Cyclopentadiene reaction with tropone

Tropolone has been made from 1,2-cycloheptanedione by bromination and reduction, and by reaction with A -bromosuccinimide from cyolo-heptanone by bromination, hydrolysis, and reduction from diethyl pimelate by acyloin condensation and bromination from cyclo-heptatriene by permanganate oxidation from 3,5-dihydroxybenzoic acid by a multistep synthesis from 2,3-dimethoxybenzoic acid by a multistep synthesis from tropone by chlorination and hydrolysis, by amination with hydrazine and hydrolysis, or by photooxidation followed by reduction with thiourea from cyclopentadiene and tetra-fluoroethylene and from cyclopentadiene and dichloroketene. - ... 

The Diels-Alder reaction may be the best known cycloaddition, but other types of cycloaddition reactions are also synthetically important. Consistent with the predictions of the selection rules, the [ 6 -f 4J cycloaddition was seen in the reaction of cyclopentadiene with tropone (61) ... 

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. 

Tropone will undergo some condensation reactions, for example it reacts with a cyclopentadiene tetraester in the presence of... 

Cyclobutadiene generated by cerium (iv) oxidation of the cyclobutadieneiron tricarbonyl complex undergoes a [4 + 2] addition to tropone to give the endo-adduct (179). Tropone reacts with l,3-diphenylindan-2-one to give the decarbonyl-ated [4 + 2]adduct (180). " The activation volume of the [4 + 6] cycloaddition of tropone to cyclopentadiene is —7.5+1 cm mol in p-dioxan at 60 °C and the overall reaction volume is —4.3 1 cm mol . The small partial molar volume of tropone is responsible for these small negative values. ... 

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