Catalysts ionic liquid-palladium

Chlorostannate ionic liquids have been used in hydroformylation reactions [23], Acidic [bmimjCl-SnCb and [l-butyl-4-methylpyridinium]Cl-SnCl2 were prepared from mixing the respective [cation]+ Cl with tin(II)chloride in a ratio of 100 104, much in the same way that the chloroaluminates are made (see Chapter 4). Both these chlorostannate ionic liquids melt below 25 °C. Addition of Pd(PPh3)2Cl2 to these chlorostannate ionic liquids leads to a reaction medium that catalyses the hydroformylation of alkenes such as methyl-3-pentenoate as shown in Scheme 8.9. The ionic liquid-palladium catalyst solution is more effective than the corresponding homogeneous dichloromethane-palladium catalyst solution. The product was readily separated from the ionic liquid by distillation under vacuum. This is an important reaction as it provides a clean route to adipic acid. 

A butoxylcarbonylation reaction was conducted in a liquid-liquid biphasic system under process conditions, but the removal of the product was conducted in a liquid-solid biphasic system at a lower temperature (84). lodobenzene or 4-bromoacetophenone reacted with CO at a pressure of 1-8 atm in the presence of a palladium-benzothiazole complex catalyst in the ionic liquid [TBA]Br (m.p. = 110°C) in the presence of Et3N base. The catalyst/ionic liquid system was recycled by extractive removal of the butyl ester product with diethyl ether. The solid residue, containing the catalyst, [TBA]Br, and Et3N.HBr, remained effective in subsequent carbonylation tests. After each cycle, the yields were still close to the initial value. A slight decrease in yield was attributed to a loss of catalyst during handling. 

Of the heterogeneous catalysts employed in conjunction with ionic liquids, palladium on activated charcoal is most frequently used.[60,75,86,88]... 

Scheme 5.6-3 Preparation of grafted supported ionic liquid palladium catalyst [78].

Using a quite different approach, polymeric beads of supported ionic liquid palladium catalysts comprised of polymerized ionic liquid monomers and palladium complexes have been synthesized using traditional suspension polymerization methods [86]. Here, polymeric ionic liquid beads were made from polymerization of l-butyl-3-vinylimidazolium bis(trifluoromethyl sulfonyl)imide and poly(vinylalcohol) by heating with AlBN (2,2 -azobis(2-methylpropionitrile)) in the presence of l,l -bis[l,8-octyl)-3-vinylimidazolium bis(trifluoromethyl sulfonyl)imide as cross-linker (Scheme 5.6-6). The ionic liquid support beads proved to be thermally stable up 250 °C which is significantly higher than conventional vinyl resins. 

Chauvin et al. reported the asymmetric hydrogenation of acetamidocinnamic acid to (S)-phenylalanine with a cationic chiral rhodium catalyst in [C4-mim][SbFJ ionic liquid, more recently the 2-arylacryUc acid has been produced with a reasonable 64% yield using a chiral ruthenium catalysts in [C4-mim][BF4] ionic liquids. Palladium catalysts immobilized in an ionic Uquid-polymer gel membrane containing either [C2-mim][CFjS03] or [C2-mim][BF4] have also been reported as catalysts for heterogeneous hydrogenation reactions. 

Palladium-catalyzed carbonylation of aryl halides with nucleophiles such as alcohols, amines, and water can be performed in ionic liquid media. Several systems have been designed so that the ionic phase can be isolated and recycled. Once carbonylation substrates/products form homogeneous mixtures with ionic liquids, the experimental protocols for catalyst/ionic liquid mixture recycling involve separation of the product by either distillation or extraction procedures using organic solvents or supercritical CO2. 

The authors describe a stabilizing effect of the ionic liquid on the palladium catalyst. In almost all reactions no precipitation of elemental palladium was observed, even at complete conversion of the aromatic halide. The reaction products were isolated by distillation from the nonvolatile ionic liquid. 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

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. 

Although the Heck reaction is synthetically very useful, it requires quite high molar quantities of palladium catalyst to be effective. As such, one of the main goals is to find a solvent that helps to increase the lifetime of the catalyst and consequently reduce the amount of catalyst required. In this respect, ionic liquids show considerable promise. Another key goal in this area is to be able to replace iodo- and bromoarenes, usually used as substrates in these reactions, with chloroarenes, which are more environmentally acceptable. Again, ionic liquids show some promise in this respect. Scheme 10.2 shows the Heck reaction between styrene and chlorobenzene that has been investigated in a number of ionic liquids. 

The Mizoroki-Heck reaction in liquid imidazolium salts as the solvent is a special case of an in situ system Under the reaction conditions NHC complexes of palladium are formed as the active catalyst from the solvent and the ligand-free palladium precursor. In general, ionic liquids are novel reaction media for homogeneous catalysis. They allow easy separation of product and catalyst after the reaction. ... 

