Catalysts for alkene metathesis

Two observations initiated a strong motivation for the preparation of indenylidene-ruthenium complexes via activation of propargyl alcohols and the synthesis of allenylidene-ruthenium intermediates. The first results from the synthesis of the first indenylidene complexes VIII and IX without observation of the expected allenylidene intermediate [42-44] (Schemes 8.7 and 8.8), and the initial evidence that the well-defined complex IX was an efficient catalyst for alkene metathesis reactions [43-44]. The second observation concerned the direct evidence that the well-defined stable allenylidene ruthenium(arene) complex Ib rearranged intramo-lecularly into the indenylidene-ruthenium complex XV via an acid-promoted process [22, 23] (Scheme 8.11) and that the in situ prepared [33] or isolated [34] derivatives XV behaved as efficient catalysts for ROMP and RCM reactions. 

NMR studies of degenerate ligand exchange in generation I and generation II ruthenium alkylidene pro-catalysts for alkene metathesis... 

It is not the aim of this review article to discuss the chemistry of the initiators (or catalysts) for alkene metathesis, but some comments on the initiators used might contribute to our understanding of why this remarkable polymerization technique has not attracted more attention up to now. 

General aspects and new metathesis catalysts. For alkene metathesis Grubbs I (1) and Grubbs II (2, 3) complexes, and the Grubbs-Hoveyda catalyst (4A) and Grela catalyst (4B) remain the workhorses. 

Schrock and Fischer type carbyne tungsten or molybdenum complexes are very interesting catalysts for alkene metathesis or alkyne polymerisation reactions. Within the first reaction steps they form carbene complexes and on these carbene complexes further metathesis or polymerisation occur. 

A related tungsten complex (R 0)2W(=NR)(=CHBu ) is an exceptionally active catalyst for alkene metathesis. Its activity is unchanged over periods of at least 1 day and the turnover is in hundreds of conversions per minute. A trigonal bipyramidal tungstenacyclobutane intermediate is again involved. 

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