Catalyst Systems Containing Rhodium

As the first transition metal-based homogeneous catalysis of hydroamination, in the early 1970s Coulson from the Du Pont laboratories had described the addition of secondary aliphatic amines to ethylene in the presence of various rhodium compounds [15, 16]. Definite results were reported with RhCl3 3 H2O as pre-catalyst in tetrahydrofuran as solvent under starting ethylene pressures of 5-14 MPa at 180-200 °C for different secondary amines (Table 3). 

Apparently the reaction is very sensitive to the nature of the amine. The highest activity, which was obtained with piperidine, corresponds to a TON of about 20 mol amine/(mol Rh) per h. Ammonia and primary amines did not react with ethylene under the conditions used, and higher olefins were also found to be essentially unreactive. For the reaction with piperidine the reaction rate was found to be independent of the piperidine concentration and first-order with respect to rhodium trichloride. 

More recent investigations in the author s group [2, 17] have shown that under the reaction conditions the tra 5-bis(piperidine) ethylene rhodium(I) complex 

Molar ratio Ethylene/amine/RhCb Appreciable side reactions noted. 

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Furthermore, in the case of the asymmetric catalytic system containing rhodium and (—)-DIOP always the same, prochiral face (re) is preferentially formylated for six other monosubstituted olefins (Table 7, column 1). Similar results are obtained with rhodium catalysts when monophosphines are used instead of DIOP. The only... 

In addition, catalysts have been developed that use only palladium as the precious metal component, with Pd loadings of about 1.8-10.6gl catalyst volume. These catalysts are mainly used as light-off catalysts, which are typically small catalysts located close to the engine outlet, and used in combination with a main catalyst that contains rhodium together with platinum which is located in the vehicle underbody downstream of the light-off catalyst. However, these palladium-only catalysts are sometimes also used as the main catalyst, in conjunction with particular engine management systems. 

Polymeric polyolefins, such as polybutadiene, secondary amines, and synthesis gas, are reacted in the presence of a catalyst system comprising a ruthenium-containing compound, a rhodium-containing compound, a steri-cally hindered phosphine, and a solvent [1191]. Preferred polybutadiene feedstocks are those with a predominance of main chain, rather than pendant olefin groups and in particular, those polymers containing both the 1,2-polybutadiene and 1,4-polybutadiene units. These polymers of high amine content are useful as down-hole corrosion inhibitors. 

Recently, a rhodium-catalyzed tandem cyclization has been reported with an arylboronic ester bearing a pendant Michael-type acceptor olefin and acetylenic65 or olefinic66 derivatives. This transformation proceeds in a water-containing medium as solvent and proton source. This catalyst system is optimized with electron-rich and bulky ligands to stabilize the rhodium intermediate and decrease the protonolysis of boron derivatives in a protic solvent. 

Hsu et al. [15] applied a bimetallic catalyst comprising rhodium and ruthenium for the hydrogenation to combine the high selectivity of the rhodium complex with the lower cost of the ruthenium complex. When the amount of each metal is identical, the catalytic activity of the bimetallic complex catalyst system was similar to that of the single rhodium-complex catalyst, containing... 

Other recent reports have also indicated that mixed-metal systems, particularly those containing combinations of ruthenium and rhodium complexes, can provide effective catalysts for the production of ethylene glycol or its carboxylic acid esters (5 9). However, the systems described in this paper are the first in which it has been demonstrated that composite ruthenium-rhodium catalysts, in which rhodium comprises only a minor proportion of the total metallic component, can match, in terms of both activity and selectivity, the previously documented behavior (J ) of mono-metallic rhodium catalysts containing significantly higher concentrations of rhodium. Some details of the chemistry of these bimetallic promoted catalysts are described here. 

Rhodium complexes that contain these ligands have demonstrated moderate to high enantio-selectivities (24-96%) in the reduction of enamides to protected amino acids (cf Scheme 12.1).7-70 Despite the moderate degree of asymmetric induction, ANIC S.p.A. (EniChem) developed an industrial process with this catalyst system for the production of (S)-phenylalanine for the synthesis of aspartame.1145 The process uses cationic Rh-7a for the reduction of 14b at 28 psig H2 and 22°C for 3 hours (S/C = 15,000) to give 15b in 83.3% ee that is enriched to 98.3% by recrystallization.72... 

Rhodium and palladium catalysts that contain 4 display high enantioselectivities for the asymmetric hydrogenation of enamides, itaconates, P-keto esters, asymmetric hydroboration, and asymmetric allylic alkylation,80 82 but this ligand system distinguishes itself from other chiral bisphos-phines in the asymmetric reduction of tetrahydropyrazines and tetrasubstituted olefins (see also Chapter 15). The reduction of tetrahydropyrazines produces the piperazine-2-carboxylate core,... 

Ethylene dimerization catalysis has, however, been more thoroughly investigated for the broader range of homogeneous catalysts. For example, active metal complexes containing titanium, nickel, iron, cobalt, rhodium, ruthenium, and palladium, are all known (133). Where possible, comparisons will be made with the relevant homogeneous catalyst systems. 

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