Cis-trans transformations

As a rule, the cis-trans equilibrium of acyclic azo compounds (—N=N—) lies extensively on the trans side. Thus, the reaction enthalpy for cis -> trans transformation of azobenzene amounts to almost 40 kJ/mol (20). The same is probably true for triazene and its derivatives (—N=N—The disilylaryltriazenes are exceptions (cf. Table I), wherein the cis forms are only slightly higher in energy than the trans forms and a moderate concentration of the cis isomer is present even at room temperature [Eq. (24)] (22). Thus, the free reaction enthalpy (AG) of the... 

As pointed out above, the spontaneous cis trans-transformation requires proportionality between dCeu/dt and Ccu at any time. Hence,... 

The main product of 1,4-butynediol hydrogenation for this reaction is cis-butenediol. Simultaneously, the parallel reaction of 1,4-butenediol hydrogenation as well as cis-trans transformation takes place at the second stage. For this process, cis-1,4-butenediol formation is most interesting from a practical point of view. Processes... 

Fig. 8. Schematic reperesentation of a possible trans-cis-trans transformation...

Zhurkov et al. [42] derived the degree of chain orientation from the dichroism of deformed-IR bands of PETP (cf. Chapter SIB and Fig. 8.5) they observed that for highly stressed segments an initial orientation with cos d = 0.75 had turned into practically complete orientation (cos — 1.0). The problems posed by an application of this method are discussed in detail by Read et al. [43]. Bouriot [44] deduced from IR measurements a reversible cis-trans transformation of the ethylene glycol segment in PETP fibers under the effect of tensile stresses. In polyamide 66 he observed a reversible increase of free (non-assodated) NH groups and an increase in the average chain orientation with chain tension. 

Cis-Trans Transformations in Azo-Dye 5i(k-Chain Polymers and Aza-Dye in a Polymer Matrix... 

Isomerases catalyse reactions such as cis-trans isomerization or more complex transformations such as D-glucose to D-fructose. 

In many cases the transformations may be more complex than indicated by Eqs. (9.89)-(9.100). An example of this is the photochemistry of cis,cis-1,3-cyclooctadiene [Eq. (9.94)].<169) A close examination of this reaction indicates that bicyclo[4.2.0]oct-7-ene is formed but in low relative yields during the initial reaction (see Table 9.9). In addition, the cis,trans-1,3-cyclooctadiene is formed and then consumed as the reaction proceeds. Fonken showed that the bicyclooctene initially formed, however, was not from thermal isomerization of the cis,trans-diene. Still a third reaction was the 1,3 sigmatropic hydrogen shift to form the cis, cis-1,4-cyclooctadiene ... 

When the cis/trans stereoselectivity of cyclopropanation with ethyl diazoacetate in the presence of CuCl P(0-z-Pr)3, Rh6(CO)16 or PdCl2 2 PhCN was plotted against that obtained with Rh2(OAc)4, a linear correlation was observed in every case, with slopes of 1.74,1.04 and 0.59, respectively (based on 22 olefins, T = 298 K) S9). These relationships as well as the results of regioselectivity studies carried out with 1,3-dienes point to the similar nature of the intermediates involved in Cu-, Rh-and Pd-catalyzed cyclopropanation. Furthermore, obvious parallels in reactivity in the transformations of Scheme 45 for a variety of catalysts based on Cu, Rh, Fe, Ru, Re and Mo suggest the conclusion that electrophilic metal carbenes are not only involved in cyclopropanation but also in ylide-forming reactions66. ... 

Photodissociation is accompanied by the cis-trans isomerization of azoalkanes. Azoalkanes have the Znmv-configuration. During photodecomposition, they are transformed into the rxs-configuration [66]. The excited molecule of Znmv-asopropane is transformed into cis-asopropane with 0 = 0.31, into the trans-con liguration with 0 = 0.51, and into free radicals with 0 = 0.18 (gas phase, 600 tor C02, room temperature). The following scheme of photophysical stages was proposed [205] ... 

It is generally believed that Pt(IV) complexes are prodrugs which are transformed to Pt(II) species before binding to DNA. However, the direct reaction of Pt(IV) complexes such as cis,trans,cis-[Pt(NH3)2(OH)2Cl2] with double-stranded oligonucleotides is reported to give Pt(IV) intra- and interstrand cross-links (119). 

MO but in the transformation cis - trans the energy of 03 increases. Consequently, the energy gap between the ns and 3 MO s will be comparable in both cases. However, the overlap integral S s03 will be greater for the cis conformation than the tram. 

On one-electron reduction followed by one-electron oxidation, cis isomers of 2- and 4-nitrostilbenes turn into the neutral nitrostilbene molecules, but in the trans forms. On oxidation of the naked anion-radicals, the neutral trans forms are the only products (cis trans conversion degrees were 100%). In the case of the coordination complexes, the trans isomers are formed only up to 40% (Todres 1992). Scheme 3.45 describes these transformations. 

As seen from Scheme 7.2, the epoxy-ring cleavage and nickel oxidation proceed simultaneously. The nickel-oxygen bond is formed. This results in the formation of the carbon-nickel biradical in which Ph-CH fragment can rotate freely. The cleavage of the (NiO)-C bond leads to the formation of a mixture of styrenes. At early reaction stages (30 min), cis and trans olefins are formed in 50 50 ratio. After a prolonged contact (30 h), when all possible transformations should be completed, the trans isomer becomes the main product and cis trans ratio becomes 5 95. Such enrichment of the mixture with the trans isomer follows from the formation of the di-P-(trimethylsilyl)styrene anion-radical and its isomerization. The styrene formed interacts with an excess of the nickel complex. 

Bode and co-workers rendered this transformation asymmetric allowing access to a>cyclopentenes 244 with high enantioselectivity (Table 19) [128], Optimized reaction conditions include the use of A-mesityl substituted aminoindanol derived triazo-lium catalyst 214. When chalcone and derivatives we re subjected to the reaction conditions, ax-cyclopentenes were formed selectively. Although the substrate scope is also limited to P-aryl substituted enals, cis. trans ratios of up to >20 1 are observed. 

The required 1-oxa-l,3-diene precursor was synthesized according to the synthesis design (Scheme 8). Cycloadditon with enol ether furnished exclusively the endo-isomer. Raney nickel treatment in refluxing ethanol yielded in one step the desired tetrahydropyran derivative in a favorable 6 1 cis/trans ratio. Transformation into the lactone and ring closure with potassium tert.-butoxide afforded (+)-ramulosin. 

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