Biphasic catalysis hydrogenation

The first example of biphasic catalysis was actually described for an ionic liquid system. In 1972, one year before Manassen proposed aqueous-organic biphasic catalysis [1], Par shall reported that the hydrogenation and alkoxycarbonylation of alkenes could be catalysed by PtCh when dissolved in tetraalkylammonium chloride/tin dichloride at temperatures of less than 100 °C [2], It was even noted that the product could be separated by decantation or distillation. Since this nascent study, synthetic chemistry in ionic liquids has developed at an incredible rate. In this chapter, we explore the different types of ionic liquids available and assess the factors that give rise to their low melting points. This is followed by an evaluation of synthetic methods used to prepare ionic liquids and the problems associated with these methods. The physical properties of ionic liquids are then described and a summary of the properties of ionic liquids that are attractive to clean synthesis is then given. The techniques that have been developed to improve catalyst solubility in ionic liquids to prevent leaching into the organic phase are also covered. 

The first results of the batch hydrogenation of prenal and citral to geran-iol and nerol provided evidence for the use of aqueous biphasic catalysis to increase the selectivity of the conversion of the substrates the accumiilation of byproducts can be nearly suppressed by the fast extraction of the product from the catalyst phase. For this reason, the distribution of the product between extraction and catalyst phase had been studied in detail. 

As already shown by Wiese et al. [17] mass transport rates in biphasic catalysis can be dramatically influenced by hydrodynamics in a tube reactor with Sulzer packings. Above all, the volume rate of the catalyst phase in which the substrates are transported by diffusion plays a decisive role in accelerating the mass transport rate. This effect was also investigated for citral hydrogenation in the loop reactor. Overall reaction rates and conversions as a function of the catalyst volume rate can be seen in Fig. 15. 

In this section we describe two approaches for using dendrimer-encapsulated metal particles to perform biphasic catalysis. The first is hydrogenation catalysis using dendrimers rendered soluble in the fluorous phase by electrostatic attachment of perfluoroether groups [103]. The second demonstrates the use of perfluoroether groups covalently hnked to the dendrimer exterior to carry out a Heck reaction [100]. 

In most aqueous/organic biphasic systems, the catalyst resides in the aqueous phase and the substrates and products are dissolved in (or constitute) the organic phase. In a few cases a reverse setup was applied i.e. the catalyst was dissolved in the organic phase and the substrates and products in the aqueous one. This way, in one of the earliest attempts of liquid-liquid biphasic catalysis an aqueous solution of butane-diol was hydrogenated with a [RhCl(PPh3)3] catalyst dissolved in benzene [22]. 

The use of surfactants in hydrogenation and hydroformylation immediately followed the practical implementation of the original idea of aqueous biphasic catalysis [57, 118]. Not only the effect of well-known tenzides (SDS, CTAB, etc.) was studied, but new amphiphilic phosphine... 

The complex [mer-IrH2Cl(PMe3)3] was used as a catalyst for the hydrogenation of alkynes and alkenes in water, and water-soluble ethylenediamine (en) complexes of iridium, [Ir(COD)(en)]Cl, were found to be excellent catalysts for aqueous hydrogenations (94). It would be interesting to determine the loss of iridium during application of these complexes in biphasic catalysis. 

Aqueous biphasic catalysis is also used in homogeneous hydrogenations.117-119 In new examples Ru clusters with the widely used TPPTN [tris(3-sulfonatophenyl) phosphine] ligand120 and Rh complexes with novel carboxylated phosphines121 were applied in alkene hydrogenation, whereas Ru catalysts were used in the hydro-genation of aromatics. Aerobic oxidation of terminal alkenes to methyl ketones was carried out in a biphasic liquid-liquid system by stable, recyclable, water-soluble Pd(II) complexes with sulfonated bidentate diamine ligands.124... 

