Comparison Among Different Types of Catalysis

The two most important characteristics of a catalyst are its activity, expressed in terms of turnover number (TON) or turnover frequency (TOF), and selectivity. The TON is the number of product molecules produced per molecule of the catalyst. The TOF is the TON per unit 

Selectivity could be of different types such as chemoselectivity, regioselectivity, diastereoselectivity, andenantioselectivity. Reactions 1.4.1-1.4.4 are representative examples where homogeneous catalysts are used. In all these reactions, the possibility of forming more than one product exists. 

In reaction 1.4.1, a mixture of normal and isobutyraldehyde rather than propane, the hydrogenation product from propylene, is formed. This is an example of chemoselectivity. Furthermore, under optimal conditions using an Rh-based homogeneous catalyst, n-butyraldehyde may be obtained with more than 95% selectivity. This is an example of regioselectivity. 

Similarly, in reaction 1.4.2, the alkene rather than the alcohol func-tionahty of the allyl alcohol is selectively oxidized. However, the epoxide product is a mixture of two enantiomers. In reaction 1.4.3, only one enantiomer of the epoxide is formed. This is an example of an enan-tioselective reaction. 

In reaction 1.4.4, a mixture of four diastereomers is formed. If one of the enantiomeric pair, let us say the first two enantiomers on the left, are selectively produced, then the reaction is diasteroselecive. Notice that in this reaction if a chiral catalyst is used, two enantiomers may not be produced in equal quantities. In other words, in such a situation both enantioselectivity and diastereoselectivity have to be measured. 

As indicated by step 1.7, there are a number of small-volume but value-added fine chemicals, intermediates, and pharmaceuticals, where homogeneous catalytic reactions play a very important role. Some of these products, listed in Table 1.1, are optically active, and for these homogeneous catalysts exhibit almost enzymelike stereoselectivities. Asymmetric or stereoselective homogeneous catalytic reactions are discussed in Chapter 9. 

Another important aspect of any catalytic process is the ease with which the products could be separated from the catalyst. For heterogeneous catalysts this is not a problem, since a solid catalyst is easily separated from liquid products by filtration or decantation. In some of the homogeneous catalytic processes, catalyst recovery is a serious problem. This is particularly so when an expensive metal like rhodium or platinum is involved. In general, catalyst recovery in homogeneous catalytic processes requires careful consideration. 

Finally, for an overall perspective on catalysis of all types, here are a few words about biochemical catalysts, namely, enzymes. In terms of activity, selectivity, and scope, enzymes score very high. A large number of reactions are catalyzed very efficiently, and the selectivity is high. For chiral products enzymes routinely give 100% enantioselectivity. However, large-scale application of enzyme catalysis in the near future is unlikely for many reasons. Isolation of a reasonable quantity of pure enzyme is often very difficult and expensive. Most enzymes are fragile and have poor thermal stability. Separation of the enzyme after the reaction is also a difficult problem. However, in the near future, catalytic processes based on thermostable enzymes may be adopted for selected products. 

---