Examples of Catalysis

There are many important examples of catalysis in the Hquid phase, but catalysis in the gas phase is unusual. From an engineering viewpoint, most of the hquid-phase processes have the foUowing characteristics in common. 

There are only a few weU-documented examples of catalysis by metal clusters, and not many are to be expected as most metal clusters are fragile and fragment to give metal complexes or aggregate to give metal under reaction conditions (39). However, the metal carbonyl clusters are conceptually important because they form a bridge between catalysts commonly used in solution, ie, transition-metal complexes with single metal atoms, and catalysts commonly used on surfaces, ie, small metal particles or clusters. 

This chapter focuses on heterogeneous catalysis, which is most important in fine chemicals production. Table 3.1 presents a number of examples of catalysis in fine chemistry. These examples are divided in heterogeneously catalysed processes and homogeneously catalysed processes. A detailed treatment of heterogeneously catalysed processes for the production of fine chemicals is also given in the book edited by Sheldon and van Bekkum (2001). 

Zinc catalysts have shown a major increase in their possible applications. Some examples of catalysis using halide complexes, including immobilized catalytic systems, have been discussed in Section 6.8.8. Some areas such as asymmetric alkynylation of aldehydes have been recently reviewed.930... 

The most well-developed recent examples of catalysis concern catalysts for oxidation reactions these are essentially achiral or chiral metal-salen complexes. Taking into account a number of results suggesting the importance of a degree of mobility of the bound complex, Sherrington et al. synthesized a series of polymer-supported complexes in which [Mn(salen)Cl] units are immobilized in a pendant fashion by only one of the aromatic rings, to polystyrene or poly(methacrylate) resin beads of various morphology (Figure 6).78,79... 

While palladium, ruthenium, and rhodium are the most common metal catalysts used to facilitate Alder-ene cyclization, a few successful examples of catalysis using different metals have been published. Both of the references reviewed in this section demonstrate chemistry that is novel and complimentary to the patterns of reactivity exhibited by late transition metals in the Alder-ene cyclization. 

In his pioneering paper, Kurz (1963) considered examples of catalysis by protons, water, hydroxide ion, and, to a limited extent, general acids and... 

Since the hydrogen iodide so formed is immediately reconverted by chlorine into iodine, iodine monochloride being then re-formed, we have here a good example of catalysis by a carrier readily interpreted from the chemical standpoint. 

At this point, we can integrate much of what has been discussed above in a single case study. Antibody NPN43C9 was reported in 1988 as the first example of catalysis of hydrolysis of an amide bond, in fact of an active anilide. Its structure and mode of action have been well studied (Janda et al., 1988b), which makes it an appropriate example for this purpose. 

Examples of catalysis involving concerted cyclic proton tremsfer in water have recently been reported in the hydrolysis of N-phenyl-iminotetrahydrofuran (Cunningham and Schmir, 1966, 1967). There is no effect of HjPO, HCO3, and CH3COOH on the rate of hydrolysis, but the type of product obtained, butyrolactone or 7-hydroxybutyranilide is dependent on their concentration. The following scheme was proposed [equation (8)]. 

Control of reaction selectivities with external reagents has been quite difficult. Unsolved problems remaining in the held of nitrile oxide cycloadditions are (a) Nitrile oxide cycloadditions to 1,2-disubstituted alkenes are sluggish, the dipoles undergoing facile dimerization to furoxans in most cases (b) the reactions of nitrile oxides with 1,2-disubstituted alkenes nonregioselective (c) stereo- and regiocontrol of this reaction by use of external reagents are not yet well developed and (d) there are few examples of catalysis by Lewis acids known, as is true for catalyzed enantioselective reactions. 

In a rather loose sense it can be stated that these successive types of reactions impose increasingly stringent requirements on a catalyst. Thus, catalysts which are active in reactions of the last type are normally effective also for the first two types, but the converse of this is not as generally valid. There are, however, in each class of reaction, examples of catalysis by all three types of catalysts (homogeneous, heterogeneous, and enzymic). 

The first example of catalysis by a polymer-metal complex was presented by Lautsch et al. u3 Metalloporphyrin was linked to a poly(phenylalanine) chain by a peptide bond. The catalytic properties of this polymer-Fe(III)porphyrin complex were compared with Fe(III)porphyrin in the oxidative reaction of phenylenedi-amine. The catalytic activity of the polymer complex was twice as large as that for the corresponding analog. 

Aldol products do not have to come from an aldol condensation. In another example of catalysis by a small organic molecule, Jeffrey Bode of UC Santa Barbara reports (J- Am. Chem. Soc. 2004,126, 8126) that the thioazolium salt 7 effects the rearrangement of an epoxy aldehyde such as 6 to the aldol product 8. This is a net oxidation of the aldehyde, and reduction of the epoxide. As epoxy aldehydes such as 6 are readily available by Sharpless asymmetric epoxidation, this should be a general route to enantiomerically-aldol products. The rearrangement also works with an aziridine aldehyde such as 9, to give the ff-amino ester 10. 

Chiral catalysts also generate reactive intermediates by forming covalent bonds with substrates. A very successful example of catalysis... 

This reaction was one of the first examples of catalysis by a supported organometallic compound. In 1964 it was observed that Mo (CO) 6/ A1203, after activation by heating in vacuo at 120°C, catalyzed the conversion of propylene into ethylene and 2-butene (82). The nature of the active site in this catalyst system is still not fully defined (83). Since the initial discovery many heterogeneous and homogeneous catalyst systems have been reported (84, 85), the latter being more amenable to kinetic and mechanistic studies. 

Most examples of catalysis can be readily characterised as homogeneous or heterogeneous but there are examples of catalysis which overlap the two types. Consider a system in which intermediates are formed at the surface and then are desorbed into the gas phase and react there. Such intermediates might generate a chain reaction in the gas phase, i.e., chain initiation and chain termination occur at the surface but chain propagation occurs in the gas phase. 

Ask the average man on the street for an example of catalysis, and you will most likely hear about the thing on the exhaust pipe that reduces engine emissions . As we shall see, the remarkable story of the catalytic converter is not only an excellent example of catalysis, but also highlights the connection between heterogeneous catalysis, green chemistry, and sustainable development. 

Yet another example of catalysis by cluster Grignard reagents is isomerization of alkyl halides. It turned out that, in the presence of these reagents, n-chloroalkanes undergo partial isomerization to produce a mixture of secondary chloroalkanes. The catalytic turnover number is as large as 20. 

Another example of catalysis over a bifunctional heteropolyacid is the direct synthesis of acetic acid from ethene. 

Other examples of catalysis by metal ions refer to hydrolytic reactions. In phenanthro line-1-nitrile, the hydration of the nitrile group (to form the amide group) is catalyzed by metal ions (Cu2+, Ni2+, Zn2+) which form complexes with the substrate [270]. The rate equation is... 

In the examples of catalysis that follow, the reader will find it useful to identify the catalysts, the species regenerated in each complete reaction cycle. In addition, the individual steps in these cycles will provide examples of the various types of organometallic reactions introduced earlier in this chapter. In each case, the proposed mechanisms presented in this section should be viewed as subject to modification as additional research is conducted. 

The iodide-catalyzed decomposition of hydrogen peroxide is an example of catalysis that does not involve a surface. It probably works by the following mechanism. 

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