Metathesis, alkene olefin reaction

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. 

Since the discovery of ruthenium and molybdenum carbene complexes that efficiently catalyze olefin metathesis under mild reaction conditions and that are compatible with a broad range of functional groups, olefin metathesis has increasingly been used for the preparation of alkenes on insoluble supports. In particular, the ruthenium complexes Cl2(PCy3)2Ru=CHR, developed by Grubbs, show sufficient catalytic activity even in the presence of air and water [781] and are well suited for solid-phase synthesis. 

With the discovery by Grubbs of ruthenium carbene complexes such as Cl2(PCy3)2Ru=CHR, which mediate olefin metathesis under mild reaction conditions and which are compatible with a broad range of functional groups [111], the application of olefin metathesis to solid-phase synthesis became a realistic approach for the preparation of alkenes. Both ring-closing metathesis and cross-metathesis of alkenes and alkynes bound to insoluble supports have been realized (Figure 5.12). 

In the mid to late 1980s, transition-metal catalysts were developed that were particularly useful for carrying out olefin (alkene) metathesis reactions (Rouhi 2002). Metathesis is a reaction in which two molecules containing carbon-carbon double bonds exchange carbon atoms along with any groups attached to them ... 

A similar type of cascade reaction has been carried out with cyclic alkenes bearing only one olefinic side chain to obtain substituted heterocycles via ruthenium-catalyzed ring closing-ring opening metathesis (RCM-ROM) reactions. The preparation of enantiomerically pure cis- or trans-a,a -disubstituted piperidines has been achieved in the same yield for the two diastereoisomers [35] (Scheme 17). This reaction has also been used as a key step for the synthesis of natural products [36-39]. 

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. 

Alkene metathesis, more commonly called olefin metathesis, is a reaction between two alkene molecules that results in the interchange of the carbons of their double bonds. Two o and two n bonds are broken and two new o and two new n bonds are formed. 

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... 

There are many sigmatropic rearrangements and all are -selective in open chain compounds but can give Z-alkenes if the structure of the compound demands it. In this way they resemble the McMurry reaction and olefin metathesis. Many such reactions are used to transmit three-dimensional stereochemistry rather than for E/Z control in 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. 

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. 

In metathesis polymerization, the catalyticaUy active species is a stable metal-carbene bond that is formed between the metal and the alkene. Upon reaction with cycloalkane, a living moiety capable of chain growth is formed. The olefin metathesis reaction mechanism is shown in Scheme 3.18. 

The 2005 Nobel Prize in Chemistry was jointly awarded to Robert H. Grubbs (Caltech), Yves Chauvin (French Petroleum Institute), and Richard R. Schrock (MIT) for establishing olefin metathesis as a reaction of synthetic versatility and contributing to an understanding of the mechanism of this novel process. Olefin metathesis first surfaced in the late 1950s when industrial researchers found that alkenes underwent a novel reaction when passed over a heated bed of mixed metal oxides. Propene, for example, was converted to a mixture of ethylene and 2-butene (cis + trans). 

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. 

In Section 24.12, we introduced alkene (olefin) metathesis, i.e. metal-catalysed reactions in which C=C bonds are redistributed. The importance of alkene and alkyne metathesis was recognized by the award of the 2005 Nobel Prize in Chemistry to Yves Chauvin, Robert H. Grubbs and Richard R. Schrock for the development of the metathesis method in organic synthesis . Examples of alkene metathesis are shown in Figure 27.3. The Chauvin mechanism for metal-catalysed alkene metathesis involves a metal alkyli-dene species and a series of [2 + 2]-cycloadditions and cycloreversions (Figure 27.4). Scheme 27.6 shows the mechanism for alkyne metathesis which involves a high oxidation state metal alkylidyne complex, L M=CR. 

---