Cis to trans isomer

Changes from cis- to trans-isomers of some 12-S-6 (C2F4) were reported.63 10.3.4.5 Higher Coordinated Species... 

Several cyclodipeptides have been subjected to base-catalyzed epimerization (EtOH/NaOEt at 30-75°C) and the ratio of cis-to-trans isomers at equilibrium has been determined (74JA3985). The results have been correlated with the conformation of the molecules. Thus, cyclo(Pro-Pro) NMR studies (73JA6142) have indicated a boat form in the cis and a planar form in the trans diastereomer. In the latter, the pyrrolidine rings take up a half-chair conformation, which is greatly strained as long as the amide bonds are planar. This renders the trans less stable than the cis diastereomer. Consequently, at equilibrium, only cis diastereomer is found the trans isomer occurs to the extent of less than 0.5%. 

The ratio of cis to trans isomers was found to depend on the nature of the diene and the reaction conditions. Thus, 2,4-hexadiene thermally gave only the trans isomer whereas the tetramethylbutadiene afforded, under... 

The thermodynamic stability of isomeric hydrocarbons is determined by burning them to C02 and H20 and comparing the heat evolved per mole (—AH combustion). The more stable isomer has the smaller (—AH) value. Trans alkenes have the smaller values and hence are more stable than the cis isomers. This is supported by the exothermic (AH negative) conversion of cis to trans isomers by ultraviolet light and some chemical reagents. 

Ph3Sb)W[C(OR)Ph] is the only product. On heating in solution, cis-(CO)4LM-[C(OR)R ] (M = Cr, W L = phosphine R = Me, Et R = alkyl) isomerizes to yield a solution containing an equilibrium mixture of both cis and trans isomers. The equilibrium ratio of cis to trans isomers varies depending on the steric requirements of the carbene and phosphine ligand, on the central metal and on the solvent used. The isomers can be separated by column chromatography. ... 

Prolonged irradiation did not change the equihbrium steady-state ratio cis to trans isomers. 

Irradiation of particles of semiconductors like Ti02 or CdS suspended in a solution of stilbene led to isomerization of its cis- to trans-isomer (29-30). The reaction seems to be initiated by electron transfer from cis-stilbene to the positive hole generated in the semiconductor by irradiation. The resulting cis radical cation (c ) is several kcal mol" higher in energy than a trans radical cation (t ) over the ground state trans isomer (31). Accordingly, the isomerization from c " to t " is exothermic. 

Recently 1,2-diarylethylenes are reported to undergo isomerization only from cis to trans in small monocrystals (38). The isomerization occurs within a limit of cis-lsomer crystalline lattice. The reaction seems to proceed through excitation walk or by addition-elimination mechanism resulting in the change of the crystal form from the cis- to trans-isomer. 

The change in the pattern of spectra is also observed using H NMR, as can be observed in the Fig. 14.7 for the cis and trans [0s(0)2(bipy)2]C104 complexes. This change in the pattern of resonances is consistent with the change from C2 to D2h symmetry upon isomerization from the cis to trans isomers [28],... 

On the other hand, in the triplet state of 5b, t is the most stable and is in equilibrium with the slightly less stable p [25] therefore, the triplet state deactivates from both p and t, several thousand times faster from p (kjp = 10" s ) than from t (k, - 10 s ). The deactivation from p gives trans and cis isomers as the typical two-way mode, and that from t gives solely the trans isomer by either unimolecular deactivation or transferring energy to the cis isomer to regenerate c, leading to quantum chain isomerization of cis to trans isomers. The isomerization proceeds with the dual mechanism depicted in Scheme 3. 

Malic acid (1) reacts with aldehydes or ketones under acidic catalysis to form dioxo-lanones. Of particular interest is the reaction of 1 with pivaldehyde in the presence of PTSA and sulfuric acid, which produces cw-dioxolanone 170 in 67% yield [59]. The choice of solvent is crucial to ensuring high cw-stereoselectivity. If the reaction is carried out in benzene, a 3 2 equilibrium mixture of cis and trans isomers is obtained. Under heterogeneous conditions in pentane the ratio of cis to trans isomers increases dramatically to 50 1 ( > 98% ds). 

Cycloalkenylcyctopropanes. Substances of this type have been prepared by reaction of (8-chlorocycloalkenones with dimethyloxosulfonium methylide to form an intermediate that reacts with an electrophilic olefln to give a cyclopropane derivative. Yields range from 20 to 80% the ratio of cis- to trans-isomers is about 25 75. ... 

What would be the expected ratio of cis to trans isomers in the product. ... 

The preparation of DEQ is well known in the patent art. The reaction of about two equivalents of fatty acid (or fatty acid methyl ester) with one mole of triethanolamine (followed by quater-nization by dimethyl sulfate) results in a mixture of mono-, di-, and triesteramines quaternary ammonium salts. U.S. Patent 5750492 and U.S. Patent application 2002/0025915A exemplify that by adjusting the reaction conditions (molar ratio of fatty acid fraction to tertiary amine, temperature, ratio of cis to trans isomer in the unsaturated fatty group) to limit the amount of triesterquat content, liquid softener concentrates with as high as 22-28% actives can be produced. Some other useful examples of liquid concentrates can be found in patents U.S. 545340 and U.S. 5574179. 

One disadvantage of azobenzene molecules is the slow and unavoidable thermal relaxation from the cis to trans isomer, which gives rise to the problem of lacking longterm stability or slow restoration rate. White et al. found that polymer stabilization could dramatically fasten the thermal relaxation of azobenzene-doped CLCs [35]. The polymer-stabilized CLC relaxed to the initial stage from the isotropic state in 2 h compared to 72 h required for the relaxation of CLC without polymer stabilization. 

Johnson, D.W. and Poulos, A. (1992) Tris(trimethylsilyl)silane in the Deoxygenation of Long Chain Unsaturated Alcohols Converts cis to trans Isomers, Tetrahedron Lett. 33, 2045-2048. 

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