Examples of Olefin Metathesis

Scheme 1 General examples of olefin metathesis reactions.

Figure 6.1 Examples of olefin metathesis processes. Ring-closing metathesis (RCM), cross metathesis (CM), acyclic diene metathesis polymerization (ADMET), and ring-opening metathesis polymerization (ROMP).

Examples of Olefin Metathesis 21.2.5.1. Ring-Closing Olefm Metathesis ... 

Scheme 4.11 Examples of olefin metathesis with acyclic or cyclic substrates in the gas phase.

Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

Although the number of applications of olefin metathesis to transition metal complexes is small compared to the number of applications in organic synthesis, this field is becoming increasingly important. Spectacular examples are the double RCM reactions of copper phenanthroline complexes as a synthetic route to catenanes [113] or a recently reported approach to steric shielding of rhenium complex terminated sp-carbon chains [114]. 

If the cycloaddition and cycloreversion steps occurred under the same conditions, an equilibrium would establish and a mixture of reactant and product olefins be obtained, which is a severe limitation to its synthetic use. In many cases, however, the two steps can very well be separated, with the cycloreversion under totally different conditions often showing pronounced regioselectivity, e.g. for thermodynamic reasons (product vs. reactant stability), and this type of olefin metathesis has been successfully applied to organic synthesis. In fact, this aspect of the synthetic application of four-membered ring compounds has recently aroused considerable attention, as it leads the way to their transformation into other useful intermediates. For example aza[18]annulene (371) could be synthesized utilizing a sequence of [2 + 2] cycloaddition and cycloreversion. (369), one of the dimers obtained from cyclooctatetraene upon heating to 100 °C, was transformed by carbethoxycarbene addition to two tetracyclic carboxylates, which subsequently lead to the isomeric azides (368) and (370). Upon direct photolysis of these, (371) was obtained in 25 and 28% yield, respectively 127). Aza[14]annulene could be synthesized in a similar fashion I28). 

Such cases are not uncommon, but full quantitative treatments are rare, since often relatively large amounts of Y must be added to obtain measurable effects. Complications may then arise from the effects of the added Y on the nature of the medium (see Chapters 2 and 3). These are particularly notable when Y and I are charged, as is often the case. Under those circumstances, maintenance of the constant ionic strength of the medium with a known non-participating ionic species is essential. The classic case of common ion depression in solvolysis of benzhydryl chloride is dealt with in Chapter 2. A more recent example of this kind of treatment with neutral reactants occurs in the elucidation of the mechanism of olefin metathesis [20], catalysed by the ruthenium methylidene 9, Scheme 9.6. With ca. 5% of 9, disappearance of diene 10 was clearly not first order. However, reactions run in the presence of large excesses of phosphine 11 were much slower and showed first-order kinetics. The plot of kQ K against 1/ [ 11 ] was linear, consistent with dissociation of 9 to yield an active catalytic species prior to engagement with the diene, with k t [11] 3 > fc2[diene]. Because first-order kinetics were observed under these conditions, determination of order with respect to the catalytic species (as well as the diene) was simplified, and an outline for the mechanism could be constructed (see also Chapter 12 for more detailed consideration of catalysed olefin metathesis). 

Apart from the development of new catalysts, the examples of application of olefin metathesis in the literature are uncountable, including total synthesis, polymer, and... 

Olefin metathesis has become one of the most important large-scale technical processes for the manufacture of olefins in the petrochemical industry [123]. When cyclic olefins are used as substrates, high-molecular polymers, which are formed by the so-called ring-opening metathesis (ROM), have found applications as elastomers and plastics. Gas-phase studies on the mechanism of olefin metathesis had been confined to simple metal carbenes, for example [Mn=CH2]+, [Fe=CH2]+,and [Co=CH2]+ [124-127]. Most of the metatheses have been observed with deuterated ethylene. 

Table 2. Examples of metal carbene complexes as initiators of olefin metathesis.

The forward reaction (1) is an example of cross-metathesis between two different olefins and provides a route to styrene. The reverse of (1) is a self-metathesis reaction such reactions may be either productive as in (4), or non-productive (also called degenerate) as in (5). 

Table 2.2 Examples of metal carbene complexes with a covmt of less than 18 electrons effectiveness as initiators of olefin metathesis/ROMP ... 

Table 9.3 Examples of cross-metathesis of higher olefins with each other... 

Derivatives of oleic acid that imdergo cross-metathesis with simple olefins are listed in Table 9.4, while Table 9.5 gives some examples of cross-metathesis reactions of simple olefins with other functional olefins. Such cross-metathesis reactions may provide useful routes to speciality chemicals such as synthetic perfumes, insect pheromones (Crisp 1988), prostaglandin intermediates (Dalcanale 1985), etc. (see Mol 1982, 1991). Of special interest are ethenolysis reactions, which allow the synthesis of compoimds with terminal double bonds. Ethenolysis (and cross-metathesis with lower olefins) has been investigated extensively for... 

Table 9.4 Examples of cross-metathesis reactions of oleic acid and its derivatives with simple olefins... 

Tebbe found that titanocene complexes promoted olefin metathesis in addition to carbonyl olefination. Despite the fact that these complexes have low activity, they proved to be excellent model systems. For example, the Tebbe complex exchanges methylene units with a labeled terminal methylene at a slow rate that can be easily monitored (Eq. 4.6) [54]. This exchange is the essential transformation of olefin metathesis. When reactions with olefins are performed in the presence of a Lewis base, the intermediate titanium metallacycle can be isolated and even structurally characterized (Eq. 4.7) [61] These derivatives were not only the first metathesis-active metallacyclobutane complexes ever isolated, but they were also the first metallacyclobutanes isolated from the cycloaddition of a metal-carbene complex with an olefin. These metallacycles participate in all the reactions expected of olefin metathesis catalysts, especially exchange with olefins... 

One synthetic advantage of olefin metathesis is that the catalyst tolerates the presence of a variety of functional groups in the reactant. In a synthesis of the antiinfluenza drug Tamiflu (Section 23.25), for example, ring-closing metathesis was used to prepare the highly functionalized cyclohexene derivative shown in the equation. What was the reactant ... 

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. 

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