Ethylene cross metathesis with alkenes

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 ... 

It is noteworthy that the double-bond isomerization step is faster than the overall elementary steps of alkane metathesis. Formation of lower alkanes is due to the tungsten hydride intermediate, favoring chain walking with double-bond migratirHi followed by fast cross metathesis with coordinated ethylene leading to lower alkenes in turn giving lower alkanes on hydrogenation. This intramolecular reaction pathway, without formation of the free olefin, probably is the difference between alkane and olefin metathesis. 

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 ... 

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. 

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. 

Treatment of an alkyne/alkene mixture with ruthenium carbene complexes results in the formation of diene derivatives without the evolution of byproducts this process is known as enyne cross-metathesis (Scheme 22). An intramolecular version of this reaction has also been demonstrated, sometimes referred to as enyne RCM. The yield of this reaction is frequently higher when ethylene is added to the reaction mixture. The preferred regiochemistry is opposite for enyne cross-metathesis and enyne RCM. The complex mechanistic pathways of Scheme 22 have been employed to account for the observed products of the enyne RCM reaction. Several experiments have shown that initial reaction is at the alkene and not the alkyne. The regiochemistry of enyne RCM can be attributed to the inability to form highly strained intermediate B from intermediate carbene complex A in the alkene-first mechanism. Enyne metathesis is a thermodynamically favorable process, and thus is not a subject to the equilibrium constraints facing alkene cross-metathesis and RCM. In a simple bond energy analysis, the 7r-bond of an alkyne is... 

Olefin metathesis is a reaction that is over fifty years old and has been developed over this time period from a process nm at high temperatures with ill-defined catalysts by unknown mechanisms to a process that can be conducted imder nuld conditions with designed catalysts by mechanisms that occur by established steps. Olefin metathesis, and the related alk3me metathesis, fully cleaves carbon-carbon double and triple bonds and reforms these bonds to generate new alkenes and alkynes. The reaction is often under equilibrium control, but certain classes of reactions can be conducted in a selective fashion that is controlled by relative rates or thermod)mamic preferences. This reaction can open strained rings to form polymers or small dienes. It can close small rings and macrocycles by a reaction that is driven by the expulsion of ethylene that makes the reaction favored entropically or by running in an open system under non-equilibrium conditions. It can also be run as a "cross metathesis" to form imsymmetrical alkenes when the steric or electronic properties of the two alkenes properly match. 

Many researchers put forward proposals to explain how metathesis could take place,but it was Herisson and Chauvin who proposed that in the catalyst the metal was bound to the carbon through a double bond (metal alkylidene)." In the catalytic cycle of cross-metathesis, this active species first reacts with the olefin to form a four-membered ring (Scheme 7.7). This metallacyclobutane intermediate then cleaves, yielding ethylene and a new metal alkylidene, which reacts with a new alkene substrate to yield another metallacyclobutane. On decomposition in the forward direction, this second intermediate yields the internal alkene product and regenerates the initial metal alkylidene. 

Synthesis of anthramycin derivative 69a was achieved using RCM and alkene cross-metathesis (CM) (Scheme 6.16) [19]. L-Methionine was converted to ene-yne 61, and subsequent RCM with catalyst [Ruj-I under an atmosphere of ethylene gave pyrrolidine derivative 62. After deprotection and condensation with the commercially available acid chloride 63, the resulting amide 64 underwent reductive cyclization using Zn/AcOH followed by treatment with diluted HCl to... 

Cross-Enyne Metathesis/Diels-Alder MCRs Ruthenium-based complexes are known to catalyze the metathesis between alkynes and alkenes to afford 1,3-dienes. Fnstero, del Pozo et al. further exploited this cross-enyne metathesis (CEYM) by trapping the 1,3-diene with a dieno-phile via an intermolecnlar Diels-Alder reaction [198]. Thns, the Ru-catalyzed MCR between alkynes 220-221 and 1,7-octadiene as an in situ sonrce of ethylene by RCM generates a 1,3-diene that can snbseqnently nndergo a Diels-Alder reaction with a wide variety of dienophiles 222... 

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