E, Z, or neither

Label each double bond E, Z, or neither. (It may help to draw in some missing H s.)... 

All /1-eliminations from the benzyl derivative in Figure 4.5 exhibit a certain stereoselectivity, in this case -stcreoselectivity. This is true regardless of whether the elimination is syn- or awft -selective or neither. The reason for the preferred formation of the /. -product is again product-development control. This comes about because there is a significant energy difference between the isomeric elimination products due to the presence (E isomers) or absence (Z isomers) of styrene resonance stabilization. 

A similar argument can be made for the [1,7] sigmatropic rearrangement. The (S,E) starting material rearranges antarafacially with respect to the seven-atom component. The H can move from the bottom face at one terminus to the top face at the other or it can move from the top face at one terminus to the bottom face at the other. Either the (E,R) or the (Z,S) product, but neither the (E,S) nor the (Z,R) product, is obtained. 

The (Z,Z)-isomers provided either the corresponding dimers or (E,E)-isomers during UV irradiation. During the photoreaction of (E,E)-isomers, some crystals provided the dimer and others showed no reaction. Neither (Z,Z)- nor (E,Z)-isomers were formed because the photoisomerization of muconates occurs via a one-way reaction mechanism. On the basis of the results of single crystal structure analysis, various molecular packing structures were observed, depending on the number and... 

Problem 3.8. Are either (or both, or neither) E)- or (Z)-l,2-dimethylcyclopropane superimposable upon its realized mirror images ... 

Draw another example of an alkene that cannot be named using the cis/trans nomenclature. (Such a molecule is not cis, trans or neither, but instead requires the E/Z nomenclature.)... 

Describe the characteristics of a molecule that falls outside the cis, trans or neither naming scheme and requires the E/Z naming scheme. 

The substituted carbons are stereogenic carbons. This means that there are not only two isomers. In the most general case, where W, X, Y, and Z are all different, there are four isomers since neither the cis nor the trans isomer is superimposable on its mirror image. This is true regardless of ring size or which carbons are involved, except that in rings of even-numbered size when W, X, Y, and Z are at opposite comers, no chirality is present, (e.g., 68). In this case, the substituted carbons are not chiral carbons. Note also that a plane of symmetry exists in such compounds. When W = Y and X=Z, the cis isomer is always superimposable on its mirror image, and hence is a meso compound, while the trans isomer consists of a dl pair, except in... 

Takaya and co-workers46 found that BINAP-based Ru(II) dicarboxylate complexes 31 can serve as efficient catalyst precursors for enantioselective hydrogenation of geraniol (2E)-32 and nerol (2Z)-32. (R)- or (iS )-citroncllal 33 is obtained in nearly quantitative yield with 96-99% ee. The nonallylic double bonds in geraniol and nerol were intact. Neither double bond migration nor (fi)-/(Z)-isomerization occurred during the catalytic process. Furthermore, the S/C ratio was extremely high, and the catalyst could easily be recovered (Scheme 6-18). This process can be applied to the asymmetric synthesis of a key intermediate for vitamin E. 

The rearrangement of E and Z N-crotylamines 245 (R, E or Z=Me) gave the corresponding nitriles 247 with 82 and 68% yield, respectively. Disappointingly, the product was obtained as an inseparable mixture of synlanti diastereomers 247 indicating a low simple diastereoselectivity. Obviously, the intermediate ketene imine fitted neither a chair- nor a boat-like conformation. Hence, a low axis-to-center chirality induction was operative, and E and Z reactants gave a... 

E is an activation energy, and Z the collision number for molecules in the gas phase, and neither has been modified to account for either electrostatic interactions, or the effect of the relative permittivity of the solvent. 

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