Metathesis, alkene olefin double

Two of the most characteristic reactions of carbene complexes are olefin metathesis and olefin cyclopropanation. Olefin metathesis is a reaction in which the C-C double bond of an alkene is cleaved, and one of the resulting alkylidene fragments combines with the metal-bound carbene to form a new alkene. The second alkylidene fragment forms a new carbene eomplex with the metal. Olefin cyclopropanation is a reaction in which a a bond is formed between the metal-bound alkylidene and each of the two carbon atoms of the alkene, to yield a cyclopropane. 

In the metathesis reaction, alkenes are converted to mixtures of lower and higher alkenes through the interchange of groups on the double bonds in an equilibrium process. The number of moles of product lighter than the feed is roughly the same as the number of moles of heavier alkenes. Olefin metathesis is a thermoneutral reaction and produces an equilibrium mixture of products. In simple cases this mixture is close to the statistical distribution of substituent groups. 

The term alkene (olefin) metathesis refers to the equilibrium reaction shown in equation (1) in which the alkylidene groups of a pair of alkenes are exchanged with one another in the presence of a transition metal-containing catalyst. The reaction involves the net cleavage of the bonds of the substrate(s) and formation of the new carbon-carbon double bonds of the prodncts. Once equilibrium has been established, the resultant prodnct mixture has a distribution of alkenes (including isomers) that is determined solely by the relative thermodynamic stabilities of the prodncts. 

Alkene metathesis, also called olefin metathesis, is a reaction that breaks the strongest bond in an alkene (the double bond) and then rejoins the fragments. When the fragments are joined, each new double bond is formed between two sp carbons that were not previously bonded. Metathesis is a Greek word that means transposition. ... 

Intermolecular-enyne metathesis, if it is possible, is very unique because the double bond of the alkene is cleaved and each alkylidene part is then introduced onto each alkyne carbon, respectively, as shown in Scheme 9. If metathesis is carried out between alkene and alkyne, many olefins, dienes and polymers would be produced, because intermolecular enyne metathesis includes alkene metathesis, alkyne metathesis and enyne metathesis. The reaction course for intermolecular enyne metathesis between a symmetrical alkyne and an unsym-metrical alkene is shown in Scheme 9. The reaction course is very complicated, and it seems impossible to develop this reaction in synthetic organic chemistry. 

Olefin metathesis enables the catalytic formation of C=C double bonds under mild conditions.1 After the development of well-defined catalysts,1 2 selective cross-couplings between functionalized terminal alkenes (CM) have been noted.2 A general problem... 

Olefin (alkene) metathesis is an important reaction in which all the carbon-carbon double bonds in an alkene are cut and then rearranged in a statistical fashion as shown in Scheme 10.12. 

The main strategies used for the preparation of alkenes by cleavage from insoluble supports are (3-elimination and olefin metathesis (Figure 3.35). Because some of these strategies enable the preparation of pure alkenes, devoid of additional functional groups, the linkers are sometimes also called traceless linkers, although the C=C double bond reveals the original point of attachment to the support. 

It was shown that the mixture was an equilibrium mixture. Thus it appears that the alkenes are being broken apart at the double bonds and the pieces reassembled randomly. This process was termed olefin metathesis because the ends of the carbon-carbon double bonds are being interchanged. 

A major drawback of alkene metathesis is lack of control over the stereochemistry of the newly formed double bond. For unstrained systems, E/Z ratios are virtually unpredictable. Alkyne metathesis, on the other hand, can always be combined with subsequent Lindlar hydrogenation, thereby giving access to stereochemically pure 2-olefins. In 1998, Ftirstner and Seidel were the first to report a ring-closing alkyne metathesis [7]. Under high-dilution conditions (0.02 m) and reduced pressure (20 mbar, removal of 2-butyne, solvent 1,2,4-trichlorobenzene (b.p. 214 °C)) the Schrock catalyst was applied to assemble macrocydic... 

In the mid to late 1980s, transition-metal catalysts were developed that were particularly useful for carrying out olefin (alkene) metathesis reactions (Rouhi 2002). Metathesis is a reaction in which two molecules containing carbon-carbon double bonds exchange carbon atoms along with any groups attached to them ... 

The double bond is the strongest bond in an alkene, yet it is also the most reactive bond. Imagine how useful it would be if we could break molecules at their double bonds and reassemble them as we please. That is the goal of olefin metathesis. We can think of an alkene as two alkylidene groups (=CHR) held together by the double... 

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... 

Alkene metathesis, more commonly called olefin metathesis, is a reaction between two alkene molecules that results in the interchange of the carbons of their double bonds. Two o and two n bonds are broken and two new o and two new n bonds are formed. 

The mechanism for olefin metathesis is complex, and involves metal-carbene intermediates— intermediates that contain a metal-carbon double bond. The mechanism is drawn for the reaction of a terminal alkene (RCH=CH2) with Grubbs catalyst, abbreviated as Ru=CHPh, to form RCH = CHR and CH2 = CH2. To begin metathesis, Grubbs catalyst reacts with the alkene substrate to form two new metal-carbenes A and B by a two-step process addition of Ru=CHPh to the alkene to yield two different metallocyclobutanes (Step [1]), followed by elimination to form A and B (Steps [2a] and [2b]). The alkene by-products formed in this process (RCH=CHPh and PhCH=CH2) are present in only a small amount since Grubbs reagent is used catalytically. 

Olefin metathesis is a catalytic reaction in which alkenes are converted into new products via the rupture and reformation of carbon-carbon double bonds. A simple example is the metathesis of propene into ethylene and 2-butene (cis and trans) eq. (1). 

Rhenium oxide-alumina catalysts are reduced at ambient temperatures and sub-atmospheric pressure by propene and higher alkenes, generating metathesis activity. Ethylene at these conditions did not show any reduction capabilities. Reduction with CO or NH3 at 300-500° C did not result in metathesis activity. At room temperature CO did not adsorb on reduced catalysts however, NO adsorbs and is a poison for the olefin metathesis reaction. Water generated in reducing catalysts with alkenes is mainly associatively adsorbed and, at ambient temperatures, exchanges hydrogen atoms with propene and butene. Activity for double-bond isomerization is partly accounted for by associatively adsorbed water, which generates acidity. ... 

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