Ethylene catalysis, rhodium complexes

As the next example of soft catalysis, we shall discuss the dimerization of ethylene to 1-butene, which is catalyzed by rhodium complexes in a redox cycle (Scheme 2-6). The active Rh catalyst A undergoes oxidative addition of HCl and insertion of ethylene into the Rh-H bond to give the Rh alkyl complex B. The following ethylene insertion reaction is the rate-determining step and is favored by the medium-hard Rh center. The resulting Rh butyl complex C has a hard-soft dis-... 

Several differences between the cobalt- and rhodium-catalyzed processes are noteworthy with regard to mechanism. Although there is a strong dependence in the cobalt system of the ethylene glycol/methanol ratio on temperature, CO partial pressure, and H2 partial pressure, these dependences are much lower for the rhodium catalyst. Details of the product-forming steps are therefore perhaps quite different in the two systems. It is postulated for the cobalt system that the same catalyst produces all of the primary products, but there seems to be no indication of such behavior for the rhodium system. Indeed, the multiplicity of rhodium species possibly present during catalysis and the complex dependence on promoters make it... 

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. 

Among the most significant developments in the field of catalysis in recent years have been the discovery and elucidation of various new, and often novel, catalytic reactions of transition metal ions and coordination compounds 13, 34). Examples of such reactions are the hydrogenation of olefins catalyzed by complexes of ruthenium (36), rhodium (61), cobalt (52), platinum (3, 26, 81), and other metals the hydroformylation of olefins catalyzed by complexes of cobalt or rhodium (Oxo process) (6, 46, 62) the dimerization of ethylene (i, 23) and polymerization of dienes (15, 64, 65) catalyzed by complexes of rhodium double-bond migration in olefins catalyzed by complexes of rhodium (24,42), palladium (42), cobalt (67), platinum (3, 5, 26, 81), and other metals (27) the oxidation of olefins to aldehydes, ketones, and vinyl esters, catalyzed by palladium chloride (Wacker process) (47, 48, 49,... 

Quite stable catalytic reaction solutions were obtained in THF with the starting pressure for ethylene of 6-6.5 MPa at a reaction temperature of 120 °C. Under these conditions and with the ratios piperidine/rhodium of 100 1 and 1000 1 in 36 and 72 h, yields of 70 and 50 % ethylpiperidine were reached, which correspond to TONs of 2 and 7 mol amine/(mol Rh) per h, respectively. Total conversion is also possible if the reaction time is prolonged further. As a side reaction, ethylene dimerization to butene was observed. This indicates the formation of a hydrido rhodium(III) complex in the hydroamination reaction, as formulated in Scheme 3, route (b). Hydrido rhodium(III) complexes are known as catalysts for ethylene dimerization [19], and if the reductive elimination of ethylpiperidine from the hydrido-y9-aminoethyl rhodium(III) complex is the rate-limiting step in the catalytic cycle of hydroamination, a competitive catalysis of the ethylene dimerization seems possible. In the context of these mechanistic considerations, an increase of the catalytic activity for hydroamination requires as much facilitation of the reductive elimination step as possible. 

Extrusion of CO from acyl-2-phenylpyridines is another way to form an Ar-C(sp ) bond. Rh(I) catalysis was found to be effective in the conversion of styryl ketones into the corresponding stil-benes [12]. Aroyl chlorides react with acyclic alkenes in the presence of a rhodium-ethylene complex, [ RhCljCjH lj Ij], in refluxing o-xylene under to give Mizoroki-Heck-type products [27a]. 

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