Imidazolium-based catalysts

Among these in situ protocols are those using ionic liquids as the solvent, or as both the solvent and the ligand. It was shown that the use of PdCOAc) in imidazolium-based ionic liquids forms in situ NHC-Pd(II) species [42], The use of methylene-bridged bis-imidazolium salt ionic liquids to form chelated complexes has also been reported [43], although better results have been obtained when Bu NBr is used as the solvent [44] and imidazolium salts were added together with PdCl in catalytic amounts [45]. Other related catalytic species such as bis-NHC complexes of silica-hybrid materials have been tested as recyclable catalysts [46,47]. 

Iridium nanoparticles prepared in imidazolium-based ILs have been also used in the catalytic hydrogenation of ketones under mild conditions [50]. Firstly, cyclohexanone was chosen as the model substrate to optimize the reaction conditions (temperature, hydrogen pressure, catalyst concentration). Initially, isolated lr(0) nanoparticles were tested in a solventless system for the hydrogenation of cyclohexanone the prehminarily results are listed in Table 15.6. 

This level of ionization is particularly relevant in some enzymic reactions where histidine residues play an important role (see Section 13.4.1). This means that the imidazole ring of a histidine residue can act as a base, assisting in the removal of protons, or, alternatively, that the imidazolium cation can act as an acid, donating protons as required. The terminology used for such donors and acceptors of protons is general acid catalyst and general base catalyst respectively. 

The salts were investigated in the Diels-Alder reaction of crotonaldehyde with cyclopentadiene (Scheme 67). The yields obtained were between 35% and 40% with an endo. exo ratio of 90 10. The control reaction without the salt at -25 °C gave no product. The observed ee with the enantiopure salt 66 was less than 5%. Nevertheless, this was the first example which showed, that imidazolium-based ionic liquids can be used in substoichiometric amounts as Lewis acid catalysts. 

The imidazolium-based ionic liquid [bmim][BF ] has been used as a catalyst in the aza-Michael reaction of various aliphatic amines to unsaturated compounds with different electron withdrawing groups in good yields as shown in Scheme 76. Water was used as the solvent in order to obtain up to 98% yield in 7 h. In the presented example, 95% yield in 7 h was achieved [198], The ionic liquid could be recovered and reused five times without loss of activity. 

The Robinson annulation of ethyl acetoacetate and tra i -chalcone was investigated with pulverized NaOH in [BMIMjPFg as the base catalyst at 100°C 110). The mixture was neutralized before extraction with toluene. The product, 6-ethoxycarbonyl-3,5-diphenyl-2-cyclohexenone, was obtained by purification in a silica gel chromatography column. A yield of 48% was obtained (Scheme 7). The ionic liquid could be recycled and reused with no diminution of product yield. The C2 position in imidazolium cations is an acidic proton donor and may have reacted... 

Alleti, R., W.S. Oh, M. Perambuduru, Z. Afrasiabi, E. Sinn and V.P. Reddy, Gadolinium Triflate Immobilized in Imidazolium Based Ionic Liquids A Recyclable Catalyst and Green Solvent for Acetylation of Alcohols and Amines, Green Chemistry, 7, 203-206 (2005). 

Functionalised ionic liquids based on cations other than imidazolium have also been developed. For example, pyridinium cations functionalised with pentafluorosulfanyl[89] or alkyl-nitrile groups1901 have been prepared as cheaper alternatives to their imidazolium-based counterparts (see Figure 2.8). The latter have been evaluated in palladium catalysed C-C cross coupling reactions and improved catalyst retention and stability were observed in the nitrile-functionalised ionic liquid compared to the simple alkyl-analogue. Consequently, the nitrile-functionalised ionic liquid solution can be reused repeatedly without significant decrease in activity (see Chapter 6 for further information). 

Imidazolium-based ionic liquids, mostly the [BF4] and [PF6] salts, are commonly employed for hydrogenation reactions and it has been observed, that anions like triflate and acetate, which are reasonably good ligands, can deactivate the catalyst.[15]... 

Apart from the omnipresent imidazolium based ionic liquids, other classes of low melting salts have been successfully applied in carbon-carbon coupling reactions, notably tetraalkyl ammonium and phosphonium salts. The former include pyrrolidinium and piperidinium salts, but particularly tetrabutylammonium bromide, [(C4)4N]Br (mp. 103-105°C), which has been evaluated quite extensively in Heck reactions and a remarkable increase in reactions rates is frequently observed with this solvent. Those examples where [(C4)4N]Br acts merely as co-catalyst[2"61 rather than as reaction medium shall not be discussed here. 

The same reaction was also investigated using the palladium-carbene complex, 58, as catalyst (Scheme 9.12).1401 Of the different ionic liquids studied, tetrabutylammonium bromide gave by far the best results while imidazolium based solvents afforded only poor conversions. At atmospheric pressure, only iodobenzene was carbonylated. The conversion of less reactive arylhalides not only required higher CO-pressures, but also the addition of a phosphine ligand. Reuse of the catalyst after extraction of the product with diethyl ether was possible for at least 6 runs with only a moderate decrease in activity. 

Of course, the opposite has also been observed whereby an imidazolium salt oxidatively adds to a transition metal complex. [135,136]. This was first noticed in catalytic reactions involving transition metal phosphane complexes as catalysts and imidazolium based ionic liquids. Unexpected improvements in catalytic performance prompted investigations to find out whether the phosphane ligands had been replaced by more electron-rich NHC... 

Further investigations on reductive elimination processes showed that this reductive elimination could be the side reaction leading to degradation of active species in C-C cross-coupling reactions. As illustrated in Scheme 31, palladium-based catalyst (93) underwent oxidative addition in the presence of iodobenzene, providing the reaction intermediate (183), which could be involved in the catalytic cycle but also affords the imidazolium salt (184) by direct reductive elimination. Since then, a few other examples of... 

The palladium-catalysed arylation of olefins with aryl halides, the Heck reaction, is usually performed in polar solvents such as acetonitrile or dimethyl sulfoxide, in combination with a base and a Pd(ii) pre-catalyst that may or may not be associated with a phosphorus ligand. Given that quaternary ammonium or phosphonium salts are known to increase reaction rates, ILs emerged as promising solvents for this reaction.In the case of imidazolium-based ionic liquids, the solvent, beside providing an unusual coulombic environment. 

In particular, highly stable Pd nanoparticles, protected by an imidazolium based ionic polymer in a functionalized IL, can be easily prepared. These Pd nanoparticles are excellent pre-catalysts for Suzuki, Heck and Stille coupling reactions and can be stored without undergoing degradation for at least two years. 

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],... 

Fig. 4 Activation and tuning of the nickel catalyst shown in Eq. (3) for hydrovinylation by combination of different imidazolium-based ILs and compressed C02 [49] cv —conversion, sel. = selectivity.

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