Mechanism cis-trans isomerization

Two essentially different cis-trans isomerization mechanisms may be derived from the state correlation diagram of ethylene ... 

The singlet cis-trans isomerization mechanisms of conjugated olefins appear to be quite different from those of isolated olefins. 

The stereochemistry of pyrazolines and pyrazolidines has already been discussed (Section 4.04.1.4.3). Optically active A - and A -pyrazolines have seldom been described (77JA2740, 79CJC360), but cis-trans isomeric pairs are common. The C-4 acid-catalyzed epimerization involves the mechanism shown in Scheme 38 (70TL3099), but in spite of some inconclusive arguments the C-5 epimerization has never been established with certainty. 

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). 

Consider a nucleus that can partition between two magnetically nonequivalent sites. Examples would be protons or carbon atoms involved in cis-trans isomerization, rotation about the carbon—nitrogen atom in amides, proton exchange between solute and solvent or between two conjugate acid-base pairs, or molecular complex formation. In the NMR context the nucleus is said to undergo chemical exchange between the sites. Chemical exchange is a relaxation mechanism, because it is a means by which the nucleus in one site (state) is enabled to leave that state. 

When elimination takes place on a compound of the form CH3—CABX or CHAB—CGGX, the new alkene does not have cis-trans isomerism, but for compounds of the form CHEG—CABX (E and G not H) (23) and CH2E—CABX (24), cis and trans isomers are possible. When the anti E2 mechanism is in operation. 

Scotter, M.J., Characterization of the coloured thermal degradation products of bixin from annatto and a revised mechanism for their formation, Food Chem., 53, 111, 1995. Zechmeister, L., Cis-trans isomerization and stereochemistry of carotenoids and diphenylpolyenes, Chem. Rev., 34, 267, 1944. 

Weber FJ, S Isken, JAM de Bout (1994) Cis/trans isomerization of fatty acids as a defence mechanism of Pseudomonas putida strains to toxic concentrations of toluene. Microbiology (UK) 140 2013-2017. 

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... 

More recently, a new mode of cis-trans isomerization of a disilene has been suggested for the extremely hindered disilene 27. As will be detailed in Section VIII. B, 27 undergoes thermal dissociation into the corresponding silylenes. Monitoring the thermolysis of (Z)-27 at 50°C by H and 29Si NMR reveals a competitive formation of the isomerized ( >27 and benzosilacyclobutene 37, which is most likely formed by intramolecular insertion of silylene 36 into the C—H bond of the o-bis(trimethylsilyl)-methyl group (Scheme 3).22,59 This suggests the possible occurrence of cis-trans isomerization via a dissociation-association mechanism. 

Density functional theory (B3LYP) has been used to investigate the mechanism of the cis-trans isomerization in Ir(H)2(CO)(C6H3 CH2P(H2) 2).300 The preferred mechanism involves two consecutive trigonal twists in which the complex passes through a distorted octahedral intermediate, as shown in reaction Scheme 19. 

The mechanism for the direct photochemical cis-trans isomerization of stilbene has been a highly controversial subject. However, a recent review by Saltiel and co-workers greatly helps to clarify this area of research by painting a detailed and beautifully consistent picture. We will make extensive reference to this review.U)... 

Evidence that eliminates the triplet mechanism as the mode for the cis-trans isomerization of stilbene upon direct photolysis has been provided by azulene quenching studies.(48) Using the experimentally determined decay ratio a/(l — a) and the triplet mechanism, it is possible to calculate what the effect of azulene is upon the pss. The predicted and observed azulene effects on the direct photoisomerization are shown in Figure 9.6. The failure of the triplet mechanism in predicting the very small changes observed in the pss provides a crucial test that is the basis for rejecting the triplet mechanism. 

FIGURE 2.1 Classical Horiuti-Polanyi half-hydrogenated state mechanism for hydrogenation, double bond migration, cis-trans isomerization, and deuterium exchange. 

Examples of this flipping mechanism are seen in cis-trans isomerizations from less stable to more stable isomers which when the reactions are carried out under deuterium. Already mentioned are the isomerizations of oleic acid. Additionally, methyl-(Z)-but-2-enoate isomerizes to its more stable E-isomer with incorporation of substantial amounts of deuterium during deuteriuma-tion over Pd/C (Fig. 2.16). At the same percentage deuteriumation, the saturated product contains in its P-position 90% of the two deuteriums added to... 

Cis-trans isomerization occurs either by formation of a half-hydrogenated state (Horiuti-Polanyi mechanism) followed by rotation around the newly formed single bond and abstraction of an appropriate hydrogen onto the surface or by double bond migration (either Horiuti-Polanyi or Jt-allyl) from a cis (trans) position to an adjacent trans (cis) position (deuterium exchange studies favor the rotation mechanism). 

Foote, C. S., Y. C. Chang, and R. W. Denny. 1970. Chemistry of singlet oxygen. XI. Cis-trans isomerization of carotenoids by singlet oxygen and a probable quenching mechanism. J. Am. Chem. Soc. 92 5218-5219. 

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