Homogeneous catalysis individual reactions

The basic notions of homogeneous catalysis have been amplified and discussed more extensively elsewhere, as have examinations of individual reaction steps. In briefly reviewing these concepts, we are constructing a framework in which to consider the activation of CO, COz, and NO by metal complexes in solution and homogeneously catalyzed conversions of these simple oxides. 

Usually, the optimization of a given catalyst for a given chemical reaction requires independent synthesis of each individual complex. This means that for typical organometallic complexes in homogeneous catalysis the synthesis is the most time-consuming reaction. 

Because the constitution of the active site of a heterogeneous catalyst is difficult to determine, the elucidation of reaction mechanisms involving these catalysts can be troublesome indeed. The field of homogeneous catalysis, in contrast, has advanced rapidly over the past few decades because chemists have developed many techniques that are useful for studying reaction mechanisms. Elucidating the mechanism of a homogeneously-catalyzed reaction requires studying the mechanism of each individual step, in a series of relatively elementary chemical... 

The concept that a catalyst provides an alternate mechanism for accomplishing a reaction, and that this alternate path is a more rapid one, has been developed in many individual cases. The basis of this idea is that the catalyst and one or more of the reactants form an intermediate complex, a loosely bound compound which is unstable, and that this complex then takes part in subsequent reactions which result in the final products and. the regenerated catalyst. Homogeneous catalysis can frequently be explained in terms of this concept. For example, consider catalysis by acids and bases.. In aqueous solutions acids and bases can increase the rate of hydrolysis of sugars, starches, and esters. The kinetics of the hydrolysis of ethyl acetate catalyzed by hydrochloric acid can be explained by the following mechanism ... 

The basic question that has to be answered for each individual reaction examined is whether or not the products produced are of high enough value to warrant the use of immobilized catalysts. It is likely that the commercial impact of such catalyst systems will be greatest in the pharmaceutical area where the selectivities inherent in homogeneous catalysis are needed, the value of the products is high and the ability to easily separate the catalyst could pay for itself after a limited number of recycles. Impact in this area has not been obvious, probably because the polymer immobilized catalyst systems have not been in the hands of the typical synthetic organic chemist. 

As we have already seen, catalytic processes generally consist of complicated series of reactions, whereby the activation of individual steps can place different demands on the catalyst. Ugo has classified the homogeneous catalysis of organic reactions on the basis of the HSAB concept [21]. 

In homogeneous catalysis, stoichiometric model reactions with well-defined transition metal complexes can be used to elucidate individual steps of the catalytic cycle. Other methods for testing the validity of an assumed reaction mechanism are the use of labelled compounds and the spectroscopic identification of intermediates [13]. An advantage of such investigations is that they can generally be carried out imder mild conditions, for example, standard pressure and low temperatures. 

Carbon dioxide is a ubiquitous and environmentally benign compound. Several attempts have been made to use it as medium or as support for individual steps of the hydroformylation. The acidic properties of CO2 in solution have been advantageously employed to remove homogeneous catalysts with basic properties from the neutral organic reaction products (see Section A Posteriori Separation of Products and Catalysts ). BASF claimed supercritical carbon dioxide (SCCO2) for the extraction of the so-called heavy ends from low-boiling hydroformylation products and rhodium catalyst [55]. In recent years, also implementations of reactions in compressed carbon dioxide in supercritical or near-critical conditions have attracted particular attention [56]. A first review on the application of supercritical fluids (SFCs) in homogeneous catalysis was authored by Jessop, Ikariya, and Noyori in 1999 [57]. Later on, Leitner and Abraham [58,59] provided updates with special focus on the use of compressed carbon dioxide. 

Central to catalysis is the notion of the catalytic site. It is defined as the catalytic center involved in the reaction steps, and, in Figure 8.1, is the molybdenum atom where the reactions take place. Since all catalytic centers are the same for molecular catalysts, the elementary steps are bimolecular or unimolecular steps with the same rate laws which characterize the homogeneous reactions in Chapter 7. However, if the reaction takes place in solution, the individual rate constants may depend on the nonreactive ligands and the solution composition in addition to temperature. 

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