Metathesis equilibrium

If one of the newly formed alkenes is removed selectively from the reaction, however, then the productive metathesis equilibrium is shifted further and faster to product. When applying the scenario to an acyclic reactant containing two metathesizable alkenes (dienes), polymerization should result if high conversions can be achieved. This reaction (equation 16) falls under the category of equilibrium step polymerization chemistry, which, regardless of the chemistry involved, is dictated by strict statistical requirements. ... 

Experimental conditions catalyst 0.02 mmol cw-2-pentene/catalyst molar ratio = 500 reaction temperature = 25 C solvent C6H5CI (5 mL). Reaction time necessary to reach the metathesis equilibrium (2-butene/2-pentene/3-hexene = 1/2/1). Initial turnover rate for the conversion of a>2-pentene. translcis-2-butene (or rra/25/d5-3-hexene) ratio obtained at 0% conversion. 

Terminal olefins that are very common and yield, as one of the metathesis products, ethylene whose volatility allows to shift the metathesis equilibrium towards the products. Note that the olefins RCH=CHCH3 yield, upon metathesis, 2-butene, another gas whose formation also shifts the metathesis equilibrium towards products. 

The synthetic utility of the alkene metathesis reaction may in some cases be limited because of the formation of a mixture of products. The steps of the catalytic cycle are equilibrium processes, with the yields being determined by the thermodynamic equilibrium. The metathesis process generally tends to give complex mixtures of products. For example, pent-2-ene 8 disproportionates to give, at equilibrium, a statistical mixture of but-2-enes, pent-2-enes and hex-3-enes ... 

The reversal of ring-closing metathesis, namely ring-opening metathesis, is also a synthetically useful reaction. With strained (small-ring) cycloalkenes, e.g. 12, the equilibrium of the reaction lies on the side of the open-chain product 13 ... 

Olefin metatheses are equilibrium reactions among the two-reactant and two-product olefin molecules. If chemists design the reaction so that one product is ethylene, for example, they can shift the equilibrium by removing it from the reaction medium. Because of the statistical nature of the metathesis reaction, the equilibrium is essentially a function of the ratio of the reactants and the temperature. For an equimolar mixture of ethylene and 2-butene at 350°C, the maximum conversion to propylene is 63%. Higher conversions require recycling unreacted butenes after fractionation. This reaction was first used to produce 2-butene and ethylene from propylene (Chapter 8). The reverse reaction is used to prepare polymer-grade propylene form 2-butene and ethylene ... 

The most thoroughly studied reactions are the metathesis of propene to ethene and 2-butene, and the metathesis of 2-pentene to 2-butene and 3-hexene. Generally, the thermodynamic equilibrium ratio of the trans and cis components of the products is obtained. The reacting alkene molecules need not be identical, two different alkenes react with each other in the same way. 

Calculated Equilibrium Distributions for the Metathesis of Some Lower Alkenes at 898.16 Ka... 

Of course, even in the case of acyclic alkenes reaction enthalpy is not exactly zero, and therefore the product distribution is never completely statistically determined. Table V gives equilibrium data for the metathesis of some lower alkenes, where deviations of the reaction enthalpy from zero are relatively large. In this table the ratio of the contributions of the reaction enthalpy and the reaction entropy to the free enthalpy of the reaction, expressed as AHr/TASr, is given together with the equilibrium distribution. It can be seen that for the metathesis of the lower linear alkenes the equilibrium distribution is determined predominantly by the reaction entropy, whereas in the case of the lower branched alkenes the reaction enthalpy dominates. If the reaction enthalpy deviates substantially from zero, the influence of the temperature on the equilibrium distribution will be considerable, since the high temperature limit will always be a 2 1 1 distribution. Typical examples of the influence of the temperature are given in Tables VI and VII. 

Many authors have observed that the cis-trans ratio of the products of the metathesis reaction is equal to the thermodynamic equilibrium value. This suggests that the reaction is not highly stereoselective. However, under certain conditions the product distribution is influenced by kinetic factors. For instance, it proves to be possible to prepare from cyclopentene... 

