E and Z conformers

A similar approach has been used in the case of azolides (A-acylazoles). The experimental dipole moment of 1-acetylpyrazole (1.55 D, Table 3) was compared with the CNDO/2 calculated values for the E and Z conformations, respectively 1.87 and 5.46 D. Fayet et al. concluded that the E conformer (41) largely predominates (77RRC471). 

In an ab initio study of the substituent effects on the relative stability of the E and Z conformers of 4-X-substituted phenyl acetates (16 R = Me) and trifluoroacetates (16 R = CF3) (Scheme 6), it was shown that increasing electron-withdrawing ability... 

A theoretical study on the dimerization of thioformylketene 5a has been performed at the B3LYP and G3MP2B3 level <20050L5817>. Authors considered the [4+2] reaction pathway involving the formation of thietenone 4a and calculated the participation of both (E)- and (Z)- conformations of thioformylketene. Obtained results have shown that thietenone 4a is less stable than thioformylketene 5a and that the barrier for ring closure is low... 

Figure 5 Calculated energy differences in kcal/mol between the E and Z conformer of N-methylacetamide. The dashed line indicates the experimental value.

The radical anions of (43) and (44) have been examined by electron spin resonance spectroscopy <86JCS(P2)185i>. The ESR spectrum of the radical anion of (43) has been interpreted in terms of the presence of both (E,E)- and (Z, )-conformers and the activation parameters for the interconversion have been determined. 

The n.m.r. spectra of E/Z mixtures of twelve compounds were assigned and the angles of twist about the aryl-imino-carbon bond were determined.Subsequently, the pure (E)- and (Z)-isomers were obtained by preparative t.l.c. at 258 K or by stereospecific syntheses from (E)- and (Z)-thioamides. The chemical shifts of the protons in the methyl groups were measured and the (E) (Z) ratios and the barriers to isomerization were determined by n.m.r. spectroscopy. The conformational states of some AW-di(primary alkyl)-amides and -thioamides were studied by dynamic H n.m.r. spectroscopy, and the barriers to the interconversion of enantiomers that were thus found were in good agreement with molecular mechanics calculations. Lr. and n.m.r. data for optically active thioamides R CSNR CHMeR indicate that there are mixtures of (E) and (Z) conformers, and i.r. and P n.m.r. studies of phosphorylated thiobenzamides provide evidence of a tautomeric equilibrium [reaction (4)], the amide form being favoured in more... 

Heteroatom substituents also introduce polar effects. In the case of a-alkoxy aldehydes the preferred TS appears to be F and G for the E- and Z-enolates, respectively. These differ from the normal Felkin TS for nucleophilic addition. The reactant conformation is believed to be determined by minimization of dipolar repulsion between the alkoxy substituent and the carbonyl group.96 This model predicts higher 3,4-anti ratios for Z-enolates, and this is observed. 

The stereochemistry of acyclic anionic oxy-Cope rearrangements is consistent with a chair TS having a conformation that favors equatorial placement of both alkyl and oxy substituents and minimizes the number of 1,3-diaxial interactions.214 For the reactions shown below, the double-bond configuration is correctly predicted on the basis of the most stable TS available in the first three reactions. In the fourth reaction, the TSs are of comparable energy and a 2 1 mixture of E- and Z-isomers is formed. 

The preferred stereoelectronic arrangement is perpendicular alignment of the acetate with respect to the double bond. For example, the cis and trans isomers of 1-vinyl-2-methylcyclohexyl acetate show divergent stereochemical results. Only the exocyclic E-isomer is formed from the cis compound, whereas the trans compound gives a 1 1 mixture of the E- and Z-isomers. This is the result of a strongly preferred conformation for the cis isomer, as opposed to a mixture of conformations for the trans isomer.32... 

This chemistry was mechanistically interesting, and in order to rationalize the observed diastereoselectivities, molecular modeling calculations were conducted on the proposed oxonium ion intermediates.4 There are E- and Z-oxonium ion conformers. Since the major Z-oxonium ion conformers (86, 87) from these calculations were approximately 3 kcalmol-1 higher in energy, these conformers most likely did not significantly contribute to the stereochemical outcome of the reaction (Figure 7.3). 

The difference in activity of the E and Z olefins against HRV-14 was explained by examining the relatively low energy virus-bound conformations. The result of an overlay of WIN-54954 (based on x-ray crystallography data), minimize E- and Z-olefinic structures and the butyne analogue, suggested that the E isomer showed a reasonable fit while the Z isomer did not. Furthermore, when the Z isomer was inserted into the HRV-14 pocket, unfavorable interactions occurred. 

Photochemical d.s jraw.v-confonnational interconversion is also known to occur in larger polyenes. Brouwer and Jacobs have reported the results of irradiation of E- and Z-2,5-dimethyl-l,3,5-hexatriene (14) in argon matrices at 10 K108. Irradiation of the -isomer gives rise to various retainers, while irradiation of the Z-isomer results only in ,Z-isomerization. Photochemical translcis conformer interconversion has also been observed for , -1,3,5,7-octatricnc in matrices at temperatures below 10 K32. 

When the C2=C3 bond was fixed by ring closure in the Z conformation as in 67, both E and Z forms with respect to the Q—C2 bond could be observed,... 

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... 

SB. Actually, these tautomeric equilibria are more complex because each of these three forms can exist as two geometric isomers ( and Z for 75A and 75A) or two conformers (rotamers E and Z for 75B). When + H, two diastereomers cis and trans and R ) are possible for the cyclic tautomer 75B. These equilibria have been thoroughly investigated (88TH2) by means of electronic, IR, H-, and C-NMR spectroscopy. The structure of the solid 5-hydroxypyrazoline 74B- (R = Me R = H R = Ph R" = i-Pr) was confirmed by X-ray diffraction (87MI5). An approximate generalization of the influence of the structural factor on these equilibria is shown in Table X. 

It should be noted that, after the introduction of a COOMe group at the C = N bond in 76A, the designation of the conformations E and Z has been reversed on comparison with those of hydrazones 75A. 

On the other hand, the EjZ ratio was independent of the type of catalyst and the conversion and determined only by the reaction temperature, indicating that the interconversion of E and Z isomers is fast and that both conformers are in equilibrium during the reaction. 

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