Relay chlorine transfer

Breslow s template-directed remote oxidation of steroids utilizes an aryl iodide as a template to direct the oxidation of steroid tertiary carbons by the radical relay mechanism, in which a chlorine radical is transferred from a [9-1-2] [PhICl] radical to the iodine atom of the template and then relayed to a geometrically accessible hydrogen atom. This method allows a highly regioselective functionalization of nonactivated carbon atoms of steroids [Eq. (78)] [137,138]. 

Since this study led to a number of extensions, it is desirable to explain why the process works, and what its advantages are. The rate advantage of a radical relay process is that the hydrogen abstraction is intramolecular, rather than the intermolecular abstraction that would occur without the relay by the template. However, this explains it only in part, since the relaying of a chlorine atom from the radical in solution to the iodine of the template is of course an intermolecular process. Why is the two-step sequence - intermolecular chlorine atom transfer, then intramolecular hydrogen abstraction - faster than an intermolecular hydrogen abstraction by the free radical in solution The answer is relat-... 

We showed that radical relay in the presence of phenyliodine dichloride but with an excess of CBr4 afforded the bromosteroid, with bromine at the point that is normally chlorinated [55]. When (SCN)2 was the additive, rather than CBr4, an SCN group was formed [55]. The mechanism shown (Scheme 6-8) involves a bromine atom transfer to... 

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