Configurations of cis-trans isomers

There is no general method for determining the configuration of cis-trans isomers. A number of methods are employed to get reliable results. The methods employed depend on the nature of the compound. The following are some of the methods employed. 

A number of physical methods are also employed to determine the configurations of cis-trans isomers. Some of these are described below. 

The three-dimensional arrangement of groups about a stereocenter in a molecule is termed its configuration. One way to designate the configurations of cis-trans isomers is to use the terms cis and tram. However, these terms are ambiguous in many instances. For example, is the following isomer of 1-chloro-l-fluoro-l-propene the cis- or the trans-stereoisomer ... 

The AG values for the rotation around the C3-N bond were obtained by the coalescence method and kinetic measurements of the equilibration of isolated and configurationally established cis-trans isomers of type 232. The barrier of rotation is considerably lower than in ordinary immonium cations, e.g. 233 AG is decreased by electron-withdrawing substituents at the nitrogen atom, whilst at the three-ring the opposite effect is observed. 

All of this structural work is backed up by total synthesis, which will be developed later. These syntheses often produce mixtures of cis-trans isomers. The natural products have a trans configuration and have the longest GC retention times. This attribution is based on an analogy with the hydrogenation of 2,5-dimethylpyrrole. 

Given a pair of cis-trans isomers, assign the E or Z configuration. 

Configuration of cis-trans Isomeric Ozonides. In symmetrically di-substituted trioxolanes, the cis isomer is a meso and the trans isomer is a racemic form. Therefore, resolution into antipodes makes it possible to determine the configuration. In 1966, Loan, Murray, and Story (4) were able to obtain an optically active form of one of the diisopropyl-trioxolanes, proving it to be the trans isomer. Their method—partial decomposition of the ozonide by brucine—could not be applied to the stilbene ozonides. 

Physical Methods of Determination of Configurations Stability and Inter Conversion of cis-trans Isomers... 

The more interesting isobutene copolymers described in Sect. A.2. contain as repeat units 1,4-di-substituted-l, 3-dienes (Table 2). The common feature of these copolymers is the prevalent trans-trans configuration of the unsaturated system (infrared band at 985 cm ), because the infrared band at 970 cm , due to the ds-trans dienic system, is very weak Since it is well known that Diels-Alder reactions of 1,4-trans-trans substituted derivatives of 13-butadiene are more humred than those of cis-trans isomers , we studied Diels-Alder reactions of the copolymers of Table 2 under mild and other conditions. Namely ... 

KatayamaN, Hashimoto H, Koyama Y and Shimamura T (1990) High performance hquid chromatography of cis-trans isomers of neurosporene Discrimination of cis and tram configurations at the end of an open conjugated chain. J Chromatogr 519 221-227... 

Boric acid and borates form very stable complexes exceedingly rapidly with polyols10 and a-hydroxy carboxylic acids.11 These are mainly 1 1 and of type (8-V). The acidity of boric acid is thereby increased glycerol is commonly used analytically as the acid can then be titrated by aqueous NaOH. Steric considerations are very critical in the formation of these complexes. Thus 1,2- and 1,3-diols in the cw-form only, such as c/j-l,2-cyclo-pentanediol, are active, and only o-quinols react. Indeed, the ability of a diol to affect the acidity of boric acid is a useful criterion of the configuration where cis-trans-isomers are possible. 

A second type of isomerism is stereoisomerism. Stereoisomers have the same molecular formula and the same connectivity but different orientations of their atoms in space. One example of stereoisomerism we have seen thus far is that of cis,trans isomers in cycloalkanes (Section 2.6), which arise because substituents on a ring are locked into one of two orientations in space with respect to one another by the ring. Isomers of this type are called configurational isomers because they differ by the configuration of substituents on an atom. 

Disubstituted cyclohexanes can exist as cis-trans isomers as well as axiaEequatorial conformers. Two isomers are predicted for 1,4-dimethylcyclohexane (see Fig. 1.9). For the trans isomer the diequatorial conformer is the energetically favorable form. Only one cis isomer is observed, since the two conformers of the cis compound are identical. Interconversion takes place between the conformational (equatoriaEaxial) isomers but not configurational (cis-trans) isomers. 

Protonation of the anion [SN2] by acetic acid in diethyl ether produces the thermally unstable sulfur diimide S(NH)2. Like all sulfur diimides, the parent compound S(NH)2 can exist as three isomers (Scheme 5.5). Ab initio molecular orbital calculations indicate that the (cis,cis) configuration is somewhat more stable than the (cis,trans) isomer, while the (trans,trans) isomer is expected to possess considerably higher energy. The alternative syn,anti or E,Z nomenclatures may also be used to describe these isomers. The structures of organic derivatives S(NR)2 (R = alkyl, aryl) are discussed in Section 10.4.2. 

No cases of cis/trans isomerism have yet been reported among organo-cobalt(III) complexes, even among compounds not covered by this review. Complexes such as [R2Co(bipy)2] all have the cis configuration steric hindrance would be greatly increased by the coplanarity of the two bipyridyl ligands in the trans isomer 123). 

Koyama, Y., Kito, M., Takii, T., Saili, K., Tsukida, K., and Yamashita, J. 1983. Configuration of the carotenoid in the reaction centers of photosynthetic bacteria. 2. Comparison of the resonance Raman lines of the reaction centers with those of the 14 different cis-trans isomers of (i-carotene. Photobiochem. Photobiophys. 5 139-150. 

A common example of Cope rearrangement involving [3, 3] sigmatropic rearrangement in a 1, 5 diene is the pyrolysis of meso 3, 4 dimethyl hexa-1, 5 diene giving exclusively cis, trans isomer of 2, 6 octadiene. The process involves a six electron transition state which has a chair like configuration and substituents occupy equatorial sites. 

Mislow and co-workers (232) have shown that the assignment of configuration in the series of diastereomeric cis- and rrans-thianthrene-5,10-dioxides 206 substituted in the aromatic ring is possible on the basis of UV or IR spectra. The authors cited found that the principal absorption band in the UV spectra of the trans isomer of 206 is shifted with respect to that of the cis isomer toward longer wave-... 

Mislow and co-workers (258) and Hammond (259) have shown that optically active diaryl sulfoxides, which are configurationally stable in the dark at 200°C, lose their optical activity after 1 hr at room temperature on irradiation with ultraviolet light. Similarly, an easy conversion of the trans isomer of thianthrene-5,10-oxide 206a into the thermodynamically more stable cis isomer takes place upon irradiation in dioxane for 2 hr. However, the behavior of a-naphthylethyl p-tolyl sulfoxide under comparable irradiation conditions is different, namely, it is completely decomposed after 4 min. These differences are not surprising because the photochemical racemization of diaryl sulfoxides occurs by way of the pyramidal inversion mechanism whereas decomposition of the latter sulfoxide occurs via a radical mechanism with the cleavage of the sulfur-carbon bond. It is interesting to note that photoracemization may be a zero-order process in which the rate depends only on the intensity of the radiation and on the quantum yield. 

When we considered trans- and c -1,2-dimethyl-cyclohexane, we found that only three configurational isomers exist, enantiomeric forms of the trans isomer, together with the cis isomer, which is an optically inactive meso compound (see Section 3.4.5). The meso relationship could be deduced from the plane of symmetry in the hexagon representation. 

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