Butene isomerization, base catalyzed

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-... 

Extrapolation of the rate data in Fig. 24 to zero conversion shows that the initial ratio of butene-1 to frans-butene formation is about unity. Thus, butene-1 is not an intermediate in the cis-trans isomerization and direct cis-trans isomerization occurs. Similar results are found for the heterogeneous base catalyzed isomerization over sodium on alumina (17). 

The experimental probe of the conformational preferences of allyl anions is based primarily on the kinetics of base catalyzed isomerization reactions of olefins and many results have been summarized by Bank180). Kinetic data of base catalyzed isomerization of 1-butene181) show that the less stable cis isomer of 2-butene is formed through a thermodynamically more stable allylic anion ... 

Additional evidence for the greater stability of the cis conformation of allylic anions is provided by other base catalyzed isomerization studies of 1-butene and 1-pentene. It was found that the thermodynamically less stable cis isomers of 2-butene and 2-pentene were the major products of the reaction182-186). Furthermore, m-2-butene isomerizes, under the same conditions, faster than the tram isomer to give 1-butene. 

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,... 

Base catalysts are extremely sensitive to the atmosphere in which they have been stored and generally require some sort of activation procedure, usually heating, prior to application. For materials stored in air, the formation of surface carbonates via reaction with atmospheric carbon dioxide is problematic. Figure 21.1 illustrates the effect of pre-treatment upon base-catalyzed n-butene isomerization [9]. It can be seen that activity increases up to a maximum of circa 650 °C. 

Studies on the use of SrO as a base catalyst are much less common than those using MgO. However, Mohri and coworkers [54] investigated the use of SrO for the base-catalyzed isomerization of 1-butene. The isomerization of alkenes by base catalysts has been known for many years [55] and usually requires a strong base. Mohri and coworkers found that activity was only detected after heating SrO in vacuo to 650 °C and that a temperature treatment of 1000 °C was required to achieve maximum activity. Selective poisoning revealed that both the Sr and the 0 ions had discrete roles with the tra s-2-butene yield associated with the Sr, while the cis-2-butene yield was associated with the O. ... 

When an unsymmetrical secondary alcohol is formed, depending on which carbon-oxygen bond is cleaved. With propylene oxide, for example, a base-catalyzed reaction favors the formation of the secondary alcohol almost exclusively, whereas, a non-catalytic or acid-catalyzed alcoholysis yields a mixture of the isomeric ethers. However, the reactions of other a-epoxides, such as 3,4-epoxy-l-butene, 3,4-epoxy-l-chloropropane (epichlorohydrin), 3,4-epoxy-l-propanol (glycidol), and styrene oxide, are more complicated with respect to which isomer is favored. ... 

Modified Zeolites. As described above, alkali ion-exchanged zeolites are weak bases. Various efforts have been made to increase the base strength of alkali metal-ion exchanged zeolites. Metallic sodium particles in zeolites are formed by the decomposition of occluded sodium azide (59). These sodium particles are capable of performing base-catalyzed reactions. These catalysts catalyze the isomerization of butenes at 300 K and the side-chain alkylation of toluene with ethylene at 523 K. 

Isomerization of Alkenes and Alkynes. Isomerization of alkenes proceeds through anion intermediates by abstraction of an allylic proton from alkene molecules by solid bases. In the case of 1-butene isomerization, high cisitrans ratio of 2-butene is characteristic of the base-catalyzed isomerization. On the... 

Scheme 9 Base catalyzed 1-butene double bond isomerization...

Base-catalyzed reactions were reviewed in general by Hattori [208,222], and from the reactions described butene isomerization, alcohol conversion, toluene alkylation, acetone condensation/diacetone alcohol decomposition, Knoevenagel condensations, diketone cyclization, ring transformation reactions, and dimerization of aldehydes to the corresponding esters were used to evaluate the acid-base properties in molecular sieves. 

Butene isomerization has already been discussed in Sect. 2.3.1 and the limitations for the characterization of acidity and basicity with this test reaction were outlined. As the products of the acid- and base-catalyzed reactions are the same (see Scheme 9), the combined presence of both types of sites would make the determination of acidity or basicity very difficult, if possible at all. 

Rb20 is shown in Fig. 3.2. In the base-catalyzed isomerization of butene, the intermediates are primarily the cis form of allylic anions, because the cis allylic anion is more stable than the trans allylic anion. A curve convex to 1-butene— V-2-butene axis is caused by the intermediate being cis form of allylic anion and characteristic of base-catalyzed butene isomerization. Essentially the same curves are observed for the other alkali metal oxides such as Li20, Na20, K2O, and CS2O. 

The reaction network for butene isomerization over KCg is also similar to those of alkali oxides. The curves are convex to l-butene-m-2-butene axis, and therefore, base-catalyzed isomerizaiton is suggested. 

Butene isomerization over alkaline earth oxides has been studied extensively. The activity and selectivity variations in 1-butene isomerization as a function of pretreatment temperature are shown in Fig. 3.10 for CaO. Similar variations have been observed for MgO, SrO and BaO. The activities appear as H2O and CO2 are removed from the surfaces. A high cis to trans ratio is characteristic of base-catalyzed 1-butene isomerization. This is caused by the high stability of as allylic anions as compared with trans allylic anion as described further on. 

Solid bases As shown in Table 4.1, the cisitrans ratio from 1-butene is usually very large in the case of solid bases. This is because the allyl intermediate formed by the abstraction of proton from butene (eq. 2) is more stable in the anti (cw) form than in the syn (trans) form (eq. 4). Superbases such as CsiO and Rb20 are reported to catalyze butene isomerization in a similar way. ... 

The selectivity of the metal sulfate catalyst is influenced by many factors besides its acidic property, such as geometric structure involving a pore structure, arrangement of basic sites, polarity of the surface, etc. For example, the relative values of the first-order rate constants (per imit acidity at pK — 3) of the depolymerization catalyzed by nickel sulfate, cupric sulfate, and silica-alumina were found to be 1100 300 1. The difference may be attributed to the differences in acid-base bi-functional catalysis of these catalysts. This view may be said to have originated in 1948 when Turkevich and Smith (45) showed that the isomerization of 1-butene to 2-butene is catalyzed by metal sulfates, sulfuric acid, phosphoric acid, etc., but little by acetic acid, hydrogen chloride, etc. The high catalytic activity of the catalysts of the former group is considered as due to acid-base bifunctional catalysis as illustrated by Fig. 14. Independently, Horiuti (45a) advanced the same idea... 

For use of this type of anthracene based ligands in the rhodium catalyzed hydroformylation of isomeric butenes in ILs as solvent, compare Dyballa,... 

The acidity and basicity increase when ZnO is mixed with other oxides. ZnO —Ti02 becomes more acidic and reveals high activity for hydration of ethylene. Both acid and base amounts increase upon mixing with Si02 and isomerization of butene is catalyzed by the acid sites of ZnO — SiOz Addition of K to ZnO increased the base amount, and the catalytic activity for oxidation of formic acid sharply increased.A mixed oxide catalyst consisting of ZnO and FezOa is a good basic catalyst for the methylation by methanol of phenol to 2,6-xylenol. ... 

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