Use of Unsaturated Systems Cyclization as a Free Radical Proof

Comments on the Use of Unsaturated Systems Cyclization as a Free Radical Proof 

The conclusion reached in Section C was that the cyclization of heteroatomic unsaturated systems could not be used as unambiguous proof of 

The first question is if no cyclization to the (Cy5) compounds is observed when the 5-hexenyl radical is chosen, is it possible to rule out the formation of an alkyl radical R on the reaction pathway The answer is no, not if fast competitive intermolecular reactions are expected. In this case, it is necessary to work at low concentrations in order to favor the intramolecular process but even under these conditions, very low yields of cyclized products are sometimes obtained. The use of the faster ring-opening processes (see Section XII. 1.D) will be then a useful complementary probe. But there is a case where even these faster reactions cannot afford a positive answer, when the radical intermediate reacts, by dimerization or disproportionation, with another radical in the solvent cage, since these processes are faster than the rearrangement processes. 

The second question is if cyclization of the 5-hexenyl compound to the (Cy5) compound is observed, is it possible to conclude safely to a homolytic pathway The answer is also no, since, as discussed in Section XII. l.D, other selective cyclizations to the (Cy5) compounds in the Cy5/Cy6 case, are known which do not involve free radical intermediates. 

These warnings should not lead to the conclusion that the free radical cyclization (or ring-opening) tool must be avoided. Quite the contrary, the 5-hexenyl cyclization is certainly one of the best probes for the presence of alkyl free radical intermediates, but in dubious cases, as for instance those involving organometallic compounds, other probes are needed. 

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