Hydrogenation using Water-Soluble Catalysts

Hydrogenation of unsaturated phosphohpids dispersed in aqueous systems using a water-soluble homogeneous catalyst was first reported by Madden and Quinn [1]. 

The catalyst was a sulfonated derivative of Wilkinson s catalyst which did not appear to affect the structure of bilayers with respect to their permeability barrier properties [2]. The catalyst was found to hydrogenate oil-in-water emulsions and two-phase oil-water systems without the need for organic co-solvents [3]. The reaction rate could be increased significantly by screening the electrostatic charge on the sulfonate groups with inorganic cations added to the aqueous phase. This allowed the catalyst to penetrate into the substrate at the interface partition of the catalyst from the aqueous into the lipid phase could not be detected. 

The reactivity of water-soluble palladium catalyst, Pd(QS)2 (palladium di(sodirmi) alizarine monosulfonate) has been examined in multilamellar dispersions of unsaturated phospholipids [4]. With substrates of dioleoylphosphatidylcholine there is a transient appearance of trans co9 but no cis double bonds were observed when the trans 9 derivative of phosphatidylcholine was used as substrate. 

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Other salts of formic acid have been used with good results. For example, sodium and preferably potassium formate salts have been used in a water/organic biphasic system [36, 52], or with the water-soluble catalysts discussed above. The aqueous system makes the pH much easier to control minimal COz is generated during the reaction as it is trapped as bicarbonate, and often better reaction rates are observed. The use of hydrazinium monoformate salts as hydrogen donors with heterogeneous catalysts has also been reported [53]. 

A reverse set-up has also been developed for the hydrogenation of water soluble substrates. The catalyst is dissolved in the organic phase and the substrates and products in the aqueous phase. Such a protocol has been used to hydrogenate an aqueous solution of 1-butene-1,1-diol, as shown in Scheme 8.6 [4], Since the... 

For a decade or so [CoH(CN)5] was another acclaimed catalyst for the selective hydrogenation of dienes to monoenes [2] and due to the exclusive solubility of this cobalt complex in water the studies were made either in biphasic systems or in homogeneous aqueous solutions using water soluble substrates, such as salts of sorbic add (2,4-hexadienoic acid). In the late nineteen-sixties olefin-metal and alkyl-metal complexes were observed in hydrogenation and hydration reactions of olefins and acetylenes with simple Rii(III)- and Ru(II)-chloride salts in aqueous hydrochloric acid [3,4]. No significance, however, was attributed to the water-solubility of these catalysts, and a new impetus had to come to trigger research specifically into water soluble organometallic catalysts. 

Hydrogenation of the carbonyl function is an important synthetic transformation and can be catalyzed by complexes of several transition metals including -among others- Co, Rh, Ru, Ir, and Os. In aqueous organometallic catalysis the first examples were given by the hydrogenation of water-soluble 2-oxo-carboxylic acids, 1,3-dihydroxyacetone and fmctose [47-54], later the same substrates were also used for testing new catalysts [29]. 

Kalck et /.96,359,362 studied the biphasic hydroformylation of 1-hexene using water soluble dinuclear species such as Rh2(m-S-lBu)2(CO)2(tppts)2 and C0/H20 where H20 acts both as a solvent and hydrogen source according to the water gas shift reaction (Equation 2). The turnover frequency (TOF) obtained was 40 h 1 and the n/i ratio of the aldehyde 96/4.96 Using RhH(CO)(tppts)3 catalysts lower rates were obtained under the same conditions.96... 

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-... 

Water-soluble rhodium complexes bearing sulfonated triphenylphos-phine ligands can catalyze the reduction of cyclohexene in a two-phase system. It is also possible to use Wilkinson s catalyst [(Ph3P)3RhCl] for the hydrogenation of water-soluble olefins in an aqueous-benzene solvent system (46). 

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