J-Trans conformation

In the absence of a Lewis acid catalyst, both j-cis and j-trans conformers are present. 

Note that the illustrated conformation has the acrolein oriented in an 5-cis conformation. This is in contrast to the usual 5-trans conformation of acroleins coordinated to a Lewis acid (Figure 6.13a), but it is supported by the fact that cyclo-pentadiene adds to the opposite face of acrolein itself [216]. It is likely that both s-cis and 5-trans dienophile conformers are present, and that the -cis conformer is more reactive. In other words, Curtin-Hammett kinetics [235] are operative. The rationale for this increased reactivity is as follows the j-trans conformation of 2-bromoacrolein would place the bromine above the indene ring. Cycloaddition to the top (Si) face of the 5-trans conformer would force the bromine into closer proximity to the indene as C2 rehybridizes from sp2 to sp3, a situation that is avoided in cycloaddition to the top (Re) face of the 5-cis conformer. 

The 5-trans conformation is 12 kJ/mol (2.8 kcal/mol) more stable than the s-cis conformation, which shows interference between the two nearby hydrogen atoms. The rotational barrier for these conformers (rotation about the C2 — C3 bond) is only about 20 kJ/mol (about 5 kcal/mol) compared with about 250 kJ/mol (60 kcal/mol) for rotation of a double bond in an alkene. The j-cis and j-trans conformers of butadiene (and all the skew conformations in between) easily interconvert at room temperature. 

Structural features that aid or hinder the diene in achieving the j-cis conformation affect its ability to participate in Diels-Alder reactions. Figure 15-16 shows that dienes with functional groups that hinder the j-cis conformation react more slowly than butadiene. Dienes with functional groups that hinder the j-trans conformation react faster than butadiene. 

The j -trans conformation is 12 kJ/mol (2.8 kcal/mol) more stable than i -cis, with van der Waals strain between the C(l) and C(4) interior hydrogens contributing to the decreased stability of the i -cis conformation. The two conformations are interconvertible by rotation about the C(2)-C(3) single bond with an activation energy for the 5-trans i -cis conversion of 25 kJ/mol (6 kcal/mol). This energy cost reflects the loss of ir-electron delocalization in going from a coplanar C=C—C=C arrangement to a nonplanar one at the transition state. 

Solution The most reactive diene has its double bonds locked in an 5-cis conformation, whereas the least reactive diene has its double bonds locked in an s-trans conformation. The other two compounds are of intermediate reactivity because they can exist in both s-cis and j-trans conformations. 1,3-Pentadiene is less apt to be in the required 5-cis conformation because of steric interference between the hydrogen and the methyl group, so it is less reactive than 2-methyl-1,3-butadiene. 

Ramani, R., and R. J. Boyd. 1981. Ab-initio Molecular Orbital Study of the cis/trans Conformations of the Peptide Bond. Int. J. Quantum Chem. Quantum Biol. Symp. 8, 117-127. 

Jolibois F, Voituriez L, Grand A, Cadet J (1996) Conformational and electronic properties of the two cis 5S,6R) and trans 5R,6S) diastereomers of 5,6-dihydroxy-5,6-dihydrothymidine X-ray and theoretical studies. Chem Res Toxicol 9 298-305... 

S. J. Angyal and T. Q. Tran, Conformational analysis in carbohydrate chemistry. 5. Formation of glycosidic anhydrides from heptoses, Can. J. Chem., 59 (1981) 379-383. 

In the case of poly-L-proline II (the amide group being in the planar trans conformation), the angle is fixed by the rigid geometry of the pyrrolidine ring hence, the conformational energy depends only on tfi. 

Although s-trans is the more favorable conformer, reaction occurs with s-cis because this conformation has its double bonds on the same side of the single bond connecting them hence, the stable form of cyclohexene with a cis double bond is formed. Reaction of the s-trans conformer with ethene would give the impossibly strained ra j -cyclohexene [Problem 9.10(a)]. As the s-cis conformer reacts, the equilibrium between the two conformers shifts toward the s-cis side, and in this way all the unreactive s-trans reverts to the reactive s-cis conformer. 

