Olefin isomerization equilibration

The method of preparation of the alumina has a marked effect on the product distribution as shown in Table VIII (47). Over the pure alumina (P) the olefinic products are nearly equilibrated. The alkali-containing catalysts, however, give kinetically controlled products. The very low activity of these catalysts for olefin isomerization had been ascertained independently. It may, therefore, be concluded that the compo.sition of the olefins produced at 350° is very nearly that of the primary dehydration products. 

Olefin isomerization has also been mediated by the photolysis of Fe(C0)s.144 Recently, a detailed study of alkene isomerization by photolysis of Fe(CO)5 has shown that the reaction is truly photocatalytic.14S The very high quantum yields ( 1.0), Table 24, and the fact that the pentenes are ultimately equilibrated to the thermodynamic ratio support the notion that the role of the light is to generate a thermally active catalyst. A mechanism similar to that in reactions (53)-(57) involving Fe(CO)3 as the repeating unit can be used to account for the results. 

Many isomerization reactions occur by double-bond migration. Two common mechanisms of double-bond migration are (Parshall and Ittel, 1992) (1) equilibration between internal and terminal olefins by hydrogen migration and (2) interconversion between branched and linear chains of an olefin. Another mechanism of olefin isomerization is the shift of an allylic hydrogen from the 3- to the 1-position. We confine our discussion to the more important doublebond migration mechanism. 

In contrast, carbon vapor generated in a carbon arc deposits mixtures of thermally equilibrated C(1S), C(3D), and C(3P) atoms on the walls of the reaction vessel, where they can be reacted with olefins.16 The most energetic and shortest-lived species, CX S), apparently forms allenes and inserts into C—H bonds. The Q1/)) atoms yield spiro-pentanes by two stereospecific addition steps. After long periods only ground state C(3P) atoms remain, and they add to olefins partially stereospecifically as shown below to yield isomeric spiropentanes. 

There are few examples of isomerization of achiral unfunctionalized olefins into enantiomerically enriched olefins. The most successful is the transformation of meso,trans-4-tert-butyl-l-vinylcyclohexane (1) into the corresponding alkene (S)-2 (Scheme 1) [6]. When a C2-symmetric titanocene catalyst, which can easily be prepared in three steps from enantiopure starting material, is used in the presence of LiAlH4 as activator the product is obtained in 80% ee. The rate of the reaction and the optical purity of the product are highly dependent on reaction temperature. The lower enantiomeric purity obtained at higher temperatures is apparently because of racemization by equilibration of the product, a process which is promoted by longer reaction times. 

A useful generality in predicting selectivity is that the rate of hydrogenation falls, both absolutely and competitively, as steric hinderance around the double bond is increased (23). On this basis in a mixture of olefins, terminal olefins would be expected to be saturated preferentially to internal olefins, if a prior rapid equilibration by isomerization did not ensue. 

The following facts are the basis for butene isomerization (I) There is a basic similarity in the composition of alkylates produced from all four butene isomers. (2) Alkylate molecules, once formed, are relatively stable under alkylation conditions and do not isomerize to any appreciable extent alkylate fractions having the same carbon number ore not equilibrated (see Table I). (3) Thermodynamic equilibrium between the butene olefins highly favors isobutene formation at alkylation temperatures. (4) Normal butenes p>roduce only small and variable amounts of normal butane, thus indicating only a small and variable amount of chain initiation from normal butenes. Yet the alkylate composition shows a high concentration of trimethylpentanes and a low concentration of dimethylhexanes. (5) A few of the octane isomers can be explai.ned only by isomerization of the eight-carbon skeletal structure this isomerization occurs while isobutene dimer is in ionic form. For example, 2,3,3- and 2,3,4-trimethylpentanes... 

Migrations of the double bond and carbon-skeleton rearrangements are important in the preparation of several olefinic compounds. A number of alkyl cyclopentenes are available in quantities suitable for synthetic work by the isomerization of cyclohexene and its homologs over alumina at 470- 480°. o-Allylphenol is isomerized by methanolic potassium hydroxide at 110° to o-propenylphenol (75%). Several /5, y-olefinic acids are conveniently obtained from the corresponding a /3-isomers by equilibration in basic media. The two isomeric acids are readily separated by partial esterification of the resulting mixtures since the /3,y-isomers are more easily esterified. "... 

Arene oxides show the characteristic reactions of epoxides (isomerization to ketones, reductions to alcohols, nucleophilic additions, deoxygenations) and olefins or conjugated dienes (catalytic hydrogenation, photochemical isomerization, cycloaddition, epoxidation, metal complexation). Where a spontaneous, rapid equilibration between the arene oxide and oxepin forms exists, reactivity typical of a conjugated triene is also found. 

Complications may arise where rearrar ement occurs in a step subsequent to nucleophilic substitution. For example Gagneux, Winstein and Young have shown that y,y-dimethylallyl chloride (39) affords a mixture of the azides 40 and 41 on treatment with azide ion (equation 29). The pure azides, separated by g.l.c., rapidly equilibrated to the isomeric mixture. On the basis of the known ability of aryl and alkyl azides to add to olefins, these authors suggested that the isomerization process was conceivably similar in character to an intramolecular addition for which intermediates such as 42 and 43 could be visualized. A possible alternative pathway for this... 

MOGD olefin product distribution is determined by thermodynamic, kinetic, and shape-selective limitations. The equilibrium calculation was greatly simplified by assuming the isomers for a given carbon number to be at equilibrium (ref. 19). At low pressure and high temperature, olefin equilibrium is reached, while at higher pressure kinetic limits prevent equilibrations at commercially feasible space velocities. Isomerization reactions are fast at all carbon numbers, and isomer equilibrium is achieved for low carbon numbers. Shape selectivity determines isomer equilibrium for higher carbon... 

The metalation of tetramethylsilane is another dramatic example of the effect of chelation on reactivity since both n-BuLi and sec-BuLi are nearly inert under the same conditions. Chelated sec-BuLi is the most reactive soluble metalating agent we have found. TMED Li-sec-Bu reacts with tetramethylsilane about 1000 times faster than TMED Li-n-Bu and yields purer product (47). Broaddus (48, 49) has discussed kinetic metalation of olefins and alkyl aromatic compounds using TMED LiBu, and he also observed the slow equilibration to the thermodynamically favored isomers. The extent of ring metalation in toluene and the conditions for isomerization to benzyllithium are discussed in Chapter 2, Smith. 

When metathesis is effected with tra i-2-pentene, rather than cis-, and (diphenylcarbene)pentacarbonyltungsten is the initiator, the 2-butene and 2-hexene products are largely trans. The stereospecificity (73-83% trans) is not as great as for cw-olefin metathesis, but it is appreciable (63). The ratios of the stereoisomers in the products are close to the equilibrium ratios, but they probably are not determined by the products equilibrating, for in the short time the metathesis was run to determine the stereochemistry of the initial product, the precursor, tranj-2-pentene, underwent only negligible isomerization. The stereochemistries therefore are determined by the kinetics, which in turn should be affected by conformational factors similar to those in Scheme... 

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