Radical domino processes

In an anionic/radical domino process an interim single-electron transfer (SET) from the intermediate of the first anionic reaction must occur. Thus, a radical is generated which can enter into subsequent reactions. Although a SET corresponds to a formal change of the oxidation state, the transformations will be treated as typical radical reactions. To date, only a few true anionic/radical domino transformations have been reported in the literature. However, some interesting examples of related one-pot procedures have been established where formation of the radical occurs after the anionic step by addition of TEMPO or Bu3SnH. A reason for the latter approach are the problems associated with the switch between anionic and radical reaction patterns, which often do not permit the presence of a radical generator until the initial anionic reaction step is finished. 

Usually, free-radical domino processes are characterized by a sequence of intramolecular steps, the overall propagation coordinate being unimolecular (excluding initiation and termination steps) (Scheme 3.1) [1],... 

A dramatic improvement in this new round trip radical domino processes developed by Curran s group was presented by Takasu, Ihara and coworkers. The new method relies on the introduction of a conjugated ester moiety at the terminal olefm, thereby effecting an acceleration of the domino reaction accompanied with an enhancement of the regio- and stereoselectivity [81]. Thus, reaction of 3-196 with Bu3SnH led to a 4 3 mixture of the two diastereomeric tricycles 3-197 and 3-198 in 83% yield. In this process, the vinyl radical 3-199 is initially formed, but this smoothly cyclizes in 5-exo-trig manner to give radical 3-200 (Scheme 3.52). Due to... 

Scheme 17. Unexpected C-arylation by a radical domino process...

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