Homogeneous catalysis alkene olefin and alkyne metathesis

3 Homogeneous catalysis alkene (olefin) and alkyne metathesis 

The catalysts that have played a dominant role in the development of this area of chemistry are those designed by Schrock (e.g. catalysts 27.1 and 27.2) and Grubbs (catalysts 27.3 and 27.4). Catalyst 27.3 is the traditional Grubbs catalyst , and related complexes are also used. The more recently developed second-generation catalyst 27.4 exhibits higher catalytic activities in alkene metathesis reactions. Catalysts 27.1, 27.3 and 27.4 are commercially available. 

A catenate is a related molecule that contains a coordinated 

Topologically, the chemical assembly of a catenand is nontrivial because it requires one molecular chain to be threaded through another. Molecule 27.5 contains two terminal alkene functionalities and can also act as a bidentate ligand by using the A. A -donor set. 

Since nature depends on FeMo-nitrogenase, complexes containing these metals are of particular interest in terms of investigating N2 to NH3 conversion. Complexes of type 27.6 have been a starting point for a number of studies involving intermediates such as 27.7 and 27.8. However, such interconversions produce only moderate 3rields of NH3 when 27.8 is protonated. 

If the whole cycle in Fig. 25.2 is cxmsidered with species in balanced against species out , the net reaction is reaction 25.1. 

A reaction is not usually catalysed by a unique species and a number of criteria must be considered when choosing the most effective catalyst, especially for a commercial process. Moreover, altering a catalyst in an industrial plant already in operation may be costly (e.g. a new plant design may be required) and the change must be guaranteed to be financially viable. Apart from the changes in reaction conditions that the use of a catalyst may bring about (e.g. pressure and temperature), other factors that must be considered are  

The catalytic turnover number (TON) is the number of moles of product per mole of catalyst. This number indicates the number of catalytic cycles for a given process, e.g. after 2 h, the TON was 2400. 

Defining the catalytic turnover number and frequency is not without problems. For example, if there is more than one product, one should distinguish between values of the total TON and TOF for all the catalytic products, and specific values for individual products. The term catalytic turnover number is usually used for batch processes, whereas catalytic turnover frequency is usually applied to continuous processes (flow reactors). 

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