Alkenes double-bond isomers

Fig. 1.30. In some cases the allyl radical intermediate of Wohl-Ziegler brominations is available from alkene double bond isomers, which can profitably be used when one of the substrates is more easily accessible or cheaper than its isomer.

A combination of a metathesis and a Diels-Alder reaction was published by North and coworkers [263]. However, this is not a true domino reaction, as the dienophile (e. g., maleic anhydride) was added after the in situ formation of the his-butadiene 6/3-89 from the fois-alkyne 6/3-88 and ethylene. The final product is the fois-cycloadduct 6/3-90, which was obtained in 34% yield. Using styrene as an un-symmetrical alkene instead of ethylene, the mono-cycloadduct 6/3-91 was formed as a mixture of double-bond isomers, in 38% yield (Scheme 6/3.26). 

The trends in alkene isomer stability can be easily seen in plots of the enthalpies of formation versus the number of carbons, nc (equation 4). For example, in Figure 3 which includes the gaseous 1-, cis- and trans-2-, and the cis- and trans-2-n-alkenes, the relatively more negative enthalpies of formation for the 2- and 3 -n-alkenes show them to be more stable than their 1-n-alkene double bond positional isomers. (Only the C4-C8 1-n-alkene members are shown for clarity.) Another well-known relationship is depicted as well acyclic trans hydrocarbons are more stable than their cis isomers. 

Part II of Figure 17.75 shows the side reactions that occur when the Pd-catalyzed hydrogenation is not completely cis-selective. The start is the formation of the -complex F from the hydropalladation product E. In a way, this reaction is the reverse of the reaction type that formed E from the -complex C (cf. part I of Figure 17.75). In an equilibrium reaction, the isomerized -complex F subsequently releases the alkene iso-B, which is a double bond isomer of the substrate alkene B—and represents a type of compound that could well be the side product of an alkene hydrogenation, too. 

Eplivanserin contains three stereogenic units. In addition to the E-config-ured alkene double bond there is a Z-configured substituted oxime. The single bond between both double bonds has partial double bond character. The formula shown below represents the s-trans isomer. This means that it represents an antiperiplanar conformation. 

The Orbital Description of the Alkene Double Bond 286 7-3 Elements of Unsaturation 287 7-4 Nomenclature of Alkenes 289 7-5 Nomenclature of Cis-Trans Isomers 291 Summary Rules for Naming Alkenes 293 7-6 Commercial Importance of Alkenes 294 7-7 Stability of Alkenes 296 7-8 Physical Properties of Alkenes 302... 

The Hofmann elimination yields alkenes and amines from larger amines. The major alkene product has the less substituted double bond, but all possible products may be formed. The hydrogens that can be eliminated are starred. When possible, cis and trans double bond isomers are both formed. 

Mercury(II) trifluoroacetate was found to be an effective catalyst for the equilibration of allylic A7, V-dimethyl carbamic esters at room temperature10 11. Tertiary allylic carbamates isomer-ized cleanly to the more stable, internal double-bond isomers at low concentration of mer-cury(II) trifluoroacetate (0.3 equiv). By use of an excess of mercury(ll) trifluoroacetate (3 equiv) contra-thermodynamic allylic isomerization of the carbamic ester of 2-alken-l-ols, which contained a disubstituted double bond, to the corresponding l-alken-3-ol derivatives could be achieved. Formation of a covalent adduct of the carbamic ester with the mercury(II) ion is responsible for this rearrangement. 

In the synthesis of novel antifimgal agent Ambruticin by Jacobsen and coworkers [114] the major building blocks were connected using the Julia-Kocienski olefination reaction (Scheme 87). It was found that high selectivity for either double bond isomer could be obtained NaHMDS in THF providing Z alkene (Table 24, entries 1 and 2) whereas LHMDS in polar solvents afforded the desired E isomer almost exclusively (entries 3 and 4). 

The formation of the sultone (160) probably involves addition of the complex across the alkene double bond, a 1,2-hydride shift and an intramolecular nucleophilic substitution reaction. The sultone (161) is formed by addition of sulfur trioxide to give the unstable p-sultone which rearranges to the more stable y-isomer (161). Another useful route to sultones is by metallation of alkanesulfonate esters for example, butane-1,3-dimethylsulfonate (162), prepared from butanel,3-diol, yields the 8-sultone, namely 6-methyl-l,2-oxathiin-2,2-dioxide (163) (Scheme 67). 

The requirement that the p orbitals overlap causes a very large barrier to rotation about the double bond, about 63 kcal/mol (264 kJ/mol). Cis (two groups on the same side) and trans (two groups on opposite sides) double-bond isomers do not interconvert at any reasonable temperatures. The trans isomer tends to be slightly more stable than the cis isomer because the groups may bump into one another when they are cis. More alkyl substitution on the double bond (replacing an H on the double bond by an R) makes the alkene slightly more stable. For example, an equilibrium mixture of butenes is found to contain 3% 1-butene, 23% ds-2-butene (R equals CHj above), and 74% /rans-2-butene. 

Two distinct types of intramolecular Heck reactions are important in the chemistry of furans. The most straightforward are transformations involving halofurans reacting intramolecularly with alkenes. For example, Muratake et al. exploited the intramolecular Heck cyclization to establish the tricyclic core structure en route to the synthesis of a furan analog of duocarmycin SA, a potent cytotoxic antibiotic [139], Under Jeffery s phase-transfer catalysis conditions, substrate 158 is converted to tricyclic derivatives 159 and 160 as an inseparable mixture (ca. 4 1) of two double bond isomers. 

Unlike the alkene double bond, there is no possibility of cis and trans isomers at the triple bond since atoms attached at the carbons of a triple bond lie in a straight line with the triple bond. 

Catalysts. In the original study of benzylation, both Pd and Ni complexes were shown to be satisfactory catalysts.t In the synthesis of benzylated alkenes, however, the Ni-catalyzed reaction shown in Scheme 25 provided the desired product in 46% yield along with a double bond isomer formed in 15% yield, while the corresponding Pd-catalyzed reaction cleanly produced the desired product in 86% yield. ° ... 

The n x interaction can be switched off if two strongly acceptor antiperiplanar a orbitals are available for the lone pair of nitrogen (Figure 6.110b). It was also suggested that the alkene x-system orients itself perpendicular to the lone pair to decrease steric repulsion. " This enamine is so uiueactive that it cannot be isomerized into the trans double bond isomer without fission of the heterocyclic ring. ""... 

In contrast to the substrate-type presented in Scheme 5.6, intramolecular Mizoroki-Heck reactions with cychc alkene moieties are quite common. Negishi and coworkers [21, 29] screened numerous substrates with different substitution patterns, out of which four are shown in Scheme 5.7. Cychzation of aryl iodide 35 proceeded well and furnished tricyclic 36 in good yield, including 10% of a double-bond isomer (not shown) (35 36). Mizoroki-Heck reactions of cyclohexenones 37 and 39 provided 68% and 82% yields respectively and, probably, due to conjugation with the carbonyl group in isomerically pure form (37,39 38,40). The two analogous cyclohexenone derivatives of aryl iodide 35 (not shown) cyclized under identical conditions in 50% and 71% yields respectively. Substrate 41a even allowed for formation of spirocyclic 42a in good yield, yet with poor... 

---