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

Radical cascades terminated by β‐scission of exocyclic CC bonds allow for the formation of aromatic products. Whereas β‐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 σ‐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. Half‐bonds to halve bonds: Cyclization followed by fragmentation allows the use of alkenes as alkyne equivalents. The energetic penalty for breaking a strong CC bond is compensated by the gain of aromaticity in the product and by stabilizing 2c–3e half‐bonds in the radical fragments. Kinetic acceleration of the fragmentation is provided by selective transition‐state stabilization through 2c–3e through‐bond interactions (see scheme).