Design of Leaving Groups in Radical C-C Fragmentations: Through- Bond 2c-3e Interactions in Self-Terminating Radical Cascades

Radical cascades terminated by beta-scission of exocyclic C-C bonds allow for the formation of aromatic products. Whereas beta-scission is common for weaker bonds, achieving this reactivity for carbon-carbon bonds requires careful design of radical leaving groups. It has now been found that the energetic penalty for breaking a strong sigma-bond can be compensated by the gain of aromaticity in the product and by the stabilizing two-center, three-electron "half-bond" present in the radical fragment. Furthermore, through-bond communication of a radical and a lone pair accelerates the fragmentation by selectively stabilizing the transition state. The stereoelectronic design of radical leaving groups leads to a new, convenient route to Sn-functionalized aromatics.