Alkene olefin metathesis

Exchange of alkylidene groups of alkenes—metathesis of olefins... 

Two of the most characteristic reactions of carbene complexes are olefin metathesis and olefin cyclopropanation. Olefin metathesis is a reaction in which the C-C double bond of an alkene is cleaved, and one of the resulting alkylidene fragments combines with the metal-bound carbene to form a new alkene. The second alkylidene fragment forms a new carbene eomplex with the metal. Olefin cyclopropanation is a reaction in which a a bond is formed between the metal-bound alkylidene and each of the two carbon atoms of the alkene, to yield a cyclopropane. 

The most relevant catalytic reactions approached by SOMC are olefin polymerization (and depolymerization), alkane activation (including a new reaction, discovered thanks to SOMC-alkane metathesis), alkene metathesis and epoxidation. All these reactions are discussed in this chapter. 

In the metathesis reaction, alkenes are converted to mixtures of lower and higher alkenes through the interchange of groups on the double bonds in an equilibrium process. The number of moles of product lighter than the feed is roughly the same as the number of moles of heavier alkenes. Olefin metathesis is a thermoneutral reaction and produces an equilibrium mixture of products. In simple cases this mixture is close to the statistical distribution of substituent groups. 

The term alkene (olefin) metathesis refers to the equilibrium reaction shown in equation (1) in which the alkylidene groups of a pair of alkenes are exchanged with one another in the presence of a transition metal-containing catalyst. The reaction involves the net cleavage of the bonds of the substrate(s) and formation of the new carbon-carbon double bonds of the prodncts. Once equilibrium has been established, the resultant prodnct mixture has a distribution of alkenes (including isomers) that is determined solely by the relative thermodynamic stabilities of the prodncts. 

Metathesis of Alkenes (Alkene or Olefin Metathesis) Alkene metathesis... 

Evidence for the migratory insertion of ethylene [121], vinylsilane [122], and styrene [123] into the Ru-Si bond (yielding vinylsilane and two (1,2- and 1,1-bis(silyl)ethene) regioisomers, respectively) showed that in the reaction first reported in 1984 as the metathesis (disproportionation) of vinylsilanes and their co-metathesis with olefins [124], instead of the C=C bond cleavage (formally characterizing alkene metathesis (eq. (8)), a new type of olefin conversion was revealed - silylative coupling of olefins with vinylsilanes. 

Carbonyl compounds the McMurry reaction Alkenes olefin metathesis Stereoselective Methods for E-Alkenes... 

Structures 25.55 (Grubbs catalyst) and 25.56 (a complex developed by Schrock) show two important carbene compounds that are used as catalysts in alkene olefin) metathesis, i.e. metal-catalysed reactions in which C=C bonds are redistributed. Examples include ring-opening metathesis polymerization (ROMP) and ring-closing metathesis (RCM). We look at these reactions in detail in Section 26.3. 

Chapter 26 Homogeneous catalysis alkene (olefin) metathesis 789... 

Fig. 26.3 Examples of alkene (olefin) metathesis reactions with their usual abbreviations.

The catalytic activity of various complex compounds of technetium was tested in the metathesis of olefins [17], epoxide ring opening reactions [18], epoxide ring formation by reaction of cyclohexene with fe/7-butylhydroperoxide [19,20], and the preferred production of tram-epoxides in the liquid phase oxidation of cw/tram-n-alkenes [21 j. 

Alkylidenes (and alkylidynes) can react with alkenes (or alkynes) to produce metallacyclobutanes (or metallacy-clobutenes), as shown in Equation (12). This is the key first step in alkene (olefin) metathesis. 

As with other transition metal-catalyzed reactions (Ziegler-Natta polymerization of alkenes, olefin metathesis), the mechanism of the Heck reaction is complicated. In brief, the species that reacts with the aryl halide is I Pd, where L is a ligand such as tiiphenylphosphine. By a process known as oxidative addition, palladium inserts into the carbon-halogen bond of the aryl halide. 

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