Trans alkenes

Figures 7 13 and 7 14 depict the stereochemical relationships associated with anti addition of bromine to (E) and (Z) 2 butene respectively The trans alkene (E) 2 butene yields only meso 2 3 dibromobutane but the cis alkene (Z) 2 butene gives a racemic mixture of 2R 3R) and 2S 3S) 2 3 dibromobutane...

A useful alternative to catalytic partial hydrogenation for converting alkynes to alkenes IS reduction by a Group I metal (lithium sodium or potassium) m liquid ammonia The unique feature of metal-ammonia reduction is that it converts alkynes to trans alkenes whereas catalytic hydrogenation yields cis alkenes Thus from the same alkyne one can prepare either a cis or a trans alkene by choosing the appropriate reaction conditions... 

Alkyne Sodium Ammonia Trans alkene Sodium amide ... 

Group I metals—sodium is the one usually employed—in liquid ammonia as the solvent convert alkynes to trans alkenes The reaction proceeds by a four step sequence in which electron transfer and proton transfer steps alternate... 

We saw m Section 9 10 that the combination of a Group I metal and liquid ammonia is a powerful reducing system capable of reducing alkynes to trans alkenes In the pres ence of an alcohol this same combination reduces arenes to nonconjugated dienes Thus treatment of benzene with sodium and methanol or ethanol m liquid ammonia converts It to 1 4 cyclohexadiene... 

Gas-phase thermolysis of (69) and (70) at 600 C/12 Torr occurs to give the corresponding cis and trans alkenes respectively, suggesting a concerted [<, +<,2a] cycloreversion process (70JA1763). 

Part B of Table 1.5 gives heats of formation for the C4, C5, and some of the Cg alkenes. A general relationship is also observed for the alkenes. The more highly substituted the double bond, the more stable is the compound. There are also other factors that enter into alkene stability. trans-Alkenes are usually more stable than cis-alkenes, probably largely because of increased nonbonded repulsion in the cis isomer. ... 

Catalytic hydrogenation of alkynes on a metal surface provides cis alkenes (see Chapter 7, Problem 13), while treatment with sodium in liquid ammonia nearly always leads to trans alkenes, e.g., hydrogenation of 2-butyne. 

Normally, a trans alkene is more stable than its cis isomer. tra/75-Cyclooctene, however, is less stable than c/s-cyclooctene by 38.5 kj/mol. Explain. 

If dichlorocarbene is generated in the presence of an alkene, addition to the double bond occurs and a dichlorocyclopropane is formed. As the reaction of dichlorocarbene with ds-2-pentene demonstrates, the addition is stereospecific, meaning that only a single stereoisomer is formed as product. Starting from a cis alkene, for instance, only cis-disubstituted cyclopropane is produced starting from a trans alkene, only trans-disubstituted cyclopropane is produced. 

Mechanism of the lithium/ ammonia reduction of an alkyne to produce a trans alkene. 

Alkynes can be reduced to yield alkenes and alkanes. Complete reduction of the triple bond over a palladium hydrogenation catalyst yields an alkane partial reduction by catalytic hydrogenation over a Lindlar catalyst yields a cis alkene. Reduction of (he alkyne with lithium in ammonia yields a trans alkene. 

Occasionally, chemists need to invert the stereochemistry of an alkene—that is, to convert a cis alkene to a trans alkene, or vice versa. There is no one-step method fordoing an alkene inversion, but the transformation can be carried out by combining several reactions in the proper sequence. How would you carry out the following reactions ... 

In 1970, it was disclosed that it is possible to achieve the conversion of dimethylformamide cyclic acetals, prepared in one step from vicinal diols, into alkenes through thermolysis in the presence of acetic anhydride." In the context of 31, this two-step process performs admirably and furnishes the desired trans alkene 33 in an overall yield of 40 % from 29. In the event, when diol 31 is heated in the presence of V, V-dimethylforrnamide dimethyl acetal, cyclic dimethylformamide acetal 32 forms. When this substance is heated further in the presence of acetic anhydride, an elimination reaction takes place to give trans olefin 33. Although the mechanism for the elimination step was not established, it was demonstrated in the original report that acetic acid, yV, V-dimethylacetamide, and carbon dioxide are produced in addition to the alkene product."... 

Different substrate geometries can even result in alternate reaction pathways operating. The reactions between trans-a, (3-epoxytrimethylsilane 115 and organo-metals (metal = Li, Ce, or La) give predominantly trans-alkene 116 in high yields (Scheme 5.25) [38]. In contrast, treatment of cis-115 with some of the same organo-metals produces (Z)-vinylsilanes. The use of a bulkier substituent on silicon (e. g.,... 

The formation of alkenes from thiirane dioxides may not be stereospecifically controlled in the presence of a sufficiently strong base and sufficiently acidic protons in the three-membered ring. Under such conditions (essentially those typical for the Ramberg Backlund reaction), epimerization via a carbanion intermediate produces an equilibrium mixture of thiirane dioxides19,99 and consequently a mixture of cis- and trans-alkenes. 

It is also important to note that trans-alkenes are often more stable than cis-alkenes due to diminished steric hindrance (p. 190), but this is not always the case. It is known, for example, that c -l,2-difluoroethene is thermodynamically more stable than tra j-l,2-difluoroethene. This appears to be due to delocalization of halogen lone-pair electrons and an antiperiplanar effect between vicinal antiperiplanar bonds. 

Of course, the trans isomer will give the opposite results the threo pair if the addition is syn and the erythro pair if it is anti. The threo and erythro isomers have different physical properties. In the special case where Y=W (as in the addition of Br2), the erythro pair is a meso compound. In addition to triple-bond compounds of the type ACsCA, syn addition results in a cis alkene and anti addition in a trans alkene. By the definition given on page 166 addition to triple bonds cannot be stereospecific, though it can be, and often is, stereoselective. 

Catalytic hydrogenation of triple bonds and the reaction with DIBAL-H usually give the eis alkene (15-11). Most of the other methods of triple-bond reduction lead to the more thermodynamically stable trans alkene. However, this is not the case with the method involving hydrolysis of boranes or with the reductions with activated zinc, hydrazine, or NH2OSO3H, which also give the cis products. 

For a catalyst that leads to trans alkenes, see Burch, R.R. Muetterties, E.L. Teller, R.G. ... 

Elimination of HBr from m 50-l,2-dibromo-l,2-diphenylethane gave cis-2-bromostilbene, while the ( + ) or ( —) isomer gave the trans alkene. This stereospecific result, which was obtained in 1904, demonstrates that in this... 

Other examples of syn elimination have been found in medium-ring compounds, where both cis and trans alkenes are possible (p. 158). As an illustration, we can look at experiments performed by Zavada et al. These workers subjected 1,1,4,4-tetra-methyl-7-cyclodecyltrimethylammonium chloride (9) to elimination and... 

As we have indicated with our arrows, the mechanism of the uncatalyzed Cope rearrangement is a simple six-centered pericyclic process. Since the mechanism is so simple, it has been possible to study some rather subtle points, among them the question of whether the six-membered transition state is in the boat or the chair form. ° For the case of 3,4-dimethyl-l,5-hexadiene it was demonstrated conclusively that the transition state is in the chair form. This was shown by the stereospecific nature of the reaction The meso isomer gave the cis-trans product, while the ( ) compound gave the trans-trans diene. If the transition state is in the chair form (e.g., taking the meso isomer), one methyl must be axial and the other equatorial and the product must be the cis-trans alkene ... 

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