Trans-cis effect

Adv. Inorg. Chem. Radiochem., 1969, 12, 375 (cis-trans effects in Co1" complexes). 

From the colorless state it can be switched with light of short wavelength (A = 380 nm) via an electrocycHc ring opening and cis/trans rotation of one half of the molecule into a state with violet/purple color. The reverse reaction is effected by visible light (A = 580 nm). Since the system is metastable, one of the two reaction directions is matched by a rival thermal reaction, the thermoreversion. This progresses, however, in the case of benzospiropyran, at room temperature by a factor of 10 slower than the light-induced reaction. 

Qiana, introduced by Du Pont in 1968 but later withdrawn from the market, was made from bis(4-aminocyclohexyl)methane and dodecanedioic acid. This diamine exists in several cis—trans and trans—trans isomeric forms that influence fiber properties such as shrinkage. The product offered silk-like hand and luster, dimensional stabiUty, and wrinkle resistance similar to polyester. The yam melted at 280°C, had a high wet glass-transition temperature of - 85° C and a density of 1.03 g/cm, the last was lower than that of nylon-6 and nylon-6,6. Qiana requited a carrier for effective dyeing (see Dye carriers). 

There are at least two mechanisms available for aziridine cis-trans isomerism. The first is base-catalyzed and proceeds via an intermediate carbanion (235). The second mechanism can be either thermally or photochemically initiated and proceeds by way of an intermediate azomethine ylide. The absence of a catalytic effect and interception of the 1,3-dipole intermediate provide support for this route. A variety of aziridinyl ketones have been found to undergo equilibration when subjected to base-catalyzed conditions (65JA1050). In most of these cases the cis isomer is more stable than the trans. Base-catalyzed isotope exchange has also been observed in at least one molecule which lacks a stabilizing carbonyl group (72TL3591). 

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

Figure 13.37 shows the UV spectrum of the conjugated diene cis,trans-, 3-cyc o-octadiene, measured in ethanol as the solvent. As is typical of most UV spectra, the absorption is rather broad and is often spoken of as a band rather than a peak. The wavelength at an absorption maximum is refened to as the X ax of the band. There is only one band in the UV spectrum of 1,3-cyclooctadiene its X ax is 230 ran. In addition to UV-VIS bands are characterized by their- absorbance (A), which is a measure of how much of the radiation that passes through the sfflnple is absorbed. To correct for concentration and path length effects, absorbance is converted to molar absorptivity (e) by dividing it by the concentration c in moles per liter and the path length I in centimeters. 

The classic application of the trans-effect lies in the synthesis of the cis- and trans-isomers of Pt(NH3)2Cl2, known as Peyrone s salt and Reiset s salt after their respective discoverers in 1844. 

The cis- and trans-isomers of [Pt(NH3)(N02)Cl2]- have been synthesized from PtCl - merely by choice of the order of ligand substitution (Figure 3.87). (In the second step, chloride trans to chloride is more labile.) The second substitution is dictated by N02 having a higher position in the trans-effect series than chloride [144],... 

A seminal paper [155] examined platinum-phosphorus NMR coupling constants in a series of cis- and trans-platinum(II and IV) complexes. The trans-influence had hitherto been explained in terms of d7r-p7r bonding, in other words, such a mechanism dominated with trans-effect... 

Like the isoelectronic Pd2+ and Pt2+, Au3+ exhibits both trans-effects and trans-influence. Table 4.13 (above) lists structural data for a number of complexes AuL3L, showing how the disparity in Au-X distances between cis-and trans-X depends on the position of L in the trans-effect series for the compounds listed, the effect is least noticeable in AuC13NH3 as these two ligands are proximate in the series. 

The mechanism of B polymerization is summarized in Scheme 4,9. 1,2-, and cis- and trews-1,4-butadiene units may be discriminated by IR, Raman, or H or nC MMR speclroseopy.1 92 94 PB comprises predominantly 1,4-rra//.v-units. A typical composition formed by radical polymerization is 57.3 23.7 19.0 for trans-1,4- c7a -1,4- 1,2-. While the ratio of 1,2- to 1,4-units shows only a small temperature dependence, the effect on the cis-trans ratio appears substantial. Sato et al9J have determined dyad sequences by solution, 3C NMR and found that the distribution of isomeric structures and tacticity is adequately described by Bernoullian statistics. Kawahara et al.94 determined the microslructure (ratio // measurements directly on PB latexes and obtained similar data to that obtained by solution I3C NMR. They94 also characterized crosslinked PB. 

Cis, trans and metal effects in transition metal porphyrins. J. W. Buchler, W. Kokisch and P. D. Smith, Struct. Bonding (Berlin), 1978, 34, 80-134 (158). 

H. W. Quinn and J. H. Tsai Cis and Trans Effects in Cobalt(Ill) Complexes... 

However, eclipsing effects are not the only factors that affect the cis/trans ratio in anti E2 eliminations. Other factors are the nature of the leaving group, the base, the solvent, and the substrate. Not all these effects are completely understood. ... 

Buchler JW, Kokisch W, Smith PD (1978) Cis, Trans, and Metal Effects in Transition Metal Porphyrins. 34 79-134... 

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