Despite the remarkable activity and selectivity attainable through sophisticated ligand design in homogeneous catalysis, broad commercial application has been hampered by the need to recover and/or recycle expensive catalysts and ligands. A number of approaches, termed biphasic catalysis , have been advanced where a soluble catalyst is immobilized in one liquid phase (often aqueous) and the substrates and products are isolated in a separate immiscible phase (Baker and Tumas, 1999 Cornils and Herrmann, 1998). Several recent reports have focused on systems using only water and C02, where water-soluble catalysts and C02-soluble substrates and products can be isolated in separate phases (Table 2.4). Hydrogenation of cinnamal-... 

The utility of the methodology for biphasic catalysis has been demonstrated with the RhCl (PPh3)3-catalyzed hydrogenation of styrene to ethyl benzene in PEG (MW = 900) (reaction conditions 30 bar H2, 50 bar C02, 40 °C and 19 h) [12], A total of five cycles are performed with one batch of catalyst/PEG solution, without replenishing either the catalyst or the PEG. The catalyst keeping active as >99 % conversion is found for all five cycles. 

Industrial Chemicals Co. on biphasic catalysis can be found in a 1962 edition of the Journal of the American Chemical Society describing hydrogenations with [Co(CN)5H]3 under aqueous-organic conditions.161 Although we are not yet aware of any publication prior to that, it is highly likely that biphasic catalysis has a longer history than thought until recently. Yet, it was the systematic studies of Joo and Manassen that really laid the foundations to the field. 

Ideally, biphasic catalysis is performed in such a way that mass-transfer from one phase to the other does not restrict the rate of the reaction. An elegant solution to overcome this potential limitation is reversible two phase-single phase reaction conditions. An example of a temperature-controlled reversible ionic liquid-water partitioning system has been demonstrated for the hydrogenation of 2-butyne-l,4-diol, see Figure 3.2.1251... 

C4CAim][PF6] Chiral Ru catalyst with imidazolium tag on the r 6-arene 35-40°C acetophenone as substrate iPrOH/KOH or formic acid/NEt3 azeotrope as hydrogen source use of general precursor for biphasic catalysis excellent ee catalyst reuse possible four times. [50]... 

The use of ionic liquids with SCCO2 has recently been reviewed.More information on ionic liquids can be found in Chapter 6. However, their use in biphasic catalysis with SCCO2 is discussed here. They have been used most extensively for hydrogenation and hydroformylation reactions. 

In a first approximation, the new methods correspond to the conventional solvent techniques of supported catalysts (cf Section 3.1.1.3), liquid biphasic catalysis (cf Section 3.1.1.1), and thermomorphic ( smart ) catalysts. One major difference relates to the number of reaction phases and the mass transfer between them. Owing to their miscibility with reaction gases, the use of an SCF will reduce the number of phases and potential mass transfer barriers in processes such as hydrogenation, carbonylations, oxidation, etc. For example, hydroformylation in a conventional liquid biphasic system is in fact a three-phase reaction (g/1/1), whereas it is a two-phase process (sc/1) if an SCF is used. The resulting elimination of mass transfer limitations can lead to increased reaction rates and selectiv-ities and can also facilitate continuous flow processes. Most importantly, however, the techniques summarized in Table 2 can provide entirely new solutions to catalyst immobilization which are not available with the established set of liquid solvents. 

Examples of aqueous-organic catalyzed reactions include oxidations,8 polymerisations,9 hydrogenations,10 hydroformylations,11 C—C coupling,12 and olefin metathesis.13,14 The use of water as a solvent for conducting biphasic catalysis has many advantages it is cheap, easily purified, readily obtained, and disposed. The main drawback is that trace amounts of organic compounds dissolved in water are difficult to remove, although this does not tend to be a problem when the aqueous-catalyst phase is reused repeatedly for the same reaction. 

SAPC can perform a broad spectrum of reactions such as hydroformylation, hydrogenation and oxidation, for the synthesis of bulk and fine chemicals, pharmaceuticals and their intermediates. Rhodium complexes are the most extensively used, but complexes of ruthenium, platinum, palladium, cobalt, molybdenum and copper have also been employed [63-65]. Owing to interfacial reactions, one of the main advantages of SAPC upon biphasic catalysis is that the solubility of the reactant in the catalytic aqueous-phase does not limit the performance of the supported aqueous phase catalysts. 

---