A similar coupling may be observed for a, /J-unsaturated aldehydes and ketones. Table 4.1-3 shows that the s-cis or s-trans conformation of the C=C-C=0 systems is indicated by the difference of the frequencies of the C=0 and the C=C stretching vibration as well as by its relative intensity in the infrared and the Raman spectrum and the depolarization factor in the Raman spectrum (Oelichmann et al., 1982). 

Bernstein, J. and Schmidt, G. M. J. (1972). Conformational studies. Part IV. The crystal and molecular structure of the metastable form of N-(j9-Chlorobenzylidene)-p -chloroaniline, a planar anil. / Chem. Soc., Perkin Trans. 2, 951-5. [72, 227]... 

Conformational analysis of substituted 1,3,2-dioxaphosphorinanes has been achieved using a topological approach (429) rather than the more elaborate random search method. This simplified approach only considers chemically and physically accessible positions for the lanthanide. Using this approach the LIS calculations are found to predict the conformational equilibrium for complexed cis conformers which is in good agreement with that based on the response of the coupling constants J(H-H) and J(P-H) to lanthanide addition. For the trans conformers the two approaches are not in agreement. 

Perhydrothiazolo[3,4-a]pyridine adopts a 60% trans-fused (179) 40% cis-fused (180) equilibrium in CDCI3-CFCI3 at 193 K. The c/j-(1-H, 8a-H)-1-methyl derivative favors exclusively the trans-conformer (181), whereas the epimer adopts an 87% trans-fused (182) 13% cis-fused (183) conformational equilibrium at 193 K. The pair of 6-ethyl substituted derivatives adopts the trans- and cis-fused conformations (184) and (185). These assignments were based on H- and C-NMR spectral data [68T2485 88JCS(P1)1173],... 

Sundararajan has studied silane polymers including polymethylphenyl-silylene)243 and polysilapropylene. " " Different tacticities and conformations of polymethylphenylsilylene were examined and the relative stabilities of each were reported. The most stable configuration was for the syndiotactic polymer with an all-trans conformation. Helix parameters, a priori bond probability, and characteristic ratios were calculated for this polymer. Polysilapropylene was studied in a similar manner. MM2 energies of the minimum for each of the different rotational isomers were included. As with the first study, contour plots of both energy and helix parameters versus (j) and < ) + 1 angles were given. 

An interesting aspect of this reaction is the formation of substantial amounts of cw-2-butene, which would appear to require the intermediacy of the j-cis-1,3-butadiene anion radical, even though butadiene exists almost exclusively in the s-trans conformation (98 %). At —33°C, 13 % of the 2-butene mixture is the cis alkene, and at -78 °C 50 % of the mixture is cw-2-butene. In the case of 1,3-pentadiene, 68 % of the 2-pentene is the cis isomer. The most plausible explanation for these stereochemical results appears to be the reversible reduction of the diene to the diene anion radical at -78 °C by the pool of solvated electrons, which yields an equilibrium mixture of the s-cis and j-tran5-anion radicals (ca. 50 50), which are... 

Reaction with a,P-unsaturated ketones and lactones. The reactivity of ot,/ -enones to singlet oxygen depends on the conformation. Systems that exist in s-trans-conformations (e.g., A4-3-ketosteroids) react slowly if at all. However, j-cis-enones react readily. For example, (R)-(+)-pulegone (1) reacts to give the products 2-4. The same products are obtained by oxidation with triphenyl phosphite ozonide (3, 324-325). 

FIGURE 15 (PLATE 4). Localization domains of ort/io-X-substituted phenols (from left to right X = F, Cl, Br, I top—trans conformer, bottom—cis conformer). The ELF value defining the boundary isosurface, j(r) = 0.659 corresponds to the critical point of index 1 on the separatiix between adjacent V(C, C) basins of benzene. Colour code magenta = core, orange = monosynaptic, blue = protonated disynaptic, green = disynaptic. Adapted from Reference 220 with permission... 

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