Ethene dimerization

Streitwieser, A., Jr. Science1981,214,627. The Diels-Alder reaction of ethene and 1,3-butadiene is exothermic by 40 kcal/mol. 

Symmetry-correct MOs for [2 + 2] cycloaddition of two ethenes. (Adapted from reference 101.) 

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Fig. 6 A schematic of TS orbital interactions between the it MOs of two ethene molecules approaching each other in parallel planes. At infinite separation, the two MOs are degenerate. At closer distances, the degeneracy is lifted by an amount AE(tt), the so-called splitting energy, which may be equated to twice the magnitude of the electronic coupling matrix element, for hole transfer in the radical cation of the ethene dimer. The (3C value was obtained from HF/6-31 + G calculations.14...

A more general rule emerges from these considerations Concerted cycloaddition reactions are "forbidden" if involving 4n T-electrons, as in ethene dimerization (n is any integer), and are "allowed" if involving 4n + 2, as in cyclo-hexene formation [32,35,36]. 

Figure 7.15 Impossibility of antarafacial attack in ethene dimerization...

When the rate of active site formation, B sites min , is constant and the rate of ethene dimerization on an active site is constant, R mol min site", then the amount of butene formed, n mol, at a time of t min, is expressed by n=j R< t-r)BdT = (1/2 )RBt. This relation well explains the change of the amount of butenes formed on Nb205/PVG during the irradiation(displayed in Fig.4). Therefore, the Nb(IV) species must be closely related to the intermediate of the reaction. 

The prototypical ethene -i- ethene cycloaddition has been explored computationally and a somewhat different picture has emerged. The Cl for ethene dimerization is calculated to be rhomboid. ... 

Related Ru systems have been prepared on Na X-zeolite. While these were active for the hydroformylation of ethene at 10 MPa and 200 °C, appreciable amounts of products from competing ethene dimerization or Fischer-Tropsch reactions were also observed and significant Ru elution occurred. 

It is customary to describe cycloaddition reactions by indicating in brackets for each component the type of orbital system involved in the reaction, the number of electrons, and the stereochemistry with respect to that component. Therefore, the pathway for the ethene dimerization shown in Figure 11.56 is termed a [,j23 - - 2J cycloaddition because each ethene reacts by way of its n system, each n bond has two electrons, and the stereochemistry is suprafacial with respect to each n system. We can see that the reaction in Figure 11.56 takes place suprafacially because the product has the same stereochemical relationships of the alkene substituents as do the reactants. That is, H is cis to H and H is cis to H in both the reactants and the product. 

Cvcloaddition reactions This type of reaction, the typical examples of which are, e.g, the Diels-Alder reaction or 2 + 2 ethene dimerization, is characterized by the fact... 

First, the most important step in the analysis of the above scheme requires us to characterize the structure of the intermediate since it is only when its structure is known with sufficient certainty that the predictions based on the value of the overlap determinants can be reliable. In general, the question of the structure of the intermediate can, of course, be quite complicated, but in the case of pericyclic reactions, which are of concern here, the situation is slightly more simple. This is due to the fact that the set of structures which could play the role of the eventual intermediates is restricted only to species of a biradical and/or zwitterionic nature [60,61], so that the proposal of the structure of the eventual intermediate need not be so complicated. Thus, e.g., in the case of 2j + 2g ethene dimerization, the corresponding intermediate can be naturally identifi with the tetramethylene biradical. In such a case, the whole two step reaction scheme can be desribed as follows ... 

As can be seen from the Table, the values of g p do indeed remedy the insufficiency of the first order index r and correctly predict the reaction preferred for the concerted process by the Woodward-Hofi nann rules. Much more interesting than this reproduction of Woodward-Hofl nann rules is, however, the comparison of relative ease of concerted and stepwise reaction mechanisms. Thus, e.g., in the case of 2+2 ethene dimerization where the original criterion failed, the new enhanced criterion based on the second order similarity indices clearly suggests that the preferred mechanism of forbidden s + s dimerization is the stepwise one. This is in a complete agreement with both experimental and theoretical data for this process [125,126]. 

The other definition is due to Hashimoto [164] who define the same reference in the form formally similar to the one derived from one-determinantal wave fimction. The similarity index (116) was then applied to a series of selected pericyclic reactions (both forbidden and allowed) and its variation with the systematic variation of the reaction coordinate q> for both types of reference standard is discussed in the study [165]. Since all the details of this discussion can again be found in this stud we remind here only the most important results. They concern, above all, the form of the dependence g(cp) vs. 9. An example of such a dependence, for the case of simple 2+2 ethene dimerization, is given in Fig. 14, but practically the same type of dependence is observed for remaining reactions as well. Also the change of the reference standard has only negligible effect and the most important difference between the Kutzelnigg s and Hashimoto s type of reference standard is the systematic vertical shift of the curves conditioned by the evident change in the values of similarity indices for 9 = 0 and 9 = ti/2 (117). 

Figure 14 Variation of similarity index g(cp) (standard of the McWeeny type) with the systematic change of reaction coordinate q> for the allowed and forbidden 2+2 ethene dimerization. The Ml line corresponds to forbidden and the dashed line to allowed reaction mechanisms.

Polymerization grade 1-butene, used as comonomer in polyethylene production, is obtained by metathesis disproportionation of propene according to the triolefin process, followed by 2-butene isomerization on specific catalysts [14]. When propene is available, the method is an alternative to the above-mentioned process of ethene dimerization to 1-butene. 1-Butene and 2-butene produced by propene disproportionation constitute a valuable source for manufacturing high purity butadiene via dehydrogenation, since the product contains only trace amounts of branched hydrocarbons. 

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