Catalysis, mechanistic information from

Studies of Lewis acid catalysis have been carried out with kinetically labile metal ions (Cu(II), Ni(II), Zn(II) etc.) and with kinetically inert metal centres such as cobalt(III). The use of inert cobalt(III) complexes in kinetic studies presents many advantages as the metal complex is well defined and it is often possible to obtain more detailed mechanistic information from such systems. 

Studies of inelastic scattering are of considerable interest in heterogeneous catalysis. The degree to which molecules are scattered specularly gives information about their residence time on the surface. Often new chemical species appear, whose trajectory from the surface correlates to some degree with that of the incident beam of molecules. The study of such reactive scattering gives mechanistic information about surface reactions. 

The plot eepjod = f(eeaux) in an enantioselective reaction is a simple operation that can sometimes be very informative from both synthetic and mechanistic perspectives. This plot is now widely used in mechanistic discussions concerning enantioselective catalysis. However, some cautions are needed, since this approach has to be combined with additional studies in order to get firm conclusions. If linearity is observed, one cannot reach conclusions on the mechanism, since even with species involving several chiral auxiliaries one may remain linear, as in the ML2 model with g = 1 in Eq. (7.3). If there are deviations from linearity, this could be a piece of information on the mechanism, for example, aggregations at some level of the chemical system, or some competitive mechanisms. References 27, 44, and 68 are only three among many published examples. 

Water solubility of all the aforementioned catalysts was due to the hydrophilic nature of ligand. In a second class, the aqueous nature originates from direct interaction of water molecules with the metal center [61]. Although this area of aqueous catalysis is not as extensive, there are several representative examples illustrating its importance and potential, as well as the variation of the metal center. One of the earlier examples involved the hydrogenation of maleic and fumaric acids with [RhCl (H20)6 ]3-" and [RuCl (H20)6 n]3" [62]. These simple catalysts revealed key mechanistic information that is applicable to many other systems, such as the requirement of alkene complexation prior to H2 activation. After this important discovery, many advances have been made in the areas of hydrogenation and polymerization reactions using these types of catalysts. 

For designing enzyme-mimicking catalysts exploiting metal ions as catalytic centers, it is necessary to understand catalytic repertories (32, 33) of metal ions acting as Lewis acid catalysts in the hydrolysis of peptide bonds and related carboxyl derivatives (e.g., esters). Although esters are much easier to hydrolyze than peptides, important mechanistic information for catalysis in peptide hydrolysis can be obtained from that in ester hydrolysis. 

This volume starts with three chapters that illustrate the wealth of information which can now be obtained in some of the traditional application fields of EPR spectroscopy. Chapter 1 by S. Van Doorslaer and D. Murphy is an in-depth review on work in catalysis focusing on the mechanistic information that can be obtained from EPR spectra. Work on radical enzymes is exemplified in Chapter 2 by... 

Finally, the body of mechanistic information that has been obtained from studies on homogeneous catalysis has helped provide the knowledge base to help interpret the data obtained on heterogeneous catalysts, normally far less tractable mechanistically. 

Initially, kinetics became a major tool for deducing mechanistic information. The relation between the mechanism of a reaction and kinetics forms an important subject of this book. Rate equations such as (1.1) follow from a postulated mechanism. However, the fact that a mechanism may be able to explain kinetic experiments by no means implies that this particular mechanism is necessarily correct. It appears that many different mechanisms may result in similar kinetic behavior. Thus, additional information on the reacting species on the surface of the catalyst is necessary to reach a definitive understanding of catalysis in relation to the structure and composition of the catalyst. 

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