Fundamental Concepts of Homogeneous Catalysis

The laws of thermodynamics dictate that the product distribution resulting from a catalyzed or uncatalyzed reaction must be the same if enough time is allowed for the transformation to come to equilibrium. The presence of a catalyst, however, can influence initial product distributions, allowing preferential formation of a product that may be less stable thermodynamically than another. This 

2For more information on TON and TOF, see G. P. Chiusoli and P. M. Maitlis, Eds., Metal Catalysis in Industrial Organic Processes, RSC Publishing Cambridge, 2006, p. 270. 

S-C Substrate-Catalyst Complex P-C Product-Catalyst Complex 

Reaction Progress versus Energy Diagrams for a Catalyzed and Uncatalyzed Reaction 

The hydroformylation reaction (Section 9-2), shown in equation 9.5, demonstrates regioselectivity, whereby one regioisomer forms preferentially over another. The Co catalyst may be modified to increase the proportion of linear (anti-Markovnikov) over branched (Markovnikov) product. 

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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. 

The chemistry of organotransition metal complexes [1] has progressed in step with the development of homogeneous catalysis [2], each influencing the other. In certain cases, study of chemistry of such complexes has been motivated by the wish to understand the mechanisms of important catalytic processes that were already developed and to improve their performance. On the other hand, examination of the chemical properties of a particular type of organotransition metal complex has sometimes led to discoveries of hitherto unknown fundamental reactions. Combination of the concept of a newly found elementary process with a known process will continue to lead to discoveries of novel catalytic processes and emich the scope of organic synthesis... 

Another concept that is fundamental to an understanding of homogeneous catalysis is that of the oxidation state of a complex. Although commonly used in general chemistry, a less puristic definition of this state, which may be regarded as a formalism more suited to transition-metal complexes, has become the cornerstone of homogeneous catalysis. 

In this chapter, we discuss theoretical studies of some selected transition metal-catalyzed reactions of carbon dioxide to illustrate how important concepts and insights can be derived as a result of these studies. These selected reactions include hydrogenation of CO2 with Hj, coupling reactions of COj and epoxides, reduction of CO2 with organoborons, carboxylation of olefins with COj, and hydrocarboxy-lation of olefins with CO2 and Hj. They are fundamentally important reactions of carbon dioxide and have been intensively investigated experimentally and theoretically. This chapter is not intended to be a comprehensive review. Instead, we discuss the above-mentioned selected examples that we believe to be representative and important in the area of homogeneous catalysis of COj by transition metals from our own perspective. 

In more fundamental studies, the reaction rate should not refer to the mass of catalyst or metal, but to the active site. As the total number of the active sites is not known, it is common practice, as recently discussed by the author et to substitute it with the total number of surface metal atoms, which can be correctly calculated by Eq. (7.71). The reaction rate expressed as the number of reacting molecules transformed per surface metal atom per second is often called as trunover frequency (TOF) and is expressed as sec . TOF is currently used to draw fundamental conclusions about the intrinsic activity of metals and the mechanism of the reaction. Unfortunately, many TOF values have been so frequently affected by large errors in dav, at least for supported metal catalysts, that they require a substantial revision. It is rather obvious that the number of active sites will always be lower than that of surface metal atoms, because either the latter one is not energetically equal in the reaction conditions or the active sites consist of nanoensembles of metal atoms (this point should be taken into account for any comparison with homogeneous or enzymatic catalysis). Hence, it is convenient to introduce the concept of real turnover frequency, given by the TOFr = TOF/fAs, where /as is the fraction of surface atoms working as active sites. 

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