Hard and Soft Catalysis

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]. 

Friedel-Crafts reactions, oxidation processes, epoxidation, ester hydrolysis 

If the first step of a reaction cycle is regarded as an acid base reaction between the catalyst and the organie substrate, then a distinction can be made between hard and soft catalysis, providing a simple basis for understanding transition metal catalyzed processes (Seheme 2-4). 

Petrochemical catal5d ic reactions are predominantly soft hard catalysis with transition metal ions is less important. 

In the intermediate of higher oxidation state, the metal is harder, and a hard-soft dissymmetry of the hgands favors the irreversible elimination of the product. In this way the key role of oxidative addition and its reverse reaction in the homogeneous catalysis of C-C and C-H coupling reactions can be understood in the simplest case, the interplay of oxidative addition and reduetive elimination ean be represented by a redox reaction (Eq. 2-83). 

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Scheme 2-4 Hard and soft catalysis with transition metal compounds...

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