Mechanism of thermal rearrangement of the spiro bicyclo[2.1.0]-pentane-5,2'-methylenecyclopropanes to 6- and 7- methylenebicyclo[3.2.0]hept-1-enes

The thermal rearrangements of the bicyclo[2.1.0]pentane-5,2'-methylenecyclopropanes fall into two classes. The first occurs near 80 degrees C and consists of a double epimerization ("bridge flip") which is initiated by cleavage of the bridge bond. An alternative mechanism by way of a trimethylenemethane intermediate is ruled out by an isotopic position-marking experiment. The second rearrangement begins to be detected above 120 degrees C. It gives the isomeric 6- and 7-methylenebicyclo[3.2.0]hept-1-enes. Two possible mechanisms can operate in this complex change, but a choice between them is not yet possible.