Other Cross-Metathesis Reactions

The cis isomer of 9-tricosene is the sex pheromone of Musca domestica (housefly). It should be noted that cross-metathesis reactions involving unsymmetrical internal alkenes can lead to a complex product mixture, as self-metathesis and other cross-metathesis reactions also occur. 13-Heptacosene, the cis form of... 

Many other (cross-) metathesis reactions of functionalized olefins have been shown to be possible in the presence of rhenium-based catalysts, such as self-metathesis (or cross-metathesis with normal olefins) of allyl- and vinylsilanes, unsaturated nitriles, chlorides, bromides etc. The products of these reactions are not yet of use in fine chemistry, but this might be remedied by future developments in this area. 

While alkane metathesis is noteworthy, it affords lower homologues and especially methane, which cannot be used easily as a building block for basic chemicals. The reverse reaction, however, which would incorporate methane, would be much more valuable. Nonetheless, the free energy of this reaction is positive, and it is 8.2 kj/mol at 150 °C, which corresponds to an equihbrium conversion of 13%. On the other hand, thermodynamic calculation predicts that the conversion can be increased to 98% for a methane/propane ratio of 1250. The temperature and the contact time are also important parameters (kinetic), and optimal experimental conditions for a reaction carried in a continuous flow tubiflar reactor are as follows 300 mg of [(= SiO)2Ta - H], 1250/1 methane/propane mixture. Flow =1.5 mL/min, P = 50 bars and T = 250 °C [105]. After 1000 min, the steady state is reached, and 1.88 moles of ethane are produced per mole of propane consmned, which corresponds to a selectivity of 96% selectivity in the cross-metathesis reaction (Fig. 4). The overall reaction provides a route to the direct transformation of methane into more valuable hydrocarbon materials. 

Although the application of tungsten catalyst 5 to the cross-metathesis reaction of other alkenes has not been reported, Basset has demonstrated that to-un-saturated esters [18] and glycosides [21], as well as allyl phosphines [22], are tolerated as self-metathesis substrates. 

The success of the cross-metathesis reactions involving styrene and acrylonitrile led to an investigation into the reactivity of other Ji-substituted terminal alkenes [27]. Vinylboranes, enones, dienes, enynes and a,p-unsaturated esters were tested, but all of these substrates failed to undergo the desired cross-metathesis reaction using the molybdenum catalyst. 

Where there is no spacer group between the C=C bond and the functional group, productive self-metathesis does not occur, but cross-metathesis reactions with other olefins are still possible. Recent impressive examples of this are the cross-metathesis reactions of acrylonitrile (equation 19). The reaction occurs with a wide variety of R groups. For 15 different compounds the yield of the new nitrile after 3 h at room temperature is 40-90%, with the cis isomer always strongly preferred (75-90%). Only minor amounts of RCH2CH=CHCH2R are formed, and no NCCH=CHCN182. The fact that acrylonitrile... 

There are other variations of olefin metathesis. In the cross-metathesis reaction (CM), two olefins are coupled. This reaction gives good yields and nonsta-tistical mixtures if the two alkenes have electronically different properties. 

Cross-metathesis reactions. The valuable cinnamylation reagent and other homologous allylation reagents are readily accessible from cross-metathesis using the Grubbs II catalyst ... 

The simplest example of a productive cross-metathesis reaction between acyclic olefins is that between ethene and but-2-ene reaction (1). In this case only one product is possible, apart from cis/trans isomerization of the but-2-ene the equilibrium mixture thus consists of four compounds. At the other extreme, the reaction of two unsymmetrical olefins, R CH=CHR and R CH=CHR , with R, R, R, R all different, can produce cis/trans isomers of four different unsymmetrical olefins by cross-metathesis as well as four symmetrical olefins by self-metathesis. Counting the cis/trans isomers of the reactants as well, this means that the equilibrium mixture will contain 20 different compounds. Side reactions, such as double-bond shift reactions, will complicate the situation still further. The main value of cross-metathesis reactions, apart from their use in the proof of mechanism, lies in their application to the synthesis of olefins that are otherwise expensive or difficult to prepare. A number of higher olefins, useful as insect sex attractants, have been made in this way. 

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... 

For the cross-metathesis reactions we used two different homogeneous ruthenium catalysts one multicomponent catalyst prepared in situ, and the other catalyst consisting of one single component. Both are based on ruthenium(II) and the latter can be isolated in pure form. Figure 1 shows the ruthenium catalysts that were used in cross-metathesis reactions to synthesize silicon-containing ot,(o-dienes. 

