Cyclization cyclobutyl radicals

Step. Pigou utilized the BS model in order to determine the likelihood of cyclization and selectivity during the ring closure in some substituted cyclobutyl radicals 25 (Scheme 3) [12], While high selectivity as well as Thorpe-Ingold rate enhancement was predicted, this route proved not to be synthetically viable due to other competing rearrangement processes [12]. 

In many cases, particularly for the study of fast reactions (ky> key in Scheme 164), the opening of the cyclopropylmethyl radical to the isomeric allyl-carbinyl radical, one thousand times faster than the cyclization of the 5-hexenyl radical, is a useful complementary tool. It has been used mainly by Kochi for the study of cage reactions of alkyl radicals in solution as well for the study of ligand and electron transfer oxidations of alkyl radicals. Furthermore, in the last three studies the knowledge of the fate of the isomeric cyclobutyl intermediate was very useful in distinguishing between a homolytic (no rearrangement of the cyclobutyl radical) and a heterolytic pathway (fast... 

Like many ionic cyclizations, the formation of cyclobutane rings by radical cyclizations is very slow, and powerful accelerating effects will be required for any synthetic applications (see Scheme 7).2 These cyclizations are again reversible, and, in the absence of any other effects, the equilibrium lies to the side of the open radical due to ring strain. The fragmentations of simple cyclobutyl carbinyl radicals are slow,... 

The pent-l-en-5-yl radical does not cyclize in any of the matrices. Non-terminal addition would involve very severe steric factors and necessitate the formation of a cyclobutyl structure which would be highly strained. Addition to the terminal end to form the cyclopentyl radical might be expected to occur but steric models again show that approach in the plane of the 77-orbital is impossible and that only approach in the nodal plane can occur. 

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