NONCONCERTED COPE REARRANGEMENT

The observation of practically identical activation parameters for the Cope rearrangement 2-->6 and its ''frustrated'' counterpart 1-->5 ind icates a two-step mechanism for the reaction 2-->6. Direct proof of th is interpretation comes from trapping experiments, which demonstrate t he intermediate formation of the diradicals 5 and 9. From the temperat ure and oxygen dependences of the trapping rates, recombination barrie rs of Delta H-not equal = 11.5 and 6.5 kcal.mol(-1) have been derived for 5-->1 and 9-->6, respectively. These results agree well with ab in itio calculations (CASPT2).