Alkenes double bond isomerization

In the case of aryl-substituted alkenes, double bond isomerization can be effected. Thus, rearrangement of allylbenzene to cis- and/or traws -jS-methylstyrene has been effected with some ruthenium(II) complexes <89IC3831>. 

Cyclic alkenes give different regioisomers depending on the reaction conditions owing to double bond isomerization caused by syii elimination of Pd—H species and its readdition. The following three reaction conditions were tested for the reaction of cycloheptene (35)[18,38] ... 

No matter which of the electrophilic methods of double-bond shifting is employed, the thermodynamically most stable alkene is usually formed in the largest amount in most cases, though a few anomalies are known. However, there is another, indirect, method of double-bond isomerization, by means of which migration in the other direction can often be carried out. This involves conversion of the alkene to a borane (15-16), rearrangement of the borane (18-11), oxidation and hydrolysis of the newly formed borane to the alcohol (12-28), and dehydration of the alcohol (17-1) ... 

Double bond cis-trans isomerization occurs during hydrogenation with a relative rate dependent on structure. The less stable double bond isomerizes to the more stable one, but, of course, kinetics and thermodynamics control the extent of isomerization. In a linear carbon chain, one can expect the cis alkene to isomerize to trans and vice versa if the thermodynamics are favorable. However, in a strained cyclic system, trans will isomerize to cis (Fig. 2.13).117... 

Alkenes with two reactive carbon-carbon double bonds per molecule like 1,5-hexadiene or diallyl ether are used in the synthesis of silicone compounds which can be later crosslinked by hydrosilylation. A sufficiently high excess of double bonds helps to prevent the dienes from taking part in silane addition across both olefmic ends, but trouble comes from double bond isomerization (Eq. 2). 

Various transition metal complexes, in particular of late transition metals, were reported to be effective catalysts for such double bond isomerization. Because organic synthesis is the focus of this volume, this section will cover the transition metal-catalyzed isomerization of alkenes, which has the significant synthetic and industrial utilities. This chapter will also include the synthetic application, asymmetric reactions,4-6 and isomerization of alkynes, in particular, that of propargylic alcohols. 

Although there are many reports on the enantioselective catalytic double bond isomerization of functionalized achiral alkenes, that of alkenes bearing an isolated double bond have had limited success. The use of a chiral bis(indenyl)titanium catalyst 5 containing a chiral bridging group realized the highly enantioselective isomerizations of unfunctionalized achiral alkenes with up to 80% ee (Equation (27)).90... 

In the case of alkenes, 1-pentene reactions were studied over a catalyst with FAU framework (Si/Al2 = 5, ultrastable Y zeoHte in H-form USHY) in order to establish the relation between acid strength and selectivity [25]. Both fresh and selectively poisoned catalysts were used for the reactivity studies and later characterized by ammonia temperature programmed desorption (TPD). It was determined that for alkene reactions, cracking and hydride transfer required the strongest acidity. Skeletal isomerization required moderate acidity, whereas double-bond isomerization required weak acidity. Also an apparent correlation was established between the molecular weight of the hard coke and the strength of the acid sites that led to coking. 

Comparison of the results for catalytic isomerization of pent-l-ene to trans-pent-2-ene with the basic and one-electron donating properties of the catalysts led to the conclusion that two different reaction mechanisms operate in double bond isomerization reactions (a) an ionic mechanism which involves proton abstraction from the alkene molecule by the super base site (pAia = 37 for pentenes) and (b) a free radical mechanism which involves the abstraction of a hydrogen atom from the alkene by the one-electron donor center (Scheme 39). 

Polymerization leads to a polymer structure (VII) with a repeating alkene double bond in the polymer chain. The double bond in each repeating unit of the polymer chain is a site of steric isomerism since it can have either a cis or a trans configuration. The polymer chain segments on each carbon atom of the double bond are located on the same side of the double... 

With vinylketene adducts (2-vinylcyclobutanones) possessing an enolizable hydrogen, the presence of triethylamine used for generating the ketene will occasionally cause double-bond isomerization resulting in alkylidenecyclobutanones 22.138,139 These derivatives are not readily prepared by alkylideneketene to alkene cycloadditions because of the extreme reactivity towards dimerization of these ketenes. 

We have used our Single Turnover (STO) reaction sequence to characterize dispersed metal catalysts with respect to the numbers of alkene saturation sites, double bond isomerization sites, and hydrogenation inactive sites they have present on their surfaces (ref. 13). Comparison of the product composition observed when a series of STO characterized Pt catalysts were used for cyclohexane dehydrogenation with those observed using a number of instrumentally characterized Pt single crystal catalysts has shown that the STO saturation sites are comer atoms of one type or another on the metal surface (ref. 10). 

Alcohol addition to alkenes can also be achieved photochemically under neutral conditions (equation 245).397 This reaction appears to be a photochemically induced ionic addition to the double bond. An arene photosensitizer is required. Unfortunately, this reaction is limited to cyclic alkenes and double bond isomerization is significant. 

Another potential complication is palladium-promoted double-bond isomerization of the alkene product, which can also destroy a newly formed stereogenic center [10]. This process involves re-addition of palladium hydride to the alkene in the initially generated (alkene)palla-dium hydride complex (the reverse of P-hydride elimination). Double-bond isomerization results when re-addition occurs with different regioselectivity than the original P-hydride elimination. 

An ab initio method has been employed to study the mechanism of the thermal isomerization of buta-1,2-diene to buta-1,3-diene. The results of the study have indicated619 that the transformation proceeds in a stepwise manner via a radical intermediate. Experimental free energies of activation for the bond shift in halocyclooctatetraenes have been reported and analyzed by using ab initio MO calculations.620 The isomerization of hexene using a dihydridorhodium complex in dimethyl sulfoxide has been reported,621 and it has been suggested622 that the Pd(II)-catalysed homogeneous isomerization of hexenes proceeds by way of zr-allylic intermediates. A study has been made623 of alkene isomerization catalysed by the rhodium /-phosphine-tin dichloride dimeric complex, and the double-bond isomerization of olefinic amines over potassium amide loaded on alumina has been described.624... 

In order to prepare the bicyclic compound 9, an intramolecular Heck reaction under Grigg s conditions (Pd(OAc)2, K2CO3, Bu4NBr) is carried out (see Chapter 13).7 Starting from aryl iodide 8 a six-membered ring formation occurs providing intermediate 28, which reacts to alkene 29. Finally double bond isomerization to compound 9 is forced by the formation of a thermodynamically more stable enol ether. 

Mono- and 1,1-disubstituted olefins are the most reactive, but even tetrasubsti-tuted alkenes have been used [17]. Because double-bond isomerization is a common side-reaction, olefins with a non-isomerizable bond are usually used. 

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,... 

Although in principle any olefin can undergo such a transformation, problems often arise with higher alkenes, because of the ease of double bond isomerizations that lead to a variety of by-products. 

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