Water-soluble catalyst, rhodium-phosphine complex systems

Water-soluble complexes constitute an important class of rhodium catalysts as they permit hydrogenation using either molecular hydrogen or transfer hydrogenation with formic acid or propan-2-ol. The advantages of these catalysts are that they combine high reactivity and selectivity with an ability to perform the reactions in a biphasic system. This allows the product to be kept separate from the catalyst and allows for an ease of work-up and cost-effective catalyst recycling. The water-soluble Rh-TPPTS catalysts can easily be prepared in situ from the reaction of [RhCl(COD)]2 with the sulfonated phosphine (Fig. 15.4) in water [17]. 

Surprisingly, there are only a few catalysts known capable of hydrogenating ketones in fully or largely aqueous systems. For example, most of the water-soluble rhodium, mthenium and indium phosphine complexes preferentially hydrogenate the C=C bonds in unsaturated ketones, as does the solvated ion pair formed from aqueous rhodium trichloride and Aliquat-336 [206]. 

Fell, B., Schobben, C. and Papadogianakis, G. (1995) Hydroformylation of homologous C0-alkenecarboxylate esters with water soluble rhodium carbonyl/tertiary phosphine complex catalyst systems. J. Mol. Catal. A Chem., 101, 179. 

An example of a large scale application of this concept is the Ruhrchemie/ Rhone Poulenc process for the hydroformylation of propylene to n-butanal, which employs a water-soluble rhodium(I) complex of trisulfonated triphenyl-phosphine (tppts) as the catalyst [103]. The same complex also functions as the catalyst in the Rhone Poulenc process for the manufacture of the vitamin A intermediate, geranylacetone, via reaction of myrcene with methyl acetoacetate in an aqueous biphasic system (Fig. 1.35) [104]. 

With the introduction of water-soluble phosphine ligands [41] and their application in complex catalysts in biphasic and phase-transfer systems, the influence of amphiphiles could be expected but the results were not really encouraging [42], Quinn and Taylor [43] reported on selective hydrogenation in phospholipid bilayer membranes and Nuzzo et al. [44] described the rhodium complex of 9 as active exclusively in the presence of SDS. 

---