Survey of catalyst systems

The most important catalyst systems are derived from compounds of the nine transition elements shown in upright type in the table below those shown in bold type are generally the most effective. Catalysis by compounds of non-transition elements is very much the exception. Rare examples appear to be EtAlCl2 (Ivin 1978c), Me4Sn/Al203 (Ahn, H-G. 1992) and MgCl2 (Buchacher 1996). 

Half-lives for olefin metathesis can vary from a few seconds to several days. The activity of a particular catalyst system is dependent on a number of factors 

The activity of a given catalyst system will be determined by both the nature and the concentration of the active species. In Ch. 3 we present the evidence that the active species is a metal carbene complex, formed in some way from the transition metal compound. It is useful to keep this in mind when reviewing the different types of catalyst system. 

Actual metal carbene complex catalysts can be divided into two broad classes, Fischer-type and Schrock-type . The Fischer-type carbene complexes are low-valent and generally characterized by the presence of one or two heteroatoms (O, N, or S) bonded to the carbene carbon. Such complexes do not normally initiate the chain metathesis of olefins, since they are both coordinatively and electronically (18e) saturated. However, they can sometimes be activated for metathesis by heating, or by reaction with a cocatalyst, or photochemically. Some examples are listed in Table 2.1. 

Some very active molybdenum carbene initiators are listed in Table 2.2. A very convenient way of generating the highly active complex Mo(=CHCMe3) (=NAr)(OR)2(Me3CNH2) in situ is to react the very robust complex Mo(=NAr) (=NCMe3)(CH2CMe3)2 with ROH in the presence of the olefinic substrate (Bell, A. 1994a). 

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