Transition structure cyclopentadiene

The activation of various reactions by Lewis acids is now an everyday practice in synthetic organic chemistry. In contrast, solvent effects on Lewis acid catalysed Diels-Alder reactions have received much less attention. A change in the solvent can affect the association step leading to the transition structure. Ab initio calculations on the Diels-Alder reaction of cyclopentadiene and methyl vinyl ketone in aqueous media showed that there is a complex of the reactants which also involves one water molecule119. In an extreme case solvents can even impede catalysis120. The use of inert solvents such as dichloromethane and chloroform for synthetic applications of Lewis acid catalysed Diels-Alder reactions is thus well justified. General solvent effects, in particular those of water, will be discussed in the following section. 

Analogous studies on the dimerization of cyclopentadiene in water revealed a stabilization of the transition structure relative to the initial structure as a result of a difference in... 

Additionally, investigations into imidazolidinone catalysed Diels-Alder reactions (Schemes 2 and 6) [234] have shown that iminium ions of a,P-unsaturated aldehydes and ketones have lower activation barriers for the Diels-Alder reaction with cyclopentadiene than the parent compound (13 and 11 kCal mol", respectively). It was also noted that transition structures show the formation of the bonds is concerted but highly asynchronous. 

Alder s endo rule applies not only to cyclic dienes like cyclopentadiene and to disubstituted dienophiles like maleic anhydride, but also to open chain dienes and to mo no-substituted dienophiles diphenylbutadiene and acrylic acid, for example, react by way of an endo transition structure 2.113 to give largely (9 1) the adduct 2,114 with all the substituents on the cyclohexene ring cis, and equilibration again leads to the minor isomer 2.115 with the carboxyl group trans to the two phenyl groups. 

A secondary orbital interaction has been used to explain other puzzling features of selectivity, but, like frontier orbital theory itself, it has not stood the test of higher levels of theoretical investigation. Although still much cited, it does not appear to be the whole story, yet it remains the only simple explanation. It works for several other cycloadditions too, with the cyclopentadiene+tropone reaction favouring the extended transition structure 2.106 because the frontier orbitals have a repulsive interaction (wavy lines) between C-3, C-4, C-5 and C-6 on the tropone and C-2 and C-3 on the diene in the compressed transition structure 3.55. Similarly, the allyl anion+alkene interaction 3.56 is a model for a 1,3-dipolar cycloaddition, which has no secondary orbital interaction between the HOMO of the anion, with a node on C-2, and the LUMO of the dipolarophile, and only has a favourable interaction between the LUMO of the anion and the HOMO of the dipolarophile 3.57, which might explain the low level or absence of endo selectivity that dipolar cycloadditions show. 

All the reactions described so far have mobilised six electrons, but other numbers are possible, notably a few [8 + 2] and [6 + 4] cycloadditions involving 10 electrons in the cyclic transition structure. A conjugated system of eight electrons would normally have the two ends of the conjugated system far apart, but there are a few molecules in which the two ends are held close enough to participate in cycloadditions to a double or triple bond. Thus, the tetraene 6.17 reacts with dimethyl azodicarboxylate 6.18 to give the [8 + 2] adduct 6.19, and tropone 6.20 adds as a 6-electron component to the 4-electron component cyclopentadiene to give the adduct 6.21. 

Figure 7. Four possible transition structures in the Diels-Alder reaction of propynal and cyclopentadiene promoted by Lewis acid. ML = Lewis acid.

Analogous studies on the dimerization of cyclopentadiene in water revealed a stabilization of the transition structure relative to the initial structure as a result of a difference in solvation of 1.7 kcalmol. Unfortunately, at least to our knowledge, reliable experimental data for this process are not available. Recently, in a similar approach, the Gibbs enthalpies of hydration of the Diels-Alder reaction of cyclopentadiene with isoprene and methyl vinyl ketone were determined. Surprisingly, it was observed that water stabilized the transition structure of the cyclopentadiene - - isoprene reaction more than that of the... 

Homoantiaromaticity is even less commonly invoked. Homocyclobutadiene 1.32b and the homocyclo-pentadienyl cation 1.33b are close to the transition structures for the interconversion of cyclopentadiene 1.32a and bicyclo[2.1.0]pentene 1.32c and of the cyclohexatrienyl cation 1.33a and the bicyclo[3.1.0]hex-enyl cation 1.33c. However, homoantiaromaticity does show up in these cases, in the sense that, unlike the interconversions in 1.30 and 1.31, neither of these interconversions is rapid. 

Another way of looking at the bonds in the transition structure for the dimerisation of cyclopentadiene is to see that they develop from the best frontier orbital overlap the leading bond comes from overlap between the large lobes on C-1 and C-1 in both the HOMO/LUMO interaction marked in bold in the drawing 6.269 and the equally effective LUMO/HOMO interaction marked in bold in the drawing 6.270. The two partly formed bonds, marked with dashed lines, come from overlap between a large lobe on C-4 and a small lobe on C-2 and between a large lobe on C-4 and a small lobe on C-2,822 either of which can develop into the full bond of the product. 

Cycloadditions. Secondary orbital interactions have been cited as an explanation for the stereochemistry of [4 + 6] cycloadditions such as that between cyclopentadiene and tropone 6.45 - > 6.46, which favours the exo transition structure 6.360. The frontier orbitals have a repulsive interaction (wavy lines) between C-3, C-4 on the tropone and C-2 on the diene (and between C-5 and C-6 on the tropone and C-3 on the diene) in the endo transition structure 6.361. However, in this reaction the exo adduct is thermodynamically favoured, the normal repulsion between filled orbitals in the endo transition structure is an adequate explanation, and the electrostatic explanation given in Section 6.5.2.4 works just as well. There is no real need to invoke secondary orbital interactions. 

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