Cross-metathesis, transition metal-enyne

The general reaction equation for alkene metathesis in a simple system, cross-metathesis of two different disubstituted alkenes, is depicted in Scheme 1. In this reaction, a transition metal catalyst establishes equilibrium between the starting alkenes, the ( )- and (Z)-stereoisomers of all possible substituent combinations, and ethylene. Related reaction processes have also been reported for alkynes (aikyne metathesis) and for combinations of alkenes and alkynes (enyne metathesis). Aikyne metathesis is less well developed compared to alkene metathesis and enyne metathesis. This review has been organized according to the basic modes of metathesis depicted in Scheme 2. Alkene metathesis is the more developed process and numerous examples of all the variants have been reported. Aikyne metathesis is less well developed and three variants exist aikyne cross-metathesis, aikyne metathesis polymerization, and ring-closing aikyne metathesis. 

In the last quarter of the 20th century a new family of C-C forming reactions based on transition metal catalysts evolved as powerful tools in synthesis. Palladium-catalyzed cross-coupling reactions (Heck, Stile, Suzuki) and metathesis of olefins, enynes, and alkynes have had a profound impact on the art of total synthesis. 

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