Ethylene geometric isomerization

Complications arising from other types of isomerism. Positional and geometrical isomerism, also described in Sec. 1.6, will be excluded for simplicity. In actual polymers these are not always so easily ignored. Polymerization of 1,2-disubstituted ethylenes. Since these introduce two different asymmetric carbons into the polymer backbone (second substituent Y), they have the potential to display ditacticity. Our attention to these is limited to the illustration of some terminology which is derived from carbohydrate nomenclature (structures [IX]-[XII]) ... 

The results summarized in Table 7 indicate that the quantum yields for the geometrical isomerization of dianthrylethylenes 38/39 are markedly affected by substitution of the ethylene double bond. For trans-substituted di-9-... 

Polar solvents also may affect the geometrical isomerization of ethylenes by affecting the efficiency of intersystem crossing [96,97]. Thus, even in the absence of polar substituents, solvent polarity can be of importance in the photochemical isomerization of aryl-substituted ethylenes. For example, cis-l-(9-anthryl)-2-(l-naphthyl)ethylene 74 in cyclohexane isomerizes to the trans-isomer 75 with a quantum yield of 0.05, while the quantum yield in acetonitrile solution is 0.31. Any trans->cis isomerization 75—74 in cyclohexane has not been observed, but the reaction does proceed with a preparatively useful quantum yield of 0.05 in acetonitrile [98],... 

The biradical pathway for the cyclobutane cleavage and geometric isomerization was first pursued by Walters with cis- and ran -l,2-dimethylcyclobutane which interconvert roughly one-fifth as fast as they undergo cleavage to propylene at 400°C (Scheme 5.23). There is also 5-10% ethylene and 2-butene formed in the reaction, and, further, there is some preservation of stereochemistry at Cl and C2. The ratio of cis- to trans-2-buionQ formed from the trans compound is 0.125 while that from the cis compound is 2 at low conversions. ... 

Comparisons of the activation enthalpy for this reaction to that for geometric isomerization of ethylene and 2-butene led to an estimate of 13 2 kcal/mol for allyl radical resonance energy once correction for hexatriene resonance energy was made. 

The purpose of this paper is to point out that the problem of cis-trans isomerism in 1,2-disubstituted ethylenes, CHX = CHX, is very analogous to the problem of conrotation-disrotation in 1,3-butadiene and that geometric isomerism, examined from the vantage point of MOVB theory, is one of the simplest stereochemical problems one is likely to encounter, a rather surprising statement in... 

Conjugation as well as geometric and positional isomerization occur when an alkadienoic acid such as linoleic acid is treated with a strong base at an elevated temperature. CycHc fatty acids result from isomerization of linolenic acid ia strong base at about 250°C (58). Conjugated fatty acids undergo the Diels-Alder reaction with many dienophiles including ethylene, propylene, acryUc acid, and maleic anhydride. 

The detection of 1,2-propylene oxide in the products from methyl ethyl ketone combustion is particularly interesting. It parallels the formation of ethylene oxide in acetone combustion (8) and of 1,2-butylene oxide in the combustion of diethyl ketone. Thus, there is apparently a group of isomerization reactions in which carbon monoxide is ejected from the transition state with subsequent closing of the C—C bond. Examination of scale molecular models shows that reactions of this type are, at any rate, plausible geometrically. 

Several aryl-substituted ethylene oxides may be cited m connexion with epoxide hydration. For instance, a-methylstyreoe oxide Fives j. 2-dihydroxy-2-phenyl propano on careful treatment with cold dilute hydrochloric acid.4 9 The isomeric substance -methylstyrene oxide, which can exist in two geometric modifications, is reported to... 

To understand the kind of isomerism that gives rise to two 2-butenes, we must examine more closely the structure of alkenes and the nature of the carbon-carbon double bond. Ethylene is a flat molecule. We have seen that this flatness is a result of the geometric arrangement of the bonding orbitals, and in particular the overlap that gives rise to the tt orbital. For the same reasons, a portion of any alkene must also be flat, the two doubly-bonded carbons and the four atoms attached to them lying in the same plane. 

For appropriately substituted bianthrones, two geometrical isomers are expected the E form and the Z form (Scheme 19). Dynamic NMR studies97 provided information on the pathways for E-Z isomerization and conformational inversion in 2,2 -disubs(i(u(cd bianthrones. These studies indicated that the high-est-energy transition state for these processes is the folded-twisted structure and not the n/2 twisted ethylene (Figure 10). 

While only two geometrical isomers exist in simple unsaturated molecules, a number of cis-trans isomers can exist in polyenes (e.g. carotenoids ). Normally, the alI-/ra/w-isomer is preponderant in carotenoids. Isomerization of the all-ftms-carotenoids by acid- or iodine-catalysis yield the so-called neo-carotenoids. The neo-carotenoids are believed to possess one c/s-ethylenic bond in the central position of the conjugated chain9. The neo-carotenoids exhibit not only a weaker long wavelength band 470 nyx) than the all... 

Other Syntheses Related to the Fischer-Tropsch Process Comparatively little is yet known of some synthetic reactions which obviously resemble the Fischer-Tropsch process very closely, but they are worth brief mention because they are also likely to be controlled by geometrical factors. The Oxo synthesis (15) of aldehydes by the interaction of ethylene or other olefins with carbon monoxide and hydrogen is carried out in contact with cobalt catalysts at temperatures in the range 110-150°, and under a pressure of 100-200 atmospheres. Cyclic olefins react similarly for example, cyclohexene gives hexahydrobenzaldehyde. There can be little doubt that a two-point adsorption of the hydrocarbon must take place and that the adsorbed molecule then reacts with carbon monoxide and hydrogen the difference between this process and that responsible for the normal hydrocarbon synthesis is that adsorbed carbon monoxide survives as such under the less drastic temperature conditions which are employed. Owing to the fact that a variety of isomeric aldehydes are produced, this system deserves further detailed study on geometrical lines. 

Fig. 5 Selected geometric parameters (A) of the optimized ri -s/n,ri (C ) transition-state structure TSefc[2] for allylic enantioface conversion and of the optimized rotational q -syn,r (C ) transition-state structure TSiso[3] for allylic isomerization occurring in the [Ni (octadienediyl)(ethylene)] complex. Activation free energies (kcal mob ) are given relative to the [Ni"(q -syn,q (C ),A-cis,-octadienediyl)(ethylene)] isomer of 2...

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