Cls-trans isomerizations

The requirement for cl -trans isomerism in alkenes. Compounds that have one of their carbons bonded to two identical groups can t exist as cis-trans isomers. Oniy when both carbons are bonded to two different groups are cis-trans isomers possible. 

In which cls-trans Isomerization Is sterlcally hindered Is not photochromlc. Preventing the rotation about the C -N bond does not hinder photochromlsm, since 2-hydroxy phenyl benzothlazole Is photochromlc. In agreement with Becker, the authors maintain that competition between cls-qulnone fluorescence and Isomerization Is not the key to understanding the mechanism of SAA photochromlsm, since... 

The photochromism of HNA seems to be simpler than that of SAA, since no cls-trans isomerization is involved (63), the cis-qulnone being the photoproduct. In this case, too, the emitting state which is produced by proton transfer from the excited phenol is similar but not identical with the excited cis-guinone. This Is demonstrated by somewhat different fluorescence spectra obtained in the presence of ultraviolet and visible exciting light. 

Data on the photochemical stability and stabilizing action of HB s, benzotriazoles, and related compounds have been reported on several occasions (6,9,70,71,7,82,87). In order to be a good stabilizer, it is not sufficient that a compound does not decompose if dissolved for example, in a plastic, it is equally important that it does not act as a sensitizer. Investigations on the sensitizing action of HB s compared to that of benzophenone and nonchelated derivatives are, therefore, of interest. Such studies are Included by Lamola (8), A. Heller (80), and Hammond (74). Cls-trans isomerization of piperylene (1,3-pentadiene) in benzene is easily sensitized by benzophenone via triplet transfer... 

Acyclic Bisolefins. irradiation of ct,(o-dlclnnamoyloxyalkanes [60] In the presence of benzophenone results In Intramolecular cyclomerlzatlon as a competitive process for cls-trans Isomerization (129-131) even if the two cinnamates are separated by 10 methylene units. 

Addition of an Unsaturated Carbon-Carbon Bond to a Carbonyl Function, intramolecular Paterno-Buchi reactions have been reported in the following systems. It should be pointed out that in most cases cls-trans isomerization, if possible, is a competitive process. 

We have synthesized and Investigated a series of cyclic hexapeptldes containing N-substltuted amino acids, which are cyclo(Sar)6, cyclo(Sar-Sar-Gly>2t cyclo(L-Pro-Sar-Gly)2 and cyclo(Sar-Gly-Gly)2 (3,4,5). It has been reported that the numbers of different conformations due to the cls/trans Isomerism of the N-substltuted peptide bonds decreased In the above order, and that the first two cyclic hexapeptldes, which are very flexible, bound alkali metal cations effectively. The prollne residue made the cyclic hexapeptlde more soluble In organic solvents. These obseirvatlons point to cyclo(L-Pro-Sar-Sar)2 which could be a synthetic lonophore transporting selectively metal Ions through a hydrophobic meud>rane. 

Chiral, N-alkylated cyclic hexapeptlde, cyclo(L-Fro-Sar-Sar)2 was synthesized and It was found to be soluble In a wide range of solvents. Owing to the cls/trans Isomerization of six N-alkylated peptide bonds and the absence of Intramolecular hydrogen bonding, cyclo(L-Fro-Sar-Sar)2 Is very flexible In a thermodynamic sense as well as In a kinetic sense. 

On carbonylation in methylene chloride it undergoes the substitution of the diene ligands, rearrangement, and cis-trans isomerization to yield 64 (97JOM(530) 259). l,r-(l,2-Ethylene)-3,3 -imidazol-2,2 -diylidene and the dimer [(T -cod)Rh (ir-Cl)] form the dinuclear species 65. 

Reality Check Notice that the structural isomer shown on the left does not show cis-trans isomerism because two Cl atoms are bonded to the same carbon. 

In summary, all the experiments expressly selected to check the theoretical description provided fairly clear evidence in favour of both the basic electronic model proposed for the BMPC photoisomerization (involving a TICT-like state) and the essential characteristics of the intramolecular S and S, potential surfaces as derived from CS INDO Cl calculations. Now, combining the results of the present investigation with those of previous studies [24,25] we are in a position to fix the following points about the mechanism and dynamics of BMPC excited-state relaxation l)photoexcitation (So-Si)of the stable (trans) form results in the formation of the 3-4 cis planar isomer, as well as recovery of the trans one, through a perpendicular CT-like S] minimum of intramolecular origin, 2) a small intramolecular barrier (1.-1.2 kcal mol ) is interposed between the secondary trans and the absolute perp minima, 3) the thermal back 3-4 cis trans isomerization requires travelling over a substantial intramolecular barrier (=18 kcal moM) at the perp conformation, 4) solvent polarity effects come into play primarily around the perp conformation, due to localization of the... 

Attempts to rearrange tne phenyl acid into the trane form by heating to 290° with fused alkali yielded a small amount of an isomeric acid. Tnis acid was unattacked by potassium permanganate and was probably the cls-phenyl cls-trans dlcarboxyllc acid not enough was at hand to Investigate its properties. 

To begin to elucidate such issues and to create a theoretical framework for them, we have focused [4-9] on a model of a protonated Schiff base (PSB) in a nonequilibrium dielectric continuum solvent. A key feature for the Sj-Sq Cl in PSBs such as retinal which plays a key role in the chromophore s cis-trans isomerization is that a charge transfer is involved, implying a strong electrostatic coupling to a polar and polarizable environment. In particular, there is translocation of a positive charge [92], discussed further below. Charge transfer also characterizes the earliest events in the photoactive yellow protein photocycle, for example [93],... 

P-Donors, As-Donors, and Sb-Donors. Formation of purple, diamagnetic [Co(pmm)2-Cl2]+ [pmm = o-C6H4(PMe2)2] has been reported. Further oxidation to cobalt(iv) could not be achieved. Bosnich et al. have reported systematic syntheses for cis- [Co-(das)2X2]"+ (X = Cl, C03, OH2, N02, N03, or MeCN). The ds-[Co(das)2Cl2] + ion was resolved with sodium arsenyltartrate, and the optically active isomers could be converted into other cis-[Co(das)2X2]+ with complete retention of optical activity. It is concluded that the cis - trans isomerization is catalysed by a cobalt(n) species, and this explains the failure to isolate pure cis-Co111 complexes with mildly reducing anions.348... 

Observations that the phosphines do not mix when a phosphine is used to catalyse the cis trans isomerism of [PtCl2(PEt3)2], and also that [PtCl(PEt3)3] + does not react with Cl" to produce cis- or frarcs-[PtCl2(PEt3)2] contradict the consecutive displacement mechanism (23) and instead suggest fluxional rotation.125... 

Skibsted and co-workers have recently presented several studies which rigorously test these stereochemical models. Complexes of the form cis- and trans-[Rh(NN)2XY]"+, (NN = en or tn (1,3-diaminopropane) X = H20, OH Y = H20, OH), plus cis and rra s-[Rh(tn)2(H20)Z]2+ (Z = Cl, Br), display similar photochemistries (a) cis to trans isomerization dominates for the diaqua and aquachloro ions (in which the H20 or halide, poorer a donors than en, occupy the axial site in the SP (square planar) intermediate) (b) trans to cis photoisomerization dominates for the hydroxoaqua species, as the OH ion is a better a donor than the en nitrogens, and hence occupies an equatorial site and (c) no photochemistry is observed for the dihydroxo ions.626 667,724 These results are summarized in Scheme 24 and Table 50. 

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