Carbanion, butene isomerization

The isomerization of butenes over MgO is a catalytic reaction which has been studied by a number of workers and is thought to occur via an anionic mechanism which involves basic sites on the surface. Baird and Lunsford (167) were able to show a correlation between the concentration of electron donors on the surface and the 1-butene isomerization rate, and they suggested that the formation of a carbanion occurred on O2- ions located on corner... 

With such strong base catalysts, carbanions can be formed even from simple olefins. Evidence for carbanionic intermediates is found in butene isomerization. Starting from 1-butene, the rate of czs-2-butene formation is twenty times higher than the rate of frans-2-butene formation (42) ... 

The binary oxides catalyze butene isomerization and 2-butanol dehydration. For butene isomerization, the alkyl cation mechanism is the main path. Contribution of the carbanion mechanism increases with increased TiOz content. A good correlation is observed between the acidity and the activity for dehydration. 

Isomerization of butene via a 7r-allyl species introduces an added dimension to the stereochemistry. The 7r-allyl species from propylene is presumed to be planar with its plane approximately parallel to the surface. Since it is attached to the electropositive zinc, it may have considerable carbanion character. A corresponding structure for adsorbed butene would lead to two isomeric forms, viz ... 

Cfs-butene should lead initially to the anti form trrms-butene should lead initially to the syn form and 1-butene should give rise initially to both. The equilibrium distribution of syn and anti forms usually differs greatly from the equilibrium distribution of cis- and frans-butene for cobalt complexes 59, 60) the syn form, precursor of irans-butene, is by far the most stable. By way of contrast for the corresponding carbanion, the cis anion seems by far the more stable. This preference for the cis carbanion is presumed to be the source of the high initial cis-to-trans ratio in the initial products of base catalyzed isomerization. In the base catalyzed isomerization of more complex cf-s-olefins (cfs-S-methyl-stilbene), the ions corresponding to syn and anti are not interconvertible and cis-trans isomeriza-... 

The selective isomerization of 1-butene to ds-2-butene in Fig. 17 and its reverse process, ds-2-butene to 1-butene (Table III), is caused by the kinetic facility of the formation of (Z)-methylallyl carbanion on a potassium-containing carbon (rz P rE reaction rates) in the presence of 02, and the slow internal rotation of (Z)-methylallyl carbanion to conformation on this catalyst. Furthermore, it is evident that the proton removed from olefin by an oxide ion does not migrate to the other oxide ions, but undergoes intramolecular hydrogen transfer as shown in Table III. 

Formation of both isomers (100 and 101) from the reaction of alkoxide ions with cis-(99) and fnms-2,3-dichlorohexafluoro-2-butene (102) (Park and Cook, 1965) should result from a genuine addition-elimination since elimination-addition is impossible, and there was no isomerization of the starting olefin. MeO-, EtO- and i-PrO- ions gave 95 + 2% retained cis ether (100) from the cis isomer (99), and 70 + 2% trans ether (101) from trans-( 102). The preferred retention points to short-lived carbanions. If CF8 interactions are the larger ones, as suggested by the models, the competition theory predicts predominant inversion for (99)... 

In isomerization reactions, an alkene is deprotonated to form an allyl anion, which is reprotonated to give the more stable alkene (double-bond migration). The most simple example is the isomerization of 1-butene producing a mixture of cis- and trans-2-butene (Scheme 3). Because the stability of the cis-allyl anion formed as an intermediate is greater than for the trans form, a high cis/trans ratio is observed for base-catalyzed reactions whereas for acid-catalyzed reactions the ratio is close to unity. Thus, the cis/trans ratio of the products has frequently been used as an indication of base-catalyzed reaction mechanisms. The carbanions formed in the course of such superbase reactions are not freely mobile in solution,... 

On alkaline earth metal oxides butene is adsorbed as methylallyl anions (CH3—CH—CH—CH2). This carbanion is an intermediate in the double bond isomerization of butene. Adsorption was shown to be stronger on CaO (higher basicity) than on MgO. Activation of CaO at 700-900 °C results in maximum Lewis basicity and optimum activity for the isomerization to 2-butene. 

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