Bond cis-trans Isomerization

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 

Examples of this flipping mechanism are seen in cis-trans isomerizations from less stable to more stable isomers which when the reactions are carried out under deuterium. Already mentioned are the isomerizations of oleic acid. Additionally, methyl-(Z)-but-2-enoate isomerizes to its more stable E-isomer with incorporation of substantial amounts of deuterium during deuteriuma-tion over Pd/C (Fig. 2.16). At the same percentage deuteriumation, the saturated product contains in its P-position 90% of the two deuteriums added to 

FIGURE 2.16 Partial deuteriumation of 2 and E isomers of methy-but-2-enoate. Saturated products (dimethyl butanoate) not shown.119 

Sometimes cis-trans isomerizations occur slowly or not at all because certain groups anchor their end of the double bond to the surface and inhibit that end from flipping. Thus, phenyl groups and oxygen-containing groups tend to inhibit cis-trans isomerization (Fig. 2.17).120 

An example of this reveals an additional substituent effect (Fig. 2.18).121 Ordinarily, the phenyl and carboxyl groups anchor the double bond approximately equally (notice the sixth entry in Table 2.3 and cinnamic acid in Fig. 

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Three novel stereo- and regioselective schemes for the total synthesis of (+ )-brefeldin A 440 have been accomplished. Each of them exploit intermolec-ular nitrile oxide cycloaddition for constructing the open chain and introducing substituents, but differ in subsequent stages. The first (480) and the second (481) use intramolecular cycloaddition for the macrocycle closure. However, in the second scheme INOC is followed by C=C bond cis-trans-isomerization. In the third scheme (481) intermolecular cycloaddition is followed by ring closing metathesis as the key step. 

The skeletal isomerization of C4 and C5 n-olefins is an acid-catalyzed reaction requiring relatively strong acid sites that proceeds via carbenium ion intermediates formed upon protonation of the double bond (17). Double bond cis-trans isomerization usually occurs on the acid sites before skeletal isomerization. The general reaction mechanism for branching isomerization is depicted in Fig. 2 2. Protonation of the double bond leads to a secondary carbenium ion, which then rearranges into a protonated cyclopropane (PCP) structure. In the case of n-butenes,... 

Monomolecular photochemistry of butadiene is rather complex. Direct irradiation in dilute solution causes double-bond cis-trans isomerization and rearrangements to cyclobutene, bicyclo[l.l.01butane, and l-methylcyclo-propene (Srinivasan, 1968 Squillacote and Semple, 1990). Matrix-isolation studies have established another efficient pathway, s-cis-s-trans isomerization (Squillacote et al., 1979 Arnold et al., 1990, 1991). 

While double-bond cis-trans isomerization in / -substituted a,/3-enones occurs at both 313 and 254 nm, oxetene forms only when short wavelengths are used (Friedrich and Schuster, 1%9, 1972). This differentiation has been explained on the basis of calculations that revealed a S,-So conical intersection lying 15 and 10 kcal/mol above the (n. ) minimum of s-trans- and s-c/s-acrolein, respectively (Reguero et al., 1994). Oxabicyclobutane, however, has not been detected. This behavior of o,/3-enones is in contrast with that of butadiene, where cyclobutene and bicyclobutane are formed simultaneously. (Cf. Sections 6.2.1 and 7.5.1.)... 

Papers (4, 47, 48) demonstrate that, while the character of the carrier (silica gel, active carbon) of the active component has no pronounced influence on the process of hydrogenation, there are distinct differences in the effect of the active components themselves. Side reactions occurred on rhodium and palladium catalysts, while on platinum catalysts they could not be observed in most cases (migration of the double bond, cis-trans isomerization). These reactions occurred only if a sufficient amount of hydrogen was present in the reaction mixture (part of hydrogen is irreversibly consumed by hydrogenation). Neither the carrier alone nor the catalyst in an inert atmosphere provoked any side reactions, which shows that hydrogen in one of its forms participates directly in the isomerization process. 

Fig. 9.1 Reaction profile for peptide bond cis-trans isomerization. The kinetic constants for the trans to cis (k, J and cis to trans (kM>) isomerization are dependent on the Gibbs free energy of activation (AG ). The ground state energy difference AG° (G°as - C°mm) determines the population of the cis and trans isomers in the equilibrium state.

Enzymes Catalyzing Peptide Bond Cis-Trans Isomerizations... 

Some Facts about Double Bonds The Orbital Model of a Double Bond the Pi Bond Cis Trans Isomerism in Alkenes A WORD ABOUT... 

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