Configuration, trans Conformation

The decalin (bicyclo[4.4.0]decane) ring system provides another important system for study of conformational effects in cyclohexane rings. Equilibration of the cis and trans isomers reveals that the trans isomer is favored by about 2.8 kcal/mol. Note that this represents a change in configuration, not conformation. The energy difference can be analyzed by noting that the cis isomer has three more gauche butane interactions that are... 

NH2OH can exist as 2 configurational isomers (cis and trans) and in numerous intermediate gauche conformations as shown in Fig. 11.7. In the crystalline form, H bonding appears to favour packing in the trans conformation. The N-O distance is 147 pm consistent with its formulation as a single bond. Above room temperature the compound decomposes (sometimes explosively) by internal oxidation-reduction reactions into a complex mixture of N2, NH3, N2O and H2O. Aqueous solutions are much more stable, particularly acid solutions in which the compound... 

For reactions of A-acyliminium ions with alkenes and alkynes one has to distinguish between A-acyliminium ions locked in an s-trans conformation and those which (can) adopt an s-cis conformation. The former type reacts as a (nitrogen stabilized) carbocation with a C —C multiple bond. Although there are some exceptions, the intramolecular reaction of this type is regarded as an anti addition to the 7t-nucleophile, with (nearly) synchronous bond formation, the conformation of the transition state determining the product configuration. 

Single- and double-selective relaxation-rates, together with n.O.e. experiments, have been used to examine the configuration and conformation of asperlin (1) in benzene solution." " Comparing experimental distances for the proton pairs H-4,H-7 and H-5,H-7 with those obtained from molecular models, it was possible to confirm earlier evidence that the oxirane ring is trans, and also to show that, of the two possible diastereoisomeric forms (49a and 49b), the data are more fully compatible with structure 49a, the... 

The carotenoid isomerase (CRTISO) was the first isomerase associated with the desaturation steps and named at a time when Z-ISO was unknown to exist ise.ws.ieo.iei (and reviewed in references ). In vitro analysis of substrate conversion " and transcript profiling in planta associated CRTISO with the desaturation steps. Isaacson demonstrated that CRTISO is specific for the 7,9 or 7,9- cis bond configuration and is not involved in the isomerization of the l5-l5-cis double bond to the trans conformation. As recently shown, Z-ISO is required for isomerization of the 15-15 cis double bond of phytoene produced in dark-grown tissues as well as in stressed photosynthetic tissues. Therefore, desaturation of phytoene to lycopene involves a two-step desaturation by PDS, followed l5-cis isomerization by Z-ISO, and then each pair of double bonds introduced by ZDS is followed by CRT-ISO-mediated isomerization of the resulting conjugated double bond pair. 

The hydrogenation of 4-alkyl substituted anilines results in the production to two possible isomeric forms, cis and trans, and both of these isomers have two possible conformations in the chair configuration of the cyclohexyl ring. However experimentally it has been found that one cis and one trans conformer dominate. The dominant isomers are shown in Figure 1. 

Au atom (see Fig. 15b). The high conductance values H represent an all-trans alkyl chain in combination with both sulfurs coordinated to two Au atoms in a bridge geometry (Fig. 15c). The theoretically predicted fourfold increase of the conductance, as compared to an atop-atop all-trans bridge, is close to the experimentally observed increase by a factor of 5. Other possible configurations with all-trans conformations (e.g., atop-bridge, multiple molecular junctions) are assumed to contribute as part of the distribution to the less-dominant peaks in the conductance histograms (e.g., M2, H2, and H3 in Fig. 13). 

Conformational mobihty, such as we get in cyclohexane rings, makes the analysis more difficult, and manipulating molecular models provides the clearest vision of the relationships. Let us look at 1,2-dimethylcyclohexane as an example. Again, we have met the cis and trans isomers when we looked at conformational aspects (see Section 3.3.2). Here, we need to consider both configuration and conformation. 

The stereochemistry of 1,3-dipolar cycloadditions of azomethine ylides with alkenes is more complex. In this reaction, up to four new chiral centers can be formed and up to eight different diastereomers may be obtained (Scheme 12.4). There are three different types of diastereoselectivity to be considered, of which the two are connected. First, the relative geometry of the terminal substituents of the azomethine ylide determine whether the products have 2,5-cis or 2,5-trans conformation. Most frequently the azomethine ylide exists in one preferred configuration or it shifts between two different forms. The addition process can proceed in either an endo or an exo fashion, but the possible ( ,Z) interconversion of the azomethine ylide confuses these terms to some extent. The endo-isomers obtained from the ( , )-azomethine ylide are identical to the exo-isomers obtained from the (Z,Z)-isomer. Finally, the azomethine ylide can add to either face of the alkene, which is described as diastereofacial selectivity if one or both of the substrates are chiral or as enantioselectivity if the substrates are achiral. 

The import of diabatic electronic states for dynamical treatments of conical intersecting BO potential energy surfaces is well acknowledged. This intersection is characterized by the non-existence of symmetry element determining its location in nuclear space [25]. This problem is absent in the GED approach. Because the symmetries of the cis and trans conformer are irreducible to each other, a regularization method without a correct reaction coordinate does not make sense. The slope at the (conic) intersection is well defined in the GED scheme. Observe, however, that for closed shell structures, the direct coupling of both states is zero. A configuration interaction is necessary to obtain an appropriate description in other words, correlation states such as diradical ones and the full excited BB state in the AA local minimum cannot be left out the scheme. 

With respect to the plane of the skeletal bonds in the planar all-trans conformation, the phenyl groups on Sij., Si , and Si +1 can be all up, all down, or alternate. A total of eight configurations are possible. 

The overall reaction is best viewed as intramolecular oxidative addition of the C(l)—H bond to the Rh(I) center, causing cyclometalation (25), followed by reductive elimination of an enamine from the Rh(III) intermediate accompanied by allylic transposition. Notably, the allylamine ligand in the initial Rh(I) complex as well as the Rh(III) intermediate has an s-trans conformation with respect to the N—C(l) and C(2)—C(3) bonds, allowing the overall suprafacial 1,3-hydrogen shift to produce the is-configured enamine product. 

Normally, the trans conformation is more stable than the cis conformation for primary amides. However, for cyclic amides (lactams), in which the ring size is small, the configuration is exclusively cis ... 

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