Ethylene metathesis reaction, transition-metal

Enyne metathesis is unique and interesting in synthetic organic chemistry. Since it is difficult to control intermolecular enyne metathesis, this reaction is used as intramolecular enyne metathesis. There are two types of enyne metathesis one is caused by [2+2] cycloaddition of a multiple bond and transition metal carbene complex, and the other is an oxidative cyclization reaction caused by low-valent transition metals. In these cases, the alkyli-dene part migrates from alkene to alkyne carbon. Thus, this reaction is called an alkylidene migration reaction or a skeletal reorganization reaction. Many cyclized products having a diene moiety were obtained using intramolecular enyne metathesis. Very recently, intermolecular enyne metathesis has been developed between alkyne and ethylene as novel diene synthesis. 

Olefin metathesis is a useful reaction for the production of propylene from ethylene and butenes using certain transition-metal compound catalysts. The two main equilibrium reactions that take place simultaneously are metathesis and isomerization. Metathesis transforms the carbon-carbon double bond, a functional group that is unreactive toward many reagents that react with many other functional groups. New carbon-carbon double bonds are formed at or near room temperature even in aqueous media from starting materials. Because olefin metathesis is a reversible reaction, propylene can be produced from ethylene and butene-2. Metathesis can be added to steam crackers to enhance the production of propylene by the transformation of ethylene and the cracking of mixed butenes. Fig. 3 shows a schematic flow diagram of a typical metathesis process. Examples of metathesis... 

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. 

This same transformation was subsequently duplicated at lower temperatures by homogeneous transition-metal catalysis. An equilibrium is established, and the same mixture is obtained regardless of whether propene or a 1 1 mixture of ethylene and 2-butene is subjected to the reaction conditions. This type of olefin metathesis is called a crossmetathesis. 

Kissin, V. V, in Proc. Symp. on Transition Metal Catalysed polymerization the Ziegler-Natta and Metathesis Polymerization, Cambridge University Press, New York, 1988. Alderson, T., Ruthenium chloride as catalyst in ethylene dimerization reaction,/. Amer Chem. Soc., 87, 5638, 1965. 

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