The strong acidity of the proton at the C2 position of a [AMIM] ion has been well recognized 183). This cation can react with palladium complexes to form inactive l,3-dialkylimidazol-2-ylidene palladium complexes 200), as confirmed in a study of the conventional Pd(OAc)2/PPh3/base catalyst in ionic liquids for the telomerization of butadiene with methanol at 85°C 201). 

The detrimental effect of the reactive proton-donor group in the [AMIM] ion was further confirmed by the use of an ionic liquid with the proton at the C2 position replaced by a methyl group. The palladium catalyst in [BDMIM]PF6 showed high activity and selectivity in a biphasic system that allowed reaction with multiple recycles and little loss of activity. 

In contrast, ionic liquids have been reported to be suitable solvents for Heck reactions because the products can be readily separated from the ionic liquids containing the homogeneous palladium catalysts. An early test with a palladium complex in ionic liquids showed remarkably improved recyclability of the catalyst (255), but palladium black still formed after several runs with recycled catalyst. 

The choice of an ionic liquid was shown to be critical in experiments with [NBuJBr (TBAB, m.p. 110°C) as a catalyst carrier to isolate a cyclometallated complex homogeneous catalyst, tra .s-di(ri-acetato)-bis[o-(di-o-tolylphosphino) benzyl] dipalladium (II) (Scheme 26), which was used for the Heck reaction of styrene with aryl bromides and electron-deficient aryl chlorides. The [NBu4]Br displayed excellent stability for the reaction. The recycling of 1 mol% of palladium in [NBu4]Br after the reaction of bromobenzene with styrene was achieved by distillation of the reactants and products from the solvent and catalyst in vacuo. Sodium bromide, a stoichiometric salt byproduct, was left in the solvent-catalyst system. High catalytic activity was maintained even after the formation of visible palladium black after a fourth run and after the catalyst phase had turned more viscous after the sixth run. The decomposition of the catalyst and the formation of palladium... 

When another palladium complex, diiodobis(l, 3-dimethylimidazolium-2-ylidene)palladium(II), was used as a catalyst (257), it resulted in a large improvement in catalyst stability in the same ionic liquid. The Heck reaction performed better in the ionic liquid than in organic solvents such as dimethylfuran (DMF). In the reaction of bromobenzene with styrene, the yield of stilbene was increased from 20% in DMF to 99% in [NBu4][Br]. The ionic liquid showed excellent solubility for all the reacting molecules. 

In experiments with a supported palladium catalyst, Pd/C, satisfactory yields were obtained without the use of phosphine ligands for the Heck reactions of aryl iodide with acrylonitrile, styrene, and methyl methacrylate in the ionic liquid [BMIM]PF6 (259). The addition of triethylamine improved the yields. The Pd/C remained in the ionic liquid only. The ionic liquid containing Pd/C can be reused as... 

Okubo et al. (260) reported that Pd(II)/Si02 was a more effective catalyst than Pd/C when two equivalents of triethylamine base were added to the same ionic liquid one equivalent was not sufficient. The reaction was carried out without phosphine ligands. The unreduced Pd(II)/Si02 catalyst with two equivalents of base in [BMIM]PF6 was more active than the supported palladium catalysts in DMF. Furthermore, the stability of [BMIM]PF6 also improved with the addition of triethylamine. 

Very high regioselectivities (> 99.5% iso) were obtained, using PdCl2(PhCN)2 in combination with (+)-neomethyldiphenylphosphine and toluene-j9-sulfonic acid, under mild conditions (70 °C and 10 bar). More recently, the palladium-catalyzed alkoxycarbonylation and amido-carbonylation of aryl bromides and iodides in [bmim][BF4] and [bmim][PF6] has been described. Enhanced reaction rates were observed compared to conventional media and the ionic liquid-catalyst could be recycled. 

Aminocarbonylation provides an efficient method for the synthesis of carboxamides from readily available alkenyl halides. This reaction finds many applications in organic synthesis, especially for the introduction of amides with a variety of A -substituents. For example, steroidal alkenyl iodide 137 was transformed to the corresponding amide derivative 138 in 88% yield through aminocarbonylation (Equation (10)). In this reaction, the palladium catalyst was recovered by using an ionic liquid, l-butyl-3-methylimidazolium salt 139, as reaction media, and reused five times with only a minor loss of activity. ... 

Ionic liquids have been used for the selenium- or palladium-catalyzed carbonylation of primary amines to form carbamates or ureas.After completion of the carbonylation, addition of water induced the precipitation of desired products, which were isolated by fdtration and separated from the ionic liquid, containing the catalytic species. Then, the catalyst could be reused after removal of residual methanol and water by distillation. Although the conversion of the reaction slightly decreased after the second run, the catalytic activity was considerably improved (from 70% to 99 %) by the addition of a small amount of the fresh catalyst. " ... 

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