Cis to trans isomerization

Photolysis of chromium alkoxycarbenes with azoarenes produced 1,2- and 1,3-diazetidinones, along with imidates from formal azo metathesis (Eq. 21) [85, 86]. Elegant mechanistic studies [87-89] indicated the primary photoprocess was trans-to-cis isomerization of the azoarene followed by subsequent thermal reaction with the carbene complex. Because of the low yields and mixtures obtained the process is of little synthetic use. 

Among thermal processes, canning caused the largest trans-to-cis isomerization of provitamin A carotenoids, increasing the total cis isomers by 39% for sweet potatoes, 33% for carrots, 19% for collards, 18% for tomatoes, and 10% for peaches 13-di-P-carotene was the isomer formed in highest amonnts. ... 

Losses of 45 to 48% in the P-carotene contents and formation of cis isomers were also verified by pasteurization of carrot juice at 110 and 120°C for 30 sec. No significant effects on trans-to-cis isomerization of a- and P-carotene isomers were observed after acidification and heating of carrot juice at 105°C for 25 sec. In addition, an increase of only 3% in the cis isomers of provitamin A carotenoids was observed after orange juice pasteurization. " ... 

Dunham and Abbott have studied ligand exchange in cis- and trans-[Pt(ox)2(H20)2] and shown that it occurs too slowly to be involved in the formation of metal-metal bonded polymers.507 They also observed a trans to cis isomerization that occurred without ligand exchange and proposed a Bailar or Ray-Dutt twist process.507... 

The initial quantum yields for cis- to tam-stilbene isomerization (O0 T) and for trans to cis isomerization (4>T-.C) are consistent with Hammond s postulate that isomerization takes place from a common state, most likely the twisted or phantom triplet state ... 

Although trans to cis isomerization per se is not expected to cause major changes in color, it is the first step for intramolecular cyclization to form cyclic volatile compounds under conditions of high temperature. The oxidation of carotenoid is also required for subsequent reorganization... 

Mango puree was produced on a laboratory scale, mimicking typical operations in continuous and small-size batches, applying pasteurization between 85°C and 93°C up to 16 min (Vasquez-Caicedo et al. 2007). Although significant trans- to cis- isomerization of p-carotenc occurred, especially by the formation of 13-m-P-carotene, provitamin A (trans- + m-p-carotene) losses... 

In general, the major consequences of food thermal processing, either at laboratory or commercial scales, on carotenoids are the transformation of the 5,6-epoxy to the 5,8-furanoid rings, trans- to cis- isomerization and oxidation. In addition, independently of the food matrix or thermal... 

Iwata K, Ozawa R, Hamaguchi H (2002) Analysis of the solvent- and temperature-dependent Raman spectral changes of S1 trans-stilbene and the mechanism of the trans to cis isomerization dynamic polarization model of vibrational dephasing and the C=C double-bond rotation. J Phys Chem A 106 3614—3620... 

An NMR investigation of water exchange at [Pt(H20)2(oxalate)2] is relevant to the mechanism of formation of one-dimensional mixed valence oxalatoplatinum polymers. In fact the rate constant for this presumably dissociative (AS = + 42 JK mol-1) reaction is considerably too low for water loss to be, as recently proposed, the first step in formation of these polymers. The mechanism of trans to cis isomerization for this oxalate complex, and for its (2 -methyl)malonate analogues, is intramolecular (Bailar or Ray-Dutt twist), since there is no concurrent incorporation of labeled solvent (177). 

Photoisomerization of the azobenzene amphiphile was found to be strongly affected by molecular packing and orientation in the aqueous bilayer solutions. A rate constant of trans to cis isomerization was extremely faster in the liquid crystalline state than in the crystalline bilayer membrane [33]. Photoreaction of the aqueous bilayer membrane of CgAzoCioN+ Br was... 

