Transalkylidenated

In the case of transalkylidenation, radioactivity will be found only in the butene ... 

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. 

Dall Asta and Motroni (44, 57) provided direct experimental evidence for the transalkylidenation mechanism in the case of cycloalkenes. With a catalyst system consisting of WOCI4, C2H6A1C12, and benzoyl peroxide they prepared a random copolymer of cyclooctene and cyclopentene, the cyclo-pentene double bond being labeled with 14C. The distribution of the radioactivity in the copolymer formed will depend on the site of ring opening. 

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 an alternative to the cyclobutane mechanism there have also been proposed mechanisms involving carbene complexes2 which cannot be considered as a more detailed description of the quasi-cyclobutane intermediate, as in the case of the tetramethylene complex. H6risson and Chauvin 88) proposed the following scheme for the transalkylidenation step ... 

For the catalyst system WCU-CsHbAICIs-CzHsOH, Calderon et al. (3, 22, 46) also proposed a kinetic scheme in which one metal atom, as the active center, is involved. According to this scheme, which was applied by Calderon to both acyclic and cyclic alkenes, the product molecules do not leave the complex in pairs. Rather, after each transalkylidenation step an exchange step occurs, in which one coordinated double bond is exchanged for the double bond of an incoming molecule. In this model the decomposition of the complex that is formed in the transalkylidenation step is specified, whereas in the models discussed earlier it is assumed that the decom-plexation steps, or the desorption steps, are kinetically not significant. 

Crotonaldehyde, hydrogenation of, 43-48 Cubane, isomerization of, 148 Cyclic dienes, metathesis of, 135 Cyclic polyenes, metathesis of, 135 Cycloalkenes, metathesis of, 134-136 kinetic model, 164 ring-opening polymerization, 143 stereoselectivity, 158-160 transalkylation, 142-144 transalkylidenation, 142-144 Cyclobutane configuration, 147 geometry of, 145, 146 Cyclobutene, metathesis of, 135 1,5,9-Cyclododecatriene, metathesis of, 135... 

Tetralin, hydrogenation of, 12 Titanium compounds as catalysts, 188 Titanium dichloride, 192, 193 number of propagation centers, 198-200 Titanium trichloride, 193, 194 Toluene in exhaust gases, 67 Transalkylation, 141, 142 Transalkylidenation, 142 Transition metal compounds as catalysts, 174... 

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. 

Give an example of coordination polymerisation via transalkylidenation. Write down the scheme. 

Confirmation that the ring-opening polymerisation of cycloolefins also proceeds by complete scission of the C=C bond (transalkylidenation) was provided soon after by Dall Asta and Motroni [44]. 

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. 

Olefin disproportionation and metathesis are very likely the heterogeneous and homogeneous counterparts of the same catalytic transformation. This process certainly involves transalkylidenation as the essential transformation. The most attractive mechanism for this... 

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. 

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