Phenanthrene Synthesis by Eosin Y‐Catalyzed, Visible Light‐ Induced [4+2] Benzannulation of Biaryldiazonium Salts with Alkynes

Abstract A metal‐free, visible light‐induced [4+2] benzannulation of biaryldiazonium salts with alkynes was developed. With eosin Y as photoredox catalyst, a variety of 9‐substituted or 9,10‐disubstituted phenanthrenes were obtained via a cascade radical addition and cyclization sequence.