Ethylene glycol, rhodium-catalyzed

Fig. 5. Plot of log(rates) vs. log(pressure) for rhodium-catalyzed CO hydrogenation. Reaction conditions 75 ml sulfolane, 3 mmol Rh, 1.25 mmol pyridine, H2/CO = 1, 240 C, 4 hr (96). Total rate includes rates to methanol, methyl formate, ethanol, ethylene glycol monoformate, and propylene glycol ( ) total ( ) methanol ( ) ethylene glycol. Open figures are for an experiment with H2/CO = 0.67.

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... 

Ruthenium complexes convert CO + Hj into methanol . Rhodium complexes at very high pressures catalyze the formation of methanol and ethylene glycol in high... 

Although isomerization of alkenes occurs simultaneously with the oxidation, rhodium and ruthenium complexes can also be used instead of palladium for the oxidation of terminal alkene [15]. With these catalysts, symmetrical quaternary ammonium salts such as tetrabutylammonium hydrogensulfate are effective. Interestingly, the rate of palladium-catalyzed oxidation of terminal alkenes can be improved by using poly(ethylene glycol) (PEG) instead of quaternary ammonium salts [16]. Thus, the rates of PEG-400-induced oxidation of 1-decene are up three times faster than those observed with cetyltrimethylammonium bromide under the same conditions. Interestingly, internal alkenes can be efficiently oxidized in this polyethylene glycol/water mixture. 

In contrast, metal clusters have several active centers or can form multi-electron systems. Metal clusters such as Rh (CO)i6, Rh4(CO)i2, It4(CO)i2, Ru3(CO)i2, and more complex structures have been successfully tested in carbonylation reactions. Rhodium clusters catalyze the conversion of synthesis gas to ethylene glycol, albeit at very high pressures up to now. 

Related to this chemistry is the hydroformylation of formaldehyde to give glycolaldehyde, which would be an attractive route from syn-gas to ethylene glycol. The reaction can indeed be accomplished and is catalyzed by rhodium arylphosphine complexes [27], but clearly phosphine decomposition is one of the major problems to be solved before formaldehyde hydrofomylation can be applied commercially. 

Another system in which intact cluster catalysis seems likely is the synthesis gas (CO/H2) production of ethylene glycol catalyzed by an anionic rhodium cluster of uncertain or undisclosed identity. From high-temperature... 

In a similar manner, Packett at Union Carbide realized a one-pot rhodium-catalyzed version of the methodology by using diols (ethylene glycol, 1,4-butane-diol, or 2,3-butanediol) as acetalization reagents and pyridinium tosylate as co-catalyst [12]. As ligands, bidentate diphosphites and diphosphines were utilized. Under these conditions, the main products were the corresponding diacetals (up to 55%), whereas in the absence of diols mainly isomeric pentenals ( 80%) and valeraldehyde ( 10%) were obtained. 

Rhodium species as Rh6(CO)i6 and [Rh5(CO)i5] catalyze the reduction of carbon monoxide to light alcohols, specially to ethylene glycol. 

A particularly significant and useful contribution of transition metals in fine organic synthesis as well at the industrial level is based on their use as catalysts. This aspect is of course particularly important with expensive transition metals (Rh, Os, Pd, etc.). Indeed, there are numerous examples of selective processes which have never been developed up to the industrial stage because of catalyst costs, especially when some (even minor) loss of the catalyst could not be avoided. This was, for example, the case for palladium-catalyzed benzylic acetoxylation reactions, and several rhodium-catalyzed reactions, such as the direct ethylene glycol production from syngas (prohibitive pressures being an additional major drawback in this latter case). 

Polyether phosphites with over 19 ethylene glycol units were used as ligands in the rhodium-catalyzed nonaqueous hydroformylation of 1-decene. The catalysts were recovered by precipitation from the reaction mixture after reaction on cooling to room temperature or lower. The precipitated catalysts could be reused up to six times without any decrease in activity. In situ formed polyetherphosphite/Ru3(CO)i2 catalyst was found to be active in hydroformylation of 1 -decene in n-heptane solution atl30°Cand50bar[20]. 

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