Ethene cross-metathesis reactions

Performing the cross-metathesis reactions at reflux (open to an argon atmosphere in a glove box) was designed to help remove the unwanted ethene produced by the reaction. 

It has been noted in a recent review that 4 successfully catalyses the cross-metathesis reactions of methyl oleate 1 or oleic acid with ethene see [1]... 

Cross-metathesis reactions between undeuterated and deuterated dienes, such as octa-1,7-diene (Grubbs 1976) and 2,2 -divinylbiphenyl (Katz 1976b) have also been studied as a means of testing the metal carbene mechanism. The initial proportions of ethene-tfo, -d2, and -d4 formed in these reactions support this mechanism. Likewise for the cross-metathesis between c/5,cw-deca-2,8-dienes... 

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. 

As stated above, olefin metathesis is in principle reversible, because all steps of the catalytic cycle are reversible. In preparatively useful transformations, the equilibrium is shifted to one side. This is most commonly achieved by removal of a volatile alkene, mostly ethene, from the reaction mixture. An obvious and well-established way to classify olefin metathesis reactions is depicted in Scheme 2. Depending on the structure of the olefin, metathesis may occur either inter- or intramolecularly. Intermolecular metathesis of two alkenes is called cross metathesis (CM) (if the two alkenes are identical, as in the case of the Phillips triolefin process, the term self metathesis is sometimes used). The intermolecular metathesis of an a,co-diene leads to polymeric structures and ethene this mode of metathesis is called acyclic diene metathesis (ADMET). Intramolecular metathesis of these substrates gives cycloalkenes and ethene (ring-closing metathesis, RCM) the reverse reaction is the cleavage of a cyclo-... 

The reaction of vinyl-substituted silanes and octavinylsilsesquioxane with vinyl-substituted amides, amines (carbazole) as welt as boronates catalyzed by I proceeds effectively to yield under optimum conditions stereo- and/or regio-selectively l-silyl-2-/V- and 1,1-silylboryl-substituted ethenes. 1-silylvinyl carbazole can also be obtained via cross-metathesis of vinylsilane with vinylcarbazole, but only in the presence of the 2nd generation Grubbs catalyst (IV). 

Cross-metathesis will then also afford ethene, which will escape the reaction because of its volatility. Alternatively, a 1,2-disubstituted (Z)-alkene bearing the same substituent in position 1 and 2 may be used as the solution component. 

Metathesis has been applied in oleochemistry for many years, but only fairly recently technical realization comes within reach [33, 34]. As typical catalysts, ruthenium carbene complexes of the Grubbs type are applied because of their very high activity (turnover numbers up to 200 000). In principle, oleochemical metathesis can be divided into two different types in self-metathesis the same fatty substrate reacts with itself and in cross-metathesis a fatty substrate reacts with, for example, a petrochemical alkene. The simplest case, the self-metathesis of methyl oleate forms 9-octadecene and dimethyl 9-octadecenedioate. The resulting diester can be used along with diols for the production of special, comparatively hydrophobic, polyesters. An interesting example of cross-metathesis is the reaction of methyl oleate with an excess of ethene, so-called ethenolysis. This provides two produds, each with a terminal double bond, 1-decene and methyl 9-decenoate (Scheme 3.3). 

All the reactions are reversible, and when volatile or insoluble products are formed displacement of the equilibrium occurs. Thus when R = H, removal of ethene from the system of Eq. (2) can drive the reaction to completion. The reverse reaction is called cross-metathesis and cross-metathesis with ethene is called ethenolysis . Both linear and branched olefins can undergo metathesis. 

Metathesis is a versatile reaction that forms the basis for several important industrial processes, such as the Phillips triolefin process, which produces propene by cross-metathesis of 2-butene with ethene, and the Shell higher olefins process (SHOP), which involves a combination process that converts ethene to detergent-range olefins. Several interesting polymeric materials are commercially produced via the ROMP of different types of unsaturated cyclic monomers, including nor-bornene, cyclooctene, and dicyclopentadiene [1]. 

