Rhodium complex-catalyzed carbonylation reaction mechanism

Forster, D. (1976) Mechanism of a rhodium-complex-catalyzed carbonylation of melhanol to acetic add. Journal of the American Chemical Society, 98, 846 Adamson, G.W., Daly, J.J. and Forster, D. (1974) Reaction of iodocarbonylrhodium ions with melhyl-iodide - structure of rhodium acetyl complex -[Me3PhN+]2[Rh2l6(MeCO)2(CO)2]2-. Journal of Organometallic Chemistry, 71, Cl7 Haynes, A., Mann, B.E., Gulliver, D.J., Morris, D.E. and Maillis, P.M. (1991) Direct observation of MeRh(CO)2l3 the key intermediate in rhodium catalyzed methanol carbonylation. Journal of the American Chemical Society 113, 8567 Haynes, A., Mann, B.E., Morris, G.E. and Maidis, P.M. (1993) Mechanistic studies on rhodium-catalyzed carbonylation reactions — spectroscopic detection and reactivity of a key intermediate, [MeRh(CO)2l3]. Journal of the American Chemical Society, 115, 4093. 

First, for reasons of clarity, the currently-accepted mechanism of transition-metal complex catalyzed-hydrosilylation reactions will be described briefly. Furthermore, consideration of selective, if not asymmetric, reduction of certain carbonyl compounds by way of rhodium(I)-catalyzed hydrosilylation (Section 4) is included in this review because the catalytic process and stereochemical course of this reaction correlate closely with those of their asymmetric reduction under similar conditions that will be described in the succeeding section. 

If ethylene is present during the carbonylation of methanol catalyzed by IrCl4, once again with Mel as promoter, methyl propionate is formed.416 The reaction depends on the presence of iridium hydride species in solution, and a rhodium analogue of the reaction exists. The full details of the mechanism are not known but the basic steps are shown in Scheme 34. The intermediates are all believed to be complexes of iridium(IIl). 

The cobalt and rhodium catalysts have one important difference between their respective mechanisms. Unlike in the rhodium-catalyzed process, there is no oxidative addition or reductive elimination step in the cobalt-catalyzed hy-droformylation reaction. This is reminiscent of the mechanistic difference between rhodium- and cobalt-based carbonylation reactions (see Section 4.2.3). The basic mechanism is well established on the basis of in situ IR spectroscopy, kinetic and theoretical analysis of individual reaction steps, and structural characterization of model complexes. 

Rhodium-catalyzed carbonylation of methanol is known as the Monsanto process, which has been studied extensively. From the reaction mechanism aspect, the study of kinetics has proved that the oxidative addition of methyl iodide to the [Rh(CO)2l2] is the rate-determining step of the catalytic cycle. It was also observed that acetyl iodide readily adds to [Rh(CO)2l2], indicating that the acetyl iodide must be scavenged by hydrolysis in order to drive the overall catalytic reaction forward. An alternative to sequential reductive elimination and the hydrolysis of acetyl iodide is the nucleophilic attack of water on the Rh acetyl complex and the production of acetic acid. The relative importance of these two alternative pathways has not yet been fully determined, although the catalytic mechanism is normally depicted as proceeding via the reductive elimination of acetyl iodide from the rhodium center. The addition of iodide salts, especially lithium iodide, can realize the reaction run at lower water concentrations thus, byproduct formation via the water gas shift reaction is reduced, subsequently improving raw materials consumption and reducing downstream separation. In addition to the experimental studies of the catalytic mechanism, theoretical studies have also been carried out to understand the reaction mechanism [17-20]. 

The important discovery by Wilkinson [1] that rhodium afforded active and selective hydroformylation catalysts under mild conditions in the presence of triphenylphosphine as a hgand triggered a lot of research on hydroformylation, especially on hgand effects and mechanistic aspects. It is commonly accepted that the mechanism for the cobalt catalyzed hydroformylation as postulated by Heck and Breslow [2] can be apphed to phosphine modified rhodium carbonyl as well. Kinetic studies of the rhodium triphenylphosphine catalyst have shown that the addition of the aUcene to the hydride rhodium complex and/or the hydride migration step is probably rate-limiting [3] (Chapter 4). In most phosphine modified systems an inverse reaction rate dependency on phosphine ligand concentration or carbon monoxide pressure is observed [4]. 

As mentioned in the previous section, the carbonylation of methanol to acetic acid is an important industrial process. Whereas the [Co2(CO)s]-catalyzed, iodide-promoted reaction developed by BASF requires pressures of the order of 50 MPa, the Monsanto rhodium-catalyzed synthesis, which is also iodide promoted and which was discovered by Roth and co-workers, can be operated even at normal pressure, though somewhat higher pressures are used in the production units.4,1-413 The rhodium-catalyzed process gives a methanol conversion to acetic acid of 99%, against 90% for the cobalt reaction. The mechanism of the Monsanto process has been studied by Forster.414 The anionic complex m-[RhI2(CO)2]- (95) initiates the catalytic cycle, which is shown in Scheme 26. 

Industrially the straight chain isomer is generally the most desired product and hence the normal/iso product ratio obtained for a given catalyst is of importance. Further, the hydrogenation activities of catalysts vary considerably such that alcohols can in some cases be obtained in a single step (222). The first catalysts developed for this reaction were based on cobalt carbonyl and later cobalt carbonyl phosphine complexes. However, more recently attention has been focused on the intrinsically much more active rhodium catalysts (222, 223). A simplified mechanism for (223) cobalt- and rhodium-catalyzed hydroformylation has been proposed which involves the following steps ... 

The mechanism of the reaction is as shown in equations (13.139) and (13.140). This reaction is also catalyzed by compounds of other metals of groups 8 and 9 such as ruthenium and iridium. Higher alcohols EtOH, Pr"OH, Pr OH also undergo carbonylation to give corresponding carboxylic acids.However, the rate of the reaction is lower. It is assumed that in this case, the oxidative addition of alkyl iodide to the rhodium(I) complex proceeds according to a radical mechanism. Hydrocarboalkoxylation, carbonylation of esters, reductive carbonylation of... 

The mechanism of the cobalt- (BASF), rhodium- (Monsanto), and iridium- (Cativa) catalyzed reaction is similar but the rate-determining steps differ and different intermediate catalyst complexes are involved. In all three processes two catalytic cycles occur. One cycle involves the metal carbonyl catalyst (II) and the other the iodide promoter (i). For a better overview only the catalytic cycle of the rhodium-catalyzed Monsanto process is presented in detail (Figure 6.15.4). Initially the rhodium iodide complex is activated with carbon monoxide by forming the catalytic active [Rhi2(CO)2] complex 4. Further the four-coordinated 16-electron complex 4 reacts in the rate-determining step with methyl iodide by oxidative addition to form the six-coordinated 18-electron transition methyl rhodium (I II)... 

Kinetic studies have allowed the optimum conditions to be defined for the synthesis of acetic anhydride by the carbonylation of methyl acetate using a variety of Group VIII metal catalysts. Such studies, complemented by IR and UV spectroscopic studies, have helped to elucidate the main catalytic pathways for the rhodium- and iridium-catalyzed reactions in the presence of iodide. Although complex, both mechanisms essentially involve oxidative addition of Mel to [M(CO)2l2] (M = Rh or Ir) followed by CO insertion into the metal-methyl bond and subsequent reductive elimination of MeCOI the latter reacts with acetate ion to give acetic anhydride and regenerate iodide. ... 

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