Exchange reactions, olefin metathesis

Keywords carbon-carbon bond formation reactions, carbonyl-olefin exchange reaction, olefin metathesis, reductive coupling of carbonyl compounds, carbonyl... 

Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

Olefin exchange reactions, 431 Olefin Metathesis and Metathesis... 

Several transition metal complexes can catalyze the exchange of partners of two double bonds. Known as the olefin metathesis reaction, this process can be used to close or open rings, as well to interchange double-bond components. 

Olefin metathesis can also be used in intermolecular reactions.299 For example, a variety of functionally substituted side chains were introduced by exchange with the terminal double bond in 5.300 These reactions gave E Z mixtures. 

Olefin metathesis refers to a process in which two alkenes exchange their alkylidene fragments. This reaction has recently been reviewed [743-747]. An exciting account on the discovery of olefin metathesis has been written by Eleuterio [748]. 

Joining olefin metathesis on the very short list of exchange reactions involving carbon-carbon bonds, the Diels-Alder reaction was studied in 2005 by Lehn and colleagues [52]. As the authors note, most Diels-Alder reactions proceed only in the forward direction at room temperature, with retro Diels-Alder reactions typically requiring elevated temperatures. Careful tuning of the diene and dienophile, however, can alter this significantly. In particular, reactions of substituted fulvenes (32) with diethylcyanofumarate (33) were... 

The two most used reversible covalent reactions are disulfide exchange and palladium-catalyzed olefin metathesis. We first probed the incorporation of olefin units into the H bonded duplexes by subjecting the modified duplexes to a Pd (Gmbb s) catalyst. Based on a duplex template with the same unsymmetrical H bonding sequence used for directing the formation of the /3-sheet structures, we prepared two groups (strands 17 and 18) of five olefins covalently linked to the two template strands (Fig. 9.13). Mixing each one of components 17 with each one of components 18 in a 1 1 fashion results in a small library of 25 (5 x 5) members. 

Metathesis is the most useful reaction in recent synthetic organic chemistry.In this reaction, bond fission of two double bonds occurs and two new double bonds are simultaneously formed after exchange (Equation (1)). Ring-closing olefin metathesis is now used for the synthesis of five-membered to macrocyclic ring compounds. 

The olefin metathesis reaction was so named by Calderon1 in 1967 following the discovery that it involved the total cleavage of the C=C bond and the apparent exchange of alkylidene moieties between two alkene molecules (equation 1). 

Olefin Metathesis (Grubbs Reaction) allows the exchange of substituents between different olefins - a transalkyiidenation. 

It has been speculated (5) that the olefin metathesis reaction mech-nism involves a four-centered quasi-cyclobutane transition state. The three basic steps postulated for the reaction, namely, formation of a bis-olefin-tungsten complex, transalkylidenation and olefin exchange, may account, in general, for the initiation and propagation steps in the ringopening polymerization of cycloolefins. Several modes of termination have been considered, but suitable data to test these are not yet available. 

Fig. 27 Examples of thermodynamically controlled reactions employed in the near-quantitative synthesis of MIMs. (a) Disulfide-exchange reaction permits equilibration between a bis(ammo-nium) disulfide dumbbell and a crown ether macrocycle to yield a mixture of [2]- and [3]rotaxanes quantitatively [194], (b) Olefin metathesis at high concentration on a benzylic amide macrocycle greatly favors the catenated species [196]. (c) Self-correcting imine bonds allow for nearly quantitative selection of a [2]rotaxane from an appropriate dynamic combinatorial library [76], (d) A weak nucleophile (E) equilibrates the components of a donor-acceptor [2]catenane in a dynamic Sn2 reaction [205]...

Exchange Reactions of Carbon-Carbon Bonds From the Olefin Metathesis to the Diels-Alder Reaction... 

Olefin metathesis is the transformation of two olefins 1 and 2 into two new olefins 3 and 4. Formally, the reaction represents a mutual exchange of alkylidene groups [Eq. (1)]. The reaction is catalyzed by various transition metal complexes. 

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. 

Alkyhdene derivatives of titanium and of phosphorus catalyse methylene exchange between olefins. Although exchange of CH2 groups is not useful for synthesis, these systems provide insight into the mechanisms of alkylidene exchange, a basic step in conventional metathesis. Titanacyclobutenes have been isolated from reactions of acetylenes with methylene-titanium complexes but titanacyclobutanes, the assumed intermediate for the case of olefins, have not been isolated. Bis(cyclopentadiene)titanacyclohexane decomposes to produce ethylene as the major product apparently via a-C-C bond cleavage. ... 

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. ... 

N. Calderon, H. Y. Chen, and K. W. Scott, Tetrahedron Lett., 1967, 3327. Calderon was the first to coin the term olefin metathesis for the olefin exchange reaction that is typified by the Triolefin Process. 

However, owing to the lack of j8-hydrogen in the intermediate metallacycle, 1,1-disubstituted ethylenes (2-methylpropene, methylenecyclohexane) do not undergo )S-hydrogen transfer instead, their methylene group is catalytically exchanged by a degenerated olefin metathesis reaction ... 

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