Homogeneous Molecular Catalysis

Catalysis takes place when a substantial current is obtained at a potential that is more positive than the direct electrochemical reduction (less positive 

FIGURE 4.2. Redox and chemical homogeneous catalysis of electrochemical reactions. 

The very fact that chemical catalysis involves the formation of an adduct opens up possibilities of selectivity, particularly stereoselectivity, that are absent in redox catalysis. Several examples of homogeneous chemical catalysis are described in the following section, illustrating the improvements that can be achieved when passing from redox to chemical catalysis. It remains true that redox catalysis has several useful applications that have already been discussed, such as kinetic characterization of fast follow-up reactions (Section 2.3) and determination of the redox properties of transient radicals (Section 2.6.4). 

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Molecular catalysis. The term molecular catalysis is used for catalytic systems where identical molecular species are the catalytic entity, like the molybdenum complex in Figure 8.1, and also large molecules such as enzymes. Many molecular catalysts are used as homogeneous catalysts (see (5) below), but can also be used in multiphase (heterogeneous) systems, such as those involving attachment of molecular entities to polymers. 

Enzyme catalysis. Enzymes are proteins, polymers of amino acids, which catalyze reactions in living organisms-biochemical and biological reactions. The systems involved may be colloidal-that is, between homogeneous and heterogeneous. Some enzymes are very specific in catalyzing a particular reaction (e.g., the enzyme sucrase catalyzes the inversion of sucrose). Enzyme catalysis is usually molecular catalysis. Since enzyme catalysis is involved in many biochemical reactions, we treat it separately in Chapter 10. 

Chapters 4 and 5 are devoted to molecular and biomolecular catalysis of electrochemical reactions. As discussed earlier, molecular electrochemistry deals with transforming molecules by electrochemical means. With molecular catalysis of electrochemical reactions, we address the converse aspect of molecular electrochemistry how to use molecules to produce better electrochemistry. It is first important to distinguish redox catalysis from chemical catalysis. In the first case, the catalytic effect stems from the three-dimensional dispersion of the mediator (catalyst), which merely shuttles the electrons between the electrode and the reactant. In chemical catalysis, there is a more intimate interaction between the active form of the catalyst and the reactant. The differences between the two types of catalysis are illustrated by examples of homogeneous systems in which not only the rapidity of the catalytic process, but also the selectivity problems, are discussed. 

The chapters in this volume illustrate how molecular concepts underlie catalysis. They illustrate how modern concepts of biology are influencing catalysis and catalyst discovery how concepts of homogeneous and surface catalysis have merged (a theme that is evident in the preceding several volumes of the Advances), exemplified by dendrimer catalysts that have properties of both molecules and surfaces and how concepts of molecular catalysis by bases have influenced the development of new solid-base catalysts and fundamental understanding of how they function. 

Sippola, V. O. and Krause, A. O. I., Oxidation activity and stability of homogeneous cobalt-sulphosalen catalyst -Studies with a phenolic and a non-phenolic lignin model compound in aqueous alkaline medium. J Molecular Catalysis A-Chemical 2003, 194 (1-2), 89-97. 

Over the last 15 years, the homogeneous studies of HDS and HDN processes have been extremely useful to understand many mechanistic details regarding the coordination of sulfur and nitrogen heterocycles to metal centers, hydrogen transfer from metal to coordinated heterocycle, metal insertion into C-S and C-N bonds, and the desulfurization/denitrogenation paths. Recently, however, there has been a qualitative leap in molecular catalysis so that crossing the border-... 

The history of heterogeneous enantioselective catalysis with chiral modification of the metal surface extends back even further than that of homogeneous molecular metal catalysis. However, successful precedents which result in a practicably useful stereoselectivity (e.g. of over 80%) involve only three types, all of which involve the hydrogenation of unsaturated bonds. Initially, these reactions were realized by achieving the correct solution to address all requirements for the chiral modifier. That is, the adsorption of the modifier must occur on all of the active... 

With our deeper understanding of the elementary processes of organotransition metal complexes and consequent advancement in clarification of mechanisms in homogeneous catalysis, we can now describe most such reactions in terms of catalytic cycles consisting of known elementary processes. The term molecular catalysis is appropriate to describe such catalysis. 

A distinct advantage of homogeneous catalysis over conventional heterogeneous catalysis is that it allows detailed clarification of the reaction mechanisms at the molecular level by catalytic cycles consisting of elementary processes. Thus the term molecular catalysis aptly describes the characteristics of homogeneous catalysis. The recent impressive advancement in catalytic asymmetric synthesis... 

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