Cross metathesis is an emerging process that could have enormous synthetic value. The utility of the cross metathesis reaction depends on the selectivity for formation of one olefin over the other possible olefin products. This selectivity can be obtained when one class of olefin is appropriately paired with a second class of olefin. 

Scheme 21.8 shows the complex network of equilibria that are possible when two different olefins are combined in a metathesis system. The relative rates and magnitudes of the individual steps are not well enough established to fully rationalize the trends in selectivities of cross-metathesis reactions. However, a few features of the [2+2] cycloadditions should be kept in mind when considering the equilibria involved in a cross metathesis. First, some of the [2+2] processes lead to metallacycles containing substituents located in positions 1,3 to each other, and these metallacycles do not lead to productive metatheses processes. Second, the [2+2] reactions are slow in some cases... 

From a synthetic point of view, cross-metathesis reactions are very useful for the production of fine chemicals, which often can hardly be obtained by other means. An example is the synthesis of 1-triacontanol, CH3(CH2)28CH20H, a plant growth stimulant. This synthesis was performed in a relatively simple two-step process by cross-metathesis of methyl erucate with 1-octadecene in the presence of a WCl6/Me4Sn catalyst, equation (7), followed by hydrogenation over a Cu/Zn catalyst of the ester thus obtained [14]. 

The chemoselectivity provided by the solid support in the cross-metathesis reaction is evident in ene-ene metathesis reactions, as both olefins are capable of a homodimerization reaction. The Mata laboratory has devoted significant effort toward the understanding and development of cross-metathesis reactions on a solid support. To probe the limits of the reaction, aliphatic, aryl, and acryloyl alkenes 8,11, and 12 were immobilized and reacted with a variety of soluble olefins 13-16 from different classifications based on homodimerization potential (Scheme 6.2). For example, the immobilized acrylate 8 was reacted with several soluble olefins to form the immobilized coupling product 9. Subsequent cleavage with TFA and esterification with diazomethane delivered the esters 10. Other immobilized alkenes explored in the same process were 11 and 12. Among other olefins, they were reacted with the soluble alkenes 13-16. Modest to good yields (shown in parentheses) were obtained, but, most importantly, no dimerization of the immobilized alkenes 8,11, and 12 was observed, and undesired homocoupling products of the alkenes 13-16 in solution were easily washed away in the workup step. 

The cross-metathesis reaction (see Chap. 15 for mechanistic understanding) of the Ru-benzylidene complexes also yields other alkylidene complexes quantitatively in the presence of a tenfold excess terminal olefin. The methylene complex [RnCl2(PCy3)2(=CH2)] could also be made (only) in this way in the presence of 100 psi ethylene at 50°C in CD2CI2 and isolated as a red-purple, air-stable solid that decomposes in CH2CI2 or CgHe solution. 

RCM usually involves the formation of a gaseous product such as ethylene. Thus the total volume of the products is more than that of the reactants, which means that the change in overall entropy is positive. Because of this, RCM too is a thermodynamically favored reaction. Unlike ROMP and RCM, cross-metathesis is a reversible reaction. Because of this, in many cross-metathesis reactions, self-metathesis and other complications may arise. 

The housefly (Musca domestica) uses the hydrocarbon (Z)-9-tricosene (318 in Scheme 58) as the major component of its sex pheromone. Simple syntheses involve Wittig coupling 153, 154) or alkylation of a terminal alkyne and subsequent reduction 155, 156) to afford the desired Z-isomer in ca. 95% purity. Erucic acid 157, 158) or oleic acid 159, 160) have served as starting material of known Z stereochemistry. The transition metal-catalyzed olefin cross-metathesis reaction has been applied by Rossi 161) to synthesize (318) as a mixture of /Z-isomers together with the other possible Cis and C28 olefins (Scheme 58). 

The enyne cross metathesis was first developed in 1997 [170,171]. Compared to CM it benefits from its inherent cross-selectivity and in theory it is atom economical, though in reality the aUcene cross-partner is usually added in excess. The inabihty to control product stereochemistry of ECM reactions is the main weakness of the method. ECM reactions are often directly combined with other transformations like cyclopropanation [172], Diels-Alder reactions [173], cychsations [174] or ring closing metathesis [175]. 

Since the alkene formed in this reaction can further react with other alkenes, many products should be formed in the cross-metathesis (CM). Therefore, in the early days, only ring-closing metathesis (RCM) of diene was investigated. It is known that the reaction is catalyzed by a transition metal. Pioneering work on olefin metathesis was undertaken by Villemin and Tsuji, who reported the synthesis of lactones using alkene metathesis ... 

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