Base-catalyzed alkene isomerizations

Allylic protons of alkenes undergo base-catalyzed exchange much more rapidly than either vinylic or saturated aliphatic protons . The enhanced kinetic acidity of such protons is due to the fact that their removal results in formation of resonance-stabilized allylic carbanions. These carbanions react with proton donors to form the original alkenes and sometimes one or more isomeric alkenes, viz. 

The mechanistic and stereochemical options available to allylic carbanions are fully as complex as those available to allylic carbonium ions. Much that is known concerning the mechanisms of base-catalyzed alkene isomerizations was obtained from stereochemical, isotope exchange, and product composition studies. The present discussion is limited to alkene isomerizations whose kinetics have been studied in homogeneous systems. Isomerizations involving allylic carbanion intermediates have been reviewed by Cram .  

The kinetics of potassium /erC-butoxide-catalyzed interconversion of cis-and /rn/7.s-a-methylstilbene and a-benzylstyrene was studied in / r/.-butyl alcohol solutions, viz. 

The kinetic data together with isotope exchange and solvent isotope elfect data are consistent with a mechanism involving non-interconverting cis and trans allylic carbanion intermediates, with partially intramolecular proton transfer, 

Labeling experiments showed that most of the isomerization does not involve exchange that is, the prototropic shift is largely intramolecular. This suggests that the protonated base formed simultaneously with the allylic carbanion is not free to exchange, but is, instead, rather tightly held by the anion XI. Proton exchange occurs by a dissociated carbanion intermediate XII, viz. 

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Most studies concerning the mechanism of base-catalyzed alkene isomerization were conducted with alkenylarenes.121... 

Most of the published kinetic data on base-catalyzed alkene isomerizations are for reactions of I-alkenes. Alkyl substitution stabilizes alkenes and products of isomerization of terminal alkenes isomerize at slower rates than their precursors. Rates of isomerization of methylcycloalkenes can be estimated from kinetic and equilibrium data or measured directly These compounds isomerize at slower rates than the corresponding methylenecycloalkanes, as expected. 

As in the case of the base-catalyzed reaction, the thermodynamically most stable alkene is the one predominantly formed. However, the acid-catalyzed reaction is much less synthetically useful because carbocations give rise to many side products. If the substrate has several possible locations for a double bond, mixtures of all possible isomers are usually obtained. Isomerization of 1-decene, for example, gives a mixture that contains not only 1-decene and cis- and franj-2-decene but also the cis and trans isomers of 3-, 4-, and 5-decene as well as branched alkenes resulting from rearrangement of carbocations. It is true that the most stable alkenes predominate, but many of them have stabilities that are close together. Acid-catalyzed migration of triple bonds (with allene intermediates) can be accomplished if very strong acids (e.g., HF—PF5) are used. If the mechanism is the same as that for double bonds, vinyl cations are intermediates. 

Ru(H20)6], which is a precursor of ROM polymerization of cyclic dienes has also been found to possess good alkene isomerization activity [1], Among others it catalyzed the isomerization of allylphenyl ether to a vinylphenyl ether (Scheme 9.1) at room temperature. Allyl ethers are stable to acids and bases, while vinyl ethers are easily cleaved in acidic solutions. Therefore this isomerization gives a mild method for removal of protecting allyl groups under exceedingly mild conditions. 

Different catalysts bring about different types of isomerization of hydrocarbons. Acids are the best known and most important catalysts bringing about isomerization through a carbocationic process. Brpnsted and Lewis acids, acidic solids, and superacids are used in different applications. Base-catalyzed isomerizations of hydrocarbons are less frequent, with mainly alkenes undergoing such transformations. Acetylenes and allenes are also interconverted in base-catalyzed reactions. Metals with dehydrogenating-hydrogenating activity usually supported on oxides are also used to bring about isomerizations. Zeolites with shape-selective characteristics... 

The situation is more complicated in the case of 5-bromohexopyranose derivatives, because products of both endo- and exo-elimination may be formed, and available evidence indicates that, with 5-bromo-/ -D-glucopyr-anose esters, base-catalyzed elimination favors the production of endo-alkenes following loss of axial hydrogen and bromine atoms. Alternatively, treatment with zinc-acetic acid gives, mainly, the products of exo-elimination.90 From the acetate 164 (R = Ac) and the benzoate 164 (R = Bz), the 4-enes (165, R = Ac, Bz) were both obtained in 65% yield following treatment with 1,5-diazabicyclo[5.4.0]undec-5-ene, whereas zinc - acetic acid afforded the 5-enes (166, R = Ac, Bz) in 59 and 67% yield. The isomeric endo products 167 (R = Ac, Bz) were isolated in 15 and 11% yield, and, from the... 

