liquid as a solvent could lead to applications in other areas of catalysis. The synthesis of imidazolium-containing ionic liquids functionalized with chiral natural amino acids has already been reported 245), as have less expensive chiral ionic liquids(2- (5). 

Recenf work by Davis and coworkers [29] has shown fhaf sulfone and sulfoxide functionalized imidazolium cations are able to enhance particular... 

New 2-hydroxypropyl-functionalized imidazolium cation ionic liquids (containing an appended hydroxyl functionality) have been made [116] by use of an atom efficient one-pot reaction befween 1-mefhylimidazole and acid with propylene oxide. Unfortunately the systems were not studied... 

I have been pursuing enzyme mimics, artificial enzymes that perform biomimetic chemistry, since starting my independent career in 1956. In the first work [52-59] my co-workers and I studied models for the function of thiamine pyrophosphate 1 as a coenzyme in enzymes such as carboxylase. We discovered the mechanism by which it acts, by forming an anion 2 that we also described as a stabilized carbene, one of its resonance forms. We examined the related anions from imidazolium cations and oxazolium cations, which produce anions 3 and 4 that can also be described as nucleophilic carbenes. We were able to explain the structure-activity relationships in this series, and the reasons why the thiazolium ring is best suited to act as a biological... 

The stability of an imidazole salt is a function of the symmetrical imidazolium cation (53) which is resonance stabilized. 

Bazureau et al. reported the synthesis of poly(ethyleneglycol) functionalized -ionic liquid phases as functionalized ionic liquids which are promising tools for synthetic applications in ionic-phase combinatorial chemistry [97], Moreover, several research groups have described ionic liquids with imidazolium cations carrying amino functionality. Davis et al. [98] reported for the first time a functionalized ionic liquid with NH2 group, n-propylamine-3-butylimidazohum tetrafluoroborate 18, for capturing C02 (Scheme 17). 

An example of this approach to TSIL formulation is the synthesis from 1-vinyl imidazole of a series of imidazolium cations with appended tertiary phosphine groups (Scheme 2.3-3). The resulting phosphines are then coordinated to a Rh (I) organometallic and dissolved in the conventional ionic liquid [BMIM PFs], the mixture constituting a TSIL. The resulting system is active for the hydroformylation of 1-octene, with no observable catalyst leaching [17]. More examples of phosphine functionalized ionic liquids that have been reported later to improve the immobilization of transition metals in ionic liquids are given in Chapter 5, Section 5.4. 

Based on the same methodologies as the pioneering examples a plethora of other TSILs with functionalized imidazolium cations have been published in the last... 

Imidazolium cation can be functionalized Possibility of using the ionic liquid as both solvent and ligand Possibility of introducing a chirality on the cation Possible combinatorial ionic-liquid development [29]... 

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