Trans-Isomerization

Initially, isomerization of 1-alkenes to 2-alkenes takes place with the formation of cw-isomers which, after some time, form an equilibrium mixture of the cis and trans compounds. Generally, cis-trans isomerization utilizing the same catalysts is slower than the migration of the double bond. A very rapid establishment of the cis-trans equilibrium was observed in the case of hydrido-type catalysts which were obtained in situ in the presence of strong acids. It is difficult to establish whether the influence of the coordination compound or that of the acid is more important. 

Isomerization by means of hydrido complexes probably occurs through the formation of alkyl compounds in which the rotation about the C-C axis is possible, followed by reformation of the trans-dlk nt molecule as a result of jS hydride elimination [equation (13.65)]. 

4- polydiene rubbers may have a variety of cis-ltrans- ratios ranging from the 100% cis- structure of natural rubber to polybutadienes with a trans- content in excess of 99%. In the late 1950s it was found that these polymers could be chemically treated in such a way that the cis-ltrans- ratio of an already formed polymer was altered. This process is known as cis-ltrans- isomerization and in the case of natural rubber leads to products with interesting properties. 

The mechanism appears to be substantially the same as for simple alkenes. It involves a free radical chain reaction resulting in the formation of a freely rotating adduct from the polybutadiene cis- or trans-) and a bromine or thiyl radical (X ) resulting from the photolysis or radiolysis of the organic bromide or mercaptan sensitizer. 

On release of the attached radical, X, the double bonds are re-established with the thermodynamically more stable configuration predominant. Unlike simple alkenes only the bromides of the possible halides are effective with polybutadiene. 

Isomerization may also be brought about in the absence of a sensitizer (Golub, 1969) both in solution and in the solid state but this is accompanied by significant side reactions such as cyclization and cross-linking. 

Two different mechanisms have been proposed to account for the sulphur-induced cis-Itrans- isomerization of polybutadienes and which will be referred to here as (a) the persulphenium ion theory, and (b) the 7r-complex theory. 

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The definitions of the empirical rate laws given above do not exclude empirical rate laws of another fomi. Examples are reactions, where a reverse reaction is important, such as in the cis-trans isomerization of 1,2-dichloroethene ... 

Zhu S-B, Lee J, Robinson G W and Lin S H 1988 A microscopic form of the extended Kramers equation. A simple friction model for cis-trans isomerization reactions Chem. Phys. Lett. 148 164-8... 

Schoenlein R W, Peteanu L A, Wang Q, Mathles R A and Shank C V 1993 Femtosecond dynamics of cis-trans Isomerization In a visual pigment analog Isorhodopsin J. Phys. Chem. 97 12 087-92... 

Figure B2.4.2. Eyring plot of log(rate/7) versus (1/7), where Jis absolute temperature, for the cis-trans isomerism of the aldehyde group in fiirfiiral. Rates were obtained from tln-ee different experiments measurements (squares), bandshapes (triangles) and selective inversions (circles). The line is a linear regression to the data. The slope of the line is A H IR, and the intercept at 1/J = 0 is A S IR, where R is the gas constant. A and A are the enthalpy and entropy of activation, according to equation (B2.4.1)...

The Cyclooctene Isomerization. A reaction that attracted some attention in recent years is the cis-trans isomerization of cyclooctene [84]. The cis isomer is much less strained than the trans, but the latter is readily fonned upon direct photolysis and also upon photosensitization. In this case, two enantiomeric trans isomers are formed. The appropriate loop is a variation of that shown in Figure 14, as shown in Figure 34. This is a phase inverting i -type loop, that... 

INORGANIC COMPLEXES. The cis-trans isomerization of a planar square form of a rt transition metal complex (e.g., of Pt " ) is known to be photochemically allowed and themrally forbidden [94]. It was found experimentally [95] to be an inhamolecular process, namely, to proceed without any bond-breaking step. Calculations show that the ground and the excited state touch along the reaction coordinate (see Fig. 12 in [96]). Although conical intersections were not mentioned in these papers, the present model appears to apply to these systems. 

Both acids 3deld succinic acid, m.p. 185°, upon catalytic reduction (see Section 111,150), thus establishing their structures. Maleic and fumaric acids are examples of compounds exhibiting cis-trans isomerism (or geometric isomerism). Maleic acid has the cm structure since inter alia it readily 3delds the anhydride (compare Section 111,93). Fumaric acid possesses the trans structure it does not form an anhydride, but when heated to a high temperature gives maleic anhydride. 

Large annulenes tend to undergo conformational distortion, cis-trans isomerizations, and sig-matropic rearrangements (p. 40 and p. 100). Methylene-bridged conjugated (4n + 2)-ic cyclopolyenes were synthesized with the expectation that these almost planar annulenes should represent stable HOckel arenes (E, Vogel, 1970, 1975). 

