Rhodium carbonylation cycle

An alternative strategy for catalyst immobilisation uses ion-pair interactions between ionic catalyst complexes and polymeric ion exchange resins. Since all the rhodium complexes in the catalytic methanol carbonylation cycle are anionic, this is an attractive candidate for ionic attachment. In 1981, Drago et al. described the effective immobilisation of the rhodium catalyst on polymeric supports based on methylated polyvinylpyridines [48]. The activity was reported to be equal to the homogeneous system at 120 °C with minimal leaching of the supported catalyst. The ionically bound complex [Rh(CO)2l2] was identified by infrared spectroscopic analysis of the impregnated resin. 

Two homogeneous metal complex water-gas shift catalyst systems have recently appeared 98, 99). The more active of these comes from our Rochester laboratory (99, 99a). It is composed of rhodium carbonyl iodide under CO in an acetic acid solution of hydriodic acid and water. The catalyst system is active at less than 95°C and less than 1 atm CO pressure. Catalysis of the water-gas shift reaction has been unequivocally established by monitoring the CO reactant and the H2 and C02 products by gas chromatography The amount of CO consumed matches closely with the amounts of H2 and C02 product evolved throughout the reaction (99). Mass spectrometry confirms the identity of the C02 and H2 products. The reaction conditions have not yet been optimized, but efficiencies of 9 cycles/day have been recorded at 90°C under 0.5 atm of CO. Appropriate control experiments have been carried out, and have established the necessity of both strong acid and iodide. In addition, a reaction carried out with labeled 13CO yielded the same amount of label in the C02 product, ruling out any possible contribution of acetic acid decomposition to C02 production (99). 

The catalytic activity of the methanol carbonylation is very dependent on the nature of the iodide promoter, and different chemistry appears to follow using HI or Nal in this regard (72). However, under otherwise identical conditions, the catalytic activity increased in the order Nal < CH3I < HI. Contrary to what is observed for the rhodium/iodide catalyst, Braca et al. did not consider CH3I to be directly involved in the catalytic carbonylation cycle (70-73). This conclusion is based on the observation that CH3I was not carbonylated under their reaction conditions. Instead, because of the necessity of a proton supplier and the promoting effect of Nal, these authors... 

The mechanism of the Li/Rh-catalyzed carbonylation of methyl acetate was formally described by a catalyst cycle by Zoeller et al. based on kinetic measurements under high pressure [41b]. The organic reaction cycle is combined with the rhodium catalyst cycle in the Li/Rh catalyst system (Figure 3). 

Mechanistic studies of rhodium-catalyzed hydroformylation of olefins have shown that the basic feature of the catalyst cycle is more or less the same as that of the cobalt-catalyzed reaction.When unmodified rhodium carbonyls, e.g., Rh4(CO)i2 and RhefCOlie, are used as catalysts, there is an equilibrium among Rh4(CO)i2, Rh6(CO)i6, and HRh(CO)n (n = 3 or 4) in the presence of carbon monoxide and hydrogen, which complicates the mechanistic study. Nevertheless, HRh(CO)n (n = 3) is postulated as the active catalyst species, and the... 

Probably the most important industrial application of a transition-metal nucleophile is the Monsanto process for carbonylating methanol, using soluble rhodium-carbonyl complexes in the presence of iodide. The catalyst is in fact [Rhl2(CO)2] (Forster 1979, and references therein), and the catalytic cycle is shown in Scheme 21. The substrate for the rhodium catalyst is methyl iodide, which oxidatively adds to yield [Rh(Me)(I)3(CO)2] . 

The presence of a lithium salt (Lil) as cocatalyst and hydrogen is very important for efficient production of acetic anhydride. The proposed reaction mechanism is shown in Figure 5 [42,43,47]. In this mechanism, there are two catalytic cycles for the formation of methyl acetate a rhodium-catalyzed cycle and a lithium-catalyzed cycle. The rhodium-catafyzed cycle is similar to the Monsanto process of methanol carbonylation (Fig. 1). The participation of the second cycle was discovered when it was found that the reaction rate was much enhanced when hydrogen and a lithium salt were added [43,44]. The role of hydrogen is to reduce the catalytically inactive Rh(CO)2l4 to the active Rh(CO)2l2. In the anhydrous medium used in the reaction, the formation of hydrogen by the reaction of carbon monoxide with water as in the water-gas shift reaction is not possible. Thus hydrogen must be added. 

Evidences for the rhodium- and iridium-catalyzed carbonylation cycles also come from isolated and model complexes. From the reaction of CHjI with 4.5, under noncatalytic conditions, a solid has been isolated. 

