Orbital interactions tropone + cyclopentadiene

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. 

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. 

The cycloaddition of cyclopentadiene and tropone (p. 91) gives the exoadduct (181) rather than the endo-adduct (182), because the secondary interactions (Fig. 4-2lb, wavy line) of the frontier orbitals are antibonding. 

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