Octenyl radicals

Fig. 10.13. MM2 models of exo and endo cyclization transition structures for 5-hexenyl, 6-heptenyl, and 7-octenyl radicals. Reproduced from Tetrahedron, 41, 3925 (1985), by permission of Elsevier.

Generally, most radical cyclizations proceed kinetically in an exo fashion to provide the smaller of the two possible rings. Nonetheless, as an important exception, 1-endo and 8-endo cyclizations do occur when the (conformationally restrained) intermediate radicals possess geometrical restrictions, i.e. less degrees of freedom relative to normal heptenyl radicals. It also should be noted that the rate constants for 1-exo and 8-endo cyclization of 7-octenyl radical are almost identical—the preference for exo-closure decreases as the rings get bigger [86]. 

The structure of the cyclo-octenyl radical (7) is such that it can adopt a configuration in which intramolecular addition across the ring may occur to give c bicyclic radical (8). 

It was thus of interest to determine whether a transannular addition could occur for the cyclo-octenyl radical. The cyclo-octenyl radical has been prepared from its parent halohydrocarbon in several matrices in the rotating cryostat. At 77°K the radical was stabilized in all of the matrices but when the samples were warmed reaction took place. The e.s.r. spectra showed that the main reaction in matrices of water, benzene or camphane was hydrogen abstraction adjacent to the double bond to give a cyclic allyl radical (9) rather than the bicyclic radical. However, in a matrix of bicyclopentadiene dimer cyclization occurs to give the bicyclic radical and in a matrix of adamantane both allylic and bicyclic radicals were formed. 

Unlike the case of the hexenyl radical, the mode of reaction of the cyclo-octenyl radical is affected by the matrix. However, the results show that transannular addition can occur in a suitable matrix at 77°K, and thus can probably occur readily in the liquid phase where there is less steric control on the conformation of the cyclo-octenyl radical. 

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