The cross-metathesis of ethene with higher olefins has been termed ethenolysis (Bradshaw 1967), and provides a useful means of reducing the extent of substitution of an olefin feedstock and of converting cyclic olefins into linear dienes, trienes, etc. Some examples for acyclic olefins are shown in Table 9.1. The reactions are best done at 50 bar in order to drive the reaction to the right and to minimize self-metathesis of the substrate. 

The reaction of ds-pent-2-ene with 4-methylpent-l-ene has been investigated in some detail under conditions that allowed most of the ethene and propene to escape reaction (7). As may be seen from Fig. 9.2, the reaction is very rapid when catalyzed by Bu4N[MoCl(CO)5]/MeAlCl2 and reaches a steady state after 8 min. The cross-metathesis products, 6-methylhept-3-ene and 5-methylhex-2-ene, are formed in somewhat larger amount than the self-metathesis products, hex-3-ene and 2,7-dimethyloct-4-ene. The initial and final trans/cis ratios, obtained by plotting Doyle s data according to the method of Fig. 6.2, are (final values shown in brackets) but-2-ene 0.9 (2.5), 5-methylhex-2-ene 1.6 (3.3), 6-methylhept-3-ene 2.5... 

As discussed in Section 11.2.1, the polymerization of cyclohexene does not occur because of its very low strain energy. For the same reason, cross-metathesis of cyclohexene with acyclic olefins is strongly disfavoured. Nevertheless, using a high pressure of ethene, it is possible to displace the equilibrium sufficiently to be able to detect a trace of octa-1,7-diene product (Crain 1972). Likewise a small yield of dodeca-1,7-diene may be detected in its reaction with hex-l-ene (Bykov 1994) and of undeca-2,8-diene in its reaction with pent-2-ene (Herisson 1971). 

The metathesis reaction of cycloalkenes yields linear unsaturated polymers, so-called polyalkenamers. This ROMP is driven by the release of ring strain in the starting material. Several interesting polymers are commercially produced via the ROMP of different types of unsaturated cyclic monomers such as cyclooctene, norbornene, and dicyclopentadiene, using homogeneous catalyst systems [6]. As an alternative process, the cross-metathesis between a cyclic and an acyclic olefin allows to synthesize certain poly-unsaturated compounds for the special chemical market. Shell [7] developed the FEAST process for the manufacture of hexa-l,5-diene via cross-metathesis of cycloocta-1,5-diene with ethene. 

Mixed WOj/Al Oj/HY catalysts prepared by calcination of physically mixed WO3, Al Oj and HY zeolite showed unique behavior in the metathesis between ethene and 2-butene to produce propene [147]. Monomeric tetrahedrally coordinated surface tungstate species responsible for the metathesis activity were formed via the interaction with Bronsted acid sites of HY zeolite. Polytungstate clusters are supposed to be less active in the metathesis reaction. The best catalyst demonstrates the 2-butene conversion close to the thermodynamic equilibrium value ( 64%) at 453 K. The catalysts are bifunctional [148] they catalyze first isomerization of 1-butene to 2-butene and then cross-metathesis between 1-butene and 2-butene to produce propene and 2-pentene. 10%W03/Al203-70%HY exhibits the highest propene yield. 

For the Mo/H-Beta zeolites, the formation of the Al2(Mo04)3 phase and the decrease in the concentration of Brpnsted acid sites explains the low catalytic activity of Mo/H-Beta in metathesis of ethylene and 2-butylene to propylene [149]. A promoting effect of Mg was revealed in the Mo/H-Beta-Al203 catalyst for cross-metathesis of ethene and butene-2 to propene [150]. The stability is improved at the Mg content of l-2wt.% due to the elimination of weak acid sites and suppression of the side olefin oligomerization reaction. 

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