Imidazolium moieties

With respect to the ionic liquid s cation the situation is quite different, since catalytic reactions with anionic transition metal complexes are not yet very common in ionic liquids. However, an imidazolium moiety as an ionic liquid cation can act as a ligand precursor for the dissolved transition metal. Its transformation into a lig-... 

Scheme 18). The formation can be explained by the initial conjugate umpolung of the aldehyde and subsequent 1,4-addition to the unsaturated ketone. After proton transfer, an intramolecular aldol-type addition results in the formation of the aforementioned zwitterions. Nucleophilic displacement of the imidazolium moiety by the alkoxide provides the p-laclonc, which exhibits increased strain, since it is annulated to a cyclopentane ring. Consequently, the P-lactone breaks apart and liberates CO2 and the observed cyclopentene products (Scheme 19). 

Improved catalyst recycling was also achieved in the reduction of acetophenone with formic acid/triethyl amine azeotrope with the r 6-benzene analogue of 14 bearing an imidazolium moiety on the diamine ligand instead of on the coordinated arene (see Figure 3.4).[51l In C4C im [PF6] the stability of the catalyst was superior in repeated runs when a mixture of [RuCl2(n6-benzene)]2 and 15 was employed rather than the neutral complex RuCl(r 6-benzene)(T sDPEN). 

The corresponding reactions with iridium(I) precursors again behave differently. When the phosphino functionalised imidazolium salt is reacted with [IrlcodlCl], the phosphane adduct with a pendant imidazolium moiety is formed. A similar reaction with the more reactive [Ir(cod)( Li-H)(p,-Cl)2]2 yields a five coordinate iridium(I) complex that might be described as having square pyramidal geometry with the bromide in apical position and the carbene in abnormal coordination mode [47-49] (see Figure 3.97). 

Crabtree and Faller developed a similar route using [IrH5(PPh3)2] and the intidazolium salt (153) (equation 23). Whereas the expected product (154) has been isolated, another complex was formed and identified as (155), showing an unusual chelation of the imidazolium moiety. Using benzimidazolium salts, because of the protection of the C4 and C5 positions, only the expected complex was observed. 

In the same publication, the Bliimel group also explored the structure of imidazolium ILs that have been immobilized on silica [11]. By applying II and 13C HRMAS NMR it could be demonstrated that the imidazolium moiety stays largely intact. Likewise, Le Bideau et al. used II MAS NMR to investigate the structural properties of [bmim] BF4 being confined in monolithic silica ionogels [12], Much to surprise, the IL showed only slightly slower dynamics compared with neat liquid [bmim]BF4 the IL s liquid-like properties seemed to be preserved in the confined state. 

The catalytic efficiency of the RTILs derived from l-butyl-3-methylimidazolium (bmim) cation is influenced by the structure of the imidazolium moiety and the counteranion following the order [bmim][MeSO ]>[bmim][HSO ] [bmim]... 

This asymmetric Mannich reaction could also proceed by an enamine pathway because nucleophilic addition of the in situ-generated enamine would be faster to an imine than to an aldehyde. As shown in the Fig. 12.59, the reaction starts with enamine 34 activation of the cyclohexanone by the proline anion and an electrostatic interaction with the imidazolium moiety of the catalyst In a second pre-equilibrium, the aldehyde and aniline produce an imine. Then enamine-activated 35 reacts with the imine to form 35 via transition state A. The last step is a dehydration reaction to afford the corresponding product. The catalyst is regenerated in the subsequent step. 

The stereochemical results can be explained by the plausible transition state A (Fig. 12.57). Because additional water is added and the reaction is conducted in wet solvents, the transition state is stabilized by hydrogen bonding between the nitrogen atom of the imine and the nitrogen atom of the imidazolium moiety of the catalyst. A switch of the facial selectivity is disfavored because of steric repulsion between the Ar group of the imine and the imidazolium moiety of the catalyst. 

Yu P, Zhou H, Mao LQ et al (2009) A facile approach to construction of a stable water-miscible ionic hquid/electrolyte interface with interactions between imidazolium moiety and carbon nanotubes. Electrochem Commun 11 1393-1396... 

Examples in which the cation of the ionic liquid contains the transition metal complex for catalysis have also been published. For example, Forbes and coworkers [63] synthesized a Rh-containing ionic liquid cation by replacing an acetate ligand at the Rh center by a carboxyhc acid functionalized imidazolium moiety. The modified dirhodium(ii) dimer of this kind was applied as an effective catalyst in the intermolecular cydopropanation reaction of styrene using ethyl diazoacetate. 

MW heating of supported thioureas w-substituted by an ester function used as a grafting element to the imidazolium moiety in the presence of diethylamine... 

In order to achieve good solubility in ionic liquid, the incorporation of one or more ionic substituents in the organometallic species is required. One of the most trivial steps is the incorporation of an imidazolium moiety in the ligand of the transition metal complex (Figure 32) or the addition of functionalized ligands. ... 

A very recent contribution to materials based on ricinoleic acid [128] revived two main aspects. The first was a search for optimum conditions related to its homopolymerisation and copolymerisation with butylene succinate. This was attained by using titanium tetrabutoxide at >200 °C to yield products with high molecular weight. Secondly, these materials displayed high biocidal activity, which was enhanced further by appending imidazolium moieties to their macromolecules. 

As an example, chloride has been shown fo be an effective template for the preparation of macrocycles incorporating heterocycles, and where the templating anion is generated in the course of the reaction. In the case of the macrocycle incorporating two imidazolium moieties and one triazole residue (Scheme 2), the yield of the product was shown to depend critically on the precursors used for the reaction. When a bis-imidazole incorporating triazole as a spacer was used with mefa-dichloroxylylene, the yield of the reaction was never >20% even in the presence of additional chloride ions. On the other hand, when a bis-imidazole with a mefa-xylylene spacer was used with bis-chloromethyl-triazole, the yield was 50%. In the presence... 

Recently, Roy S R et al. [116] reported [BMIm]-based ILs in the Biginelli reaction of aldehydes, p-keto ester or p-diketone, and urea or thiourea (Scheme 7.13). It was found that both connter anion and 1,3-disubstituted imidazolium moiety played a key role in determining the catalytic activity of the IL. The best IL was proved to be [BMIm]MeS04, and a very small amount of the IL (1 mol%) was sufficient enough to finish the Biginelli reaction at 100 °C for 30 min, affording dihydropy-rimidinones in good yields. 

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