Stereoselectivity heterogeneously catalyzed

Fig. 17.75. Part II Mechanism of the non-stereoselective heterogeneous Pd-catalyzed hydrogenation of C=C double bonds.

Fig. 3.22. Stereoselectivity and stereospecificity of a pair of heterogeneously catalyzed hydrogenations.

Research activities on heterogeneously-catalyzed stereoselective reductions have generally increased during the past years[l]. Among various functionalities the stereoselective aromatic ring reduction is of importance in the synthesis of fine chemicals such as pharmaceuticals, herbicides, fragrances and liquid crystals. Particularly, substituted phenol derivatives offer the possibility to control the product selectivity via reduction of the prochiral intermediate cyclohexanone [2,3]. 

This chapter presents a comprehensive overview of heterogeneously catalyzed MPVO reactions. It includes, apart from the use of various metal oxides, the more recent application of chemically anchored co-ordination compounds, hydro-talcites, mesoporous materials, and zeolites as recyclable solid catalysts. Some remarkable examples of shape-selective conversions resulting in high stereoselectivities illustrate the progress made in this field. 

I 9 Stereoselective Hfrirolase-Catalyzed Processes in Continuous-Flow Mode Table 9.1 Comparison of homogeneous and heterogeneous continuous-flow systems. 

Enzymes are known for their exquisite shape selectivity and regio- and stereoselectivity with respect to their substrates [16,17], For example, different versions of the cytochrome P-450 enzyme family catalyze the hydroxylation of substrates as diverse as P-naphthylamine and vitamin D [18,19], The specificity of the P-450 enzymes derives from the shape of the substrate-binding cavity, which is contiguous with the active site. (Unfortunately, in heterogeneous catalysis, substrate is often the term used to describe the stratum or support upon which the catalyst is mounted. The analog of the enzymatic substrate in heterogeneous catalysis is called the feed or reactant.)... 

Stereoselectivity in homogeneous catalytic reactions is well documented. Oftentimes mechanisms can be postulated regarding individual steps in contrast to heterogeneous systems. A recent detailed investigation of the stereoselective formation of C-C bonds catalyzed by (C5H5)2ZrCl2 has been reported by Hoveyda and Xu.12 The initial step in these reactions is the addition of an alkyl Grignard reactant to an unactivated olefin followed by further bond formation as in Equation (Eq.) [I] 12... 

As for heterogeneous catalysts, the addition of hydrogen is catalyzed by a large variety of materials, but synthetically useful procedures generally employ nickel or the platinum metals. In the latter case, the best results are obtained if the metal is finely divided over the surface of an inert support. Many materials can be used as catalyst supports, however, carbon or alumina are suitable for the majority of reactions. Calcium and barium carbonate or sulfate are also frequently used if less reactive catalysts are desired. The influence of the support is generally small compared to the effect of the metal3. The choice of metal is especially important when stereoselectivity is desired because different metals can catalyze the formation of different diastereomers upon hydrogenation. 

By analogy with the results obtained with the phenyl-substituted analogues, chemisorption of isobutane might be expected to proceed by hydride ion abstraction to form a tertiary-butyl carbonium ion and it would indeed be convenient to assign the 3000 A band to this ion. Although such alkyl carbonium ions have been useful for the mechanistic description of a variety of reactions of hydrocarbons over heterogeneous acid catalysts, in recent years numerous observations have been reported which apparently are not in accord with this concept. Notable in this respect is the remarkable degree of stereoselectivity associated with the acid-catalyzed double bond isomerization of butenes... 

The hydroesteriflcation and hydrocarboxylation reactions catalyzed by transition metals and their complexes demonstrate the versatility of these processes. Palladium complexes are particularly useful catalysts for these reactions. Indeed, the hydrocarbonylation of a large variety of substrates has been selectively achieved by using palladium catalysts in homogeneous, heterogeneous, or biphasic systems. The results obtained for the thiocar-bonylation showed excellent regio- and stereoselectivity control for most substrates. It is anticipated that the prochiral nature of most of the reactants and products will open a new venue for the asymmetric synthesis of acids, esters, and thioesters of substantial development in the future. 

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