Mechanisms and Catalytic Cycles

The acetic acid-forming part of the catalytic cycle for methanol carbonylation consists of reactions between acetyl iodide and water to give acetic acid and HI (Fig. 4.2, bottom left). The hydroiodic acid reacts with methanol to regenerate CH3I and water. A similar mechanism operates for the carbonylation of methyl acetate. Acetic acid and acetyl iodide react to give acetic anhydride and HI. The latter reacts with methyl acetate to regenerate acetic acid and methyl iodide. These reactions are shown in Fig. 4.9 by the large, left-hand-side loop. 

In addition to this, there is another lithium salt promoted pathway (Fig. 4.9) that contributes signihcantly to product formation. Here the product-forming reaction between lithium acetate and acetyl iodide is followed by the reaction between Lil and methyl acetate. These reactions are shown by the inner loop on the left-hand side. In fact, the inner loop is the dominant product-forming pathway, and lithium salts play a crucial role in the overall catalysis. Note that the right-hand-side loop of the catalytic cycle is exactly the same as in Fig. 4.1(a). 

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Figure 6.5 Mechanism and catalytic cycle for propylene polymerization with a model metallocene catalyst. Conversion of 6.16 to 6.17 and 6.19 to 6.20 involve insertion of (n + 1) propylene molecules.

Figure 21.1 Mechanism and catalytic cycle of flavin-containing BVMOs.

Ligand association, dissociation, and substitution processes are facile, so the exact number of ligands on a metal center is usually not a major concern when mechanisms and catalytic cycles involving transition metals are drawn. 

In addition, to demonstrate the reaction principle for a wider range of applications including in particular asymmetric catalysis, future efforts will be directed toward the utilization of the chemical characteristics of the system in synthetic procedures. The carbonic acid equilibria (Scheme 1) provide many opportunities to interact directly with substrates, products, or catalytically active intermediates as, for example, demonstrated in Scheme 2. Up to now, the main focus has been to avoid detrimental interference of such processes in the reactions under scrutiny. As our imderstanding of the mechanisms and catalytic cycles in this medium increases, it should, however, become more and more possible to take advantage of the CO2/H2O reactivity similarly to the interaction of SCCO2 with secondary amines [47, 48],... 

The book targets scientists who are active in molecular catalysis both in aca-demic/research institutions and industry. It focuses on a deeper understanding of reaction mechanisms and catalytic cycles at the molecular level. This level of insight is required for a semi-rational development of novel catalysts for many key transformations. It also provides the basis for rational improvements of existing catalysts in order to boost their performance. 

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