The effects of nitro substituents on the cis-trans isomerization of stilbenes has been reviewed70 (equation 63). The trans-to-cis isomerization occurs from a triplet excited state, whereas the reverse cis-to-trans isomerization occurs through a main route which bypasses the triplet state. A nitro substituent usually causes a significant enhancement of the quantum yield of the intersystem crossing. Nitro substituent effects on the photoisomerization of trans-styrylnaphthalene71 (equation 64), trans-azobenzenes72 and 4-nitrodiphenylazomethines73 (equation 65) have been studied for their mechanisms. 

The presence of a nitro substituent can enhance the intramolecular charge transfer in the excited state dramatically, so that the normal trans-to-cis isomerization of l-[2(4-nitrophenyl)ethenyl]pyrene in cyclohexane is completely suppressed74 in polar solvents such as acetonitrile (equation 66). 

UV-photoirradiation causes trans-to-cis isomerization and the resulting solution exhibits a new voltammetric profile, Figure 2b, in which traces of the original waves are preceded by an anodic process (Eof = +0.03 V), which has been attributed to the single-stepped two-electron oxidation of cw-[Fc—N=N—Fc].3b This suggests that the apparent simple isomerization really involves important electronic effects in that (based on the discussions in Section 1.3, Chapter 4) one must conclude that in trans-[Fc—N=N—Fc] the two ferrocenyl units are interacting with each other, while in cz>[Fc—N=N—Fc] they do not interact. 

Cyanide anation in DMSO has proved useful in the preparation of cyano complexes (Scheme 29). 25 Cyanide displaces coordinated DMSO cleanly but deprotonates rather than displaces coordinated water. The preparation of trans- and ds-[Cr(CN)2(NH3)4]C104 starts from [Cr(DMSO)2(NH3)4]3+ or [CrCl(DMSO)(NH3)4]2+. The insolubility of trans-[Cr(CN)2(NH3)4]C104 permitted its isolation from solution, but it was necessary to separate the cis isomer by ion exchange. Rapid trans to cis isomerization of bww-[Cr(DMSO)2(NH3)4]3+ occurs on the addition of CNT before anation produces the dicyano complexes stepwise, both apparently via c -[CrCN(DMSO)(NH3)4]2+, which in an uncommon rearrangement gives a mixture of trans- and cis-dicyano complexes. Substitution of DMSO by CN" in water is stereoretentive. 

The Photoactive Yellow Protein (PYP) is the blue-light photoreceptor that presumably mediates negative phototaxis of the purple bacterium Halorhodospira halophila [1]. Its chromophore is the deprotonated trans-p-coumaric acid covalently linked, via a thioester bond, to the unique cystein residue of the protein. Like for rhodopsins, the trans to cis isomerization of the chromophore was shown to be the first overall step of the PYP photocycle, but the reaction path that leads to the formation of the cis isomer is not clear yet (for review see [2]). From time-resolved spectroscopy measurements on native PYP in solution, it came out that the excited-state deactivation involves a series of fast events on the subpicosecond and picosecond timescales correlated to the chromophore reconfiguration [3-7]. On the other hand, chromophore H-bonding to the nearest amino acids was shown to play a key role in the trans excited state decay kinetics [3,8]. In an attempt to evaluate further the role of the mesoscopic environment in the photophysics of PYP, we made a comparative study of the native and denatured PYP. The excited-state relaxation path and kinetics were monitored by subpicosecond time-resolved absorption and gain spectroscopy. 

The conclusion that the ac electrolysis of Re rans-SP CCCOjCl proceeds via excited states is also supported by the direction of isomerization. In thermal reactions of stilbene derivatives and radicals cis to trans conversions are generally observed (61). Contrary to this behavior the photolysis and ac electrolysis lead to energetically uphill trans to cis isomerization. 

Trans to cis isomerization of azo and ethylene linkages associated complexes of the type [Re(diimine)(CO)3L]+, where L are ligands shown in Fig. 16, was examined by several groups [34-36,63,77]. The process is out-fined in Scheme 3. Optical excitation in the visible caused trans to cis isomerization along with a marked enhancement of emission. 

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