Reaction rates rhodium catalysis

Information published from several sources about 1970 presented details on both the halide-containing RhCl(CO)(PPh3)2- and the hydride-containing HRh(CO)(PPh3)3-catalyzed reactions. Brown and Wilkinson (25) reported the relative rates of gas uptake for a number of different olefinic substrates, including both a- and internal olefins. These relative rates are listed in Table XV. 1-Alkenes and nonconjugated dienes such as 1,5-hexadiene reacted rapidly, whereas internal olefins such as 2-pentene or 2-heptene reacted more slowly by a factor of about 25. It should also be noted that substitution on the 2 carbon of 1-alkene (2-methyl-l-pentene) drastically lowered the rate of reaction. Steric considerations are very important in phosphine-modified rhodium catalysis. 

In this reaction, a rhodium atom complexed to a chiral diphosphine ligand ( P—P ) catalyzes the hydrogenation of a prochiral enamide, with essentially complete enan-tioselectivity and limiting kinetic rates exceeding hundreds of catalyst turnovers per second. While precious metals such as Ru, Rh, and Ir are notably effective for catalysis of hydrogenation reactions, many other transition-metal and lanthanide complexes exhibit similar potency. 

It is interesting to note that, when operating at approximately 5% water content, and with good reaction rates, it is possible to save about 30% of the energy costs of the process when compared to the Monsanto process. In addition, the current price of rhodium is 20-fold that of iridium. Even with annual unit capacity productions as large as 500 000 tons, it is still possible to improve catalysis and to achieve significantly better performances. 

The presence of 4e as the predominant species during the catalysis is also in accord with the observed kinetic behavior of this catalyst with 1-octene and styrene as the substrates. The observation of this saturated acyl rhodium complex is in line with the positive dependence of the reaction rate on the hydrogen concentration and the zero order in alkene concentration. It was concluded previously that this saturated acyl complex is an unreactive resting state [18]. Before the final hydro-genolysis reaction step can occur, a CO molecule has to dissociate in order to form... 

A group at Monsanto has also studied the catalysis of the water-gas shift reaction by rhodium carbonyl iodide (103b). The main difference between their work and our own is the choice of reaction conditions. Their study was conducted at 185°C under 200-400 psig carbon monoxide. Despite this drastic difference in reaction conditions, the studies are surprisingly consistent. In particular, the Monsanto group also finds evidence for two rate-limiting reactions. They did not find this by temperature variation, but instead, consistent with our own work, find that at low acid and iodide... 

The carbonyl [Ru3(CO),2] is a good cocatalyst for the low pressure hydroformylation of internal alkenes using the classic rhodium phosphine [HRh(CO)(PPh3),] system in the presence of an excess of triphenylphosphine (P/Rh = 200) (22). Starting from a mixture of hex-2- and hex-3-ene, the addition of [Ru3(CO),2l (Rh/Ru = 1/1) increased both the reaction rate and the n/iso ratio of heptanals. More recently, Poilblanc and coworkers (23) have prepared a mixed ruthenium-rhodium complex formulated as [CIRh(/i-CO)(//-dppm)2Ru(CO)2] (dppm is Ph2PCH2PPh2). This species shows catalytic activity in the hydroformylation of pent-l-ene at 40 bar (H2/C0= 1/1) and 75°C. Conversion to hexanals was 90% in 24 hours and the linearity reached 70%. No further report has appeared to determine the role of the two metals in this catalysis. 

Later work by Stevenson [72] supported this hypothesis. The preparation of PET catalysed by antimony trioxide was studied in thin films on metal surfaces that were carefully selected to avoid catalysis by surface effects or by dissolved metal as mentioned earlier, a large number of metals and their oxides, salts or other derivatives catalyse the polyesterification reaction. On inactive surfaces like silver or rhodium the catalysed polycondensation rate increased with decrease in film thickness. In the absence of added catalyst there was no tendency for the rate to increase with decreasing film thickness. Stevenson proposed that in thin films the catalyst-deactivating component was more readily lost, thereby increasing the reaction rate. 

In a further variation, the PVP-supported rhodium catalyst was used for methanol carbonylation in supercritical carbon dioxide [100]. This reaction medium has complete miscibility with CO and dissolves high concentrations of methanol and methyl iodide, while being a poor solvent for ionic metal complexes. Catalytic reaction rates up to half of those obtained in conventional liquid-phase catalysis were achieved with minimal catalyst leaching. 

The first bisphosphine calixarenes that have been used in catalysis are di(amide)-phosphine hybrids calix[4]ar-ene. Reaction of [RhCl(norbornadiene)]2 with these calixarene derivatives gave an organometallic complex whose norbornadiene-rhodium moiety lies above the cavity defined by the four substituents of the calixarene and between the two amide functionalities. This complex was applied in the hydroformylation reaction of styrene. The rather low reaction rate observed (7.5 turnovers per Rh per hour) has been attributed to a partial encapsulation of the metal center preventing the approach of the substrate. Indeed, the metal center may be viewed as located in a hemispherical ligand environment. 

Although reaction rate and selectivities of heterogeneous catalysis do depend on ligand dimensions ligand/metal complex ratios, these values are in most cases unknown. In order to clarify this, the hydrogenation of olefins on rhodium/phosphine catalysts has been studied [231]. The compounds />-/i-butyl- and p- -dodecylphenyl-diphenylphosphine as well as triphenylphosphine have been used as models of triphenylphosphine polymeric ligands... 

The rhodium/TPPTS-catalyzed hydroformylation of higher olefins in organic/ aqueous biphasic system in the presence of double long-chain cationic surfactants (37) was studied at 100 °C and 20 bar CO H2 = 1 1 pressure. The reaction rate was comparable with that in homogeneous catalysis system [111]. 

The Wilkinson hydrogenation cycle shown in Figure 3 (16) was worked out in experiments that included isolation and identification of individual rhodium complexes, measurements of equiUbria of individual steps, deterrnination of rates of individual steps under conditions of stoichiometric reaction with certain reactants missing so that the catalytic cycle could not occur, and deterrnination of rates of the overall catalytic reaction. The cycle demonstrates some generally important points about catalysis the predominant species present in the reacting solution and the only ones that are easily observable by spectroscopic methods, eg, RhCl[P(CgH 2]3> 6 5)312 (olefin), and RhCl2[P(CgH )2]4, are outside the cycle, possibly in virtual equiUbrium with... 

---