The hydroformylation or oxo process was discovered in 1938 by Otto Roelen of Ruhrchemie AG in Germany. Oxo means carbonyl in German and the hydroformylation (or oxonation) initially produces carbonyl compounds. Commercially both cobalt and rhodium carbonyl catalysts have been exploited since World War II. The same catalyst can be regenerated and reused through many cycles but the catalyst will eventually degrade and lose its activity or it can also be poisoned by impurities that are likely present in the feedstock. 

Cases of the S-coordinated rhodium and iridium are quite scarce. To complete the picture, we next consider the possibilities of S-coordination using complicated derivatives of thiophene. 2,5-[Bis(2-diphenylphosphino)ethyl]thiophene is known to contain three potential donor sites, two phosphorus atoms and the sulfur heteroatom, the latter being a rather nucleophilic center (93IC5652). A more typical situation is coordination via the phosphorus sites. It is also observed in the product of the reaction of 2,5-bis[3-(diphenylphosphino)propyl]thiophene (L) with the species obtained after treatment of [(cod)Rh(acac)] with perchloric acid (95IC365). Carbonylation of [Rh(cod)L][C104]) thus prepared yields 237. Decarbonylation of 237 gives a mixture of 238 and the S-coordinated species 239. Complete decarbonylation gives 240, where the heterocycle is -coordinated. The cycle of carbonylation decarbonylation is reversible. 

The generation of the initial metal-carbon bond in the catalytic cycle by reaction of methyl iodide with a metal carbonyl-containing species has been proposed as a key step in both the cobalt (2) and rhodium (4) catalyzed systems. 

This catalytic cycle, generating acetyl iodide from methyl iodide, has been demonstrated by carbonylation of anhydrous methyl iodide at 80°C and CO partial pressure of 3 atm using [(C6H5)4As][Rh(CO)2X2] as catalysts. After several hours reaction, acetyl iodide can be identified by NMR and infrared techniques. However, under anhydrous conditions some catalyst deactivation occurs, apparently by halogen abstraction from the acetyl iodide, giving rhodium species such as frans-[Rh(CO)2I4] and [Rh(CO)I4] . Such dehalogenation reactions are common with d8 and d10 species, particularly in reactions with species containing weak... 

The rate of the methanol carbonylation reaction in the presence of iridium catalysts is very similar to that observed in the presence of rhodium catalysts under comparable conditions (29). This is perhaps initially surprising in view of the well-recognized greater nucleophilicity of iridium(I) complexes as compared to their rhodium(I) analogues. It can be seen from the above studies that the difference in the chemistry of the metals at the trivalent stage of the catalytic cycle serves to produce faster rates of alkyl migration with the rhodium system thus, overall the two metal catalysts give comparable rates. 

Although detailed mechanistic studies are not reported, the postulated mechanism for the reductive cyclization of allenic carbonyl compounds involves entry into the catalytic cycle via silane oxidative addition. Allene silylrhodation then provides the cr-allylrhodium hydride A-18, which upon carbometallation of the appendant aldehyde gives rise to rhodium alkoxide B-14. Oxygen-hydrogen reductive elimination furnishes the hydrosilylation-cyclization product... 

A mechanistic study by Haynes et al. demonstrated that the same basic reaction cycle operates for rhodium-catalysed methanol carbonylation in both homogeneous and supported systems [59]. The catalytically active complex [Rh(CO)2l2] was supported on an ion exchange resin based on poly(4-vinylpyridine-co-styrene-co-divinylbenzene) in which the pendant pyridyl groups had been quaternised by reaction with Mel. Heterogenisation of the Rh(I) complex was achieved by reaction of the quaternised polymer with the dimer, [Rh(CO)2l]2 (Scheme 11). Infrared spectroscopy revealed i (CO) bands for the supported [Rh(CO)2l2] anions at frequencies very similar to those observed in solution spectra. The structure of the supported complex was confirmed by EXAFS measurements, which revealed a square planar geometry comparable to that found in solution and the solid state. The first X-ray crystal structures of salts of [Rh(CO)2l2]" were also reported in this study. 

The commercialisation of an iridium-based process is the most significant new development in methanol carbonylation catalysis in recent years. Originally discovered by Monsanto, iridium catalysts were considered uncompetitive relative to rhodium on the basis of lower activity, as often found for third row transition metals. The key breakthrough for achieving high catalytic rates for an iridium catalyst was the identification of effective promoters. Recent mechanistic studies have provided detailed insight into how the promoters influence the subtle balance between neutral and anionic iridium complexes in the catalytic cycle, thereby enhancing catalytic turnover. 

The reductive elimination/oxidative addition is of practical importance in catalytic cycles, for example the rhodium/methyl iodide catalysed carbonylation of methanol. In organic synthesis the palladium or nickel catalysed cross-coupling presents a very common example involving oxidative addition and reductive elimination. A simplified scheme is shown in Figure 2.19 [17],... 

Figure 8.1 Catalytic cycle for the rhodium-catalyzed methanol carbonylation.

---