Carbonyl cycle

The influence of steric effects on the rates of oxidative addition to Rh(I) and migratory CO insertion on Rh(III) was probed in a study of the reactivity of a series of [Rh(CO)(a-diimine)I] complexes with Mel (Scheme 9) [46]. For a-diimine ligands of low steric bulk (e.g. bpy, L1, L4, L5) fast oxidative addition of Mel was observed (103-104 times faster than [Rh(CO)2l2] ) and stable Rh(III) methyl complexes resulted. For more bulky a-diimine ligands (e.g. L2, L3, L6) containing ortho-alkyl groups on the N-aryl substituents, oxidative addition is inhibited but methyl migration is promoted, leading to Rh(III) acetyl products. The results obtained from this model system demonstrate that steric effects can be used to tune the relative rates of two key steps in the 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. 

Figure 4. Absorption spectra showing the "carbonyl cycling behavior of the dioxygen adduct [Cu 2(XYL-0-)(02 9, reference 64.

Spectroscopic Studies of Model Reaction Steps of the Rh Carbonylation Cycle I 205... 

Spectroscopic Studies of the Model Reaction Steps of the Ir Carbonylation Cycle 209 5.6... 

The result of these studies has been to show how the differences between these apparently very similar processes arise. In the Rh catalysed carbonylation of MeOH to AcOH, it is the control of [HI] which determines how much of the catalyst is present in the active form as well as the relative rate of the competing water gas shift cycle and it is the property of HI as an acid, which is important. In the Ir catalysed carbonylation of MeOH to AcOH, it is again the control of [HI] which is important, not so much because of the shift between active and inactive forms of the catalyst as with Rh but because of the inhibition of the carbonylation cycle by F and thus because of the property of HI as an iodide rather than as an acid. 

While the main carbonylation cycles are now understood in considerable detail for these apparently simple catalytic systems, there will undoubtedly be considerably more work done on these and related sytems to understand the factors influencing the principal steps of oxidative addition, migratory insertion and reductive elimination and, in particular, further work to understand the unwanted reactions that lead to by-products. 

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... 

A simplified mechanistic scheme is shown in Fig. 22-9. Note that the process is the result of two cycles coupled to each other (a) the iodide cycle, which converts methanol to iodomethane and acetyliodide to acetic acid, and (b) the main carbonylation cycle. The species [Rh(CO)2I2] is actually connected to a third cycle, the water gas shift, which removes water from the system to give C02 and H2 it also ties up some of the catalyst as [Rh(CO)2I4] . 

Figure 22-10 Free energy profile of part of the Rh catalyzed methanol carbonylation cycle derived from kinetic data for the interconversion of A, B, and C (cf. Fig. 22-9) at 35°C in CH2Cl2-MeI. All three species are in equilibrium with each other (adapted from P. M. Maitlis et al., J. Chem. Soc., Dalton Trans. 1996, 2187).

The resting state of the iridium catalyst is the anionic methyl complex, [Ir(CO)2l3Me], which is rapidly formed by oxidative addition of Mel to [I CO U]-- The complex is isolated as its cisfac isomer, and an X-ray crystal structure has been determined [144], Stoichiometric carbonylation of this species (Equation (13)) is regarded as the rate-determining step of the catalytic carbonylation cycle. 

On the left hand of Scheme 2 is shown the catalytic cycle to produce a-keto amide (Cycle 1), whereas the right-hand catalytic cycle shows the route to amide (Cycle II). The process common to both processes is oxidative addition of aryl halide to give arylpalladium halide. Further CO coordination to the arylpalladium intermediate gives a CO-coordinated complex. If CO insertion into the aryl-palladium bond takes place, an acylpalladium complex is produced to drive the double carbonylation cycle. Further coordination of CO followed by attack of amine on the carbonyl ligand produces the aroyl(carbamoyl)palladium species as the bis-acyl-type intermediate. Reductive elimination of the a-keto amide by combination of the benzoyl ligand with the carbamoyl ligand regenerates the Pd(0) species that carries the catalytic cycle. 

Water gas shift reactions compete with both carbonylation cycles (cycles C and D). In essence this is competition between CH3I and HI for oxidative... 

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. 

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