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,... 

The discovery that sodium in the presence of small amounts of organosodium compounds, produced in situ or deposited on alumina, acts as an effective catal for double bond isomerization of alkenes and cyclenes U) triggered much research in this field (2). It was subsequently discovered that base-catalyzed isomerization of olefins may proceed in homogeneous solutions using lithium ethylenediamine (3) or potassium tert-butoxide (UBuOK) in dimethyl sulfoxide (DMSO) (4). 

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. ... 

As Peterson outlined in his preliminary communication of the method, either basic (KH, KOBu or NaH) or acidic conditions (acetic acid, sulfuric acid or boron trifluoride etherate) may be utilized to effect the elimination of the silylcarbinol. Alternatively the initial adduct may be treated in situ with thio-nyl or acetyl chloride. This procedure may be advantageous in cases where isomerization of the alkene is problematic, and is particularly useful in the synthesis of terminal alkenes. As discussed in Section 3.1.3.4.2, the Johnson group has successfully employed aqueous HF to effect the elimination and this method may also have advantages in situations complicated by base-catalyzed isomerization. ... 

Rates of potassium / r/.-butoxide-catalyzed isomerization of l-alkenes and methylenecycloalkanes in dimethylsulfoxide at 55°C are summarized in Table 2. For the acyclic as well as exocyclic alkenes, isomerization rates parallel rates of base-catalyzed enolization of structurally analogous ketones. This is further evidence that the rate-limiting step in the alkene isomerizations is proton abstraction to form the allylic carbanion. 

AUylic hydroperoxides. The product ratios of the allylic hydroperoxides obtained on oxidation of alkenes with singlet oxygen differ significantly from those obtained by base-catalyzed isomerization of /3-halo hydroperoxides, which involves perepoxide intermediates. A third mechanism must be operating in the reaction of triphenyl phosphite ozonide (3, 323-324 4, 559). This last reaction presumably proceeds by an ionic mechanism, since singlet oxygen is not formed at — 70° from the ozonide. ... 

Besides various iron and ruthenium complexes [43, 44], nickel-based catalysts have recently been shown to be highly reactive in this respect as well [45]. Thus, a nickel catalyst prepared in situ from equimolar amounts of NiCl2(dppe) (dppe, l,2-bis(diphenylphosphino)ethane) and LiBHEtj (5 mol% each), which presumably resulted in the formation of NiHCl(dppe) as the active catalyst, efficiently catalyzed the isomerization/aldol event of allyl alcohols 82 and aldehydes 83 in combination with the Lewis acid MgBr2 (5 mol%) to furnish aldol products 84 in typically excellent yields and variable isomeric ratios (Table 8.11). Allyl alcohols with a terminal alkene reacted much faster than those with an internal olefin, and the aldol reaction occurred exclusively on the side of the former allyl alcohol. 

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... 

Although alkali metal amides cannot catalyze intermolecular hydroamination of higher unactivated alkenes, allylbenzene derivatives react smoothly via base-catalyzed isomerization into p-methyl styrene derivatives, which are active enough to form hydroamination products (22) [170]. 

The protocol developed by Jacobsen and Katsuki for the salen-Mn catalyzed asymmetric epoxidation of unfunctionalized alkenes continues to dominate the field. The mechanism of the oxygen transfer has not yet been fully elucidated, although recent molecular orbital calculations based on density functional theory suggest a radical intermediate (2), whose stability and lifetime dictate the degree of cis/trans isomerization during the epoxidation <00AG(E)589>. 

Hosokawa, Murahashi, and coworkers demonstrated the ability of Pd" to catalyze the oxidative conjugate addition of amide and carbamate nucleophiles to electron-deficient alkenes (Eq. 42) [177]. Approximately 10 years later, Stahl and coworkers discovered that Pd-catalyzed oxidative amination of styrene proceeds with either Markovnikov or anti-Markovnikov regioselectivity. The preferred isomer is dictated by the presence or absence of a Bronsted base (e.g., triethylamine or acetate), respectively (Scheme 12) [178,179]. Both of these reaction classes employ O2 as the stoichiometric oxidant, but optimal conditions include a copper cocatalyst. More recently, Stahl and coworkers found that the oxidative amination of unactivated alkyl olefins proceeds most effectively in the absence of a copper cocatalyst (Eq. 43) [180]. In the presence of 5mol% CUCI2, significant alkene amination is observed, but the product consists of a complicated isomeric mixture arising from migration of the double bond into thermodynamically more stable internal positions. 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

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