Thermal isomerizations of vinylcycloalkanes

Thermal isomerizations of vinylcyclopropanes usually yield cyclopentenes as the principal products, although acyclic dienes are sometimes among the products and in some instances are the major products. These reactions have been reviewed in part by Frey and Walsh - and by Sarel et al..  

Isomerizations of most vinylcyclopropanes are first order and are unaffected by glass surfaces and isomerization products. In two instances they were shown to be insensitive to added nitric oxide or oxygen In the gas phase, their rates are independent of pressure above about 5 torr but fall off with diminishing pressure at lower pressures.  

Most vinylcyclopropane isomerizations have energies of activation near 50 kcal.mole and entropies of activation near zero. Activation parameters and reaction products of vinyl cyclopropane isomerizations are summarized in Table 9. 

These rearrangements may involve formation of diradicals as discrete intermediates, or they may be concerted processes in which bond breaking and bond making occur simultaneously. Their insensitivity to nitric oxide and oxygen and the absence of dimeric products suggest that long-lived radicals are not intermediates. The effects of substituents on reactivity can be accounted for in terms of either a concerted or a diradical mechanism. However, the 

According to this mechanism, the initial step in typical vinylcyclopropane isomerizations is cleavage of the cyclopropane ring to form an allylic diradical, which undergoes intramolecular reorganization more rapidly than it enters into bimolecular reactions with other radicals, viz- 

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