Cis conformation

The Diels-Alder cycloaddition is one exanple of a pericyclic reaction, which is a one-step reaction that proceeds through a cyclic transition state. Bond formation occurs at both ends of the diene system, and the Diels-Alder transition state involves a cyclic ariay of six carbons and six tt electrons. The diene must adopt the 5-cis conformation in the transition state. 

But in the substituted dienes like cis-1-substituted dienes, the 5 -cis conformation is disfavoured with respect to the 5 -cis form of corresponding transform due to the nonbonding interactions between the substituent and the 4-hydrogen. In a concerted reaction the trans isomer should be therefore more reactive than the cis-derivative. 

Many (i7 -diene)metallocenes (5) of conjugated dienes, fixed in their 5-cis conformation by a rigid carbon framework, have been obtained in high yields by most of the synthetic routes used for the preparation of 3... 

Conjugated Dienes. General Observations, The catalytic hydrogenation of conjugated dienes by pentacyanocobaltate(II) is completely selective, yielding only monoolefin, which cannot be reduced further (19, 20, 21), Lack of reduction of 2,5-dimethyl-2,4-hexadiene indicates that dienes in which the 5-cis conformation is sterically hindered might not be catalytically hydrogenated (21). However, reduction of 3-methylene cyclohexene, a diene with a fixed s-trans conformation, disproves this and indicates that other steric factors are probably involved (24). 

In contrast to those unreactive dienes that can t achieve an 5-cis conformation, other dienes are fixed only in the correct s-cis geometry and are therefore highly reactive in the Diels-Alder cycloaddition reaction. 1,3-Cyclopentadiene, for example, is so reactive that it reacts with itself. At room temperature, 1,3-cyclopentadiene dimerizes. One molecule acts as diene and a second molecule acts as dienophile in a self Diels-Alder reaction. 

Clearly each case must be analyzed separately, but these transition structures serve as a starting point for such analyses. Note also that the structures of Scheme 5.29 all have enones in an 5-cis conformation, which is not available to cyclic acceptors such as cyclohexenone, cyclopentenone, and unsaturated lactones. 

Theoretical studies indicate that these transition structures are probably influenced by frontier molecular orbitals (in addition to steric effects), as indicated in Scheme 5.34c [182]. For the reaction of aminoethylene (a primary enamine) and acrolein, the enamine HOMO and the enone LUMO have the most attractive interactions when aligned in the chair configuration shown, which has the enone in an 5-cis conformation. Note that this orientation places the NH and the electrophile a-carbon in close proximity for proton transfer via the ene transition structure. 

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 equilibrium favors the 5-trans conformation of (Z)-l,3-pentadiene more than it does that of the E isomer because the 5-cis conformation of the Z isomer has more van der Waals strain. 

In order to achieve the necessary geometry in the Diels-Alder transition state, the diene must be able to adopt the 5-cis conformation. In Section 10.10, we saw that the s-cis conformation of 1,3-butadiene is 12 kJ/mol (2.8 kcal/mol) less stable than the 5-trans form. This is a relatively small energy difference, so 1,3-butadiene is reactive in the Diels-Alder reaction. Dienes that cannot readily adopt the s-cis conformation are less reactive. For example, 4-methyl-1,3-pentadiene is a thousand times less reactive in the Diels-Alder reaction than ra/25-1,3-pentadiene because its 5-cis conformation is destabilized by the steric effect imposed by the additional methyl group. 

Cyclic dienes yield bridged bicyclic Diels-Alder adducts. Since they are constrained to the 5-cis conformation, cyclic dienes such as 1,3-cyclopentadiene are highly reactive in the Diels-Alder reaction. 

For dienes that are predominantly in the -trans conformation (either free to rotate or held in the 5-trans conformation). Woodward and Fieser used a base value of 217 nm, the Amax for unsubstituted 1,3-butadiene. To this value, add 5 nm for each alkyl substituent. For dienes that are held in the 5-cis conformation by a six-membered ring, the base value is 253 nm for the diene, plus 5 nm for each alkyl substituent. 

FIG. 5 Cis conformation with respect to the amide bond of sodium IV-lauroylsarcosi-nate upon vesicle formation in an equimolar mixture with N-dodecylpyridinium chloride. 

In order to participate in a Diels-Alder reaction, the conjugated diene must be in an 5 -cis conformation because C-1 and C-4 in an. v-trans conformation are too far apart to react with the dienophile in a concerted reaction. Therefore, a conjugated diene that is locked in an s-trans conformation cannot undergo a Diels-Alder reaction because it cannot achieve the required c/s-conformation. 

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. 

Figure 10.3 illustrates the classification of the MO s of butadiene and cyclobutene. We will use ( + ) and ( - ) to specify orbital phase when we are considering individual molecular orbitals. There are two elements of symmetry which are common both to the 5-cis conformation of butadiene and cyclobutene. These are a plane of symmetry and a two-fold rotation axis. 

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