Free radical cyclization kinetic control

The most important process (which accounts for most of the uses of radical cychzations in synthesis) is the selective 5-exo-cyclization of the 5-hexenyl radical to give the cyclopentyl methyl radical. This occurs even though the alternative - a 6-endo cyclization to give a more stable, cyclohexyl radical - is thermodynamically more favorable. Thus, the 5-ex o-cycHzation proceeds under kinetic control. The preference for 5-ex o-cyclization is explained by an early transition state with little product character. The transition state is a strain-free chair-like arrangement, which nicely accommodates the stereoelectronically required attack angle on the alkene. This model also nicely explains the stereochemical outcome of the cycHzation reaction. Assuming that substituents prefer to adopt pseudo-equatorial positions in the chair-Uke transition state, we see why ... 

Even when a free radical process can reasonably be assumed, the results are generally complicated by the possible occurrence of reversible cyclizations leading to a mixture of the two thiaheterocyclic compounds. And even when the cyclization process may be assumed to occur under kinetic control, the reaction appears to be much less selective than in the cases discussed in the preceding sections, although the ratio of the two cyclized products may be dramatically altered compared to reversible processes. With these difficulties in mind, and because the relatively small number of studies devoted to cyclizations d unsaturated thiyl radicals, any attempt at rationalization must be considered as tentative at this time. 

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