The first identified complexes of unsubstituted thiazole were described by Erlenmeyer and Schmid (461) they were obtained by dissolution in absolute alcohol of both thiazole and an anhydrous cobalt(II) salt (Table 1-62). Heating the a-CoCri 2Th complex in chloroform gives the 0 isomer, which on standirtg at room temperature reverses back to the a form. According to Hant2sch (462), these isomers correspond to a cis-trans isomerism. Several complexes of 2,2 -(183) and 4,4 -dithiazolyl (184) were also prepared and found similar to pyridyl analogs (185) (Table 1-63). Zn(II), Fe(II), Co(II), Ni(II) and Cu(II) chelates of 2.4-/>is(2-pyridyl)thiazole (186) and (2-pyridylamino)-4-(2-pyridy])thiazole (187) have been investigated. The formation constants for species MLr, and ML -" (L = 186 or 187) have been calculated from data obtained by potentiometric, spectrophotometric, and partition techniques. 

Number of carbon atoms number of double bonds (geometric (cis, trans) isomerism). 

Qiana, introduced by Du Pont in 1968 but later withdrawn from the market, was made from bis(4-aminocyclohexyl)methane and dodecanedioic acid. This diamine exists in several cis—trans and trans—trans isomeric forms that influence fiber properties such as shrinkage. The product offered silk-like hand and luster, dimensional stabiUty, and wrinkle resistance similar to polyester. The yam melted at 280°C, had a high wet glass-transition temperature of - 85° C and a density of 1.03 g/cm, the last was lower than that of nylon-6 and nylon-6,6. Qiana requited a carrier for effective dyeing (see Dye carriers). 

The three isomerizations, ct5-2-butene trans-2-huiene, 1-butene 2-butene, and butenes isobutylene, require increasingly severe reaction conditions. When the position of the double bond is shifted, cis—trans isomerization also occurs, and mixtures of butenes result when the carbon skeleton... 

Photochromism Based on Geometric Isomerism. The simplest examples of a photochromic reaction involving reversible cis-trans isomerization is the photoisomerization of azobenzene [103-33-3] C22H2QN2 (16). 

The cis-trans isomerization of stilbenes is technically another photochromic reaction (18). Although the absorption bands of the stilbene isomers, occur at nearly identical wavelengths, the extinction coefficient of the lowest energy band of cis-stilbene [645-49-8] is generally less than that of stilbene [103-30-0],... 

The stereochemistry of pyrazolines and pyrazolidines has already been discussed (Section 4.04.1.4.3). Optically active A - and A -pyrazolines have seldom been described (77JA2740, 79CJC360), but cis-trans isomeric pairs are common. The C-4 acid-catalyzed epimerization involves the mechanism shown in Scheme 38 (70TL3099), but in spite of some inconclusive arguments the C-5 epimerization has never been established with certainty. 

There are at least two mechanisms available for aziridine cis-trans isomerism. The first is base-catalyzed and proceeds via an intermediate carbanion (235). The second mechanism can be either thermally or photochemically initiated and proceeds by way of an intermediate azomethine ylide. The absence of a catalytic effect and interception of the 1,3-dipole intermediate provide support for this route. A variety of aziridinyl ketones have been found to undergo equilibration when subjected to base-catalyzed conditions (65JA1050). In most of these cases the cis isomer is more stable than the trans. Base-catalyzed isotope exchange has also been observed in at least one molecule which lacks a stabilizing carbonyl group (72TL3591). 

Figure 6 Thermodynamic cycle for multi-substate free energy calculation. System A has n substates system B has m. The free energy difference between A and B is related to the substate free energy differences through Eq. (41). A numerical example is shown in the graph (from Ref. 39), where A and B are two isomers of a surface loop of staphylococcal nuclease, related by cis-trans isomerization of proline 117. The cis trans free energy calculation took into account 20 substates for each isomer only the six or seven most stable are included in the plot.

In the native protein these less stable ds-proline peptides are stabilized by the tertiary structure but in the unfolded state these constraints are relaxed and there is an equilibrium between ds- and trans-isomers at each peptide bond. When the protein is refolded a substantial fraction of the molecules have one or more proline-peptide bonds in the incorrect form and the greater the number of proline residues the greater the fraction of such molecules. Cis-trans isomerization of proline peptides is intrinsically a slow process and in vitro it is frequently the rate-limiting step in folding for those molecules that have been trapped in a folding intermediate with the wrong isomer. 

Related to stereoregularity is the possibility of cis, trans isomerism. The molecule of natural rubber is a c/s-1,4-polyisoprene whilst that of gutta percha is the trans isomer. 

When two or more substituents are present on a cyclohexane ring, the interactions between the substituents must be included in the analysis. The dimethylcyclohexanes provide an example in which a straightforward interpretation is in complete agreement with the experimental data. For 1,2-, 1,3-, and 1,4-dimethylcyclohexane, the free-energy change of the equilibrium for the cis trans isomerization is given below. ... 

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