Reaction transalkylideneation

In experiments with [2- 4C] propene in the presence of a Re207-Al2C>8 catalyst, the ethene formed showed no radioactivity, while the butene showed a specific radioactivity twice as high as that of the starting material. This result is completely consistent with a transalkylidenation scheme, and excludes a transalkylation reaction. Similar results were obtained by Clark and Cook (62) in experiments with radioactive propene on a M0O3-C0O-AI2O3 catalyst. 

Calderon et al. (3, 46) obtained analogous results with the homogeneous system WC16-C2H6A1C12-C2H60H. In the reaction between 2-butene and 2-butene-ds, the only new product was 2-butene-d4, from which they concluded that a transalkylidenation reaction had taken place. 

It can be concluded that the metathesis reaction of acyclic alkenes and cycloalkenes proceeds via the rupture and formation of carbon-carbon double bonds, i.e. that the metathesis of alkenes is a true transalkylidenation reaction. 

Bradshaw et al. 67) were the first to propose a reaction pathway that is compatible with a transalkylidenation scheme. They suggested that the reaction proceeds via a quasi-cyclobutane intermediate. Applied to linear alkenes, this is pictured as follows ... 

As opinions regarding the metathesis reaction pathway have shifted in recent years to favor a nonpairwise carbene-to-metallocyclobutane transformation, increasing attention has been given to the mechanistic significance of cyclopropane olefin interconversions. This interconversion process seems to occur primarily when certain relatively inefficient catalysts are employed, which in itself raises questions. Under ideal conditions, "good metathesis catalysts are remarkably efficient promoters of transalkylidenation, and consequently are well suited for olefin and polymer syntheses. Thus, most early studies focused primarily on applications. When side reactions did occur, they were usually ignored or presumed to be trivial cationic processes. 

Such reactions of cycloolefins are called metathesis polymerization by analogy with the transalkylidenation reaction of linear olefins which proceed under comparable conditions and in which the double bonds of the olefins are broken and new double bonds are formed (metathesis) ... 

The metathesis reaction consists of a movement of double bonds between different molecules, as shown in Figure 1.4. Thus, the metathesis reaction can be addressed as a transalkylideneation reaction. The cleavage of the carbon-carbon double bonds was established using isotopic labelled compounds that were subjected to ozonolysis after reaction (9). 

Numerous studies have dealt with the mechanism of metathesis.6,7 14 20-31 The first important question to answer in an understanding of the mechanism is whether the carbon-carbon single bonds or the carbon-carbon double bonds are cleaved during the reaction. The corresponding transformations taking place are transalkylation [Eq. (12.9)] or transalkylidenation [Eq. (12.10)], respectively ... 

Both 2H and 14C labeling experiments have shown that the reaction proceeds via the cleavage and reformation of the double bonds, i.e., it is a transalkylidenation process (5,6,7). The reaction is reversible, essentially thermoneutral, and attains a thermodynamic equilibrium typified by a statistical distribution of the alkylidene moieties. 

The transalkylidenation of two terminal alkenes under release of ethene, catalyzed by ruthenium carbenoids (Grubbs Catalyst). Statistically, the reaction can lead to three possible pairs of geometric isomers, i.e. EJZ pairs for two homocouplings and the cross-coupling (R-CH=CH-R, R -CH=CH-R and R-CH=CH-R ) - a total of 6 products. 

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. 

The metathesis or transalkylidenation reaction is catalyzed by complexes of tungsten, molybdenum or rhenium, in a heterogeneous or homogeneous phase, and consists in the. scission of the double bond with the formation of two new olefins. 

The mechanism of transalkylidenation is not known, but metal-carbene or 3- or 4-carbon-metal heterocycles may be intermediates in the reaction (Cardin et a ., 1973). The cocatalyst appears to generate the catalytically active transition metal species, but such activation can also be achieved photolytical-ly in the absence of a cocatalyst [Eq. (155) (Krausz et al., 1975)]. A mechanism for this latter reaction is suggested in Scheme 15. There is good precedent for... 

Olefin metathesis can be thought of formally as a reaction in which the carbon-carbon double bonds in two olefins are broken and rearranged subsequently by interchange of the aUcylidene residues (transalkylidenation), the process resulting in the formation of two new olefins (Scheme 1). If one of the olefin products is volatile and can be easily removed, then the reversible metathesis reaction can be driven completely to the right. The reaction needs to be catalyzed, routinely by a transition metal complex. 

The ring-opening process involves a metathesis reaction in which two cyclo-olefinic double bonds are simultaneously cleaved, this being followed by transalkylidenation of the fragments. Thus for cyclopentene the process may be represented as ... 

A metathesis reaction is catalyzed by transition metal derivatives and results in a transalkylidenation of olefinic hydrocarbons as shown hereafter ... 

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