Ethylene cross-metathesis

The total synthesis of Stemona alkaloid (-)-tuberostemonine was accomplished by P. Wipf. Late in the synthesis, the introduction of an ethyl sidechain was required. This could be achieved in a novel four-step sequence. First, the allyl sidechain was introduced by the Keck radical allylation. To this end, the secondary alkyl phenylselenide substrate was treated with allyltriphenyltin in the presence of catalytic amounts of AIBN. This was followed by the introduction of a methyl group onto the lactone moiety. The allyl group then was transformed into the desired ethyl group as follows the terminal double bond was isomerized to the internal double bond by the method of R. Roy. This was followed by ethylene cross metathesis and catalytic hydrogenation to provide the desired ethyl sidechain. 

Cross-metathesis of ethylene and methyl oleate (ethenolysis) and associated... 

The treatment of equivalent amounts of two different alkenes with a metathesis catalyst generally leads to the formation of complex product mixtures [925,926]. There are, however, several ways in which cross metathesis can be rendered synthetically useful. One example of an industrial application of cross metathesis is the ethenolysis of internal alkenes. In this process cyclic or linear olefins are treated with ethylene at 50 bar/20 80 °C in the presence of a heterogeneous metathesis catalyst. The reverse reaction of ADMET/RCM occurs, and terminal alkenes are obtained. 

Cross-metathesis, however, is usually a nonselective reaction. Transformation of two terminal alkenes in the presence of a metathesis catalyst, for instance, can give six possible products (three pairs of cis/trans isomers) since self-metathesis of each alkene and cross-metathesis occur in parallel. It has been observed, however, that terminal olefins when cross-metathesized with styrene yield trans-P-alkylstyrenes with high selectivity.5 A useful synthetic application of cross-metathesis is the cleavage of internal alkenes with ethylene called ethenolysis to yield terminal olefins ... 

Acyclic dienes are the products in cross-metathesis of cycloalkenes and acyclic alkenes. With ethylene, a,co-dienes are formed ... 

The trend of structural selectivity can be summarized as degenerate metathesis of terminal alkenes (exchange of methylene groups) > cross-metathesis of terminal and internal alkenes > metathesis of internal alkenes > productive metathesis of terminal alkenes (formation of internal alkene and ethylene).87 Since different catalyst systems exhibit different selectivities, a simple general picture accounting for all stereochemical phenomena of metathesis is not feasible. 

As discussed in Section 12.3, the triolefin process to transform propylene to ethylene and 2-butene developed by Phillips135,136 is not practiced at present because of the increased demand for propylene. The reverse process, that is, cross-metathesis of ethylene and 2-butene, however, can contribute to satisfy the global demand for propylene. Lyondell Petrochemical operates a 136,000-t/y (ton/year) plant for the production of propylene.236 In a joint project by BASF and FINA, Phillips metathesis technology will be used to enhance propylene production.237 A similar project was also announced by DEA.238 In a continuous process jointly developed by IFP and Chines Petroleum Corporation, cross-metathesis of ethylene and 2-butene is carried out in the liquid phase over Re207-on-Al203 catalyst (35°C, 60 bar).239,240... 

A very useful cross-metathesis is the reaction involving ethylene, which is called ethenolysis. Reaction of ethylene with internal alkenes produces the more useful terminal alkenes. Two terminal alkenes 45 and 42 are formed from the unsymmetric alkene 6 and ethylene. The symmetric alkenes 11 are converted to single terminal alkenes 45. The terminal dienes 46 are formed by ethenolysis of the cyclic alkenes 43. 

Cross-metathesis using terminal alkenes or ethylene... 

Scheme 14. Ring-closing olefin metathesis of 62 detached the allyl glycoside 63. A cross-metathesis with 64 and ethylene liberated target-oligosaccharides such as 65 in form of their n-pentenyl glycosides.

W(OAr)2Cl4 [C4Ciim]Cl-AlCl3- Cross-metathesis of linear olefins, e.g. conversion of 1-pentene to form ethylene and Et AlCl . . , , 4-octene no reaction details given system active tor several runs [22]... 

Metathesis is a versatile reaction applicable to almost any olefinic substrate internal, terminal or cyclic alkenes, as well as dienes or polyenes. (Alkyne metathesis is a growing area, but will not be dealt with here.) The reaction is also known as olefin disproportionation or olefin transmutation, and involves the exchange of fragments between two double bonds. Cross metathesis (CM, Figure 1) is defined as the reaction of two discrete alkene molecules to form two new alkenes. Where the two starting alkene molecules are the same it is called self-metathesis. Ethenolysis is a specific type of cross metathesis where ethylene... 

Cross metathesis of ethylene with internal alkenes provides a facile route to terminal alkenes. A number of processes have been described that use this transformation however, the only products, besides neohexene,that appear to be important are the a,o>-dienes that result from metathesis of cyclic alkenes with an excess of eAylene. This family of compounds should find a wide variety of applications. 

An example of the second category is the cross metathesis of cyclooctene and ethylene giving 1, 9-decadiene (60) in about 75% yield (Eqn. 22.55). 42 This reaction between cyclic olefins and ethylene provids an excellent method for the preparation of a, -diolefins. 2 ... 

Dimersol E is used to upgrade C2 + C3 fuel gas. Co-oligomerization of ethylene and propene leads to a gasoline stream very similar to the Dimersol G product. Mixed butenes are also obtained with Dimersol E (from ethylene dimerization). They can be used in paraffinic alkylation or to make propene through a subsequent cross-metathesis reaction with ethylene. 

In contrast to the reliable, high-yielding, and selective intramolecular, n-enyne metathesis reaction, intermolecular enyne metathesis (enyne cross-metathesis) has seen less use in the synthesis of complex molecules due to limited selectivity, despite its potential in fragment-coupling processes (404). The most common use of intermolecular enyne metathesis employs ethylene as the alkene component, providing a particularly convenient method for the production of... 

Both catalysts 1 and 2 are effective in promoting cross-metathesis leading to various conjugated dienes from alkenes and alkynes. Chiral 2-(a-acetoxybenzyl)-1,3-butadiene is obtained from (/f)-3-acetoxy-3-phenylpropyne via cross-metathesis with ethylene. - Furthermore, the reaction of 1,6-diynes with alkenes is even more intriguing ... 

Apart from Ru-catalyzed metathesis in ionic liquids two examples of tungsten catalyzed metathesis reactions have also been reported. Vasnev and coworkers studied the metathesis of Thexene catalyzed by WCle in tetrafluoroborate ionic liquids [266]. In the reaction sequence under investigation, l-hexene first isomerized to 2-hexene which then formed 4-octene and ethylene by cross-metathesis. The yields of the metathesis product were demonstrated to increase with increasing reaction temperature and with addition of a tin-containing promoter. 4-octene was obtained in selectivies up to 97% (at 25% l-hexene conversion) when tetrabutyl tin was applied as the promoter in this reaction. 

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