Cis-trans isomerisation and

Detailed studies of 1 1 complex formation between and maleic and fumaric acids, which precedes reduction to succinic acid, cis-trans isomerisation and exchange of the double bond hydrogens, are relevant to the complex kinetics (A = substrate)... 

In some early work, conjugated trienes were reported to undergo cis-trans isomerisation and double bond migration on packed columns containing polyester stationary phases [609,619]. On the other hand, when an "all-glass" WCOT system was used with the non-polar stationary phase, OV-1, it proved possible to separate a number of geometrical isomers, eluting in the order cfc-9,11,13- (ECL = 18.95), cff-9,11,13- (18.99), c-8,10,12- (19.09), ttc-9,11,13- (19.10), fff-9,11,13- (19.36) and fff-8,10,12-18 3 (19.39), without any losses [900]. Comparable results have been obtained by others [226,282,394]. Indeed, somewhat better resolution was obtained with a WCOT column coated with Carbowax 20M (ECLs = 21.50 to 22.19), from which components eluted without loss in the same order as on an OV-1 column [282]. 

In unsaturated fatty acids, positional isomerisation is also possible, when the double bond shifts by one carbon atom either away from the carboxyl or towards the carboxyl. Positional isomerisation is usually associated with cis-trans isomerisation and takes place under similar conditions to the cis-trans isomerisation, on heating to temperatures higher than about 240 °C. Isolated double bonds are relatively stable, but the pentadiene systems easily isomerise to conjugated systems. Two conjugated isomers are formed from linoleic acid, whereas linolenic acid yields four conjugated isomers (Figure 3.14). 

The reaction types which includes 2 + 2 cycloaddition and carried out by photochemical means have also great synthetic potentiality. The photodimerisation of cinnamic acids were among the earliest photochemical reactions to be studied. These compounds give good yield of dimers when irradiated in the crystalline state. In solution, cis-trans isomerisation is the dominent reaction. 

Consider the photosensitised cis-trans isomerisation of stilbene The two isomers have different triplet energy levels (247kJmoT1 for the cis and 205kjmol-1 for the trans). The proportion of the cis isomer in the photostationary state varies with the energy of the photosensitiser (ET), as shown in Figure 8.5. 

In the simple four-electron systems, a route for cis-trans isomerisation of a diene is made available by the photochemical reaction usually being a disrotatory ring closure and the thermal reaction being a conrotatory ring opening ... 

If the products formed from the direct and sensitised reactions differ then the triplet state formed on sensitisation is not the reactive excited state formed by direct irradiation. This situation is illustrated in Sections 8.2 and 8.3, where direct and photosensitised cis-trans isomerisation of alkenes was considered. 

Non-geminal bis, tris, and tetrakis secondary amino derivatives are found to undergo reversible cis trans isomerisations in the presence of amine hydrochlorides [141-145]. It is reported that aluminium trichloride also acts as a catalyst for these interconversions (Eq. 26) [142]. 

The processes of olefin exchange, double bond migration and cis—trans isomerisation, observed to occur concomitant with hydrogenation, may be accounted for by considering that the formation of the half-hydrogenated state is reversible. For olefin exchange we can write... 

The initial rate kinetics for hydrogenation, double bond migration and ci —trans isomerisation are shown in Table 10. The activation energies for isomerisation and hydrogenation are shown in Table 11. 

Conclusive evidence for the participation of 7r-allylic intermediates in double bond migration has been obtained from a study of the nickel-catalysed hydrogenation of the isomeric olefinic esters methyl oleate and methyl elaidate using tritium as a tracer [147]. It was also concluded that in this system cis—trans isomerisation occurred by an addition—abstraction mechanism. 

The possible involvement of oxygen in cis-trans isomerisation of polyacetylene and the related question of whether cis- and tram-polymers oxidize at different rates... 

Y. Zhao, Smart Light-Responsive Materials, Wiley-Blackwell, Indianapolis 2009 Ch. Dugave (ed.), cis-trans Isomerisation in Biochemistry, Wiley-VCH, Weinheim 2006 For photomobile polymer materials and light driven plastic motors, see M. Yamada, Y. Yu, M. Kinoshita, C.J. Barret,... 

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