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 ]. 

As stated above, olefin metathesis is in principle reversible, because all steps of the catalytic cycle are reversible. In preparatively useful transformations, the equilibrium is shifted to one side. This is most commonly achieved by removal of a volatile alkene, mostly ethene, from the reaction mixture. An obvious and well-established way to classify olefin metathesis reactions is depicted in Scheme 2. Depending on the structure of the olefin, metathesis may occur either inter- or intramolecularly. Intermolecular metathesis of two alkenes is called cross metathesis (CM) (if the two alkenes are identical, as in the case of the Phillips triolefin process, the term self metathesis is sometimes used). The intermolecular metathesis of an a,co-diene leads to polymeric structures and ethene this mode of metathesis is called acyclic diene metathesis (ADMET). Intramolecular metathesis of these substrates gives cycloalkenes and ethene (ring-closing metathesis, RCM) the reverse reaction is the cleavage of a cyclo-... 

Potentially, phosphaalkenes can be precursors to phosphinidenes in the same manner that carbenes can be formed from alkenes. This latter metathesis route deserves more attention in light of the recognition that stable carbenes can be in equilibrium with their dimers [11]. However, a discussion on phosphaalkenes is outside the scope of the present survey. 

It also explains the /Z selectivity of products at low conversions (kinetic ratio. Scheme 19). In the case of propene, a terminal olefin, E 2-butene is usually favoured (E/Z - 2.5 Scheme 19), while Z 3-heptene is transformed into 3-hexene and 4-octene with EjZ ratios of 0.75 and 0.6, respectively, which shows that in this case Z-olefins are favoured (Scheme 20). At full conversion, the thermodynamic equilibriums are reached to give the -olefins as the major isomers in both cases. For terminal olefins, the E olefin is the kinetic product because the favoured pathway involved intermediates in which the [ 1,2]-interactions are minimized, that is when both substituents (methyls) are least interacting. In the metathesis of Z-olefins, the metallacyclobutanes are trisubstituted, and Z-olefins are the kinetic products because they invoke reaction intermediates in which [1,2] and especially [1,3] interactions are minimized. 

One problem in the combination of metathesis transformations using alkenes is the fact that they are equilibrium reactions. In contrast, metathesis reactions of ene-ynes are irreversible as they give 1,3-butadienes, which are usually inert under the reaction conditions. Thus, the combination of a RCM and a ROM of ene-ynes of type 6/3-48 in the presence of an alkene (e. g., ethylene) led to 6/3-49 in good yield (Scheme 6/3.13) [242]. In these transformations the terminal triple bond reacts first. The process is not suitable for the formation of six-ring heterocycles. 

Reaction pathways apparently analogous to d and f of Eq. (26) yield a mixture of propylene and cyclopropane. Only when photochemical activation was employed were the major products olefins derived from metathesis-decomposition of the metallocycle. The failure to form metathesis olefins under moderate conditions is significant. It may be that either unimolecular dissociation of the olefin from the complex (in the absence of excess olefin to restabilize the carbene) is energetically unfavored, or the metallocyclobutane structure in the equilibrium given by steps a and b in Eq. (26) is highly stabilized and favored. These results... 

Stereospecificity as applied to olefin metathesis may be considered in two ways (a) How does the cis/trans isomer ratio of a product olefin compare with its equilibrium ratio, or (b) how does this cis/trans value differ from 1.0, which is the statistically expected value in terms of probabilities. In the present discussion, the latter definition applies. 

With but few notable exceptions (75-77), an inherent characteristic of the metathesis of acyclic olefins with both homogeneous and heterogeneous catalysts is the tendency for attainment of thermodynamic equilibrium in the composition of cis and trans isomers in reactions carried to high conversion. Therefore, any inherent stereospecificity can only be evaluated by extrapolating compositional data to zero percent reaction. 

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