ABSTRACT
Chemists have observed that some compounds react as if they possessed two rapidly equilibrating structures which differ only by the arrangement of the a o i r electrons. Such an example is cyclooctatetraene 15 which can be hydrogenated to cyclooctane, but which, being devoid of conjugation (because of its tub-shaped geometry), cannot undergo Diels-Alder cycloaddition. Its valence isomer, bicyclo-[420]-octatriene 16 possesses a conjugated diene system which reacts normally with dienophiles. The activation parameters for the interconversion 15-»10 are 28.1 kcal/mol and -|-1 cal/mol grad; the reverse reaction 16-»15 has an activation energy of 18.7 kcal/mol [88,89]. The low activation energies for the thermal interconversion have been rationalized by Woodward and Hoffmann [90].
